Anxiolytic properties of agomelatine, an antidepressant with melatoninergic and serotonergic properties: role of 5-HT2C receptor blockade

RATIONALE

The novel antidepressant agent, agomelatine, behaves as an agonist at melatonin receptors and as an antagonist at serotonin (5-HT)(2C) receptors.

OBJECTIVES

To determine whether, by virtue of its antagonist properties at 5-HT(2C) receptors, agomelatine elicits anxiolytic properties in rats.

METHODS

Employing a combined neurochemical and behavioural approach, actions of agomelatine were compared to those of melatonin, the selective 5-HT(2C) receptor antagonist, SB243,213, and the benzodiazepine, clorazepate.

RESULTS

In unfamiliar pairs of rats exposed to a novel environment, agomelatine enhanced the time devoted to active social interaction, an action mimicked by clorazepate and by SB243,213. In a Vogel conflict procedure, agomelatine likewise displayed dose-dependent anxiolytic activity with a maximal effect comparable to clorazepate, and SB243,213 was similarly active in this procedure. In a plus-maze procedure in which clorazepate significantly enhanced percentage entries into open arms, agomelatine revealed only modest activity and SB243,213 was inactive. Further, like SB243,213, and in contrast to clorazepate, agomelatine did not suppress ultrasonic vocalizations emitted by rats re-exposed to an environment associated with an aversive stimulus. Whereas clorazepate reduced dialysate levels of 5-HT and noradrenaline in hippocampus and frontal cortex of freely moving rats, agomelatine did not affect extracellular levels of 5-HT and elevated those of noradrenaline. SB243,213 acted similarly to agomelatine. Melatonin, which did not modify extracellular levels of 5-HT or noradrenaline, was ineffective in all models of anxiolytic activity. Furthermore, the selective melatonin antagonist, S22153, did not modify anxiolytic properties of agomelatine in either the social interaction or the Vogel Conflict tests.

CONCLUSIONS

In contrast to melatonin, and reflecting blockade of 5-HT(2C) receptors, agomelatine is active in several models of anxiolytic properties in rodents. The anxiolytic profile of agomelatine differs from that of benzodiazepines from which it may also be distinguished by its contrasting influence on corticolimbic monoaminergic pathways.

Contrasting mechanisms of action and sensitivity to antipsychotics of phencyclidine versus amphetamine: importance of nucleus accumbens 5-HT2A sites for PCP-induced locomotion in the rat

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.

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.

Blockade of the discriminative stimulus effects of DOI by MDL 100,907 and the 'atypical' antipsychotics, clozapine and risperidone

In a drug discrimination paradigm, the 5-HT2A/2C receptor antagonists, ritanserin, ICI 169,369 (2-(2-dimethylaminoethylthio-3-phenylquinoline hydrochloride) and mianserin, and the preferential 5-HT2A receptor antagonist, ketanserin, antagonised the discriminative stimulus effects of the 5-HT2A/2C receptor agonist, DOI ((2,5-dimethoxy-4-iodohenyl)-2-aminopropan) (0.63 mg/kg i.p.). Effective dose50 (ED50) values were: 0.32, 0.39, 0.15 and 0.03 mg/kg s.c., respectively. While the novel, selective 5-HT2C receptor antagonist, SB 200,646 (N-(1-methyl-5-iodolyl)-N'-(3-pyridyl) urea hydrochloride) was inactive (10 mg/kg s.c. and 20 mg/kg p.o.), the highly selective 5-HT2A receptor antagonist, MDL 100,907 (R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl]-4- piperidine-methanol) very potently (ED50 = 0.0006) abolished the action of DOI. MDL 100,907 may display antipsychotic properties and the 'atypical' antipsychotics, clozapine, risperidone and sertindole, each of which possesses marked affinity at 5-HT2A receptors, abolished the discriminative stimulus effects of DOI (ED50 values of 0.07, 0.03 and 0.33 mg/kg, respectively). In contrast, haloperidol (0.16) was ineffective. These data demonstrate that 5-HT2A receptors mediate the discriminative stimulus effects of DOI and support the hypothesis that an antagonistic action at 5-HT2A receptors contributes to the in vivo actions of 'atypical' antipsychotics.

