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Prefrontal control of cognitive functions critically depends upon glutamatergic transmission and N-methyl D-aspartate (NMDA) receptors, the activity of which is regulated by dopamine. Yet whether the NMDA receptor coagonist d-serine is implicated in the dopamine-glutamate dialogue in the prefrontal cortex (PFC) and other brain areas remains unexplored. Here, using electrophysiological recordings, we show that d-serine is required for the fine-tuning of glutamatergic neurotransmission, neuronal excitability, and synaptic plasticity in the PFC through the actions of dopamine at D 1 and D 3 receptors. Using in vivo microdialysis, we show that D 1 and D 3 receptors exert a respective facilitatory and inhibitory influence on extracellular levels and activity of d-serine in the PFC, with actions expressed primarily via the cAMP/protein kinase A (PKA) signaling cascade. Further, using functional magnetic resonance imaging (fMRI) and behavioral assessment, we show that d-serine is required for the potentiation of cognition by D 3 R blockade as revealed in a test of novel object recognition memory. Collectively, these results unveil a key role for d-serine in the dopaminergic neuromodulation of glutamatergic transmission and PFC activity, findings with clear relevance to the pathogenesis and treatment of diverse brain disorders involving alterations in dopamine-glutamate cross-talk., Competing Interests: Competing interest statement: M.J.M. and J.-M.R., respectively, were and are full-time employees of Institut de Recherche Servier and have no other interests to declare. S. Schmidt and J.K. are employees of Pronexus Analytical AB but declare no conflicts of interests.

Frontocortical NMDA receptors are pivotal in regulating cognition and mood, are hypofunctional in schizophrenia, and may contribute to autistic spectrum disorders. Despite extensive interest in agents potentiating activity at the co-agonist glycine modulatory site, few comparative functional studies exist. This study systematically compared the actions of the glycine reuptake inhibitors, sarcosine (40-200 mg/kg) and ORG24598 (0.63-5 mg/kg), the agonists, glycine (40-800 mg/kg), and D-serine (10-160 mg/kg) and the partial agonists, S18841 (2.5 mg/kg s.c.) and D-cycloserine (2.5-40 mg/kg) that all dose-dependently prevented scopolamine disruption of social recognition in adult rats. Over similar dose ranges, they also prevented a delay-induced impairment of novel object recognition (NOR). Glycine reuptake inhibitors specifically elevated glycine but not D-serine levels in rat prefrontal cortical (PFC) microdialysates, while glycine and D-serine markedly increased levels of glycine and D-serine, respectively. D-Cycloserine slightly elevated D-serine levels. Conversely, S18841 exerted no influence on glycine, D-serine, other amino acids, monamines, or acetylcholine. Reversal of NOR deficits by systemic S18841 was prevented by the NMDA receptor antagonist, CPP (20 mg/kg), and the glycine modulatory site antagonist, L701,324 (10 mg/kg). S18841 blocked deficits in NOR following microinjection into the PFC (2.5-10 μg/side) but not the striatum. Finally, in rats socially isolated from weaning (a neurodevelopmental model of schizophrenia), S18841 (2.5 and 10 mg/kg s.c.) reversed impairment of NOR and contextual fear-motivated learning without altering isolation-induced hyperactivity. In conclusion, despite contrasting neurochemical profiles, partial glycine site agonists and glycine reuptake inhibitors exhibit comparable pro-cognitive effects in rats of potential relevance to treatment of schizophrenia and other brain disorders where cognitive performance is impaired.

The highly-interconnected and neurochemically-rich frontal cortex plays a crucial role in the regulation of mood and cognition, domains disrupted in depression and other central nervous system disorders, and it is an important site of action for their therapeutic control. For improving our understanding of the function and dysfunction of the frontal cortex, and for identifying improved treatments, quantification of extracellular pools of neuromodulators by microdialysis in freely-moving rodents has proven indispensable. This approach has revealed a complex mesh of autoreceptor and heteroceptor interactions amongst monoaminergic pathways, and led from selective 5-HT reuptake inhibitors to novel classes of multi-target drugs for treating depression like the mixed α 2 -adrenoceptor/5-HT reuptake inhibitor, S35966, and the clinically-launched vortioxetine and vilazodone. Moreover, integration of non-monoaminergic actions resulted in the discovery and development of the innovative melatonin receptor agonist/5-HT 2C receptor antagonist, Agomelatine. Melatonin levels, like those of corticosterone and the "social hormone", oxytocin, can now be quantified by microdialysis over the full 24 h daily cycle. Further, the introduction of procedures for measuring extracellular histamine and acetylcholine has provided insights into strategies for improving cognition by, for example, blockade of 5-HT 6 and/or dopamine D 3 receptors. The challenge of concurrently determining extracellular levels of GABA, glutamate, d-serine, glycine, kynurenate and other amino acids, and of clarifying their interactions with monoamines, has also been resolved. This has proven important for characterizing the actions of glycine reuptake inhibitors that indirectly augment transmission at N-methyl-d-aspartate receptors, and of "glutamatergic antidepressants" like ketamine, mGluR5 antagonists and positive modulators of AMPA receptors (including S47445). Most recently, quantification of the neurotoxic proteins Aβ42 and Tau has extended microdialysis studies to the pathogenesis of neurodegenerative disorders, and another frontier currently being broached is microRNAs. The present article discusses the above themes, focusses on recent advances, highlights opportunities for clinical "translation", and suggests avenues for further progress., (© The Author(s) 2016.)

D2/D3 dopamine receptors (D2R/D3R) agonists regulate Akt, but their effects display a complex time-course. In addition, the respective roles of D2R and D3R are not defined and downstream targets remain poorly characterized, especially in vivo. These issues were addressed here for D3R. Systemic administration of quinelorane, a D2R/D3R agonist, transiently increased phosphorylation of Akt and GSK-3β in rat nucleus accumbens and dorsal striatum with maximal effects 10 min after injection. Akt activation was associated with phosphorylation of several effectors of the mammalian target of rapamycin complex 1 (mTORC1): p70S6 kinase, ribosomal protein-S6 (Ser240/244), and eukaryotic initiation factor-4E binding protein-1. The action of quinelorane was antagonized by a D2/D3R antagonist, raclopride, and the selective D3R antagonist S33084, inactive by themselves. Furthermore, no effect of quinerolane was seen in knock-out mice lacking D3R. In drd1a-EGFP transgenic mice, quinelorane activated Akt/GSK-3β in both neurons expressing and lacking D1 receptor. Thus, the stimulation of D3R transiently activates the Akt/GSK-3β pathway in the two populations of medium-size spiny neurons of the nucleus accumbens and dorsal striatum. This effect may contribute to the influence of D3R ligands on reward, cognition, and processes disrupted in schizophrenia, drug abuse, and Parkinson's disease., (© 2013 International Society for Neurochemistry.)

Studies of 5-HT-glutamate interactions suggest that activation of brain 5-HT(2A) receptors leads to an AMPA receptor-mediated induction of the immediate early (activity-dependent) gene, Arc (Arg3.1). In this respect, noradrenaline-glutamate interactions are poorly characterised. Here we investigated the influence on regional brain Arc gene expression of selective blockade of α(2)-adrenoceptors in rats. Several complementary techniques were used: qPCR (mRNA, discrete tissue punches), in situ hybridisation (mRNA, sections) and immunocytochemistry. The α(2)-adrenoceptor antagonist, RX 821002, dose-dependently and time-dependently (maximal effect 2 h) increased Arc mRNA levels as demonstrated both by qPCR and in situ hybridisation. The α(2)-adrenoceptor antagonist, atipamezole, also increased Arc mRNA in in situ hybridisation studies. Changes in Arc mRNA after RX 821002 were of similar magnitude in punches and intact tissue sections and region-specific, with effects being most pronounced in parietal cortex and caudate putamen, less robust in frontal cortex, and not detectable in hippocampal sub-regions. Both qPCR and in situ hybridisation studies demonstrated that RX 821002-induced Arc mRNA was blocked by the AMPA antagonist, GYKI 52466. Pretreatment with the NMDA antagonist MK 801 also prevented RX 821002-induced Arc mRNA, as did the mGluR5 antagonist MPEP, whilst the mGluR2/3 antagonist, LY341495, had no effect. Finally, immunocytochemical studies showed that RX 821002 increased Arc-immunoreactivity in cells in close apposition to α(2)-adrenoceptor-positive processes. Thus, employing three complementary techniques, these observations demonstrate that blockade of α(2)-adrenoceptors triggers brain expression of the immediate early gene, Arc, and that this effect involves the recruitment of AMPA, NMDA and mGluR5 but not mGluR2/3 glutamatergic receptors., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Cognitive deficits in schizophrenia severely compromise quality of life and are poorly controlled by current antipsychotics. While 5-HT(6) receptor blockade holds special promise, molecular substrates underlying their control of cognition remain unclear. Using a proteomic strategy, we show that 5-HT(6) receptors physically interact with several proteins of the mammalian target of rapamycin (mTOR) pathway, including mTOR. Further, 5-HT(6) receptor activation increased mTOR signalling in rodent prefrontal cortex (PFC). Linking this signalling event to cognitive impairment, the mTOR inhibitor rapamycin prevented deficits in social cognition and novel object discrimination induced by 5-HT(6) agonists. In two developmental models of schizophrenia, specifically neonatal phencyclidine treatment and post-weaning isolation rearing, the activity of mTOR was enhanced in the PFC, and rapamycin, like 5-HT(6) antagonists, reversed these cognitive deficits. These observations suggest that recruitment of mTOR by prefrontal 5-HT(6) receptors contributes to the perturbed cognition in schizophrenia, offering new vistas for its therapeutic control., (Copyrights © 2012 The Authors. Published by John Wiley and Sons, Ltd on behalf of EMBO.)

Corticosterone influences emotion and cognition via actions in a diversity of corticolimbic structures, including the amygdala. Since extracellular levels of corticosterone in brain have rarely been studied, we characterized a specific and sensitive enzymatic immunoassay for microdialysis quantification of corticosterone in the basolateral amygdaloid complex of freely-moving rats. Corticosterone levels showed marked diurnal variation with an evening (dark phase) peak and stable, low levels during the day (light phase). The "anxiogenic agents", FG7142 (20 mg/kg) and yohimbine (10 mg/kg), and an environmental stressor, 15-min forced-swim, induced marked and sustained (1-3 h) increases in dialysis levels of corticosterone in basolateral amygdaloid complex. They likewise increased dialysis levels of dopamine and noradrenaline, but not serotonin and GABA. As compared to basal corticosterone levels of ~200-300 pg/ml, the elevation provoked by forced-swim was ca. 20-fold and this increase was abolished by adrenalectomy. Interestingly, stress-induced rises of corticosterone levels in basolateral amygdaloid complex were abrogated by combined but not separate administration of the corticotrophin releasing factor(1) (CRF(1)) receptor antagonist, CP154,526, and the vasopressin(1b) (V(1b)) receptor antagonist, SSR149,415. Underpinning their specificity, they did not block forced-swim-induced elevations in dopamine and noradrenaline. In conclusion, extracellular levels of corticosterone in the basolateral amygdaloid complex display marked diurnal variation. Further, they are markedly elevated by acute stressors, the effects of which are mediated (in contrast to concomitant elevations in levels of monoamines) by co-joint recruitment of CRF(1) and V(1b) receptors., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The present studies characterized the functional profile of N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-1,2-dihydro-3-H-benzo[e]indole-3-carboxamide) (S32212), a combined serotonin (5-HT)(2C) receptor inverse agonist and α(2)-adrenoceptor antagonist that also possesses 5-HT(2A) antagonist properties (J Pharmacol Exp Ther 340:750-764, 2012). Upon parenteral and/or oral administration, dose-dependent (0.63-40.0 mg/kg) actions were observed in diverse procedures. Both acute and subchronic administration of S32212 reduced immobility time in a forced-swim test in rats. Acutely, it also suppressed marble burying and aggressive behavior in mice. Long-term administration of S32212 was associated with rapid (1 week) and sustained (5 weeks) normalization of sucrose intake in rats exposed to chronic mild stress and with elevated levels of mRNA encoding brain-derived neurotrophic factor in hippocampus and amygdala (2 weeks). S32212 accelerated the firing rate of adrenergic perikarya in the locus coeruleus and elevated dialysis levels of noradrenaline in the frontal cortex and hippocampus of freely moving rats. S32212 also elevated the frontocortical levels of dopamine and acetylcholine, whereas 5-HT, amino acids, and histamine were unaffected. These neurochemical actions were paralleled by "promnemonic" properties: blockade of scopolamine-induced deficits in radial maze performance and social recognition and reversal of delay-induced impairments in social recognition, social novelty discrimination, and novel object recognition. It also showed anxiolytic actions in a Vogel conflict procedure. Furthermore, in an electroencephalographic study of sleep architecture, S32212 enhanced slow-wave and rapid eye movement sleep, while decreasing waking. Finally, chronic administration of S32212 neither elevated body weight nor perturbed sexual behavior in male rats. In conclusion, S32212 displays a functional profile consistent with improved mood and cognitive performance, together with satisfactory tolerance.

