antinociceptive effect of morphine microinjections into the dorsal hippocampus in the formalin-induced orofacial pain in rats

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.