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Leptin directly acts within the hypothalamus to stimulate gonadotropin-releasing hormone secretion in vivo in rats

Hajime Watanobe

It is still not known whether leptin, an adipocyte-derived hormone, acts directly within the hypothalamus to stimulate the gonadotropin-releasing hormone (GnRH)-luteinizing hormone (LH) system. In order to address this question, the present study examined the effects of direct intrahypothalamic perfusions with leptin on the in vivo release of GnRH in ovarian steroid-primed ovariectomized rats utilizing the push-pull perfusion technique. Both -melanocyte-stimulating hormone (-MSH) and neuropeptide Y were also measured in the hypothalamic perfusates. In normally fed animals, the leptin infusion was without effect on the release of these three hypothalamic peptides and also without effect on plasma LH and prolactin (PRL), whether leptin was infused into the medial preoptic area (where the majority of GnRH neuronal cell bodies exist) or the median eminence-arcuate nucleus complex (where axon terminals of GnRH neurons are located). In contrast, in 3-day fasted rats leptin was effective in stimulating the secretion of GnRH, -MSH, and LH, regardless of the site of perfusion. These three hormones were increased in a temporal order of -MSH, GnRH and LH. Irrespective of the site of perfusion, leptin was without effect on the release of neuropeptide Y. Only when leptin was infused into the median eminence-arcuate nucleus complex was PRL secretion also stimulated, although its onset was 1 h behind that of LH. The leptin-induced elevations of GnRH, -MSH, LH and PRL were all dose-dependently stimulated by subnormal (1.0 ng ml^-1) and normal (3.0 ng ml^-1) concentrations of leptin, but at higher concentrations (10 ng ml^-1) it did not produce additional effects. Leptin infusion into the anterior hypothalamic area, a control site equidistant from both the medial preoptic area and the median eminence-arcuate nucleus complex, did not produce a significant change in any of the hormones in either the fed or fasted rats. These results demonstrate for the first time that leptin can act at both the cell bodies and axon terminals of GnRH neurons to stimulate the release of the neurohormone in vivo, and they also suggest that -MSH may play a significant intermediary role in linking leptin and GnRH secretion.

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