Studies to determine the site of increased estradiol negative feedback in the underfed female rat
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 (E$\sb2$) 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 E$\sb2$ levels increased, the number of animals pulsing decreased. These results suggest that the suppression of LH pulses by E$\sb2$ 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 E$\sb2$ 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 E$\sb2$ either in vivo or in vitro. In summary, by studying the effect of chronic low plasma levels of E$\sb2$ 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 E$\sb2$ negative feedback at the CNS.
Patricia Ann Spillar,
"Studies to determine the site of increased estradiol negative feedback in the underfed female rat"
(January 1, 1990).
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