title: Influence of inhibitory serotonergic inputs to orexin/hypocretin neurons on the diurnal rhythm of sleep and wakefulness.
abstract: OBJECTIVE
  Serotonergic (5HT) neurons of the dorsal raphe nuclei receive excitatory input from hypothalamic orexin (hypocretin) neurons and reciprocally inhibit orexin neurons through the 5HT1A receptor. However, the physiological significance of this negative feedback circuit for sleep/wakefulness regulation is little understood.
  
  
  METHODS
  5HT1A receptor expression level was specifically and reversibly controlled in the orexin neurons using the Tet-off system. The responsiveness of orexin neurons to 5HT in vitro and the sleep/wakefulness patterns were compared between 5HT1A-overexpressing and control mice.
  
  
  RESULTS
  When the 5HT1A receptor was overexpressed in orexin neurons of Orexin-EGFP; orexin-tTA; TetO Htr1a mice, 5HT-induced inhibition of orexin neurons was prolonged. In the absence of doxycycline, Orexin-tTA; TetO Htr1a mice exhibited severe fragmentation of sleep/wakefulness during the first half of the dark period-the time of maximal activity in nocturnal rodents-without affecting sleep/wakefulness during the light period when sleep time is maximal. However, when the 5HT1A receptor in orexin neurons was reduced to basal expression levels in the presence of doxycycline, sleep/wakefulness patterns in Orexin-tTA; TetO Htr1a mice during the early active period were indistinguishable from those of littermate TetO Htr1a mice. These results strongly suggest that enhancement of inhibitory serotonergic input to orexin neurons caused fragmentation of wakefulness. In contrast, sleep/wakefulness architecture in the light period was unaffected by 5HT1A receptor overexpression in the orexin neurons.
  
  
  CONCLUSIONS
  Inhibitory serotonergic input likely functions as negative feedback to orexin neurons in the early dark period and helps stabilize wakefulness bouts, thereby contributing to the diurnal rhythm of sleep and wakefulness.
authors.affiliation: National Institute for Physiological Sciences, National Institute of Natural Sciences, Okazaki, Japan.
authors.forename: Sawako
authors.initials: S
authors.lastname: Tabuchi
authors.forename: Tomomi
authors.initials: T
authors.lastname: Tsunematsu
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Kilduff
authors.forename: Shouta
authors.initials: S
authors.lastname: Sugio
authors.forename: Ming
authors.initials: M
authors.lastname: Xu
authors.forename: Kenji F
authors.initials: KF
authors.lastname: Tanaka
authors.forename: Satoru
authors.initials: S
authors.lastname: Takahashi
authors.forename: Makoto
authors.initials: M
authors.lastname: Tominaga
authors.forename: Akihiro
authors.initials: A
authors.lastname: Yamanaka
mesh_terms: D000818:Animals; D001921:Brain / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology*; D004569:Electroencephalography; D004576:Electromyography; D025461:Feedback, Physiological / Q000502:physiology; D005260:Female; D047908:Intracellular Signaling Peptides and Proteins / Q000502:physiology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009474:Neurons / Q000502:physiology*; D009479:Neuropeptides / Q000502:physiology*; D000068797:Orexins; D018408:Patch-Clamp Techniques; D044282:Receptor, Serotonin, 5-HT1A / Q000502:physiology; D059326:Serotonergic Neurons / Q000502:physiology*; D012890:Sleep / Q000502:physiology*; D014851:Wakefulness / Q000502:physiology*
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D000068797:Orexins; D044282:Receptor, Serotonin, 5-HT1A
keywords: 5HT1A receptor; Orexin; TetO; doxycycline; hypocretin; serotonin; tTA; transgenic