Discriminative stimulus properties of the atypical antipsychotic drug clozapine in rats trained to discriminate 1.25 mg/kg clozapine vs. 5.0 mg/kg clozapine vs. vehicle

Clozapine, the prototype for atypical antipsychotic drugs, is used in the drug discrimination paradigm as a model for screening atypical from typical antipsychotic drugs. Previous drug discrimination studies in rats have shown that a 1.25 mg/kg clozapine training dose provides full stimulus generalization (i.e.) >or=80% condition-appropriate responding) to most atypical antipsychotic drugs, although a 5.0 mg/kg clozapine training dose appears necessary to provide stimulus generalization to other atypical antipsychotic drugs. The present study sought to characterize the pharmacological mechanisms that mediate these clozapine training doses. In rats trained to discriminate 1.25 vs. 5.0 mg/kg clozapine vs. vehicle in a three-choice drug discrimination task, various receptor-selective compounds were tested for stimulus generalization. The antidepressant mianserin was also tested. Full stimulus generalization from the 1.25 mg/kg clozapine training dose occurred only to mianserin (98.8%). Partial substitution (i.e. >or=60% and <80% condition-appropriate responding) to the 5.0 mg/kg clozapine training dose occurred for the muscarinic receptor antagonist scopolamine. The combined total percentage of responding on the 1.25 and 5.0 mg/kg clozapine levers, however, was well above the full substitution criteria at the 0.25, 0.5, and 1.0 mg/kg scopolamine doses. The M1 agonist N-desmethylclozapine, the nicotinic antagonist mecamylamine, the D1 antagonist SCH 23390, the D4 antagonist LU 38-012, the 5-HT1A agonist (+)-8-OH-DPAT, the 5-HT1A antagonist WAY 100 635, the 5-HT2A/2B/2C antagonist ritanserin, the 5-HT6 antagonist RO4368554, the alpha1 antagonist prazosin, the alpha2 antagonist yohimbine, and the histamine H1 antagonist pyrilamine all failed to substitute for either the 1.25 or the 5.0 mg/kg clozapine training doses. These results are consistent with previous evidence that antidepressant drugs have a tendency to substitute for clozapine and that muscarinic receptor antagonism may mediate the discriminative stimulus properties of 5.0 mg/kg clozapine. The lack of stimulus generalization from either clozapine training dose to other receptor-selective compounds, however, fails to explain how this model screens atypical from typical antipsychotic drugs and suggests that the discriminative stimulus properties of clozapine consist of a compound cue.     

Clozapine discrimination with a low training dose distinguishes atypical from typical antipsychotic drugs in rats

Rationale: Previous drug discrimination studies with clozapine have not reliably distinguished between atypical and typical antipsychotics. Objectives: The present study was conducted to determine whether low-dose clozapine drug discrimination could distinguish atypical from typical antipsychotics. Methods: Rats were trained to discriminate 1.25 mg/kg clozapine from vehicle in a two-lever drug discrimination procedure. Results: Generalization testing revealed full substitution with the atypical antipsychotics olanzapine (90.3% maximum generalization), sertindole (99.8%), and risperidone (87.1%) and partial substitution for quetiapine (seroquel, 66.4%) and the typical antipsychotics haloperidol (56.8%) and thioridazine (74.3%). Remoxipride (23.1%) and the typical antipsychotics chlorpromazine (27.9%) and fluphenazine (29.5%) did not reliably substitute for clozapine. Conclusions: In contrast to previous clozapine drug discrimination studies with higher training doses, the atypical antipsychotics olanzapine, sertindole, and risperidone reliably substituted for clozapine while typical antipsychotics did not. These results suggest that low-dose clozapine drug discrimination may be a more sensitive assay for distinguishing atypical from typical antipsychotic drugs. Received: 3 August 1999 / Final version: 9 December 1999     

Discriminative-stimulus effects of clozapine in squirrel monkeys: comparison with conventional and novel antipsychotic drugs

