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A high proportion of depressed patients fail to respond to antidepressant drug treatment. Treatment-resistant depression (TRD) is a major challenge for the psychopharmacology of mood disorders. Only in the past decade have novel treatments, including deep brain stimulation (DBS) and ketamine, been discovered that provide rapid and sometimes prolonged relief to a high proportion of TRD sufferers. In this review, we consider the current status of TRD from four perspectives: the challenge of developing an appropriate regulatory framework for novel rapidly acting antidepressants; the efficacy of non-pharmacological somatic therapies; the development of an animal model of TRD and its use to understand the neural basis of antidepressant non-response; and the potential for rapid antidepressant action from targets (such as 5-HT1A receptors) beyond the glutamate receptor.

Background and Purpose Pattern separation, that is, the formation of distinct representations from similar inputs, is an important hippocampal process implicated in cognitive domains like episodic memory. A deficit in pattern separation could lead to memory impairments in several psychiatric and neurological disorders. Hence, mechanisms by which pattern separation can be increased are of potential therapeutic interest. Experimental approach 5-HT1A receptors are involved in spatial memory. Herein we tested the ‘biased’ 5-HT1A receptor agonists F15599, which preferentially activates post-synaptic heteroreceptors, and F13714, which preferentially activates raphe-located autoreceptors, in rats in a novel spatial task assessing pattern separation, the object pattern separation (OPS) task. Key Results The acetylcholinesterase inhibitor donepezil, which served as a positive control, significantly improved spatial pattern separation at a dose of 1 mg·kg−1, p.o. F15599 increased pattern separation at 0.04 mg·kg−1, i.p., while F13714 decreased pattern separation at 0.0025 mg·kg−1, i.p. The selective 5-HT1A receptor antagonist WAY-100635 (0.63 mg·kg−1, s.c.) counteracted the effects of both agonists. These data suggest that acute preferential activation of post-synaptic 5-HT1A heteroreceptors improves spatial pattern separation, whereas acute preferential activation of raphe-located 5-HT1A autoreceptors impairs performance. Conclusions and Implications We successfully established and validated a novel, simple and robust OPS task and observed a diverging profile of response with ‘biased’ 5-HT1A receptor agonists based on their targeting of receptors in distinct brain regions. Our data suggest that the post-synaptic 5-HT1A receptor consists of a potential novel molecular target to improve pattern separation performance.

Summary Aims Positron emission tomography (PET) imaging using 5-HT1A receptor radioligands shows a decreased expression of this serotonin receptor in the hippocampus of patients with Alzheimer's disease (AD) at advanced stages. However, previous 5-HT1A receptor radioligands used in human imaging were antagonists, thought to bind to 5-HT1A receptors in different functional states (i.e., both the one which displays high affinity for agonists and is thought to mediate receptor activation, as well as the functional state which has low affinity for agonists). Comparing the PET imaging obtained using an agonist radioligand, which binds selectively to the functional state of the receptors, with the PET imaging obtained using an antagonist radioligand would therefore provide original information on 5-HT1A receptor impairment during AD. Methods Quantitative autoradiography using 18F-F15599 and 18F-MPPF, a 5-HT1A agonist and antagonist, respectively, was measured in hippocampi of 18 patients with AD. Results Functional 5-HT1A receptors, labeled by 18F-F15599, represented ~35% of total receptors, as estimated by 18F-MPPF labeling. 18F-F15599 binding decreased in dentate gyrus of patients with AD, as indicated by Braak's stages. In contrast, binding of 18F-MPPF was statistically unchanged. Conclusion These in vitro results support testing the concept of functional PET imaging using agonist radioligands in clinical studies.

