Interactions of 5-HT2 receptor agonists with acetylcholine in spinal analgesic mechanisms in rats with neuropathic pain.

Serotonin type 2 (5-HT(2)) receptors reportedly inhibit neuropathic pain in the spinal cord, but little is known about how spinal 5-HT(2) receptors might act against such abnormal sensitivity. We examined whether the cholinergic and tachykinin systems were involved in the antiallodynic effect of intrathecally administered 5-HT(2) receptor agonists in rats with nerve injury. Allodynia was produced by tight ligation of the left L5 and L6 spinal nerves, and determined by applying von Frey hairs to the left hindpaw. Effects of intrathecal pretreatment with 5-HT(2) receptor antagonists (ketanserin and RS-102221), muscarinic receptor antagonists (atropine and scopolamine), a choline uptake blocker (hemicholium-3), and an NK(1) receptor antagonist (L-706336) were assessed in rats subsequently given a 100- micro g intrathecal dose of a 5-HT(2) receptor agonist either alpha-methyl-5-HT or iododimethoxy aminopropane (DOI). Antiallodynic effects of 5-HT(2) receptor agonists were attenuated by the 5-HT(2A) receptor antagonist ketanserin (30 micro g), but not by the 5-HT(2C) receptor antagonist RS-102221 (40 micro g). Muscarinic receptor antagonists (30 micro g each), the choline uptake blocker (10 micro g), and the NK(1) receptor antagonist (30 micro g) also inhibited the antiallodynic effects of 5-HT(2) receptor agonists. Antiallodynic effects of intrathecally administered 5-HT(2) receptor agonists may be mediated by spinal release of acetylcholine induced via 5-HT(2A) and NK(1) receptors.