Luteinizing hormone-releasing hormone (LHRH) neurons and fibers are located in the anteroventral hypothalamus, specifically in the preoptic medial area and the organum vasculosum of the lamina terminalis. Most luteinizing hormone-releasing hormone neurons project to the median eminence where they are secreted in the pituitary portal system in order to control the release of gonadotropin. The aim of this study is to provide, using immunohistochemistry and female brain rats, a new description of the luteinizing hormone-releasing hormone fibers and neuron localization in the anterior hypothalamus. The greatest amount of the LHRH immunoreactive material was found in the organum vasculosum of the lamina terminalis that is located around the anterior region of the third ventricle. The intensity of the reaction of LHRH immunoreactive material decreases from cephalic to caudal localization; therefore, the greatest immunoreaction is in the organum vasculosum of the lamina terminalis, followed by the dorsomedial preoptic area, the ventromedial preoptic area, and finally the ventrolateral medial preoptic area, and in fibers surrounding the suprachiasmatic nucleus and subependymal layer on the floor of the third ventricle where the least amount immunoreactive material is found. 1. Introduction The luteinizing hormone-releasing hormone (LHRH) is a gonadotropin releasing hormone (GnRH), which acts on the pituitary hormones as a follicle stimulating hormone (FSH) and luteinizing hormone (LH), which act on the gonads, [1]. The GnRH neurons are originated in the nasal epithelium and migrate accompanying the fibers of the vomeronasal and terminal nerves [2, 3] up to the anterobasal part of the brain, where they enter the brain together with nerve terminals and then move caudally to the preoptic hypothalamus, where GnRH neurons are definitively located [2, 4]. These GnRH neurons and fibers are mainly located in the anteroventral third ventricle region, specifically in the preoptic area (PA) and the organum vasculosum of the lamina terminalis (OVLT) [5]. The anterior hypothalamus is the major region of the diencephalon implicated in the development of the olfactory system and the sexual differentiation of the brain. Most of the GnRH neurons axons project to the external zone of the median eminence where is GnRH secreted into the pituitary portal vasculature to control the release of gonadotropin [6–8]. The preoptic area (PA) is part of the anterior hypothalamus and is confined to the anteroventral region of the third ventricle (AV3V); the PA is divided into, the medial
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