Assesment of orexin receptor 1 in stress attenuated nociceptive behaviours in formalin test

Introduction: It is known that acute and chronic stress induce hormonal and neuronal changes which affecting both pain threshold and nociceptive behaviours. Orexin plays an important role in modulation of pain and stress. Considering pain modulation during stress and the role of orexin in pain and stress, orexin might be involved in pain modulation during stress.We evaluated the involvement of orexin receptor-1in acute immobilization stress on the tonic pain model. Methods: Adult male, Wistar rats (200–300 g), placed in a stereotaxic apparatus and canulla were inserted into their left cerebral ventricle. After 1 week of recovery, animals were initially submitted to one session of acute restraint stress (30 min) and immediately submitted to formalin injection in the hind paw to evaluate nociceptive behaviours. Orexin receptor 1 antagonist (SB 334867) was injected intracerebroventricularly, 5 minute before formalin injection, while the solvent was injected in the control group. Results: two percent formalin produced typical biphasic pain responses in rats that was observed for more than 1 hour. Acute exposure to restraint stress reduced the nociceptive behaviour by chemical stimulation in phase 1, interphase and phase 2. The short-term stress induced analgesia was reflected in a decrease in the nociceptive behaviour during phase 1, whereas the long-term stress induced analgesia was reflected in a decrease in the nociceptive behaviours during phase 2. Pretreatment with orexin receptor 1 antagonist (SB 334867) attenuated the antinociceptive behavioral effect of restraint stress. Conclusion: Our results indicate that orexin receptor 1 antagonist attenuated antinociceptive effect of restraint stress assessed by formalin. These findings show that orexin receptor 1 might mediate an opioid-independent stress-induced analgesia.