These data suggest that the relative extent of 5-HT(2A) and 5-HT(2C) antagonism, as well as the extent of D(2) receptor blockade, has a critical influence on DA release in the mPFC and NAC and may be a determining factor in the action of this class of atypical APDs on these two potentially clinically relevant parameters.
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 ED 50 of 1.14 mg/kg. The atypical APDs olanzapine (ED 50 =0.24 mg/ kg), risperidone (ED 50 =0.072 mg/kg), and ziprasidone (ED 50 =0.33 mg/kg) fully substituted for CLZ's 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 (ED 50 =2.08 mg/ kg) and that the 5-HT receptor agonist quipazine significantly attenuated CLZ's 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.
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.
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