S32006, a novel 5-HT2C receptor antagonist displaying broad-based antidepressant and anxiolytic properties in rodent models

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.

Induction of hyperlocomotion in mice exposed to a novel environment by inhibition of serotonin reuptake. A pharmacological characterization of diverse classes of antidepressant agents

This study characterized the influence of acute administration of diverse classes of antidepressant agent upon the spontaneous locomotor activity (LA) of mice in a novel, open-field environment. The selective serotonin (5-HT) reuptake inhibitors (SSRIs), citalopram, fluoxetine, paroxetine, fluvoxamine, litoxetine and zimelidine, dose-dependently enhanced LA. Their actions were mimicked by the mixed 5-HT/noradrenaline (NA) reuptake inhibitors (SNRIs), venlafaxine, duloxetine and S33005. In contrast, clomipramine only slightly elevated LA and two further tricyclics, imipramine and amitriptyline, were inactive. Further, the selective NA vs. 5-HT reuptake inhibitors (NARIs), reboxetine, desipramine, maprotiline, nisoxetine and nortriptyline all failed to increase LA. The "atypical antidepressants," mianserin and mirtazapine, neither of which modify 5-HT reuptake, as well as the mixed SSRI/5-HT(2) antagonists, nefazodone and trazodone, also failed to increase LA. Doses of SSRI and SNRI which increased LA did not modify motor performance in the rotarod test. Further, they did not enhance LA in rats, suggesting that this response is characteristic of mice. Finally, upon prehabituation of mice to the activity chamber, the SSRI, citalopram, and the SNRI, venlafaxine, failed to increase LA. In conclusion, in mice exposed to a novel environment, inhibition of 5-HT reuptake by SSRIs and SNRIs enhances spontaneous LA in the absence of a generalized influence upon motor function. This response provides a simple parameter for characterization of SSRIs and SNRIs, and differentiates them from other classes of antidepressant agent. Although an influence upon arousal and/or anxiety is likely related to the increase in LA, the functional significance of this response requires additional elucidation.

Anxiolytic properties of the selective, non-peptidergic CRF(1) antagonists, CP154,526 and DMP695: a comparison to other classes of anxiolytic agent

The selective, non-peptidergic corticotropin-releasing factor (CRF)(1) receptor antagonists, CP154,526 and DMP695, dose-dependently increased punished responses of rats in a Vogel conflict test and enhanced social interaction (SI) of rats in an unfamiliar environment. They were, however, inactive in a plus-maze procedure and failed to reduce ultrasonic vocalizations (USV) associated with an aversive environment. In contrast, the benzodiazepine, chlordiazepoxide, was effective in all these procedures. Further, the serotonin (5-HT)(1A) agonist, flesinoxan, was active in each paradigm (except the plus-maze) while the 5-HT(2C) antagonist, SB242,084, was effective in the SI and Vogel but not the plus-maze and USV procedures. In contrast to chlordiazepoxide, flesinoxan and SB242,084, CP154,526 did not modify dialysate levels of 5-HT, norepinephrine (NE) and dopamine (DA) in the frontal cortex (FCX) of freely moving rats. In conclusion, CP154,526 and DMP695 possess a common and distinctive profile of anxiolytic action expressed in the absence of an intrinsic influence upon monoamine release.

The role of dopamine D3 compared with D2 receptors in the control of locomotor activity: a combined behavioural and neurochemical analysis with novel, selective antagonists in rats

BACKGROUND

The role of dopamine D(3)/D(2) receptors in the control of locomotion is poorly understood.

OBJECTIVES

To examine the influence of selective antagonists at D(3) or D(2) receptors on locomotion in rats, alone and in interaction with the preferential D(3) versus D(2) receptor agonist, PD128,907.

METHODS

Affinities of ligands at rat D(2) and cloned, human hD(3), hD(2S), hD(2L) and hD(4) sites were determined by standard procedures. Locomotion was monitored automatically in rats pre-habituated for 30 min to an open-field environment. Extracellular levels of dopamine (DA) were determined by dialysis in the nucleus accumbens and striatum. Drugs were given acutely via the systemic route.

