Chronological Changes in Sex Steroid, Gonadotropin and Prolactin Secretion in Aging Female Rats Displaying Different Reproductive States1

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Abstract
Longitudinal studies were performed in a colony of aging female rats, from 4-33 months of age, to determine the chronological change in reproductive patterns and the changes in sex steroid, prolactin and gonadotropin secretion associated with different reproductive states. The present study demonstrates that the incidence (65%) of irregular estrous cycles in aging rats increased abruptly from 10-12 months of age. Subsequently, female rats became chronically anovulatory with persistent vaginal cornifications and their ovaries contained developed follicles but no corpora lutea. The highest incidence (65%) of constant estrous (CE) rats occurred at the age of about 19 months. During the anovulatory state, CE rats displayed low to medium levels of serum estradiol, estrone, testosterone and androstenedione, low levels of progesterone and minimal levels of 20α-hydroxyprogesterone. Preovulatory increases in gonadotropin and prolactin release, similar to those seen in young cycling rats on proestrus, were not observed in CE rats. Whereas serum basal LH levels remained unaltered, morning FSH levels were increased in CE rats. The latter may account for the persistent follicular development in aging rats during chronic anovulatory state. Serum basal prolactin levels were normal in CE rats during the early phase (11-16 months of age) of the anovulatory state, but were subsequently increased 3 to 4-fold beyond 24 months of age. Moreover, ovariectomy at a young age prevented the increased pituitary prolactin release in old female rats. These results suggest that continuous exposure to medium levels of estrogens, particularly in the presence of sustained low progesterone secretion, may alter pituitary secretion of prolactin in aging rats.

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