 The effects of conventional and novel atypical antipsychotic drugs were compared to clozapine in squirrel monkeys that discriminated IM injections of clozapine (1.0 mg/kg) from saline in a two-lever drug discrimination procedure. Clozapine (0.03–3.0 mg/kg) produced dose-related increases in responding on the clozapine-associated lever with full substitution at the training dose in all monkeys. Dose-related increases in responding on the clozapine-associated lever and full substitution also were observed with structural analogues of clozapine including perlapine and fluperlapine (0.1–3.0 mg/kg), seroquel (0.1–5.6 mg/kg), and JL 5, JL 8 and JL 18 (0.1–3.0 mg/kg). Other clozapine analogues, including olanzapine, amoxapine, loxapine and clothiapine, and conventional antipsychotic drugs, including phenothiazines such as chlorpromazine and thioridazine, produced some clozapine-associated responding up to the highest doses that could be studied, but did not substitute for clozapine. Olanzapine did produce full clozapine-lever responding following pretreatment with the dopamine D₂-receptor agonist (+)-PHNO (0.003–0.01 mg/kg). Putatively atypical antipsychotics that are structurally unrelated to clozapine including risperidone (0.003–0.1 mg/kg), sertindole (0.03–1.0 mg/kg) and remoxipride (0.1–5.6 mg/kg) similarly failed to substitute for clozapine up to the highest doses. The present results indicate that some, but not all, structural analogs of clozapine have clozapine-like discriminative-stimulus effects and that novel antipsychotic drugs which purportedly have clozapine-like clinical efficacy may not produce its interoceptive stimulus effects. Received: 2 November 1996 / Final version: 13 January 1997     

The discriminative stimulus properties of the atypical antipsychotic olanzapine in rats

Rationale: Analysis of the preclinical behavioral effects of atypical antipsychotic agents will provide a better understanding of how they differ from typical antipsychotics and aid in the development of future atypical antipsychotic drugs. Objectives: The present study was designed to provide information about the discriminative stimulus properties of the atypical antipsychotic olanzapine. Methods: Rats were trained to discriminate the atypical antipsychotic olanzapine (either 0.5 mg/kg OLZ or 0.25 mg/kg OLZ, i.p.) from vehicle in a two- lever drug discrimination procedure. The atypical antipsychotic clozapine fully substituted for olanzapine in both the 0.5-mg/kg OLZ group (99.3% drug lever responding [DLR]) and the 0.25-mg/kg OLZ group (99.9% DLR). The typical antipsychotic chlorpromazine also substituted for olanzapine in both the 0.5-mg/kg OLZ group (87.5% DLR) and in the 0.25-mg/kg OLZ group (98.9% DLR); whereas, haloperidol displayed partial substitution for olanzapine in the 0.5-mg/kg OLZ group (56.1% DLR) and in the 0.25-mg/kg OLZ group (76.4% DLR). The 5.0-mg/kg dose of thioridazine produced olanzapine-appropriate responding in the 0.5-mg/kg OLZ group (99.6% DLR), but only partial substitution was seen with the 0.25-mg/kg OLZ training dose (64.0% DLR). The atypical antipsychotics raclopride (53.9% DLR) and risperidone (60.1% DLR) displayed only partial substitution in the 0.5-mg/kg OLZ group. Both the muscarinic cholinergic antagonist scopolamine (90.0% DLR) and the 5-HT_2A/2C serotonergic antagonist ritanserin (86.0% DLR) fully substituted for olanzapine in the 0.5-mg/kg OLZ group. Conclusions: In contrast to previous discrimination studies with clozapine-trained rats, the typical antipsychotic agents chlorpromazine and thioridazine and the serotonin antagonist ritanserin substituted for olanzapine. These results demonstrate that there are differences in the mechanisms underlying the discriminative stimulus properties of clozapine and olanzapine. Specifically, olanzapine’s discriminative stimulus properties appear to be meditated in part by both cholinergic and serotonergic mechanisms. Received: 1 March 1999 / Final version: 6 September 1999     

Generalization testing with atypical and typical antipsychotic drugs in rats trained to discriminate 5.0 mg/kg clozapine from vehicle in a two‐choice drug discrimination task

