Gonadal steroids do not affect basal growth hormone response to naloxone in humans

Evidence has accumulated that endogenous hypothalamic opioid activity fluctuates through the menstrual cycle depending upon the ovarian steroid milieu. In fact, naloxone, the specific opiate antagonist, is more effective in producing neuroendocrine changes in the late follicular and midluteal phases of the menstrual cycle, when the functional activity of hypothalamic opiate system is high. In order to investigate a possible regulatory function of endogenous opioids on basal growth hormone (GH) secretion in humans, we studied the basal GH response to naloxone (2 mg iv as a bolus) in different phases of the menstrual cycle in ten regularly menstruating women and in eight hypogonadal (postmenopausal) females before and after estrogen treatment. This protocol was carried out to test the hypothesis that estrogens could sensitize basal GH response to opiate receptor blockade. The results do not support this view and suggest that, under basal conditions, hypothalamic opiates have minimal influence on GH secretion in humans.