Cocaine-like discriminative stimulus properties of the δ-selective opioid receptor agonist, [D-Pen 2, L-Pen 5]enkephalin, in the rat

Abstract

The δ-selective opioid receptor agonist, [D-Pen 2, L-Pen 5]enkephalin (DPLPE) (10 μg i.c.v.), fully generalized to cocaine cue in the rat trained to discriminate 10 mg/kg of cocaine from vehicle. The cocaine-like discriminative stimulus effects of DPLPE were almost completely reversed by naltrindole (56 μg i.c.v.), a δ-selective opioid receptor antagonist. In contrast, the μ-selective opioid receptor agonist [D-Ala 2, NMePhe 4, Gly-ol]enkephalin (DAMGO) (0.03–0.3 μg i.c.v.) failed to generalize to cocaine cue. These results suggest that the discriminative stimulus effects of cocaine are mediated through the activation of δ-opioid receptors.