RESULTS

PD128,907, which preferentially recognised D(3) versus D(2) sites, biphasically reduced and enhanced locomotion at "low" (0.01-0.63 mg/kg) and "high" (2.5-10 mg/kg) doses, respectively. L741,626 and S23199, which behaved as preferential D(2) versus D(3) receptor antagonists, enhanced the reduction in locomotion evoked by the low dose of PD128,907, blocked the increase provoked by the high dose and suppressed spontaneous locomotion alone. Analogous findings were obtained with haloperidol and raclopride which showed equilibrated affinity at D(2) and D(3) receptors. UH232 and AJ76, which showed a mild preference for D(3) versus D(2) sites, did not modify the effect of a low dose of PD128,907, slightly enhanced the hyperlocomotion elicited by the high dose and exerted little influence on locomotion alone. S14297 and U99194, which acted as preferential D(3) versus D(2) receptor antagonists, abolished the reduction in locomotion elicited by a low dose of PD128,907, potentiated the induction of locomotion by a high dose, and failed to influence locomotion alone. The actions of S14297 were stereoselective inasmuch as they were mimicked by the racemic form, S11566, but not by the inactive enantiomer, S17777. In contrast to S14297, S11566 and U99194, however, S33084, SB269,652, GR218,231 and N-[-4-['-(1-naphtyl)piperazine-1-yl]butyl] anthracene-2-carboxamide ("NGB-1"), highly selective D(3) versus D(2) receptor antagonists, were inactive under all conditions. PD128,907 (0.01-10.0 mg/kg) suppressed dialysate levels of DA in the nucleus accumbens and striatum, actions blocked by L741,626 and haloperidol, yet unaffected by S14297 and S33084.

CONCLUSIONS

The facilitatory influence of a "high" dose of PD128,907 upon locomotion is mediated by postsynaptic D(2) receptors and, possibly, countered by their D(3) counterparts. Correspondingly, selective blockade of D(2) but not of D(3) receptors alone suppresses motor function. The reduction in locomotion provoked by a "low" dose of PD128,907 may be mediated by D(2) autoreceptors, but a role of postsynaptic D(3) receptors cannot be excluded. Finally, mechanisms underlying the contrasting influence of chemically diverse D(3) receptor antagonists upon locomotion remain to be elucidated.

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), a preferential dopamine D3 versus D2 receptor antagonist and potential antipsychotic agent: III. Actions in models of therapeutic activity and induction of side effects

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.

Chronic restraint stress impairs T-cell immunity and promotes tumor progression in mice

Long-term exposure to stressful situations can affect the immune system. The T-cell response is an important component of anti-tumoral immunity. Hence, impairment of the immune function induced by a chronic stressor has been postulated to alter the immunosurveillance of tumors, thus leading to a worse neoplastic prognosis. Here, we show that chronic restraint stress affects T-cell mediated immunity in mice. This was evidenced by a decrease of mitogen-induced T-cell proliferation, a reduction in CD4(+)T lymphocyte number and a decrease of tumor necrosis factor-alpha (TNF-alpha) and Interferon-gamma (IFN-gamma) production in stressed mice. Additionally, mice subjected to chronic restraint stress displayed an enhancement of tumor growth in a syngeneic lymphoma model, i.e. an increase of tumor proliferation and a reduction of animal survival. Finally, stressed mice had a reduced specific cytotoxic response against these tumor cells. These results suggest that chronic exposure to stress promotes cancer establishment and subsequent progression, probably by depressing T-cell mediated immunity. The T-cell immunity impairment as well as the tumor progression enhancement emphasize the importance of the therapeutic management of stress to improve the prognosis of cancer patients.

The selective serotonin (5-HT)1A receptor ligand, S15535, displays anxiolytic-like effects in the social interaction and Vogel models and suppresses dialysate levels of 5-HT in the dorsal hippocampus of freely-moving rats. A comparison with other anxiolytic agents

RATIONALE

The benzodioxane, S15535, possesses low intrinsic activity and marked selectivity at 5-HT1A receptors, hippocampal populations of which are implicated in anxious states.

