Cytoplasmic and nuclear receptor-estradiol complex in the hipothalamus and pituitary: relationship with pituitary sensitivity to gonadotropin releasing hormone and gonadotropin secretion in the rat.

S.c. introduction of silastic capsules containing estradiol-17beta (E2) in the ovariectomized rat causes a surge of luteinizing hormone (LH) on the following day. The levels of cytoplasmic and nuclear E2 receptor complex were measured in the pituitary and hypothalamus at 0, 9, 24 and 33 h intervals after placing the E2 implant. The pituitary sensitivity to exogenous gonadotropin releasing hormone (GnRH) with respect to LH release in vitro was also measured. The results suggest that: (1) the pituitary sensitivity to exogenous GnRH remained higher 9, 24, and 33 h after placing the implant; (2) the circulating LH level decreased at 24 h and then increased at 33 h; (3) the pituitary and hypothalamic E2 receptor levels show striking differences during E2-induced LH release. Thus, the relationship between pituitary sensitivity to exogenous GnRH and the E2 receptor level in the pituitary and hypothalamus is complex.