Document Type : Original Article
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
Department of Physiology, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran
Department of Physiology, Faculty of Medicine, Urmia University of Medical Sciences, Urmia, Iran
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.
Materials and 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.
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.
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.