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Departments of Physiology and Biophysics (H.M.D., R.A.S.) and Obstetrics and Gynecology (D.K.C., R.A.S.), University of Washington, Seattle, Washington 98195
Address all correspondence and requests for reprints to: Robert A. Steiner, Department of Physiology and Biophysics, Health Sciences Building, G-424, School of Medicine, University of Washington, Box 357290, Seattle, Washington 98195-7290. E-mail: steiner{at}u.washington.edu.
| Abstract |
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| Introduction |
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| GPR54 Becomes an Adopted Orphan |
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These observations on the phenotype of animals lacking a functional GPR54 opened speculation about the significance of kisspeptin/GPR54 signaling in the regulation of the reproductive axis. At that point (late in 2003), it seemed reasonable to infer that kisspeptin signaling plays some sort of role in the hypothalamic pathway that controls the secretion of GnRH; however, the cellular and molecular mechanisms that mediate this phenomenon were completely unknown. Many laboratories around the world soon jumped into the fray to solve this mystery.
| Kisspeptin Stimulates GnRH Secretion |
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The results from a wide variety of studies indicate that kisspeptin stimulates gonadotropin secretion via a hypothalamic pathway that activates GnRH neurons. For example, acyline (a GnRH antagonist) blocks the ability of kisspeptin to stimulate gonadotropin release in the rat, mouse, and primate (16, 18, 22), and kisspeptin evokes the release of GnRH from rat hypothalamic explants (12). Kisspeptin also induces LH secretion from rat pituitary explants, suggesting that it may have a direct effect on gonadotropes (14). However, the ability of GnRH antagonists to abolish the rise in LH and FSH associated with central or peripheral kisspeptin administration indicates that the direct stimulation of the pituitary by kisspeptin is not a primary mechanism of gonadotropin secretion (14, 15, 16, 23, 24). Irwig et al. (22) further demonstrated in rats that central administration of kisspeptin induces Fos expression in GnRH neurons, indicative of neuronal stimulation. Not only does kisspeptin activate GnRH neurons, but it also appears to act directly on those neurons. In situ hybridization (ISH) techniques have revealed that many GnRH neurons in the rat and mouse also express the mRNA for GPR54 (17, 22). These observations in the mammal were presaged by the earlier work in the cichlid fish by Parhar and his colleagues (25), who used laser capture microscopy and RT-PCR assays to demonstrate that GnRH neurons in these animals coexpress GPR54 mRNA. Together, these experiments argue that kisspeptin binds to GPR54 expressed on GnRH neurons, which then directly stimulates the release of GnRH, presumably through some Ca2+-mediated process typical of Gq/G11-coupled receptors such as GPR54 (reviewed in Refs.26 and 27). Nevertheless, unequivocal proof that kisspeptin neurons make direct synaptic contact with GnRH neurons remains to be demonstrated.
| Anatomy and Regulation of Kiss1 Expression in the Forebrain |
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The Arc and AVPV have long been thought to play important roles in the feedback regulation of GnRH and gonadotropin secretion by estradiol (E) and testosterone (T); however, the circuitry that mediates this phenomenon remains ambiguous (31). Evidence from two independent groups of investigators suggests that kisspeptin neurons could be responsible for mediating the negative feedback effects of sex steroids on GnRH secretion. First, Navarro et al. (13) used whole-hypothalamus RT-PCR to demonstrate the inhibitory effect of E on Kiss1 expression in the female rat. Thereafter, in two ISH-based investigations, one in male and the other in female mice, Smith and coworkers (28, 29) found that reduction of circulating levels of sex steroids by gonadectomy increased the number of detectable Kiss1 mRNA-expressing neurons as well as the content of Kiss1 mRNA per cell in the Arc. Replacement of T in castrated males and E in ovariectomized females restored the expression of Kiss1 mRNA to that observed in intact, untreated animals. The suppression of kisspeptin activity by sex steroids in the Arc appears to be mediated by estrogen receptors (ER) in the female and both ER and androgen receptors in the male (28, 29). These findings are consistent with a role for kisspeptins in mediating the negative feedback effects of gonadal steroids on GnRH secretion in both the male and female.
Although kisspeptin neurons in the Arc have been implicated in the negative feedback of sex steroids, the results of high-resolution ISH analysis suggest that kisspeptin neurons in more rostral nuclei of the forebrain have a different agenda. The expression of Kiss1 mRNA in the PeN and AVPV of both male and female mice is positively regulated by sex steroids (28, 29). Gonadectomy produces a dramatic decrease in the number of Kiss1 mRNA-expressing cells in the AVPV. Replacement of E in gonadectomized animals restores the expression of Kiss1 mRNA to that of normal, intact animals in both sexes. T also restores Kiss1 mRNA levels in the AVPV to those of intact males, an effect that appears to depend on the aromatization of T to E (28). Furthermore, targeted disruptions of the ER
gene blocks the ability of E to induce the expression of Kiss1 mRNA in the AVPV (29). Thus, the inductive effects of gonadal steroids on the expression of Kiss1 in the AVPV appear to be mediated by ER in both sexes.
