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M Tezuka, M Irahara, K Ogura, M Kiyokawa, T Tamura, T Matsuzaki, T Yasui and T Aono

OBJECTIVE: 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. METHODS: 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). RESULTS: 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. CONCLUSIONS: 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.

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K Ogura, M Irahara, M Kiyokawa, M Tezuka, T Matsuzaki, T Yasui, M Kamada and T Aono

BACKGROUND: Leptin, which is the product of the obese gene, is believed to play important roles in pubertal development and reproductive function in females. In a study using adult male rats, it was found that leptin stimulated secretion of gonadotropin from the pituitary in a dose-related manner. However, there has been no such study in female rats. OBJECTIVE: To investigate the effects of leptin on the production of LH and FSH from the pituitary in female rats, using primary cultured pituitary cells. METHODS: In this study, we determined body weight, serum leptin concentration and serum estradiol (E(2)) concentration in female Wistar rats at 3, 5, 6, 7, 9 and 11 weeks of age, and cultured pituitary cells from 6-week-old female Wistar rats with leptin (0--10(-7) mol/l) and GnRH (0 or 10(-8) mol/l). Then basal and GnRH-stimulated extra- and intracellular LH and FSH were assayed by RIA. RESULTS: Serum leptin concentration increased with increases in body weight and E(2) concentration. The pubertal serum leptin concentration was about 10(-10) mol/l. At a lower or moderate concentration, leptin produced dose-related increases in both basal and GnRH-stimulated extra- and intracellular LH and FSH in pituitary cells. At a concentration of 10 mol/l, leptin significantly (P<0.05) stimulated both basal and GnRH-stimulated extra- and intracellular LH and FSH. However, at greater concentrations, these effects diminished. CONCLUSIONS: These results indicated that leptin induced pituitary cells to produce and secrete both LH and FSH, with or without GnRH. The concentration of leptin that induced the greatest production of gonadotropins by pituitary cells was 10(-10) mol/l, which was the same as the physiological pubertal concentration. Leptin may be involved in the onset of puberty. It is also conceivable that leptin may be a cause of ovulatory failure, not only in weight loss but also in weight gain.