Clozapine (CLZ) drug discrimination is used as a preclinical model to evaluate compounds for putative atypical antipsychotic properties. In rats, a 1.25 mg/kg CLZ training dose appears to have greater pharmacological specificity for atypical antipsychotic drugs than the traditional 5.0 mg/kg CLZ training dose; however, methodological differences among studies have precluded a direct comparison between these training doses. In the present study, rats were trained to discriminate a 5.0 mg/kg CLZ dose from vehicle in a two‐choice drug discrimination task using methods similar to those in a previous study from our laboratory that used a 1.25 mg/kg CLZ training dose. Clozapine produced full substitution (≥80% CLZ‐lever responding) for itself at the training dose (5.0 mg/kg). The atypical antipsychotics olanzapine, quetiapine, and ziprasidone also produced full substitution for 5.0 mg/kg CLZ, whereas the atypical antipsychotics risperidone and sertindole produced partial substitution (≥60% CLZ‐lever responding). The typical antipsychotic, thioridazine, produced full substitution for the 5.0 mg/kg CLZ training dose, but the typical antipsychotics chlorpromazine, fluphenazine, and haloperidol failed to substitute for clozapine. In a subgroup of 1.25 mg/kg CLZ‐trained rats, ziprasidone produced strong partial substitution (73.0 % CLZ‐lever responding) for the 1.25 mg/kg CLZ training dose. Based on these findings, some atypical antipsychotic drugs (i.e., quetiapine and ziprasidone) produce full substitution only for the 5.0 mg/kg CLZ training dose, whereas other atypical antipsychotic drugs (i.e., sertindole and risperidone) produce full substitution only for the 1.25 mg/kg CLZ training dose. Thus, both of these training doses are important for the screening of putative atypical antipsychotic drugs with the clozapine drug discrimination assay. Drug Dev. Res. 64:55–65, 2005. © 2005 Wiley‐Liss, Inc.     

Haloperidol both prevents and reverses quinpirole-induced nonregulatory water intake, a putative animal model of psychogenic polydipsia

RATIONALE: Polydipsia is a severe complication of long-term schizophrenia and, despite its unknown pathogenesis, is empirically treated with typical or atypical antipsychotics. In the rat, nonregulatory water intake is induced by repeated administration of amphetamine-like compounds or by the D2/3 agonist, quinpirole. OBJECTIVE: This study is aimed at determining the potential activity of antipsychotic compounds with different affinities for D2 receptors in preventing and/or reversing quinpirole-induced polydipsia. MATERIALS AND METHODS: Male Sprague-Dawley rats were treated with five injections of quinpirole (0.5 mg/kg i.p.) to induce polydipsia. The oral effects of haloperidol, olanzapine, clozapine, and ST2472 on QNP-induced polydipsia were analyzed in the following two schedules. In the preventive schedule, haloperidol (0.2, 0.4, and 0.8 mg/kg), olanzapine (1.5, 3, and 6 mg/kg), ST2472 (1 and 2 mg/kg), and clomipramine (5, 10, and 20 mg/kg) were given in combination with quinpirole from day 1 to day 5. In the reversal schedule, rats showing quinpirole-induced polydipsia on the third day received haloperidol (0.4 mg/kg), olanzapine (1.5 and 3 mg/kg), clozapine (10, 20, and 40 mg/kg), ST2472 (1, 2, 5, and 10 mg/kg), and clomipramine (5, 10, and 20 mg/kg) before quinpirole on days 4 and 5. RESULTS: Haloperidol both prevented and reversed quinpirole-induced polydipsia, whereas olanzapine and ST2472 only reversed it. Clomipramine prevented but did not reverse quinpirole-induced polydipsia, and clozapine did not reverse it either. CONCLUSIONS: We suggest that, once developed, polydipsia is governed by dopaminergic D2 mechanisms. In contrast, either an increase in the serotoninergic tone or an inhibition of D2 receptors can modulate the development of quinpirole-induced excessive drinking.     

Quinpirole, 8-OH-DPAT and ketanserin modulate catalepsy induced by high doses of atypical antipsychotics

