Dynamic changes in hypothalamic LH-RH levels associated with the ovarian steroid-induced gonadotrophin surge

Pro-oestrous-type LH surges were induced by treating ovariectomized rats (day 0) with either oestradiol benzoate (OeB, 40 micrograms/rat) or oestrogen (10 micrograms/rat followed by progesterone (4 mg/rat, day 2, OeBP). To characterize the temporal changes associated with the LH surge on day 2, LH-RH levels in the medial basal hypothalamus (MBH) and the pre-optic area (POA) were analyzed by RIA in rats sacrified at hourly intervals between 10.00--19.00 h. Following these treatments the LH-RH content of POA fluctuated somewhat but no definite pattern was observed. In OeB rats, the MBH LH-RH levels were unchanged through the initial LH surge period (13.00--15.00 h) declining significantly, as in the OeBP group, near the end of the surge period (19:00 h). However, in OeBP rats, the MBH LH-RH contents decreased at 14.00 h and these were followed by markedly elevated levels between 15.00--17.00 h coincident with maximal increments in serum LH. The OeBP group alone showed a surge of serum FSH.