Differential patterns of gonadotropin responses to ovarian steroids and to LH-releasing hormone between constant-estrous and pseudopregnant states in aging rats.

Abstract

The stimulatory feedback actions of estradiol and progesterone on gonadotropin secretion were tested in acutely ovariectomized (OVX) old constant estrous (CE) and pseudopregnant (PSP) rats of similar age (20-26 months). Studies were also performed to determine the pituitary LH responses to synthetic LU-releasing hormone (LHRI-I) in both intact and OVX, estradiol-implanted CE and PSP females. Serial blood samples were collected through chronic intra-atrial cannulae for measuring plasma LU and FSH, while single blood samples were obtained for estrogen and progestin assays. Serum concentrations of estradiol were twice as high in CE (26 ± 3 pg/mI, mean ± SEM) as in PSP rats (11 ± 2 pg/mI), whereas progesterone and 20a-hydroxyprogesterone levels were many-fold greater in PSP than in CE rats. In acutely OVX-PSP rats, estradiol and progesterone administration elicited a significant increase in LH release. PSP females that had evidence of pituitary tumors and/or lesions showed an LH response to ovarian steroidssimilartothatof animals with normal pituitaries. In contrast, steroid administration failed to increase LH release in acutely OVX-CE rats, and this lack of LH response was found in animals regardless of whether pituitary pathology was evident. Despite these differences between the CE and PSP states, the pituitary LU responses to single iv. pulses of LHRH were the same in intact CE and PSP rats. Placement of estradiol implants into OVX-CE and PSP rats significantly increased serum estradiol concentrations in both groups, but enhanced the LU response to LHRH only in the PSP rats and not in the CE animals. The present study demonstrates that the stimulatory feedback actions of estradiol and progesterone on LH secretion, which are present in acutely OVX old PS? rats, are absent in old CE females immediately following ovariectomy. Together with other data the results suggest changes in central nervous function of CE rats, which are probably related to their ovarian steroid background rather than to advanced age per Se.