Clozapine--a serotonin antagonist?

The presynaptic component of the serotonergic system is required for clozapine's efficacy

Clozapine, by virtue of its absence of extrapyramidal side effects and greater efficacy, revolutionized the treatment of schizophrenia, although the mechanisms underlying this exceptional activity remain controversial. Combining an unbiased cheminformatics and physical screening approach, we evaluated clozapine's activity at >2350 distinct molecular targets. Clozapine, and the closely related atypical antipsychotic drug olanzapine, interacted potently with a unique spectrum of molecular targets. This distinct pattern, which was not shared with the typical antipsychotic drug haloperidol, suggested that the serotonergic neuronal system was a key determinant of clozapine's actions. To test this hypothesis, we used pet1(-/-) mice, which are deficient in serotonergic presynaptic markers. We discovered that the antipsychotic-like properties of the atypical antipsychotic drugs clozapine and olanzapine were abolished in a pharmacological model that mimics NMDA-receptor hypofunction in pet1(-/-) mice, whereas haloperidol's efficacy was unaffected. These results show that clozapine's ability to normalize NMDA-receptor hypofunction, which is characteristic of schizophrenia, depends on an intact presynaptic serotonergic neuronal system.

Synthesis, SAR and unanticipated pharmacological profiles of analogues of the mGluR5 ago-potentiator ADX-47273

An iterative analogue library synthesis strategy rapidly developed comprehensive SAR for the mGluR5 ago-potentiator ADX-47273. This effort identified key substituents in the 3-position of oxadiazole that engendered either mGluR5 ago-potentiation or pure mGluR5 positive allosteric modulation. The mGluR5 positive allosteric modulators identified possessed the largest fold shifts (up to 27.9-fold) of the glutamate CRC reported to date as well as providing improved physiochemical properties.

Pimavanserin tartrate: a 5-HT2A inverse agonist with potential for treating various neuropsychiatric disorders

BACKGROUND

Pimavanserin tartrate is the first 5-HT(2A) inverse agonist to enter clinical trials as a treatment for L-dopa-induced psychosis in Parkinson's disease and for augmentation of low-dose risperidone treatment in schizophrenia. Pimavanserin is also being evaluated as a possible anti-insomnia drug.

OBJECTIVE

To discuss the potential of pimavanserin to fill multiple therapeutic needs.

METHODS

The problems with currently approved antipsychotics and sleep agents are explored to highlight how pimavanserin might address some longstanding issues in the treatment of psychosis and insomnia.

RESULTS/CONCLUSIONS

In Phase II clinical trials, pimavanserin seemed to be safe, well-tolerated and efficacious in treating L-dopa-induced psychosis without worsening motor symptoms. Pimavanserin also potentiated the therapeutic effects of low-dose risperidone, reduced haloperidol-induced akathisia, and increased slow-wave sleep in older individuals.

Differential profile of antipsychotics at serotonin 5-HT1A and dopamine D2S receptors coupled to extracellular signal-regulated kinase

The effects of antipsychotics targeting dopamine D2 and serotonin 5-HT1A receptors were compared with conventional antipsychotics on phosphorylation of Extracellular signal-Regulated Kinase 1/2 (ERK 1/2) in CHO cell lines stably expressing either the human serotonin 5-HT1A or human dopamine D2S receptor. All antipsychotics except haloperidol and olanzapine exhibited agonist properties at serotonin 5-HT1A receptors. Emax values (% effect of 10 microM 5-HT) were: bifeprunox (74), SSR181507 (73), SLV313 (72), aripiprazole (60), ziprasidone (56), clozapine (33). At dopamine D2S receptors, partial agonist activity (% effect of 10 microM dopamine) was observed for bifeprunox (76), SSR181507 (66) and aripiprazole (59). Other antipsychotics attenuated dopamine-induced ERK phosphorylation, with pK(B) values of : SLV313 (8.5), haloperidol (8.1), olanzapine (7.8), ziprasidone (7.7), and clozapine (6.4). Amongst the dopamine D2/serotonin 5-HT1A receptor compounds, aripiprazole acts as a partial dopamine D2S and serotonin 5-HT1A receptor agonist. SSR181507 and bifeprunox possess a profile of action similar to each other, efficaciously stimulating both serotonin 5-HT1A and dopamine D2S receptors. In contrast, SLV313, also an efficacious serotonin 5-HT1A receptor agonist, acted as a high potency dopamine D2 receptor antagonist. Thus, antipsychotics display varying efficacies at serotonin 5-HT1A and dopamine D2S receptors which may play a major role in their differential functional profiles in blocking the diverse symptoms of schizophrenia.

Effects of novel antipsychotics with mixed D(2) antagonist/5-HT(1A) agonist properties on PCP-induced social interaction deficits in the rat

Considerable interest has arisen in identifying antipsychotic agents with improved efficacy against negative symptoms, such as social withdrawal. In rats, a social interaction deficit can be induced by the NMDA antagonist phencyclidine (PCP). Here, we examined the effects of antipsychotics, reported to exert dual 5-HT(1A)/D(2) actions, on PCP-induced social interaction deficits. Drugs were administered daily for 3 days in combination with either vehicle or PCP (2.5mg/kg, SC) and social interaction was measured on the last day of drug treatment. Pairs of unfamiliar rats receiving the same treatment were placed in a large open field for 10 min and the number of social behaviors were scored. The results indicate that: (1) PCP significantly reduced social interaction by over 50% compared with vehicle-treated controls; (2) haloperidol (0.0025-0.16 mg/kg, SC) and clozapine (0.04-10mg/kg, IP) did not reverse PCP-induced social interaction deficits; (3) the substituted benzamide remoxipride reversed PCP-induced deficits at 0.63 and 2.5mg/kg (4) the 5-HT(1A) agonist 8-OH-DPAT was inactive (at 0.01-0.63 mg/kg, SC); (5) among compounds reported to exert dual 5-HT(1A)/D(2) actions, SSR181507 (at 0.16 mg/kg, SC) and aripiprazole (at 0.04 and 0.16 mg/kg, IP), but not ziprasidone (0.04-2.5mg/kg, IP), SLV313 (0.0025-0.16 mg/kg, SC) or bifeprunox (0.01-0.63 mg/kg, IP), significantly reversed PCP-induced social interaction deficits; and (6) the 5-HT(1A) receptor antagonist WAY100635 blocked the effects of SSR181507 and aripiprazole. These findings indicate that the balance of activity at 5-HT(1A) and D(2) receptors profoundly influences the activity of antipsychotics in this model of social withdrawal, and their potential benefit on at least some of the negative symptoms of schizophrenia.