N-methyl-D-aspartate receptors (NMDARs) subserve numerous neurophysiological and neuropathological processes in the cerebral cortex. Their activation requires the binding of glutamate and also of a coagonist. Whereas glycine and D-serine (D-ser) are candidates for such a role at central synapses, the nature of the coagonist in cerebral cortex remains unknown. We first show that the glycine-binding site of NMDARs is not saturated in acute slices preparations of medial prefrontal cortex (mPFC). Using enzymes that selectively degrade either D-ser or glycine, we demonstrate that under the present conditions, D-ser is the principle endogenous coagonist of synaptic NMDARs at mature excitatory synapses in layers V/VI of mPFC where it is essential for long-term potentiation (LTP) induction. Furthermore, blocking the activity of glia with the metabolic inhibitor, fluoroacetate, impairs NMDAR-mediated synaptic transmission and prevents LTP induction by reducing the extracellular levels of D-serine. Such deficits can be restored by exogenous D-ser, indicating that the D-amino acid mainly originates from glia in the mPFC, as further confirmed by double-immunostaining studies for D-ser and anti-glial fibrillary acidic protein. Our findings suggest that D-ser modulates neuronal networks in the cerebral cortex by gating the activity of NMDARs and that altering its levels is relevant to the induction and potentially treatment of psychiatric and neurological disorders.

"Ecstasy" [3,4-methylenedioxymetamphetamine (MDMA)] is of considerable interest in light of its prosocial properties and risks associated with widespread recreational use. Recently, it was found to bind trace amine-1 receptors (TA(1)Rs), which modulate dopaminergic transmission. Accordingly, using mice genetically deprived of TA(1)R (TA(1)-KO), we explored their significance to the actions of MDMA, which robustly activated human adenylyl cyclase-coupled TA(1)R transfected into HeLa cells. In wild-type (WT) mice, MDMA elicited a time-, dose-, and ambient temperature-dependent hypothermia and hyperthermia, whereas TA(1)-KO mice displayed hyperthermia only. MDMA-induced increases in dialysate levels of dopamine (DA) in dorsal striatum were amplified in TA(1)-KO mice, despite identical levels of MDMA itself. A similar facilitation of the influence of MDMA upon dopaminergic transmission was acquired in frontal cortex and nucleus accumbens, and induction of locomotion by MDMA was haloperidol-reversibly potentiated in TA(1)-KO versus WT mice. Conversely, genetic deletion of TA(1)R did not affect increases in DA levels evoked by para-chloroamphetamine (PCA), which was inactive at hTA(1) sites. The TA(1)R agonist o-phenyl-3-iodotyramine (o-PIT) blunted the DA-releasing actions of PCA both in vivo (dialysis) and in vitro (synaptosomes) in WT but not TA(1)-KO animals. MDMA-elicited increases in dialysis levels of serotonin (5-HT) were likewise greater in TA(1)-KO versus WT mice, and 5-HT-releasing actions of PCA were blunted in vivo and in vitro by o-PIT in WT mice only. In conclusion, TA(1)Rs exert an inhibitory influence on both dopaminergic and serotonergic transmission, and MDMA auto-inhibits its neurochemical and functional actions by recruitment of TA(1)R. These observations have important implications for the effects of MDMA in humans.

This study describes a novel analytical method permitting simultaneous HPLC-fluorimetric quantification of multiple (15) D- and L-amino acids, kynurenate, taurine and phosphoethanolamine (a marker of membrane integrity) in microdialysates of prefrontal cortex of freely-moving rats. Levels of GABA were elevated by the transporter inhibitor, nipecotic acid, and by the transaminase inhibitor, vigabatrine.Supporting a neuronal origin, they were decreased by the GABAB autoreceptor agonist, baclofen,yet unaffected by fluoroacetate which disrupts glial metabolism. Glutamate levels were elevated by the transporter inhibitor, L-trans-PDC, and mainly neuronal since they were not decreased by fluoroacetate,yet reduced by baclofen (which recruits GABAB receptors on glutamatergic terminals) and elevated by the NMDA receptor antagonist, dizocilpine. By contrast, levels of glutamine were reduced by L-trans-PDC.Consistent with glial origin, they were unaffected by baclofen, yet reduced by fluoroacetate. Administration of D-serine selectively increased its levels over L-serine, and vice versa. D-serine modestly decreased levels of glycine, which were enhanced by administration of glycine itself and of the glycine transporter-1 inhibitor, sarcosine. Kynurenate levels were increased by its precursor, kynurenine, an effect abolished by the amino-transferase inhibitor, amino-oxyacetate. Taurine and the energy drink, Red Bull®, selectively elevated levels of taurine, which were only slightly reduced by fluoroacetate. Finally, administration of NMDA increased levels of taurine, kynenurate and phosphoethanolamine, while reducing D-serine. These actions were abolished by the competitive NMDA receptor antagonist, CPP, which was inactive alone. This broad-based dialysis system should prove instructive for exploring actions of psychotropic drugs, and for characterising animal models of CNS disorders., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Though neurokinin(1) (NK(1)) receptors are implicated in depressed states and their treatment, selective antagonists have disappointed in clinical trials. Accordingly, we designed a novel ligand, S41744 (2-piperazin-1-yl-indan-2-carboxylic-acid-(3-chloro-5-fluoro-benzyl)-methyl-amide), which both blocks NK(1) receptors and interferes with serotonin (5-HT) reuptake. S41744 mimicked the selective antagonist aprepitant in binding human (h)NK(1) receptors and in antagonising Substance-P-mediated Extracellular-Regulated-Kinase phosphorylation (pK(B), 7.7). Further, it dose-dependently (0.63-40.0 mg/kg, i.p.) displaced ex vivo [(3)H]-[Sar(9),Met(O(2))(11)]-Substance P binding to gerbil striatum, attenuated formalin-induced hind-paw licking in gerbils, and antagonised locomotion induced by i.c.v. administration of the NK(1) agonist GR73632 to guinea pigs. Like paroxetine, S41744 recognised h5-HT transporters, reduced synaptosomal uptake of 5-HT (pK(B), 7.9), and dose-dependently (0.63-10.0 mg/kg) elevated dialysis levels of 5-HT in the hippocampus and frontal cortex of freely-moving guinea pigs. Further, S41744 increased extracellular levels of 5-HT in frontal cortex and hippocampus of rats to a greater extent than paroxetine, and its inhibitory influence upon serotonergic perikarya was blunted relative to its affinity for 5-HT transporters. S41744 more potently blocked stress-induced vocalizations in guinea pigs than aprepitant and paroxetine, and it was active in forced-swim and marble-burying procedures of putative antidepressant properties in mice. While aprepitant displayed anxiolytic actions in stress-induced foot-tapping and social interaction tests in gerbils, paroxetine was anxiogenic and S41744 "neutral", reflecting balanced NK(1) antagonism and suppression of 5-HT reuptake. Moreover, S41744 shared anxiolytic actions of aprepitant in the rat Vogel Conflict Test. In conclusion, S41744 is an innovative NK(1) antagonist/5-HT reuptake inhibitor justifying further evaluation for treatment of stress-related disorders., (Copyright 2010 Elsevier B.V. All rights reserved.)

Melanin-concentrating hormone (MCH)1 receptors are widely expressed in limbic structures and cortex. Their inactivation is associated with anxiolytic and antidepressive properties but little information is available concerning cognition. This issue was addressed using the selective antagonists, SNAP-7941 and GW3430, in a social recognition paradigm in rats. The muscarinic blocker, scopolamine (1.25 mg/kg s.c.), reduced social recognition, an action dose-dependently blocked by SNAP-7941 and GW3430 (0.63-10.0 and 20.0-80.0 mg/kg i.p., respectively) which did not themselves display amnesic properties. Further, in a protocol where a spontaneous deficit was induced by a prolonged inter-session delay, SNAP-7941 and GW3430 dose-dependently enhanced social recognition. In dialysis studies, SNAP-7941 (0.63-40.0 mg/kg i.p.) and GW3430 (10.0-40.0 mg/kg i.p.) elevated extracellular levels of acetylcholine (ACh) in the frontal cortex (FCX) of freely moving rats. The SNAP-7941 effect was specific, as it did not increase levels of ACh in ventral and dorsal hippocampus: moreover, it did not modify levels of noradrenaline, dopamine, serotonin and glutamate in FCX. Active doses of SNAP-7941 and GW3430 corresponded to doses (2.5-40.0 and 10.0-80.0 mg/kg i.p., respectively) exerting anxiolytic properties in Vogel conflict and ultrasonic vocalization tests, and antidepressant actions in forced swim, isolation-induced aggression and marble-burying procedures. In contrast to SNAP-7941 and GW3430, the benzodiazepine, diazepam, decreased social recognition and dialysate levels of ACh, while the tricyclic, imipramine, reduced social recognition and failed to enhance cholinergic transmission. In conclusion, at anxiolytic and antidepressant doses, SNAP-7941 and GW3430 improve social recognition and elevate extracellular ACh levels in FCX. This profile differentiates MCH1 receptor antagonists from conventional anxiolytic and antidepressant agents.

Rationale: Serotonin (5-HT)(2C) receptors are implicated in the control of mood, and their blockade is of potential interest for the management of anxiodepressive states., Objectives: Herein, we characterized the in vitro and in vivo pharmacological profile of the novel benzourea derivative, S32006., Materials and Methods: Standard cellular, electrophysiological, neurochemical, and behavioral procedures were used., Results: S32006 displayed high affinity for human (h)5-HT(2C) and h5-HT(2B) receptors (pK (i)s, 8.4 and 8.0, respectively). By contrast, it had negligible (100-fold lower) affinity for h5-HT(2A) receptors and all other sites examined. In measures of Gq-protein coupling/phospholipase C activation, S32006 displayed potent antagonist properties at h5-HT(2C) receptors (pK (B) values, 8.8/8.2) and h5-HT(2B) receptors (7.8/7.7). In vivo, S32006 dose-dependently (2.5-40.0 mg/kg, i.p. and p.o.) abolished the induction of penile erections and a discriminative stimulus by the 5-HT(2C) receptor agonist, Ro60,0175, in rats. It elevated dialysis levels of noradrenaline and dopamine in the frontal cortex of freely moving rats, and accelerated the firing rate of ventrotegmental dopaminergic and locus ceruleus adrenergic neurons. At similar doses, S32006 decreased immobility in a forced-swim test in rats, reduced the motor depression elicited by 5-HT(2C) and alpha(2)-adrenoceptor agonists, and inhibited both aggressive and marble-burying behavior in mice. Supporting antidepressant properties, chronic (2-5 weeks) administration of S32006 suppressed "anhedonia" in a chronic mild stress procedure and increased both expression of BDNF and cell proliferation in rat dentate gyrus. Finally, S32006 (0.63-40 mg/kg, i.p. and p.o) displayed anxiolytic properties in Vogel conflict and social interaction tests in rats., Conclusion: S32006 is a potent 5-HT(2C) receptor antagonist, and possesses antidepressant and anxiolytic properties in diverse rodent models.

In contrast to clinically available antipsychotics, the novel benzopyranopyrrolidine derivative, S33138 (N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenyl-acetamide), behaves as a preferential antagonist of D(3) versus D(2) receptors and does not interact with histamine H(1) and muscarinic receptors. In contrast to haloperidol, clozapine, olanzapine, and risperidone, S33138 (0.16-2.5 mg/kg s.c.) did not disrupt performance in passive-avoidance and five-choice serial reaction time procedures. Furthermore, upon either systemic administration (0.04-2.5 mg/kg s.c.) or introduction into the frontal cortex (0.04-0.63 mug/side), S33138 potently attenuated the perturbation of social recognition by scopolamine or a prolonged intersession delay. Over a comparable and low-dose range, S33138 (0.04-0.63 mg/kg s.c.) elevated dialysis levels of acetylcholine in the frontal cortex of freely moving rats. At higher doses (2.5-10.0 mg/kg s.c.), S33138 also increased frontocortical levels of histamine, whereas monoamines, glutamate, glycine, and GABA were unaffected. By analogy to the other antipsychotics, S33138 (0.63-10.0 mg/kg s.c.) inhibited conditioned avoidance responses in rats, apomorphine-induced climbing in mice, and hyperlocomotion elicited by amphetamine, cocaine, dizocilpine, ketamine, and phencyclidine in rats. S33138 (0.16-2.5 mg/kg s.c.) also blocked the reduction of prepulse inhibition elicited by apomorphine. In comparison with the above actions, only "high" doses of S33138 (10.0-40.0 mg/kg s.c.) elicited catalepsy. To summarize, reflecting preferential blockade of D(3) versus D(2) receptors, S33138 preserves and/or enhances cognitive function, increases frontocortical cholinergic transmission, and is active in models of antipsychotic properties at doses well below those inducing catalepsy. In comparison with clinically available agents, S33138 displays, thus, a distinctive and promising profile of potential antipsychotic properties.

