Clozapine blocks disruptive and discriminative stimulus effects of quipazine

Discriminative stimulus properties of the atypical antipsychotic clozapine and the typical antipsychotic chlorpromazine in a three-choice drug discrimination procedure in rats

RATIONALE

The atypical antipsychotic drug (APD) clozapine elicits a robust discriminative cue that is generally selective for other atypical APDS in two-choice drug discrimination (DD) procedures.

OBJECTIVES

The present study determined whether a three-choice DD procedure with the atypical APD clozapine (CLZ) versus the typical APD chlorpromazine (CPZ) versus vehicle (VEH) could provide greater selectivity between atypical and typical APDs.

METHODS

Sprague-Dawley rats were trained to discriminate 5.0 mg/kg CLZ from 1.0 mg/kg CPZ from VEH in a three-lever DD task with an FR30 food reinforcement schedule.

RESULTS

Generalization testing with CLZ produced CPZ-appropriate responding at lower doses (ED50=0.103 mg/kg) and CLZ-appropriate responding at higher doses (ED50=1.69 mg/kg). Generalization testing with the atypical APD olanzapine produced similar results. In contrast, the atypical APD risperidone and the typical APDs CPZ and haloperidol produced only CPZ-appropriate responding. The muscarinic antagonist scopolamine produced CPZ-appropriate responding at lower doses and CLZ-appropriate responding at higher doses in a manner similar to CLZ and olanzapine. The co-administration of haloperidol (0.00625 mg/kg) with scopolamine shifted the dose-response curve for CLZ-appropriate responding to the left. The 5-HT(2A/2C) antagonist ritanserin and the H1 histamine antagonist pyrilamine did not substitute for either CLZ or CPZ. The alpha1 adrenergic antagonist prazosin did not substitute for CLZ, but produced full substitution for CPZ.

CONCLUSIONS

The three-choice DD procedure clearly distinguished the atypical APDs CLZ and olanzapine from the typical APDs CPZ and haloperidol; however, the stimulus properties of the atypical APD risperidone were similar to CPZ, but not to CLZ. These findings further suggest that CLZ, as well as CPZ, elicits a compound cue.

The interactions of typical and atypical antipsychotics with the (-)2, 5,-dimethoxy-4-methamphetamine (DOM) discriminative stimulus

The present study was designed to test the hypothesis that atypical, but not typical, antipsychotics produce a functional in vivo blockade of 5-HT2A receptors. The magnitude of functional in vivo 5-HT2A receptor blockade elicited by representative compounds from each of the six major structural classes of typical antipsychotics, and the representative atypical antipsychotics clozapine and risperidone, was indicated by their respective abilities to block the stimulus effects of the phenylalkylamine hallucinogen (-)DOM in the rat. Chlorpromazine, thioridazine, fluphenazine, thiothixene and haloperidol did not produce a significant antagonism of the (-)DOM stimulus. The benzoxapine, loxapine (60%), and the atypical dibenzodiazepine, clozapine (62%), partially blocked and risperidone fully blocked (100%) the (-)DOM stimulus. None of these agents elicited significant levels of (-)DOM-appropriate responding when administered alone. These results indicate that the typical antipsychotics, with the exception of lozapine, fail to produce effective in vivo antagonism of 5-HT2A receptors at doses compatible with the preservation of operant behavior. In contrast, the atypical antipsychotics clozapine and risperidone elicit effective in vivo antagonism of 5-HT2A receptors without severe behavioral disruption. Thus, these data are supportive of the hypothesis that the mechanism of action of atypical, but not typical, antipsychotics involves the antagonism of 5-HT2A receptors in vivo.

Olanzapine, an atypical antipsychotic, increases rates of punished responding in pigeons

The effects of olanzapine [LY 170053; 2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2, 3b] [1,5]benzodiazepine), a potential atypical antipsychotic, were determined in pigeons whose keypeck responding was punished. These effects were compared to the anxiolytic agents chlordiazepoxide and pentobarbital, and to other antipsychotic agents. Keypeck behavior was maintained under a multiple FR30 FR30 schedule, signalled by white and red stimulus lights, respectively. Each component of the schedule alternated every 3 min with a 30-s timeout. During the white keylight component, responding was maintained by food presentation. During the red keylight component, responding was maintained by food and simultaneously suppressed by electric shock presentation, with response rates being only about 5% of those during the white stimulus light. Olanzapine (0.01-1.0 mg/kg) increased punished responding at doses below those which had an effect on unpunished responding. Clozapine (0.01-1.0 mg/kg), ritanserin (0.1-3.0 mg/kg), and, to a lesser extent, risperidone (0.1-1.0 mg/kg) were also effective at increasing punished responding. Generally, the maximum effect seen with olanzapine was equal to that seen with ritanserin, and it exceeded that seen with clozapine. However, these effects were generally less than those seen with chlordiazepoxide and pentobarbital. Haloperidol (0.01-0.1 mg/kg) was completely without effect on punished responding, while it caused decreases in unpunished behavior. These results provide further evidence that olanzapine has a profile in behavioral tests unlike the typical antipsychotic haloperidol. Moreover, this profile is similar to clozapine, a clinically effective antipsychotic with an atypical profile.

