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 (۱ %, ۵۰ μl) in the upper lip region and the time spent of face rubbing was measured in ۳-min blocks for ۴۵ min. Formalin induced a biphasic (first phase: ۰-۳ min; second phase: ۱۵-۳۳ min) pain response. Intra-hippocampal microinjections of morphine at doses of ۲ and ۴ μg significantly (P < ۰.۰۵) attenuated the first phase, and at doses of ۱, ۲ and ۴ μg, morphine significantly (P < ۰.۰۵) suppressed both phases of formalin-induced orofacial pain response. Intra-hippocampal microinjections of naloxone (۱ and ۴ μg) non-significantly increased pain when used alone, and in pretreatment microinjection, naloxone (۴ μg) reversed morphine (۲ 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.