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. 2017 May 31;8:357. doi: 10.3389/fphys.2017.00357

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Copyright © 2017 Chieffi, Carotenuto, Monda, Valenzano, Villano, Precenzano, Tafuri, Salerno, Filippi, Nuccio, Ruberto, De Luca, Cipolloni, Cibelli, Mollica, Iacono, Nigro, Monda, Messina and Messina.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Input and output of orexin neurons at interface of sleep, stress, reward, and energy homeostasis. Orexin neurons in the lateral hypothalamic area (LHA) and posterior hypothalamus (PH) are placed to provide a link among limbic system, energy homeostasis, brainstem and other systems. Arrows show excitatory projections and broken arrows inhibitory projections. Gray semicircles indicate OX1R and black semicircles indicate OX2R. Neurotransmitters/modulators are underlined. LC, DR, and TMN are wake-active regions, VLPO is sleep-active region, and LDT/PPT is REM-active region. Orexin neurons promote wakefulness through monoaminergic nuclei that are wake-active. Stimulation of dopaminergic centers by orexins modulates reward systems (VTA). Peripheral metabolic signals influence orexin neuronal activity to coordinate arousal and energy homeostasis. Stimulation of neuropeptide Y neurons by orexin increases food intake. The SCN, the central body clock, sends input to orexin neurons via the DMH. Input from the limbic system (amygdala and BST) might be important to regulate the activity of orexin neurons upon emotional stimuli to evoke emotional arousal or fear-related responses. Abbreviations: BST, bed nucleus of the stria terminalis; VLPO, ventrolateral preoptic area; LC, locus ceruleus; DR, dorsal raphe; TMN, tuberomammillary nucleus; LDT, laterodorsal tegmental nucleus; PPT, pedunculopontine tegmental nucleus; VTA, ventral tegmental area; SCN, suprachiasmatic nucleus; DMH, dorsomedial hypothalamus; Arc, arcuate nucleus.