Background and Purpose Many dementia patients exhibit behavioural and psychological symptoms (BPSD) that include psychosis, aggressivity, depression and anxiety. Antipsychotic drugs are frequently prescribed but fail to significantly attenuate mood deficits, may interfere with cognitive function and are associated with motor and cardiac side effects, which are problematic in elderly patients. A need therefore exists for drugs that are better suited for the treatment of BPSD. Experimental Approach We used in vitro cellular and in vivo behavioural tests to characterize ADN-1184, a novel arylsulfonamide ligand with potential utility for treatment of BPSD. Key Results ADN-1184 exhibits substantial 5-HT6/5-HT7/5-HT2A/D2 receptor affinity and antagonist properties in vitro. In tests of antipsychotic-like activity, it reversed MK-801-induced hyperactivity and stereotypies and inhibited conditioned avoidance response (MED = 3 mg·kg−1 i.p.). Remarkably, ADN-1184 also reduced immobility time in the forced swim test at low doses (0.3 and 1 mg·kg−1 i.p.; higher doses were not significantly active). Notably, up to 30 mg·kg−1 ADN-1184 did not impair memory performance in the passive avoidance test or elicit significant catalepsy and only modestly inhibited spontaneous locomotor activity (MED = 30 mg·kg−1 i.p.). Conclusions and Implications ADN-1184 combines antipsychotic-like with antidepressant-like properties without interfering with memory function or locomotion. This profile is better than that of commonly used atypical antipsychotics tested under the same conditions and suggests that it is feasible to identify drugs that improve BPSD, without exacerbating cognitive deficit or movement impairment, which are of particular concern in patients with dementia.

Background and purpose: F15599, a novel 5-hydroxytryptamine (5-HT)1A receptor agonist with 1000-fold selectivity for 5-HT compared with other monoamine receptors, shows antidepressant and procognitive activity at very low doses in animal models. We examined the in vivo activity of F15599 at somatodendritic autoreceptors and postsynaptic 5-HT1A heteroreceptors. Experimental approach: In vivo single unit and local field potential recordings and microdialysis in the rat. Key results: F15599 increased the discharge rate of pyramidal neurones in medial prefrontal cortex (mPFC) from 0.2 µg·kg−1 i.v and reduced that of dorsal raphe 5-hydroxytryptaminergic neurones at doses >10-fold higher (minimal effective dose 8.2 µg·kg−1 i.v.). Both effects were reversed by the 5-HT1A antagonist (±)WAY100635. F15599 did not alter low frequency oscillations (∼1 Hz) in mPFC. In microdialysis studies, F15599 increased dopamine output in mPFC (an effect dependent on the activation of postsynaptic 5-HT1A receptors) with an ED50 of 30 µg·kg−1 i.p., whereas it reduced hippocampal 5-HT release (an effect dependent exclusively on 5-HT1A autoreceptor activation) with an ED50 of 240 µg·kg−1 i.p. Likewise, application of F15599 by reverse dialysis in mPFC increased dopamine output in a concentration-dependent manner. All neurochemical responses to F15599 were prevented by administration of (±)WAY100635. Conclusions and implications: These results indicate that systemic administration of F15599 preferentially activates postsynaptic 5-HT1A receptors in PFC rather than somatodendritic 5-HT1A autoreceptors. This regional selectivity distinguishes F15599 from previously developed 5-HT1A receptor agonists, which preferentially activate somatodendritic 5-HT1A autoreceptors, suggesting that F15599 may be particularly useful in the treatment of depression and of cognitive deficits in schizophrenia.

Background and purpose: Activation of post-synaptic 5-HT1A receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT1A receptor agonist. Experimental approach: F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo. Key results: F15599 was highly selective for 5-HT1A receptors in binding experiments and in [35S]-GTPγS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT1A receptors. In cell lines expressing h5-HT1A receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT1A receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [35S]-GTPγS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated Gαi than Gαo activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT1A receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT1A receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT1A receptors in vivo almost as potently as F13714. Conclusions and implications: F15599 showed a distinctive activation profiles for 5-HT1A receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT1A receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.

