Clozapine: an atypical neuroleptic

Toward personalized medicine in schizophrenia: Genetics and epigenetics of antipsychotic treatment

Schizophrenia is a complex psychiatric disorder where genetic, epigenetic, and environmental factors play a role in disease onset, course of illness, and treatment outcome. Pharmaco(epi)genetic research presents an important opportunity to improve patient care through prediction of medication side effects and response. In this narrative review, we discuss the current state of research and important progress of both genetic and epigenetic factors involved in antipsychotic response, over the past five years. The review is largely focused on the following frequently prescribed antipsychotics: olanzapine, risperidone, aripiprazole, and clozapine. Several consistent pharmacogenetic findings have emerged, in particular pharmacokinetic genes (primarily cytochrome P450 enzymes) and pharmacodynamic genes involving dopamine, serotonin, and glutamate neurotransmission. In addition to studies analysing DNA sequence variants, there are also several pharmacoepigenetic studies of antipsychotic response that have focused on the measurement of DNA methylation. Although pharmacoepigenetics is still in its infancy, consideration of both genetic and epigenetic factors contributing to antipsychotic response and side effects no doubt will be increasingly important in personalized medicine. We provide recommendations for next steps in research and clinical evaluation.

Prescribed drugs containing nitrogen heterocycles: an overview

Heteroatoms as well as heterocyclic scaffolds are frequently present as the common cores in a plethora of active pharmaceuticals natural products. Statistically, more than 85% of all biologically active compounds are heterocycles or comprise a heterocycle and most frequently, nitrogen heterocycles as a backbone in their complex structures. These facts disclose and emphasize the vital role of heterocycles in modern drug design and drug discovery. In this review, we try to present a comprehensive overview of top prescribed drugs containing nitrogen heterocycles, describing their pharmacological properties, medical applications and their selected synthetic pathways. It is worth mentioning that the reported examples are actually limited to current top selling drugs, being or containing N-heterocycles and their synthetic information has been extracted from both scientific journals and the wider patent literature.

Clozapine pre-treatment has a protracted hypolocomotor effect on the induction and expression of amphetamine sensitization

Amphetamine locomotor sensitization is an animal model for the study of addiction and schizophrenia. The antipsychotic clozapine blocks the hyperlocomotion induced by an acute injection of amphetamine, but its effect on locomotor sensitization after repeated amphetamine administration remains unknown. In the present study we investigate the effect of repeated administration of clozapine on the induction and expression of amphetamine locomotor sensitization. We propose that repeated administration of clozapine blocks the induction and expression of amphetamine sensitization. Male Sprague-Dawley rats were classified according to their locomotor response to an acute saline injection in high responder saline (HRS) or low responder saline (LRS). Rats from both groups were injected once daily with amphetamine for 5 consecutive days. Horizontal locomotor activity was measured during 40 min. Four days after the last injection, an acute dose of amphetamine was administered to assess the expression of sensitization. Clozapine was injected once daily for 4 consecutive days before (pre-treatment) or after (treatment) induction of sensitization. Pre-treatment with clozapine significantly decreases both acute amphetamine-induced hyperlocomotion and the induction and expression of amphetamine sensitization only in LRS rats, showing a protracted hypolocomotor effect. On the other hand, clozapine treatment had no effect over locomotor response on the expression of amphetamine sensitization in either LRS or HRS rats. These data suggest that clozapine effect on amphetamine locomotor response depends on individual differences. Also, our results suggest that clozapine pre-treatment attenuates the neuroplasticity underlying amphetamine sensitization, but clozapine treatment is unable to reverse these changes once amphetamine sensitization has been induced.

Chronic cannabinoid exposure reduces phencyclidine-induced schizophrenia-like positive symptoms in adult rats

RATIONALE

Chronic cannabis use can induce psychotic states that resemble schizophrenia. Yet, schizophrenic patients often smoke cannabis as a form of self-medication to counter the aversive symptoms of schizophrenia. We recently demonstrated an ameliorating effect of cannabinoid self-administration (SA) on negative and cognitive schizophrenia-like symptoms induced experimentally by the non-competitive N-methyl-D-aspartate receptor antagonist phencyclidine (PCP). Whether cannabinoid SA alleviates or exacerbates schizophrenia-like positive symptoms is still unclear.

OBJECTIVES

This follow-up study aimed to evaluate the effect of self-administered cannabinoid on PCP-induced schizotypic positive symptoms in adult rats.

