Effects of adrenalectomy and gonadectomy on the antinociceptive effect of morphine and naloxone antagonism in mice

In the present study the effects of adrenalectomy and gonadectomy on the antinociceptive activity of morphine and its antagonism by naloxone were studied in male and female mice. In the female mice, it was found that adrenalectomy enhanced the antinociceptive activity of morphine by two-fold, while the antinociceptive effects of morphine measured in the presence of naloxone were similar to those of the sham-operated controls. The naloxone potency ratios, expressed as the ratios of morphine ED50s with naloxone to that without naloxone, were increased in adrenalectomised animals. Similar effects were observed in oophorectomised female mice though the influence of oophorectomy on naloxone antagonism was not as effective as adrenalectomy. In animals that were both adrenalectomised and oophorectomised the antinociceptive effect of morphine measured was similar to those animals receiving either operative procedure. However, the naloxone potency ratios determined were between those determined in adrenalectomised mice and oophorectomised animals. In male mice, the effect of adrenalectomy was similar to that observed in the female animals, namely both the antinociceptive activity of morphine and naloxone antagonism were enhanced. Orchidectomy enhanced the antinociceptive effect of morphine but not to the same extent as adrenalectomy. Furthermore, the enhancement of naloxone antagonism was also less than that induced by adrenalectomy. The effects of adrenalectomy and orchidectomy in male mice were similar to those of adrenalectomy alone. These results suggest that steroid hormones are involved in the actions of both morphine and naloxone.