Serotonin (5-HT)2C receptors tonically inhibit dopamine (DA) and noradrenaline (NA), but not 5-HT, release in the frontal cortex in vivo

Effect of genetic and pharmacological blockade of GABA receptors on the 5-HT2C receptor function during stress

Serotonin (5-HT)2C receptors play a role in psychoaffective disorders and often contribute to the antidepressant and anxiolytic effects of psychotropic drugs. During stress, activation of these receptors exerts a negative feedback on 5-HT release, probably by increasing the activity of GABAergic interneurons. However, to date, the GABA receptor types that mediate the 5-HT2C receptor-induced feedback inhibition are still unknown. To address this question, we assessed the inhibition of 5-HT turnover by a 5-HT2C receptor agonist (RO 60-0175) at the hippocampal level and under conditions of stress, after pharmacological or genetic inactivation of either GABA-A or GABA-B receptors in mice. Neither the GABA-B receptor antagonist phaclofen nor the specific genetic ablation of either GABA-B1a or GABA-B1b subunits altered the inhibitory effect of RO 60-0175, although 5-HT turnover was markedly decreased in GABA-B1a knock-out mice in both basal and stress conditions. In contrast, the 5-HT2C receptor-mediated inhibition of 5-HT turnover was reduced by the GABA-A receptor antagonist bicuculline. However, a significant effect of 5-HT2C receptor activation persisted in mutant mice deficient in the α3 subunit of GABA-A receptors. It can be inferred that non-α3 subunit-containing GABA-A receptors, but not GABA-B receptors, mediate the 5-HT2C -induced inhibition of stress-induced increase in hippocampal 5-HT turnover in mice.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

Effects of 5-hydroxytryptamine 2C receptor agonist MK212 and 2A receptor antagonist MDL100907 on maternal behavior in postpartum female rats

Maternal behavior in rats is a highly motivated and well-organized social behavior. Given the known roles of serotonin (5-HT) in emotion, motivation, social behavior, and major depression - and its known interaction with dopamine - it is likely that serotonin also plays a crucial role in this behavior. So far, there are surprisingly few studies focusing on 5-HT in maternal behavior, except for maternal aggression. In the present study, we examined the effects of 5-HT2C receptor agonism and 5-HT2A receptor antagonism on maternal behavior in postpartum female rats. We hypothesized that activation of 5-HT2C receptors and blockade of 5-HT2A receptors would produce a functionally equivalent disruption of maternal behavior because these two receptor subtypes often exert opposite effects on various brain functions and psychological processes relevant to rat maternal behavior. On postpartum Days 5, 7, and 9, Sprague-Dawley mother rats were given a single injection of 0.9% NaCl solution, the 5-HT2C agonist MK212 (0.5, 1.0 or 2.0 mg/kg, ip), or the 5-HT2A antagonist MDL100907 (0.05, 0.5 or 2.0 mg/kg, ip). Maternal behavior was tested 30 min before and 30 min, 120 min, 240 min after injection. Acute injection of MK212 significantly disrupted pup retrieval, pup licking, pup nursing, and nest building in a dose-dependent fashion. At the tested doses, MDL100907 had little effect on various components of rat maternal behavior. Across the 3 days of testing, no apparent sensitization or tolerance associated with repeated administration of MK212 and MDL100907 was found. We concluded that rat maternal performance is critically dependent on 5-HT2C receptors, while the role of 5-HT2A receptors is still inconclusive. Possible behavioral mechanisms of actions of 5-HT2C receptor in maternal behavior are discussed.

Dissociable effects of 5-HT2C receptor antagonism and genetic inactivation on perseverance and learned non-reward in an egocentric spatial reversal task

Cognitive flexibility can be assessed in reversal learning tests, which are sensitive to modulation of 5-HT2C receptor (5-HT2CR) function. Successful performance in these tests depends on at least two dissociable cognitive mechanisms which may separately dissipate associations of previous positive and negative valence. The first is opposed by perseverance and the second by learned non-reward. The current experiments explored the effect of reducing function of the 5-HT2CR on the cognitive mechanisms underlying egocentric reversal learning in the mouse. Experiment 1 used the 5-HT2CR antagonist SB242084 (0.5 mg/kg) in a between-groups serial design and Experiment 2 used 5-HT2CR KO mice in a repeated measures design. Animals initially learned to discriminate between two egocentric turning directions, only one of which was food rewarded (denoted CS+, CS-), in a T- or Y-maze configuration. This was followed by three conditions; (1) Full reversal, where contingencies reversed; (2) Perseverance, where the previous CS+ became CS- and the previous CS- was replaced by a novel CS+; (3) Learned non-reward, where the previous CS- became CS+ and the previous CS+ was replaced by a novel CS-. SB242084 reduced perseverance, observed as a decrease in trials and incorrect responses to criterion, but increased learned non-reward, observed as an increase in trials to criterion. In contrast, 5-HT2CR KO mice showed increased perseverance. 5-HT2CR KO mice also showed retarded egocentric discrimination learning. Neither manipulation of 5-HT2CR function affected performance in the full reversal test. These results are unlikely to be accounted for by increased novelty attraction, as SB242084 failed to affect performance in an unrewarded novelty task. In conclusion, acute 5-HT2CR antagonism and constitutive loss of the 5-HT2CR have opposing effects on perseverance in egocentric reversal learning in mice. It is likely that this difference reflects the broader impact of 5HT2CR loss on the development and maintenance of cognitive function.

Involvement of serotonin 5-HT3 receptors in the modulation of noradrenergic transmission by serotonin reuptake inhibitors: a microdialysis study in rat brain

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs), in addition to being able to enhance serotonergic neurotransmission, are able to modulate other brain systems involved in depression.

OBJECTIVES

This study evaluates the neurochemical effect of the SSRI citalopram on brain noradrenergic activity and the serotonin receptor involved in this effect.

METHODS

Dual-probe microdialysis in the locus coeruleus (LC) and prefrontal cortex (PFC) was performed in freely awake rats.

