Changes in Control of Gonadotropin Secretion in the Transition Period between Regular Cycles and Constant Estrus in Aging Female Rats1

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Abstract
The control of gonadotropin secretion in female rats was evaluated during the transition period between cyclic and acyclic reproductive function. Mid-aged irregularly cycling (IRC), mid-aged constant estrus (CE) and old CE rats showed an attenuated LH, but normal FSH, response to ovariectomy when compared with young rats. Old CE rats did not respond to the positive feedback action of estrogen (E) or progesterone (P) on LH release, while mid-aged IRC and CE rats showed a reduced LH response when compared with young rats. A similar FSH surge was induced by E or P in all groups. The data suggest that mid-aged rats are in a transition period during which alterations in the neuroendocrine system occur that eventually lead to an acyclic reproductive state.