Pharmacological evidence that nitric oxide mediates the antinociception produced by muscarinic agonists in the rostral ventral medulla of rats.

The present study was designed to characterize the antinociception produced by the administration of a muscarinic agonist, (+)-cis-methyldioxolane, into the rostral ventral medulla (RVM) of male Sprague-Dawley rats. Seven days after the implantation of 25-gauge stainless-steel guide cannulae, animals were injected with graded doses of (+)-cis-methyldioxolane, and antinociception was assessed by using the 52 degrees C hot-plate and tail-flick tests in a single-blind design. (+)-cis-Methyldioxolane produced dose-related hot-plate and tail-flick antinociception for 30 to 45 min peaking 5 to 10 min after RVM injection. The ED50 values of (+)-cis-methyldioxolane in the hot-plate and tail-flick tests were 2.4 and 1.7 nmol, respectively. Five-minute preinjections with 0.35 nmol of the muscarinic M1 receptor blocker pirenzepine competitively antagonized the antinociception produced by (+)-cis-methyldioxolane. The antinociception produced by RVM injections of the muscarinic M2 receptor blocker methoctramine was additive to the antinociceptive effects of (+)-cis-methyldioxolane when the two antinociceptive effects of (+)-cis-methyldioxolane when the two agents were combined. Twenty four-hour pretreatment of the RVM with the irreversible muscarinic receptor antagonist 4-diphenylacetoxy-N-[2-chloroethyl]-piperidine mustard blocked the antinociceptive effects of (+)-cis-methyldioxolane completely. Administration of 6 nmol of the nitric oxide synthase inhibitor L-NG-nitroarginine into the RVM competitively antagonized the antinociception produced by (+)-cis-methyldioxolane and (+)-muscarine in the hot-plate and tail-flick tests. Pretreatment with 100 nmol of L-arginine, but not D-arginine, significantly reversed the inhibitory effects of L-NG-nitroarginine on (+)-cis-methyldioxolane-produced antinociception. Pretreatment with 100 nmol of the guanylyl cyclase inhibitor methylene blue into the RVM profoundly antagonized the antinociception produced by (+)-cis-methyldioxolane. Neither buffer L-NG-nitroarginine, L-arginine-D-arginine or methylene blue altered hot-plate or tail-flick nociception when injected alone into the RVM. In contrast, either dibutyryl cyclic GMP or 8-bromo cyclic GMP, membrane-permeable cyclic GMP analogs, produced hot-plate and tail-flick antinociception when injected alone into the RVM. These data are consistent with the hypothesis that the antinociception produced by muscarinic stimulation of the RVM is mediated by an L-arginine/nitric oxide/cyclic GMP cascade.