Search

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.

In forced-swim tests in mice and rats, the novel D(3)/D(2) receptor agonist S32504 [(+)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] dose-dependently (0.04-2.5 mg/kg) and stereospecifically suppressed immobility compared with its enantiomer S32601 [(-)-trans-3,4,4a,5,6,10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth-[1,2-b]-1,4-oxazine]. Ropinirole was less potent than S32504 in this procedure, and it was likewise less potent than S32504 (0.04-2.5 mg/kg) in attenuating motor-suppressant properties of the alpha(2)-adrenoceptor agonist S18616 [(S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(1',2',3',4'-tetrahydronaphthalene)]]. In a learned helplessness paradigm, S32504 (0.08-2.5 mg/kg) suppressed escape failures. Furthermore, in a chronic mild stress model of anhedonia, S32504 (0.16-2.5 mg/kg) rapidly restored the suppression of sucrose consumption. S32504 inhibited marble-burying behavior in mice (0.04-0.16 mg/kg) and aggressive behavior in isolated mice (0.04-2.5 mg/kg): only higher doses of ropinirole mimicked these actions of S32504. In tests of anxiolytic activity, S32504 was more potent (0.0025-0.16 mg/kg) than ropinirole in suppressing fear-induced ultrasonic vocalizations, and S32601 was inactive. Furthermore, in contrast to ropinirole, S32504 modestly enhanced punished responses in a Vogel conflict procedure and increased open-arm entries in a plus-maze. At doses active in the above-described procedures, S32504 did not elicit hyperlocomotion. In the forced-swim, marble-burying, and ultrasonic vocalization models, actions of S32504 were blocked by the D(2)/D(3) antagonists haloperidol and raclopride and by the D(2) antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol], but not by the 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. Finally, chronic administration of S32504 did not, in contrast to venlafaxine, modify corticolimbic levels of serotonin(2A) receptors or brain-derived neurotrophic factor. In conclusion, S32504 displays a broad and distinctive profile of activity in models of potential antidepressive and anxiolytic properties. Its actions are more pronounced than those of ropinirole and principally involve engagement of D(2) receptors.

These studies evaluated the potential antiparkinsonian properties of the novel dopamine D(3)/D(2) receptor agonist S32504 [(+)-trans-3,4,4a,5,6, 10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth[1,2-b]-1,4-oxazine] in comparison with those of the clinically employed agonist ropinirole. In rats with a unilateral, 6-hydroxydopamine lesion of the substantia nigra, S32504 (0.0025-0.04 mg/kg, s.c.) more potently elicited contralateral rotation than S32601 [(-)-trans-3,4,4a,5,6, 10b-hexahydro-9-carbamoyl-4-propyl-2H-naphth-[1,2-b]-1,4-oxazine (its less active enantiomer)], ropinirole, and l-3,4-dihydroxyphenylalanine (l-DOPA). Rotation elicited by S32504 was blocked by the D(2)/D(3) receptor antagonists haloperidol and raclopride and by the D(2) antagonist L741,626 [4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol], but not by the D(3) 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]. As assessed by dialysis in both lesioned and nonlesioned animals, S32504 (0.04-2.5 mg/kg, s.c.) reduced striatal levels of acetylcholine. This effect was blocked by raclopride, haloperidol, and L741,626 but not S33084. In rats treated with reserpine, hypolocomotion was reversed by S32504 and, less potently, by ropinirole. In "unprimed" marmosets treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, both s.c. (0.01-0.04 mg/kg) and p.o. (0.04-1.25 mg/kg) administration of S32504 dose-dependently and rapidly (within 10 min) increased locomotor activity and reduced disability. Furthermore, S32504 dose-dependently reversed bradykinesia and improved posture in "L-DOPA-primed" animals, whereas eliciting less pronounced dyskinesia than l-DOPA. Finally, in terminally differentiated SH-SY5Y cells presenting a dopaminergic phenotype, S32504, but not S32601, abrogated the neurotoxic effects of 1-methyl-4-phenylpyridinium, an action inhibited by raclopride and S33084 but not L741,626. Ropinirole was weakly neuroprotective in this model. In conclusion, S32504 displays potent and stereospecific activity in rodent, primate, and cellular models of antiparkinsonian properties. Although activation of D(2) receptors is crucial to the motor actions of S32504, engagement of D(3) receptors contributes to its neuroprotective properties.