Competitive antagonism of serotonin (5-HT)2C and 5-HT2A receptor-mediated phosphoinositide (PI) turnover by clozapine in the rat: a comparison to other antipsychotics

The antagonist actions of clozapine and several other antipsychotics at 5-hydroxytryptamine (5-HT)2A and 5-HT2C receptors were studied using the in vitro model of 5-HT-induced phosphoinositide (PI) turnover in rat choroid plexus (5-HT2C) and frontal cortex (5-HT2A). While (-)-sulpiride and raclopride were inactive, clozapine and the other drugs behaved as antagonists both at 5-HT2A and at 5-HT2C receptors. Their order of potency (p Inhibitory Concentration (IC)50) was as follows. 5-HT2A receptors: risperidone (9.07) > spiperone > chlorpromazine > clozapine > thioridazine = fluphenazine > haloperidol (6.03). 5-HT2C receptors: clozapine (7.19) > chlorpromazine > risperidone > thioridazine > fluphenazine > spiperone > haloperidol (< 4.00). In each tissue, clozapine shifted the concentration-effect curve for 5-HT to the right in the absence of an alteration in slope or maximal effect. These findings indicate that clozapine acts as a competitive antagonist at 5-HT2A and 5-HT2C receptors and that its antagonist properties are shared, though less potently at 5-HT2C sites, by several, clinically active antipsychotics.

Interactions of clozapine with the stimulus effects of DOM and LSD

Two groups of rats were trained with the 5-HT2 agonists 2,5-dimethoxy-4-methylamphetamine (DOM) or lysergic acid diethylamide (LSD) in a two-lever discrimination task. Tests of generalization and antagonism were then carried out with clozapine. DOM did not generalize to clozapine. Partial antagonism of DOM was observed with 0.3, 1, and 2 mg/kg clozapine and statistically significant full antagonism with 3 mg/kg. LSD did not fully generalize to clozapine. Partial antagonism of LSD was observed with 3 and 4 mg/kg clozapine. Because clozapine is known to block muscarinic as well as 5-HT2 receptors, atropine was studied in DOM-trained rats. DOM partially generalized to 3 mg/kg atropine. Partial attenuation of DOM stimulus effects was observed with 3 mg/kg atropine, and no attenuation with 5 mg/kg. A combination of 2 mg/kg clozapine and 3 mg/kg atropine vs. DOM produced response suppression in five of seven rats. The atropine test results do not exclude the possibility of an antimuscarinic component in the observed attenuation of DOM and LSD stimulus effects by clozapine.

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.

The dopamine-serotonin relationship in clozapine response

The effects of clozapine on the dopamine and serotonin systems may underlie its atypical pharmacologic and clinical profile. To examine this hypothesis, we measured dopamine and serotonin plasma and cerebrospinal (CSF) metabolites and the relationship of these values to treatment response in 19 neuroleptic refractory and intolerant schizophrenic patients. Only a small change in the CSF and plasma homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5HIAA) levels was found. However, the pretreatment CSF HVA/5HIAA ratio and, to a lesser extent, the CSF HVA level predicted treatment response. These results suggest that the modest relationship between HVA and 5-HIAA and treatment response supports the involvement of both neurotransmitters in the pathophysiology of schizophrenia.

Clozapine pharmacology and tardive dyskinesia

Clozapine, an atypical neuroleptic, does not cause extrapyramidal symptoms of Parkinsonism and dystonia and appears to have a reduced or absent capacity to produce tardive dyskinesia. 37 subjects, most with chronic schizophrenia, were treated with clozapine and TD outcome was analyzed. A subset of these subjects underwent plasma and CSF studies. TD response was heterogenous, but a proportion of patients improved with clozapine treatment. Neurochemical data differed from published reports of classical neuroleptics with the most robust effect produced by clozapine seen in CSF norepinephrine levels. Other neurochemical data and implications for the mechanism of clozapine in TD are reviewed.