The effect of the selective dopamine D2 receptor agonist quinpirole, the selective 5-HT1A receptor agonist 8-OH-DPAT and the selective 5-HT2A receptor antagonist ketanserin on catalepsy induced by atypical antipsychotics clozapine, risperidone, olanzapine and sertindole at higher doses was studied in rats. Haloperidol (0.5, 1 and 2 mg/kg), clozapine (50 and 75 mg/kg) and olanzapine (15 and 30 mg/kg) produced catalepsy dose-dependently while sertindole at doses up to 40 mg/kg failed to produce catalepsy in rats. However, sertindole (15, 30 and 45 mg/kg) produced a cataleptic effect in mice in a dose-dependent manner. At a high dose (5 mg/kg), risperidone produced catalepsy in rats. Quinpirole (0.05 and 0.1 mg/kg) reversed the cataleptic effect of haloperidol (2 mg/kg), risperidone (5 mg/kg), olanzapine (30 mg/kg) and sertindole (45 mg/kg). Quinpirole (0.05 and 0.1 mg/kg) reversed clozapine (75 mg/kg)-induced catalepsy. 8-OH-DPAT (0.15 and 0.3 mg/kg) dose-dependently reversed catalepsy induced by haloperidol (2 mg/kg) and risperidone (5 mg/kg) without affecting the cataleptic effect of olanzapine. However, the higher dose (0.45 mg/kg) of 8-OH-DPAT reversed it significantly. 8-OH-DPAT (0.3 mg/kg) reversed clozapine (75 mg/kg)-induced catalepsy. 8-OH-DPAT (0.15, 0.3 and 0.45 mg/kg) failed to reverse sertindole-induced catalepsy. Ketanserin (0.75 and 1.5 mg/kg) completely reversed catalepsy induced by haloperidol (2 mg/kg) and risperidone (5 mg/kg). Ketanserin (0.75 and 1.5 mg/kg) dose-dependently reversed olanzapine (30 mg/kg) and sertindole (45 mg/kg)-induced catalepsy without any effect on clozapine (75 mg/kg)-induced catalepsy. A higher dose (3 mg/kg) of ketanserin reversed clozapine-induced catalepsy. The present study suggests that atypical antipsychotics show fewer extrapyramidal symptoms (EPS) due to greater modulation of the serotonergic system. Therefore, an antipsychotic with dopamine D2/5-HT2A antagonistic action and 5-HT1A agonistic action may prove to be superior to the existing antipsychotics.     

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 ₁ 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.     

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.     

Stimulus properties of the "atypical" antipsychotic zotepine in rats: comparisons with clozapine and quetiapine

The stimulus properties of the "atypical" antipsychotic zotepine were assessed in three studies in rats. In Study 1, the ability of zotepine to generalise to clozapine was studied. Two groups of rats were trained to discriminate clozapine at 2 and 5 mg/kg. Clozapine induced full generalisation in both groups, with the generalisation curves shifted significantly to the left in the low dose group. In generalisation tests clozapine did not suppress responding. Zotepine induced dose-related generalisation in both groups, with full generalisation in the low dose group and 50% maximal generalisation in the high dose group at the highest dose that could be tested. In contrast to clozapine, zotepine induced substantial (50% or more) substitution for clozapine only at doses which suppressed responding. In Study 2 zotepine was investigated in rats trained to discriminate quetiapine (10 mg/kg). Quetiapine induced full generalisation and zotepine only induced 54% generalisation at the highest dose that could be tested. Generalisation was accompanied by response suppression induced by both quetiapine and zotepine. In Study 3 an attempt was made to train a zotepine discrimination (1 mg/kg increased to 2 mg/kg). Even after 150 training sessions it proved impossible to obtain reliable discriminative responding with zotepine. These data suggest that: (i) The actions of zotepine in discrimination assays are similar to, but not identical with, those of clozapine and quetiapine; (ii) The differences among the actions of clozapine, quetiapine and zotepine may be related to either the unique ability of zotepine to block noradrenaline (NA) uptake, or to its more marked affinity for D(2) receptors; (iii) The finding that zotepine only mimicked quetiapine up to a level of 54% was unexpected, since quetiapine and clozapine generalise reciprocally and zotepine generalised fully to (low dose) clozapine. This finding may also be related either to zotepine's ability to inhibit NA uptake or its relatively high D(2) affinity; (iv) Although zotepine clearly possesses discriminative properties, it is not possible to train it as a reliable stimulus, in contrast to clozapine and quetiapine. This may be due to its more marked D(2) receptor affinity. Collectively, these data demonstrate both similarities and differences between zotepine and other novel atypical antipsychotics in drug discrimination assays.     

