Date of Award
8-1990
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Biological Sciences
First Advisor
Bela E. Piacsek
Second Advisor
Robert Fitts
Third Advisor
John Buntin
Fourth Advisor
Nelson Horsemen
Fifth Advisor
Brian R. Unsworth
Abstract
Reduced food intake results in altered reproductive function in female mammals, and this is due, at least in part, to an increase in the negative feedback efficacy of estradiol (E2) on pituitary luteinizing hormone (LH) secretion. The purpose of this thesis was to determine the site (pituitary or central nervous system (CNS)) at which E$\sb2$ feedback is altered in the underfed adult female rat. In the first group of studies, ovariectomized (OVX) underfed (UR) and control (CR) rats were exposed to three levels of steroid, all within the low physiological range of normally cycling intact rats. All CR exhibited pulsatile LH secretion at all doses. Within the UR group, however, as plasma E2 levels increased, the number of animals pulsing decreased. These results suggest that the suppression of LH pulses by E2 in UR may be an "all-or-none" phenomenon which is more likely to occur at a CNS site than at the pituitary level. Although the first group of studies provided evidence that underfeeding alters negative feedback at the CNS level, it did not eliminate the possibility that E2 feedback is also altered at the pituitary in the UR. The second set of studies revealed that underfeeding does not alter the ability of the pituitary to respond to exogenous gonadotropin releasing hormone (GnRH) either in the presence or absence of E2 either in vivo or in vitro. In summary, by studying the effect of chronic low plasma levels of E2 in the UR, first on LH pulses, and then on the sensitivity to exogenous GnRH, we have provided evidence that underfeeding increases the efficacy of E2 negative feedback at the CNS.