Effects of hypothalamic-pituitary aging on reproduction

Diminished reproductive performance in mammals during aging is complex and explanations are lacking for many aspects of age-related reproductive failure. Although several theories of the aging process have been offered, not all are applicable to aging in the reproductive system and to aging in the hypothalamic-pituitary system in particular. Considerable experimental effort has been directed toward the identification of events that initiate the decline in reproductive function and it appears that the level at which diminished reproductive function occurs is species-dependent. Estrogen-induced changes in the hypothalamus have been shown in both species; but it appears that they are directly of reproductive failure in the rodent only. The preponderance of the available evidence pertaining to hypothalamic-pituitary aging has been obtained in experiments involving either rodents or primates. Rodents and primates have different reproductive neuroendocrine control mechanisms and, therefore, different mechanisms of reproductive aging. The preponderance of the available evidence indicates that reproductive aging in rodents is initiated at the level of the hypothalamus whereas in primates it is relatively clear that ovarian exhaustion is responsible for decreasing reproductive function during aging. Regardless of where the process of reproductive aging is initiated, it appears that multiple cellular, molecular, and hormonal mechanisms occur simultaneously or in sequence to eliminate cyclic reproductive function as aging occurs.