Characterization of the antinociception produced by intrathecally administered muscarinic agonists in rats.

The present study was designed to characterize the antinociception produced by the administration of a potent muscarinic agonist into the intrathecal space of the lumbar spinal cord of male Sprague-Dawley rats. Seven days after surgical implantation of intrathecal catheters, animals were injected with graded doses of (+)-cis-methyldioxolane. (+)-cis-Methyldioxolane produced hot-plate and tail-flick antinociception for up to 90 min, peaking 5 to 30 min after injection. The dose of (+)-cis-methyldioxolane that inhibited nociception by 50% was 12 nmol in both the hot-plate and tail-flick tests. This antinociception was not accompanied by a general depression of other spontaneous motor responses. The tissue concentration of (+)-cis-methyl-dioxolane in the lumbar spinal cord present at the time of maximal hot-plate and tail-flick antinociception was approximately 12 microM. In similarity to (+)-cis-methyldioxolane, intrathecally administered (+)-muscarine also produced strong hot-plate and tail-flick antinociceptive responses. In contrast, intrathecally administered N-methylcarbachol, a nicotinic agonist, had no effect on nociception. Five-minute pretreatment with graded doses of pirenzepine, methoctramine, idazoxan, LY53857, or S-(-)-zacopride each significantly antagonized hot-plate and tail-flick antinociception produced by 37.5 nmol of (+)-cis-methyldioxolane in a dose-related manner with median effective antagonist doses in the range of 0.4 to 2.2 nmol. Intrathecal pretreatment with graded doses of prazosin or naloxone enhanced the antinociception produced by (+)-cis-methyldioxolane in the tail-flick but not the hot-plate tests. Intrathecal vehicle, S(-)-propranolol or mecamylamine did not alter (+)-cis-methyldioxolane-induced antinociception. The data suggest that the antinociceptive responses produced by intrathecally administered (+)-cis-methyldioxolane involve the stimulation of muscarinic M1 and/or M2 cholinergic receptors, and may also involve activation of alpha-2 adrenergic, 5-hydroxytryptamine1c/2 and 5-hydroxytryptamine3 serotonergic receptor systems at the level of the lumbar cord.