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.

In a dialysis procedure not requiring perfusate addition of acetylcholinesterase inhibitors to "boost" basal levels of acetylcholine (ACh), the influence of the antiparkinson agent piribedil upon levels of ACh in frontal cortex and dorsal hippocampus of freely moving rats was compared with those of other antiparkinson drugs and selective ligands at alpha(2)-adrenoceptors (ARs). Suggesting a tonic, inhibitory influence of alpha(2A)-ARs upon cholinergic transmission, the alpha(2)-AR agonist 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline tartrate (UK14,304), and the preferential alpha(2A)-AR agonist guanabenz reduced levels of ACh. They were elevated by the antagonists 2(2-methoxy-1,4 benzodioxan-2-yl)-2-imidazoline HCl (RX821002) and atipamezole and by the preferential alpha(2A)-AR antagonist 2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidazole (BRL44008). In contrast, trans-2,3,9,13b-tetrahydro-1,2-dimethyl-1H-dibenz[c,f]imidazo[1,5-a]azepine (BRL41992) and prazosin, preferential alpha(2B/2C)-AR antagonists, were inactive. The dopaminergic agonist and antiparkinson agent piribedil, which behaves as an antagonist at alpha(2)-ARs, dose dependently increased extracellular levels of ACh. This action was absent upon pretreatment with a maximally effective dose of RX821002. On the other hand, a further dopaminergic agonist and antiparkinson agent, talipexole, which possesses agonist properties at alpha(2)-ARs, dose dependently reduced levels of ACh. This action was also blocked by RX821002. In contrast to piribedil and talipexole, quinelorane, which interacts with dopaminergic receptors but not alpha(2)-ARs, failed to affect ACh levels. Finally, in analogy to the frontal cortex, piribedil likewise elicited a dose-dependent increase in extracellular levels of ACh in the dorsal hippocampus. In conclusion, in distinction to talipexole and quinelorane, and reflecting its antagonist properties at alpha(2A)-ARs, piribedil reinforces cholinergic transmission in the frontal cortex and dorsal hippocampus of freely moving rats. These actions may be related to its facilitatory influence upon cognitive function.

Although certain antiparkinson agents interact with serotonin (5-HT) receptors, little information is available concerning functional actions. Herein, we characterized efficacies of apomorphine, bromocriptine, cabergoline, lisuride, piribedil, pergolide, roxindole, and terguride at human (h)5-HT(1A), h5-HT(1B), and h5-HT(1D) receptors [guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding], and at h5-HT(2A), h5-HT(2B), and h5-HT(2C) receptors (depletion of membrane-bound [(3)H]phosphatydilinositol). All drugs stimulated h5-HT(1A) receptors with efficacies (compared with 5-HT, 100%) ranging from modest (apomorphine, 35%) to high (cabergoline, 93%). At h5-HT(1B) receptors, efficacies varied from mild (terguride, 37%) to marked (cabergoline, 102%) and potencies were modest (pEC(50) values of 5.8-7.6): h5-HT(1D) sites were activated with a similar range of efficacies and greater potency (7.1-8.5). Piribedil and apomorphine were inactive at h5-HT(1B) and h5-HT(1D) receptors. At h5-HT(2A) receptors, terguride, lisuride, bromocriptine, cabergoline, and pergolide displayed potent (7.6-8.8) agonist properties (49-103%), whereas apomorphine and roxindole were antagonists and piribedil was inactive. Only pergolide (113%/8.2) and cabergoline (123%/8.6) displayed pronounced agonist properties at h5-HT(2B) receptors. At 5-HT(2C) receptors, lisuride, bromocriptine, pergolide, and cabergoline were efficacious (75-96%) agonists, apomorphine and terguride were antagonists, and piribedil was inactive. MDL100,907 and SB242,084, selective antagonists at 5-HT(2A) and 5-HT(2C) receptors, respectively, abolished these actions of pergolide, cabergoline, and bromocriptine. In conclusion, antiparkinson agents display markedly different patterns of agonist and antagonist properties at multiple 5-HT receptor subtypes. Although all show modest (agonist) activity at 5-HT(1A) sites, their contrasting actions at 5-HT(2A) and 5-HT(2C) sites may be of particular significance to their functional profiles in vivo.