Serotonin receptors represent highly favorable molecular targets for cognitive enhancement in schizophrenia and other disorders

RATIONALE

Current treatments for schizophrenia adequately treat the positive symptoms of schizophrenia but only modestly improve cognitive deficits. This review provides evidence for and against the use of selective 5-HT receptor drugs as cognition enhancing agents for schizophrenia and other disorders.

METHODS

Pre-clinical and clinical literature concerned with the role of the serotonergic system in cognition and memory as it relates to schizophrenia is reviewed. Individual 5-HT receptor subtypes for which selective drugs are available that are likely to improve cognition are reviewed. Recommendations for clinical testing are proposed.

RESULTS AND CONCLUSIONS

Four 5-HT receptor systems (5-HT(1A), 5-HT(2A), 5-HT(4), 5-HT(6)) are highlighted as suitable targets for enhancing cognition and memory. Because many clinically available antipsychotic drugs already target 5-HT(1A), 5-HT(2A) and 5-HT(6) receptors, design of clinical trials will need to take into account the serotonergic pharmacology of concurrently administered antipsychotic medications. 5-HT(1A) partial agonists and 5-HT(2A) antagonists have shown modest effectiveness in improving cognition in schizophrenia. 5-HT(6)-selective compounds for cognition enhancement are in late-stage clinical trials, while 5-HT(4) compounds have not yet been tested in humans for cognition enhancement.

RECOMMENDATIONS

For stand-alone therapy for enhancing cognition, 5-HT(1A) partial agonists, 5-HT(2A) antagonists, 5-HT(4) partial agonists and 5-HT(6) antagonists are all likely to induce at least modest improvement in cognition in schizophrenia. If "add-on therapy" is contemplated, antipsychotic drugs with weak affinities for serotonin receptors should be used to avoid confounds. It is likely that serotonergic drugs will soon be available as cognition enhancing medications for disorders other than schizophrenia (e.g. dementia).

Effects of risperidone, clozapine and haloperidol on extracellular recordings of substantia nigra reticulata neurons of the rat brain

Risperidone has proven to be effective as an antipsychotic drug and has fewer extrapyramidal side-effects than classic neuroleptics. In addition to its dopamine D2 receptor antagonistic properties, this antipsychotic agent is a potent 5-HT2 receptor antagonist. The atypical antipsychotic, clozapine, also possesses both dopamine D2 and 5-HT2 receptor affinity next to affinities for other receptors. To gain an insight in the consequences for basal ganglia activity of treatment with these atypical neuroleptics vs. typical neuroleptics, the effects of cumulative doses of risperidone, clozapine and haloperidol on the firing rate of substantia nigra reticulata neurons were studied. Extracellular recordings were performed in chloralhydrate-anaesthetized male Wistar rats. Both risperidone (50-3200 micrograms/kg i.v.) and clozapine (100-6400 micrograms/kg i.v.) dose dependently decreased substantia nigra reticulata activity maximally to 70% of the basal activity. With both treatments, a dose of 800 micrograms/kg was significantly effective. In contrast, haloperidol (12.5-800 micrograms/kg i.v.) gradually induced a slight increase in substantia nigra reticulata activity, which was identical to the substantia nigra reticulata activity after saline treatment. Therefore, these results indicate that typical and atypical neuroleptics affect differentially the output of the basal ganglia in the substantia nigra reticulata. To evaluate the involvement of 5-HT2 receptors in the effect of risperidone, the 5-HT2 receptor agonist, quipazine (0.5 mg/kg i.p.), was administered 15 min preceding risperidone treatment. A 4-fold higher dose of risperidone was needed to significantly affect the substantia nigra reticulata firing rate. Thus, the 5-HT2 component of the effect of risperidone is, at least partly, responsible for the difference in effect on substantia nigra reticulata neurons in comparison to haloperidol.

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

Risperidone: regional effects in vivo on release and metabolism of dopamine and serotonin in the rat brain

The antipsychotic drug risperidone shows high affinity for both central serotonin (5-HT)2A and dopamine (DA)-D2 receptors in vivo. By employing microdialysis in freely moving rats, the effects of acute risperidone administration on regional brain DA and 5-HT release and metabolism were compared with the corresponding effects of the atypical antipsychotic drug clozapine as well as amperozide, the selective DA-D2 receptor antagonist raclopride and the selective 5-HT2A/5-HT2C receptor antagonist ritanserin. Risperidone (0.2 or 2.0 mg/kg, SC) was found to increase DA release and metabolism to about the same extent in three major projection areas of the mesotelencephalic dopaminergic system, i.e. the nucleus accumbens (NAC), the medial prefrontal cortex (MPC) and the lateral striatum (STR). In contrast, clozapine and amperozide (both 10.0 mg/kg, SC), as well as raclopride (2.0 mg/kg, SC), were all found differentially to affect DA release and metabolism in the three projections areas. Specifically, clozapine and amperozide enhanced DA release in the MPC to a greater extent than in the NAC or the STR, whereas raclopride instead preferentially increased DA release in the NAC and the STR but not in the MPC. Ritanserin (3.0 mg/kg, SC) did not exert any major effects on DA metabolism in the three areas studied. In contrast to the regionally rather homogenous activation of brain DA systems caused by risperidone, the drug was found to enhance brain 5-HT metabolism preferentially in the MPC, as indicated by the elevated extracellular concentration of 5-hydroxyindoleacetic acid (5-HIAA) in this region. A similar elevation of the 5-HIAA level in the MPC was observed after amperozide and, to some extent, after clozapine and ritanserin administration. The risperidone-induced (2.0 mg/kg, SC) elevation of 5-HIAA concentrations in the frontal cortex was found to be paralleled by an increased 5-HT release in this brain area. Consequently, our findings demonstrate a pharmacological profile of risperidone, as reflected in brain DA metabolism, in between that of clozapine and the DA-D2 antagonists. The preferential activation of 5-HT release and metabolism in frontal cortical areas might be of particular relevance for the ameliorating effect of risperidone on negative symptoms in schizophrenia, especially when associated with depression.

