Peripheral control of inflammatory but not neuropathic pain by endogenous cholinergic system.

This study investigated the role of the cholinergic system in the modulation of inflammatory and neuropathic pain. The paw pressure test was used with inflammatory pain induced by intraplantar injection of carrageenan and neuropathic pain induced by sciatic nerve constriction. All drugs were locally administered into the right hindpaw of rats. Neostigmine, an acetylcholinesterase inhibitor (2, 4, 8 or 16 μg), inhibited the inflammatory pain induced by carrageenan (250 μg/paw), but not the hyperalgesia induced by prostaglandin E₂ (2 μg/paw). Neostigmine (8 μg) increased the nociceptive threshold only in the treated paw, suggesting only a local effect. The muscarinic antagonist atropine (150, 300 and 600 μg) caused a reduction in the nociceptive threshold induced by carrageenan (125 μg/paw), but not by prostaglandin E₂ (1 μg/paw). Atropine significantly decreased the nociceptive threshold only in the treated paw. On the other hand, in the presence of neuropathic pain, atropine (300 μg) did not alter the nociceptive threshold induced by constriction of the sciatic nerve. This study suggests that a peripheral endogenous cholinergic system involving muscarinic receptors may be activated during inflammation as a modulatory negative feedback control of inflammatory pain.