title: Tail flick modification of orexin-a induced changes of electrophysiological parameters in the rostral ventromedial medulla.
abstract: OBJECTIVE
  It is well known that intracerebroventricular (ICV) and supraspinal injections of orexin-A elicit analgesia, but the mechanism(s) of action remains unidentified. This study aims to characterize the effect of orexin-A on rostral ventromedial medulla (RVM) neurons which are involved in the descending nociception modulating pathway.
  
  
  METHODS
  In this experimental study, we injected orexin-A or vehicle di- rectly into rats' ICV while the tail flick (TF) latencies were measured and the on- and off-cell firing activities were monitored for more than 60 minutes.
  
  
  RESULTS
  In response to noxious stimuli on the tail, we observed increased firing rate of on-cells and a decreased association with the firing rate of off-cells and in neutral cells the firing rate was unchanged just prior to tail flicking. ICV injection of orexin-A decreased the spontaneous firing rate of on-cells (the type of RVM neurons believed to have facilitatory action on nociception). Furthermore, orexin-A increased firing rate of off-cells (the type of RVM neurons believed to have an inhibitory action on nocicep- tion). Orexin-A reduced the TF-related responses of on-cells and TF-related pause duration of off-cells.
  
  
  CONCLUSIONS
  These results have shown that the analgesic effect produced by orexin-A may be induced by brainstem neurons. It is suggested that the orexinergic system from the hypothalamus to the RVM may have a potential role in modulation of nociceptive transmission.
authors.affiliation: Department of Physiology, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran ; Department of Basic Sciences, Torbat Heydarieh University of Medical Sciences, Torbat Heydarieh, Iran.
authors.forename: Hassan
authors.initials: H
authors.lastname: Azhdari-Zarmehri
authors.affiliation: Department of Physiology, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran.
authors.forename: Saeed
authors.initials: S
authors.lastname: Semnanian
authors.affiliation: Department of Physiology, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran.
authors.forename: Yaghoub
authors.initials: Y
authors.lastname: Fathollahi
authors.affiliation: Department of Physiology, Faculty of Medicine, Urmia University of Medical Sciences, Urmia, Iran.
authors.forename: Firouz Ghaderi
authors.initials: FG
authors.lastname: Pakdel
keywords: Orexin-A; Pain; Tail Flick