OBJECTIVE

Herein, we examined its potential anxiolytic actions in relation to its influence upon extracellular levels of 5-HT in the dorsal hippocampus of freely-moving rats. Its effects were compared with those of other anxiolytic agents: the 5-HT1A agonists, buspirone and 8-hydroxy-2-(di-n-propylamino)-tetralin HBr (8-OH-DPAT), the 5-HT2C antagonist, SB206,553 and the benzodiazepine, diazepam.

METHODS

Potential anxiolytic actions were evaluated in the Vogel conflict paradigm (increase in punished responses) and the social interaction (SI) test (increase in active SI) in rats. Extracellular levels of 5-HT were determined by microdialysis.

RESULTS

In analogy to diazepam. S15535 increased punished responses in the Vogel test. This action was dose dependently expressed over a broad (16-fold) dose range. Buspirone and 8-OH-DPAT were likewise active, but yielded highly biphasic dose-response curves. SB206,553 was dose dependently active in this model. In the SI test, S15535 similarly mimicked the anxiolytic-like effect of diazepam and was active over a broad dose range. Buspirone and 8-OH-DPAT again showed biphasic dose-response curves, as did SB206,553. In both the Vogel and SI tests, the anxiolytic-like effects of S15535 were abolished by the selective 5-HT1A receptor antagonist, WAY100,635, which was inactive alone. S15535 exerted its anxiolytic-like effects with a more pronounced separation to motor-disruptive doses than the other drugs. Finally, S15535 suppressed dialysate levels of 5-HT in the dorsal hippocampus, an action abolished by WAY100,635. Buspirone, 8-OH-DPAT and diazepam, but not SB206,553, also reduced 5-HT levels.

CONCLUSION

Likely reflecting its distinctive ability to selectively and preferentially activate pre- versus postsynaptic 5-HT1A receptors, S15535 suppresses hippocampal 5-HT release and displays marked anxiolytic-like effects over a broad dose range in the relative absence of motor perturbation.

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: III. Actions in models of potential antidepressive and anxiolytic activity in comparison with ropinirole

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.

S 15535: a highly selective benzodioxopiperazine 5-HT1A receptor ligand which acts as an agonist and an antagonist at presynaptic and postsynaptic sites respectively

The novel benzodioxopiperazine, S 15535 (4-(benzodioxan-5-yl)1-(indan-2- yl)piperazine), displayed high affinity for 5-HT1A binding sites (1.8 nM) whereas its affinity was 100-fold lower at other 5-HT receptor types, at alpha 1, alpha 2- and beta-adrenoceptors and at dopamine D1 and D2 receptors. In vivo, S 15535 (0.16-10 mg/kg s.c.) acted as an antagonist at postsynaptic 5-HT1A receptors in completely blocking the flat-body posture and hypothermia elicited by the 5-HT1A receptor agonist, 8-OH-DPAT. It had no effect when applied alone. At presynaptic 5-HT1A receptors, S 15535 acted as an agonist in inhibiting striatal accumulation of 5-hydroxytryptophan (0.04-0.63 mg/kg s.c.) and in spiperone reversibly reducing electrical activity of the dorsal raphe nucleus (0.004-0.031 mg/kg i.v.). At doses up to 40.0 mg/kg s.c., S 15535 neither inhibited methylphenidate-induced gnawing nor elicited ptosis suggesting a lack of antagonist properties at, respectively, dopamine D2 receptors and alpha 1-adrenoceptors. In conclusion, S 15535 is a potent 5-HT1A ligand which acts, in vivo, as a highly selective agonist and antagonist at presynaptic and postsynaptic 5-HT1A receptors, respectively.

Blockade of serotonin 5-HT1B and 5-HT2A receptors suppresses the induction of locomotor activity by 5-HT reuptake inhibitors, citalopram and fluvoxamine, in NMRI mice exposed to a novel environment: a comparison to other 5-HT receptor subtypes

RATIONALE

Though 5-HT plays an important role in the modulation of motor function, which is perturbed in depressive states, little is known concerning the influence of serotonin reuptake inhibitors (SSRIs) on locomotor activity (LA). Recently, we demonstrated that SSRIs, such as citalopram, enhance LA in mice exposed to a novel environment.

