Abolition of conditioned heart-rate responses in rabbits following central administration of [N-MePhe3, D-Pro4] morphiceptin.

Overview

Rabbits were initially habituated to a tone and then give 15-20 paired trials where the tone was followed by periorbital shock resulting in conditioned heart-rate decelerations to tone onset and an acceleration of heart-rate following shock offset. The animals were matched for learning performance and divided into two groups. Each animal received a microinfusion into the region of the fourth ventricle of either the opiate [N-MePhe3,D-Pro4] morphiceptin, a highly selective mu receptor agonist, or a mixture of the morphiceptin analogue and the opiate antagonist naltrexone. Administration of the morphiceptin analogue eliminated the conditioned bradycardia to the tone but not the acceleration to the shock. Rabbits given the mixture continued to show conditioned heart-rate decelerations. Previous studies have shown that opiates abolish a recently learned conditioned nictitating membrane response. These effects are consistent with the hypothesis that the opiate effect on conditioning is due to an attenuation of conditioned fear.