Receptor-selective opioid peptides fail to affect behavioral responses induced by a low dose of apomorphine in the mouse.

The effects of intracerebroventricular injections of the mu-selective opioid agonist DAMGO ([D-Ala2,NMePhe4,Gly-ol]enkephalin), the kappa-selective opioid agonist dynorphin A-(1-13), and the delta-selective opioid agonist DPLPE ([D-Pen2,L-Pen5]enkephalin) on the decrease in different behavioral responses induced by a low dose of apomorphine (0.03 mg/kg) were investigated in the mouse. A low dose (0.03 mg/kg) of apomorphine produced a marked decrease in behavioral responses such as circling and rearing. Although the dopamine D1 antagonist SCH 23390 (0.01 and 0.03 mg/kg) did not influence behavior induced by apomorphine (0.03 mg/kg), the dopamine D2 antagonist sulpiride (3.0 mg/kg) reversed the decrease in circling and rearing behavior induced by apomorphine, suggesting that the effects of apomorphine on circling and rearing are mediated through dopamine D2 autoreceptors. DAMGO (0.003 or 0.01 microgram), dynorphin A-(1-13) (3.0 or 10.0 micrograms), or DPLPE (0.3 and 1.0 microgram) had no significant effects on the apomorphine-induced decrease in circling and rearing behavior. These in vivo results suggest that opioid peptides selective for receptor types fail to influence drug effects mediated by dopamine D2 autoreceptors.