AGE-RELATED DIFFERENCES IN THE RELEASE OF LUTEINIZING-HORMONE AND GONADOTROPIN-RELEASING-HORMONE IN OVARIECTOMIZED RATS

The effect of aging on the release of gonadotropin-releasing hormone (GnRH) in vitro and of luteinizing hormone (LH) both in vivo and in vitro in ovariectomized (Ovx) rats was studied. Old (21-24 months) and young (3-4 months) rats were Ovx before use. They were injected subcutaneously with estradiol benzoate (25 ttg⁄kg) or sesame oil for 3 days and then challenged with GnRH (0.5, 2 or 10 ug⁄kg) via a jugular catheter. Blood samples were collected immediately before and at 5, 10,20, 40 and 60 min following GnRH injection. For in vitro study, Ovx rats were decapitated. The anterior pituitary glands (APs) were incubated with GnRH (0.1 or 10nAT)and estradiol (0, 0.1, 1 or 10 nM) at 37 °C for 30 min. The mediobasal hypothalamus was superfused with Locke’s solution at 37 °C for 210 min, and stimulated with 60 mM KCI at 90 and 150 min. The medium samples were collected at 10-min intervals. Concentrations of GnRH and LH in plasma and medium samples were measured by radioimmunoassay. In all rats, the basal and GnRH-stimulated levels of plasma LH were lower in old than in young rats. The spontaneous release of LH in vitro from APs of Ovx rats was increased by aging, whereas GnRH-stimulated release of LH in vitro was lower in old than in young animals. The potassium-stimulated, but not spontaneous, release of GnRH was lower in old than in young Ovx rats. These results suggest that the reduction of plasma LH level in female rats during aging is in part due to a decrease in the K+-stimulated release of GnRH, and a reduction of pituitary responsiveness to GnRH and estradiol. © 1990 S. Karger AG, Basel.