METHODS

Male rats were trained to self-administer either the cannabinoid CB1 receptor agonist WIN 55,212-2 (WIN; 12.5 μg/kg/infusion) or its vehicle (Veh) intravenously. The effects of acute and chronic intermittent intraperitoneal administration of PCP (2.5 mg/kg) on motor parameters were then tested in Veh-SA and WIN-SA.

RESULTS

Cannabinoid SA significantly attenuated the psychotomimetic effects of PCP exposure observed in control rats. Following acute PCP administration, WIN-SA animals displayed more frequent rearing and lower anxiety-like profile than Veh-SA rats. WIN-SA rats also exhibited lower behavioural sensitisation to chronic PCP treatment as demonstrated by reduced hyperlocomotion in response to an acute PCP challenge. In addition, parallel experiments performed in experimenter-administered rats that received WIN at comparable SA doses confirmed the ameliorating effects of cannabinoid exposure on PCP-induced schizotypic behaviours, indicating that motivational effects were not responsible for the ameliorative effects of cannabinoids.

CONCLUSIONS

Our results indicate that cannabis may exert protective effects on positive schizotypic symptoms in adult animals such as hypermotility and anxiety state.

Clozapine and olanzapine exhibit an intrinsic anxiolytic property in two conditioned fear paradigms: contrast with haloperidol and chlordiazepoxide

Psychotic fear and anxiety disturbances are seen at a relatively high frequency in patients with schizophrenia. Atypical anti-psychotics are believed to show superior efficacy in reducing these symptoms. However, clinical and preclinical evidence regarding their anxiolytic efficacy has been mixed. In this study, we evaluated the possible anxiolytic property of two atypicals clozapine and olanzapine and compared them with typical haloperidol and chlordiazepoxide (a prototype of sedative-anxiolytic drug) in two preclinical models of fear. In Experiment 1, we used a fear-induced passive avoidance and conditioned place aversion paradigm and examined the effects of clozapine (20 mg/kg, sc), haloperidol (0.05 mg/kg, sc) and chlordiazepoxide (10 mg/ kg, ip). In Experiments 2 and 3, we used a two-way active avoidance conditioning paradigm and further compared the effects of clozapine (20 mg/kg, sc), haloperidol (0.05 mg/kg, sc), chlordiazepoxide (10 mg/kg, ip) and three doses of olanzapine (0.5, 1.0, and 2.0 mg/kg, sc). Results show that clozapine and chlordiazepoxide, but not haloperidol, significantly attenuated the shock conditioning-induced place aversion, decreased the amount of defecations and the number of the 22-kHz vocalizations. Clozapine also reduced the shock conditioning-induced hyperthermia. Similar to clozapine, olanzapine also significantly decreased the amount of defecations and reduced the shock conditioning-induced hyperthermia, but it did not inhibit the 22-kHz vocalizations. This study demonstrates that clozapine and olanzapine possess an intrinsic anxiolytic property, which is not attributable to its superior anti-"psychotic" effect or its favorable effects on motor functions or learning and memory processes. These findings also suggest that the combined use of passive avoidance and active avoidance conditioning models can be useful in better differentiating typical and atypical anti-psychotics as well as anxiolytics.

Prevention of clozapine-induced granulocytopenia/agranulocytosis with granulocyte-colony stimulating factor (G-CSF) in an intellectually disabled patient with schizophrenia

BACKGROUND

While clozapine is an effective treatment for refractory schizophrenia, its use is limited by haematological side effects. Treatment options that allow continued prescription of clozapine by tackling these side effects will greatly aid patients for whom this medication is all too often their only hope of recovery.

METHOD

In this case report, we describe what we believe are two 'firsts' in the clozapine literature: the use of granulocyte-colony stimulating factor on a prophylactic basis in an intellectually disabled patient receiving clozapine for refractory schizophrenia.

RESULT

Treatment with granulocyte-colony stimulating factor prevented discontinuation of clozapine, enabling our intellectually disabled patient's recovery from a schizophrenic illness.