These studies examined the influence of the selective 5-hydroxytryptamine (serotonin) (5-HT)(1A) receptor partial agonist S15535 [4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine] upon cholinergic transmission and cognitive function in rodents. In the absence of acetylcholinesterase inhibitors, S15535 dose-dependently (0.04-5.0 mg/kg s.c.) elevated dialysis levels of acetylcholine in the frontal cortex and dorsal hippocampus of freely moving rats. In the cortex, the selective 5-HT(1A) receptor antagonist WAY100,635 [(N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide) fumarate] dose-dependently (0.0025-0.63) blocked this action of S15535. By contrast, in dorsal hippocampus, WAY100,635 mimicked the induction of acetylcholine release by S15535. In a social recognition paradigm, S15535 dose-dependently (0.16-10.0) improved retention, an action blocked by WAY100,635 (0.16), which was ineffective alone. Furthermore, S15535 dose-dependently (0.04-2.5) and WAY100,635 reversibly abolished amnesic properties of the muscarinic antagonist scopolamine (0.63) in this procedure. Cognitive deficits provoked by scopolamine in autoshaping and Morris water-maze procedures were likewise blocked by S15535 at doses of 0.63 to 10.0 and 0.16 to 2.5, respectively. In a two-platform spatial discrimination task, in which S15535 similarly abrogates cognitive deficits elicited by scopolamine, injection of S15535 (1.0 and 10.0 microg) into dorsal hippocampus blocked amnesic effects of the 5-HT(1A) agonist 8-hydroxy-2-dipropylaminotetralin (0.5 microg). Finally, S15535 (0.16-0.63) improved performance in a spatial, delayed nonmatching to sample model in mice, and in an operant delayed nonmatching to sample model in old rats, S15535 (1.25-5.0 mg/kg p.o.) increased response accuracy and reduced latency to respond. In conclusion, S15535 reinforces frontocortical and hippocampal release of acetylcholine and displays a broad-based pattern of procognitive properties. Its actions involve both blockade of postsynaptic 5-HT(1A) receptors and engagement of 5-HT(1A) autoreceptors.

S32504 [(+)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] displayed marked affinity for cloned, human (h)D(3) receptors (pK(i), 8.1) at which, in total G-protein ([(35)S]GTPgammaS binding, guanosine-5'-O-(3-[(35)S]thio)-triphosphate), Galpha(i3) (antibody capture/scintillation proximity), and mitogen-activated protein kinase (immunoblot) activation procedures, it behaved as an agonist: pEC(50) values, 8.7, 8.6, and 8.5, respectively. These actions were blocked by haloperidol and the selective D(3) receptor antagonist S33084 [(3aR,9bS)-N-[4-(8-cyano-1,3a,4,9b-tetrahydro-3H-benzopyrano[3,4-c]pyrrole-2-yl)-butyl]-(4-phenyl) benzamide)]. S32504 showed lower potency at hD(2S) and hD(2L) receptors in [(35)S]GTPgammaS binding (pEC(50) values, 6.4 and 6.7) and antibody capture/scintillation proximity (hD(2L), pEC(50), 6.6) procedures. However, reflecting signal amplification, it potently stimulated hD(2L) receptor-coupled mitogen-activated protein kinase (pEC(50), 8.6). These actions were blocked by haloperidol and the selective D(2) receptor antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol]. The affinity of S32504 for hD(4) receptors was low (5.3) and negligible for hD(1) and hD(5) receptors (pK(i), <5.0). S32504 showed weak agonist properties at serotonin(1A) ([(35)S]GTPgammaS binding, pEC(50), 5.0) and serotonin(2A) (G(q), pEC(50), 5.2) receptors and low affinity for other (>50) sites. In anesthetized rats, S32504 (0.0025-0.01 mg/kg, i.v.) suppressed electrical activity of ventrotegmental dopaminergic neurons. Correspondingly, S32504 (0.0025-0.63 mg/kg, s.c.) potently reduced dialysis levels (and synthesis) of dopamine in striatum, nucleus accumbens, and frontal cortex of freely moving rats, actions blocked by haloperidol and L741,626 but not by S33084. In contrast, S32504 only weakly inhibited serotonergic transmission and failed to affect noradrenergic transmission. Actions of S32504 were expressed stereospecifically versus its less active enantiomer S32601 [(-)-trans-3,4,4a,5,6,10b-hexahydro-9-carbomoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine]. Although the D(3)/D(2) agonist and antiparkinsonian agent ropinirole mimicked the profile of S32504, it was less potent. In conclusion, S32504 is a potent and selective agonist at dopamine D(3) and D(2) receptors.

This study employed [(35)S]guanosine 5'- O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding to compare the actions of antipsychotic agents known to stimulate cloned, human 5-HT(1A) receptors with those of reference agonists at postsynaptic 5-HT(1A) receptors. In rat hippocampal membranes, the following order of efficacy was observed (maximum efficacy, E(max), values relative to 5-HT=100): (+)8-OH-DPAT (85), flesinoxan (62), eltoprazine (60), S14506 (59), S16924 (48), buspirone (41), S15535 (22), clozapine (22), ziprasidone (21), pindolol (7), p-MPPI (0), WAY100,635 (0), spiperone (0). Despite differences in species and tissue source, the efficacy and potency (pEC(50)) of agonists (with the exception of clozapine) correlated well with those determined previously at human 5-HT(1A) receptors expressed in Chinese hamster ovary (CHO) cells. In contrast, clozapine was more potent at hippocampal membranes. The selective antagonists p-MPPI and WAY100,635 abolished stimulation of binding by (+)8-OH-DPAT, clozapine and S16924 (p-MPPI), indicating that these actions were mediated specifically by 5-HT(1A) receptors. Clozapine and S16924 also attenuated 5-HT- and (+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding, consistent with partial agonist properties. In [(35)S]GTPgammaS autoradiographic studies, 5-HT-induced stimulation, mediated through 5-HT(1A) receptors, was more potent in the septum (pEC(50) approximately 6.5) than in the dentate gyrus of the hippocampus (pEC(50) approximately 5) suggesting potential differences in coupling efficiency or G protein expression. Though clozapine (30 and 100 microM) did not enhance [(35)S]GTPgammaS labelling in any structure, S16924 (10 micro M) modestly increased [(35)S]GTPgammaS labelling in the dentate gyrus. On the other hand, both these antipsychotic agents attenuated 5-HT (10 microM)-stimulated [(35)S]GTPgammaS binding in the dentate gyrus and septum. In conclusion, clozapine, S16924 and ziprasidone act as partial agonists for G protein activation at postsynaptic 5-HT(1A) receptors in the hippocampus. These data support a role of postsynaptic 5-HT(1A) receptors in the functional profiles of certain antipsychotic agents.

Agomelatine (S20098) displayed pKi values of 6.4 and 6.2 at native (porcine) and cloned, human (h)5-hydroxytryptamine (5-HT)2C receptors, respectively. It also interacted with h5-HT2B receptors (6.6), whereas it showed low affinity at native (rat)/cloned, human 5-HT2A (<5.0/5.3) and 5-HT1A (<5.0/5.2) receptors, and negligible (<5.0) affinity for other 5-HT receptors. In antibody capture/scintillation proximity assays, agomelatine concentration dependently and competitively abolished h5-HT2C receptor-mediated activation of Gq/11 and Gi3 (pA2 values of 6.0 and 6.1). As measured by [3H]phosphatidylinositol depletion, agomelatine abolished activation of phospholipase C by h5-HT2C (pKB value of 6.1) and h5-HT2B (pKB value of 6.6) receptors. In vivo, it dose dependently blocked induction of penile erections by the 5-HT2C agonists (S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (Ro60,0175) and 1-methyl-2-(5,8,8-trimethyl-8H-3-aza-cyclopenta[a]inden-3-yl) ethylamine (Ro60,0332). Furthermore, agomelatine dose dependently enhanced dialysis levels of dopamine in frontal cortex of freely moving rats, whereas they were unaffected in nucleus accumbens and striatum. Although the electrical activity of ventrotegmental dopaminergic neurons was unaffected agomelatine, it abolished their inhibition by Ro60,0175. Extracellular levels of noradrenaline in frontal cortex were also dose dependently enhanced by agomelatine in parallel with an acceleration in the firing rate of adrenergic cell bodies in the locus coeruleus. These increases in noradrenaline and dopamine levels were unaffected by the selective melatonin antagonist N-[2-(5-ethyl-benzo[b]thien-3-yl)ethyl] acetamide (S22153) and likely flect blockade of 5-HT2C receptors inhibitory to frontocortical dopaminergic and adrenergic pathways. Correspondingly, distinction to agomelatine, melatonin showed negligible activity 5-HT2C receptors and failed to modify the activity of adrenergic and dopaminergic pathways. In conclusion, in contrast to melatonin, agomelatine behaves as an antagonist at 5-HT2B and 5-HT2C receptors: blockade of the latter reinforces frontocortical adrenergic and dopaminergic transmission.

Unilateral 6-hydroxydopamine (6-OHDA) lesions of substantia nigra pars compacta (SNPC) neurons in rats induce behavioural hypersensitivity to dopaminergic agonists. However, the role of specific dopamine receptors is unclear, and potential alterations in their transduction mechanisms remain to be evaluated. The present study addressed these issues employing the dopaminergic agonist, quinelorane, which efficaciously stimulated G-protein activation (as assessed by [35S]GTPgammaS binding) at cloned hD2 (and hD3) receptors. At rat striatal membranes, dopamine stimulated [35S]GTPgammaS binding by 1.9-fold over basal, but its actions were only partially reversed by the selective D2/D3 receptor antagonist, raclopride, indicating the involvement of other receptor subtypes. In contrast, quinelorane-induced stimulation (48% of the effect of dopamine) was abolished by raclopride, and by the D2 receptor antagonist, L741,626. Further, novel antagonists selective for D3 and D4 receptors, S33084 and S18126, respectively, blocked the actions of quinelorane at concentrations corresponding to their affinities for D2 receptors. Quinelorane potently induced contralateral rotation in unilaterally 6-OHDA-lesioned rats, an effect abolished by raclopride and L741,626, but not by D3 and D4 receptor-selective doses of S33084 and S18126, respectively. In functional ([35S]GTPgammaS) autoradiography experiments, quinelorane stimulated G-protein activation in caudate putamen and, to a lesser extent, in nucleus accumbens and cingulate cortex of naive rats. In unilaterally SNPC-lesioned rats, quinelorane-induced G-protein activation in the caudate putamen on the non-lesioned side was similar to that seen in naive animals (approximately 50% stimulation), but significantly greater on the lesioned side (approximately 80%). This increase was both pharmacologically and regionally specific since it was reversed by raclopride, and was not observed in nucleus accumbens or cingulate cortex. In conclusion, the present data indicate that, in rat striatum, the actions of quinelorane are mediated primarily by D2 receptors, and suggest that behavioural hypersensitivity to this agonist, induced by unilateral SNPC lesions, is associated with an increase in D2, but not D3 or D4, receptor-mediated G-protein activation.

S33005 displayed marked affinity for native, rat, and cloned human serotonin (5-HT) transporters (SERT) and less pronounced affinity for norepinephrine (NE) transporters (NET), while its affinity at dopamine (DA) transporters and >50 other sites was negligible. Reuptake of 5-HT and (less potently) NE into cerebral synaptosomes was inhibited by S33005, whereas DA reuptake was little affected. In vivo, S33005 prevented depletion of cerebral pools of 5-HT by parachloroamphetamine. Furthermore, it decreased electrical activity of raphe-localized serotonergic neurones, an action abolished by the 5-HT1A antagonist WAY100,635. At higher doses, S33005 blocked firing of locus ceruleus-localized adrenergic neurones, an action abolished by the alpha2-adrenergic antagonist idazoxan. In contrast, S33005 did not inhibit ventrotegmental dopaminergic neurones. In frontal cortex of freely moving rats, S33005 dose dependently elevated dialysate levels of 5-HT, NE, and DA. In hippocampus, levels of 5-HT and NE were similarly elevated, while in nucleus accumbens and striatum, levels of 5-HT were increased whereas DA was unaffected. Upon chronic (2 weeks) administration, basal levels of NE were elevated in frontal cortex and, therein, 5-HT2A receptor density was decreased. Comparative studies with clinically used antidepressants showed that venlafaxine possessed a profile similar to S33005 but was less potent. Clomipramine likewise interacted with SERTs and NETs but also with several other receptors types, while citalopram and reboxetine were preferential ligands of SERTs and NETs, respectively. In conclusion, S33005 interacts potently with SERTs and, less markedly, with NETs. It enhances extracellular levels of 5-HT and NE throughout corticolimbic structures and selectively elevates dialysis levels of DA in frontal cortex versus subcortical regions.

