Behavioral interactions between naloxone and dopamine agonists.

A fixed-interval schedule of food presentation was used to study the effects of apomorphine, ET-495 and clonidine and interactions between these drugs and naloxone in the rat. The rate of responding under this schedule was decreased by low doses of apomorphine (0.03--0.3 mg/kg), ET-495 (0.3-1.0 mg/kg) and clonidine (0.03--0.3 mg/kg). Naloxone, at a dose (3 mg/kg) which did not itself alter responding, potentiated the rate decreasing effects of apomorphine and ET-495 but did not alter the effects of clonidine. It is suggested that the rate decreasing effects of low doses of apomorphine and ET-495 may be related to presynaptic inhibition of dopamine neurons and that under certain circumstances naloxone may also inhibit dopaminergic activity.