Cannabinoid CB1 receptor dependent effects of the NMDA antagonist phencyclidine in the social withdrawal model of schizophrenia

Clinical and laboratory findings suggest that cannabinoid signalling is implicated in schizophrenia. However, the interaction remains poorly understood, as data are often contradictory. Here we investigated wild-type (WT) and cannabinoid CB1 receptor-knockout (CB1-KO) mice in the phencyclidine-induced social withdrawal model of schizophrenia. N-methyl-D-aspartate (NMDA) antagonists (including phencyclidine) induce psychotic symptoms in humans, and are used to model schizophrenia in a variety of experimental conditions. In WTs, 5 mg/kg phencyclidine increased locomotion and stereotyped behaviours, and decreased social interactions. These changes are consistent with a schizophrenia-like effect. In CB1-KOs, phencyclidine decreased locomotion, enhanced ataxia and stereotypy more markedly than in WTs, but did not affect social interactions. Locomotion showed a significant negative correlation with both ataxia and stereotypy, suggesting that in CB1-KOs, the locomotor suppressive effect of phencyclidine was secondary to changes in these variables. Our findings demonstrate that CB1 gene disruption dramatically alters the behavioural effects of the NMDA antagonist phencyclidine, suggesting that the CB1 receptor is involved in schizophrenia. As social disruption and stereotypy respectively are believed to model negative and positive symptoms of schizophrenia, our findings tentatively suggest that cannabinoids are differentially involved in these two symptom categories. These findings require verification by experiments involving CB1 receptor blockers, as the genetic and pharmacological blockade of receptors may not always provide similar results.

Interactions between environmental stimulation and antipsychotic drug effects on forebrain c-fos activation

The immediate-early gene product Fos is differentially induced in the rat brain by the antipsychotic drugs haloperidol and clozapine. It is often claimed that although both drugs induce Fos in the nucleus accumbens, haloperidol but not clozapine increases Fos-like immunoreactivity in the striatum, whereas clozapine but not haloperidol increases Fos-like immunoreactivity in prefrontal cortex. Investigations of antipsychotic drug effects on Fos have typically administered high doses with pronounced sedative effects to behaviorally naive animals. In the present study, we compared the effects of low doses of haloperidol (0.1 mg/kg) and clozapine (5 mg/kg) on Fos-like immunoreactivity in rats which were either behaviorally naive, exposed to a novel environment or tested for two-way active avoidance. We determined that haloperidol increased Fos in the striatum and nucleus accumbens regardless of testing condition whereas clozapine markedly reduced the induction of Fos by behavioral testing in these regions; moreover, haloperidol dramatically increased prefrontal cortical Fos expression in animals placed in a novel environment, but not in testing-naive controls. From these results we suggest that antipsychotic drug-induced patterns of Fos expression in the rat are highly dependent on animals' concurrent behavioral status, perhaps reflecting neuroanatomically specific interactions between antipsychotic drugs and environmental stressors which also may occur in the schizophrenic condition.

Low-dose clozapine pretreatment partially prevents haloperidol-induced deficits in conditioned active avoidance

The effectiveness of neuroleptics in disrupting conditioned active avoidance has led to the widespread use of this test as an index of antipsychotic efficacy, whereas the tendency for these drugs to induce catalepsy is believed to reflect their propensity to cause extrapyramidal motor side-effects. Although the typical neuroleptic haloperidol produces catalepsy as well as profound deficits in conditioned active avoidance, the atypical neuroleptic clozapine does not induce catalepsy and is less effective than haloperidol in disrupting active avoidance. Furthermore, clozapine pretreatment prevents haloperidol-induced catalepsy. We investigated whether clozapine pretreatment might also reduce the disruptive effects of haloperidol on two-way active avoidance. We assessed the avoidance acquisition of the following drug treatment groups in which all animals received two injections prior to testing: vehicle + vehicle, vehicle + haloperidol (0.1 mg/kg, i.p.), clozapine (2.5, 5.0 or 10 mg/kg, i.p.) + haloperidol (0.1 mg/kg, i.p.), or clozapine (2.5, 5.0 or 10 mg/kg, i.p.) + vehicle. Haloperidol-pretreated animals showed markedly impaired active avoidance, deficits which were improved by 2.5 and 5 mg/kg but not by 10 mg/kg clozapine pretreatment. These data suggest that the disruptive effects of haloperidol on conditioned active avoidance partially mirror its capacity to induce catalepsy and extrapyramidal motor symptoms. Furthermore, this study indicates that clozapine may be effective in reducing motor side-effects caused by typical neuroleptics.

