Dynamic alterations in luteinizing hormone-releasing hormone (LHRH) neuronal cell bodies and terminals of adult rats

Summary 1. The decapeptide lueteinizing hormone-releasing hormone (LHRH) is synthesized in neuronal cell bodies diffusely distributed across the basal forebrain and is secreted from neuronal terminals in the median eminence. Once secreted, LHRH enters the portal vessels and is then transported to the anterior pituitary, where it modulates the synthesis and secretion of gonadotropins, which are essential to gonadal function and reproduction.2. Because of the difficulties encountered in studying these diffusely distributed neurons, we have developed strategies which combine immunocytochemistry and computer-assisted techniques to examine individual LHRH neuronal cell bodies, as well as the entire population of LHRH neurons from the diagonal band of Broca to the mammillary bodies. In addition, we have examined LHRH neuronal terminals in the median eminence using computer-assisted imaging techniques to examine individual terminals by electron microscopy or across all rostral-caudal regions of the median eminence by light microscopy. In our most recent studies using confocal microscopy, we have examined the relationships of LHRH terminals to glial processes.3. These studies reveal a very dynamic system of LHRH neuronal cell bodies and terminals. The population of neurons in which LHRH can be detected varies as a function of time after gonadectomy, during the estrous cycle, and during the preovulatory surge of LH during the afternoon of proestrus. Dynamic changes are also observed in LHRH terminals in the median eminence as a function of time after gonadectomy and in specific rostral-caudal regions of the median eminence during the preovulatory surge of LH. Finally, confocal microscopy reveals that LHRH terminals are prevented from contacting the basal lamina of the brain by glial end-feet.4. We are currently examining the hypothesis that these relationships change as a function of endocrine milieu and, therefore, participate in the modulation of LHRH secretion. Ongoing studies focus on defining the sites of action and synergy of multiple sources of regulation of LHRH secretion and their relative importance to ensuring reproductive success.