SAT-417 The Gonadotrope Leptin Signal Is Critical for the Early-Morning Estrus Rise in FSH in Female Mice

Abstract

The importance of peripheral nutritional signals to the functioning of the reproductive axis is appreciated, but the mechanisms are not well understood. The three tiers of the hypothalamic-pituitary-gonadal axis are receptive to many of these nutritional signals, including leptin. We have previously shown that leptin communicates with pituitary gonadotropes to maintain numbers of gonadotropin-releasing-hormone receptors (GnRHrs), and that loss of the leptin signal (LEPR) in gonadotropes results in subfertility in females. Using our current model of gonadotrope leptin resistance (GnRHR-cre, floxed Lepr exon 1) which targets the deletion of all LEPRs specifically to gonadotropes expressing Gnrhr, we investigated the role of leptin in the follicle-stimulating hormone (FSH) surge in female mice. Pituitaries and serum from 2-3 month-old control (CTL) and gonadotrope-Lepr-null (MUT) females were collected at midnight between proestrus and estrus, and at 0400 and 0900 on the morning of estrus. The samples were assayed for LH and FSH, and pituitaries were also used to determine GnRHr levels (ELISA) and Lh, Fsh, and Gnrhr mRNA levels (qPCR). Our gonadotrope-Lepr-null females showed a dramatic decrease in serum FSH during the peak of FSH secretion (0400 estrus, CTL: 25.8 ng/mL ± 5.7, n=6, MUT: 6.9 ng/mL ± 0.8, n=5, p<0.03). This result is significant, despite the large natural variability in gonadotropin surges. MUT FSH secretion also trended down at midnight by about 40%, although this effect was not significant. Interestingly, stores of FSH at 0400 were significantly increased in MUT pituitaries compared to controls. Control FSH stores dropped by ~74% from midnight to 0400, whereas mutant stores only dropped about 14% in the same time period, suggesting that the mutants have insufficient FSH secretion beginning at midnight. Analyses of ovarian sections from diestrous or proestrous MUT females showed a significant 38-43% decline in the average number of antral follicles (ANOVA, p<0.05 n=7-8), which correlates with the low FSH secretion. We also discovered a major drop in Fsh mRNA in our MUT pituitaries at 0400 (CTL RQ: 0.68 ± 0.14, n=5; MUT RQ: 0.10 ± 0.04, n=6, p<0.02). MUT Fsh mRNA remained low at 0900 (NS), at about 40% of control mRNA. Importantly, despite finding GnRHR proteins decreased at other stages in the cycle, no differences were seen between controls and mutants with regards to GnRHR (protein or mRNA) during these specific FSH surge time points. Our study indicates that leptin signals to the reproductive axis through control of FSH secretion specifically during the time of the FSH surge in female mice. We hypothesize that leptin may also be able to regulate Fsh transcription. Future studies will test the ability of leptin to stimulate Fsh transcription and secretion from normal, early AM estrous pituitaries and will investigate the pathways that mediate these effects.