Effect of the cholinergic and opioid receptor mechanisms on nicotine-induced analgesia

  In this study, we investigated the effect of nicotinic receptor agonists and antagonists on the analgesic response to morphine in the formalin test. In experiments conducted in mice, nicotine produced an early dose-dependent analgesic effect. At a dose of 0.5 mg/kg, mecamylamine, a nicotinic receptor inhibitor, suppressed the analgesic effect induced by 0.1 mg/kg nicotine in both stages of the formalin test, while hexamethonium had no such effect. Atropine, a muscarinic receptor antagonist, reduced the nicotine response at doses of 5 and 10 mg/kg. Mecamylamine, hexamethonium and atropine had no effect on morphine-induced analgesia. Administered separately, the antagonists had no effect on the analgesic response either. High doses of mecamylamine lead to an increase of the pain response. We conclude that cholinergic and opioid receptors play a possible role in the analgesic effect induced by nicotine.