Analysis of sex and gonadectomy differences in beta-endorphin antinociception elicited from the ventrolateral periaqueductal gray in rats.

Male rats exhibit significantly greater antinociception following central administration of morphine than female rats. The present study examined potential differences in beta-endorphin (5.2-26 microg) antinociception elicited from the ventrolateral periaqueductal gray in adult sham-operated and gonadectomized male and female rats. Male rats displayed significantly greater peak (30 min) tail-flick latencies across the entire range of beta-endorphin doses administered into the ventrolateral periaqueductal gray than female rats tested during the estrous phase of the estrous cycle. Adult gonadectomy failed to appreciably change the pattern of this effect in either males of females. Thus, antinociception elicited from the ventrolateral periaqueductal gray by beta-endorphin, like morphine, is sensitive to sex differences.