Intra-paragigantocellularis lateralis injection of orexin-A has an antinociceptive effect on hot plate and formalin tests in rat.

In the present study, the effect of orexin-A (ORXA) microinjection into the paragigantocellularis lateralis (LPGI) on nociceptive behaviors, using hot-plate and formalin tests as thermal and chemical models of pain in rat, was examined. Also, we determined whether the pretreatment with SB-334867, a selective OX1-receptor antagonist, would prevent the antinociceptive effect of orexin-A. ORXA (0.1-100 nM/0.5 μL) microinjected into the LPGi nucleus, dose-dependently decreased the formalin induced nociceptive behaviors and also produced a dose-dependent antinociceptive effect in the hot-plate test. Pretreatment with a selective orexin receptor 1 (OX1R) antagonist, SB-334867, also inhibited the effect of ORXA on formalin induced nociceptive behaviors while the SB-334867 (100 μM) alone had no effect on formalin test. These data demonstrated that the ORXA-induced antinociception in formalin test is mainly mediated through the OX1R in LPGi which might play a potential role in processing the pain information associated with descending pain modulation.