RESULTS

Systemic citalopram (10 mg/kg, i.p.) increased noradrenaline (NA) in the LC (E max = 141 ± 13%) and simultaneously decreased NA in the PFC (Emax = -46 ± 7%). In the local presence into the LC of the α2-adrenoceptor antagonist RS79948 (1 μM), systemic citalopram increased NA in the LC (Emax = 157 ± 25%) and PFC (Emax = 175 ± 24%). Local citalopram (0.1-100 μM) into the LC induced NA increase in the LC (Emax = 210 ± 25%) and decrease in the PFC (Emax = -38 ± 9%). Local LC citalopram effect was abolished by LC presence of the 5-HT3 receptor antagonist MDL72222 (1 μM) but not the 5-HT1/2 receptor antagonist methiothepin (1 μM). Systemic citalopram in the LC presence of MDL72222 did not modify NA in the LC but increased NA in the PFC (Emax = 158 ± 26%). Local citalopram into the PFC enhanced NA (Emax = 376 ± 18%) in the area, which was prevented by MDL72222.

CONCLUSIONS

The SSRI citalopram modulates central noradrenergic neurotransmission by activation, through endogenous serotonin, of 5-HT3 receptors expressed in the somatodendritic (LC) and terminal (PFC) areas, which subsequently promote an enhancement of local NA. Therefore, 5-HT3 receptors and somatodendritic α2-adrenoceptors in the LC play an important role in the global effect of SSRIs.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

Evaluation of chemically diverse 5-HT₂c receptor agonists on behaviours motivated by food and nicotine and on side effect profiles

RATIONALE

Selective 5-HT2C receptor agonists, such as lorcaserin, are being developed for the treatment of obesity. Studies suggest that they may also have therapeutic potential for addictive behaviours including nicotine dependence, although few drugs of this class have been evaluated.

OBJECTIVES

The primary aim was to evaluate the highly selective 5-HT2C agonist, CP-809101, against food-motivated (operant FR5 and progressive ratio schedules, palatability-induced feeding) and nicotine-motivated (intravenous self-administration, drug discrimination) behaviours in rats and to compare with equivalent findings for the structurally distinct 5-HT2C receptor agonists lorcaserin and Ro 60-0175. The secondary aims were to evaluate the side effect profiles of lorcaserin and CP-809101 and to determine the plasma levels of lorcaserin at a dose (1 mg/kg) that reduces both food and nicotine reinforcement for comparison to plasma concentrations reported in human trials.

RESULTS

CP-809101 (0.3-3 mg/kg SC) reduced responding for both nicotine and food and blocked the discriminative stimulus properties of nicotine in a similar manner to lorcaserin and Ro 60-0175. Behaviours such as hypolocomotion, chewing and ptosis became evident following both CP-809101 and lorcaserin administration at higher doses. Plasma levels of lorcaserin were of similar range to those reported in obesity trials.

CONCLUSIONS

These studies support the utility of 5-HT2C agonists as a therapeutic approach to treat nicotine dependence. Plasma exposure levels after acute lorcaserin treatment suggest that equivalent dosages could be used to evaluate these drugs in obesity and smoking cessation trials. Finally, there may be differences in the side effect profiles between lorcaserin and CP-809101, raising the possibility for tolerability differences amongst 5-HT2C agonists.

Synthesis and evaluation of methylated arylazepine compounds for PET imaging of 5-HT(2c) receptors

The serotonin 5-HT(2c) receptor is implicated in a number of diseases including obesity, depression, anxiety, and schizophrenia. In order to ascribe the role of 5-HT(2c) in these diseases, a method for measuring 5-HT(2c )density and function in vivo, such as with positron emission tomography (PET), must be developed. Many high-affinity and relatively selective ligands exist for 5-HT(2c) but cannot be accessed with current radiosynthetic methods for use as PET radiotracers. We propose that N-methylation of an arylazepine moiety, a frequent structural feature in 5-HT(2c) ligands, may be a suitable method for producing new radiotracers for 5-HT(2c). The impact of N-methylation has not been previously reported. For the agonists that we selected herein, N-methylation was found to increase affinity up to 8-fold without impairing selectivity. Compound 5, an N-methylated azetidine-derived arylazepine, was found to be brain penetrant and reached a brain/blood ratio of 2.05:1. However, our initial test compound was rapidly metabolized within 20 min of administration and exhibited high nonspecific binding. N-Methylation, with 16 ± 3% isolated radiochemical yield (decay corrected), is robust and may facilitate screening other 5-HT(2c) ligands as radiotracers for PET.

Avoidance disruptive effect of clozapine and olanzapine is potentiated by increasing the test trials: further test of the motivational salience hypothesis

Antipsychotic drugs suppress animals' ability to avoid an aversive stimulus in the conditioned avoidance response model (CAR). This behavioral effect is thought to reflect antipsychotic activity and is suggested to be mediated by a drug's action in attenuating the motivational salience of a conditioned stimulus (CS). In the present study, we tested whether atypical antipsychotic drugs clozapine and olanzapine act through this behavioral mechanism by manipulating the number of avoidance test trials. We reasoned that more CS trials in the presence of clozapine or olanzapine would afford the drug more opportunities to decrease the motivational salience of the CS, thus avoidance decline would be greater with the increase of CS trials in each test session. In two separate experiments, adult male Sprague-Dawley rats were tested under clozapine (5.0mg/kg, sc), olanzapine (0.5mg/kg, sc) or vehicle (sterile water) for 6 consecutive days in three CS trial conditions (i.e. 3, 10, and 40 CS trials per session). Two days later, all rats were tested under the same 40-trial session after receiving clozapine (5.0mg/kg, sc) or olanzapine (0.5mg/kg, sc). Results show that repeated clozapine and olanzapine treatment persistently decreased avoidance response, and this effect was potentiated by the increase of number of CS trials in the test sessions, as the clozapine-treated or olanzapine-treated rats tested under the 40-trial or 10-trial condition had significantly lower avoidance and faster decline across-sessions than those tested under the 3-trial condition. This potentiated effect was not only seen in the total avoidance percentage, but also observed in the within-session decline pattern in the last three drug test sessions and in the final 40-trial test session. These findings suggest that the clinical efficacy of a drug can be enhanced by increasing the exposure of symptoms in the presence of the drug.