OBJECTIVES

This study examined the role of multiple classes of 5-HT receptor in citalopram-induced LA.

METHODS

The most selective antagonists currently available were used.

RESULTS

Citalopram-induced LA was dose-dependently attenuated by the 5-HT1B/1D receptor antagonists, S18127, GR125,743 and GR127,935, and by the selective 5-HT1B antagonist, SB224,289, but unaffected by the selective 5-HT1A antagonist, WAY100,635. The selective antagonists at 5-HT2A receptors, MDL100,907 and SR46,349 also dose-dependently attenuated induction of locomotion by citalopram, whereas the 5-HT2B antagonist, SB204,741, and the 5-HT2B/2C antagonist, SB206,553 were ineffective. Further, the selective 5-HT2C antagonist, SB242,084, potentiated the response to citalopram. Selective antagonists at 5-HT3 (ondansetron), 5-HT4 (GR125,487), 5-HT6 (SB271,046) and 5-HT7 (SB269,970) receptors did not significantly modify the action of citalopram. Underpinning these findings, SB224,289, GR125,743, MDL100,907 and SR46,349 likewise attenuated induction of locomotion by a further SSRI, fluvoxamine.

CONCLUSIONS

The locomotor response to SSRIs of mice exposed to a novel environment is mediated via 5-HT1B and 5-HT2A receptors. In view of the importance of motor function to the etiology and treatment of depression, the significance of these observations to the clinical actions of SSRIs will be of interest to elucidate.

S32504, a novel naphtoxazine agonist at dopamine D3/D2 receptors: II. Actions in rodent, primate, and cellular models of antiparkinsonian activity in comparison to ropinirole

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.

Stereospecific blockade of marble-burying behaviour in mice by selective, non-peptidergic neurokinin1 (NK1) receptor antagonists

By analogy with the selective serotonin reuptake inhibitor, fluvoxamine, and the tricyclic agent, clomipramine, the novel, selective, non-peptidergic NK(1) receptor antagonist, GR205,171, dose-dependently and completely blocked marble-burying behaviour in mice: Inhibitory Dose(50)s (ID(50)s), 4.5, 4.8 and 7.6 mg/kg, respectively. In contrast to GR205,171, its isomer, GR226,206, which displays substantially lower affinity for NK(1) receptors, was inactive (> 40.0 mg/kg). By analogy with GR205,171, a further, selective NK(1) antagonist, RP67,580, abolished marble-burying behaviour with an ID(50) of 11.9 mg/kg. At doses significantly reducing marble-burying behaviour, GR205,171 and RP67,580 little influenced motor behaviour. In conclusion, like fluvoxamine and clomipramine, selective, non-peptidergic NK(1) receptor antagonists block marble-burying in mice. Although the biological bases of this behaviour remain unclear, these observations underpin the contention that NK(1) receptors may be implicated in affective disorders.

Antidepressant-like properties of the anti-Parkinson agent, piribedil, in rodents: mediation by dopamine D2 receptors

