Background: Acetylcholine causes synthesis of nitric oxide in vascular endothelium, and presumptive evidence in vivo suggests spinally released acetylcholine causes antinociception and increased sympathetic nervous system activity via a nitric oxide mechanism. The purpose of this study was to determine, using a recently described bioassay system, whether acetylcholine stimulates nitric oxide release from spinal cord tissue in vitro.
Methods: Rat thoracolumbar spinal cord slices were incubated in a tissue chamber and perfused with Krebs-Henseleit solution. The perfusate was then passed through endotheliumdenuded rat aortic rings and their tension was measured. Vascular rings were preconstricted with phenylephrine, then were exposed to spinal cord perfusate with increasing concentrations (10(-12)-10(-4)M) of acetylcholine alone or with various antagonists.
Results: Acetylcholine perfusion of spinal tissue caused concentration-dependent relaxations of the aortic rings, an effect blocked by each of the muscarinic antagonists, atropine, pirenzepine, and AFDX-116. Acetylcholine-induced relaxation also was antagonized by an inhibitor of nitric oxide synthase (N-methyl-L-arginine), a nitric oxide scavenger (hemoglobin) and an inhibitor of guanylate cyclase (methylene blue).
Conclusions: These results demonstrate release of a vasorelaxant from spinal cord tissue by acetylcholine, which results from an action on muscarinic receptors and exhibits a pharmacology consistent with nitric oxide. Although precise anatomic localization of acetylcholine's action is not possible with this system, these results add to evidence that acetylcholine causes nitric oxide synthesis in the spinal cord.