Role of specific dopamine receptor subtypes in amphetamine discrimination

Biochemical, electrophysiological, and behavioral experiments suggest that the dopamine D-1 and D-2 receptor subtypes functionally interact. In rats trained to discriminate 1.0 mg/kg d-amphetamine, substitution with the D-2 agonist quinpirole (0.1–2.0 mg/kg) produces amphetaminelever responding, whereas the D-1 agonist SKF 38393 (0.3–10.0 mg/kg) elicits only saline-appropriate responding. Combining either quinpirole (0.05–0.5 mg/kg) or SKF 38393 (0.5–10.0 mg/kg) with 0.3 mg/kg d-amphetamine results in dose-dependent increases in amphetamine-lever responding. Conversely, the D-1 antagonist SCH 23390 (0.02–0.1 mg/kg) antagonizes the discrimination produced by 0.7 mg/kg d-amphetamine. Additional combination studies examined the effect of DA receptor drugs on discrimination when quinpirole is substituted in d-amphetamine trained rats. SKF 38393 (0.5–7.0 mg/kg) fails to increase the amphetamine-appropriate lever response produced by either 0.05 or 0.2 mg/kg quinpirole. Similarly, SCH 23390 (0.01–0.1 mg/kg) fails to antagonize the amphetamine-lever responding produced by either 0.2 or 0.5 mg/kg quinpirole. Haloperidol (0.02–0.2 mg/kg) does antagonize the amphetamine-appropriate response produced by quinpirole substitution. The d-amphetamine discrimination studies indicate that stimulating D-2 receptors alone or D-1 receptors in the presence of d-amphetamine yields d-amphetamine-lever responding, and suggests that D-1/D-2 receptors can functionally interact to alter discrimination behavior. Quinpirole substitution, on the other hand, shows an insensitivity to D-1 receptor manipulations.     

Discriminative stimulus properties of N-desmethylclozapine, the major active metabolite of the atypical antipsychotic clozapine, in C57BL/6 mice

N-desmethylclozapine (NDMC) is the major active metabolite of the atypical antipsychotic drug clozapine and may contribute to the therapeutic efficacy of clozapine. Although they share many pharmacological features, it is noteworthy that NDMC is a partial dopamine D2 and cholinergic muscarinic M1/M4 agonist, whereas clozapine is a weak dopamine D2 receptor inverse agonist/antagonist and a nonselective muscarinic antagonist. To better understand the in-vivo pharmacological mechanisms of these drugs, male C57BL/6NHsd-wild-type mice were trained to discriminate 10.0 mg/kg NDMC from vehicle in a two-lever drug discrimination procedure for food reward. It was found that the parent drug clozapine fully substituted for NDMC, whereas the typical antipsychotic drug haloperidol (dopamine D2 antagonist) and the atypical antipsychotic drug aripiprazole (D2 partial agonist) did not substitute for NDMC. These results demonstrated that clozapine and its major metabolite NDMC share in-vivo behavioral properties (i.e. discriminative stimulus properties) that are likely due to shared pharmacological mechanisms that differ from other antipsychotic drugs. The discriminative stimulus properties of NDMC probably reflect a compound cue similar to that of its parent drug clozapine due to its diverse binding profile.     

Serotonin receptor mechanisms mediate the discriminative stimulus properties of the atypical antipsychotic clozapine in C57BL/6 mice

Rationale The atypical antipsychotic drug (APD) clozapine (CLZ) has been shown to have a robust discriminative cue in rats, pigeons, and monkeys in two-choice drug discrimination procedures.Objectives The present study determined whether a two-choice drug discrimination procedure with CLZ could be established in C57BL/6 mice and whether this procedure could distinguish between atypical and typical APDs.Methods C57BL/6 male mice were trained to discriminate 2.5 mg/kg CLZ from vehicle in a two-lever drug discrimination procedure.Results Generalization testing with CLZ produced full substitution at the 2.5- and 5.0-mg/kg doses with an ED₅₀ of 1.14 mg/kg. The atypical APDs olanzapine (ED₅₀=0.24 mg/kg), risperidone (ED₅₀=0.072 mg/kg), and ziprasidone (ED₅₀=0.33 mg/kg) fully substituted for CLZs discriminative cue, while the typical APD haloperidol failed to substitute for CLZ. Generalization testing with selective ligands showed that the serotonin (5-HT)_2A/2B/2C antagonist ritanserin fully substituted for CLZ (ED₅₀=2.08 mg/kg) and that the 5-HT receptor agonist quipazine significantly attenuated CLZs discriminative cue without disrupting response rates. The muscarinic receptor antagonist scopolamine, the dopamine agonist amphetamine, and the 5-HT agonist quipazine failed to substitute for CLZ.Conclusions These results demonstrated that antagonism of 5-HT receptors plays an important role in mediating the discriminative stimulus properties of the atypical APD CLZ in C57BL/6 mice. The atypical APDs olanzapine, risperidone, and ziprasidone fully substituted for CLZ, while the typical APD haloperidol did not. These results suggest that CLZ drug discrimination in C57BL/6 mice may be an effective preclinical behavioral assay for screening atypical from typical antipsychotic drugs.     

