Effects of brain lesions on the antinociceptive properties of morphine in rats

Abstract

1. Electrolytic lesions were made in various brain regions of the rat, and the effects of these lesions on nociceptive threshold and the antinociceptive actions of morphine were tested using a shock titration technique. 2. Lesions in the medial thalamus, the periaqueductal grey area, or the caudate nucleus, had no effect on the nociceptive threshold; whereas, lesions in the posterior hypothalamus resulted in a small but statistically significant increase in this threshold. 3. Morphine administered intraperitoneally to rats having histologically verified lesions in the posterior hypothalamus, the caudate nucleus or the periaqueductal grey area resulted in a 15-30% increase in the nociceptive threshold. This increase was similar to that observed in unoperated control rats. On the other hand, injections of morphine into animals having greater than 50% of the medial thalamus destroyed produced a highly significant increase of 95% in the threshold. This potentiation of the antinociceptive action of morphine was not observed in rats having less than 50% destruction of the medial thalamus.