The accompanying multivariate analysis of the binding profiles of antiparkinson agents revealed contrasting patterns of affinities at diverse classes of monoaminergic receptor. Herein, we characterized efficacies at human (h)D(2SHORT(S)), hD(2LONG(L)), hD(3), and hD(4.4) receptors and at halpha(2A)-, halpha(2B)-, halpha(2C)-, and halpha(1A)-adrenoceptors (ARs). As determined by guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding, no ligand displayed "full" efficacy relative to dopamine (100%) at all "D(2)-like" sites. However, at hD(2S) receptors quinpirole, pramipexole, ropinirole, quinerolane, pergolide, and cabergoline were as efficacious as dopamine (E(max)100%); TL99, talipexole, and apomorphine were highly efficacious (79-92%); piribedil, lisuride, bromocriptine, and terguride showed intermediate efficacy (40-55%); and roxindole displayed low efficacy (11%). For all drugs, efficacies were lower at hD(2L) receptors, with terguride and roxindole acting as antagonists. At hD(3) receptors, efficacies ranged from 33% (roxindole) to 94% (TL99), whereas, for hD(4) receptors, highest efficacies (approximately 70%) were seen for quinerolane, quinpirole, and TL99, whereas piribedil and terguride behaved as antagonists and bromocriptine was inactive. Although efficacies at hD(2S) versus hD(2L) sites were highly correlated (r = 0.79), they correlated only modestly to hD(3)/hD(4) sites (r = 0.44-0.59). In [(35)S]GTPgammaS studies of halpha(2A)-ARs, TL99 (108%), pramipexole (52%), talipexole (51%), pergolide (31%), apomorphine (16%), and quinerolane (11%) were agonists and ropinirole and roxindole were inactive, whereas piribedil and other agents were antagonists. Similar findings were obtained at halpha(2B)- and halpha(2C)-ARs. Using [(3)H]phosphatidylinositol depletion, roxindole, bromocriptine, lisuride, and terguride displayed potent antagonist properties at halpha(1A)-ARs. In conclusion, antiparkinson agents display diverse agonist and antagonist properties at multiple subtypes of D(2)-like receptor and alpha(1)/alpha(2)-AR, actions, which likely contribute to their contrasting functional profiles.

Because little comparative information is available concerning receptor profiles of antiparkinson drugs, affinities of 14 agents were determined at diverse receptors implicated in the etiology and/or treatment of Parkinson's disease: human (h)D(1), hD(2S), hD(2L), hD(3), hD(4), and hD(5) receptors; human 5-hydroxytryptamine (5-HT)(1A), h5-HT(1B), h5-HT(1D), h5-HT(2A), h5-HT(2B), and h5-HT(2C) receptors; halpha(1A)-, halpha(1B)-, halpha(1D)-, halpha(2A)-, halpha(2B)-, halpha(2C)-, rat alpha(2D)-, hbeta(1)-, and hbeta(2)-adrenoceptors (ARs); and native histamine(1) receptors. A correlation matrix (294 pK(i) values) demonstrated substantial "covariance". Correspondingly, principal components analysis revealed that axis 1, which accounted for 76% variance, was associated with the majority of receptor types: drugs displaying overall high versus modest affinities migrated at opposite extremities. Axis 2 (7% of variance) differentiated drugs with high affinity for hD(4) and H(1) receptors versus halpha(1)-AR subtypes. Five percent of variance was attributable to axis 3, which distinguished drugs with marked affinity for hbeta(1)- and hbeta(2)-ARs versus hD(5) and 5-HT(2A) receptors. Hierarchical (cluster) analysis of global homology generated a dendrogram differentiating two major groups possessing low versus high affinity, respectively, for multiple serotonergic and hD(5) receptors. Within the first group, quinpirole, quinerolane, ropinirole, and pramipexole interacted principally with hD(2), hD(3), and hD(4) receptors, whereas piribedil and talipexole recognized dopaminergic receptors and halpha(2)-ARs. Within the second group, lisuride and terguride manifested high affinities for all sites, with roxindole/bromocriptine, cabergoline/pergolide, and 6,7-dihydroxy-N,N-dimethyl-2-ammotetralin (TL99)/apomorphine comprising three additional subclusters of closely related ligands. In conclusion, an innovative multivariate analysis revealed marked heterogeneity in binding profiles of antiparkinson agents. Actions at sites other than hD(2) receptors likely participate in their (contrasting) functional profiles.

