Influence of corticosterone on the response to gonadotropin-releasing hormone in rats

The objective of this study was to determine whether the diurnal rhythm of corticosterone (B) alters the response of the pituitary to gonadotropin-releasing hormone (GnRH). Mature female rats housed in constant illumination were ovariectomized (Ovx) and adrenalectomized (Adx), and maintained on either corticosterone (B, 260 micrograms/ml in drinking water) or 0.9% saline. 3 weeks later a silastic implant of estradiol (E2) was implanted subcutaneously into each animal. For the next 5 days B-maintained rats received a daily injection of either B (250 micrograms/100 g BW) or vehicle at 05.00 or 17.00 h. Groups of rats were sacrificed at 0, 1, 3, 6, 12 and 18 h after the fifth injection, and plasma B and E2 were measured. Other animals were challenged with GnRH (1.0 ng/100 g BW) at 0, 6, 12 and 18 h after the last B injection. The fifth B injection initiated changes in plasma B which closely mimicked the normal B rhythm in intact rats. E2 levels (approximately 100 pg/ml) remained fairly constant over the same period. In rats receiving B injection at 17.00 h, the response to GnRH was altered by B administration. The LH response was greatest 6 h after B was injected, and animals showed a lower response at other times. The same relationship between circulating B and the response to GnRH could not be demonstrated in rats injected with B at 05.00 h. Animals maintained on saline exhibited higher basal LH levels, a longer mean half-life of LH, and the plasma LH response to GnRH was lower than in animals receiving B therapy. These results indicated that: (1) physiological dosages of B can alter the response to GnRH in constant light rats; (2) B has a potentiating effect on this response, and (3) B may also influence the clearance rate for LH.