Oxidation sensitivity may be a useful tool for the detection of the hematotoxic potential of newly developed molecules: application to antipsychotic drugs

Some antipsychotic agents have been found to produce agranulocytosis and aplastic anemia. The oxidation phenomena and/or the formation of free radicals has been suggested to be causally related to various hematological disorders, e.g., agranulocytosis. Using five experimental conditions, we tested the oxidative potential of compounds with and without a history of hematological side effects, e.g., agranulocytosis and aplastic anemia. A statistical analysis was undertaken for each experimental condition and a multivariate analysis combining all results was performed. Two peroxidase-induced free radical models did not successfully discriminate between drugs with and without a history of causing hematologic problems (<70%). The lipid peroxidation system provided even less satisfactory discrimination, with only 56.25% correct classification. However, an 87.5% correct classification was obtained when using the oxidation potentials of these drugs determined at pH 4.7 and at pH 7.4. A multivariate analysis taking into account the five variables provided 87.5% success in classification. The two clusters were better discriminated in terms of a "distance coefficient." In a second analysis, the putative antipsychotic pyridobenzodiazepine analogues (JL5, JL8, JL18, and JL25) were classified in the cluster of toxic compounds, while the oxa- and thiazepine analogues (JL2, JL3, and JL13) were classified as nontoxic compounds. On the other hand, a few metabolites of clozapine and fluperlapine were classified in the toxic compound group. The procedure described herein is, to our knowledge, the first which classifies molecules of different structures as well as different pharmacological profiles according to their hematotoxic potential. Such a procedure could be used to predict drug-induced hematological side effects.

Clozapine: dopamine D1 receptor agonism in the prefrontal cortex as the code to decipher a Rosetta stone of antipsychotic drugs

A large number of ligand binding studies have shown that clozapine has a number of receptor affinities, including those of the dopamine (DA) D1 and D2 receptor families. The study of intrinsic efficacy at these receptors is less straight-forward. In the experiments summarised here, evidence is presented that clozapine behaves as an agonist at DA D1 receptors. Thus, the hypothermia produced by clozapine (2.5 mg kg(-1)) in the rat is fully antagonised by either of the selective DA D1 receptor antagonists SCH-23390 (0.1 mg kg(-1)) or NNC-687 (4 mg kg(-1)). These results provide an intriguing explanation for the clinical profile of clozapine as an atypical antipsychotic drug. Thus, there are supporting clinical and laboratory observations implicating DA D1 receptors in the prefrontal cortex in cognitive functions. Finally, clozapine displays features with regard to extrapyramidal motor mechanisms, and seizure thresholds, that could be explained by its properties as a DA D1 receptor agonist.

Enhancement of latent inhibition in the rat at a high dose of clozapine

The present experiments investigated clozapine (2.5, 5 and 10 mg/kg) and haloperidol (0.1 mg/kg) administration on latent inhibition (LI) in rats. Clozapine's ability to antagonize amphetamine-induced disruption of LI was also assessed. A conditioned emotional response procedure was employed. In the pre-exposure stage, 'pre-exposed' rats received 10 (Experiment 1) or 40 (Experiment 2) presentations of a flashing light stimulus without reinforcement. During the conditioning phase, the light stimulus was paired with a footshock. At test, LI was expressed by the extent of suppression of water licking during flashing light presentation. Both clozapine (10 mg/kg) and haloperidol (0.1 mg/kg) significantly facilitated LI. In addition, clozapine significantly reversed the disruption of LI induced by amphetamine (1.0 mg/kg). These results with clozapine illustrate that LI is sensitive to antipsychotics which differ in their mode of action and furthermore emphasize the value of LI as a test model for detecting the antipsychotic potential of novel drugs.

Neuroethological assessment of amphetamine-induced behavioral changes and their reversal by neuroleptics: focus on the amygdala and nucleus accumbens

1. An ethological approach was combined with intracerebral infusions of amphetamine to broaden understanding of how this drug acts on mesolimbic neuronal systems to alter behavior. 2. Rats, tested in sets of three, were allowed to interact with each other or with various novel objects in an open-field arena. Specific behavioral responses were assessed and grouped into several broad categories: motivation (movement directed toward novel objects), social (movement involving contact with other rats), and motor (movement without obvious direction toward environmental stimuli) as well as no movement (quiet rest). 3. Infusion of d-amphetamine (10 micrograms/microliter) into either the amygdala or nucleus accumbens elevated motor behavior relative to control rats in the set, but only amygdaloid infusions also increased the motivation score. Intra-amygdaloid clozapine or haloperidol blocked the increase in this score, but only clozapine also blocked the motor effects of intra-amygdaloid amphetamine. 4. Although neither neuroleptic in the accumbens blocked the amphetamine-induced increase in the motor category, both clozapine and haloperidol lowered the motivation score below the amphetamine level. 5. The results suggest a role for the amygdala in the motivational component of amphetamine-induced behavioral effects. Both neuroleptics, moreover, appear to reverse this component perhaps by acting via either amygdaloid or accumbal mechanisms. Although follow-up studies are warranted, a neuroethological approach is likely to shed new light on the neuronal systems underlying the complex behavioral changes induced by amphetamine and related stimulants.