Rats were trained to recognize a discriminative stimulus (DS) elicited by the dopamine D(2)/D(3) receptor agonist, PD128,907 (0.16 mg/kg, i.p.), which suppressed frontocortical release of dopamine (DA) but not 5-HT. The selective 5-HT1A receptor agonists, 8-OH-DPAT and flesinoxan, dose-dependently generalized to PD128,907 with effective dose(50)s (ED50s) of 0.08 and 1.5mg/kg, s.c., respectively, and inhibited the release and synthesis of 5-HT but not of DA. The 'atypical' antipsychotic, clozapine, which displays weak partial agonist properties at 5-HT1A receptors, dose-dependently, though partially, generalized to PD128,907 (50%, 2.5mg/kg, s.c.). Further, S16924 and ziprasidone, which in a like manner, display partial agonist activity at 5-HT1A receptors, generalized with ED50s of 0.6 and 2.3mg/kg, s.c., respectively. In contrast, haloperidol, which is devoid of affinity at 5-HT1A sites, was inactive. At doses equivalent to those generalizing to PD128,907, clozapine, S16924 and ziprasidone reduced serotonergic (but not dopaminergic) transmission, whereas haloperidol was inactive. In rats trained to recognize a further D2/D3 agonist, 7-OH-DPAT (0.16 mg/kg, i.p.), generalization was obtained similarly with 8-OH-DPAT (ED50 = 0.07 mg/kg, s.c.), flesinoxan (3.4) and clozapine (0.6), but not with haloperidol. In conclusion, although PD128,907 and 7-OH-DPAT do not directly interact with 5-HT1A receptors or influence serotonergic transmission, their DS properties are mimicked by 5-HT1A receptor agonists at doses activating 5-HT1A but not D2/D3 (auto)receptors. These observations likely account for generalization of clozapine, S16924 and ziprasidone to PD128,907 and 7-OH-DPAT inasmuch as they behave as antagonists at D2/D3 receptors, yet agonists at 5-HT1A (auto)receptors.

Compared with cloned, human (h)D(2) receptors (pK(i) = 6.9), the antiparkinsonian agent piribedil showed comparable affinity for halpha(2A)- (7.1) and halpha(2C)- (7.2) adrenoceptors (ARs), whereas its affinity for halpha(2B)-ARs was less marked (6.5). At halpha(2A)- and halpha(2C)-ARs, piribedil antagonized induction of [(35)S]guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) binding by norepinephrine (NE) with pK(b) values of 6.5 and 6.9, respectively. Furthermore, Schild analysis of the actions of piribedil at halpha(2A)-ARs indicated competitive antagonism, yielding a pA(2) of 6.5. At a porcine alpha(2A)-AR-Gi1alpha-Cys351C (wild-type) fusion protein, piribedil competitively abolished (pA(2) = 6.5) GTPase activity induced by epinephrine. However, at a alpha(2A)-AR-Gi1alpha-Cys351I (mutant) fusion protein of amplified sensitivity, although still acting as a competitive antagonist (pA(2) = 6.2) of epinephrine, piribedil itself manifested weak partial agonist properties. Similarly, piribedil weakly induced mitogen-activated protein kinase phosphorylation via wild-type halpha(2A)-ARs, although attenuating its phosphorylation by NE. As demonstrated by functional [(35)S]GTPgammaS autoradiography in rats, piribedil antagonized activation by NE of alpha(2)-ARs in cortex, amygdala, and septum. Antagonist properties were also expressed in a dose-dependent enhancement of the firing rate of adrenergic neurons in locus ceruleus (0.125-4.0 mg/kg i.v.). Furthermore, piribedil (2.5-4.0 mg/kg s.c.) accelerated hippocampal NE synthesis, elevated dialysis levels of NE in hippocampus and frontal cortex, and blocked hypnotic-sedative properties of the alpha(2)-AR agonist xylazine. Finally, piribedil showed only modest affinity for rat alpha(1)-ARs (5.9) and weakly antagonized NE-induced activation of phospholipase C via halpha(1A)-ARs (pK(b) = 5.6). In conclusion, piribedil displays essentially antagonist properties at cloned, human and cerebral, rat alpha(2)-ARs. Blockade of alpha(2)-ARs may, thus, contribute to its clinical antiparkinsonian profile.

The alpha(2)-adrenoceptor (AR) agonist, S18616 ((S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(8'-chloro-1' , 2',3',4'-tetrahydronaphthalene)] accompanying article), suppressed electrical activity of adrenergic neurons in the locus ceruleus, an action reversed by the alpha(2)-AR antagonist, idazoxan, which itself enhanced their firing rate. Electrical activity of serotonergic neurons in the dorsal raphe nucleus was similarly suppressed, an action likewise blocked by idazoxan, which did not, itself, influence firing. In freely moving rats, S18616 decreased extracellular levels of norepinephrine (NE), serotonin (5-HT), and dopamine (DA) in frontal cortex and hippocampus. The selective alpha(2)- versus alpha(1)-AR antagonists, atipamezole and BRL-44408 (a preferential alpha(2A)-AR antagonist), elevated levels of NE and DA but not 5-HT. In their presence, the influence of S18616 on frontocortical levels of NE, DA, and 5-HT was blocked. In contrast, prazosin, a selective alpha(1)- versus alpha(2)-AR antagonist (which also preferentially blocks alpha(2B/2C)-ARs) dose dependently decreased levels of 5-HT, but not NE and DA, and failed to modify the actions of S18616. Ultrasonic vocalizations elicited by rats in an aversive environment were inhibited by S18616, which also suppressed aggressive and marble-burying behaviors in mice. Furthermore, S18616 (biphasically) enhanced punished responses in the Vogel conflict test and active social interaction tests in rats. At higher doses, S18616 displayed sedative/hypnotic properties. Both anxiolytic and motor actions of S18616 were inhibited by atipamezole and BRL-44408 but not prazosin. Dexmedetomidine mimicked the actions of S18616 at higher doses except for more potent sedative/hypnotic properties. Clonidine also mimicked S18616, but only at markedly higher doses. In conclusion, via activation of alpha(2)-ARs, S18616 potently inhibits corticolimbic adrenergic, serotonergic, and (frontocortical) dopaminergic transmission in parallel with the expression of its anxiolytic and sedative properties.

The present study evaluated, via a combined electrophysiological and dialysis approach, the potential influence of serotonin (5-HT)(2C) as compared to 5-HT(2A) and 5-HT(2B) receptors on dopaminergic, adrenergic, and serotonergic transmission in frontal cortex (FCX). Whereas the selective 5-HT(2A) antagonist MDL100,907 failed to modify extracellular levels of dopamine (DA), noradrenaline (NA) or 5-HT simultaneously quantified in single dialysate samples of freely-moving rats, the 5-HT(2B)/5-HT(2C) antagonist SB206,553 dose-dependently increased levels of DA and NA without affecting those of 5-HT. This action was attributable to 5-HT(2C) receptor blockade inasmuch as the selective 5-HT(2C) antagonist SB242,084 likewise increased FCX levels of DA and NA, whereas the selective 5-HT(2B) antagonist SB204,741 was ineffective. Further, the preferential 5-HT(2C) receptor agonist Ro60-0175 dose-dependently depressed FCX levels of DA. The suppressive influence of 5-HT(2C) receptors on DA release was also expressed on mesolimbic and nigrostriatal dopaminergic pathways, in that levels of DA in nucleus accumbens and striatum were likewise reduced by Ro60-0175 and elevated, though less markedly, by SB206,553. In line with the above findings, Ro60-0175 dose-dependently decreased the firing rate of ventrotegmental dopaminergic and locus coeruleus (LC) adrenergic perikarya, whereas their activity was dose-dependently enhanced by SB206,553. Furthermore, SB206,553 transformed the firing pattern of ventrotegmental dopaminergic neurons into a burst mode. In contrast to SB206,553, MDL100,907 had little affect on the firing rate of dopaminergic or adrenergic neurons. In conclusion, as compared to 5-HT(2A) and 5-HT(2B) receptors, 5-HT(2C) receptors exert a tonic, suppressive influence on the activity of mesocortical - as well as mesolimbic and nigrostriatal - dopaminergic pathways, likely via indirect actions expressed at the level of their cell bodies. Frontocortical adrenergic, but not serotonergic, transmission is also tonically suppressed by 5-HT(2C) receptors., (Copyright 2000 Wiley-Liss, Inc.)

The benzopyranopyrrole S33084 displayed pronounced affinity (pK(i) = 9.6) for cloned human hD(3)-receptors, and >100-fold lower affinity for hD(2) and all other receptors (>30) examined. S33084 concentration dependently, potently, and competitively (pA(2) = 9.7) antagonized dopamine (DA)-induced [(35)S]guanosine-5'- O-(3-thio)triphosphate (GTPgammaS) binding at hD(3)-receptors. It also concentration dependently abolished stimulation by DA of hD(3)-receptor-coupled mitogen-activated protein kinase. Administered alone, S33084 did not modify dialysate levels of DA in the frontal cortex, nucleus accumbens, or striatum of freely moving rats, nor the firing rate of ventrotegmental dopaminergic cell bodies. Furthermore, it had minimal effect on DA turnover in mesocortical, mesolimbic, and nigrostriatal projection regions. However, S33084 dose dependently blocked the suppressive influence of the preferential D(3)-agonist PD128,907 on frontocortical release of DA. Furthermore, it likewise antagonized the inhibitory influence of PD128,907 on the electrical activity of ventrotegmental dopaminergic neurons. Although less potent than S33084, GR218,231 likewise behaved as a selective hD(3)- versus hD(2)-receptor antagonist and its neurochemical and electrophysiological profiles were similar. In contrast, L741,626 was a preferential antagonist at hD(2) versus hD(3) sites. In vivo, on administration alone, L741,626 increased frontocortical, mesolimbic, and (more potently) striatal DA release, enhanced the firing rate of dopaminergic perikarya, and accelerated cerebral DA synthesis. It also blocked the actions of PD128,907. In conclusion, S33084 is a novel, potent, selective, and competitive antagonist at hD(3)-receptors. Although GR218,231 behaves similarly, L741,626 is a preferential D(2)-receptor antagonist. DA D(2)- but not D(3)-(auto) receptors tonically inhibit ascending dopaminergic pathways, although the latter may contribute to phasic suppression of DA release in frontal cortex.

The selective dopamine D(3)-receptor antagonist S33084 dose dependently attenuated induction of hypothermia by 7-hydroxy-2-dipropylaminotetralin (7-OH-DPAT) and PD128,907. S33084 also dose dependently reduced 7-OH-DPAT-induced penile erections (PEs) but had little effect on 7-OH-DPAT-induced yawning and hypophagia, and it did not block contralateral rotation elicited by the preferential D(3) agonist quinpirole in unilateral substantia nigra-lesioned rats. In models of potential antipsychotic activity, S33084 had little effect on conditioned avoidance behavior and the locomotor response to amphetamine and cocaine in rats, and weakly inhibited apomorphine-induced climbing in mice. Moreover, S33084 was inactive in models of potential extrapyramidal activity in rats: induction of catalepsy and prolactin secretion and inhibition of methylphenidate-induced gnawing. Another selective D(3) antagonist, GR218,231, mimicked S33084 in inhibiting 7-OH-DPAT-induced PEs and hypothermia but neither hypophagia nor yawning behavior. Similarly, it was inactive in models of potential antipsychotic and extrapyramidal activity. In distinction to S33084 and GR218,231, the preferential D(2) antagonist L741,626 inhibited all responses elicited by 7-OH-DPAT. Furthermore, it displayed robust activity in models of antipsychotic and, at slightly higher doses, extrapyramidal activity. In summary, S33084 was inactive in models of potential antipsychotic and extrapyramidal activity and failed to modify spontaneous locomotor behavior. Furthermore, it did not affect hypophagia or yawns, but attenuated hypothermia and PEs, elicited by 7-OH-DPAT. This profile was shared by GR218,231, whereas L741,626 was effective in all models. Thus, D(2)-receptors are principally involved in these paradigms, although D(3)-receptors may contribute to induction of hypothermia and PEs. S33084 should comprise a useful tool for further exploration of the pathophysiological significance of D(3)- versus D(2)-receptors.

Mirtazapine displayed marked affinity for cloned, human alpha2A-adrenergic (AR) receptors at which it blocked noradrenaline (NA)-induced stimulation of guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]-GTPgammaS) binding. Similarly, mirtazapine showed high affinity for cloned, human serotonin (5-HT)2C receptors at which it abolished 5-HT-induced phosphoinositide generation. Alpha2-AR antagonist properties were revealed in vivo by blockade of UK-14,304-induced antinociception, while antagonist actions at 5-HT2C receptors were demonstrated by blockade of Ro 60 0175-induced penile erections and discriminative stimulus properties. Mirtazapine showed negligible affinity for 5-HT reuptake sites, in contrast to the selective 5-HT reuptake inhibitor, citalopram. In freely moving rats, in the dorsal hippocampus, frontal cortex (FCX), nucleus accumbens and striatum, citalopram increased dialysate levels of 5-HT, but not dopamine (DA) and NA. On the contrary, mirtazapine markedly elevated dialysate levels of NA and, in FCX, DA, whereas 5-HT was not affected. Citalopram inhibited the firing rate of serotonergic neurons in dorsal raphe nucleus, but not of dopaminergic neurons in the ventral tegmental area, nor adrenergic neurons in the locus coeruleus. Mirtazapine, in contrast, enhanced the firing rate of dopaminergic and adrenergic, but not serotonergic, neurons. Following 2 weeks administration, the facilitatory influence of mirtazapine upon dialysate levels of DA and NA versus 5-HT in FCX was maintained, and the influence of citalopram upon FCX levels of 5-HT versus DA and NA was also unchanged. Moreover, citalopram still inhibited, and mirtazapine still failed to influence, dorsal raphe serotonergic neurons. In conclusion, in contrast to citalopram, mirtazapine reinforces frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission. These actions reflect antagonist properties at alpha2A-AR and 5-HT2C receptors.

