Effect of Ovariectomy on Plasma LH, FSH, Estradiol, and Progesterone and Medial Basal Hypothalamic LHRH Concentrations in Old and Young Rats

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Abstract
Title hormones were measured by RIA [radioimmunoassay] in 8-12 mo. old female rats which had begun to exhibit constant estrous (CE) or prolonged diestrous (PD) vaginal smear patterns and compared to young cycling rats on proestrus, estrus or diestrus. In addition, the effect of ovariectomy on these hormonal profiles was examined. Old CE rats had normal plasma LH [luteinizing hormone], FSH [follicle stimulating hormone] and progesterone concentrations, but exhibited elevated estradiol levels and decreased MBH-LHRH [medial basal hypothalamic luteinizing hormone releasing hormone] concentrations compared to young cycling rats on the day of estrus. Ovariectomy resulted in an attenuated rise in plasma LH and FSH and a much lesser decrease in MBH-LHRH when compared to young rats, despite comparable steroid changes. Old PD rats had normal LH and FSH levels, but had elevated estradiol and progesterone concentrations and decreased MBH-LHRH levels when compared to young rats on the day of diestrus. Ovariectomy caused a normal decrease in MBH-LHRH; however, the increased gonadotropin levels were significantly less than in young controls.