Clozapine acts as a 5-HT2 antagonist by attenuating DOI-induced inhibition of male rat sexual behaviour

Evidence has been reported that clozapine may derive part of its therapeutic effects in treatment-resistant schizophrenic patients by interacting with the serotonin system. Among the few behavioural models available to test the hypothesis of an interaction of clozapine with 5-HT2 receptors, male rat sexual behaviour is particularly useful, since in this behaviour 5-HT1A and 5-HT2 receptors have opposite functions. Stimulation of 5-HT1A receptors facilitates ejaculatory behaviour and stimulation of 5-HT2 receptors inhibit ejaculation. In the present study, male rat sexual behaviour was depressed by treatment with DOI (1.0 mg/kg), a selective 5-HT2 receptor agonist. The depressive effect of DOI was attenuated by the administration of clozapine (0.1-1.0 mg/kg) in doses that by themselves did not significantly affect sexual behaviour. It was concluded that clozapine in the male rat sexual behaviour model may be interpreted as serving as a 5-HT2 antagonist.

The role of 5-HT2A receptors in antipsychotic activity

The correlation between the clinical activity of antipsychotic agents and their affinity for the D2 dopamine receptor has been the mainstay of the hypothesis that schizophrenia is due to excessive dopaminergic function. More recently, the unique clinical profile of the atypical antipsychotic clozapine has been proposed to involve actions on additional receptor systems. In particular, the high affinity of clozapine for the 5HT2A receptor subtype has been suggested to contribute to its reduced side-effect liability, greater efficacy and its activity in therapy-resistant schizophrenia. We have used the highly selective 5-HT2A antagonist MDL 100,907 to explore the contribution of 5-HT2A receptor blockade to antipsychotic activity. Biochemical, electrophysiological and behavioral studies reveal that selective 5HT2A receptor antagonists have the preclinical profile of an atypical antipsychotic. The limited clinical evidence available also suggests that compounds producing 5-HT2A receptor blockade are effective, in particular, against the negative symptoms of schizophrenia.

Clozapine decreases serotonin extracellular levels in the nucleus accumbens by a dopamine receptor-independent mechanism

By using brain microdialysis, clear differences in the effects of the systemic administration of clozapine and haloperidol on the extracellular concentration of dopamine (DA) and serotonin (5-HT) were found in the nucleus accumbens of freely moving rats. Haloperidol (0.5 mg/kg s.c.) significantly increased DA (ca. 40%) but did not modify 5-HT extracellular concentration, while clozapine (5 mg/kg s.c.) significantly decreased 5-HT (ca. 40%) but did not change DA concentration. Locally infused, clozapine (10(-5) M) significantly increased DA (75%) and reduced 5-HT extracellular concentration (50%). The reduction of 5-HT cannot be explained by a clozapine-induced blockade of DA receptors, because the local infusion (10(-5) M) of the DA D2-like antagonist raclopride and the DA D1-like antagonist SCH 23390 significantly increased DA (ca. 300% and 200%, respectively) but did not modify 5-HT extracellular concentration. Conversely, the DA D2-like agonist quinpirole and the DA D1-like agonist SKF-38393 significantly decreased (ca. 60% in both cases) DA extracellular concentration without affecting that of 5-HT. The present results indicate that clozapine displays a powerful anti-serotoninergic effect by inhibiting 5-HT release in the nucleus accumbens, an effect probably derived from the reduction of firing activity at the dorsal raphe and by local mechanisms that may involve some 5-HT receptor subtype(s).

Extrapyramidal symptoms with selective serotonin reuptake inhibitors

BACKGROUND

Several case reports in the literature suggest that selective serotonin reuptake inhibitors can produce extrapyramidal symptoms.

METHODS

Computerised literature searches were used to identify reports on extrapyramidal symptoms and serotonin reuptake inhibitors. Subsequently, manual searches were made for articles in which there was any indication of the mechanisms responsible for these extrapyramidal symptoms.

RESULTS

Only a few reports could be identified in which serotonin reuptake inhibitors were implicated in extrapyramidal symptoms in some patients.

CONCLUSIONS

Evidence is discussed from preclinical and clinical studies suggesting the interaction between serotoninergic and dopaminergic neurotransmitter system, as a possible mechanism for production of extrapyramidal symptoms.

Clozapine injected into the nucleus accumbens potentiates apomorphine-induced jaw movements

The effects of clozapine injected into the nucleus accumbens on apomorphine-induced jaw movements were studied. Jaw movements induced by apomorphine (0.5 mg/kg i.v.) were potentiated by clozapine (10 micrograms/0.2 microliters) injected into the nucleus accumbens 10 min before apomorphine. Enhancement of the apomorphine-induced jaw movements was also found with the muscarinic acetylcholine receptor antagonist, methylscopolamine (2.5 micrograms), whereas the acetylcholine receptor agonist, carbachol (2.5 micrograms), inhibited the effects of apomorphine. Injection of a smaller dose of carbachol (0.1 microgram) alone into the nucleus accumbens 10 min before failed to alter the effects of apomorphine but prevented the potentiation induced by clozapine. Both the 5-hydroxytryptamine(5-HT)2A receptor antagonist, 2-(2-dimethylaminoethylthio)-3-phenylquinoline hydrochloride (ICI 169,369, 0.1 and 0.2 microgram), and the alpha 1-adrenoceptor antagonist, prazosin (0.05 and 0.2 microgram), failed to affect the effects of apomorphine(0.5 mg/kg i.v.). In contrast, clozapine (1, 5 and 10 micrograms), ICI 169,369 (0.1 and 0.2 microgram) or prazosin (0.05 and 0.2 microgram) given into the ventral striatum inhibited the effects of apomorphine (0.5 mg/kg i.v.). It is suggested that the clozapine-induced potentiation in the nucleus accumbens might be due to its antimuscarinic properties.

Clozapine inhibits serotoninergic transmission by an action at alpha 1-adrenoceptors not at 5-HT1A receptors

This study examined the mechanism underlying the influence of clozapine upon serotoninergic transmission in the rat. In vitro, clozapine manifested weak affinity at 5-HT1A receptors (pKi = 6.5) as compared to the agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (9.0), but high affinity at alpha 1-adrenoceptors (8.2) as compared to the alpha 1-adrenoceptor antagonist, prazosin (9.7). Ex vivo, clozapine (inhibitory dose (ID)50 = 0.7 mg/kg s.c.) mimicked prazosin (0.5) in potently occupying central alpha 1-adrenoceptors whereas, as compared to 8-OH-DPAT (0.2), it failed to occupy 5-HT1A receptors (> 10.0). The firing of serotoninergic neurones in the dorsal raphe nucleus was abolished by 8-OH-DPAT, clozapine and prazosin with ID50 values of 0.006, 0.09 and 0.07 mg/kg i.v., respectively. At comparable doses, they reduced striatal turnover of 5-HT. While the 5-HT1A receptors antagonists, (-)-tertatolol (2.0 mg/kg i.v.) and spiperone (0.63 mg/kg i.v.), blocked the action of 8-OH-DPAT upon dorsal raphe nucleus firing, they failed to modify the effect of clozapine and prazosin. In contrast, the alpha 1-adrenoceptor agonist, cirazoline (0.005 mg/kg i.v.) prevented the actions of clozapine and prazosin, but not that of 8-OH-DPAT. It is concluded that clozapine only weakly interacts with 5-HT1A receptors and that its potent alpha 1-adrenoceptor antagonist properties underlie inhibition of serotoninergic transmission.