Background and purpose: The D2/D3 receptor antagonist, D4 receptor partial agonist, and high efficacy 5-HT1A receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the ‘serotonin syndrome’. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia. Experimental approach: F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems. Key results: Through 5-HT1A activation, F15063 partially alleviated (MED: 0.04 mg kg−1) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg−1, F15063 reduced interaction by itself. F15063 (0.16 mg kg−1) selectively re-established PCP-impaired ‘cognitive flexibility’ in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04–0.63 mg kg−1) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D4 partial agonism, as it was blocked by the D4 antagonist L745,870. Finally, F15063 up to 40 mg kg−1 did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating. Conclusions and implications: The balance of D2/D3, D4 and 5-HT1A receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia. British Journal of Pharmacology (2007) 151, 266–277. doi:10.1038/sj.bjp.0707160

Background and purpose: F15063 is a high affinity D2/D3 antagonist, D4 partial agonist, and high efficacy 5-HT1A agonist, with little affinity (40-fold lower than for D2 receptors) at other central targets. Here, the profile of F15063 was evaluated in models of positive symptoms of schizophrenia and motor side-effects. Experimental approach: Rodent behavioural tests were based on reversal of hyperactivity induced by psychostimulants and on measures of induction of catalepsy and ‘serotonin syndrome’. Key results: F15063 potently (ED50s: 0.23 to 1.10 mg kg−1 i.p.) reversed methylphenidate-induced stereotyped behaviors, blocked d-amphetamine and ketamine hyperlocomotion, attenuated apomorphine-induced prepulse inhibition (PPI) deficits, and was active in the conditioned avoidance test. In mice, it reversed apomorphine-induced climbing (ED50 = 0.30 mg kg−1 i.p.). F15063, owing to its 5-HT1A agonism, did not produce (ED50 > 40 mg kg−1 i.p.) catalepsy in rats and mice, a behavior predictive of occurrence of extra-pyramidal syndrome (EPS) in man. This absence of cataleptogenic activity was maintained upon sub-chronic treatment of rats for 5 days at 40 mg kg−1 p.o. Furthermore, F15063 did not induce the ‘serotonin syndrome’ in rats (flat body posture and forepaw treading: ED50 >32 mg kg−1 i.p.). Conclusions and implications: F15063 conformed to the profile of an atypical antipsychotic, with potent actions in models of hyperdopaminergic activity but without inducing catalepsy. These data suggest that F15063 may display potent antipsychotic actions with low EPS liability. This profile is complemented by a favourable profile in rodent models of negative symptoms and cognitive deficits of schizophrenia (companion paper). British Journal of Pharmacology (2007) 151, 253–265. doi:10.1038/sj.bjp.0707159

Background and purpose: Combining 5-HT1A receptor activation with dopamine D2/D3 receptor blockade should improve negative symptoms and cognitive deficits in schizophrenia. We describe the in vitro profile of F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine). Experimental approach: F15063 was characterised in tests of binding affinity and in cellular models of signal transduction at monoamine receptors. Key results: Affinities (receptor and pKi values) of F15063 were: rD2 9.38; hD2L 9.44; hD2S 9.25; hD3 8.95; hD4 8.81; h5-HT1A 8.37. F15063 had little affinity (40-fold lower than D2) at other targets. F15063 antagonised dopamine-activated G-protein activation at hD2, rD2 and hD3 receptors with potency (pKb values 9.19, 8.29 and 8.74 in [35S]GTPγS binding experiments) similar to haloperidol. F15063 did not exhibit any hD2 receptor agonism, even in tests of ERK1/2 phosphorylation and G-protein activation in cells with high receptor expression. In contrast, like (±)8-OH-DPAT, F15063 efficaciously activated h5-HT1A (Emax 70%, pEC50 7.57) and r5-HT1A receptors (52%, 7.95) in tests of [35S]GTPγS binding, cAMP accumulation (90%, 7.12) and ERK1/2 phosphorylation (93%, 7.13). F15063 acted as a partial agonist for [35S]GTPγS binding at hD4 (29%, 8.15) and h5-HT1D receptors (35%, 7.68). In [35S]GTPγS autoradiography, F15063 activated G-proteins in hippocampus, cortex and septum (regions enriched in 5-HT1A receptors), but antagonised quinelorane-induced activation of D2/D3 receptors in striatum. Conclusions and implications: F15063 antagonised dopamine D2/D3 receptors, a property underlying its antipsychotic-like activity, whereas activation of 5-HT1A and D4 receptors mediated its actions in models of negative symptoms and cognitive deficits of schizophrenia (see companion papers). British Journal of Pharmacology (2007) 151, 237–252. doi:10.1038/sj.bjp.0707158

