Evidence for a role of the ventro-medial posterior hypothalamus in nociceptive processes in the rat

Bilateral, radio-frequency destruction of the ventro-medial posterior hypothalamus (VMPH) resulted, as compared to sham-operated and control rats and evaluated in the tail-flick and vocalization tests, in a significant decrease in basal nociceptive threshold on day 4 post-surgery. By day 12, however, no significant difference between sham and lesioned rats was seen. At this time the antinociception elicited by either acute foot-shock or cold-water-immersion stress was profoundly attenuated. The antinociceptive response to various doses of morphine was not, in contrast, diminished. As established by use of radioimmunoassay, these lesions did not significantly alter hypothalamic levels of beta-endorphin, met-enkephalin, dynorphin or alpha-neo-endorphin. They did, however, produce a pronounced and significant fall in the hypothalamic content of substance P. These data are indicative that the VMPH may, via a mechanism not involving endorphins, be of importance in the determination of basal nociceptive threshold and in the generation of stress-, but not morphine-, evoked antinociception. The relationship of these findings to the interconnections of the VMPH, and to the possible significance of substance P and the pituitary in nociceptive processes, is discussed.