Sarah A. Vunck

A comparison of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in DBA/2 and C57BL/6 inbred mice.

Inbred mouse strain comparisons are an important aspect of pharmacogenetic research, especially in strains known to differ in regard to specific neurotransmitter systems. DBA/2 mice differ from C57BL/6 mice in terms of both functional and anatomical characteristics of dopamine systems. Given the importance of D2 antagonism in the action of antipsychotic drugs and in theories regarding schizophrenia (i.e. the dopamine hypothesis), this study compared the discriminative stimulus properties of the atypical antipsychotic drug clozapine (CLZ) in C57BL/6 and DBA/2 inbred mice. DBA/2 and C57BL/6 mice were trained to discriminate 2.5 mg/kg of CLZ from vehicle in a two-lever drug discrimination procedure and tested with a variety of antipsychotic drugs and selective ligands. Both strains of mice readily acquired the CLZ discrimination. The atypical antipsychotic drugs olanzapine and risperidone fully substituted for CLZ in both DBA/2 and C57BL/6 mice, but ziprasidone fully substituted only in the C57BL/6 mice. The typical antipsychotic drug haloperidol produced partial substitution for CLZ in the DBA/2 mice, and the dopamine agonist amphetamine required a higher dose to reduce response rates significantly in DBA/2 mice as compared with C57BL/6 mice. Antagonism of serotonergic (5-HT2A/2B/2C) receptors with ritanserin and &agr;1-adrenergic receptors with prazosin engendered CLZ-appropriate responding only in the C57BL/6 mice. Thus, while serotonergic and &agr;-adrenergic antagonism were shown to be important for CLZs discriminative cue in C57BL/6 mice, none of the selective ligands produced CLZ-appropriate responding in DBA/2 mice. Differences in dopamine-mediated functions between the two strains of mice may explain some of the findings in this study.

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.

Behavioral metabolomics analysis identifies novel neurochemical signatures in methamphetamine sensitization.

Behavioral sensitization has been widely studied in animal models and is theorized to reflect neural modifications associated with human psychostimulant addiction. While the mesolimbic dopaminergic pathway is known to play a role, the neurochemical mechanisms underlying behavioral sensitization remain incompletely understood. In this study, we conducted the first metabolomics analysis to globally characterize neurochemical differences associated with behavioral sensitization. Methamphetamine (MA)‐induced sensitization measures were generated by statistically modeling longitudinal activity data for eight inbred strains of mice. Subsequent to behavioral testing, nontargeted liquid and gas chromatography–mass spectrometry profiling was performed on 48 brain samples, yielding 301 metabolite levels per sample after quality control. Association testing between metabolite levels and three primary dimensions of behavioral sensitization (total distance, stereotypy and margin time) showed four robust, significant associations at a stringent metabolome‐wide significance threshold (false discovery rate, FDR <0.05). Results implicated homocarnosine, a dipeptide of GABA and histidine, in total distance sensitization, GABA metabolite 4‐guanidinobutanoate and pantothenate in stereotypy sensitization, and myo‐inositol in margin time sensitization. Secondary analyses indicated that these associations were independent of concurrent MA levels and, with the exception of the myo‐inositol association, suggest a mechanism whereby strain‐based genetic variation produces specific baseline neurochemical differences that substantially influence the magnitude of MA‐induced sensitization. These findings demonstrate the utility of mouse metabolomics for identifying novel biomarkers, and developing more comprehensive neurochemical models, of psychostimulant sensitization.

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.