Background and purpose: Desensitization of somatodendritic 5-HT1A receptors is involved in the mechanism of action of several antidepressants, but the rapidity of this effect and the amount of agonist stimulation needed are unclear. We evaluated the capacity of the high-efficacy 5-HT1A agonist, F13714 (3-chloro-4-fluorophenyl-(4-fluoro-4-{[(5-methyl-6-methylaminopyridin-2-ylmethyl)-amino]-methyl}-piperidin-1-yl-methanone) and of the partial agonist, flesinoxan, to desensitize somatodendritic 5-HT1A receptors involved in the control of 5-HT release. Experimental approach: Intracerebral microdialysis in the hippocampus of freely moving rats was used to examine the acute and chronic effects of the two compounds (administered by osmotic pumps for 3, 7 or 14 days) on extracellular 5-HT levels, measured by HPLC with electrochemical detection. Key results: When given acutely, F13714, flesinoxan and the low-efficacy 5-HT1A agonist, buspirone, dose-dependently decreased extracellular 5-HT concentrations (ED50 values: 0.04, 0.77 and 5.6 mg kg � 1 , respectively). The selective 5-HT1A antagonist WAY100635 inhibited the effects of the three compounds. F13714 (2.5 mg kg � 1 per day for 3, 7 or 14 days and 0.63 mg kg � 1 for 7 days) significantly attenuated the inhibition of 5-HT release induced by buspirone (10 mg kg � 1 ). In contrast, flesinoxan (10 mg kg � 1 per day) failed to alter the response to buspirone at any of the treatment durations. Conclusions and implications: Rat somatodendritic 5-HT1A receptors controlling hippocampal 5-HT release were rapidly desensitized by chronic activation with a high-efficacy 5-HT1A agonist, but not by chronic activation with a partial agonist. Thus, rapid 5-HT1A autoreceptor desensitization by high-efficacy agonists may accelerate the onset of the therapeutic effects of antidepressants.

Serotonin 5-HT1B receptors couple to G-proteins of the Gi/o family. However, their activation of specific G-protein subtypes is poorly characterised. Using an innovative antibody capture/guanosine-5′-0-(3-[35S]thio)-triphosphate ([35S]GTPγS) binding strategy, we characterised Gαi3 subunit activation by h5-HT1B receptors stably expressed in Chinese hamster ovary (CHO) cells. The agonists, 5-HT, alniditan and BMS181,101, stimulated Gαi3, whereas methiothepin and SB224,289 behaved as inverse agonists. The selective 5-HT1B receptor ligand, S18127, modestly stimulated Gαi3 and reversed the actions of both 5-HT and methiothepin. S18127 (1 μM) also produced parallel, dextral shifts of the 5-HT and methiothepin isotherms. Isotopic dilution experiments ([35S]GTPγS versus GTPγS) revealed high-affinity [35S]GTPγS binding to Gαi3 subunits in the absence of receptor ligands indicating constitutive activity. High-affinity [35S]GTPγS binding was increased 2.8-fold by 5-HT with an increase in the affinity of GTPγS for Gαi3 subunits. In contrast, methiothepin halved the number of high-affinity binding sites and decreased their affinity. h5-HT1B receptor-mediated Gαi3 subunit activation was dependent on the concentration of NaCl. At 300 mM, 5-HT stimulated [35S]GTPγS binding, basal Gαi3 activation was low and methiothepin was inactive. In contrast, at 10 mM NaCl, basal activity was enhanced and the inverse agonist activity of methiothepin was accentuated. Under these conditions, 5-HT decreased Gαi3 activation. In conclusion, at h5-HT1B receptors expressed in CHO cells: (i) inverse agonist induced inhibition of Gαi3, and its reversal by S18127, reveals constitutive activation of this Gα subunit; (ii) constitutive Gαi3 activation can be quantified by isotopic dilution [35S]GTPγS binding and (iii) decreasing NaCl concentrations enhances Gαi3 activation and leads to protean agonist properties of 5-HT: that is a switch to inhibition of Gαi3. British Journal of Pharmacology (2003) 138, 1077–1084. doi:10.1038/sj.bjp.0705140

