Effects of neuropeptide Y on the discriminative stimulus and antinociceptive properties of morphine

Previous research indicates that opioid receptor blockade diminishes the ef fects of neuropeptide Y (NPY) on feeding and memory. Conversely, NPY attenu ates naloxone-precipitated morphine withdrawal. The present study evaluated the effects of NPY on the discriminative stimulus and antinociceptive effe cts produced by the prototypical mu opioid, morphine. Rats were trained to discriminate 5.6 mg/kg morphine (IP) from saline using a standard two-lever , food-reinforced, drug discrimination procedure. Across a range of doses ( 3.0, 5.0, and 10 mu g), intracerebroventricular (ICV) injection of NPY fail ed to substitute for, antagonize, or potentiate the discriminative stimulus effects of morphine. A warm-water tail-withdrawal procedure was used to ex amine the antinociceptive effects of morphine and NPY, alone and in combina tion. NPY (3.0 and 10 mu g, ICV) failed to alter tail-withdrawal latencies from 52 degrees and 56 degrees C water, whereas morphine (1.0-30 mg/kg, IF) produced a dose-related increase in latencies at both water temperatures. A 10-mu g dose of NPY also failed to alter the antinociceptive effects of m orphine. This study does not support the idea that the discriminative stimu lus and antinociceptive effects of morphine are dependent on an NPYergic pa thway. (C) 1999 Elsevier Science Inc.