Although serotonin 5-HT(1A) receptors couple to several Gi/o G-protein subtypes, little is known concerning their differential activation patterns. In this study, in membranes of Chinese hamster ovary cells expressing h5-hydroxytryptamine(1A) receptors (CHO-h5-HT(1A)), isotherms of 5-HT-stimulated guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) binding were biphasic, suggesting coupling to multiple G-protein subtypes. The high potency component was abolished by preincubation with an antibody recognizing Galpha(i3) subunits and was resistant to induction of [(35)S]GTPgammaS dissociation by unlabeled GTPgammaS, thus yielding a bell-shaped concentration-response isotherm. To directly investigate Galpha(i3) activation, we adopted an antibody-capture/scintillation proximity assay. 5-HT and other high-efficacy agonists yielded bell-shaped [(35)S]GTPgammaS binding isotherms, with peaks at nanomolar concentrations. As drug concentrations increased, Galpha(i3) stimulation progressively returned to basal values. In contrast, the partial agonists (-)-pindolol and 4-(benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) displayed sigmoidal stimulation isotherms, whereas spiperone and other inverse agonists sigmoidally inhibited [(35)S]GTPgammaS binding. Agonist-induced stimulation and inverse agonist-induced inhibition of Galpha(i3) activation were i) abolished by pretreatment of CHO-h5-HT(1A) cells with pertussis toxin; ii) reversed by the selective 5-HT(1A) antagonist (N-[2-[4-(2-methoxy-phenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclohexane-carboxamide) fumarate (WAY100,635), and iii) absent in nontransfected CHO cell membranes. 5-HT isotherms could be modified by altering sodium concentration; only stimulatory actions were observed at 300mM NaCl, whereas only inhibitory actions were seen at 10 mM NaCl. Furthermore, bell-shaped isotherms were not detected at short incubation times, suggesting time-dependent changes in receptor/Galpha(i3) coupling. Taken together, these data show that low but not high concentrations of high-efficacy 5-HT(1A) agonists direct receptor signaling to Galpha(i3). In contrast, partial agonists favor h5-HT(1A) receptor signaling to Galpha(i3) over a wide concentration range, whereas inverse agonists inhibit constitutive Galpha(i3) activation.

As determined by a guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding assay, which does not distinguish G protein subtypes, 5-hydroxytryptamine (5-HT) and 2(S)- 1-(6-chloro-5-fluoro-1H-indol-1-yl)-2-propanamine fumarate (Ro600175) behaved as full agonists at human 5-HT(2C) (h5-HT(2C)) receptors (VSV isoform) stably expressed in Chinese hamster ovary (CHO) cells, whereas 1-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), d-lysergic acid diethylamide (LSD), and lisuride exhibited partial agonist properties. After treatment with pertussis toxin to uncouple 5-HT(2C) receptors from Gi/Go but not Gq/11, DOI and LSD were as efficacious as 5-HT and Ro600175 in stimulating [(35)S]GTPgammaS binding, whereas lisuride still exhibited low efficacy (40%). Correspondingly, in a scintillation proximity assay employing specific antibodies against Gq/11, 5-HT, Ro600175, DOI, and LSD behaved as high-efficacy agonists, whereas lisuride showed efficacy of 36%. In contrast, when employing a specific antibody recognizing Gi(3), DOI and LSD were less efficacious (80 and 30%, respectively) than 5-HT and Ro600175, and lisuride was inactive. Agonist actions were specifically mediated by h5-HT(2C) receptors inasmuch as the selective 5-HT(2C) antagonist SB242,084 blocked [(35)S]GTPgammaS binding at both Gq/11 and Gi(3). Agonist potency for stimulation of Gi(3) was ~6- to 8-fold less than for Gq/11, indicating that the latter was preferentially engaged by h5-HT(2C) receptors. Inactivation of h5-HT(2C) receptors with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline did not modify the efficacy of 5-HT, Ro600175, and DOI at Gq/11, whereas their efficacies were substantially reduced at Gi(3), indicating a greater receptor reserve for the former. Finally, the preferential activation of Gq/11 versus Gi(3) by DOI, LSD, and lisuride was diminished in the presence of lower receptor number. In conclusion, h5-HT(2C) receptors couple to both Gq/11 and Gi(3) in CHO cells, and efficacy for G protein subtype activation is both ligand- and receptor reserve-dependent.