Olanzapine attenuates the reinforcing effects of cocaine

The possibility that the atypical neuroleptic olanzapine can antagonize the ability of cocaine to produce both conditioned place preference and self-administration in rats was investigated. Pre-treatment with olanzapine (3.0, 4.5 mg/kg, but not 1.5 mg/kg) significantly attenuated conditioned place preference produced by cocaine (10 mg/kg). However, the higher dose of olanzapine administered alone resulted in conditioned place aversion. Pre-treatment with olanzapine also produced a dose-dependent decrease in cocaine self-administration (0.33 mg/infusion) under a fixed-ratio 2 schedule of reinforcement. Olanzapine produced a similar dose-responsive attenuation in operant responding for food (fixed-ratio 10) suggesting that olanzapine produces a nonspecific decrease in operant behavior. Pre-treatment with 4.5 mg/kg olanzapine significantly attenuated cocaine-induced hyperactivity, whereas lower olanzapine doses had little effect upon cocaine-induced hyperactivity. These results suggest that pre-treatment with olanzapine is capable of blocking the reinforcing effects of cocaine and illustrates the value of using multiple tests of reinforcement when evaluating the pharmacological effects of newer psychotherapeutic agents.

JL 13, a potential successor to clozapine, is less sensitive to oxidative phenomena

The oxidation behaviour of JL 13, a promising antipsychotic, was investigated in comparison with clozapine and loxapine, by measuring their direct "radical scavenging" abilities and their efficacies in inhibiting the lipid peroxidation. In the lipid peroxidation system, the reactivity of these compounds with free radicals produced by gamma-irradiation of linoleic acid may be presented as follows: JL 13 = loxapine < clozapine. In two enzymatic systems (HRP/GSH and HRP/H2O2/ GSH) which generate the thiyl free radicals, clozapine produces a strong enhancement of the thiyl-radical EPR signal intensity while JL 13 and loxapine exhibit no or minimal effect on this signal. The redox potential values for the three derivatives confirm the spectro-photometric and EPR results. Following this study, we show that JL 13, although presenting a preclinical clozapine-like profile, appears less sensitive to oxidation than clozapine.

The latent inhibition model of schizophrenia: further validation using the atypical neuroleptic, clozapine

Latent inhibition (LI) refers to retarded conditioning to a stimulus that has been repeatedly presented without reinforcement. LI is impaired in schizophrenia patients and in rats treated with amphetamine. Neuroleptic drugs produce two effects in this test paradigm: antagonism of amphetamine-induced disruption of LI, and enhancement of LI when administered on their own. The present experiments tested the effects of the atypical neuroleptic, clozapine, on LI. The experiments used a conditioned emotional response procedure in rats licking for water, consisting of three stages: preexposure, in which the to-be-conditioned stimulus (tone) was repeatedly presented without reinforcement; conditioning, in which the preexposed stimulus was paired with reinforcement (foot shock); and test, in which LI was indexed by animals' degree of suppression of licking during tone presentation. In experiments 1 and 2, the effects of 5.0 and 10.0 mg/kg clozapine on LI were assessed following 20 or 10 tone preexposures, respectively. Experiments 3 and 4 used 40 preexposures and investigated antagonism of amphetamine-induced disruption of LI by 5.0 and 10.0 mg/kg clozapine, respectively. The results demonstrated that clozapine possesses a neuroleptic profile in the LI model, namely, it facilitates the development of LI and antagonizes amphetamine-induced disruption of LI.

Neutropenia and agranulocytosis in patients receiving clozapine in the UK and Ireland

BACKGROUND

Clozapine can cause reversible agranulocytosis and neutropenia. This study documents the occurrence of blood dyscrasias and identifies predisposing risk factors.

METHOD

An analysis was made of the haematological, demographic, and dosage data from a central database on 6316 patients receiving clozapine over four and a half years in the UK and Ireland.

RESULTS

During the study period, 2.9% of the patients developed neutropenia and 0.8% developed agranulocytosis. The peak incidence of both disorders was in the first 6-18 weeks of treatment. Fatal agranulocytosis occurred in 0.03% of patients. After the first year of treatment, the incidence of agranulocytosis significantly decreased to the order noted with some phenothiazines.

CONCLUSIONS

The use of a patient monitoring service kept the haematological risks associated with using clozapine within acceptable limits, particularly in view of the benefits of this medication in treatment-resistant schizophrenia.

