Role of leptin in hypothalamic–pituitary function

W Yu

M Kimura

A Walczewska

S Karanth

S McCann

Pennington Biomedical Research Center, Louisiana State University, 6400 Perkins Road, Baton Rouge, LA 70808-4124

^{To whom reprint requests should be addressed. e-mail:}ude.crbp.shm@nnaccm.

S. M. McCann

Accepted 1996 Nov 27.

Abstract

A defect in the structure of the obese gene is responsible for development of obesity in the ob/ob mouse. The product of expression of the gene is the protein hormone leptin. Leptin causes weight loss in ob/ob and normal mice, it is secreted by adipocytes, and it is an important controller of the size of fat stores by inhibiting appetite. The ob/ob mouse is infertile and has a pattern of gonadotropin secretion similar to that of prepubertal animals. Consequently, we hypothesized that leptin might play a role in the control of gonadotropin secretion and initiated studies on its possible acute effects on hypothalamic–pituitary function. After a preincubation period, hemi-anterior pituitaries of adult male rats were incubated with leptin for 3 hr. Leptin produced a dose-related increase in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release, which reached peaks with 10^{and 10}^{M leptin, respectively. Gonadotropin release decreased at higher concentrations of leptin to values indistinguishable from that of control pituitaries. On the other hand, prolactin secretion was greatly increased in a dose-related manner but only with leptin concentrations (10}^–10^{M). Incubation with leptin of median eminence–arcuate nuclear explants from the same animals produced significant increases in LH-releasing hormone (LHRH) release only at the lowest concentrations tested (10}^–10^{M). As the leptin concentration was increased, LHRH release decreased and was significantly less than control release at the highest concentration tested (10}^{M). To determine if leptin can also release gonadotropins}in vivo, ovariectomized females bearing implanted third ventricle cannulae were injected with 10 μg of estradiol benzoate s.c., followed 72 hr later by microinjection into the third ventricle of leptin (0.6 nmol in 5 μl) or an equal volume of diluent. There was a highly significant increase in plasma LH, which peaked 10–50 min after injection of leptin. Leptin had no effect on plasma FSH concentrations, and the diluent had no effect on either plasma FSH or LH. Thus, leptin at very low concentrations stimulated LHRH release from hypothalamic explants and FSH and LH release from anterior pituitaries of adult male rats in vitro and released LH, but not FSH, in vivo. The results indicate that leptin plays an important role in controlling gonadotropin secretion by stimulatory hypothalamic and pituitary actions.

Keywords: obese mouse, follicle-stimulating hormone, luteinizing hormone, luteinizing hormone-releasing hormone

Abstract

In late 1994, Zhang et al. (1) published an extremely important paper in which the structure of the mouse obese (ob) gene and its human homologue by positional cloning was described. The protein named leptin is the product of the ob gene. The obesity in the ob/ob mouse is due to a mutation in the gene, resulting in failure of secretion of leptin from adipocytes. Leptin causes weight loss in mice by reducing food intake and increasing energy expenditure (2–5). Furthermore, ob gene expression is increased in human obesity (6–9) and in various animal models of obesity (10). The accumulated evidence suggests that leptin is important in controlling body weight.

Not only is the ob/ob mouse infertile, but it has atrophic reproductive organs (11). Gonadotropin secretion is impaired and very sensitive to the negative feedback of gonadal steroids, much as in prepubertal animals (12). It has recently been shown that chronic treatment with leptin can induce recovery in the reproductive system in the ob/ob mouse by promoting growth and function of the reproductive organs and fertility (13, 14) by increasing secretion of gonadotropins (15).

The critical weight hypothesis of the development of puberty states that when body weight reaches a certain level, puberty occurs (16). This hypothesis in its original form does not hold since, if rats are underfed, puberty is delayed, but with access to food, rapid weight gain leads to onset of puberty at weights well below the critical weight under normal nutritional conditions (17). Therefore, puberty is probably induced when fat stores reach a certain level (16, 17). We hypothesized that when fat stores reach the critical point, there is increased release of leptin from adipocytes into the bloodstream. Leptin then acts on hypothalamic cells to stimulate release of luteinizing hormone (LH)-releasing hormone (LHRH), thereby triggering gonadotropin release. The subsequent release of follicle-stimulating hormone (FSH) and LH stimulates gonadal steroid secretion, leading to development of the reproductive tract and induction of puberty.

Therefore, we initiated studies of the possible effects of leptin on hypothalamic–pituitary function. We hypothesized that leptin would also play a role in control of gonadotropin secretion in adult animals, and we therefore studied its effects on the release of FSH and LH from hemi-anterior pituitaries (APs) and also its possible action in releasing LHRH from medial basal hypothalamic explants in vitro. To determine if leptin is active in vivo, we used a model which we have often employed to evaluate stimulatory effects of peptides on LH release, namely the ovariectomized, estrogen-primed rat (18). We microinjected the protein into the third ventricle (3V) of conscious animals bearing implanted 3V cannulae and also catheters in the external jugular vein extending to the right atrium, so that we could remove blood samples before and after the injection of leptin and measure its effect on the concentrations of plasma FSH and LH.

Acknowledgments

We thank Judy Scott and Jason Holland for their excellent secretarial assistance. This work was supported by National Institutes of Health Grants DK43900 and MH51853.

Acknowledgments

Footnotes

Abbreviations: LH, luteinizing hormone; LHRH, LH-releasing hormone; FSH, follicle-stimulating hormone; 3V, third ventricle; AP, hemi-anterior pituitary; KRB, Krebs–Ringer bicarbonate buffer; ME, median eminence; Arc, arcuate nuclear; AN, arcuate nucleus; PVN, paraventricular nuclei; NPY, neuropeptide Y.

Footnotes

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