Reflecting its potent inhibition of serotonin (5-HT) reuptake (accompanying paper), S33005 blocked spontaneous tail-flicks induced by parachloroamphetamine in rats. This action was mimicked by the 5-HT reuptake inhibitor, citalopram, and the 5-HT/norepinephrine (NE) reuptake inhibitor, venlafaxine, whereas the preferential NE reuptake inhibitor, reboxetine, was inactive. Consistent with its less potent interaction with NE transporters, higher doses of S33005 attenuated induction of hypothermia by reserpine, an action mimicked by reboxetine and venlafaxine, whereas citalopram was ineffective. In mice, S33005 reduced immobility in forced-swim and tail-suspension procedures. It also inhibited marble-burying behavior and suppressed aggressive behavior between resident and intruder animals. In rats, S33005 generalized to a discriminative stimulus elicited by citalopram and attenuated hypnotic-sedative actions of the alpha2-adrenoceptor agonist, S18616. For these parameters, S33005 was a more potent agent (median, 1.2 mg/kg, s.c.) than venlafaxine, citalopram, reboxetine, or the tricyclic agent, clomipramine. Even at markedly higher doses (40.0-80.0 mg/kg, s.c.), S33005 little affected motor behavior. S33005 (10.0 mg/kg, s.c.) also increased responses in a learned helplessness paradigm in rats, whereas venlafaxine was ineffective. Finally, in a rat chronic mild-stress model, S33005 dose- (2.5-40.0 mg/kg) and time- (2-5 weeks) dependently enhanced sucrose consumption. Venlafaxine was likewise active in this procedure. In conclusion, in line with its inhibition of 5-HT and (less potently) NE reuptake, S33005 is active in a broad range of models suggestive of antidepressant activity. It exerts its actions more potently than venlafaxine and clomipramine, and its overall profile is distinct from those of citalopram and reboxetine.

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.

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.

S18616 ((S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4, 2'-(8'-chloro-1',2',3',4'-tetrahydronaphthalene)]) displayed high affinity at native rat alpha(2)-adrenoceptors (AR)s (pK(i), 9.8), native human (h)alpha(2A)-ARs (9.6), and cloned halpha(2A)- (9.5), halpha(2B)- (9.2), and halpha(2C)- (9.0) ARs. It showed 40-fold lower affinity for halpha(1A)-ARs (8.4) and >/=100-fold lower affinity for rat alpha(1)-ARs (7.1), halpha(1B)-ARs (7.7), halpha(1D)-ARs (7.6), imidazoline(1) (7.4), and imidazoline(2) (7.4) sites and >100-fold lower affinity for all other (>50) sites. At halpha(2A)-ARs, in guanosine-5'-O-(3-[(35)S]thio)triphosphate binding studies, S18616 was a potent (partial) agonist: log effective concentration (pEC(50)), 9.3/maximal effect, 51%. This observation was corroborated employing a halpha(2A)-Gi1alpha fusion protein/GTPase assay (9.0/40%) in which the actions of S18616 were blocked by pertussis toxin. Employing guanosine-5'-O-(3-[(35)S]thio)triphosphate binding assays, S18616 was also a partial agonist at halpha(2C)-ARs (8.2/63%) but a full agonist (8.4/124%) at halpha(2B)-ARs. At halpha(2A)-, halpha(2B)-, and halpha(2C)-ARs, the selective alpha(2)-AR antagonist, atipamezole, abolished the actions of S18616: pK(b) values of 9.1, 9. 1, and 9.4, respectively. As determined by depletion of membrane-bound [(3)H]phosphatidyl inositols, S18616 behaved as a (less potent) agonist (7.8/79%) at halpha(1A)-ARs, an action abolished by prazosin (pK(b), 8.9). Reflecting alpha(2)-AR agonist properties, S18616 potently (>/=1 microg/kg, s.c.) and dose dependently elicited hypothermia and antinociception (nine diverse models) in rodents. These actions were dose dependently inhibited by chemically diverse alpha(2)- versus alpha(1)-AR antagonists, atipamezole, idazoxan, RX821,002, and BRL44418 (a preferential alpha(2A)-AR ligand). In contrast, the actions of S18616 were unaffected by the alpha(1)-AR antagonists, ARC239 and prazosin (which preferentially block alpha(2B/2C)- versus alpha(2A)-ARs). Although the affinity of dexmedetomidine at alpha(2)-ARs was lower than S18616; it displayed a similar receptor and functional profile. Clonidine displayed lower efficacy than S18616, was substantially less potent, and had marked affinity for imidazoline(1) sites and alpha(1)-ARs. In conclusion, S18616 is a novel, selective, and highly potent agonist at alpha(2)-ARs.

