DIFFERENTIAL-EFFECTS OF ADRENALECTOMY ON THE PROLACTIN-INDUCED SUPPRESSION OF LH AND FSH-SECRETION AFTER CASTRATION IN MALE-RATS

Hyperprolactinaemia inhibits gonadotrophin secretion in males and fema les of many species. The aim of this study was to determine the role o f the adrenal gland in mediating the inhibitory effects of prolactin b y contrasting the effects of acute hyperprolactinaemia on LH and FSH s ecretion in adrenal-intact and adrenalectomized rats with and without physiological corticosterone replacement. Adult male rats were adminis tered purified ovine prolactin every 12 h (2.4 mg per injection s.c.) beginning at the time of castration. Blood samples were collected ever y 3 h for 36 h, then every 12 h until 10 days after castration. Ovine prolactin significantly reduced LH secretion in all groups from approx imately 15 to 48 h after castration. In contrast, plasma FSH concentra tions were reduced by ovine prolactin from 21 to 48 h only in the adre nal-intact rats and not in the adrenalectomized or adrenalectomized pl us corticosterone groups. In all groups, ovine prolactin inhibited end ogenous prolactin secretion in rats by short-loop autofeedback as soon as 3 h after the first ovine prolactin injection and throughout the 1 0 days of the study. Adrenalectomy per se, with or without corticoster one replacement, also had a differential effect on LH and FSH secretio n after castration, causing only a transient delay in the rise in LH a fter castration, but inducing a significant and long-lasting inhibitio n of FSH secretion. The results demonstrate that ovine prolactin-induc ed suppression of LH secretion after castration occurs with or without the adrenal glands. Suppression of FSH secretion after castration by ovine prolactin, however, may involve an adrenal component. The rise i n FSH after castration, but not of LH, requires the involvement of sti mulatory factors from the adrenal gland. The adrenal gland, therefore, appears to have a differential role in the control of LH and FSH secr etion following castration and also in mediating the inhibitory effect of hyperprolactinaemia on gonadotrophin secretion. In contrast to the transient suppression of LH and FSH by acutely administered ovine pro lactin, endogenous prolactin secretion in rats is continuously suppres sed by exogenously induced hyperprolactinaemia.