1. The interaction of serotonergic ligands at human (h) 5-HT(1A) receptors expressed in Chinese hamster ovary cells was examined with the selective 'neutral' 5-HT(1A) antagonist [(3)H]-WAY100,635. Its binding was saturable (K(D)=0.056 nM) with a B(max) (3.65 pmol mg(-1)) significantly higher than that of two other selective 5-HT(1A) radioligands: the partial agonist, [(3)H]-S15535 (2.77 pmol mg(-1)) and the agonist, [(3)H]-8-OH-DPAT (2.02 pmol mg(-1)). 2. The influence of GTPgammaS (100 microM) on the binding affinity of 15 serotonergic agonists, partial agonists, antagonists and inverse agonists was investigated in competition binding experiments with [(3)H]-WAY100,635. 3. Agonists, including 5-HT, 8-OH-DPAT and buspirone, displayed biphasic isotherms which shifted to the right in the presence of GTPgammaS. In contrast, isotherms of the inverse agonists, methiothepin, (+)butaclamol and spiperone, were shifted to the left in the presence of GTPgammaS. Unlabelled WAY100,635 was the only ligand that was unaffected by GTPgammaS, consistent with 'neutral' antagonist properties. 4. The magnitude of affinity changes induced by GTPgammaS for 13 ligands was highly correlated (r = 0.98) with their efficacy (positive and negative) previously determined by [(35)S]GTPgammaS binding. 5. In contrast, the napthylpiperazine derivative and high efficacy agonist, S14506, displayed only a modest GTPgammaS shift, in accordance with previous indications of 'atypical' binding properties of this ligand. A further full agonist, S14671, which is chemically closely-related to S14506, also displayed a minimal GTPgammaS shift, underpinning this observation. 6. In conclusion, [(3)H]-WAY100,635 constitutes a useful neutral antagonist radioligand for the characterization of drug actions at h5-HT(1A) receptors. GTPgammaS-induced affinity changes of agonist and inverse agonist competition isotherms generally correlate well with ligand efficacy, with the notable exception of two chemically-similar agents, S14506 and S14671, which are efficacious agonists, yet relatively insensitive to h5-HT(1A) receptor/G-protein coupling changes.

The present study evaluated, via a combined electrophysiological and dialysis approach, the potential influence of serotonin (5-HT)2C as compared to 5-HT2A and 5-HT2B receptors on dopaminergic, adrenergic, and serotonergic transmission in frontal cortex (FCX). Whereas the selective 5-HT2A 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-HT2B/5-HT2C antagonist SB206,553 dose-dependently increased levels of DA and NA without affecting those of 5-HT. This action was attributable to 5-HT2C receptor blockade inasmuch as the selective 5-HT2C antagonist SB242,084 likewise increased FCX levels of DA and NA, whereas the selective 5-HT2B antagonist SB204,741 was ineffective. Further, the preferential 5-HT2C receptor agonist Ro60-0175 dose-dependently depressed FCX levels of DA. The suppressive influence of 5-HT2C 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-HT2A and 5-HT2B receptors, 5-HT2C 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-HT2C receptors. Synapse 36:205–221, 2000. © 2000 Wiley-Liss, Inc.