The dopamine D2/D3 receptor agonist and alpha2 adrenergic receptor antagonist, piribedil, is used clinically as monotherapy and as an adjunct to L-3,4-dihydroxyphenylalanine in the treatment of Parkinson's disease. As it appears to improve mood, we examined its actions in rodent models of antidepressant properties, in comparison with the prototypical anti-Parkinson agent, apomorphine, the D2/D3 receptor agonist, quinpirole, and the antidepressants, imipramine and fluvoxamine. In the mouse forced-swim test, acute administration of imipramine, fluvoxamine, apomorphine or quinpirole decreased immobility time, actions dose dependently mimicked by piribedil (2.5-10.0 mg/kg, subcutaneously). In rats, acute and subchronic administration of piribedil similarly reduced immobility (0.63-10.0 mg/kg, subcutaneously) and apomorphine, quinpirole and imipramine were also active in this test, whereas fluvoxamine was inactive. Both in mice and in rats, the D2/D3 receptor antagonist, raclopride, and the D2 receptor antagonist, L741,626, dose dependently blocked the antidepressant properties of piribedil, whereas the selective D3 receptor antagonists, S33084 and SB277,011, were ineffective. In a chronic mild stress model in rats, piribedil (2.5-40.0 mg/kg, subcutaneously) restored sucrose intake in stressed animals exerting its actions more rapidly (by week 1) than imipramine. Imipramine, fluvoxamine, apomorphine, quinpirole and piribedil dose dependently (0.63-10.0 mg/kg, subcutaneously) suppressed aggressive and marble-burying behaviour in mice. In the latter procedure, raclopride and L741,626, but not S33084, attenuated the actions of piribedil. Over a dose range (0.63-10.0 mg/kg, subcutaneously) equivalent to those active in models of antidepressant activity, piribedil did not stimulate locomotor behaviour. In conclusion, principally via recruitment of D2 receptors, piribedil exerts robust and specific antidepressant-like actions in diverse rodent models.

S33138 [N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]-benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenylacetamide], a preferential dopamine D3 versus D2 receptor antagonist and potential antipsychotic agent. II. A neurochemical, electrophysiological and behavioral characterization in vivo

The novel benzopyranopyrrolidine, S33138 [N-[4-[2-[(3aS,9bR)-8-cyano-1,3a,4,9b-tetrahydro[1]benzopyrano[3,4-c]pyrrol-2(3H)-yl)-ethyl]phenylacetamide], is a preferential antagonist of cloned human D(3) versus D(2L) and D(2S) receptors. In mice, S33138 (0.04-2.5 mg/kg i.p.) increased levels of mRNA encoding c-fos in D(3) receptor-rich Isles of Calleja and nucleus accumbens more potently than in D(2) receptor-rich striatum. Furthermore, chronic (3 weeks) administration of S33138 to rats reduced the number of spontaneously active dopaminergic neurones in the ventral tegmental area (0.16-10.0 p.o.) more potently than in the substantia nigra (10.0). In primates treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, antiparkinson actions of the D(3)/D(2) agonist, ropinirole, were potentiated by low doses of S33138 (0.01-0.16 p.o.) but diminished by a high dose (2.5). Consistent with antagonism of postsynaptic D(3)/D(2) sites, S33138 attenuated hypothermia and yawns elicited by the D(3)/D(2) agonist 7-OH-DPAT [(+)-7-dihydroxy-2-(di-n-propylamino)-tetralin] in rats, and it blocked (0.01-0.63, s.c.) discriminative properties of PD128,907 [(+)-(4aR,10bR)-3,4, 4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol; trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolininecarboxamide]. Suggesting antagonist properties at D(3)/D(2) autoreceptors, S33138 prevented (0.16-2.5 s.c.) the inhibitory influence of PD128,907 upon dopamine release in frontal cortex, nucleus accumbens, and striatum and abolished (0.004-0.25 i.v.) its inhibition of ventral tegmental dopaminergic neuron firing. At higher doses, antagonist actions of S33138 (0.5-4.0 i.v.) at alpha(2C)-adrenoceptors were revealed by an increased firing rate of adrenergic perikarya. Finally, antagonism of 5-hydroxytryptamine (5-HT(2A) and 5-HT(7)) receptors was shown by blockade of 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane-induced head twitches (0.63-10.0 s.c.) and 5-carboxytryptamine-induced hypothermia (2.5-20.0 i.p.), respectively. In conclusion, S33138 displays modest antagonist properties at central alpha(2C)-adrenoceptors, 5-HT(2A) and 5-HT(7) receptors. Furthermore, in line with its in vitro actions, it more potently blocks cerebral populations of D(3) versus D(2) receptors.

