Small doses of apomorphine induce catalepsy and antagonize methamphetamine stereotypy in rats.

Small doses of apomorphine (AP, 31.25-125 micrograms/ kg IP) induced dose-dependent catalepsy in rats. However, unlike the stereotyped behavior induced by high doses of AP which has a rapid onset and is short-lasting, the cataleptic effect induced by small doses of AP was evident 30 min after AP injection and was unusually long-lasting. Further, AP (31.25-125 micrograms/kg) administered 60 min before methamphetamine was found to significantly antagonize the methamphetamine-induced stereotyped behavior. Pretreatment with molindone (0.45 and 0.8 mg/kg IP), in doses reported to selectively block the presynaptic DA receptors, not only decreased the cataleptic effect of AP but also reversed the AP antagonism of methamphetamine stereotypy. The results suggest that small doses of AP induce catalepsy and antagonize methamphetamine stereotypy probably by an action at presynaptic DA receptor sites.