Characterization of muscarinic receptor subtypes that mediate antinociception in the rat spinal cord

Spinal cholinergic receptors are involved in mediating antinociceptive effects. To characterize the receptor subtypes modulating nociceptive transmission, we first determined the antinociceptive effects of intrathecally administered muscarinic agonists (McN-A-343 and carbachol) and a cholinesterase inhibitor (neostigmine) in rats. The antagonist potencies of muscarinic antagonists with different preferences for muscarinic receptors [atropine, pirenzepine (M-1), methoctramine (M-2), and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; M-3)] were then examined for McN-A-343, carbachol, and neostigmine. After determining the time of peak effect for each antagonist, the just maximally effective (JME) dose of each agonist was given in conjunction with one of the several doses of the antagonists. The three agents produced a dose-dependent increase in paw withdrawal latency, with the following 50% effective dose and the following JME doses: neostigmine 6 and 14 nmol, carbachol 29 and 110 nmol, and McN-A-343 354 and 630 nmol. The rank of order of potency (and median infective dose in nanomoles) for the antagonists was: neostigmine = (atropine 14 > 4-DAMP 44 >> methoctramine > 137, and pirenzepine > 236); carbachol = (pirenzepine 0.5 = atropine 0.6 > 4-DAMP 5 >> methoctramine > 137); McN-A-343 = (pirenzepine 0.5 > atropine 3 > 4-DAMP 6 >> methoctramine > 137). Our results suggest that the M-1 and possibly the M-3 muscarinic receptors mediate spinal antinociception in the rat. Implications: Spinal muscarinic agonists, such as carbachol and the cholinesterase inhibitor neostigmine, induce a potent analgesia in the rat. Using selective receptor antagonists, we have shown that these effects are likely mediated by spinal M-1 and/or M-3 receptor subtypes.