Differential effects of low serum levels of estradiol-17 beta on hypothalamic LHRH levels and LH secretion in castrated male rats

Abstract

In castrated male rats, gonadal steroid treatment raises the medial basal hypothalamic (MBH) LHRH levels and suppresses LH release. We have now examined whether the steroid-induced MBH LHRH response could occur concomitantly with normal pituitary LH secretion. Of the various concentrations tested, serum estradiol-17 beta (E2) levels in the range of 9-16 pg/ml consistently and selectively stimulated the accumulation of LHRH in the MBH after a long latency period of 60-72 h. These low E2 levels did not suppress LH release at any time interval. Serum LH levels and the episodic pattern of LH secretion were similar to those of castrated control rats. It is unlikely that enhanced pituitary sensitivity in E2-treated rats may have compensated for a decrease in LHRH release since the LH response to exogenous LHRH was similar in E2-treated and control rats. These studies show that dissociation of the feedback effects on the MBH LHRH and pituitary gland LH release may be obtained with low serum concentrations of E2 in male rats; low E2 levels stimulate LHRH accumulation in the MBH by mechanisms which may not involve inhibition of LHRH release.