KiSS-1 neurones are direct targets for leptin in the ob/ob mouse.

Leptin is an adipocyte-derived hormone that acts on the hypothalamus to influence feeding, metabolism and reproduction, but the cellular and molecular targets for the action of leptin in the brain have yet to be fully elucidated. Kisspeptins are encoded by the Kiss1 gene, which is expressed in the hypothalamus and has been implicated in the neuroendocrine regulation of gonadotrophin-releasing hormone secretion. We tested the hypothesis that kisspeptin-expressing neurones are targets for leptin. First, we examined whether leptin regulates the expression of Kiss1 by comparing levels of KiSS-1 mRNA in the arcuate nucleus among groups of mice having different circulating levels of leptin: (i) wild-type (WT); (ii) leptin-deficient ob/ob; and (iii) ob/ob mice treated with leptin. All mice were castrated to control for endogenous concentrations of gonadal steroids. KiSS-1 mRNA was significantly reduced in ob/ob compared to WT mice and levels of KiSS-1 mRNA in ob/ob mice treated with leptin were increased, but not fully restored to that found in WT animals. Second, we performed double-label in situ hybridisation for KiSS-1 mRNA and the leptin receptor (Ob-Rb) mRNA and found that almost one-half (approximately 40%) of KiSS-1 mRNA-expressing cells in the arcuate nucleus expressed Ob-Rb mRNA. These results demonstrate that KiSS-1 neurones are direct targets for regulation by leptin and suggest that the reproductive deficits associated with leptin-deficient states may be attributable, in part, to diminished expression of Kiss1.

Stimulation of sexual behavior in the male rat by galanin-like peptide.

Galanin-like peptide (GALP) is a recently described neuropeptide, which shares a partial sequence identity with galanin but is derived from a separate gene. Central injections of GALP stimulate the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and induce the expression of Fos in several brain areas known to regulate male sexual behavior in the rat. We postulated that GALP may also stimulate sexual behavior in concert with its stimulatory effect on the hypothalamic-pituitary-gonadal (HPG) axis. To test this hypothesis, we administered GALP, galanin, or the vehicle (artificial cerebrospinal fluid, aCSF) alone to sexually experienced male rats and assessed the effects of these agents on sexual behavior. We observed that compared to aCSF alone, GALP significantly increased all aspects of male-typical sexual behavior, whereas galanin inhibited all of these same behaviors. To examine whether the stimulatory effects of GALP on sexual behavior were mediated by GALP's stimulatory effects on the HPG axis, we castrated the same male rats and repeated the behavioral experiment. We found that GALP maintained its inductive action on male-typical sexual behaviors in the castrated animals, suggesting that the effects of GALP on sexual behavior are not the result of GALP's ability to stimulate testosterone secretion. These observations suggest that GALP neurons are part of the hypothalamic circuitry controlling sexual behavior in the male rat.

A role for kisspeptins in the regulation of gonadotropin secretion in the mouse.

Kisspeptins are products of the KiSS-1 gene, which bind to a G protein-coupled receptor known as GPR54. Mutations or targeted disruptions in the GPR54 gene cause hypogonadotropic hypogonadism in humans and mice, suggesting that kisspeptin signaling may be important for the regulation of gonadotropin secretion. To examine the effects of kisspeptin-54 (metastin) and kisspeptin-10 (the biologically active C-terminal decapeptide) on gonadotropin secretion in the mouse, we administered the kisspeptins directly into the lateral cerebral ventricle of the brain and demonstrated that both peptides stimulate LH secretion. Further characterization of kisspeptin-54 demonstrated that it stimulated both LH and FSH secretion, at doses as low as 1 fmol; moreover, this effect was shown to be blocked by pretreatment with acyline, a potent GnRH antagonist. To learn more about the functional anatomy of kisspeptins, we mapped the distribution of KiSS-1 mRNA in the hypothalamus. We observed that KiSS-1 mRNA is expressed in areas of the hypothalamus implicated in the neuroendocrine regulation of gonadotropin secretion, including the anteroventral periventricular nucleus, the periventricular nucleus, and the arcuate nucleus. We conclude that kisspeptin-GPR54 signaling may be part of the hypothalamic circuitry that governs the hypothalamic secretion of GnRH.