Amygdaloid neurons respond to clozapine rather than haloperidol in behaving rats pretreated with intra-amygdaloid amphetamine

Single-unit activity was recorded from the amygdaloid complex in freely moving rats during an infusion of amphetamine directly into the recording site. Relative to the quiet resting period prior to the infusion, amphetamine routinely increased neuronal activity within 5-15 min after infusion onset, and this response continued for at least another 30 min. It was generally accompanied by marked increases in sniffing, rearing, locomotion, and grooming as well as by a tendency to turn to the ipsilateral side. Haloperidol and clozapine, typical and atypical antipsychotic drugs, respectively, were then tested in their ability to reverse these neuronal and behavioral effects. Both antipsychotics were administered subcutaneously at behaviorally effective doses within 10 min after termination of the amphetamine infusion. Haloperidol (1.0 mg/kg) failed to reverse the amphetamine-induced increase in amygdaloid neuronal activity and required more than 20 min to exert a partial blockade of the accompanying behavioral activation. Clozapine (10.0 mg/kg), in contrast, blocked the excitatory effects of amphetamine on all tested neurons and also blocked most amphetamine-induced behaviors within 10 min. Taken together, these results, which support other lines of electrophysiological evidence, point to the amygdala as a critical site in the differential behavioral effects of typical and atypical antipsychotic drugs.

Serotonergic mechanisms in neuroleptic-induced catalepsy in the rat

The present paper reviews a series of experiments aimed at elucidating the interaction between specific dopamine (DA) and 5-hydroxytryptamine (5-HT) receptors in the mediation of extrapyramidal motor functions in the rat. There is strong evidence to suggest that (1) the catalepsy produced by dopamine D1 or D2 receptor antagonists can be completely antagonized by the administration of 5-HT1A receptor agonists acting at 5-HT autoreceptors in the median raphe nucleus; (2) the catalepsy produced by a dopamine D2 receptor antagonist can be completely antagonized by treatment with a 5-HT2A/C receptor agonist; and (3) the catalepsy produced by blockade of either dopamine D1 or D2 receptors is not affected by the administration of a 5-HT2A/C receptor antagonist. The emerging picture of DA/5-HT receptor interactions in the mediation of extrapyramidal motor functions is of great interest in relation to present efforts to develop new atypical neuroleptics with affinity for brain 5-HT receptor subtypes, and also for the observations that new serotonin selective re-uptake inhibiting antidepressants can produce parkinson-like symptoms in vulnerable individuals.

Antipsychotics and neuropeptides: the atypical profile of CI-943 and its relationship to neurotensin

CI-943 is a new drug candidate with antipsychotic-like activity in a variety of behavioural tests in rodents and primates, but without any affinity for brain dopamine receptors. CI-943 does not cause dystonia in monkeys, a predictive symptom of extrapyramidal side effects (EPS). Its mechanism of action remains unclear. Neurotensin (NT) concentration in nucleus accumbens and caudate is increased by CI-943; this may be associated with its antipsychotic effect. Indeed various observations suggest that the clinical action of antipsychotic drugs may at least be partially mediated by some neuropeptides. Various actions of neurotensin are reviewed. The hypothesis on the role of neurotensin represents a new strategy in the development of pharmacological tools for the treatment of schizophrenia.

Differential effects of repeated administration of novel antipsychotic drugs on the activity of midbrain dopamine neurons in the rat

Five potential antipsychotics (i.e. risperidone, olanzapine, seroquel, ziprasidone and amperozide) were given daily for 21 days to rats and the effect on the number of spontaneously active dopamine neurons in ventral tegmental area and substantia nigra pars compacta was determined. Standard electrophysiological measurements (i.e. single unit recording technique) were used. Risperidone, olanzapine and amperozide showed some selectivity (at one particular dose) for decreasing the number of active dopamine neurons in the ventral tegmental area. However, risperidone induced a U-shaped dose-response curve. The highest dose of amperozide inhibited the activity in substantia nigra pars compacta, showing a liability to induce extrapyramidal side-effects. Seroquel and ziprasidone inhibited the activity in both areas indicating a classical antipsychotic profile (i.e. high liability to cause extrapyramidal side-effects).