Herein, we evaluate the interaction of the alpha(2)-AR antagonist, yohimbine, as compared to fluparoxan, at multiple monoaminergic receptors and examine their roles in the modulation of adrenergic, dopaminergic and serotonergic transmission in freely-moving rats. Yohimbine displays marked affinity at human (h)alpha(2A)-, halpha(2B)- and halpha(2C)-ARs, significant affinity for h5-HT(1A), h5-HT(1B), h5-HT(1D), and hD(2) receptors and weak affinity for hD(3) receptors. In [(35)S]GTPgammaS binding protocols, yohimbine exerts antagonist actions at halpha(2A)-AR, h5-HT(1B), h5-HT(1D), and hD(2) sites, yet partial agonist actions at h5-HT(1A) sites. In vivo, agonist actions of yohimbine at 5-HT(1A) sites are revealed by WAY100,635-reversible induction of hypothermia in the rat. In guinea pigs, antagonist actions of yohimbine at 5-HT(1B) receptors are revealed by blockade of hypothermia evoked by the 5-HT(1B) agonist, GR46,611. In distinction to yohimbine, fluparoxan shows only modest partial agonist actions at h5-HT(1A) sites versus marked antagonist actions at halpha(2)-ARs. While fluparoxan selectively enhances hippocampal noradrenaline (NAD) turnover, yohimbine also enhances striatal dopamine (DA) turnover and suppresses striatal turnover of 5-HT. Further, yohimbine decreases firing of serotonergic neurones in raphe nuclei, an action reversed by WAY100,635. Fluparoxan increases extracellular levels of DA and NAD, but not 5-HT, in frontal cortex. In analogy, yohimbine enhances FCX levels of DA and NAD, yet suppresses those of 5-HT, the latter effect being antagonized by WAY100,635. The induction by fluoxetine of FCX levels of 5-HT, DA, and NAD is potentiated by fluparoxan. Yohimbine likewise facilitates the influence of fluoxetine upon DA and NAD levels, but not those of 5-HT. In conclusion, the alpha(2)-AR antagonist properties of yohimbine increase DA and NAD levels both alone and in association with fluoxetine. However, in contrast to the selective alpha(2)-AR antagonist, fluparoxan, the 5-HT(1A) agonist actions of yohimbine suppress 5-HT levels alone and underlie its inability to augment the influence of fluoxetine upon 5-HT levels., (Copyright 2000 Wiley-Liss, Inc.)

We examined the involvement of multiple monoaminergic receptors in the induction of spontaneous tail-flicks (STFs) by the open channel blocker at N-methyl-D-aspartate (NMDA) receptors, dizocilpine, and the NMDA recognition site antagonist 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). At doses eliciting a maximal STF response, dizocilpine and CPP elevated levels of norepinephrine, but not dopamine or serotonin, in dialysates of nucleus accumbens, their known locus of action in eliciting STFs. Chemically diverse alpha(2)-adrenergic receptor (AR) antagonists atipamezole, L745,743, RX821,002, idazoxan, and desfluparoxan abolished induction of STFs by dizocilpine, whereas the preferential alpha(1)-AR antagonists prazosin, WB4101, and ARC239 were weakly active: relative potencies in blocking STFs correlated significantly with affinity at alpha(2)-ARs. The D(1)/D(5) receptor antagonists SCH23390, SCH39166, and NNC756 potently abolished STFs, whereas the D(2) antagonist L741,626, the D(3) antagonists GR218,231 and S14297, and the D(4) antagonists S18126 and L745,870 were inactive. D(1) and alpha(2)-AR antagonists also blocked induction of STFs by CPP. Blockade of dizocilpine-induced STFs was specific inasmuch as idazoxan and SCH 23390 did not modify induction of ataxia by dizocilpine. Antagonists at multiple 5-hydroxytryptamine receptors failed to modify induction of STFs. Finally, dizocilpine-induced STFs were blocked by clozapine and 11 other antipsychotics, the potency of which correlated significantly with affinity at alpha(2)-ARs. In conclusion, STFs evoked by interruption of transmission at NMDA receptors are dependent on D(1) receptors and alpha(2)-ARs for their expression. Antagonism of the alpha(2)-ARs is involved in their blockade by antipsychotics. This model should facilitate exploration of interrelationships between glutamatergic and monoaminergic mechanisms involved in psychiatric and neurologic disorders.

S18327 was dose-dependently active in several models of potential antipsychotic activity involving dopaminergic hyperactivity: inhibition of apomorphine-induced climbing in mice, of cocaine- and amphetamine-induced hyperlocomotion in rats, and of conditioned avoidance responses in rats. Furthermore, reflecting its high affinity at serotonin(2A) sites, S18327 potently blocked phencyclidine-induced locomotion and 1-[2, 5-dimethoxy-4-iodophenyl]-2-aminopropane-induced head-twitches in rats. In models of glutamatergic hypoactivity, S18327 blocked hyperlocomotion and spontaneous tail-flicks elicited by the N-methyl-D-aspartate antagonist dizocilpine. The actions of S18327, together with its binding profile at multiple monoaminergic receptors (15 parameters in total), were compared with those of clozapine, haloperidol, and 11 other antipsychotics by multiparametric analysis, and the resulting dendrogram positioned S18327 close to clozapine. Consistent with a clopazine-like profile, S18327 generalized to a clozapine discriminative stimulus and evoked latent inhibition in rats, blocked aggression in isolated mice, and displayed anxiolytic properties in the ultrasonic vocalization and Vogel procedures in rats. Relative to the above paradigms, only markedly (>20-fold) higher doses of S18327 were active in models predictive of potential extrapyramidal side effects: induction of catalepsy and prolactin secretion, and inhibition of methylphenidate-induced gnawing in rats. S18327 showed only modest affinity for histaminic and muscarinic receptors. Multiparametric analysis of these data distinguished S18327 from both haloperidol (high extrapyramidal potential) and clozapine (high histaminic and muscarinic affinity). In conclusion, S18327 displays a broad-based pattern of potential antipsychotic activity at doses appreciably lower than those eliciting extrapyramidal side effects. In this respect, S18327 closely resembles clozapine, but it is chemically distinct and displays weak affinity for histaminic and muscarinic receptors.

S18327 displayed modest affinity for human (h)D(2) and hD(3) receptors and high affinity for hD(4) receptors. At each, S18327 antagonized stimulation of [(35)S]guanosine-5'-O-(3-thio)triphosphate binding by dopamine (DA). It also blocked activation of mitogen-activated protein kinase at hD(3) receptors. The affinity of S18327 at hD(1) and hD(5) sites was modest. S18327 showed pronounced affinity for human serotonin (h5-HT)(2A) receptors and human alpha(1A)-adrenergic receptors (hARs), at which it antagonized increases in intracellular Ca(2+) concentration levels elicited by 5-HT and norepinephrine (NE), respectively. S18327 presented significant affinity for halpha(2A)-ARs and antagonized NE-induced[(35)S]guanosine-5'-O-(3-thio)triphosphate binding both at these sites and at alpha(2)-ARs in rat amygdala. Reflecting blockade of alpha(2)-autoreceptors, S18327 enhanced firing of adrenergic neurons in locus ceruleus, accelerated hippocampal synthesis of NE, and increased dialysate levels of NE in hippocampus, accumbens, and frontal cortex. S18327 abolished inhibition of ventrotegmental area-localized dopaminergic neurons by apomorphine. However, S18327 alone did not affect their activity and only modestly enhanced cerebral turnover of DA and dialysate levels of DA in striatum and accumbens. In contrast, S18327 markedly increased dialysate levels of DA in frontal cortex, an action abolished by the selective alpha(2)-AR agonist, S18616. Finally, S18327 reduced synthesis and dialysate levels of 5-HT in striatum and suppressed firing of dorsal raphe-localized serotonergic neurons, an action attenuated by the alpha(1)-AR agonist cirazoline. In conclusion, S18327 possesses marked antagonist activity at alpha(1)-ARs and D(4) and 5-HT(2A) receptors and less potent antagonist activity at alpha(2)-ARs and D(1) and D(2) receptors. Antagonism by S18327 of alpha(2)-ARs enhances adrenergic transmission and reinforces frontocortical dopaminergic transmission, whereas blockade of alpha(1)-ARs inhibits dorsal raphe-derived serotonergic pathways. As further described in the accompanying paper, this profile of activity may contribute to the potential antipsychotic properties of S18327.

The serotonin1A receptor partial agonist, buspirone, also displays antagonist properties at D2 receptors and is metabolized to the alpha2-adrenergic receptor antagonist, 1-(2-pyrimidinyl-piperazine). Herein, we examined mechanisms underlying the influence of buspirone alone, and in association with the serotonin reuptake inhibitor, fluoxetine, upon extracellular levels of serotonin, dopamine and noradrenaline simultaneously quantified in the frontal cortex of freely moving rats. Buspirone (0.01-2.5 mg/kg, s.c.) dose-dependently decreased dialysate levels of serotonin (-50%), and increased those of dopamine (+100%) and noradrenaline (+140%). The reduction by buspirone of serotonin levels was abolished by the serotonin1A receptor antagonist, WAY 100,635 (0.16), which did not, however, modify its influence upon dopamine and noradrenaline. In contrast to buspirone, the serotonin reuptake inhibitor, fluoxetine (10.0), increased frontocortical levels of serotonin (+ 120%), dopamine (+55%) and noradrenaline (+90%). Buspirone dose-dependently (0.01-2.5) decreased the induction by fluoxetine of serotonin levels yet potentiated (three-fold) its elevation of dopamine and noradrenaline levels. The serotonin1A agonist, 8-hydroxy-2-(di-n-propyl-amino)-tetralin (0.16), mimicked the action of buspirone in reducing resting levels of serotonin (-60%) and in enhancing those of dopamine (+135%) and noradrenaline (+165%). Like buspirone, it attenuated the influence of fluoxetine upon serotonin levels, yet facilitated its influence upon dopamine and noradrenaline levels. In contrast, WAY 100,635 selectively potentiated the increase in levels of serotonin (two-fold) versus dopamine and noradrenaline elicited by fluoxetine. Further, WAY 100,635 abolished the inhibitory influence of buspirone upon fluoxetine-induced serotonin release, but only partly interfered with its potentiation of fluoxetine-induced increases in dopamine and noradrenaline levels. The D2/D3 receptor antagonist, raclopride (0.16), increased basal dopamine (+60%) levels but little influenced those of serotonin and noradrenaline, and failed to modify the action of fluoxetine. The alpha2-adrenergic receptor antagonist, 1-(2-pyrimidinyl-piperazine) (2.5), which did not modify resting levels of serotonin, markedly increased those of dopamine (+90%) and noradrenaline (+190%) and potentiated (two-fold) the increases in dialysate levels of dopamine, noradrenaline and serotonin provoked by fluoxetine. Further, the alpha2-adrenergic receptor agonist, S18616, attenuated the enhancement by buspirone of the fluoxetine-induced increase in levels of dopamine and noradrenaline. In conclusion, the inhibitory influence of buspirone upon resting and fluoxetine-stimulated serotonin levels reflects its agonist properties at serotonin1A autoreceptors. The facilitatory influence of buspirone upon resting and fluoxetine-stimulated dopamine and noradrenaline levels may also involve its serotonin1A properties. However, its principal mechanism of action in this respect is probably the alpha2-adrenergic antagonist properties of its metabolite, 1-(2-pyrimidinyl-piperazine). The present observations are of significance to experimental and clinical studies of the influence of buspirone upon depressive states, alone and in association with antidepressant agents.

