Abstract
Reproduction, controlled by the excitatory action of the neuropeptide kisspeptin, is an energy costly function for the organism and, therefore, it is tightly regulated by metabolic factors. Among those, hypothalamic melanocortins are essential in the control of metabolism. Inactivating mutations in the proopiomelanocortin (Pomc) gene or melanocortin receptor 4 (Mc4r) cause severe obesity and subfertility in humans and mice. However, whether the reproductive impairment is a direct consequence of the lack of melanocortin signaling, or indirect resulting from the obesity phenotype, remains unknown. α-MSH (derived from Pomc) stimulates LH release and this action has been shown to be mediated by Kiss1 neurons. We evaluated the expression of Mc4r in Kiss1 neurons, and using dual fluorescence in situ hybridization (RNAscope), we showed that Kiss1 neurons of the arcuate nucleus (Kiss1(ARC)) and the anteroventral periventricular/periventricular nucleus (Kiss1(AVPV/PeN)) express Mc4r. Mc4r colocalized with 84% of Kiss1(ARC) and with 74% of Kiss1(AVPV/PeN), suggesting that Kiss1 neurons are a direct target of melanocortins. In order to evaluate the involvement of Kiss1 neurons in the reproductive role of melanocortins, we ablated Mc4r specifically from Kiss1 neurons by generating a specific Kiss1-Mc4rKO (Kiss1-cre(+)/Mc4r-lox(+/+)) mouse line. We found that Kiss1-Mc4rKO females exhibit disrupted estrous cycles with prolonged diestrous phases and delayed puberty onset compared to their control littermates, without developing obesity during the time of the study. The reproductive phenotype of Kiss1-Mc4rKO males was not affected. When central injections of an Mc4r selective agonist were performed on Kiss1-Mc4rKO intact males and their control littermates, LH levels were significantly increased to the same extend in both groups at 15 min post-injections, suggesting that, in the male, α-MSH controls LH levels through a Kiss1 neuron independent mechanism. Taken together, these results support a direct role of melanocortins on Kiss1 neurons in the control of GnRH release in female mice. Overall, understanding the neuroendocrine mechanisms underlying the metabolic regulation of reproduction will help improve the fertility rates in patients affected by metabolic disturbances. Keywords: Kisspeptin, α-MSH, Melanocortins, Mc4r, Reproduction