Systemic administration of amperozide, a new atypical antipsychotic drug, preferentially increases dopamine release in the rat medial prefrontal cortex

The putative atypical antipsychotic drug amperozide (APZ) shows high affinity for serotonin 5-HT2 receptors but only low affinity for dopamine (DA) D2 receptors. By employing microdialysis, we examined the effects of APZ on extracellular concentrations of DA in the nucleus accumbens (NAC), the dorsolateral striatum (STR) and the medial prefrontal cortex (MPC) of awake rats. A 5.0 mg/kg (SC) dose of APZ failed to affect DA concentrations in the NAC, while it increased DA outflow in the STR (by 46%) and the MPC (by 207%). A higher dose of APZ (10 mg/kg, SC) enhanced dialysate DA from the NAC and the STR by 30%, and from the MPC by 326%. Similarly, clozapine (2.5 and 10 mg/kg, SC) produced a greater release of DA in the MPC (+ 127 and + 279%) than in the NAC (+ 52 and + 98%). The selective 5-HT2 receptor antagonist ritanserin (1.5 and 3.0 mg/kg, SC) also produced a slightly higher increase of DA output in the MPC (+ 25 and + 47%) compared with the NAC (+ 19 and + 21%). In contrast, the selective D2 receptor antagonist raclopride (0.5 and 2.0 mg/kg, SC) increased DA release in the NAC (+ 65 and + 119%) to a greater extent than in the MPC (+ 45 and + 67%). These data suggest that the 5-HT2 receptor antagonistic properties of APZ and clozapine may contribute to their preferential effects on DA transmission in the MPC. Infusion of low doses (1, 10 microM, 40 min) of APZ through the probe in the DA terminal areas did not affect significantly DA outflow, while infusion of high doses (100, 1000 microM, 40 min) resulted in a more pronounced elevation of DA levels in the NAC (up to 961%) and the STR (up to 950%) than in the MPC (up to 316%). These findings indicate that the selective action of systemically administered APZ on DA in the MPC is most likely mediated at a level other than the terminal region. Taken together, the present results provide support for the notion that 5-HT2 receptor antagonism may be of considerable significance for the action of atypical antipsychotic drugs on mesolimbocortical dopaminergic neurotransmission.

The role of serotonin in schizophrenia: an overview of the nomenclature, distribution and alterations of serotonin receptors in the central nervous system

Serotonin is a neurotransmitter that is widely distributed throughout the central nervous system. The role of serotonin in schizophrenia is still unclear. Postmortem studies of serotonin receptor subtypes in schizophrenia have been inconclusive for the most part. The most promising findings involve a reduction in 5-HT2 receptors and 5-HT reuptake sites in the prefrontal cortex of schizophrenic patients. In this paper we review the function, distribution and pharmacological characteristics of serotonin receptors. Postmortem studies are also reviewed, focusing upon the role of these receptors in schizophrenia.

Serotonergic drugs do not substitute for clozapine in clozapine-trained rats in a two-lever drug discrimination procedure

The atypical neuroleptic clozapine has been shown to have cue properties in two-lever drug discrimination procedures. Although it has been demonstrated that clozapine acts at several types of receptors in vitro and in vivo, including dopamine, serotonin [5-hydroxytryptamine (5-HT)], and acetylcholine receptors, the mechanism of action for its discriminative stimulus properties has not yet been determined. The present study examined the effects of haloperidol (D2 dopamine antagonist), ritanserin (5-HT2 antagonist), 1-alpha H,3-alpha,5-alpha H-tropan-3yl-3,5-dichlorobenzoate (MDL 72222) (5-HT3 antagonist), and buspirone (5-HT1A agonist) in stimulus substitution tests with rats trained to discriminate clozapine (5.0 mg/kg, IP) from vehicle in a two-lever drug discrimination procedure under a fixed ratio 30 schedule of food reinforcement. Analysis of the results revealed that, while clozapine produced dose-dependent responding on the clozapine lever, haloperidol and the three serotonin drugs failed to produce full substitution for clozapine at any of the doses tested. These results suggest that the discriminative stimulus properties are not mediated by D2 dopamine receptor blockade, antagonism at 5-HT2 or 5-HT3 receptors, or agonistic activity at 5-HT1A receptors. The neural basis of clozapine's discriminative stimulus properties has not yet been determined.

Multiple neurochemical action of clozapine: a quantitative autoradiographic study of DA2, opiate and benzodiazepine receptors in the rat brain after long-term treatment

The atypical neuroleptic clozapine has clinical and behavioral properties that differ not only from the typical compounds, but also from atypical ones. It interacts with the dopaminergic systems, but also produces effects on the serotoninergic, GABA-ergic, cholinergic systems. In spite of the amount of papers devoted to its study, the profile of the neurochemical action of this drug is still confuse. In this paper we investigated the DA2-, opiate- and benzodiazepine-receptor modifications induced by the long term (21 days) treatment with clozapine 20 mg/kg/day in the rat brain. We found a decrease of DA2 receptor density in the target areas of the mesolimbocortical system (ventral n. caudate-putamen, cerebral cortex except for the anterior cingulate at the most anterior level and the n. accumbens) and a decrease of opiate and benzodiazepine receptors in the cerebral cortex and in the olfactory tubercle. Opiate receptors increase in the patches of the striatum. We also compared these effects with those produced by long-term (21 days), low-dosage (0.5 mg/kg day) haloperidol.

Clozapine: an atypical neuroleptic

Since it was synthesized in 1960, much has been written about clozapine. Although a number of its properties are those of a neuroleptic, it displays marked differences from classical antipsychotics to the extent that it is currently listed as an atypical neuroleptic. A classical neuroleptic has been defined in man according to its antipsychotic properties, accompanied by extrapyramidal effects, and in animals according to its cataleptic properties, its ability to antagonize apomorphine and amphetamine stereotypies and to suppress the conditioned avoidance response. Moreover, the classical neuroleptic exerted depressive and anhedonic effects in most conditioning schedules. With clozapine, most of these properties are no longer strictly in force to the point that they call in question the validity of the tests carried out to detect the potential of neuroleptics. This article attempts to compare the characteristics of clozapine with those of classical neuroleptics from a toxicological, neuropharmacological, psychopharmacological and clinical point of view.

