Effects of chronic stimulation or antagonism of opiate receptors on GH secretion in male and female rats.

The present study was undertaken to assess the role of endogenous opioid systems in the sexually dimorphic pattern of growth hormone (GH) secretion. To this end, male rats were treated chronically (6 to 12 h) with morphine and estrogen-exposed, ovariectomized female rats with morphine or naloxone. Chronic morphine exposure of male rats caused a 12-fold increase in basal GH levels and a modest rise in GH pulse frequency. These two changes resulted in a 3-fold increase in both mean GH concentration and total GH secretion over 6 h. In female rats, chronic morphine reduced GH pulse amplitudes but did not significantly affect other parameters of GH secretion. By contrast, chronic naloxone treatment of female rats reduced basal GH levels by 64% without affecting GH pulse amplitudes or pulse frequency. These data suggest that endogenous opioid systems are involved in the regulation of the basal GH secretion in both male and female rats.