The benzofurane (+)-S 14297, the benzamide nafadotride, the aminoindane U 99194 and the arylpiperazine GR 103,691 have been proposed as "selective" antagonists at dopamine D3 vs. D2 receptors. Herein, we compared their in vitro affinities and in vivo actions to those of the aminotetralin D3 antagonists (+)-AJ 76 and (+)-UH 232. Affinities at recombinant, human (h)D3 and/or hD2 sites were determined by employing the mixed D2/D3 antagonist [125I]-iodosulpride and the preferential D3 ligands [3H]-(+)-PD 128, 907 and [3H]-(+)-S 14297. [3H]-(+)-PD 128,907, [3H]-(+)-S 14297 and [125I]-iodosulpride yielded an essentially identical pattern of displacement at D3 sites, which suggests that they recognize the same population of receptors. The rank order of potency (Ki values in nM vs. [3H]-(+)-PD 128,907) was GR 103,691 (0.4) approximately nafadotride (0.5) > haloperidol (2) approximately (+)-UH 232 (3) approximately (+)-S 14297 (5) > (+)-AJ 76 (26) > U 99194 (160). The rank order of preference (Ki ratio, D2:D3) for D3 receptors (labeled by [3H]-PD 128,907) vs. D2 sites (labeled by [125I]-iodosulpride) was (+)-S 14297 (61) approximately GR 103,691 (60) > U 99194 (14) > nafadotride (9) approximately (+)-UH 232 (8) approximately (+)-AJ 76 (6) > haloperidol (0.2). (+)-S 14297 and GR 103,691 also showed greater than 100-fold selectivity at dopamine hD3 vs. hD4 and hD1 sites. However, GR 103,691 showed marked affinity for serotonin1A receptors (5.8 nM) and alpha-1 adrenoceptors (12.6 nM). In vivo, all antagonists except GR 103,691 prevented the induction of hypothermia by (+)-PD 128,907 (0.63 mg/kg s.c.) and a further preferential D3 agonist, (+)-7-OH-DPAT (0.16 mg/kg s.c.). On the other hand, haloperidol, (+)-AJ 76, (+)-UH 232 and nafadotride all induced catalepsy in rats, whereas (+)-S 14297, U 99194 and GR 103,691 were inactive. Haloperidol, (+)-AJ 76, (+)-UH 232, nafadotride and (weakly) U 99194 also enhanced prolactin secretion and striatal dopamine synthesis, whereas (+)-S 14297 and GR 103,691 were inactive. However, despite its high affinity at 5-HT1A receptors and alpha-1 adrenoceptors, both of which are present on raphe-localized serotonergic neurons, GR 103,691 (0.5 mg/kg i.v.) failed to influence their basal firing rate or the inhibition of their electrical activity by the 5-HT1A agonist (+/-)-8-OH-DPAT (0.005 mg/kg i.v.), a result that casts doubt on its activity in vivo. In conclusion, both (+)-S 14297 and GR 103,691 are markedly selective ligands that permit the characterization of actions at hD3 vs. hD2 receptors in vitro, but (+)-S 14297 appears to be of greater utility for the evaluation of their functional significance in vivo. Nevertheless, to develop a better understanding of the respective roles of dopamine D3 and D2 receptors, we need additional, chemically diverse antagonists of improved potency and selectivity.

The novel benzoindane S 18126 possessed > 100-fold higher affinity at cloned, human (h) D4 (Ki = 2.4 nM) vs. hD2 (738 nM), hD3 (2840 nM), hD1 (> 3000 nM) and hD5 (> 3000 nM) receptors and about 50 other sites, except sigma1 receptors (1.6 nM). L 745,870 similarly showed selectivity for hD4 (2.5 nM) vs. hD2 (905 nM) and hD3 (> 3000 nM) receptors. In contrast, raclopride displayed low affinity at hD4 (> 3000 nM) vs. hD2 (1.1 nM) and hD3 receptors (1.4 nM). Stimulation of [35S]-GTPgammaS binding at hD4 receptors by dopamine (DA) was blocked by S 18126 and L 745,870 with Kb values of 2.2 and 1.0 nM, respectively, whereas raclopride (> 1000 nM) was inactive. In contrast, raclopride inhibited stimulation of [35S]-GTPgammaS binding at hD2 sites by DA with a Kb of 1.4 nM, whereas S 18126 (> 1000 nM) and L 745,870 (> 1000 nM) were inactive. As concerns presynaptic dopaminergic receptors, raclopride (0.01-0.05 mg/kg s.c. ) markedly enhanced DA synthesis in mesocortical, mesolimbic and nigrostriatal dopaminergic pathways. In contrast, even high doses (2. 5-40.0 mg/kg s.c.) of S 18126 and L 745,870 were only weakly active. Similarly, raclopride (0.016 mg/kg i.v.) abolished inhibition of the firing rate of ventrotegmental dopaminergic neurons by apomorphine, whereas even high doses (0.5 mg/kg i.v.) of S 18126 and L 745,870 were only weakly active. As regards postsynaptic dopaminergic receptors, raclopride potently (0.01-0.3 mg/kg s.c.) reduced rotation elicited by quinpirole in rats with unilateral lesions of the substantia nigra, antagonized induction of hypothermia by PD 128, 907, blocked amphetamine-induced hyperlocomotion and was effective in six further models of potential antipsychotic activity. In contrast, S 18126 and L 745,870 were only weakly active in these models (5.0-> 40.0 mg/kg s.c.). In six models of extrapyramidal and motor symptoms, such as induction of catalepsy, raclopride was likewise potently active (0.01-2.0 mg/kg s.c.) whereas S 18126 and L 745,870 were only weakly active (10.0-80.0 mg/kg s.c.). In freely moving rats, raclopride (0.16 mg/kg s.c.) increased levels of DA by + 55% in dialysates of the frontal cortex. However, it also increased levels of DA in the accumbens and striatum by 70% and 75%, respectively. In contrast to raclopride, at a dose of 0.16 mg/kg s.c. , neither S 18126 nor L 745,870 modified frontal cortex levels of DA. However, at a high dose (40.0 mg/kg s.c.), S 18126 increased dialysate levels of DA (+ 85%) and noradrenaline (+ 100%), but not serotonin (+ 10%), in frontal cortex without affecting DA levels in accumbens (+ 10%) and striatum (+ 10%). In conclusion, S 18126 and L 745,870 behave as potent and selective antagonists of cloned, hD4 vs. other dopaminergic receptor types in vitro. However, their in vivo effects at high doses probably reflect residual antagonist actions at D2 (or D3) receptors. Selective blockade of D4 receptors was thus associated neither with a modification of dopaminergic transmission nor with antipsychotic (antiproductive) or extrapyramidal properties. The functional effects of selective D4 receptor blockade remain to be established.

S 16924 showed a pattern of interaction at multiple (>20) native, rodent and cloned, human (h) monoaminergic receptors similar to that of clozapine and different to that of haloperidol. Notably, like clozapine, the affinity of S 16924 for hD2 and hD3 receptors was modest, and it showed 5-fold higher affinity for hD4 receptors. At each of these sites, using a [35S]GTPgammaS binding procedure, S 16924, clozapine and haloperidol behaved as antagonists. In distinction to haloperidol, S 16924 shared the marked affinity of clozapine for h5-HT2A and h5-HT2C receptors. However, an important difference to clozapine (and haloperidol) was the high affinity of S 16924 for h5-HT1A receptors. At these sites, using a [35S]GTPgammaS binding model, both S 16924 and clozapine behaved as partial agonists, whereas haloperidol was inactive. In vivo, the agonist properties of S 16924 at 5-HT1A autoreceptors were revealed by its ability to potently inhibit the firing of raphe-localized serotoninergic neurones, an action reversed by the selective 5-HT1A receptor antagonist, WAY 100,635. In contrast, clozapine and haloperidol only weakly inhibited raphe firing, and their actions were resistant to WAY 100,635. Similarly, S 16924 more potently inhibited striatal turnover of 5-HT than either clozapine or haloperidol. Reflecting its modest affinity for D2 (and D3) autoreceptors, S 16924 only weakly blocked the inhibitory influence of the dopaminergic agonist, apomorphine, upon the firing rate of ventrotegmental area-localized dopaminergic neurones. Further, S 16924 only weakly increased striatal, mesolimbic and mesocortical turnover of dopamine (DA). Clozapine was, similarly, weakly active in these models, whereas haloperidol, in line with its higher affinity at D2 (and D3) receptors, was potently active. In the frontal cortex (FCX) of freely moving rats, S 16924 dose-dependently reduced dialysate levels of 5-HT, whereas those of DA and NAD were dose-dependently increased in the same samples. In contrast, although S 16924 also suppressed 5-HT levels in the striatum and nucleus accumbens, DA levels therein were unaffected. Clozapine mimicked this selective increase in DA levels in the FCX as compared to striatum and accumbens. In contrast, haloperidol modestly increased DA levels in the FCX, striatum and accumbens to the same extent. In distinction to S 16924, clozapine and haloperidol exerted little influence upon 5-HT levels. Finally, the influence of S 16924 upon FCX levels of 5-HT, DA (and NAD) was attenuated by WAY 100,635. In conclusion, S 16924 possesses a profile of interaction at multiple monoaminergic receptors comparable to that of clozapine and distinct to that of haloperidol. In addition, S 16924 is a potent, partial agonist at 5-HT1A receptors. Correspondingly, acute administration of S 16924 decreases cerebral serotoninergic transmission and selectively reinforces frontocortical as compared to subcortical dopaminergic transmission. In line with these actions, S 16924 shows a distinctive profile of activity in functional (behavioral) models of potential antipsychotic activity (companion paper).

S 16924 antagonized locomotion provoked by dizocilpine and cocaine, reduced conditioned avoidance responses and blocked climbing elicited by apomorphine, models predictive of control of the positive symptoms of schizophrenia: its median inhibitory dose (ID)50 was 0.96 mg/kg, s.c. vs. 1.91 for clozapine and 0.05 for haloperidol. Rotation elicited in unilateral, substantia nigra-lesioned rats by the D1 agonist, SKF 38393, and by the D2 agonist, quinpirole, was blocked equipotently by S 16924 (0.8 and 1. 7) and clozapine (0.6 and 2.0), whereas haloperidol preferentially blocked quinpirole (0.02) vs. SKF 38393 (1.8). S 16924 more potently inhibited the head-twitches elicited by 1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the locomotion provoked by phencyclidine than it inhibited the locomotion elicited by amphetamine (ID50s = 0.15 and 0.02 vs. 2.4). Clozapine showed a similar preference (0.04 and 0.07 vs. 8.6), but not haloperidol (0. 07 and 0.08 vs. 0.04). The discriminative stimulus (DS) properties of DOI were also blocked by S 16924 (ID50 = 0.17) and clozapine (0. 05) but not by haloperidol (>0.16). S 16924 fully (100%) generalized [effective dose (ED)50 = 0.7] to a clozapine DS and clozapine (0.23) fully generalized to a S 16924 DS whereas haloperidol (>/=0.08) only partially generalized (/=80.0) or clozapine (>/=80.0). Further, S 16924 (ID50 = 3.2) and clozapine (5.5) inhibited induction of catalepsy by haloperidol. This action of S 16924 was abolished by the 5-HT1A receptor antagonist, WAY 100,635 (0.16), which less markedly attenuated the anticataleptic action of clozapine. Further, although gnawing elicited by methylphenidate was inhibited by S 16924 (ID50 = 8.4), clozapine (19.6) and haloperidol (0.04), only the action of S 16924 was blocked by WAY 100,635 (0.16). Haloperidol potently (0.01-0.16, approximately 24-fold) increased prolactin levels whereas they were less markedly affected by S 16924 (2.5-40.0, 4-fold) and clozapine (10.0-40.0, 3-fold). Clozapine displayed high affinity at cloned, human, muscarinic (M1) and native, histamine (H1) receptors (Kis = 4.6 and 5.4 nM, respectively), whereas S 16924 (>1000 and 158) and haloperidol (>1000 and 453) displayed low affinity. In conclusion, S 16924 displays a profile of activity in diverse models of potential antipsychotic and extrapyramidal properties similar to that of clozapine and different to that of haloperidol. In particular, reflecting its partial agonist actions at 5-HT1A receptors, S 16924 inhibits rather than induces catalepsy in rats. However, in contrast to clozapine, S 16924 displays only low affinity for muscarinic and histaminic receptors.

In the present study, a novel and exceptionally sensitive method of high-performance liquid chromatography coupled to coulometric detection, together with concentric dialysis probes, was exploited for an examination of the role of autoreceptors and heteroceptors in the modulation of dopamine, noradrenaline and serotonin levels in single samples of the frontal cortex of freely-moving rats. The selective D3/D2 receptor agonist, CGS 15855A [(+/-)-trans-1,3,4,4a,5,10b-hexahydro-4-propyl-2H-[1]benzopyrano[3 ,4-b]-pyridin-9-ol], and antagonist, raclopride, respectively decreased (-50%) and increased (+60%) levels of dopamine without significantly modifying those of serotonin and noradrenaline. The selective alpha2-adrenergic receptor agonist, dexmedetomidine, markedly decreased noradrenaline levels (-100%) and likewise suppressed those of serotonin and dopamine by -55 and -45%, respectively. This effect was mimicked by the preferential alpha2-adrenergic receptor agonist, guanabenz (-100%, -60% and -50%). Furthermore, the alpha2-adrenergic receptor antagonist, RX 821,002 [2(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline], and the preferential alpha2A-adrenergic receptor antagonist, BRL 44408 [2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidaz ole], both evoked a pronounced elevation in levels of noradrenaline (+212%, +109%) and dopamine (+73%, +85%). In contrast, the preferential alpha(2B/2C)-adrenergic receptor antagonist, prazosin, did not modify noradrenaline and dopamine levels. RX 821,002 and BRL 44408 did not significantly modify levels of serotonin, whereas prazosin decreased these levels markedly (-55%), likely due to its alpha1-adrenergic receptor antagonist properties. The selective serotonin-1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), reduced serotonin levels (-65%) and increased those of dopamine and noradrenaline by +100%), and +175%, respectively. The selective serotonin-1A antagonist, WAY 100,635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo- hexanecarboxamide], which had little affect on monoamine levels alone, abolished the influence of 8-OH-DPAT upon serotonin and dopamine levels and significantly attenuated its influence upon noradrenaline levels. Finally, the selective serotonin-1B agonist, GR 46611 [3-[3-(2-dimethylaminoethyl)-1H-indol-5-yl]-N-(4-methoxybenzyl)acrylamid e], decreased serotonin levels (-49%) and the serotonin-1B antagonist, GR 127,935 [N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-me thyl-1,2,4-oxadiazol-3-yl)-biphenyl-4-carboxamide], which did not significantly modify serotonin levels alone, abolished this action of GR 46611. Levels of dopamine and noradrenaline were not affected by GR 46611 or GR 127,935. In conclusion, there is a complex pattern of reciprocal autoreceptor and heteroceptor control of monoamine release in the frontal cortex. Most notably, activation of alpha2-adrenergic receptors inhibits the release of noradrenaline, dopamine and serotonin in each case, while stimulation of serotonin-1A receptors suppresses serotonin, yet facilitates noradrenaline and dopamine release. In addition, dopamine D2/D3 autoreceptors restrain dopamine release while (terminal-localized) serotonin-1B receptors reduce serotonin release. Control of serotonin release is expressed phasically and that of noradrenaline and dopamine release tonically.

