Reversal of pertussis toxin-induced thermal allodynia by muscarinic cholinergic agonists in mice

The intrathecal administration of pertussis toxin (PTX) not only blocks the antinociceptive effects of the muscarinic cholinergic receptor agonist oxotremorine administered systemically, but also produces a long-lasting thermal allodynia in mice. The purpose of the present studies was to determine both the antinociceptive effects in normal mice and the antiallodynic effects in PTX-treated mice of systemically administered muscarinic cholinergic receptor agonists and cholinesterase inhibitors. In normal, mice, antinociceptive effects were tested using a 55 degrees C water-bath tail-flick test. In mice treated 7 days previously with PTX (0.3 mu g i.t.), antiallodynic effects were tested using a 45 degrees C water-bath tail-flick test. The nonselective high-efficacy muscarinic agonists oxotremorine, H-TZTP (3-(1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine oxalate), and methylthio[3.2.1], (exo (+)3-(3-methylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[2.2.1]heptane oxalate), as well as vedaclidine, a mixed M-2/M-4 muscarinic receptor partial agonist and M-1/M-3/M-5 muscarinic receptor antagonist, the nonselective partial agonists RS86 and pilocarpine, and the cholinesterase inhibitors physostigmine and tacrine all produced dose-related antinociception. Oxotremorine, H-TZTP and methylthio[2.2.1] produced dose-related reversals of PTX-induced thermal allodynia whereas vedaclidine produced a partial reversal and RS86 and pilocarpine, as well as physostigmine and tacrine, failed to reverse the allodynia. The present results provide further evidence that decrements in PTX-sensitive G(i/o)-protein functioning may be involved in initiating and/or maintaining some persistent or neuropathic pain states. Moreover, the present results suggest that muscarinic receptor agonists such as vedaclidine may be useful in the treatment of persistent pain states that are due at least in part to dysfunction of inhibitory second messenger systems. (C) 2000 Elsevier Science Ltd. All rights reserved.