Chlorpromazine as a discriminative stimulus in rats: Generalization to typical and atypical antipsychotics

The discriminative stimulus properties of the typical antipsychotic chlorpromazine were examined in a two‐lever drug discrimination procedure for food reward. Six of nine rats readily acquired the discrimination between 1.0 mg/kg chlorpromazine (i.p.) and vehicle in a mean of 29.7 training sessions. The chlorpromazine generalization curve was dose‐dependent and yielded an ED₅₀ of 0.305 mg/kg (95% confidence interval (CI) = 0.201–0.463 mg/kg). The chlorpromazine cue generalized to the atypical antipsychotics clozapine (ED₅₀ for the clozapine curve was 0.258 mg/kg [95% CI = 0.047–1.420 mg/kg]) and olanzapine (ED₅₀ for the olanzapine curve was 0.199 mg/kg [95% CI = 0.076–0.522 mg/kg]) and to the typical antipsychotic thioridazine (ED₅₀ for the thioridazine curve was 3.103 mg/kg [95% CI = 1.993–4.832 mg/kg]). Haloperidol (a typical antipsychotic) and raclopride (an atypical antipsychotic) did not substitute for chlorpromazine. It is clear from the present results that the discriminative stimulus properties of chlorpromazine share similarities both with the atypical antipsychotics clozapine and olanzapine and with the typical antipsychotic thioridazine. The extent to which the discriminative stimulus properties of antipsychotic drugs reflect or are predictive of their therapeutic effects in schizophrenic patients remains unclear. Drug Dev. Res. 48:38–44, 1999. © 1999 Wiley‐Liss, Inc.     

Anti-cataleptic effects of clozapine, but not olanzapine and quetiapine, on SCH 23390- or raclopride-induced catalepsy in rats.

The present study investigated potential anti-cataleptic properties of the prototype atypical antipsychotic clozapine and two newly developed atypical antipsychotics, olanzapine and quetiapine, which are structurally related and display similar pharmacological profiles to clozapine. Clozapine (2.5 mg kg(-1), s.c.), but not olanzapine (2.0 mg kg(-1), s.c.) and quetiapine (20.0 mg kg(-1), s.c.), blocked catalepsy induced either by the dopamine D(1/5) receptor antagonist SCH 23390 (50.0 microg kg(-1), s.c) or the selective dopamine D(2/3) receptor antagonist raclopride (4.0 mg kg(-1), s.c.). Such findings are consistent with the beneficial effects of clozapine in the management of drug-induced psychosis in parkinsonian patients, and suggest that neither olanzapine nor quetiapine may be a safe alternative to clozapine in this field. Furthermore, the results indicate that clozapine has a unique pharmacological profile that distinguishes it from olanzapine and quetiapine. The mechanisms underlying anti-cataleptic or anti-parkinsonian properties of clozapine are unclear but may be related to dopamine D(1) receptor agonism of clozapine.     

Clozapine and N-methyl-D-aspartate have positive modulatory actions on their respective discriminative stimulus properties in C57BL/6 mice

The impairment of N-Methyl-D-Aspartate receptors is thought to contribute to negative symptoms and cognitive deficits. In vitro studies suggest that atypical antipsychotic drugs like clozapine may help to alleviate these deficits by enhancing glutamatergic function. The present study examined the in vivo interaction of clozapine with N-Methyl D-aspartate by training one group of C57BL/6 mice to discrimination 2.5 mg/kg clozapine from vehicle and another group to discriminate 30 mg/kg N-Methyl D-aspartate from vehicle in a two-lever drug discrimination task. Cross-generalization testing revealed that N-Methyl D-aspartate (3-56 mg/kg) failed to substitute for clozapine in the clozapine-trained mice, while clozapine (0.625 mg/kg) produced partial substitution in the N-Methyl D-aspartate-trained mice. Interestingly, administration of a low, non-generalizing dose of each training drug in combination with the full range of doses of the alternate training drug produced full and dose-dependent substitution in both clozapine- and N-Methyl D-aspartate-trained mice. The α(1) antagonist prazosin fully and dose-dependently substituted for both clozapine and N-Methyl D-aspartate. These results suggest that the shared discriminative stimulus properties between clozapine and N-Methyl D-aspartate may be mediated through indirect mechanisms, possibly in part through α(1) adrenergic antagonism.     