Although antagonism of mesolimbic dopamine D(2) receptors by neuroleptics such as haloperidol attenuates positive symptoms of schizophrenia, a significant population of "resistant" patients fails to respond while negative and cognitive symptoms are little modified. Furthermore, concomitant blockade of striatal D(2) receptors is associated with extrapyramidal motor side effects. The superior "atypical" antipsychotic profile of clozapine appears to reside in its broad pattern of interaction with D(2) receptors and a diversity of other monoaminergic sites. In this regard, serotonergic mechanisms are of particular relevance both in view of their modulation of dopaminergic transmission and their key role in the control of mood, cognition, and motor behavior. While most attention has focused on potential advantages of preferential 5-HT(2A) versus D(2) receptor blockade, 5-HT(1A) receptors likewise represent a valid target for improved antipsychotic agents. In this regard, rather than selective agents, ligands interacting with both 5-HT(1A) and D(2) receptors appear of interest. A modest level of efficacy appears optimal, that is, sufficient to engage highly sensitive 5-HT(1A) autoreceptors while blocking their low-sensitivity postsynaptic counterparts. Such a profile may counter negative and cognitive symptoms, improve mood, diminish extrapyramidal 5-HT(1A) motor side effects, and, perhaps, enhance efficacy in refractory patients. Notably, "partial agonist" properties of clozapine at 5-HT(1A) receptors may contribute to its distinctive functional profile. However, notwithstanding this compelling body of experimental data, clinical studies of antipsychotics interacting with 5-HT(1A) receptors are required to establish their genuine pertinence to the-hopefully improved-treatment of schizophrenia.

This study evaluated the influence of receptor/G-protein (R:G) stoichiometry on constitutive activity and the efficacy of agonists, partial agonists, and inverse agonists at human (h) 5-hydroxytryphamine 1B (5-HT(1B)) receptors. Two Chinese hamster ovary cell lines were used; they expressed 8.5 versus 0.4 pmol h5-HT(1B) receptors/mg (determined by [(3)H]GR125,743 saturation analysis) and 3.0 versus 1.5 pmol receptor-activated G-proteins/mg [determined by guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) isotopic dilution], respectively. Thus, they displayed R:G ratios of approximately 3.0 (RGhigh) and approximately 0.3 (RGlow), respectively. In competition-binding experiments, the agonists, 5-HT and sumatriptan, displayed fewer high-affinity (HA)-binding sites and the partial agonists, BMS181, 101 and L775,606, displayed decreased affinity in RGhigh versus RGlow membranes. In contrast, the inverse agonists, SB224,289 and, to a lesser extent, methiothepin, showed increased affinity. In G-protein activation experiments, both basal and 5-HT-activated [(35)S]GTPgammaS binding were higher in RGhigh than in RGlow membranes. Constitutive activity (determined by inhibition of basal [(35)S]GTPgammaS binding with GTPgammaS in the absence of receptor ligands) was more pronounced in RGhigh versus RGlow membranes, as revealed by the >5-fold greater proportion of HA sites. Correspondingly, the negative efficacy of inverse agonists was strikingly augmented, inasmuch as they suppressed approximately two-thirds of HA [(35)S]GTPgammaS binding in RGhigh membranes, but only approximately one-third in RGlow membranes. Furthermore, the efficacy of partial agonists was greater at RGhigh versus RGlow membranes, as estimated by their ability to enhance [(35)S]GTPgammaS binding. In conclusion, an increase in R:G ratios at h5-HT(1B) receptors was associated with an increase in relative efficacy of partial agonists and, most notably, an increase in both constitutive G-protein activation and negative efficacy of inverse agonists.

