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Neuropeptide glutamic acid-isoleucine (NEI) is a 14-amino acid peptide processed from prepro-melanin-concentrating hormone (ppMCH). In males, the localization of NEI is almost identical to that of MCH, the cell bodies of both being located primarily in the lateral hypothalamic area and zona incerta, projecting fibers throughout the brain. Although MCH has been widely studied, the role that NEI plays in brain circuitry has been poorly investigated. Recently, we showed that intracerebroventricular injection of NEI increases serum luteinizing hormone (LH) levels. In order to identify the anatomical substrate underlying this effect, we used combined immunohistochemistry methods to analyze the forebrains of females on the diestrus and proestrus days, as well as those of ovariectomized females treated with estradiol benzoate, with estradiol benzoate plus progesterone or with sesame oil (control animals). We found that ovariectomized females with no steroid treatment showed an increased number of NEI-immunoreactive neurons in the medial zona incerta. In addition, we observed dense to moderate NEI innervation of areas related to reproduction, including the organum vasculosum of the lamina terminalis, the anteroventral periventricular nucleus (AVPV) and the median eminence. The NEI fibers were in close apposition with the AVPV and gonadotropin-releasing hormone (GnRH) neurons expressing Fos in the afternoon of the proestrus day or following administration of estradiol benzoate plus progesterone. In the median eminence, NEI varicosities and terminal-like structures were in close proximity to blood vessels and GnRH fibers. Our results suggest that NEI might induce LH secretion in one of the following ways: by direct release into the median eminence, by modulation of GnRH neurons located in the preoptic area, by modulation of the GnRH terminals located in the median eminence or by an additive effect involving other neurotransmitters or neurohormones. Release of NEI might also induce LH secretion indirectly by modulating AVPV neurons.

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