Endocrinology, Vol 123, 2132-2139, Copyright © 1988 by Endocrine Society

ARTICLES

Episodic prolactin secretion in the adult male rat is affected by orchidectomy and testosterone replacement

PM Grosser and B Robaire
Department of Pharmacology and Therapeutics, McGill University, Montreal, Quebec, Canada.

To characterize the plasma PRL pattern in adult male rats and elucidate the modulatory effects of testosterone on this circulating PRL pattern, serial blood samples were obtained from both intact rats and rats orchidectomized and given various doses of testosterone via sustained release polydimethylsiloxane implants. Adult male rats were equipped with chronic indwelling jugular catheters. One week later the animals were orchidectomized and given either empty (12-mm) or testosterone- filled polydimethylsiloxane implants measuring 3, 5, or 12 mm that maintain plasma testosterone at 0%, 20%, 30%, or 60%, respectively, of those concentrations found in normal animals. Plasma samples for PRL RIA were obtained every 5-10 min for 3 h, before (intact control) and 3, 6, 9, 15, 21, and 28 days after orchidectomy and testosterone replacement. The plasma PRL pattern in intact animals was pulsatile; on the average, three or four pulses per 3 h, with amplitudes of 3.6 ng/ml on a 2.7 ng/ml nadir, were seen. After orchidectomy PRL pulse nadir, peak, and amplitude were rapidly attenuated. These parameters stabilized between days 6 and 15 at levels approximately 40% of those recorded in intact rats. In contrast, PRL pulse frequency remained in the control range for the first 9 days after orchidectomy. Thereafter, pulse frequency accelerated and reached stable plateau levels by day 15 at 145% of the values seen before orchidectomy. The administration of 3- mm testosterone implants completely prevented the effects of orchidectomy on PRL pulse nadir, peak, and amplitude, but only partially prevented the postorchidectomy rise in pulse frequency. Although the two larger implants (5 and 12 mm) had no further effect on pulse nadir, peak, and amplitude over that seen with the 3-mm implant, only the 12-mm implant completely prevented the acceleration in PRL pulse frequency accompanying orchidectomy. These results indicate that testosterone is intimately involved in regulation of the circulating PRL pattern and that this steroid has effects on the neuroendocrine system controlling PRL pulse frequency independent of those regulating pulse nadir, peak, and amplitude.