Effects of microdialyzed oxotremorine, carbachol, epibatidine, and scopolamine on intraspinal release of acetylcholine in the rat

Intrathecally administered cholinergic agonists such as oxotremorine (musca rinic), carbachol (mixed nicotinic and muscarinic agonist), and epibatidine (nicotinic) have all been shown to reduce nociception in behavioral studie s. Thus, there is substantial evidence for a role of acetylcholine (ACh) in the control of nociception in the spinal cord, but the mechanisms regulati ng ACh release are not known. The present study was initiated to establish a rat model to study which mechanisms are involved in the control of ACh re lease. Spinal microdialysis probes were inserted intraspinally at the C1-C5 spinal level in isoflurane-anesthetized rats. The probes were perfused wit h Ringer's solution containing 10 mu M neostigmine to prevent degradation o f ACh. Oxotremorine, carbachol, epibatidine, and scopolamine, dissolved in Ringer's solution, were administered intraspinally via dialysis and 30 mu l /10-min samples of dialysate were collected for HPLC analysis of ACh conten t. The release of ACh was found to be constant in the control (Ringer's onl y) situation during the experimental period of 150 min. Oxotremorine (100-1 000 mu M), carbachol (1 mM), and epibatidine (50-5000 mu M) enhanced but sc opolamine (50-200 nM) decreased the intraspinal release of ACh. Oxotremorin e (ED50 = 118 mu M) and epibatidine (ED50 = 175 mu M) were found to produce a dose-dependent increase of ACh release. Cholinergic agonists caused an i ncrease of intraspinal ACh and the antagonist scopolamine caused a decrease d release of ACh. The data do not support an autoreceptor function of eithe r nicotinic or muscarinic receptors in the spinal cord, contrary to what ha s been observed in the brain.