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.

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.

Site-selective serotonin agonists as discriminative stimuli

Various direct- and indirect-acting serotonin (5-HT) agonists serve as training drugs in tests of stimulus control of behavior; such agents include: 5-hydroxytryptophan, 5-methoxy-N,N-dimethyltryptamine, and fenfluramine. However, with the recent discovery of multiple populations of central 5-HT binding sites, the concept of site-selective serotonergic agents needs to be addressed. Certain 4-substituted 1-(2,5-dimethoxyphenyl)-2-aminopropanes such as DOM (4-methyl), DOB (4-bromo), and DOI (4-iodo) appear to be 5-HT2-selective agonists and serve as effective training drugs in rats. Stimulus generalization occurs among these agents regardless of which is used as the training drug, although stimulus generalization does not occur with 5-HT1A-selective agonists [e.g., 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH DPAT)] or with 5-HT1B-selective agonists [e.g., 1-(3-trifluoromethylphenyl)piperazine (TFMPP)]. 8-OH DPAT and TFMPP also serve as training drugs; the 8-OH DPAT-stimulus generalizes to other 5-HT1A agonists, but not to 5-HT1B or 5-HT2 agonists, whereas the TFMPP-stimulus generalizes to other 5-HT1B agonists, but not to 5-HT1A or 5-HT2 agonists. Classical serotonin antagonists, most of which are rather selective for 5-HT2 sites, and 5-HT2-selective antagonists are able to block the stimulus effects of DOM, DOB, and DOI, but not those of 8-OH DPAT or TFMPP. The results of such studies reveal that, in rats, site-selective 5-HT agonists produce stimulus effects that are also selective; although generalization may occur with nonselective 5-HT agonists, animals trained to discriminate site-selective 5-HT agonists apparently do not recognize other 5-HT agonists that are selective for a different site. Animals trained to discriminate such agents from saline might be useful for the identification and/or investigation of novel site-selective agonists and antagonists (for example, the 8-OH DPAT-stimulus generalizes to members of a new class of anxiolytics that display high affinity for 5-HT1A binding sites), and might also aid in the overall understanding of central serotonergic mechanisms.

Differential effects of acute clozapine and haloperidol on the activity of ventral tegmental (A10) and nigrostriatal (A9) dopamine neurons

Extracellular single-unit recording techniques were used to evaluate the effects of acute intravenous (i.v.) clozapine (CLOZ) and haloperidol (HAL) on the activity of dopamine (DA) neurons of the ventral tegmental area (VTA or A10) and the substantia nigra pars compacta (SNC or A9). CLOZ increased the firing rate of A10 but not A9 cells, and drove 9/23 (39%) of A10 cells into an apparent depolarization blockade. HAL, on the other hand, produced a rate elevation and, at higher doses, depolarization inactivation in both subpopulations of DA neurons. Cell firing was restored in inactivated cells with i.v. apomorphine (APO) or iontophoretic GABA. CLOZ always fully reversed APO-induced suppression of A10 DA activity, but in many cases only partially reversed suppression of A9 DA neurons. Scopolamine did not mimic the effects of CLOZ on A10 neurons, and it also failed to block the activating effect of HAL on A9 units, indicating that the selective action of CLOZ cannot be interpreted simply by its anticholinergic properties. After hemi-transections of the diencephalon, which severed the medial forebrain bundle and other feedback pathways to the DA somata, CLOZ was still ineffective in altering A9 DA activity. This suggests that the lack of effect on CLOZ on A9 cells is not due to the inhibitory influence of forebrain feedback pathways. This hemi-transection also left intact the activation of A10 neurons produced by HAL and CLOZ, but it did prevent the excitatory action of HAL on most A9 neurons sampled. This indicates that forebrain feedback pathways are less critical in mediating the action of APDs on A10 DA neurons. Finally, iontophoretic application of CLOZ and HAL into the vicinity of DA cell bodies blocked the rate-reducing effects of locally applied DA, but not those of GABA. This suggests that both APDs block somatodendritic DA autoreceptors. However, HAL was considerably more potent than CLOZ in producing this blockade. It is suggested that the different pharmacological and clinical properties of HAL and CLOZ may be partially explained by a differential mode of action on the A10 and A9 subpopulations of DA cells. The data also provide pharmacological evidence that these 2 groups of DA cells are regulated by different mechanisms.