(S)-amisulpride as a discriminative stimulus in C57BL/6 mice and its comparison to the stimulus effects of typical and atypical antipsychotics

Amisulpride, a substituted benzamide derivative, exerts atypical antipsychotic and antidepressant clinical effects and its (S)-stereoisomer is thought to underlie these actions. In the present study, male C57BL/6 mice were trained to discriminate (S)-amisulpride (10mg/kg, s.c.) from vehicle in a two-lever drug discrimination task for food reward. The (S)-amisulpride stimulus was rapidly acquired and was shown to be dose-related, time dependent (effective between 30 and 120min) and stereoselective: (S)-amisulpride (ED50=1.77mg/kg; 4.2µmol/kg) was about three times more potent than rac-amisulpride (ED50=4.94mg/kg; 13.4µmol/kg) and ten times more potent than (R)-amisulpride (ED50=15.84mg/kg; 42.9µmol/kg). In tests of stimulus generalization, the (S)-amisulpride stimulus generalized completely to sulpiride (ED50=12.67mg/kg; 37.1µmol/kg), a benzamide analog that also is purported to be an atypical antipsychotic, but did not fully generalize to the typical antipsychotic drug haloperidol (maximum of 45% drug-lever responding) nor to the atypical antipsychotic drugs clozapine (partial substitution of 65% drug-lever responding) or aripiprazole (~30% drug-lever responding). These results demonstrated that (S)-amisulpride appears to exert a unique discriminative stimulus effect that is similar to other benzamides, but which differs from other structural classes of antipsychotic drugs.