DYNAMIC CHANGES IN HYPOTHALAMIC LH-RH LEVELS ASSOCIATED WITH THE OVARIAN STEROID-INDUCED GONADOTROPHIN SURGE

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Abstract
Pro-estrous-type LH [luteinizing hormone] surges were induced by treating ovariectomized rats (day 0) with either estradiol benzoate (EB, 40 .mu.g/rat) or estrogen (10 .mu.g/rat) followed by progesterone (4 mg/rat, day 2, EBP). To characterize the temporal changes associated with the LH surge on day 2, LH-RH [LH-releasing hormone] levels in the medial basal hypothalamus (MBH) and the pre-optic area (POA) were analyzed by RIA [radioimmunoassay] in rats sacrificed 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 EB rats, the MBH LH-RH levels were unchanged through the initial LH surge period (13.00-15.00 h) declining significantly, as in the EBP group, near the end of the surge period (19.00 h). However, in EBP 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 EBP group alone showed a surge of serum FSH [follicle stimulating hormone].