Date of Award

Summer 1993

Degree Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Biology

Abstract

Reductions in food intake interrupts ovulation and causes cessation of menstrual and estrous cycles in adult mammals. The mechanism by which this underfeeding acts to disrupt fertility is not completely understood. It is known that there is an increased negative feedback efficacy by estrogen for LH secretion in the underfed adult female rat. The role of progesterone in mediating the effects of underfeeding has not been investigated. The focus of this work has been to investigate the possible role of progesterone in suppressing LH secretion in the underfed adult female rat. The first study was undertaken to determine plasma estrogen and progesterone levels in intact underfed rats. Mean plasma estrogen levels were 11.8 pgjml and mean plasma progesterone levels were 8.62 ngjml . Both of these values are near basal levels for each hormone measured in normally fed intact controls. The primary source of progesterone, in both the underfed and the control, was the ovary itself and not the adrenal. Underfed animals in subsequent studies were treated with approximately intact levels of progesterone and estrogen via subcutaneously implanted Silastic capsules. Blood samples for all studies were collected, under ether anesthesia, via cardiac puncture, exsanguination, or carotid cannula, and were then assayed by RIA for LH, progesterone , and estrogen . These studies have shown that by itself, progesterone has no effect on plasma LH levels in the underfed, as has already been demonstrated for the normally fed animal. When progesterone was administered in combination with estrogen, either sequentially or simultaneously, there were small, but not significant, reductions in mean plasma LH levels as compared to estrogen treatment alone. Thus progesterone does not act to augment estrogen ' s negative feedback effect on LH secretion. Analysis of the LH pulse profile revealed no effects attributable to progesterone on LH pulse frequency, amplitude, or length . It did, however, provide evidence to support the hypothesis that the effect of underfeeding, in terms of estrogen's negative feedback effect, is mediated at the level of the hypothalamus or at other higher brain centers that affect GnRH secretion. Thus, any minor effect of progesterone may be exerted at the initiation of, or in the long-term maintenance of diet- induced anestrous, and it may well be that estrogen is the primary mediator of dietinduced anestrus' effects in the short-term in the adult female rat .

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