Binding of typical and atypical antipsychotics to 5-HT1C and 5-HT2 sites: clozapine potently interacts with 5-HT1C sites

We determined the affinity of several typical and atypical antipsychotics for the 5-HT1C and 5-HT2 sites using radioligand binding assays. Most of the antipsychotics tested appeared to bind to 5-HT2 sites with affinities that were fairly high (i.e. pKi values between 7 and 9) and significantly higher than for 5-HT1C sites. In contrast, clozapine was found to have a significantly higher affinity for 5-HT1C than for 5-HT2 sites. Clozapine had the highest affinity for 5-HT1C sites of all the compounds tested. These findings are consistent with the hypothesis that an interaction with 5-HT2 receptors may be relevant to the clinical activity of typical antipsychotics. The findings also suggest, however, that an interaction with 5-HT1C sites may be relevant to the mechanism of clinical action of clozapine and, perhaps, of other atypical antipsychotics.

Interaction of the atypical neuroleptic clozapine with 5-HT3 receptors in the cerebral cortex and superior cervical ganglion of the rat

Clozapine, an atypical neuroleptic drug devoid of extrapyramidal side effects, was a moderately potent, competitive inhibitor of the binding of [3H]quaternised ICS 205-930 to 5-HT3 receptor sites in rat cortical membranes, possessing a pKi value of 7.0. In contrast, several other antipsychotic agents, including fluphenazine, alpha-flupenthixol, haloperidol, spiperone and (-)-sulpiride were essentially inactive. Clozapine also antagonised the 2-methyl 5-HT-induced depolarisation of the rat isolated superior cervical ganglion, a response known to be mediated via 5-HT3 receptors. Clozapine (0.1-1 microM) induced parallel displacements to the right of the dose-response curve to 2-methyl 5-HT in this tissue, possessing a pKb value of 7.3. These data suggest that the atypical antipsychotic profile of clozapine may be related, at least, in part to its ability to interact with central 5-HT3 receptor sites.

Anxiolytic potential of sulpiride, clozapine and derivatives in the open-field test

Recently acquired data question the sharp dichotomy between anxiolytics and neuroleptics, since disinhibitory effects have been measured in the rat with very low doses of haloperidol and higher doses of atypical neuroleptics in FI and DRL schedules, but also in the open-field test. That the DA transmission in certain brain regions is involved in some aspects of anxiety has recently been suggested. The present study confirms this hypothesis particularly with high doses of sulpiride (80 mg/kg) and clozapine (24 mg/kg) when tested in the open-field test. Moreover, the results show how a slight chemical modification of clozapine can give a direction to pharmacological activity with one derivative still resembling clozapine and the second one resembling haloperidol. As neuroleptics do not seem to influence the synthesis and utilization of GABA, the higher entry score observed with them would seem to depend above all on DA antagonism in the mesolimbic system.

Acute effects of typical and atypical antipsychotic drugs on the release of dopamine from prefrontal cortex, nucleus accumbens, and striatum of the rat: an in vivo microdialysis study

In vivo microdialysis has been used to study the acute effects of antipsychotic drugs on the extracellular level of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. (-)-Sulpiride (20, 50, and 100 mg/kg i.v.) and haloperidol (0.1 and 0.5 mg/kg i.v.) enhanced the outflow of dopamine in the striatum and nucleus accumbens. In the medial prefrontal cortex, (-)-sulpiride at all doses tested did not significantly affect the extracellular level of dopamine. The effect of haloperidol was also attenuated in the medial prefrontal cortex; 0.1 mg/kg did not increase the outflow of dopamine and the effect of 0.5 mg/kg haloperidol was of shorter duration in the prefrontal cortex than that observed in striatum and nucleus accumbens. The atypical antipsychotic drug clozapine (5 and 10 mg/kg) increased the extracellular concentration of dopamine in all three regions. In contrast to the effects of sulpiride and haloperidol, that of clozapine in the medial prefrontal cortex was profound. These data suggest that different classes of antipsychotic drugs may have distinct effects on the release of dopamine from the nigrostriatal, mesolimbic, and mesocortical terminals.

Utilization of microdialysis for assessing the release of mesotelencephalic dopamine following clozapine and other antipsychotic drugs

1. In vivo microdialysis was utilized to assess the effect of clozapine, haloperidol, and sulpiride on the release of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. 2. The results suggest that acute administration of various classes of antipsychotic drugs may differentially increase the extracellular concentration of dopamine in mesotelencephalic systems. Haloperidol and sulpiride were more effective in releasing dopamine from the terminals of the nigrostriatal dopamine system while clozapine had a more prefrontal effect on the mesolimbic and particularly the mesocortical dopamine systems.

Atypical neuroleptics suppress dopaminergic behavioral supersensitivity

Seven days after bilateral 6-OHDA denervation of the nucleus accumbens locomotor activity was recorded in rats. 6-OHDA lesion strongly enhanced hypermotility induced by apomorphine (1.0 mg/kg IP) as a sign of behavioral dopaminergic supersensitivity. The potency of the classical neuroleptic haloperidol (0.03-0.25 mg/kg IP) to antagonize apomorphine-induced hypermotility was reduced in 6-OHDA-pretreated rats. The atypical neuroleptics sulpiride (5.0-20.0 mg/kg IP), thioridazine (1.0-5.25 mg/kg IP) and clozapine (0.5-2.0 mg/kg IP) and the 5-HT antagonists cyproheptadine (0.2 mg/kg IP) and ritanserin (0.01 mg/kg IP) suppressed the augmented apomorphine response in 6-OHDA-lesioned animals to the level of the apomorphine effect in controls. It is concluded that the model of denervation supersensitivity is capable of differentiating typical and atypical neuroleptics. The abolition of the 6-OHDA-induced increase of the apomorphine hypermotility by the atypical neuroleptics cannot be explained solely by postsynaptic dopamine receptor antagonism. Serotonergic mechanism may be involved in this action.

