Hypothalamic alterations and reproductive aging in female rats: evidence of altered luteinizing hormone-releasing hormone neuronal function.

Prior to the age-related loss of regular estrous cycles, female rats exhibit an attenuated preovulatory LH surge, a sign that reproductive decline is imminent. Numerous studies have revealed an important role for the hypothalamus in aging of the reproductive axis in this species. Because LHRH represents the primary hypothalamic signal that regulates gonadotropin release, assessments of LHRH neuronal activity can provide a window into hypothalamic function during reproductive aging. Studies of the dynamic activity of LHRH neurons during times of enhanced secretion have revealed deficits in middle-aged females. Available data are consistent with a decline in LHRH synthesis, transport, and secretion in middle-aged females during times of increased demand for LHRH output. Moreover, the alterations noted in LHRH neuronal function could account, in part, for the attenuation and eventual loss of the preovulatory LH surge with age. Elements extrinsic to LHRH neurons undoubtedly contribute to the decline in the parameters of LHRH neuronal function observed in middle-aged females. Whether alterations intrinsic to LHRH neurons also play a role in the age-associated reduction in LHRH synthesis and secretion remains to be determined. Recent examinations of hormone profiles during the perimenopausal period suggest that a potential hypothalamic contribution to aging of the reproductive axis in women warrants further examination.