Evidence exists that a reinforcement in monoaminergic transmission in the frontal cortex (FCX) is associated with antidepressant (AD) properties. Herein, we examined whether blockade of alpha2-adrenergic receptors modified the influence of monoamine reuptake inhibitors on FCX levels of serotonin (5-HT), noradrenaline (NAD), and dopamine (DA). The selective alpha2-adrenergic receptor agonist S 18616 (0.16 mg/kg, s.c.) suppressed extracellular levels of NAD, DA, and 5-HT (by 100, 51, and 63%, respectively) in single dialysates of FCX of freely moving rats. In contrast, the selective alpha2-adrenergic receptor antagonists atipamezole (0.16 mg/kg, s.c.) and 1-(2-pyrimidinyl)piperazine (1-PP; 2.5 mg/kg, s.c.) increased levels of NAD (by 180 and 185%, respectively) and DA (by 130 and 90%, respectively), without affecting 5-HT levels. Duloxetine (5.0 mg/kg, s.c.), a mixed inhibitor of 5-HT and NAD reuptake, and fluoxetine (10.0 mg/kg, s.c.), a selective 5-HT reuptake inhibitor, both increased levels of 5-HT (by 150 and 120%, respectively), NAD (by 400 and 100%, respectively), and DA (by 115 and 55%, respectively). Atipamezole (0.16 mg/kg, s.c.) markedly potentiated the influence of duloxetine and fluoxetine on levels of 5-HT (by 250 and 330%, respectively), NAD (by 1,030 and 215%, respectively), and DA (by 370 and 170%, respectively). 1-PP similarly potentiated the influence of duloxetine on 5-HT, NAD, and DA levels (by 290, 1,320, and 600%, respectively). These data demonstrate that alpha2-adrenergic receptors tonically inhibit NAD and DA and phasically inhibit 5-HT release in the FCX and that blockade of alpha2-adrenergic receptors strikingly potentiates the increase in FCX levels of 5-HT, NAD, and DA elicited by reuptake inhibitors. Concomitant alpha2-adrenergic receptor antagonism and inhibition of monoamine uptake may thus provide a mechanism allowing for a marked increase in the efficacy of AD agents.

The novel, potential anxiolytic, S 15535 (4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine), is an agonist and antagonist (weak partial agonist) at pre- and postsynaptic serotonin (5-HT)1A receptors, respectively. Herein, we characterized its influence on dialysate levels of 5-HT, dopamine (DA) and NAD simultaneously determined in single samples of the frontal cortex (FCX) of freely moving rats, and compared its activity in several other models of potential antidepressant (AD) properties with those of the 5-HT reuptake inhibitor (SSRI), fluoxetine. S 15535 displayed high affinity at cloned human (h) 5-HT1A receptors (Ki = 0.7 nM) and >250-fold lower affinity at cloned hD2 (400 nM), hD3 (248 nM) and h alpha2A-adrenergic (AR) (190 nM) receptors. S 15535 (0.08-5.0 mg/kg s.c.) markedly and dose-dependently suppressed dialysate levels of 5-HT in the FCX, nucleus accumbens and striatum of freely moving rats, whereas fluoxetine (10.0 mg/kg s.c.) elevated levels of 5-HT in each structure. In contrast to 5-HT, dialysate levels of DA and NAD in the FCX were dose-dependently increased by S 15535, and this effect was mimicked by fluoxetine. The influence of S 15535 and fluoxetine on FCX levels of DA was regionally specific inasmuch as dialysate levels of DA in the accumbens and striatum were not modified. The selective 5-HT1A antagonist, WAY 100,635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (0.16) transiently elicited a slight increase in cortical levels of 5-HT, an action opposite to that of S 15535. Further, in the presence of WAY 100,635 (0.16), the influence of S 15535 (0.63) on cortical levels of 5-HT, DA and NAD was markedly attenuated. Upon chronic administration of S 15535 or fluoxetine (10.0 mg/kg s.c. daily for 14 days, in each case), there was no significant alteration in the density of beta-AR receptors in the FCX. However, in contrast to fluoxetine, S 15535 elicited a significant (25%) decrease in the density (Bmax) of 5-HT2A receptors labeled by [3H]ketanserin in the cortex; there was no alteration in Kd. In a learned helplessness paradigm in rats, S 15535 (0.63-40.0 mg/kg p.o.) markedly reduced escape deficits on each of three consecutive days of testing. Fluoxetine (2.0-8.0 mg/kg i.p.) was also active in each session, but presented a biphasic dose-response curve. Finally, under the conditions used, neither S 15535 (0.63-10.0) nor fluoxetine (0.63-10.0) decreased immobility time in the forced swim test. In conclusion, S 15535 is a selective ligand of cloned, h5-HT1A receptors. Its agonist actions at 5-HT1A autoreceptors underlie its ability to decrease extracellular levels of 5-HT in the FCX, and likely contribute to the increase in extracellular levels of DA and NAD evoked by S 15535 in this structure. Further, S 15535 is active in several other, although not all, models of potential AD activity. Thus, although S 15535 is under development as an anxiolytic agent, a further characterization of its putative AD actions would be of interest.

A serotonin (5-HT)1A receptor partial agonist, buspirone, potentiates the clinical antidepressant properties of 5-HT reuptake inhibitors (SSRIs). Herein, we examined the interaction of buspirone with two SSRIs, duloxetine and fluoxetine, on extracellular levels of 5-HT, dopamine (DA), and noradrenaline (NAD) in single dialysate samples of freely moving rats. Duloxetine (5.0 mg/kg. s.c.) and fluoxetine (10.0 mg/kg, s.c.) increased dialysate levels of DA (65 and 60% vs. basal values, respectively). NAD (400 and 90%, respectively), and 5-HT (130 and 110%, respectively) in the frontal cortex (FCX). Buspirone (2.5 mg/kg, s.c.) similarly elevated levels of DA (100%) and NAD (160%) but reduced those of 5-HT (-50%). Administered with buspirone, the ability of duloxetine and fluoxetine to increase 5-HT levels was transiently inhibited (over 60 min), although by the end of sampling (180 min) their actions were fully expressed. In contrast, buspirone markedly and synergistically facilitated the elevation in DA levels elicited by duloxetine (550%) and fluoxetine (240%). Furthermore, buspirone potentiated the induction of NAD levels by duloxetine (750%) and fluoxetine (350%). These data suggest that a reinforcement in the influence of SSRIs on DA and possibly. NAD but not 5-HT release in FCX may contribute to their increased antidepressant activity in the presence of buspirone. More generally, they support the hypothesis that a reinforcement in dopaminergic transmission in the FCX contributes to the actions of SSRIs and other antidepressant drugs.

In this study, we examined the influence of blockade of serotonin (5-HT)1A and/or 5-HT1B autoreceptors on the fluoxetine-induced increase in dialysate levels of 5-HT as compared with dopamine (DA) and noradrenaline (NAD) in single samples of the frontal cortex (FCx) of freely moving rats. Fluoxetine (10.0 mg/kg, s.c.) elicited a twofold increase in dialysate levels of 5-HT relative to baseline values. The selective 5-HT1A antagonist WAY 100,635 (0.16 mg/kg, s.c.) did not influence 5-HT release alone but doubled the influence of fluoxetine on basal levels. Similarly, the selective 5-HT 1B/1D antagonist GR 127,935 (2.5 mg/kg, s.c.) did not alter basal 5-HT levels alone and doubled the fluoxetine-induced increase in 5-HT levels. Combined administration of WAY 100,635 and GR 127,935 elicited an (at least) additive rise in the fluoxetine-induced increase in 5-HT levels to eightfold basal values, without modifying resting 5-HT levels. These changes were selective for 5-HT inasmuch as the parallel (twofold) increase in DA and NAD levels provoked by fluoxetine was not potentiated. The present data demonstrate that combined blockade of 5-HT1A and 5-HT1B autoreceptors markedly and selectively potentiates the fluoxetine-induced increase in dialysate levels of 5-HT versus DA and NAD in the FCx of freely moving rats. These observations suggest that 5-HT 1A/1B antagonism may represent a novel strategy for the improvement in the therapeutic profile of 5-HT reuptake inhibitor antidepressant agents and that 5-HT may be primarily involved in such interactions.

In freely moving rats, the novel, selective dopamine (DA) D3 receptor agonist PD 128,907 dose-dependently [effective dose (ED25) = 0.07 mg/kg, s.c.] reduced dialysate levels of DA in the frontal cortex, a structure innervated by the ventral tegmental area (VTA). This action of PD 128,907 (0.16 mg/kg, s.c.) was abolished by a selective DA D3 receptor antagonist S 14297 (1.25 mg/kg, s.c.), which alone did not modify levels of DA. In contrast to S 14297, its inactive distomer, S 17777, did not modify the actions of PD 128,907. In addition, PD 128,907 dose-dependently and potently inhibited the firing rate of VTA-localized neurons in anesthetized rats (ED50 = 0.001 mg/kg, i.v.). S 14297, but not S 17777, completely reversed the actions of PD 128,907 (0.005 mg/kg, i.v.) with a 50% inhibitory dose of 0.03 mg/kg, i.v. and did not itself significantly modify the firing rate. In conclusion, these data provide the first direct evidence that DA D3 (auto) receptors modulate (inhibit) the release of DA in the frontal cortex.

Using [125I]-iodosulpride as a radioligand, the novel naphthofurane, (+/-)-S 11566 [(+/-)-[7-(N,N-dipropylamino)-5,6,7,8-tetra-hydro- naphtho(2,3b)dihydro,2,3-furane]) showed a marked preference for human, recombinant D3 as compared with D2 receptors stably transfected into Chinese hamster ovary cells (Kis = 24/529 nM). This activity resided in its (+)-eutomer, (+)-S 14297 (13/297 nM) as compared with its (-)-distomer, (-)-S 17777 (406/3544 nM). In contrast, (+)-AJ 76 manifested only a mild 2-fold preference for D3 sites (70/154 nM), whereas haloperidol and six additional antagonists showed a mild (2-7-fold) preference for D2 sites. As concerns agonists, (+)-7-OH-DPAT, (+/-)-CGS 15855A, quinelorane, (-)-quinpirole and N-0434 displayed a preference (6-40-fold) for D3 receptors, whereas piribedil showed a slight, 2-fold, preference for D2 sites (243/126 nM). (+)-S 14297 showed low (> 1.0 microM) affinity at rat D1 and D2 sites and at cloned, human D4 and D5 receptors and only low affinity (145 to > 10,000 nM) at all other sites examined. In vivo, administered s.c., (+)-7-OH-DPAT, CGS 15855A, quinelorane, (-)-quinpirole and N-0434 potently evoked hypothermia. Across all (8) agonists tested, potency correlated significantly with affinity at D3 sites (r = .84, P < .001) but not D2 sites (r = .50, P > .05). (+)-S 14297 (0.16-1.25 mg/kg, s.c.) blocked the induction of hypothermia by (+)-7-OH-DPAT, CGS 15855A and (-)-quinpirole, but not by the alpha 2-adrenergic agonist, clonidine, without influencing core temperature alone. In contrast, (-)-S 17777 (10.0 mg/kg, s.c.) was only partially active. Across all (9) antagonists, potency for inhibition of (+)-7-OH-DPAT-induced hypothermia correlated more strongly with affinity at D3 (r = .96, P < .001) than D2 (r = .75, P < .02) sites. Whereas haloperidol and the other antagonists provoked prolactin secretion and elicited catalepsy, (+)-S 14297 and (+/-)-S 11566 at doses of up to 10.0 and 40.0 mg/kg, s.c., respectively, were not significantly effective (P > .05). Across all antagonists, potency for eliciting prolactin secretion and catalepsy correlated better with affinity at D2 (r = .95 and .96) than D3 (r = .76 and .91) sites. In conclusion, these data demonstrate that the novel naphtofurane, (+)-S 14297, is a selective ligand (antagonist) at dopamine D3 receptors and suggest that their activation mediates hypothermia in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)