Continuous infusion of clozapine increases mu and delta opioid receptors and proenkephalin mRNA in mouse brain

The biochemical mechanisms involved in the actions of the atypical antipsychotic clozapine are still unclear. Because elevated levels of enkephalin in certain areas of the central nervous system may be necessary for antipsychotic activity, we have examined the effect of clozapine on certain receptors and mRNA transcripts involved in the opioid peptidergic system. Clozapine was infused continuously into mice for 21 days and the density of mu and delta opioid receptors was measured in the brains by quantitative receptor autoradiography, and the level of proenkephalin mRNA and dopamine D1 and D2 receptor mRNA were measured by in situ hybridization histochemistry. The results showed that continuous infusion of clozapine increased the density of D1 but not D2 receptors. However, it failed to alter the levels of either D1 or D2 dopamine receptor mRNA. By contrast, clozapine increased the density of mu and delta opioid receptors and increased the levels of proenkephalin mRNA. These results indicate that continuous treatment with clozapine increases opioid peptidergic activity in mouse brain and suggest that alteration of peptidergic activity as well as alteration of dopaminergic activity may be involved in its antipsychotic action.

Modulation of the clozapine structure increases its selectivity for the dopamine D4 receptor

Clozapine has a more marked affinity for the recently cloned dopamine D4 receptor than for the dopamine D2 receptor. In the search for a selective ligand for the dopamine D4 receptor, useful as a pharmacological tool or as a potent atypical antipsychotic, a pyridobenzodiazepine derivative bioisoster of clozapine, JL 18, 8-methyl-6-(4-methyl-1-piperazinyl)-11H-pyrido [2,3-b][1,4]benzodiazepine, was found to be the most dopamine D4-selective ligand belonging to the diarylazepine class. Indeed, JL 18 binds to the dopamine D4 receptor with affinity up to 25 times superior to that for the dopamine D2 receptor and presents reduced affinities for other receptors.

Peroxidase-catalysed oxidation of different dibenzazepine derivatives

According to a recent hypothesis suggesting the potential role of free radical formation in the clozapine-induced agranulocytosis, we have evaluated the susceptibility to the peroxidase-mediated oxidation of different dibenzazepine analogues. On the one hand, compounds with an arylamine group such as clozapine or isoclozapine present a high reactivity in the horseradish peroxidase or myeloperoxidase systems and, on the other hand, fluperlapine, though known to induce agranulocytosis, and other dibenzothiazepine and dibenzoxazepine derivatives appear insensitive to oxidation. Consequently, among tricyclic derivatives, the way of diaryloxa- and diarylthiazepine compounds could be an alternative for the development of safer drugs such as antipsychotics.

Differentiation of haloperidol and clozapine using a complex operant schedule in the dog

This study aimed to differentiate chronically administered typical (haloperidol) and atypical (clozapine) neuroleptics in the dog using a complex temporal regulation schedule combining operant, voluntary, and involuntary motor parameters. Although clozapine and haloperidol showed some characteristics of neuroleptics, justifying their adherence to the same class of compounds, differences have also been highlighted and compared to the clinical observations. Haloperidol induced catalepsy, tremor, dystony, hyperkinesia, and stereotypy. Subjects produced anticipated responses before any stimulus. Incomplete and delayed responses were also produced. An interpretation in terms of akathisia and anhedonia has been suggested. Clozapine induced tremor, exploration, dystony, and hypersalivation. Subjects produced disinhibitory responses to the negative stimulus and incomplete responses but these latter were submitted to tolerance. The simultaneous presence of tranquilizing and disinhibitory effects has been reported on the clinical potential of clozapine both in cases of positive and negative schizophrenic symptomatologies.

BMY-14802, a sigma ligand and potential antipsychotic drug, reverses amphetamine-induced changes in neostriatal single-unit activity in freely moving rats

The effects of BMY-14802 (5, 10, or 20 mg/kg), a sigma-receptor ligand showing preclinical evidence of antipsychotic efficacy, were tested on single-unit activity in the neostriatum of freely moving rats with or without pretreatment with 1.0 mg/kg D-amphetamine. Relative to resting baseline, amphetamine activated the large majority of neurons that changed firing rate in close temporal association with movement. All doses of BMY-14802 reversed this neuronal response, but the effect was most pronounced at 20 mg/kg. This dose, however, was equally likely to reverse or to induce a haloperidol-like potentiation of those neurons inhibited by amphetamine. In contrast, 10 mg/kg BMY-14802 consistently reversed amphetamine-induced neuronal inhibitions. All doses of BMY-14802 attenuated the locomotor effects of amphetamine, but only the higher doses also blocked other aspects of the amphetamine behavioral response. By itself, BMY-14802 dose dependently inhibited motor-related neurons, but elicited less behavioral activation than amphetamine. BMY-14802 (20 mg/kg) also induced hindlimb ataxia and occasional backwards locomotion. Haloperidol (1.0 mg/kg) reliably suppressed both behavior and neuronal activity when injected 30 min after BMY-14802, whether or not amphetamine pretreatment was given. Thus, BMY-14802 shares with other neuroleptics the capacity to reverse amphetamine-induced excitations of neostriatal motor-related neurons, whereas other effects of BMY-14802 reveal some haloperidol-like actions at 20 mg/kg that do not occur at lower doses.

