The present study examined the gonadotropin surge-inducing actions of estradiol (E2), both alone and with progesterone (P4), in middle-aged, early persistent-estrous (PE) female rats that had become PE within 35 days. In addition, we also assessed the effect of P4 on the mating-induced gonadotropin surges in these acyclic animals. Early PE rats were ovariectomized and received E2 implants (Day 0). On Day 4, an s.c. injection of P4 (0.5 mg/ 100 g body weight) at 1200 h markedly increased plasma P4 and elicited both LH and FSH surges, whereas vehicle-treated controls displayed no rise in P4 or gonadotropins. This observation confirms that at middle age, female rats no longer respond to the positive-feedback stimulation of E2 on gonadotropin surges whenever the estrous cyclicity ceases. As PE continued, such a surge-inducing action of E2 plus P4 became diminished after 75 days of PE and disappeared thereafter. When caged with males, vehicle-treated early PE rats display a mating-induced increase in P4 from the adrenal along with small gonadotropin surges. The amplitude of these mating-induced gonadotropin surges was enhanced by supplementation with exogenous P4 in early PE rats. Our findings indicate that during the early phase of PE, the surge-inducing action of E2 and P4 remains intact but deteriorates as PE continues. Thus, a deficiency in P4 secretion during aging may contribute to the diminished gonadotropin surge response in the hypothalamic-pituitary axis and the subsequent cessation of estrous cyclicity.
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