In vitro release of LHRH from the hypothalamus of female rats during prepubertal development

In this study changes in serum FSH concentration, hypothalamic LHRH content and in vitro release of LHRH from hypothalami of female Wistar rats during prepubertal development are described. The concentration of FSH in serum was determined by radioimmunoassay. Extremely high levels (1.800 ng/ml) were found between 12 and 16 days of age. Thereafter, FSH levels rapidly fell to low levels (135 ng/ml) on day 32. LHRH was measured by radioimmunoassay in extracts of mediobasal hypothalami (MBH) and showed a gradual increase during the first 10 days of life. Between 10 and 16 days of age, hypothalamic LHRH content did not change. After day 16, LHRH content sharply increased to a maximum found on day 32. The release of LHRH from MBH of developing rats was studied during in vitro incubation in Krebs-Ringer bicarbonate buffer. LHRH in the medium was measured by radioimmunoassay. The spontaneous release rate of LHRH was maximal from MBH of 12-day-old rats and minimal from those of 32 days. Depolarization of neural membranes, by medium containing 45 mM K+ induced a stimulation of LHRH release. This K+ -induced release of LHRH was also most prominent on day 12. These observations are consistent with the hypothesis that the high circulating FSH levels found in female rats between 12 and 16 days of age are at least in part caused by a high secretion rate of LHRH during this period.