Clozapine's mechanisms of action: non-dopaminergic activity rather than anatomical selectivity

The mechanisms of clozapine's unusual actions were investigated in anesthetized rats. Sensory field potentials were recorded in the nucleus accumbens, striatum and somatosensory cortex. Both haloperidol and clozapine suppressed responses in the striatum and accumbens. Clozapine's but not haloperidol's subcortical actions were unaffected by dopamine depletion. In addition, the cortical effects of the two drugs differed. These data indicate that clozapine has non-dopaminergic activity but no particular specificity for limbic basal ganglia structures. While it is known that clozapine acts as a serotonergic and cholinergic antagonist, several considerations suggest that action on still another transmitter system, possibly glutamate, needs to be investigated.

Endocrine and receptor pharmacology of serotonergic anxiolytics, antipsychotics and antidepressants

Several classes of drugs that modify serotonin (5-HT) neurotransmission are either currently used, or are being evaluated for their potential use in the treatment of anxiety, schizophrenia, and depression. 5-HT1A agonists are considered potential anxiolytics, while some atypical antipsychotics are potent 5-HT2 antagonists (and also have modest dopamine D2 affinity). Furthermore, there is a diverse group of serotonergic drugs that may be effective antidepressants. Secretion of ACTH, corticosterone/cortisol, prolactin, renin, oxytocin and vasopressin are stimulated by activation of different 5-HT receptor subtypes, while other neurotransmitter receptors also influence the secretion of these hormones. We compared the receptor binding profiles of 5-HT anxiolytics, antipsychotics and antidepressants with their endocrine effects. These comparisons could aid in understanding both the therapeutic and side effects of these drugs.

Neuroleptics increase C-FOS expression in the forebrain: Contrasting effects of haloperidol and clozapine

The mechanisms by which the atypical neuroleptic clozapine produces its therapeutic effects in the treatment of schizophrenia without causing the extrapyramidal side effects that are characteristic of most antipsychotic drugs remain unclear. Recently, a single injection of the typical antipsychotic haloperidol has been shown to increase c-fos expression in the striatum [Dragunow et al. (1990) Neuroscience 37, 287-294]. C-fos is a proto-oncogene that encodes a 55,000 mol. wt phosphoprotein, Fos, which is thought to assist in the regulation of "target genes" containing an AP-1 binding site. Because a wide variety of physiological and pharmacological stimuli increase c-fos expression, it has been proposed that Fos immunohistochemistry might be useful in mapping functional pathways in the central nervous system. The present experiments examined some potential neuroanatomical differences in the actions of clozapine and haloperidol by comparing their effects on c-fos expression in the medial prefrontal cortex, nucleus accumbens, striatum and lateral septum. The effects of the selective dopamine receptor antagonists SCH 23390 (D1) and raclopride (D2) were also examined. Haloperidol (0.5, 1 mg/kg) and raclopride (1, 2 mg/kg) produced large increases in the number of Fos-containing neurons in the striatum and nucleus accumbens. SCH 23390 (0.5, 1 mg/kg) reduced the number of Fos-positive neurons in the nucleus accumbens and striatum, and had no effect in the other regions. Neither haloperidol nor raclopride increased the number of Fos-positive neurons in the medial prefrontal cortex. Haloperidol, but not raclopride, produced a modest increase in c-fos expression in the lateral septal nucleus. Clozapine (10, 20 mg/kg) was without effect in the striatum; however, it significantly increased the number of Fos-positive neurons in the nucleus accumbens, medial prefrontal cortex and lateral septal nucleus. Destruction of mesotelencephalic dopaminergic neurons with 6-hydroxydopamine abolished the increase in Fos expression in the nucleus accumbens and striatum produced by haloperidol and raclopride, and also blocked the clozapine-induced increase in the nucleus accumbens. However, the inductive effects of clozapine and haloperidol on c-fos expression in the lateral septal nucleus and of clozapine in the medial prefrontal cortex were not affected by the 6-hydroxydopamine lesions. These results suggest that clozapine's unique therapeutic profile may be related to its failure to induce Fos in the striatum as well as its idiosyncratic actions in the lateral septum and medial prefrontal cortex. The effects of clozapine in these latter regions do not appear to be mediated by dopaminergic mechanisms.