Melperone and clozapine: neuroendocrine effects of atypical neuroleptic drugs

The effects of atypical neuroleptics within the neuroendocrine axis of rodents can be distinguished from those of typical neuroleptics by the production of: 1) a shortlived increase in serum PRL concentrations, 2) an acute increase in the activity of TIDA neurons, and 3) a marked increase in serum corticosterone concentrations. It is of interest to speculate that the pharmacological properties of atypical neuroleptics which mediate the unique neuroendocrine responses are of relevance to an understanding of the mechanisms which underlie the clinical profile of these antipsychotic agents.

Clozapine: new research on efficacy and mechanism of action

Clozapine can produce greater clinical improvement in both positive and negative symptoms than typical antipsychotic drugs in neuroleptic-resistant schizophrenic patients. The clinical response may occur rapidly in some patients but is delayed in others. Clozapine has also been reported to produce fewer parkinsonian symptoms, to involve a lower risk of producing tardive dyskinesia, and to produce no serum prolactin elevations in man. It seems likely that these effects are the result of a common biological mechanism or related mechanisms, rather than unrelated effects. Other atypical antipsychotic drugs, such as melperone and fluperlapine, share at least some of these properties. A relatively low affinity for the D-2 dopamine (DA) receptor and high affinity for the 5-HT2 receptor, producing a high 5-HT2/D-2 ratio, best distinguishes atypical antipsychotics like clozapine from typical antipsychotic drugs. Through its weak antagonist action on D-2DA receptors and a potent inhibitory effect on 5-HT2 receptors, as well as its ability to increase DA and 5-HT2 release, clozapine may be able to permit more normal dopaminergic function in the anterior pituitary, the mesostriatal, mesolimbic and mesocortical regions. The numerous advantages of clozapine over typical neuroleptics are consistent with the primary importance of DA to the pathophysiology of schizophrenia. The secondary but still significant role of 5-HT in the action of clozapine may either be direct or via the effect of 5-HT on dopaminergic mechanisms. Some aspects of schizophrenia could be due to a dysregulation of the interaction between serotonergic and dopaminergic neurotransmission.

Effects of acute and chronic treatments with clozapine and haloperidol on serotonin (5-HT2) and dopamine (D2) receptors in the rat brain

The effects of acute and chronic treatments with haloperidol or clozapine on the binding of [3H]spiperone to D2 and 5-HT2 receptors were examined in 6 discrete regions of the striatum, n. accumbens and frontal cortex using quantitative autoradiography. Acute treatment with haloperidol, 0.1-2.0 mg/kg, i.p., produced a dose-dependent reduction to 60% of control in the binding of [3H]spiperone to D2 receptors in the striatum and n. accumbens and no effect on the binding of [3H]spiperone to 5-HT2 receptors in the striatum, n. accumbens or frontal cortex. Acute treatment with clozapine, 10-40 mg/kg, i.p., produced a dose-dependent reduction in D2-specific binding in both the n. accumbens and the striatum and also significant reductions to 24% of control in the binding of [3H]spiperone to cortical 5-HT2 receptors. Chronic treatment with haloperidol, 1 mg/kg/day, i.p., significantly increased (40-65%) the maximal number of D2-specific [3H]spiperone binding sites in the n. accumbens and the dorsolateral and ventrolateral regions of the striatum, whereas small increases (20-29%) were seen in the ventromedial, dorsomedial, rostral and caudal regions of the striatum. Chronic treatment with clozapine, 20 mg/kg/day, i.p., did not change the maximal number of D2 receptors in the n. accumbens or any region of the striatum. Chronic treatments with clozapine produced a decrease in the maximal number of cortical 5-HT2 receptors to 55% of control whereas haloperidol had no effect. This study demonstrates regional differences in the up-regulation of striatal D2 receptors following chronic treatment with haloperidol and different effects of a typical and atypical neuroleptic on 5-HT2 receptors following acute and chronic treatments.

Clozapine antagonism of D-1 and D-2 dopamine receptor-mediated behaviors

When tested in rats supersensitive to dopamine agonists, the atypical neuroleptic clozapine displayed pharmacological properties expected of both a D-1 and D-2 receptor antagonist. The locomotor response induced by the D-1 receptor agonist SKF-38393 in neonatal-6-hydroxydopamine (6-OHDA)-lesioned rats was reversed in a dose-related fashion, although a complete blockade of this behavior was not observed indicative for only a partial antagonism of D-1 receptor function. Clozapine also blocked the self mutilation resulting from L-dihydroxyphenylalanine (L-DOPA) administration to neonatal-6-OHDA-lesioned rats, an effect previously linked to D-1 receptor activation. At higher doses, clozapine blocked the locomotor activity elicited by the D-2 agonist LY-171555 in adult-6-OHDA-lesioned rats. Therefore, the action of clozapine on D-1 as well as D-2 receptor-mediated behaviors contributes to its pharmacological effects. The ability of clozapine to stop self-mutilatory behavior in neonatal-6-OHDA-lesioned rats suggests that this drug might be an effective treatment for self-injurious behavior associated with the Lesch-Nyhan syndrome and mental retardation.

Clinical studies on the mechanism of action of clozapine: the dopamine-serotonin hypothesis of schizophrenia

Clozapine administration to schizophrenic patients was found to produce dopamine2 (D-2) and serotonin2 (5-HT2) receptor blockade, as evidenced by the ability to block the increases in growth hormone and cortisol secretion produced by apomorphine and MK-212, respectively, direct acting dopamine (DA) and 5-HT2 agonists. Clozapine did not increase plasma prolactin (PRL) levels nor did it block the apomorphine-induced decrease in plasma PRL concentration, as would be expected from a D-2 receptor antagonist. These PRL results are consistent with the observation that clozapine may increase DA release. Clozapine also decreased plasma tryptophan, plasma homovanillac acid (HVA) and basal plasma cortisol levels. Rodent studies suggest clozapine also increases 5-HT release. We hypothesize that antagonism of D-2 and 5-HT2 receptors and enhancement of DA and 5-HT release are critical elements in the action of clozapine to minimize both positive and negative symptoms without producing significant extrapyramidal symptoms or plasma PRL increases. It is proposed that schizophrenia may also involve a dysregulation of 5-HT2- and D-2-mediated neurotransmission, and that a more normal balance in serotonergic and dopaminergic neurotransmission is at least partially restored by clozapine.