WAY 100,635 enhances both the 'antidepressant' actions of duloxetine and its influence on dialysate levels of serotonin in frontal cortex

The mixed serotonin and noradrenaline reuptake inhibitor, duloxetine, (5.0 mg/kg, s.c.), increased levels of serotonin (220%), dopamine (180%) and noradrenaline (470%) in individual dialysates of frontal cortex of freely moving rats. Its influence on serotonin, but not dopamine or noradrenaline, levels was enhanced by the 5-HT1A receptor antagonist, WAY 100,635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide 3HCl) (0.16 mg kg(-1), s.c). In the forced swimming test, although duloxetine was inactive alone, it dose dependently reduced immobility in the presence of WAY 100,635. Thus, blockade of 5-HT1A (auto)receptors selectively facilitates the influence of duloxetine on serotonin levels in the frontal cortex in rats and, in the forced swimming model, enhances its 'antidepressant' properties in parallel.

The dopamine D3 receptor antagonist, (+)-S 14297, blocks the cataleptic properties of haloperidol in rats

In contrast to haloperidol, the selective dopamine D3 receptor antagonist, (+)-S 11566 [(+/-)-[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro- naphtho(2,3b)dihydro,2,3-furanel]] and its active isomer, (+)-S 14297, induced neither catalepsy nor reduced conditioned avoidance responses in rats. (+)-S 11566 and (+)-S 14297 did, however, dose-dependently abolish the cataleptic actions of haloperidol. This action was expressed stereospecifically inasmuch as (-)-S 17777, the inactive distomer of (+)-S 14297, was ineffective. Further, the influence of haloperidol upon conditioned avoidance responses was not affected by (+)-S 14297. These data suggest that blockade of dopamine D3 receptors may inhibit the extrapyramidal but not-as based on the conditioned avoidance response paradigm-antipsychotic actions of neuroleptics.

Characterization of potent and selective antagonists at postsynaptic 5-HT1A receptors in a series of N4-substituted arylpiperazines

Benzocycloalkyl and benzocycloalkenyl moities linked, directly or via an alkyl chain, to oxygen-bearing heteroarylpiperazines were synthesized, in an attempt to obtain potent and selective antagonists at postsynaptic 5-HT1A receptors. From the numerous arylpiperazines described in the literature, 1-(2,3-dihydro-1,4-benzodioxin-5-yl)piperazine (3a) was chosen as a model of an arylpiperazine in view of its selectivity for 5-HT1A receptors versus alpha 1-, alpha 2-, and beta-adrenergic receptors, as well as dopamine D1 and D2 receptors. Two other closely-related arylpiperazines, 1-(1,5-benzodioxepin-6-yl)piperazine (3b) and 1-(benzofuran-7-yl)piperazine (3c), were also examined in this study. All compounds showed high affinity at 5-HT1A sites (8.10 < or = pKis < or = 9.35), and the majority behaved as antagonists in vivo in blocking the hypothermia induced by the 5-HT1A agonist 8-OH-DPAT in the absence of a marked effect alone at equivalent doses. An in vivo evaluation of dopamine D2 receptor antagonist properties revealed that the majority of compounds was devoid of activity at this site, in marked contrast to BMY 7378 which displayed virtually no selectivity for 5-HT1A versus dopamine D2 receptors. Moreover, six compounds of the present series, 8, 10, 11, 14, 25, and 37, showed > 10-fold selectivity in vitro for 5-HT1A versus alpha 1-adrenergic receptors. Compound 14 displayed an optimal compromise between potency (pKi = 8.75), marked antagonist activity, and selectivity toward alpha 1-adrenergic (81-fold) and dopamine D2 (195-fold) receptors. These characteristics clearly distinguish 14 from previously-reported ligands such as the postsynaptic 5-HT1A antagonist BMY 7378 and the weak partial agonist NAN 190 which, in contrast to the compounds of this series, belong to the well-exemplified class of imido derivatives of (o-methoxyphenyl)piperazines. The availability of 14 (S 15535) should facilitate the further elucidation of the functional role and potential therapeutic significance of 5-HT1A receptors.

S32212, a novel serotonin type 2C receptor inverse agonist/α2-adrenoceptor antagonist and potential antidepressant: II. A behavioral, neurochemical, and electrophysiological characterization

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.

Dopamine D2 receptor-mediated G-protein activation in rat striatum: functional autoradiography and influence of unilateral 6-hydroxydopamine lesions of the substantia nigra

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.