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.

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 mitogen-activated protein kinase (MAPK) cascade is stimulated by both receptor tyrosine kinases and G protein-coupled receptors. We show that recombinant human dopamine D(3) receptors expressed in Chinese hamster ovary cells transiently activate MAPK via pertussis toxin-sensitive Gi and/or Go proteins. The involvement of D(3) receptors was confirmed by use of the D(3) agonists PD 128,907 and (+)-7-hydroxy-2-dipropylaminotetralin, which mimicked the response to dopamine (DA). Furthermore, haloperidol and the selective D(3) receptor antagonists S 14297 and GR 218,231 attenuated DA-induced MAPK activation; however, when tested alone, S 14297 weakly stimulated MAPK activity, suggesting partial agonist activity. The transduction mechanisms by which hD(3) receptors activate MAPK were explored with specific kinase inhibitors. Genistein and lavendustin A, inhibitors of tyrosine kinase activity, did not reduce DA-induced MAPK activation. In contrast, PD 98059, an inhibitor of MAPK kinase, and Ro 31-8220 and G 6983, inhibitors of protein kinase C (PKC), blocked DA-induced MAPK activation. However, MAPK activation was insensitive to PKC down-regulation by phorbol esters, indicating the involvement of an "atypical" PKC. Furthermore, MAPK activation involved phosphatidylinositol 3-kinase inasmuch as its inhibition by LY 294002 and wortmannin reduced DA-induced MAPK activation. In conclusion, this study demonstrates that stimulation of hD(3) receptors activates MAPK. This action is mediated via an atypical isoform of PKC, possibly involving cross-talk with products of phosphatidylinositol 3-kinase activation.

The novel benzodioxopyrrolidine (S-16924) displays a clozapine-like profile of interaction with multiple monoaminergic receptors, in addition to potent agonist activity at serotonin (5-HT)1A receptors. S-16924 (2.5 mg/kg i.p.) and clozapine (5.0 mg/kg i.p.) generated robust discriminative stimuli (DS) and displayed full mutual generalization. The D4 antagonists L-745,870 and S-18126, the D1/D5 antagonist SCH-39166, and the D3 antagonist S-14297 showed at most partial generalization to S-16924 and clozapine. The D2/D3 antagonist raclopride fully generalized to S-16924, but only partially generalized to clozapine. The 5-HT2A antagonist MDL-100, 907 fully generalized to S-16924 and two further 5-HT2A antagonists, fananserin and SR-46349, showed partial generalization. However, MDL-100,907, fananserin, and SR-46349 showed less pronounced generalization to clozapine. Similarly, the 5-HT2C antagonists SB-200,646 and SB-206,553 more markedly generalized to S-16924 than to clozapine. The 5-HT1A receptor agonist (+/-)-8-dihydroxy-2-(di-n-propylamino) tetralin generalized fully to S-16924 but not to clozapine. Full generalization was obtained to both S-16924 and clozapine for the clozapine congeners, olanzapine and quetiapine. In distinction, the benzisoxazole, risperidone, and the phenylindole, sertindole, weakly generalized to S-16924 and clozapine. However, the benzisoxazole ziprasidone, which possesses 5-HT1A agonist properties, generalized fully to S-16924 but not to clozapine. Finally, the muscarinic antagonist scopolamine generalized fully to clozapine and partially to S-16924. In conclusion, S-16924 and clozapine display both communalities and differences in their "compound" DS; this likely reflects their respective complex patterns of interaction with multiple monoaminergic receptors. Although no specific receptor was identified as underlying the clozapine DS, 5-HT1A agonist as well as D2 and 5-HT2A/2C antagonist properties contribute to the S-16924 DS.