Diurnal rhythmicity and hypothalamic deficits in glucose utilization in aged ovariectomized rats

Aging is associated with a loss of cyclic gonadotropin release in female animals. This deficit may reflect dampened circadian rhythmicity of neuroendocrine events and/or altered function in hypothalamic nuclei important to regulation of cyclic female reproduction. The purpose of this study was to determine if diurnal periodicity and glucose metabolism in the hypothalamus are altered with age and whether such changes could help to explain the age-related deficits in gonadotropin release. Young (3-4-month-old) and old (18-21-month-old) rats were ovariectomized and subjected to the 2-deoxy-D-1-¹⁴C-glucose technique to measure rates of cerebral glucose utilization (GU), an index of neural function (Sokoloff et al., 1977) in various brain areas and in the pineal gland. We measured GU during the light (1400 hours) and the dark (2200 hours) in 17 anatomical regions including the following hypothalamic areas: medial preoptic nucleus, suprachiasmatic preoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, arcuate nucleus, and median eminence. Serum concentrations of luteinizing hormone (LH) and prolactin were measured in the same rats to determine the effect of age on both of these hormones. Diurnal periodicity of GU was observed in the suprachiasmatic nucleus and the pineal gland in young and old rats. Although there was no age difference in GU of the pineal gland, GU was reduced during the light and dark in the suprachiasmatic nucleus and all other hypothalamic areas examined except the suprachiasmatic preoptic nucleus and the median eminence. Ovariectomy induced an attenuated increase in concentrations of LH in old, compared to young rats. Serum concentrations of prolactin were the same in both young and old ovariectomized rats. The results indicate that the functional activity of several specific hypothalamic nuclei important to luteinizing hormone-release hormone (LHRH) release and reproductive function declines with age. In particular, diminished GU in the suprachiasmatic nucleus may reflect changes in neural function in this important pacemaker area of the brain.