Relationship between Circulating Estrogens and the Central Mechanisms by which Ovarian Steroids Stimulate Luteinizing Hormone Secretion in Aged and Young Female Rats*

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Abstract
The present study was undertaken to determine the relationship between a persistent elevation in circulating estrogens and the lack of a stimulatory feedback of ovarian steroids on LH secretion in old persistent estrous (PE) rats and to investigate whether, under experimental conditions in young cyclic females, a chronic rise in circulating estradiol inhibits the positive feedback action of steroids on LH secretion. Eighteenmonth-old PE rats and 4-month-old regularly cyclic females were ovariectomized (OVX) and immediately primed with estradiol benzoate (EB; 8 /μg/100 g BW, sc). Three days later, progesterone injection (0.5 mg/100 g BW, sc) elicited a LH surge in formerly cyclic but not PE rats. Five weeks later, however, EBprogesterone administration to the same animals elicited a LH surge not only in OVX young rats but also in OVX old females which had previously been in the PE state, although the magnitude of the LH surge was significantly less in OVX-PE than in OVX young rats. In long term OVX old and young rats, sc placement of Silastic implants containing 17β-estradiol produced a chronic (7-week) elevation in circulating estradiol (∼30 pg/ml) similar to that found in intact old PE females, and completely inhibited the stimulatory actions of EB-progesterone on LH secretion. However, both groups of animals with estradiol implants showed significant increases in LH release after iv LHRH administration, suggesting a central nervous system rather than a pituitary defect(s) of LH secretion. It is concluded that chronic elevation in circulating estrogens inhibits the central nervous system mechanism(s) which mediates the stimulatory feedback actions of ovarian steroids on LH secretion in both old and young rats, and that a lack of positive feedback on LH secretion in old PE females is not because of advanced age per se. (Endocrinology108: 836, 1981)

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