5-HT2 ligands in the treatment of anxiety and depression

INTRODUCTION

One third of depressed patients do not respond adequately to conventional antidepressants including the selective serotonin reuptake inhibitors (SSRIs). Therefore, multi-target drugs or augmentation strategies have been developed for the management of SSRIs-resistant patients. In this context, the 5-HT(2) receptor subtypes represent promising targets but their precise roles have yet to be determined.

AREAS COVERED

The aim of this review is to shed some light on the preclinical evidence supporting the use of 5-HT(2A) and/or 5-HT(2C) receptor antagonists such as antipsychotics, as potential effective adjuncts in SSRIs-resistant depression. This review synthesizes the current literature about the behavioral, electrophysiological and neurochemical effects of 5-HT(2) receptors ligands on the monoaminergic systems but also on adult hippocampal neurogenesis.

EXPERT OPINION

Although studies support the hypothesis that the inactivation of 5-HT(2A) and/or 5-HT(2C) receptors might be of interest to reinforce different facets of the therapeutic activity of SSRIs, this pharmacological strategy remains debatable notably because of the lack of chronic data in relevant animal models. Conversely, emerging evidence suggests that the activation of 5-HT(2B) receptor is required for antidepressant-like activity, opening the way to new therapeutic approaches. However, the potential risks related to the enhancement of monoaminergic neurotransmissions could represent a major concern.

Modulation of haloperidol-induced patterns of the transcription factor Nur77 and Nor-1 expression by serotonergic and adrenergic drugs in the mouse brain

Different patterns of expression of the transcription factors of Nur77 and Nor-1 are induced following acute administration of typical and atypical antipsychotic drugs. The pharmacological profile of atypical antipsychotics suggests that serotonergic and/or adrenergic receptors might contribute to these reported differences. In order to test this possibility, we examined the abilities of serotonin 5-HT(1A) and 5-HT(2A/2C), and α₁- and α₂-adrenergic receptor drugs to modify the pattern of Nur77 (NR4A1) and Nor-1 (NR4A3) mRNA expression induced by haloperidol. Various groups of mice were treated with either saline, DOI, a 5-HT(2A/2C) agonist, MDL11939, a 5-HT(2A) antagonist, 8-OH-DPAT, a 5-HT(1A) agonist, prazosin, an α₁-adrenergic antagonist and idazoxan, an α₂-adrenergic antagonist, alone or in combination with haloperidol. The 5-HT(2A/2C) agonist DOI alone significantly increased Nur77 expression in the medial striatum and nucleus accumbens. DOI reduced Nor-1 expression, while MDL11939 increased the expression of this transcript in the cortex. Prazosin reduced Nur77 expression in the dorsal striatum and nucleus accumbens. Interestingly, 8-OH-DPAT and MDL11939 partially prevented haloperidol-induced Nur77 up-regulation, while MDL11939 completely abolished Nor-1 expression in the striatum. In addition, MDL11939 decreased haloperidol-induced Nur77 and Nor-1 mRNA levels in the ventral tegmental area. On the contrary, idazoxan (α₂ antagonist) consistently potentiated haloperidol-induced Nur77, but not Nor-1 mRNA levels in the striatum, whereas prazosin (α₁ antagonist) remained without effect. Taken together, these results show the ability of a 5-HT(1A) agonist or a 5-HT(2A) antagonist to reduce haloperidol-induced Nur77 and Nor-1 striatal expression, suggesting that these serotonin receptor subtypes participate in the differential pattern of gene expression induced by typical and atypical antipsychotic drugs.

Sertraline delays relapse in recently abstinent cocaine-dependent patients with depressive symptoms

AIMS

Whether the selective serotonin re-uptake inhibitor sertraline at 200 mg/day delays relapse in recently abstinent cocaine-dependent individuals.

DESIGN

The study involved a 12-week, double-blind, placebo-controlled clinical trial with 2-week residential stay followed by 10-week out-patient participation.

SETTING

Veterans Affairs residential unit and out-patient treatment research program.

PARTICIPANTS

Cocaine-dependent volunteers (n = 86) with depressive symptoms (Hamilton score > 15), but otherwise no major psychiatric or medical disorder or contraindication to sertraline.

MEASUREMENTS

Participants were housed on a drug-free residential unit (weeks 1-2) and randomized to receive sertraline or placebo. Participants then participated on an out-patient basis during weeks 3-12 while continuing to receive study medication. Patients participated in a day substance abuse/day treatment program during weeks 1-3 and underwent weekly cognitive behavioral therapy during weeks 4-12. The primary outcome measure was thrice-weekly urine results and the secondary measure was Hamilton Depression scores.

FINDINGS

Pre-hoc analyses were performed on those who participated beyond week 2. Generally, no group differences in retention or baseline characteristics occurred. Sertraline patients showed a trend towards longer time before their first cocaine-positive urine ('lapse', χ(2) = 3.67, P = 0.056), went significantly longer before having two consecutive urine samples positive for cocaine ('relapse', χ(2) = 4.03, P = 0.04) and showed significantly more days to lapse (26.1 ± 16.7 versus 13.2 ± 10.5; Z = 2.89, P = 0.004) and relapse (21.3 ± 10.8 versus 32.3 ± 14.9; Z = 2.25, P = 0.02). Depression scores decreased over time (F = 43.43, P < 0.0001), but did not differ between groups (F = 0.09, P = 0.77).

CONCLUSIONS

Sertraline delays time to relapse relative to placebo in cocaine-dependent patients who initially achieve at least 2 weeks of abstinence.