Actions of novel antipsychotic agents on apomorphine-induced PPI disruption: influence of combined serotonin 5-HT1A receptor activation and dopamine D2 receptor blockade.

The dopamine D1/D2 agonist apomorphine (0.63 mg/kg) disrupted prepulse inhibition (PPI) of acoustic startle in rats, a model of sensorimotor gating deficits observed in schizophrenia. All current antipsychotics, which antagonize D2 receptors, prevent this apomorphine-induced deficit. A novel class of antipsychotics possesses, in addition to D2 antagonist property, various levels of 5-HT1A agonist activity. Considering that the latter itself produces PPI deficits, it appeared necessary to assess the potential of this novel class of antipsychotics to reverse apomorphine-PPI deficits. Potent D2 antagonists, like haloperidol (0.63-2.5 mg/kg), risperidone (0.63-10 mg/kg), and olanzapine (0.63-40 mg/kg) prevented apomorphine PPI disruption. The atypical antipsychotics, clozapine (40 mg/kg), nemonapride (0.01-2.5 mg/kg), ziprasidone (10 mg/kg), and aripiprazole (0.01 and 10 mg/kg), which all exhibit 5-HT1A agonist properties, reversed PPI deficits at some doses only, whereas the anti-dyskinetic agent sarizotan (0.16-10 mg/kg), an efficacious 5-HT1A agonist, did not. New generation antipsychotics with marked 5-HT1A agonist properties, such as SLV313 and SSR181507 (0.0025-10 mg/kg and 0.16-10 mg/kg, respectively) did not reverse these deficits whereas bifeprunox (0.04-2.5 mg/kg) did. To reveal the contribution of 5-HT1A agonist properties in the lack of effects of SLV313 and SSR181507, we pretreated rats with the 5-HT1A antagonist WAY100635 (0.63 mg/kg). Under these conditions, significant reversal of PPI deficit was observed, indicating that D2 antagonist properties of SLV313 and SSR181507 are now sufficient to overcome the disruptive effects of apomorphine. To summarize, antipsychotics possessing agonist efficacy at 5-HT1A receptors exhibit diverse profiles against apomorphine-induced PPI deficits, depending on the balance between D2 and 5-HT1A activities, suggesting that they may display distinct activity on some aspects of gating deficits in schizophrenic patients.     

Comparison of the novel antipsychotic ziprasidone with clozapine and olanzapine: inhibition of dorsal raphe cell firing and the role of 5-HT1A receptor activation.

Ziprasidone is a novel antipsychotic agent which binds with high affinity to 5-HT1A receptors (Ki = 3.4 nM), in addition to 5-HT1D, 5-HT2, and D2 sites. While it is an antagonist at these latter receptors, ziprasidone behaves as a 5-HT1A agonist in vitro in adenylate cyclase measurements. The goal of the present study was to examine the 5-HT1A properties of ziprasidone in vivo using as a marker of central 5-HT1A activity the inhibition of firing of serotonin-containing neurons in the dorsal raphe nucleus. In anesthetized rats, ziprasidone dose-dependently slowed raphe unit activity (ED50 = 300 micrograms/kg i.v.) as did the atypical antipsychotics clozapine (ED50 = 250 micrograms/kg i.v.) and olanzapine (ED50 = 1000 micrograms/kg i.v.). Pretreatment with the 5-HT1A antagonist WAY-100,635 (10 micrograms/kg i.v.) prevented the ziprasidone-induced inhibition; the same dose of WAY-100,635 had little effect on the inhibition produced by clozapine and olanzapine. Because all three agents also bind to alpha 1 receptors, antagonists of which inhibit serotonin neuronal firing, this aspect of their pharmacology was assessed with desipramine (DMI), a NE re-uptake blocker previously shown to reverse the effects of alpha 1 antagonists on raphe unit activity. DMI (5 mg/kg i.v.) failed to reverse the inhibitory effect of ziprasidone but produced nearly complete reversal of that of clozapine and olanzapine. These profiles suggest a mechanism of action for each agent, 5-HT1A agonism for ziprasidone and alpha 1 antagonism for clozapine and olanzapine. The 5-HT1A agonist activity reported here clearly distinguishes ziprasidone from currently available antipsychotic agents and suggests that this property may play a significant role in its pharmacologic actions.     

