Faculty of Veterinary Medicine, Urmia University Veterinary Research Forum 2008-8140 1 2 2010 09 01 Antinociceptive Effect of Morphine Microinjections into the Dorsal Hippocampus in the Formalin-Induced Orofacial Pain in Rats 83 89 1488 EN Amir Erfanparast Department of Basic Sciences, Faculty of Veterinary Medicine, Urmia University, Urmia, Iran Esmaeal Tamaddonfard Department of Basic Sciences, Faculty of Veterinary Medicine, Urmia University, Urmia, Iran Amir Abbas Farshid Department of Pathobiology, Faculty of Veterinary Medicine, Urmia University, Urmia, Iran Emad Khalilzadeh Department of Basic Sciences, Faculty of Veterinary Medicine, Urmia University, Urmia, Iran Journal Article 2012 12 25 In the present study, the effects of intra-hippocampal microinjections of morphine (an opioid agonist) and naloxone (an opioid antagonist) were investigated in the formalin-induced orofacial pain in rats. Orofacial pain was induced by subcutaneous injection of formalin (1 %, 50 μl) in the upper lip region and the time spent of face rubbing was measured in 3-min blocks for 45 min. Formalin induced a biphasic (first phase: 0-3 min; second phase: 15-33 min) pain response. Intra-hippocampal microinjections of morphine at doses of 2 and 4 μg significantly (P < 0.05) attenuated the first phase, and at doses of 1, 2 and 4 μg, morphine significantly (P < 0.05) suppressed both phases of formalin-induced orofacial pain response. Intra-hippocampal microinjections of naloxone (1 and 4 μg) non-significantly increased pain when used alone, and in pretreatment microinjection, naloxone (4 μg) reversed morphine (2 μg)-induced antinociception. These results indicate that at the level of hippocampus of the brain, morphine through a naloxone-reversible mechanism produced an antinociceptive effect confronting the pain induced by formalin in the orofacial region in rats. https://vrf.iranjournals.ir/article_1488_8a146f1a3da4700cbf03cdc55e2daae6.pdf