Luteinizing Hormone Regulation and Sexual Behavior in Middle-Aged Female Rats*

Abstract

LH regulation and lordosis were compared in groups of young (3 months old) and middle-aged (8–14 months old) female rats, all of whom were exhibiting regular estrous cycles at the beginning of experimentation. There were four main findings. First, in females having 4-day estrous cycles, the preovulatory surges of LH and progesterone were significantly lower in middle-aged animals. Second, phasic LH release, elicited in ovariectomized females with either estradiol benzoate (EB) and progesterone or EB and mating, was significantly reduced in middle-aged females by 8 months of age. This effect was evident in short and long term ovariectomized females. Third, the postovariectomy rise in LH and the negative feedback response to EB were attenuated in middle-aged females, but not until 11 months of age. Fourth, middle-aged and young females were equivalent in lordotic responses after ovariectomy and EB administration. Plasma steroid measurements at selected time points during the experiments confirmed that the steroid injections produced statistically equivalent levels of estrogen and progesterone in the various age groups. Thus, the deficiencies in LH regulation reflect changes in gonadotropin control mechanisms and are associated with reduced estrogen sensitivity. Although tonic LH regulation, phasic LH regulation, and sexual behavior all require the action of estrogen on neuroendocrine mechanisms, differences are evident in the developmental rate and degree of aging effects in these systems, and aging does not involve a general loss of estrogen sensitivity in neuroendocrine mechanisms.