Role of D1 and D2 dopamine receptors in the discriminative stimulus properties of the atypical antipsychotic clozapine in rats

The purpose of the present study was to assess the role of dopamine D₁ and D₂ receptors in the discriminative stimulus properties of the atypical antipsychotic clozapine (CLZ). Two groups of rats were trained to discriminate either a moderate dose of clozapine (5.0 mg/kg) from vehicle or a high dose of clozapine (10.0 mg/kg) from vehicle in a two‐lever drug discrimination paradigm. Generalization testing with clozapine yielded an ED₅₀ of 0.9 mg/kg (95% confidence limits = 0.5–2.0 mg/kg) for the 5.0 CLZ group and 2.0 mg/kg (95% confidence limits = 1.4–2.8 mg/kg) for the 10.0 CLZ group. Substitution testing with the D₁ antagonist SCH 23390 and the D₂ dopamine antagonist haloperidol failed to produce clozapine‐appropriate responding for either of the clozapine training doses. The antipsychotic drug thioridazine (which binds to a number of neurotransmitters in addition to dopamine) produced partial substitution (64.5% drug lever responding) in the 5.0 CLZ group at the 5.0 mg/kg dose. These results suggest that antagonism of D₁ and D₂ dopamine receptors alone is not sufficient to produce clozapine‐appropriate responding, even with the higher training dose of 10.0 mg/kg. Drug Dev. Res. 46:139–147, 1999. © 1999 Wiley‐Liss, Inc.     

Discriminative stimulus properties of 1.25 and 5.0 mg/kg doses of clozapine in rats: examination of the role of dopamine, serotonin, and muscarinic receptor mechanisms

Clozapine (CLZ), an atypical antipsychotic drug (APD), produces minimal extrapyramidal side effects (EPS) and has significant advantages for treating both positive and negative symptoms in schizophrenic patients. CLZ has been established as a discriminative cue in the drug discrimination paradigm and in generalization tests the CLZ cue is more selective for atypical, rather than typical, APDs. However, greater selectivity for atypical antipsychotics has been demonstrated with a lower (1.25 mg/kg) CLZ training dose in rats [Psychopharmacology, 149 (2000) 189], rather than the traditional, higher training dose (5.0 mg/kg). It is therefore of interest to evaluate the properties mediating the 1.25 mg/kg CLZ discriminative cue. In the present study, rats were trained to discriminate either 1.25 mg/kg (N=7) or 5.0 mg/kg (N=7) CLZ from vehicle in a two-lever drug discrimination task. The typical antipsychotic haloperidol (0.1-0.4 mg/kg) did not substitute for either CLZ cue, whereas the atypical antipsychotic melperone (0.37-3.0 mg/kg) provided full substitution in both groups (>80% CLZ-appropriate responding). The 5-HT(1A) receptor agonist (+)-8-OH-DPAT (0.04-0.16 mg/kg), and the selective 5-HT(2A) receptor antagonist M100907 (0.03-1.0 mg/kg) did not produce substitution in either group. (+)-8-OH-DPAT combined with haloperidol (0.05 mg/kg) engendered only partial substitution (>60% CLZ-appropriate responding) for both CLZ cues, and M100907 combined with haloperidol (0.05 and 0.1 mg/kg doses) failed to provide substitution in either group. Trihexyphenidyl (0.18-6.0 mg/kg), a muscarinic M(1)-preferring receptor antagonist, engendered full substitution for the 1.25 mg/kg CLZ cue, but only partial substitution for the 5.0 mg/kg CLZ cue. These results provide evidence that antagonism at the muscarinic M(1) receptor is sufficient to provide 1.25 mg/kg CLZ-like discriminative stimulus effects.