Connections To Histamine Neurons That Underlie Wakefulness
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Histamine neurons are located in the posterior hypothalamus of the brain, specifically the tuberomammillary nuclei. These neurons play a fundamental role in arousal. This project addresses the question of what regulates histamine neurons in order for them to effect arousal. There are two main signals that prompt wakefulness: the daily rhythm of the biological clock (suprachiasmatic nucleus, SCN, that contain vasopressin and vasoactive intestinal polypetide, VIP) and arousal after external stimuli (brainstem reticular activating system: which express the noradrenergic markers, dopamine beta hydroxylase (DBH) and neuropeptide Y (NPY)). Understanding if either or both systems innervate histamine neurons could provide an explanation of how histamine neurons promote wakefulness. Using immunocytochemical techniques to co-localize histamine neurons and axons from neurons within the SCN (vasopressin or VIP) and brainstem catecholamine neurons (DBH or NPY), this project aims to identify in rat brain whether either or both of the two possible regulatory systems innervate the histamine neurons. The results of the study showed that vasopressin neurons of the SCN send axons to the dorsal and ventral tuberomammillary histamine neurons; however, neurons containing VIP had no axon projections in this area. The catecholamine neurons containing both dopamine beta hydroxylase and neuropeptide Y sent axons to the ventral and dorsal tuberomammillary nuclei. Collectively, these data indicate that both dorsal and ventral histamine neurons are connected to regions of the brain that are involved in timing and arousal.