Discovery of potential antipsychotic agents possessing pro-cognitive properties

Current antipsychotic drug therapies for schizophrenia have limited efficacy and are notably ineffective at addressing the cognitive deficits associated with this disorder. The present study was designed to develop effective antipsychotic agents that would also ameliorate the cognitive deficits associated with this disease. In vitro studies comprised of binding and functional assays were utilized to identify compounds with the receptor profile that could provide both antipsychotic and pro-cognitive features. Antipsychotic and cognitive models assessing in vivo activity of these compounds included locomotor activity assays and novel object recognition assays. We developed a series of potential antipsychotic agents with a novel receptor activity profile comprised of muscarinic M(1) receptor agonism in addition to dopamine D(2) antagonism and serotonin 5-HT(2A) inverse agonism. Like other antipsychotic agents, these compounds reverse both amphetamine and dizocilpine-induced hyperactivity in animals. In addition, unlike other antipsychotic drugs, these compounds demonstrate pro-cognitive actions in the novel object recognition assay. The dual attributes of antipsychotic and pro-cognitive actions distinguish these compounds from other antipsychotic drugs and suggest that these compounds are prototype molecules in the development of novel pro-cognitive antipsychotic agents.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Mirtazapine in combination with perospirone synergistically enhances dopamine release in the rat prefrontal cortex via 5-HT1A receptor activation

AIMS

The purpose of this study was to elucidate the mechanism underlying the clinical efficacy of mirtazapine-perospirone combination therapy for treatment-resistant depression in a rat model.

METHODS

We studied the effect of combination therapy of the noradrenergic and specific serotonergic antidepressant mirtazapine and the serotonin-dopamine antagonist perospirone on serotonin (5-HT) and dopamine release in the rat medial prefrontal cortex (mPFC) by using microdialysis. Male Wistar rats (250-330 g bodyweight) underwent implantation of a guide cannula in the mPFC, and a microdialysis probe was then inserted into the guide cannula to ensure its final placement in the mPFC. Microdialysis and subsequent chromatographic analysis were performed to estimate the extracellular 5-HT and dopamine concentrations.

RESULTS

When they were used individually, perospirone (0.25 mg/kg, intraperitoneally) and mirtazapine (4 mg/kg, intraperitoneally) induced increased dopamine release up to 134% and 190% relative to the pretreatment level, respectively. Their combination therapy synergistically and significantly (P < 0.0001) increased the dopamine concentration up to 397% of the pretreatment level compared with that induced by the individual drugs. This combination-induced dopamine release was attenuated by 5-HT(1A) antagonist WAY 100635 (0.5 mg/kg, intraperitoneally), to a peak dopamine release of 151%. Extracellular 5-HT release in the mPFC was not altered in any of the treatment groups.

CONCLUSIONS

The large increase in the dopamine concentration in the rat mPFC after the combination therapy was unique and may be responsible for the profound antidepressive effect observed in patients with treatment-resistant depression.

Determining the subjective and physiological effects of BZP combined with TFMPP in human males

RATIONALE

‘Party Pills’ containing benzylpiperazine (BZP) and trifluoromethylphenylpiperazine (TFMPP) have been used in a recreational context since the 1990s and, prior to April 2008, were legally available in New Zealand. Taken together, they have been reported to produce a ‘high’ similar to that produced by 3,4-methylenedioxymethamphetamine (MDMA).

OBJECTIVES

There has been little research on the subjective effects of piperazines in humans. The purpose of this study is to further investigate the subjective and physiological responses following an oral dose of BZP combined with TFMPP in males.

METHODS

In a randomised, double-blind, placebo-controlled study the subjective and physiological effects of BZP/TFMPP were investigated in 36 healthy, non-smoking males (mean age 22 ± 4 years). Participants were tested before and approximately 120 min after administration of a single dose of placebo (n = 16) or 100/30 mg BZP/TFMPP (n = 20). Participants were required to comment on the subjective effects using three rating scales—the Addiction Research Centre Inventory (ARCI), the Visual Analogue Scale (VAS) and the Profile of Mood States (POMS). Participants' blood pressure, heart rate and body temperature were also measured.

RESULTS

Statistical analysis using repeated-measures analysis of variance (ANOVA) and planned comparisons revealed that BZP/TFMPP significantly increases blood pressure and heart rate (p < 0.05). Likewise, the subjective rating scales revealed that BZP/TFMPP has significant dexamphetamine-like effects, increases dysphoria and feelings of self-confidence (p < 0.05).

CONCLUSION

These physiological and subjective data reflect clear similarities between the effects of BZP/TFMPP and commonly known stimulants such as dexamphetamine and MDMA.

REVIEW: Apathy diagnosis, assessment, and treatment in Alzheimer's disease

Apathy is defined as a disorder of motivation. There is wide acknowledgement that apathy is an important behavioral syndrome in Alzheimer's disease and in various neuropsychiatric disorders. In light of recent research and the renewed interest in the correlates and impacts of apathy and in its treatments, it is important to develop criteria for apathy that will be widely accepted, have clear operational steps, and be easy to apply in clinical practice and in research settings. Meeting these needs was the focus for a task force that included members of the European Psychiatric Association, the European Alzheimer's Disease Consortium and experts from Europe, Australia and North America.

The role of serotonin-2 (5-HT2) and dopamine receptors in the behavioral actions of the 5-HT2A/2C agonist, DOI, and putative 5-HT2C inverse agonist, SR46349B

RATIONALE

Atypical antipsychotic efficacy is often attributed to actions at serotonin-2 (5-HT(2)) and dopamine receptors, indicating a potential benefit of understanding the interplay between these systems. Currently, it is known that 5-HT(2) receptors modulate dopamine release, although the role of specific dopamine receptors in 5-HT(2)-mediated behavior is not well understood.

OBJECTIVES

We examined the role of 5-HT(2A), 5-HT(2C), and dopamine (D1 and D2) receptors in the behavioral response to a 5-HT(2A/2C) agonist (DOI) and 5-HT(2A/2C) antagonist (SR46349B).

