comparing the analgesic effects of periaqueductal gray matter injection of orexin a and morphine on formalin- induced nociceptive behaviors.

introduction: orexin-a and b (hypocretin 1 and 2) are neuropeptides that are mostly expressed in the posterior and lateral hypothalamus (lh). intracisternal (icv) and intratechal (it) injections of orexin-a (hypocretin-1) have been shown to elicit analgesic responses in formalin test. however, the locations of central sites that may mediate these effects have not been clearly elucidated. orexin-containing fibers are projected to periaqueductal gray matter (pag), which is involved in pain modulation. methods: behavioral study was done on male sprague dawley rats (200-300 g) in formalin induced nociceptive behaviour. results: intra-pag microinjection of orexin-a produced a dose-dependent inhibition of formalin-evoked behaviour in interphase and phase 2, but not in phase 1, indicating an antinociceptive role of exogenous orexin-a in the pag. analgesic effect of orexin-a was less than and specific to inter- and late phase of formalin test, when compared with that of morphine (5 μg/0.5μl) after intra-pag administration. conclusion: the obtained results suggest that orexin-a plays an anti-nociceptive role in pag, on the interphase and late phase of formalin test in rats. so it is possible that orexin-a might be involved in the mechanisms of inter- and last phases of formalin induced behaviours.