Leptin controls rabbit ovarian function in vivo and in vitro: Possible interrelationships with ghrelin

The aim of these in vivo and in vitro studies was to examine the role of leptin in the control of plasma hormone concentrations, reproduction, and secretory activity of ovarian granulosa cells.In in vivo experiments, 15 female European domestic rabbit (Oryctolagus cuniculus) were treated with leptin (5 μg animal−1 d−1 for 1 wk before induction of ovulation with 25 IU equine chorionic gonadotropin and 0.25 IU human chorionic gonadotropin), and 15 females constituted the control group (treated with phosphate-buffered saline). Plasma concentrations of progesterone (P4), testosterone (T), estradiol (E2), estrone sulfate (ES), and insulin-like growth factor I (IGF-I) were determined at the estimated day of ovulation by radioimmunoassay (RIA), and number, viability, and body weight of newborns were recorded at parturition. In in vitro experiments, granulosa cells were isolated from periovulatory ovarian follicles of five control and five females treated with ghrelin (10 μg animal−1 d−1 for 1 wk before induced ovulation). Isolated cells were cultured for 2 d with and without leptin (0, 1, 10, or 100 ng/mL medium). Secretion of P4, T, E2, IGF-I, and prostaglandin F (PGF) was assessed in culture medium by RIA.In in vivo experiments, leptin administrations reduced plasma P4, T, E2, ES, and IGF-I levels. Leptin treatments did not affect ovarian weight or total number and body mass of newborns, but the proportion of pregnant females and number of live newborns were significantly higher in leptin-treated females than that in control females. In in vitro experiments, leptin significantly decreased (at 1 and 10 ng/mL) or increased (at 100 ng/mL) P4 secretion, promoted E2 and IGF-I (both at 100 ng/mL) secretion, and reduced T (at 1 and 10 ng/mL) and PGF (at 10 ng/mL) secretion. Granulosa cells from ghrelin-treated animals secreted less P4, T, E2, and PGF, but not IGF-I, than that secreted by granulosa cells from control animals. Furthermore, pretreatment of animals with ghrelin suppressed or even reversed subsequent leptin effects on P4, T, E2, IGF-I, and PGF secretion by cultured granulosa cells.These observations (1) show for the first time that leptin can increase the number of live newborns in rabbits, (2) confirm previous data on the ability of leptin to control ovarian secretory activity both directly and via upstream mechanisms, (3) demonstrate the involvement of ghrelin in the control of rabbit ovarian secretory functions, and (4) suggest an antagonistic interrelationship between leptin and ghrelin in the rabbit.