Analgesic action of nicotine on tibial nerve transection (TNT)-induced mechanical allodynia through enhancement of the glycinergic inhibitory system in spinal cord

The activation of cholinergic pathways by nicotine elicits various physiological and pharmacological effects in mammals. For example, the stimulation of nicotinic acetylcholine receptors (nAChRs) leads to an antinociceptive effect. However, it remains to be elucidated which subtypes of nAChR are involved in the antinociceptive effect of nicotine on nerve injury-induced allodynia and the underlying cascades of the nAChR-mediated antiallodynic effect. In this study, we attempted to characterize the actions of nicotine at the spinal level against mechanical allodynia in an animal model of neuropathic pain, tibial nerve transection (TNT) in rats. It was found that the intrathecal injection of nicotine, RJR-2403, a selective α4β2 nAChR agonist, and choline, a selective α7 nAChR agonist, produced an antinociceptive effect on the TNT-induced allodynia. The actions of nicotine were almost completely suppressed by pretreatment with mecamylamine, a non-selective nicotinic antagonist, or dihydro-β-erythroidine, a selective α4β2 nAChR antagonist, and partially reversed by pretreatment with methyllycaconitine, a selective α7 nAChR antagonist. Furthermore, pretreatment with strychnine, a glycine receptor antagonist, blocked the antinociception induced by nicotine, RJR-2403, and choline. On the other hand, the GABAA antagonist bicuculline did not reverse the antiallodynic effect of nicotine. Together, these results indicate that the α4β2 and α7 nAChR system, by enhancing the activities of glycinergic neurons at the spinal level, exerts a suppressive effect on the nociceptive transduction in neuropathic pain.