Effect of typical and atypical antipsychotic drugs on 5-HT2 receptor density in rat cerebral cortex

The effect of acute treatment with seven atypical antipsychotic drugs and four typical antipsychotic drugs on serotonin2 (5-HT2) receptor binding sites in rat cerebral cortex was studied. Among the atypical antipsychotic drugs examined, clozapine, fluperlapine, RMI-81582 and setoperone decreased the density of 5-HT2 receptors, but ticspirone, amperozide and melperone did not. None of the drugs affected the Kd value. Among the typical antipsychotic drugs, loxapine decreased Bmax and increased the Kd of 5-HT2 receptor binding sites, whereas chlorpromazine and cis-flupenthixol had no effect. Clothiapine, a typical antipsychotic drug of the same chemical class as clozapine, decreased Bmax without increasing Kd. The downregulation of 5-HT2 receptor binding sites following a single injection of clozapine, 20 mg/kg, remained almost unchanged during the first 72 hrs and was still significantly decreased for up to 120 hrs. There was no relationship between the affinity for the downregulation of rat cortical 5-HT2 receptor binding site and 5-HT2 receptor density. Coadministration of the D1 dopamine agonist, SKF-38393, did not affect the clozapine-induced downregulation. It is suggested that rapid and prolonged downregulation of 5-HT2 receptor sites is characteristic of some but not all atypical antipsychotic drugs and is not specific to atypical antipsychotic drugs. Dibenzo-epines (clozapine, loxapine, amoxapine, chlothiapine) consistently downregulate 5-HT2 receptors in frontal cortex after acute treatment.

Effects of 5-HT1A agonists and 5-HT2 antagonists on haloperidol-induced dyskinesias in squirrel monkeys: no evidence for reciprocal 5-HT-dopamine interaction

Dyskinetic movements and dystonic postures may be induced by neuroleptics in monkeys that have undergone previous neuroleptic treatment, and these motor abnormalities constitute a primate model of drug-induced extrapyramidal symptomatology. In view of previous suggestions that brain serotonergic systems may tonically inhibit dopamine neurons, the effects of several new and selective 5-HT2 receptor antagonists and 5-HT1A receptor agonists were investigated in this model. Setoperone, a dopamine D2 receptor antagonist with extremely potent 5-HT2 antagonism, caused dyskinetic movements. Although ritanserin is a potent 5-HT2 antagonist with very weak dopamine antagonist properties, this drug did not antagonize dyskinesias but induced them when administered at a high dose (30 mg/kg). Buspirone induced dyskinesias and blocked apomorphine-induced climbing, supporting prior reports that it has dopamine antagonist effects. Gepirone, a 5-HT1A agonist with less marked dopamine antagonist properties, induced dyskinesias in only one of six monkeys at 30 mg/kg and did not block haloperidol-induced dyskinesias. 8-OH-DPAT partly attenuated haloperidol-induced dyskinesias, an effect possibly attributable to its weak dopamine agonist properties. Tonic inhibition of brain extrapyramidal dopamine systems by serotonin systems does not appear to characterize neuroleptic-related dyskinesias in squirrel monkeys.

Antagonism of serotonin receptor mediated neuroendocrine and temperature responses by atypical neuroleptics in the rat

The ability of atypical and typical antipsychotics to antagonize serotonin (5-HT) receptor-mediated temperature and neuroendocrine responses was tested in rats. Clozapine, melperone and setoperone, three atypical neuroleptics, blocked in a dose-dependent manner, the hyperthermic response to the 5-HT agonist, MK-212, whereas chlorpromazine and haloperidol were ineffective. The hypothermic response to the 5-HT1A agonist, 8-OH-DPAT, was unaltered by any of the atypical neuroleptics tested. Similarly, MK-212-induced corticosterone secretion was blocked in a dose-related manner by clozapine, melperone and setoperone but was relatively unaffected by either haloperidol or chlorpromazine. The increase in corticosterone secretion observed following 8-OH-DPAT administration was not attenuated by pretreatment with the atypical or typical antipsychotics tested. These data indicate that atypical neuroleptics are effective 5-HT2 but not 5-HT1A antagonists in vivo. Conversely, the typical neuroleptics, haloperidol and chlorpromazine do not block the 5-HT receptors involved in activation of the hypothalamic-pituitary-adrenal axis or thermoregulation.

Central dopamine systems and gastric stress pathology in rats

Acute treatments with haloperidol (1 mg/kg), clozapine (10 mg/kg) and metoclopramide (10 mg/kg) significantly facilitated cold-restraint-induced gastric ulcer formation in rats. In addition, haloperidol and clozapine also produced gastric mucosal erosions in non-stressed rats. Bilateral lesions of the ventral tegmental area (VTA) and substantia nigra also aggravated stress ulcerogenesis--VTA lesions also being effective in inducing gastric ulcers in non-stressed rats. Long-term treatment with dopaminergic blockers showed variable effects. Clozapine potentiated the gastric stress pathology, whereas no significant facilitation was observed with haloperidol or metoclopramide. In addition, withdrawal from haloperidol did not influence the gastric ulcer formation when compared to controls. The role of central dopaminergic involvement in gastric stress pathology is discussed in light of the present results.

Differential effects of haloperidol, clozapine, and fluperlapine on tuberoinfundibular dopamine neurons and prolactin secretion in the rat

Two atypical neuroleptic agents, clozapine and fluperlapine, produced rapid elevations in plasma PRL concentrations that were similar in magnitude to those produced by haloperidol. However, the PRL response to clozapine or fluperlapine was of much shorter duration than that elicited by haloperidol. Clozapine, but neither fluperlapine nor haloperidol, produced a rapid increase in the activity of tuberoinfundibular dopamine (TIDA) neurons, as evidenced by an enhanced accumulation of dihydroxyphenylalanine (DOPA) in the median eminence after the inhibition of DOPA decarboxylase. The clozapine-induced increase in DOPA accumulation was evident within 30 minutes after its administration and persisted for at least 4 hours. The clozapine-induced increase in the activity of TIDA neurons may account, in part, for the abbreviated PRL response to this neuroleptic. In addition, ability to produce a short-lived increase in PRL secretion in the rat appears to be common to the atypical neuroleptic drugs.

Evidence against serotonin involvement in the hyperactivity produced by injections of muscimol into the median raphe nucleus

Microinjections of muscimol into the median raphe nucleus were found to result in pronounced hyperactivity which could not be attenuated by the serotonin depletion produced either by systemic treatment with p-chlorophenylalanine or by intra-raphe injections of 5,7-dihydroxytryptamine. Furthermore, hyperactivity could not be produced by intra-median raphe injections of serotonin or of fenfluramine, compounds which would be expected to inhibit serotonergic raphe cells. These results argue strongly against an essential involvement of serotonin in mediating the effects of intra-median raphe muscimol injections. Muscimol failed to produce hyperactivity, however, when injected into rats who had previously received an electrolytic median raphe lesion. This finding suggests that muscimol injected into the median raphe produces hyperactivity as a result of an action on local cell bodies, rather than by diffusion to a distant site. The simplest explanation of the current results is that muscimol injected into the median raphe produces hyperactivity as a result of an inhibition of nonserotonergic cells within the median raphe nucleus.

