Effects of leptin on gonadotropin secretion in juvenile female rat pituitary cells

Leptin is an adipocyte-derived hormone, which is the product of the obese gene and it is thought to play important roles in pubertal development and maintenance of reproductive function in the female. In a study using adult male or female rats, it was found that leptin stimulated the secretion of gonadotropin directly from the pituitary in a dose-related manner. However, there is no study in juvenile female rats before puberty.

In this study, we cultured pituitary cells from 4-, 6- and 8-week-old female Wistar rats with leptin (0-10(-7)mol/l) and gonadotropin-releasing hormone (GnRH) (0 or 10(-8) mol/l). Basal or GnRH-stimulated secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and their synthesis within cells were determined by radioimmunoassay (RIA).

Leptin induced bell-shaped dose--response curves of basal LH and FSH secretion from cultured cells of every age-group of rats studied. The most effective concentration of leptin on the basal secretion of LH and FSH from 6- and 8-week-old cultured pituitary cells was 10(-10) mol/l. This leptin concentration was consistent with circulating physiological serum leptin levels at each age. As for juvenile 4-week-old pituitary cells, the most effective concentration was 10(-11) mol/l which was lower than that of 6- and 8-week-old rats. It was consistent with the circulating serum leptin levels of 4-week-old rats. Also, the synthesis and the GnRH-stimulated secretion of LH and FSH were effectively controlled by leptin at concentrations similar to the serum leptin levels of given ages.

Leptin induced pituitary cells to synthesize and secrete both LH and FSH regardless of the presence or absence of GnRH. The concentration of leptin that induced the greatest synthesis and secretion of gonadotropins from pituitary cells changed around the pubertal period. The most effective leptin concentrations in each experiment were similar to the physiological serum leptin level at each animal age. These results indicate that leptin stimulates gonadotrophs not only in the pubertal and the mature period but also in the juvenile period before puberty. It is also conceivable that leptin may modulate the sensitivity of gonadotrophs until the appearance of GnRH stimulation, and may be the factor that brings about puberty onset.