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 pro duced by the administration of a muscarinic agonist, (+)-cis-methyldio xolane, into the rostral ventral medulla (RVM) of male Sprague-Dawley rats. Seven days after the implantation of 25-gauge stainless-steel gu ide cannulae, animals were injected with graded doses of (+)-cis-methy ldioxolane, and antinociception was assessed by using the 52 degrees C hot-plate and tail-flick tests in a single-blind design. (+)-cis-Meth yldioxolane produced dose-related hot-plate and tail-flick antinocicep tion for 30 to 45 min peaking 5 to 10 min after RVM injection. The ED( 50) values of (+)-cis-methyldioxolane in the hot-plate and tail-flick tests were 2.4 and 1.7 nmol, respectively. Five-minute preinjections w ith 0.35 nmol of the muscarinic M(1) receptor blocker pirenzepine comp etitively antagonized the antinociception produced by (+)-cis-methyldi oxolane. The antinociception produced by RVM injections of the muscari nic M(2) receptor blocker methoctramine was additive to the antinocice ptive effects of (+)-cis-methyldioxolane when the two agents were comb ined. Twenty four-hour pretreatment of the RVM with the irreversible m uscarinic receptor antagonist 4-diphenylacetoxy-N-[2-chloroethyl]-pipe ridine mustard blocked the antinociceptive effects of (+)-cis-methyldi oxolane completely. Administration of 6 nmol of the nitric oxide synth ase inhibitor L-N-G-nitroarginine into the RVM competitively antagoniz ed the antinociception produced by (+)-cis-methyldioxolane and (+)-mus carine in the hot-plate and tail-flick tests. Pretreatment with 100 nm ol of L-arginine, but not D-arginine, significantly reversed the inhib itory effects of L-N-G-nitroarginine on (+)-cis-methyldioxolane-produc ed antinociception. Pretreatment with 100 nmol of the guanylyl cyclase inhibitor methylene blue into the RVM profoundly antagonized the anti nociception produced by (+)-cis-methyldioxolane. Neither buffer, L-N-G -nitroarginine, L-arginine, D-arginine or methylene blue altered hot-p late or tail-flick nociception when injected alone into the RVM. In co ntrast, either dibutyryl cyclic GMP or 8-bromo cyclic GMP, membrane-pe rmeable cyclic GMP analogs, produced hot-plate and tail-flick antinoci ception when injected alone into the RVM. These data are consistent wi th the hypothesis that the antinociception produced by muscarinic stim ulation of the RVM is mediated by an L-arginine/nitric oxide/cyclic GM P cascade.