Generalization of D-, L- and DL-chlorpheniramine and zolantidine to the discriminative stimulus effects of cocaine and methamphetamine.

Abstract

We recently demonstrated that some H-antagonists have cocaine or methamphetamine-like discriminative stimulus effects. In the present study, the effects of optical isomers of chlorpheniramine (D-, L- and DL-forms) on the discriminative stimulus effects of cocaine and methamphetamine were examined in rats trained to discriminate between cocaine (10.0 mg/kg) or methamphetamine (1.0 mg/kg) and saline, to determine whether these effects of H1-antagonists are mediated by the blockade of H-receptors. In generalization tests with optical isomers of chlorpheniramine, the D-, L- and DL-forms all completely generalized to the discriminative stimulus effects of cocaine, but did not generalize to those of methamphetamine. Dose-generalization by the optical isomers of chlorpheniramine to the discriminative stimulus effects of cocaine did not correlate with the H-antagonistic potency of these drugs. These results suggest that all of the optical isomers of chlorpheniramine have cocaine-like discriminative stimulus effects, but that these effects are not mediated by H1-receptor blockade. On the other hand, the H2-antagonist, zolantidine, generalized to the discriminative stimulus effects of methamphetamine, but not to those of cocaine, suggesting that zolantidine may have methamphetamine-like discriminative stimulus effects. In the present study, GBR12909 (dopamine uptake inhibitor) completely generalized to the discriminative stimulus effects of cocaine, but not to those of methamphetamine, whereas apomorphine (dopamine receptor agonist) generalized more potently to the discriminative stimulus effects of methamphetamine than to those of cocaine. These findings imply that although the dopaminergic system plays an important role in the discriminative stimulus effects of both cocaine and methamphetamine, there may be differences between their effects.