A modulation of the expression of immediate-early genes (IEGs) such as c-fos is likely involved in the long-term influence of dopaminergic ligands on the activity of basal ganglia neurons. The roles of individual dopamine receptor types in this regard remain unclear, and the present study employed primary cultures of rat striatal neurons to examine the potential modulation of c-fos expression by D(3) receptors. Neurons were treated with the preferential D(3) receptor agonists, (+/-)-7-OH-DPAT and PD 128,907, which showed, respectively, 16-fold and 56-fold selectivity for recombinant rat D(3) vs. D(2) receptors (K(i) values, rD(2)/rD(3) = 59.5/3.7 nM and 251/4.5 nM, respectively). At concentrations of 3 and 30 nM, respectively, (+/-)-7-OH-DPAT and PD 128,907 significantly increased the expression of c-fos mRNA. The action of (+/-)-7-OH-DPAT was expressed stereospecifically; its (+)-isomer (K(i) values, D(3)/D(2) = 1.6/56.7 elicited a 26% +/- 7.6% increase in c-fos expression whereas its (-)-isomer (K(i) values, D(3)/D(2) = 59/1,060 nM) was ineffective. Further, stimulation of c-fos mRNA expression by PD 128,907 (20 nM) was markedly and significantly (P < 0.05) attenuated (-91.8% +/- 5.3%) by 30 nM of the selective D(3) receptor antagonist, (+)-S 14297 (K(i) values, D(3)/D(2) = 11/401 nM). In contrast, the action of PD 128,907 was significantly less affected (-24.5% +/- 7.8%) by 30 nM of its less active distomer, (-)-S 17777 (K(i) values, D(3)/D(2) = 294/3,191 nM). An examination of the concentration-response relationship revealed that (+/-)-7-OH-DPAT and PD 128,907 both produced bell-shaped dose-response curves for c-fos induction. The sequential activation of D(2) receptors-which inhibit striatal c-fos expression (Simpson and Morris [1995] Neuroscience 68:97-106)-by higher concentrations of (+/-)-7-OH-DPAT and PD 128,907 is presumably involved in the inflexion at higher doses. In conclusion, the present data demonstrate that activation of D(3) receptors results in a selective induction of c-fos mRNA expression in cultured striatal neurons. These data show that neuronal D(3) receptors control the expression of IEGs and suggest that D(3) receptors may mediate long-term adapative changes in the activity of neurons in the basal ganglia.

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.

In the present study, the comparative mechanisms of action of phencyclidine (PCP) and amphetamine were addressed employing the parameter of locomotion in rats. PCP-induced locomotion (PLOC) was potently blocked by the selective serotonin (5-HT)2A vs. D2 antagonists, SR46349, MDL100,907, ritanserin and fananserin, which barely affected amphetamine-induced locomotion (ALOC). In contrast, the selective D2 vs. 5-HT2A antagonists, eticlopride, raclopride and amisulpride, preferentially inhibited ALOC vs. PLOC. The potency of these drugs and 12 multireceptorial antipsychotics in inhibiting PLOC vs. ALOC correlated significantly with affinities at 5-HT2A vs. D2 receptors, respectively. Amphetamine and PCP both dose dependently increased dialysate levels of dopamine (DA) and 5-HT in the nucleus accumbens, striatum and frontal cortex (FCX) of freely moving rats, but PCP was proportionally more effective than amphetamine in elevating levels of 5-HT vs. DA in the accumbens. Further, whereas microinjection of PCP into the accumbens elicited locomotion, its introduction into the striatum or FCX was ineffective. The action of intra-accumbens PCP, but not intra-accumbens amphetamine, was abolished by SR46349 and clozapine. Parachloroamphetamine, which depleted accumbens pools of 5-HT but not DA, likewise abolished PLOC without affecting ALOC. In contrast, intra-accumbens 6-hydroxydopamine (6-OHDA), which depleted DA but not 5-HT, abolished ALOC but only partially attenuated PLOC. In conclusion, PLOC involves (indirect) activation of accumbens-localized 5-HT2A receptors by 5-HT. PLOC is, correspondingly, more potently blocked than ALOC by antipsychotics displaying marked affinity at 5-HT2A receptors.