MATERIALS AND METHODS

Effects were assessed by measuring rabbit head bobs (previously characterized as 5-HT(2A) receptor-mediated) and body shakes (5-HT(2C)-mediated).

RESULTS

As expected, DOI produced head bobs and body shakes, and these DOI-elicited behaviors were attenuated by the SR46349B pretreatment. Unexpectedly, SR46349B also induced head bobs when administered alone. However, SR46349B-elicited head bobs are distinguishable from those produced by DOI since the 5-HT(2A) antagonist, ketanserin, only attenuated DOI-elicited head bobs. Conversely, 5-HT(2C) ligands (SB242084 and SB206553) inhibited SR46349B but not DOI-induced head bobs. Furthermore, when administered alone, SB206553 (a 5-HT(2C) inverse agonist) produced head bobs, indicating the behavior can be either 5-HT(2A) or 5-HT(2C) mediated. Next, it was revealed that D1 and D2 receptors play a role in DOI-elicited head bobs, but only D1 receptors are required for SR46349B-elicited head bobs.

CONCLUSIONS

5-HT(2A) receptor agonism and 5-HT(2C) inverse agonism produce the same behavior, likely due to similar downstream actions at D1 receptors. Consequently, 5-HT(2C) agonism or D1 agonism may be effective therapies for disorders, such as schizophrenia, currently being treated with 5-HT(2A) antagonists.

Opponency revisited: competition and cooperation between dopamine and serotonin

Affective valence lies on a spectrum ranging from punishment to reward. The coding of such spectra in the brain almost always involves opponency between pairs of systems or structures. There is ample evidence for the role of dopamine in the appetitive half of this spectrum, but little agreement about the existence, nature, or role of putative aversive opponents such as serotonin. In this review, we consider the structure of opponency in terms of previous biases about the nature of the decision problems that animals face, the conflicts that may thus arise between Pavlovian and instrumental responses, and an additional spectrum joining invigoration to inhibition. We use this analysis to shed light on aspects of the role of serotonin and its interactions with dopamine.

5-HT(2A) and 5-HT(2C) receptor stimulation are differentially involved in the cortical dopamine efflux-Studied in 5-HT(2A) and 5-HT(2C) genetic mutant mice

Both 5-HT(2A) and 5-HT(2C) receptors modulate cortical dopamine efflux, but in opposite directions. We have now compared the ability of the three 5-HT(2A/2C) receptor agonists, DOI (R(-)-2,5-dimethoxy-4-iodoamphetamine), mCPP (meta-chlorophenylpiperazine) and MK-212 (6-Chloro-2-(piperazinyl) pyrazine), to modulate cortical dopamine efflux in 5-HT(2A) and 5-HT(2C) genetic mutant mice. In the 5-HT(2A) mice, the preferential 5-HT(2A) receptor agonist DOI (2.5mg/kg, s.c.) induced a slight but significant increase in cortical dopamine efflux only in the wild type (WT) mice; MK-212 (2.5mg/kg) reduced dopamine efflux in both WT and receptor knockout (KO) mice; moreover, MCPP, 2.5mg/kg, had no effect in either types. In 5-HT(2C) mice, DOI increased dopamine efflux in both types; while MK-212 decreased dopamine efflux in the WT, but not the receptor KO mice. These results provide new evidence that 5-HT(2A) receptor stimulation enhances and 5-HT(2C) receptor stimulation inhibits cortical dopamine efflux, and suggest the effects of DOI, MK-212 and mCPP on the cortical dopamine efflux are due to their different abilities on 5-HT(2A) and 5-HT(2C) receptors stimulation. Of these three agents, only DOI, the more selective 5-HT(2A) receptor agonist, is hallucinogenic. The absence of hallucinations with mCPP may be due to its relatively more potent 5-HT(2C) receptor agonist effect, inhibiting the ability of mCPP to enhance dopamine efflux in cortical and perhaps limbic regions as well. The present data provide additional evidence that hallucinations are due, in part, to 5-HT(2A) rather than 5-HT(2C) receptor stimulation. These findings suggest that 5-HT(2C) receptor agonists may be useful as antipsychotics, consistent with previous suggestions.

A new strategy for antidepressant prescription

From our research and literature search we propose an understanding of the mechanism of action of antidepressants treatments (ADTs) that should lead to increase efficacy and tolerance. We understand that ADTs promote synaptic plasticity and neurogenesis. This promotion is linked with stimulation of dopaminergic receptors. Previous evidence shows that all ADTs (chemical, electroconvulsive therapy, repetitive transcranial magnetic stimulation, sleep deprivation) increase at least one monoamine neurotransmitter serotonin (5-HT), noradrenaline (NA) or dopamine (DA); this article focuses on DA release or turn-over in the frontal cortex. DA increased dopaminergic activation promotes synaptic plasticity with an inverted U shape dose–response curve. Specific interaction between DA and glutamate is mediated by D1 receptor subtypes and Glutamate (NMDA) receptors with neurotrophic factors likely to play a modulatory role. With the understanding that all ADTs have a common, final, DA-ergic stimulation that promotes synaptic plasticity we can predict that (1) AD efficiency is related to the compound strength for inducing DA-ergic stimulation. (2) ADT efficiency presents a therapeutic window that coincides with the inverted U shape DA response curve. (3) ADT delay of action is related to a “synaptogenesis and neurogenesis delay of action.” (4) The minimum efficient dose can be found by starting at a low dosage and increasing up to the patient response. (5) An increased tolerance requires a concomitant prescription of a few ADTs, with different or opposite adverse effects, at a very low dose. (6) ADTs could improve all diseases with cognitive impairments and synaptic depression by increasing synaptic plasticity and neurogenesis.

