Opioid peptidergic control of gonadotropin-releasing hormone pulse generator activity in the ovariectomized goat

The role of endogenous opioid peptides in the control of gonadotropin-releasing hormone pulse generator activity was examined in long-term ovariectomized goats by recording the hypothalamic multiple-unit activity specifically associated with pulsatile luteinizing hormone secretion (MUA volleys). Administration of naloxone, an opioid receptor antagonist, at a rate of 1.0 mg/kg/h for 2 h resulted in shorter (p < 0.05) intervals between MUA volleys than during the control period (61.6 +/- 0.7 vs. 92.5 +/- 7.2 min) in animals treated with progesterone and estradiol that had reproduced a steroidal milieu during the luteal phase. Similarly, in the absence of steroidal treatment, the interval between MUA volleys was shortened (p < 0.01) in a dose-dependent manner from 42.3 +/- 3.7 min (control) to 35.1 +/- 3.9, 32.6 +/- 4.1, and 27.0 +/- 1.7 min when infused with 0.2, 1.0, and 5.0 mg/kg/h of naloxone, respectively. On the other hand, the duration of each MUA volley was lengthened by naloxone. The results indicate that the hypothalamic gonadotropin-releasing hormone pulse generator activity is under a tonic suppression by endogenous opioid peptides in ovariectomized goats.