J. Neurochem. (2010) 114, 1323–1332. Abstract The function of the serotonin2B receptor (5-HT2BR) in the mammalian brain is poorly characterized, especially with regard to its influence on dopamine (DA) neuron activity. Here, we assessed this issue by evaluating effects of 5-HT2BRs ligands in the control of striatal and accumbal DA outflow, using in vivo microdialysis in halothane-anesthetized rats, and amphetamine-induced hyperlocomotion in vigil rats. The selective 5-HT2BR antagonist 1-[(2-chloro-3,4-dimethoxyphenyl)methyl]-2,3,4,9-tetrahydro-6-methyl-1H-pyrido[3,4-B]indole (LY 266097; 0.16 mg/kg, i.p.) had no influence on basal accumbal and striatal DA outflow but reduced significantly accumbal DA outflow when injected at 0.63 mg/kg. A significant reduction of basal DA outflow in the nucleus accumbens was also observed after i.p. administration of 0.16 mg/kg 2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyrimidine, another selective 5-HT2BR antagonist. In contrast, the 5-HT2BR agonist α-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (3 mg/kg, s.c.) had no influence on basal DA outflow in either brain region. The increase in striatal and accumbal DA outflow induced by the 5-HT2CR inverse agonist 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f] indole (5 mg/kg, i.p.) was unaltered by LY 266097 (0.63 mg/kg) pre-treatment. Conversely, LY 266097 (0.63 mg/kg) significantly diminished the increase in DA outflow induced by haloperidol (0.01 mg/kg, s.c.) or amphetamine (0.5 mg/kg, i.p.) in the nucleus accumbens, but not in the striatum. Amphetamine-induced hyperlocomotion (1 mg/kg) was also attenuated by LY 266097 (0.63 mg/kg). These findings demonstrate that 5-HT2BRs exert a facilitatory control on mesoaccumbens DA pathway activity, and suggest that they may constitute a new target for improved treatment of DA-related neuropsychiatric disorders.

A new series of benzopyrano[3,4-c]pyrrole derivatives were synthesized and evaluated for their interaction with dopamine D3 versus D2 receptors. Amongst these compounds, 4x (S 33084) was found to be a potent and selective dopamine D3 receptor antagonist.

At recombinant human 5-hydroxytryptamine (5-HT)5-HT1A receptors expressed in Chinese hamster ovary cells (CHO-5-HT1A), 5-carboxamidotryptamine (5-CT), acted as a full agonist (relative to 5-HT=100%) for stimulation of receptor-mediated [35S]-GTPγS (guanylyl 5′-[γ-thio]-tryphosphate) binding. In contrast, spiperone inhibited basal [35S]-GTPγS binding by 30.2% (IC50=55.5 nm) in CHO-5-HT1A membranes but not in control untransfected membranes. The antagonist, N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)-cyclohexane-carboxamide (WAY 100,635), blocked both 5-CT-induced stimulation and spiperone-induced inhibition of [35S]-GTPγS binding without itself modifying [35S]-GTPγS binding. It is concluded that, in this heterologous expression system, 5-HT1A receptors display ‘constitutive’ activation of G-proteins and that spiperone displays inverse agonist activity whereas WAY 100,635 acts as a ‘neutral’ antagonist at this site. British Journal of Pharmacology (1997) 120, 737–739; doi:10.1038/sj.bjp.0701025