The preferential dopamine (DA) D3 versus D2 receptor agonist, (+)-7-OH-DPAT, dose-dependently decreased DA synthesis in the nucleus accumbens, olfactory tubercles, striatum and frontal cortex. This action was potently mimicked by several other high-potency D3 agonists: CGS 15855A, (-)-quinpirole, quinelorane and N-0434. In contrast, piribedil, which displays a mild preference for D2 sites, was less active. Across eight agonists, potency for inhibition of DA synthesis correlated more potently to affinity at D3 (r = .82 +/- .04) than D2 receptors (r = .60 +/- .06, P < .05). Correlations were also marked to potency for induction of a further D3-mediated response, hypothermia (r = .93 +/- .02). The novel and selective D3 versus D2 antagonist, (+/-)-S 11556, attenuated the action of (+)-7-OH-DPAT in each structure. This action was shared by its active (+)-eutomer, (+)-S 14297, whereas its inactive (-)-distomer, (-)-S 17777, was ineffective. (+)-S 14297 similarly attenuated the inhibitory action of CGS 15855A and (-)-quinpirole upon DA synthesis, whereas it failed to modify inhibition of striatal DA synthesis by the alpha 2-adrenergic receptor agonist, clonidine. As compared with the D2/D3 receptor antagonist, haloperidol, neither (+/-)-S 11566 nor (+)-S 14297 modified DA turnover upon administration alone. Furthermore, across (nine) antagonists, potency in facilitating DA synthesis more powerfully correlated to affinity at D2 (r = .94 +/- .01) than D3 (r = .73 +/- .01) sites (P < .01). Correlations were also marked to potency for induction of catalepsy (r = .91 +/- .01) and prolactin secretion (r = .89 +/- .01) but not for antagonism of (+)-7-OH-DPAT-induced hypothermia (r = .60 +/- .01). In freely moving rats, (+)-7-OH-DPAT dose-dependently reduced dialysate concentrations of DA in the nucleus accumbens and contralateral striatum: this action was potently mimicked by CGS 15855A, but only weakly so by piribedil. (+)-S 14297 markedly attenuated the action of (+)-7-OH-DPAT, whereas (-)-S 17777 was inactive. In contrast, haloperidol completely blocked the action of (+)-7-OH-DPAT. Finally, in distinction to haloperidol, upon administration alone, (+)-S 14297 did not significantly enhance the release of DA. In conclusion, these data suggest that D3 (auto)receptors control synthesis and release of DA in dopaminergic pathways innervating the limbic system, cortex and striatum.(ABSTRACT TRUNCATED AT 400 WORDS)

In this study, we examined the influence of the actions of the halogenated phenylpiperazine LY 165,163 at dopamine D1, D2 and D3 receptors on its 5-HT1A agonist properties in vivo. LY 165,163 displayed marked affinity at striatal rat (r)D2 (pKi = 7.0), cloned rD2 (pKi = 7.3), cloned rD3 (pKi = 8.0), cloned human (h)D2 (pKi = 7.2) and cloned hD3 (pKi = 7.8) receptors, whereas its affinity at striatal rD1 receptors was weak (pKi = 5.7). In contrast, the prototypical 5-HT1A agonist 8-OH-DPAT displayed low affinity at each of these sites (pKi < 5.5). In vivo, LY 165,163 behaved as an antagonist at D2 autoreceptors in enhancing the synthesis of dopamine throughout the brain. Consistently with antagonist properties at postsynaptic D2 receptors, in unilateral substantia nigra-lesioned rats, the rotation elicited by the D2 agonist quinpirole was potently blocked by LY 165,163, whereas that elicited by the D1 agonist SKF 38393 was only weakly inhibited. In addition, LY 165,163 potently evoked prolactin secretion in rats and abolished apomorphine-induced climbing in mice. At native rat 5-HT1A receptors and cloned human 5-HT1A receptors, LY 165,163 (pKi values of 8.7 and 8.9, respectively) mimicked the high affinity of 8-OH-DPAT (pKi values of 9.0 and 9.2). Further, like 8-OH-DPAT, LY 165,163 acted as an agonist in inhibiting the firing of dorsal raphé nucleus neurons, in reducing striatal turnover of 5-HT and in eliciting both hypothermia and corticosterone secretion. However, in contrast to 8-OH-DPAT, LY 165,163 failed to evoke spontaneous tail-flicks (STFs). This difference probably reflects its D2 antagonist properties because the induction of STFs by 8-OH-DPAT was, in contrast to its induction of hypothermia and corticosterone secretion, blocked by the preferential D2 antagonist haloperidol. Moreover, in the presence of quinpirole and in the presence of a further dopaminergic agonist, apomorphine, LY 165,163 did elicit STFs. Further, STFs evoked by LY 165,163 in the presence of apomorphine were abolished by the 5-HT1A antagonists (-)-alprenolol, BMY 7378 and NAN-190. We conclude that LY 165,163 exhibits marked activity at both pre- and postsynaptic dopaminergic D2 (D3 and D1) receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

In this study, we used a complementary in vivo electrophysiological and (in individual rats) neurochemical approach to characterize the actions of chemically diverse serotonin (5-HT)1A receptor ligands at central 5-HT1A autoreceptors as compared to dopamine (DA) D2 autoreceptors and presynaptic alpha-2 adrenergic receptors (ARs). The novel, high efficacy, 5-HT1A agonists, WY 48,723 (an arylpiperazine), (+)-flesinoxan (a benzodioxane) and S 14671 and S 14506 (methoxynaphtylpiperazines) mimicked the aminotetralin, 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), in inhibiting the firing of dorsal raphe nucleus (DRN) neurons. Similarly, the firing rate of DRN neurons was reduced by the "partial" agonists, MDL 73005EF, BMY 7378, NAN-190, tandospirone and the novel pyrimidinylpiperazine, zalospirone. Furthermore, S 14489, S 15535 and S 15931, novel benzodioxopiperazines, which behave as antagonists at postsynaptic 5-HT1A receptors, inhibited completely DRN firing, whereas the methoxyphenylpiperazine, WAY 100,135, and the aryloxoarylamine, (-)-tertatolol, were ineffective. Indeed, in analogy to spiperone, both WAY 100,135 and (-)-tertatolol behaved as apparently competitive antagonists in that, in their presence, the dose-response curves for inhibition of DRN firing by S 14671, S 14506 or 8-OH-DPAT were shifted in parallel to the right with no loss of maximal effect. In distinction to WAY 100,135 and (-)-tertatolol, a further novel, putative "antagonist," SDZ 216-525 (a benzoisothiazolpiperazine) weakly inhibited the electrical activity of the DRN. With the exception of (-)-tertatolol, which behaved as a weak agonist, a very similar pattern of inhibition of 5-HT turnover was seen in the striatum (innervated by the DRN), the hippocampus and the hypothalamus (DRN and median raphe nucleus) and the spinal cord (nucleus raphe magnus), with the striatum displaying the greatest sensitivity. Drug potency for inhibition of firing and turnover was highly correlated (r = 0.80-0.82) and these actions were significantly correlated to affinity at (hippocampal) 5-HT1A receptors (r = 0.62-0.73). As concerns DA D2 autoreceptors, the agonist action of apomorphine in reducing DA turnover were mimicked only by 8-OH-DPAT, whereas the majority of the other 5-HT1A ligands, in analogy to raclopride, enhanced DA turnover. The facilitation of DA turnover appeared to reflect direct blockade of DA D2 autoreceptors because potency was correlated powerfully to affinity at these D2 sites (r = 0.89).(ABSTRACT TRUNCATED AT 400 WORDS)

In Chinese hamster ovary cells stably transfected with recombinant rat dopamine D2 or D3 receptors, the naphthofurane antagonist, (+)-S 14297 [(+)-[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro- naphtho(2,3b)dihydro,2,3-furane]], displayed a pronounced preference for dopamine D3 versus D2 receptors: Ki values = 13 and 365 nM, respectively. In contrast, its distomer, (-)-S 17777, showed low affinity (296 versus 3403 nM). The aminotetralin agonist, (+)-7-OH-DPAT (7-hydroxy-2-(di-n-propylamino)tetralin), also showed high affinity at dopamine D3 (1.8 nM) versus D2 (96 nM) receptors while its (-)-isomer showed low affinity (71 and 1461 nM). In freely moving rats, (+)-7-OH-DPAT (0.16 mg/kg s.c.)-but not (-)-7-OH-DPAT-decreased dialysate levels of dopamine in the nucleus accumbens. (+)-S 14297 (1.25 mg/kg s.c.) markedly inhibited the action of (+)-7-OH-DPAT without influencing dopamine levels alone. Further, this action was stereospecific in that (-)-S 17777 (20.0 mg/kg s.c.) was inactive. In conclusion, data obtained with the novel, selective dopamine D3 receptor antagonist, (+)-S 14297 suggest that dopamine D3 autoreceptors modulate the release of dopamine from mesolimbic dopaminergic neurones.

This study examined the influence of N-methyl-D-aspartate (NMDA) receptors upon the activity of serotoninergic neurones projecting from the rat dorsal raphe nucleus (DRN) to the striatum of rats. The channel blocker (+)-MK 801 (0.04-0.63 mg/kg, s.c.) augmented striatal accumulation of the serotonin (5-HT) precursor, 5-hydroxytryptophan (5-HTP), in rats treated with the inhibitor of decarboxylase, NSD 1015: the maximal effect of (+)-MK 801 was 164% relative to vehicle values (= 100%). In analogy, (+)-MK 801 (0.01-0.5 mg/kg, i.v.) increased the firing rate of DRN neurones with a maximal effect of 204%. This action was stereospecific in that (-)-MK 801, which shows lower affinity at NMDA receptors, enhanced firing only at higher doses. The selective, competitive antagonist at the NMDA recognition site, CPP (0.5-8.0 mg/kg, i.v.), also facilitated the firing rate of DRN neurones, though with a maximal effect (137%) less than that of (+)-MK 801. Further, CPP (40.0 mg/kg, s.c.) did not significantly modify striatal 5-HT synthesis. While NMDA did not significantly modify DRN firing alone, it abolished the facilitatory action of CPP, consistent with a competitive interaction at the NMDA recognition site. In conclusion, blockade of NMDA receptors specifically facilitates the activity of ascending serotoninergic neurones.

In this study, we attempted to identify of the subtype(s) of alpha-2 adrenergic receptor (AR) involved in the control of motor behavior, nociception and the hippocampal synthesis of noradrenaline (NA) in the rat. The high efficacy alpha-2 AR agonists, xylazine and UK 14,304 [5-bromo-6-[2-imidazolin-2-yl-amino]quinoxaline], inhibited striatal accumulation of L-dopa in rats pretreated with NSD 1015 (an inhibitor of aromatic amino acid-decarboxylase), elicited a loss of the righting reflex in rats, provoked ataxia in the rotarod test in mice and elicited antinociception in the writhing and hot-plate tests in mice. Guanfacine and guanabenz, agonists acting preferentially at rat alpha-2A (R alpha-2A)/human alpha-2A (H alpha-2A) AR, mimicked the antinociceptive and motor actions of xylazine and UK 14,304 and likewise inhibited NA synthesis. The preferential R alpha-2A/H alpha-2A AR antagonist, [2-(2H-(1-methyl-1, 3-dihydroisoindole)methyl)-4, 5-dihydro-imidazole (BRL 44408), enhanced hippocampal synthesis of NA and blocked the antinociceptive and motor effects of UK 14,304, xylazine, guanfacine and guanabenz. Similarly, fluparoxan and des-fluorofluparoxan, preferential antagonists at R alpha-2A AR as compared to H alpha-2A AR, were highly active. In contrast, the preferential alpha-2B/alpha-2C AR antagonists, ARC 239 [2-(2-(4-o-methoxyphenyl)piperazine-1-yl)-ethyl)-4,4-dimethyl-1,3- (2H,4H)-isoquinolinedione] prazosin, corynanthine, spiroxatrine and [1,2-dimethyl-2,3,9,13-betetrahydro-1H-dibenzo(c,f)- imidazo(1,5-a)azepine (BRL 41992)], as well as the preferential H alpha-2A AR antagonist, [2-(2,6-dimethoxyphenoxyethyl)- aminomethyl-1,4-benzodioxane] (WB 4101), were only weakly active. Based on the actions of a total of 16, structurally diverse alpha-2 AR antagonists, a correlation matrix was constructed. This revealed a strong correlation among the tests (median r = 0.82) and allowed for a comparison between drug potency in inhibiting these alpha-2 AR-mediated actions and affinity at various populations of alpha-2 AR subtypes (see companion paper). Correlations for potency in the two motor tests were pronounced with R alpha-2A sites (0.85), modest with H alpha-2A sites (0.60) and alpha-2B sites (0.58) and poor with alpha-2C sites (0.35). For the two antinociceptive tests, correlations were likewise pronounced with R alpha-2A sites (0.80) but less marked with H alpha-2A sites (0.73), alpha-2B sites (0.62) and alpha-2C sites (0.62).(ABSTRACT TRUNCATED AT 400 WORDS)