An obligatory role for spinal cholinergic neurons in the antiallodynic effects of clonidine after peripheral nerve injury

Background: Indirect evidence supports a role of spinal cholinergic neurons in tonically reducing response to noxious mechanical stimulation and in ef fecting analgesia from alpha (2)-adrenergic agonists, This study directly a ssessed the role of cholinergic neurons in regulating the level of mechanic al allodynia and in participating in the antiallodynic effect of the clinic ally used alpha (2)-adrenergic agonist, clonidine, in an animal model of ne uropathic pain. Methods: Allodynia was produced in rats by ligation of the left L5 and L6 s pinal nerves. Rats received a single intrathecal injection of saline or one of three different doses of the cholinergic neurotoxin, ethylcholine musta rd aziridinium ion (AF64-A; 2, 5, and 15 nmol). Seven days later, allodynia was assessed before and after intrathecal injection of 15 pg clonidine, Th e spinal cord was removed, and spinal cord acetylcholine content, cholinerg ic neuron number and distribution, and alpha (2)-adrenergic receptor expres sion were determined. Results: AF64-A administration reduced both the number of cholinergic cells and the acetylcholine content of the lumbar dorsal spinal cord by 20-50% b ut did not affect level of mechanical allodynia. AF64-A did, however, compl etely block the antiallodynic effect of clonidine, AF64-A did not reduce al pha (2)-adrenergic ligand binding in dorsal lumbar cord. Conclusions: These data suggest that spinal cholinergic tone does not affec t the level of mechanical allodynia after peripheral nerve injury. There is a quantitative reliance on spinal cholinergic neurons in the allodynia rel ieving properties of intrathecal clonidine, and this reliance does not depe nd on alpha (2)-adrenergic receptors colocalized on spinal cholinergic inte rneurons.