The opposite effects of sex steroids on expression of Kiss1 mRNA between the AVPV and Arc beg several questions. First, how does E induce the expression of Kiss1 mRNA in the AVPV but suppress it in the Arc? Although we dont know the answer to this question, it may be that E, acting through ER
, recruits coactivators of transcription in kisspeptin neurons of the AVPV and corepressors of transcription in the Arc (32). Second, why does E up-regulate Kiss1 mRNA in the AVPV and down-regulate it in the Arc? It seems likely that kisspeptin neurons in these two areas must perform different functions, with those in the Arc being clearly implicated in the negative feedback regulation of gonadotropin secretion. Because the AVPV is thought to play a vital role in the generation of the preovulatory gonadotropin surge, it is conceivable that kisspeptin neurons in this region mediate the positive feedback effects of E on gonadotropin secretion. Evidence presented by Kinoshita et al. (20) demonstrating that a blockade of kisspeptin signaling in the female rat abolishes the preovulatory LH surge is consonant with a role in positive feedback for kisspeptin in the female. However, the notion of a positive (or inductive) feedback effect of E (and T) on Kiss1 expression in the AVPV of the male is more difficult to understand. It may be that the stimulatory effect of sex steroids in the AVPV of the male occurs as a remnant (or vestige) of the positive feedback pathway, which is mostly ablated in the developing male during the critical period of sexual differentiation (30, 33). Establishing the functional significance of the inductive effects of sex steroids on Kiss1 expression in the AVPV and PeN of both the male and female is an important objective for future research.
| Does Puberty Begin with a Kiss? |
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Evidence that kisspeptin signaling does, in fact, increase across puberty has been reported in several species. Navarro and coworkers (13) have reported that both male and female rats experience a sharp increase in the hypothalamic content of Kiss1 mRNA at the time of puberty. In the AVPV of mice, Han and colleagues (34) have found that both the number of neurons that express Kiss1 mRNA and their content of Kiss1 mRNA per cell is greater in adult compared with prepubertal animals. In addition, Kiss1 mRNA content is higher in the hypothalamus of pubertal monkeys than in juveniles of both sexes (18). The changes in the expression of hypothalamic Kiss1 mRNA found in these studies seem likely to reflect increased kisspeptin neuronal activity over pubertal development.
As in the case of Kiss1, the expression of GPR54 is also amplified over pubertal development. Hypothalamic content of GPR54 mRNA increases at puberty in the male and female rat (13). In the macaque, Shahab and his colleagues (18) observed an increase in hypothalamic content of GPR54 mRNA in intact females, but not in castrated males, suggesting that sex steroids (at least in the male) could mediate the pubertal induction of GPR54 in the macaque. As is the case of the castrated male macaque, in the hypothalamus of intact male mice, the expression of GPR54 mRNA mice does not appear to change over puberty (34), which would argue against a role of GPR54 in triggering the onset of puberty in this species. However, a recent investigation in the male mouse has revealed that kisspeptin depolarizes more than 90% of GnRH neurons from adults in vitro, but the same treatment depolarizes fewer than 30% of GnRH neurons in juveniles (34). These results would argue that despite the fact that the expression of GPR54 apparently doesnt change with puberty, the ability of GPR54 to stimulate neuronal activity nevertheless increases over development. This idea is supported by evidence that much higher doses of kisspeptin are required to stimulate LH secretion in prepubertal compared with adult mice (34). Taken together, these observations suggest that an increase in the efficacy of the kisspeptin-GPR54 signaling accompanies the onset of puberty, although the molecular mechanisms may be species dependent. Thus, the activities of both kisspeptin and its receptor appear to ramp up during pubertal development; however, the mechanisms that initiate and sustain this process are currently unknown. Although an intact kisspeptin-GPR54 pathway appears to be a prerequisite for puberty, it is likely to be only one of many factors required to admit a juvenile into adulthood.
| Kisspeptin Neurons as Central Processors |
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Studies on the interaction of kisspeptin and GPR54 with adipocytokines and adrenal hormones may shed more light on the functioning of the hypothalamic-pituitary-gonadal axis before and after puberty. The relationship between kisspeptin and the action of other endogenous GnRH secretagogues, such as excitatory amino acids (e.g. glutamate), galanin-like peptide, and vasopressin, are worthy of exploration, because it is conceivable that the effects of these neurotransmitters on GnRH secretion are mediated through kisspeptin-GPR54 signaling. In conclusion, the kisspeptin-GPR54 pathway plays a vital role in the neuroendocrine regulation of reproduction in the male and female mammal.
| Acknowledgments |
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| Footnotes |
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The authors have no potential conflicts of interest to declare.
First Published Online December 22, 2005
Abbreviations: Arc, Arcuate nucleus; AVPV, anteroventral periventricular nucleus; E, estradiol; ER, estradiol receptor; ISH, in situ hybridization; PeN, periventricular nucleus; T, testosterone.
Received October 11, 2005.
Accepted for publication November 10, 2005.
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