CGS 10746B: an atypical antipsychotic candidate that selectively decreases dopamine release at behaviorally effective doses

CGS 10746B, a benzothiadiazepine, has a behavioral profile in mice and monkeys similar to the atypical antipsychotic clozapine. Unlike clozapine, CGS 10746B suppresses dopamine neuron firing rates and, when administered at behaviorally effective doses by the oral or intraperitoneal route, decreases neostriatal dopamine release without changing dopamine metabolism or occupying D2 receptors. CGS 10746B is the first atypical antipsychotic candidate that selectively decreases dopamine release.

Differential response of amygdaloid neurons to clozapine and haloperidol: effects of repeated administration

Rats were pretreated with saline or with behaviorally equivalent doses of clozapine (10.0 mg/kg) or haloperidol (1.0 mg/kg) twice daily for six consecutive days. On the following day, amygdaloid neurons in clozapine-pretreated rats responded to a challenge injection of this drug with a significantly greater increase in firing rate than saline controls. In contrast, amygdaloid neurons generally remained unresponsive to haloperidol even when pretreatment with this drug was extended to 13 days. Neither clozapine nor haloperidol pretreatment, however, altered the response of amygdaloid neurons to d-amphetamine administered after a four-day washout period. Amphetamine inhibited amygdaloid activity to a comparable extent in all rats. Taken together, these results implicate the amygdaloid complex as an important site of action of clozapine and related antischizophrenic drugs.

125I-LSD autoradiography confirms the preferential localization of caudate-putamen S2 receptors to the caudal (peripallidal) region

The in vitro binding of 125I-lysergic acid diethylamide (LSD) to horizontal sections of rat brain was quantified with computer-assisted autoradiography. Specific binding of 125I-LSD to D2 and S2 sites, defined with 5 microM (+)-butaclamol, was 65-94% of the total binding. Identification of S2 sites with 50 nM ketanserin showed that over 90% of the butaclamol-displaced 125I-LSD binding in the frontal, cingulate and parietal neocortex was to S2 sites (22-55 fmol/mg protein). 125I-LSD also labeled a dense population of S2 sites (16 fmol/mg protein) in the caudal caudate-putamen at the level of the globus pallidus which exceeded by 5-fold the concentration of S2 sites (3 fmol/mg protein) in more rostral portions of the caudate-putamen. The peripallidal distribution of S2 sites was identical to that observed previously with the less selective S2 label, [3H]spiperone. The dense concentration of S2 sites in the caudal caudate-putamen and their overlap with D2 binding sites indicates that the peripallidal neostriatum may play an important role in interactions between dopamine and serotonin.

Typical and atypical antipsychotic occupancy of D2 and S2 receptors: an autoradiographic analysis in rat brain

In thin sections of rat brain, [3H]spiperone binds to D2 sites in the basal ganglia (caudate-putamen, nucleus accumbens, olfactory tubercle) and S2 sites in the claustrum and motor cortex. The in vitro displacement of [3H]spiperone from these regions was quantified autoradiographically with the "atypical" neuroleptics clozapine and thioridazine, which ameliorate psychosis, a "typical" neuroleptic, haloperidol, which also induces extrapyramidal side effects, or with metoclopramide, which induces extrapyramidal side effects but is an ineffective antipsychotic. Whereas metoclopramide was equipotent at D2 sites, haloperidol was less potent and clozapine and thioridazine more potent by 2- to 3-fold at competing for D2 sites in the nucleus accumbens or olfactory tubercle than in the caudate-putamen. As measured autoradiographically or with tissue homogenates, clozapine, thioridazine, and five other atypical neuroleptics were 4- to 800-times more potent at competing for S2 sites in the frontal cortex than for D2 sites in the basal ganglia. A preference of atypical antipsychotics for D2 receptors in the nucleus accumbens and olfactory tubercle and for the S2 receptor may explain the relative lack of extrapyramidal side effects produced by these compounds.

Comparison of circling induced by unilateral intrastriatal microinjections of haloperidol, clozapine and CCK-8 in rats

The existence of the neuropeptide cholecystokinin (CCK) within a subpopulation of central dopamine (DA) neurons has led to speculations that the peptide may serve as an endogenous modulator of DA functions. To test this possibility, the present study examined the pharmacological action of CCK-8 by comparing its effects on DA-mediated circling behavior with those of a typical (haloperidol; HAL) and an atypical (clozapine; CLZ) dopamine antagonist neuroleptic drug. Rats received unilateral intrastriatal infusions of either sulfated CCK-8 (1, 2, or 8 micrograms), HAL (5 micrograms) or CLZ (5 or 20 micrograms) 15 minutes after systemic injection of d-amphetamine (1 mg/kg). Animals were then placed into rotational chambers where the number and direction of complete 360 degree turns was automatically recorded over a 1 hour session. HAL produced strong and almost exclusive ipsilateral circling while the responses after CLZ and CCK-8 were reliably more variable in rotational direction. More specifically, the results suggest that CLZ is only a weak antagonist of behaviors mediated by striatal DA activation while CCK seems to be devoid of antidopaminergic properties in the striatum.

Locomotor effects of lisuride: a consequence of dopaminergic and serotonergic actions

The open-field test was used to study the involvement of serotonergic and dopaminergic mechanisms in the action of lisuride on locomotor activity in the rat. Lisuride produced a biphasic locomotor effect. The maximum locomotor stimulatory response of lisuride was stronger than that of apomorphine and comparable with that of apomorphine and LSD combined. Hypermotility induced by high doses of lisuride was partially suppressed by the serotonin antagonist cyproheptadine and not further enhanced by LSD. A moderate dose of lisuride potentiated apomorphine-induced hypermotility in the same manner as has been shown for LSD. Lesion of dopaminergic structures within the median raphe nucleus by 6-OHDA produced a potentiation of lisuride-induced hypermotility. This effect was suppressed by cyproheptadine. The locomotor inhibitory effect of low doses of lisuride may be related to a stimulation of presynaptic mesolimbic dopamine receptors. It is concluded that the locomotor stimulant effect of higher doses of lisuride may depend on stimulation of postsynaptic dopamine receptors and a serotonergic action and that the locomotor effects of lisuride reflect a complex interaction at dopaminergic and serotonergic transmission systems.