Effects of centrally administered neuropeptides on discriminative stimulus properties of cocaine in the rat

Abstract

The present study was designed to investigate the effects of centrally administered neuropeptides on the discriminative stimulus properties of cocaine in the rat. Rats were trained to discriminate 10.0 mg/kg of cocaine from vehicle in a shock avoidance paradigm. The μ-selective opioid agonist [ d-Ala 2, NMePhe 4,Gly-ol]enkephalin (DAMGO) (0.03–0.3 μg, ICV) or the κ-selective opioid agonist dynorphin A-(1–13) (1.0–10.0 μg, ICV) did not generalize to cocaine cue, although the δ-selective opioid agonist [ d-Pen 2, l-Pen 5] enkephalin (DPLPE) (10.0 μg, ICV) reportedly generalizes to it through the mediation of δ-opioid receptors. Thyrotropin-releasing hormone (10.0–56.0 μg, ICV), somatostatin (0.3–3.0 μg, ICV), substance P (3.0–17.5 μg, ICV), or neurotensin (3.0–17.5 μg, ICV) did not produce any stimulus effects in common with cocaine. It appears that neuropeptides other than the δ-selective opioid do not play a major role in the discriminative stimulus properties of cocaine.