Evidence that the reward attenuating effect of the D1-like antagonist, SCH-23390, is not mediated by its agonist action at the 5-HT2c receptors

This study investigated the effect of the 5-HT2c receptor antagonist, SB-242,084, on the attenuation of brain stimulation reward by SCH-23390. An additional experiment determined the effectiveness of SB-242,084 at blocking the reward attenuating effect of 5-HT2c agonist, CP-809,101. Results show that SB-242,084 blocked the reward attenuating effect of CP-809,101 but failed to alter that of SCH-23390. These findings provide evidence that the agonist action of SCH-23390 at the 5-HT2c receptors does not contribute to its attenuating effect on reward.

5-HT2C receptor activation prevents stress-induced enhancement of brain 5-HT turnover and extracellular levels in the mouse brain: modulation by chronic paroxetine treatment

Stress is known to activate the central 5-hydroxytryptamine (5-HT) system, and this is probably part of a coping response involving several 5-HT receptors. Although 5-HT(2C) receptors are well known to be implicated in anxiety, their participation in stress-induced changes had not been investigated in parallel at both behavioral and neurochemical levels. We show here that the preferential 5-HT(2C) receptor agonist, m-chlorophenylpiperazine, as well as restraint stress increased anxiety in the mouse social interaction test. The selective 5-HT(2C) receptor antagonist, SB 242,084, prevented both of these anxiogenic effects. Restraint stress increased 5-HT turnover in various brain areas, and this effect was prevented by the 5-HT(2B/2C) receptor agonist RO 60-0175 (1 mg/kg), but not the preferential 5-HT(2A) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1 mg/kg), and in contrast potentiated by SB 242,084 (1 mg/kg), which also blocked the effect of RO 60-0175. Using microdialysis, RO 60-0175 was shown to inhibit cortical 5-HT overflow in stressed mice when 5-HT reuptake was blocked locally. Chronic paroxetine prevented both the anxiogenic effect of m-chlorophenylpiperazine and the inhibitory effect of RO 60-0175 on locomotion and stress-induced increase in 5-HT turnover. The anxiolytic action of chronic paroxetine might be associated with an enhancement of 5-HT neurotransmission caused by a decreased 5-HT(2C) receptor-mediated inhibition of stress-induced increase in 5-HT release.

Role of serotonin in central dopamine dysfunction

The interaction between serotonin (5-HT) and dopamine (DA)-containing neurons in the brain is a research topic that has raised the interest of many scientists working in the field of neuroscience since the first demonstration of the presence of monoamine-containing neurons in the mid 1960. The bulk of neuroanatomical data available clearly indicate that DA-containing neurons in the brain receive a prominent innervation from serotonin (5-hydroxytryptamine, 5-HT) originating in the raphe nuclei of the brainstem. Compelling electrophysiological and neurochemical data show that 5-HT can exert complex effects on the activity of midbrain DA neurons mediated by its various receptor subtypes. The main control seems to be inhibitory, this effect being more marked in the mesocorticolimbic DA system as compared to the DA nigrostriatal system. In spite of a direct effect of 5-HT by its receptors located on DA cells, 5-HT can modulate their activity indirectly, modifying gamma-aminobutyric (GABA)-ergic and glutamatergic input to the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Although 5-HT/DA interaction in the brain has been extensively studied, much work remains to be done to clarify this issue. The recent development of subtype-selective ligands for 5-HT receptors will not only allow a detailed understanding of this interaction but also will lead to the development of new treatment strategies, appropriate for those neuropsychiatric disorders in which an alteration of the 5-HT/DA balance is supposed.

The serotonin 2C receptor potently modulates the head-twitch response in mice induced by a phenethylamine hallucinogen

RATIONALE

Hallucinogenic serotonin 2A (5-HT(2A)) receptor partial agonists, such as (+ or -)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), induce a frontal cortex-dependent head-twitch response (HTR) in rodents, a behavioral proxy of a hallucinogenic response that is blocked by 5-HT(2A) receptor antagonists. In addition to 5-HT(2A) receptors, DOI and most other serotonin-like hallucinogens have high affinity and potency as partial agonists at 5-HT(2C) receptors.

OBJECTIVES

We tested for involvement of 5-HT(2C) receptors in the HTR induced by DOI.

RESULTS

Comparison of 5-HT(2C) receptor knockout and wild-type littermates revealed an approximately 50% reduction in DOI-induced HTR in knockout mice. Also, pretreatment with either the 5-HT(2C) receptor antagonist SB206553 or SB242084 eradicated a twofold difference in DOI-induced HTR between the standard inbred mouse strains C57BL/6J and DBA/2J, and decreased the DOI-induced HTR by at least 50% in both strains. None of several measures of 5-HT(2A) receptors in frontal cortex explained the strain difference, including 5-HT(2A) receptor density, Galpha(q) or Galpha(i/o) protein levels, phospholipase C activity, or DOI-induced expression of Egr1 and Egr2. 5-HT(2C) receptor density in the brains of C57BL/6J and DBA/2J was also equivalent, suggesting that 5-HT(2C) receptor-mediated intracellular signaling or other physiological modulators of the HTR may explain the strain difference in response to DOI.

CONCLUSIONS

We conclude that the HTR to DOI in mice is strongly modulated by 5-HT(2C) receptor activity. This novel finding invites reassessment of hallucinogenic mechanisms involving 5-HT(2) receptors.

The use of antidepressants in clinical practice: focus on agomelatine

OBJECTIVE

Agomelatine (Valdoxan) is licensed by the European Medicines Agency for the treatment of major depressive episodes in adults. The objective of this review was to consider how the drug should be used in clinical practice in particular starting, stopping and switching to and from the drug.

METHODS

The existing clinical evidence was reviewed.

RESULTS

Data suggest that when switching to agomelatine from other antidepressants consideration should be given to tapering the previous antidepressant in order to minimize the risk of the original drug causing discontinuation/withdrawal symptoms. The risk of pharmacological interactions between most antidepressants and agomelatine is low and so tapering the previous antidepressant can usually be done after agomelatine has been started. An exception is fluvoxamine which should not be concurrently prescribed with agomelatine. As agomelatine appears to cause no significant discontinuation symptoms, it can probably be stopped abruptly when treatment is completed or when switching to another antidepressant.

