MODULATORY EFFECT OF STEROID-HORMONES ON GNRH-INDUCED LH-SECRETION BY CULTURED RAT PITUITARY-CELLS

The purpose of the present experiments was to examine the short- and long-term effects of estradiol-17beta (E2), progesterone (P), and 5alpha-dihydrotestosterone (DHT). alone and in combination, on the gonadotrophin-releasing hormone (GnRH)-induced luteinizing hormone (LH) secretion. using an ovariectomized rat pituitary cells culture model. After 72 h in steroid-free medium, pituitary cells were further cultured for 24 h in medium with or without E2 (1 nM), P (100 nM), or DHT (10 nM). Cultures were then incubated for 5 h in the absence or presence of 1 nM GnRH with or without steroids. LH was measured in the medium and celt extract by radioimmunoassay. The results show that the steroid hormones exert opposite effects on the release of LH induced by GnRH, which seems to be dependent upon the length of time the pituitary cells have been exposed to the steroids. In fact, short-term (5 h) action of E2 resulted in a partial inhibition 64% of control) of LH release in response to GnRH, while long-term (24 h) exposure enhanced (158%) GnRH-induced LH release. Similar results were obtained with DHT, although the magnitude of the effect was lower than with E2. Conversely, P caused an acute stimulatory action (118%) on the LH released in response to GnRH and a slightly inhibitory effect (90%) after chronic treatment. GnRH-stimulated LH biosynthesis was also influenced by steroid treatment. Significant increases in total (cells plus medium) LH were observed in pituitary cells treated with E2 or DHT. While the stimulatory effect of E2 was evident after both acute (133 %) and chronic (119 %) treatment, that of DHT appears to be exerted mainly after long-term priming (118 %). These results suggest that the steroids modulate GnRH-induced LH secretion by acting on both synthesis and release of LH. On the other hand, total hormone content was not affected by P. The acute (5 h) effects of E2, P, and DHT on the GnRH response in E2-primed (24 h) cells during a short-term incubation, were also tested. Addition of P to the pituitary cells primed with E2 led to an acute potentiation of the stimulatory effect of E2 on GnRH-induced LH release and total content. Conversely, the augmentative E2 effect on pituitary responsiveness to GnRH was abolished by DHT. Taken together, these findings suggest that the physiological significance of the stimulatory action of progesterone could be to define the final magnitude of the LH preovulatory surge, while the inhibition by DHT could be required to limit the LH surge to that day of proestrus.