Effects of intracerebroventricular injection of opioid peptides selective for ?, ? and ? receptors on discriminative stimulus properties of pentazocine in the rat

The present study was designed to investigate the effects of centrally administered morphine and opioid peptides on the discriminative stimulus properties of pentazocine in the rat. Rats were trained to discriminate 3 mg/kg (s.c.) of pentazocine from vehicle in a shock avoidance paradigm. A 3 mg/kg (s.c.) dose of pentazocine produced stimulus effects in common with those induced by a training dose (3 mg/kg, s.c.) of pentazocine. Morphine (0·1–3 μg, i.c.v.) produced a dose-dependent increase in responding appropriate for pentazocine lever. The μ-selective opioid receptor agonist [D-Ala2,NMePhe4,Gly-ol] enkephalin (DAMGO) (0·0003–0·03 μg, i.c.v.) generalized to pentazocine cue. d-Pen2, l-Pen5] enkephalin (DPLPE) (3 and 10 μg, i.c.v.), a δ-selective opioid receptor agonist, produced partial generalization to pentazocine cue. However, the κ-selective opioid receptor agonist dynorphin A-(1–13) (3 and 10 μg, i.c.v.) did not generalize to pentazocine cue. The pentazocine-like discriminative stimulus effects of morphine (3 μg, i.c.v.) and DAMGO (0·03 μg, i.c.v.) were fully reversed by intracerebroventricular injection of the μ-selective opioid receptor antagonist β-funaltrexamine (5 μg, i.c.v.). These results suggest that μ-opioid receptors play a major role in the discriminative stimulus effects of pentazocine, while δ-opioid receptors only partially contribute to them. © 1998 John Wiley & Sons, Ltd.