CONCLUSIONS

While this guidance may change as clinical evidence and experience grows, currently agomelatine appears to have a good tolerability profile and is relatively easy to use, though prescribers should note the requirement to conduct liver function tests (LFTs) in accordance with the Summary of Product Characteristics (SPC).

In-vivo effects of the 1,2,4-piperazine derivatives MM5 and MC1, putative 5-HT agonists, on dopamine and serotonin release in rat prefrontal cortex

Two 1,2,4-substituted derivatives of piperazine were tested for their effect on dopamine and serotonin (5-HT) release in rat prefrontal cortex. Both compounds, 1-[4-(4-chinolin-2-yl-piperazin-1-yl)-butyl]piperidin-2-on (MM5) and 1-[4-(2-methyl-4-chinolin-2-yl-piperazin-1-yl)-butyl]-8-azaspiro [4.5]decano-7,9-dion (MC1), produced hypothermia in mice and showed affinity for 5-HT1A receptors in-vitro. Like the selective 5-HT1A agonist 8-OH-DPAT (0.1 mg kg−1), MM5 given peripherally (30 mg kg−1) decreased the extracellular 5-HT level in rat prefrontal cortex, while MC1 suppressed 5-HT release at a higher dose (40 mg kg−1), but not at a lower one (30 mg kg−1). The effect of both compounds on 5-HT release was abolished by WAY 100635 (0.3 mg kg−1). MC1 (30 and 40 mg kg−1), but not MM5, raised cortical dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and extracellular homovanillic acid (HVA) levels. The effect of MC1 on dopamine release was reversed by neither WAY 100635 nor the non-selective 5-HT2 antagonist ritanserin (2 mg kg−1). However, ritanserin prevented the effect of the higher dose of MC1 on 5-HT release. The results of this study suggest that MM5 exhibits the profile of a 5-HT1A agonist devoid of dopaminergic activity. MC1 seems to possess moderate agonist activity at 5-HT1A and 5-HT2A receptors, while acting on 5-HT release in the rat prefrontal cortex. However, the facilitation of dopamine release by this compound does not seem to be related to its affinity for 5-HT1A and 5-HT2A receptors.

Antipsychotic drug development

Schizophrenia typically manifests itself with a wide array of symptoms--positive, negative, cognitive, and affective--and may also involve neurodevelopmental and neurodegenerative aspects. Each of these symptom dimensions may be derived from pathology at one or more receptor types, localized in different regions of the brain. The absence of a single therapeutic target for schizophrenia has therefore prompted the de-emphasis of selective "magic bullets" and a critical re-examination of the intramolecular polypharmacy afforded by antipsychotics. In this chapter, we present a review of some of the receptor targets that are currently thought to mediate symptoms of schizophrenia, and discuss their possible implications for future antipsychotic drug development. Therapeutic strategies for schizophrenia that successfully exploit the multifunctionality of antipsychotics will take into account the entire receptor activity "portfolio" of the agent and provide a total therapeutic response that, like the elephant of the Buddhist parable, is greater than the sum of its parts.

A novel multilevel statistical method for the study of the relationships between multireceptorial binding affinity profiles and in vivo endpoints

The present work introduces a novel method for drug research based on the sequential building of linked multivariate statistical models, each one introducing a different level of drug description. The use of multivariate methods allows us to overcome the traditional one-target assumption and to link in vivo endpoints with drug binding profiles, involving multiple receptors. The method starts with a set of drugs, for which in vivo or clinical observations and binding affinities for potentially relevant receptors are known, and allows obtaining predictions of the in vivo endpoints highlighting the most influential receptors. Moreover, provided that the structure of the receptor binding sites is known (experimentally or by homology modeling), the proposed method also highlights receptor regions and ligand-receptor interactions that are more likely to be linked to the in vivo endpoints, which is information of high interest for the design of novel compounds. The method is illustrated by a practical application dealing with the study of the metabolic side effects of antipsychotic drugs. Herein, the method detects related receptors confirmed by experimental results. Moreover, the use of structural models of the receptor binding sites allows identifying regions and ligand-receptor interactions that are involved in the discrimination between antipsychotic drugs that show metabolic side effects and those that do not. The structural results suggest that the topology of a hydrophobic sandwich involving residues in transmembrane helices (TM) 3, 5, and 6, as well as the assembling of polar residues in TM5, are important discriminators between target/antitarget receptors. Ultimately, this will provide useful information for the design of safer compounds inducing fewer side effects.

Electrophysiological studies in the rat brain on the basis for aripiprazole augmentation of antidepressants in major depressive disorder

RATIONALE

Aripiprazole (ARI) is an atypical antipsychotic approved by the Food and Drug Administration for use in major depressive disorder as an adjunct to antidepressants. However, the precise mechanisms responsible for the effectiveness of ARI augmentation are not fully understood.

OBJECTIVES

The current study was aimed at examining the effects of ARI administration alone and in combination with the selective serotonin reuptake inhibitors (SSRI) escitalopram (ESC) on the firing of serotonin (5-HT), norepinephrine (NE), and dopamine (DA) neurons.

METHODS

Electrophysiological experiments were carried out in anesthetized Sprague-Dawley rats. ESC was delivered via subcutaneously implanted osmotic minipumps at a dose of 10 mg/kg/day. ARI was subcutaneously injected daily at a dose of 2 mg/kg/day. Both drugs were administered for 2 and 14 days alone and in combination. Control rats received physiological saline in analogous regimens.

