The role of nicotinic acetylcholine and opioid systems of the ventral orbital cortex in modulation of formalin-induced orofacial pain in rats

Nicotinic acetylcholine and opioid receptors are involved in modulation of pain. In the present study, we investigated the effects of microinjection of nicotinic acetylcholine and opioid compounds into the ventral orbital cortex (VOC) on the formalin-induced orofacial pain in rats. For this purpose, two guide cannulas were placed into the left and right sides of the VOC of the brain. Orofacial pain was induced by subcutaneous injection of a diluted formalin solution (50 μl, 1.5%) into the right vibrissa pad and face rubbing durations were recorded at 3-min blocks for 45 min. Formalin produced a marked biphasic pain response (first phase: 0-3 min and second phase: 15-33 min). Epibatidine (a nicotinic receptor agonist) at doses of 0.05, 0.1 and 0.2 μg/site, morphine (an opioid receptor agonist) at doses of 0.5, 1 and 2 μg/site and their sub-analgesic doses (0.025 μg/site epibatidine with 0.25 μg/site morphine) combination treatment suppressed the second phase of pain. The antinociceptive effect induced by 0.2 μg/site of epibatidine, but not morphine (2 μg/site), was prevented by 2 μg/site of mecamylamine (a nicotinic receptor antagonist). Naloxone (an opioid receptor antagonist) at a dose of 2 μg/site prevented the antinociceptive effects induced by 2 μg/site of morphine and 0.2 μg/site of epibatidine. No above-mentioned chemical compounds affected locomotor activity. These results showed that at the VOC level, epibatidine and morphine produced antinociception. In addition, opioid receptor might be involved in epibatidine-induced antinociception, but the antinociception induced by morphine was not mediated through nicotinic acetylcholine receptor.