RESULTS

Two-day ESC administration resulted in a significant decrease in the firing rate of 5-HT, NE, and DA neurons. Following 14 days of ESC administration, 5-HT firing returned to the baseline. The firing rate of NE and DA neurons remained significantly decreased. ARI administered for 2 or 14 days significantly increased the firing rate of 5-HT neurons by 36% and 48%, respectively, but not those of DA and NE neurons. Desensitization of somatodendritic 5-HT autoreceptors was observed after 2 days of ARI administration. The combination of the two drugs reversed the inhibitory action of ESC on the firing rate of 5-HT, NE, and DA neurons.

CONCLUSION

The present study showed that addition of ARI to an SSRI regimen reverses the inhibitory action of the SSRI on monoaminergic neuronal firing.

5-HT(2A) and 5-HT(2C) receptors exert opposing effects on locomotor activity in mice

Although it is well established that hallucinogens act as 5-HT(2A) and 5-HT(2C) receptor agonists, little is known about the relative contributions of 5-HT(2A) and 5-HT(2C) receptors to the acute behavioral effects of these drugs. The behavioral pattern monitor was used to characterize the effects of the hallucinogen 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on locomotor and investigatory behavior in mice. Studies were also conducted to assess the contributions of 5-HT(2A) and 5-HT(2C) receptors to the behavioral effects of DOI. DOI produced an inverted U-shaped dose-response function, with lower doses (0.625-5.0 mg/kg) increasing and higher doses (> or =10 mg/kg) decreasing locomotor activity. The increase in locomotor activity induced by 1.0 mg/kg DOI was absent in 5-HT(2A) receptor KO mice, suggesting the involvement of 5-HT(2A) receptors. The reduction in locomotor activity produced by 10 mg/kg DOI was potentiated in 5-HT(2A) KO mice and attenuated by pretreatment with the selective 5-HT(2C/2B) antagonist SER-082. These data indicate that the decrease in locomotor activity induced by 10 mg/kg DOI is mediated by 5-HT(2C) receptors, an interpretation that is supported by the finding that the selective 5-HT(2C) agonist WAY 161,503 produces reductions in the locomotor activity that are potentiated in 5HT(2A) KO mice. These results show for the first time that 5-HT(2A) and 5-HT(2C) receptors both contribute to the effects of DOI on locomotor activity in mice. Furthermore, these data also suggest that 5-HT(2A) and 5-HT(2C) receptors exert opposing effects on locomotor activity.

Asenapine elevates cortical dopamine, noradrenaline and serotonin release. Evidence for activation of cortical and subcortical dopamine systems by different mechanisms

RATIONALE

Asenapine, a psychopharmacologic agent developed for schizophrenia and bipolar disorder, has higher affinity for 5-HT(2A/C,6,7) and alpha(2) adrenergic receptors than for D(2) receptors. Asenapine exhibits potent antipsychotic-like effects without inducing catalepsy, increases cortical and subcortical dopamine release, and facilitates cortical glutamatergic transmission in rats. In this study, we further analyzed the effects of asenapine on dopaminergic, noradrenergic, and serotonergic systems in the rat brain.

MATERIALS AND METHODS

We studied the effects of asenapine on (1) dopaminergic neurons in the ventral tegmental area (VTA) and noradrenergic neurons in the locus coeruleus using in vivo single cell recording, (2) release of dopamine and noradrenaline (medial prefrontal cortex), serotonin (frontal cortex), and dopamine (nucleus accumbens), using in vivo microdialysis.

RESULTS

Systemic asenapine increased dopaminergic (0.001-0.2 mg/kg, i.v.) and noradrenergic (0.025-0.05 mg/kg i.v.) neuronal firing, and asenapine (0.1-0.2 mg/kg, s.c) increased cortical noradrenaline and serotonin output. Local asenapine administration increased all three monoamines in the cortex but did not affect accumbal dopamine output. Intra-VTA tetrodotoxin perfusion blocked asenapine-induced accumbal but not cortical dopamine outflow.

CONCLUSION

Asenapine at doses associated with antipsychotic activity enhanced cortical monoamine efflux. Whereas the asenapine-induced dopamine increase in nucleus accumbens is dependent on activation of dopaminergic neurons in the VTA, the increase of cortical dopamine outflow involves largely a local action at nerve terminals. Our data provide further insight on the pharmacologic characteristics of asenapine that may have bearing on its clinical efficacy in the treatment of schizophrenia and bipolar disorder.

WAY-163909, a 5-HT2C agonist, enhances the preclinical potency of current antipsychotics

INTRODUCTION

5-HT(2C) agonists, by decreasing mesolimbic dopamine without affecting nigrostriatal dopamine, are predicted to have antipsychotic efficacy with low extrapyramidal side effects (EPS). Combining 5-HT(2C) agonists with low doses of existing antipsychotics could increase treatment efficacy while reducing treatment liabilities such as EPS (typical antipsychotics), and the propensity for weight gain (atypical antipsychotics).

OBJECTIVES

The objectives of these studies were to combine WAY-163909, a selective 5-HT(2C) agonist, with either the typical antipsychotic haloperidol, or the atypical antipsychotic clozapine, at doses that were ineffective on their own, with the expectation that a shift in potency in several rodent behavior models predictive of antipsychotic activity would occur.

RESULTS AND DISCUSSION

In mice, co-administration of either haloperidol, or clozapine, produced a significant leftward shift in the ability of WAY-163909 to block apomorphine-induced climbing behavior, without any affect on apomorphine-induced stereotypy or an increased propensity for catalepsy. In the rat-conditioned avoidance model, WAY-163909 was combined with either haloperidol or clozapine at doses that individually produced reductions in avoidance response on the order of 10%, while the combination of WAY-163909 and either of the antipsychotics resulted in a greater than 70% reduction in avoidance, with no evidence of response failures, or pharmacokinetic interaction.

CONCLUSION

Doses of either haloperidol or clozapine, that failed to antagonize an MK-801 induced deficit in prepulse inhibition, significantly attenuated the sensory gating deficit when combined with WAY-163909. Data support the notion that 5-HT(2C) receptor agonists, co-administered with other marketed antipsychotics, allow for dose sparing with a more favorable side-effect profile.