Spinal muscarinic cholinergic and nitric oxide systems in cardiovascular regulation

Pharmacological activation of muscarinic receptors located in the thoracic spinal cord evokes a marked increase in blood pressure and heart rate. We have previously demonstrated that the cardiovascular response to stimulation of spinal cord muscarinic cholinergic receptors is dependent upon a pharmacologically described ascending spino-bulbar pathway. The purpose of the study was to determine whether the blood pressure and heart rate responses to intrathecal (i.t.) injection of the muscarinic cholinergic receptor agonist carbachol are mediated by a local nitric oxide (NO)-generating system. Freely moving rats were previously prepared with chronic indwelling i.t. and intra-arterial catheters. Both the pressor and tachycardic responses produced by i.t. injection of carbachol were inhibited in a dose-dependent manner by i.t. pre-treatment with the NO synthase inhibitor N-omega-Nitro-L-arginine methylester (L-NAME). To confirm the site of action of the drugs employed in conscious rats, a separate group of rats was anesthetized, and using surgical procedures previously developed in this laboratory, drug distribution was limited specifically to the lower thoracic spinal cord. When carbachol was administered by i.t. injection and localized to the lower thoracic area, muscarinic cholinergic receptor stimulation again produced a marked pressor response, but without the accompanying tachycardia. The ability of N-omega-Nitro-L-arginine methylester (L-NAME) to inhibit the pressor response to carbachol in conscious rats was confirmed in anesthetized rats, although higher doses of L-NAME than those employed in conscious rats were required. L-NAME-induced inhibition of the carbachol-evoked pressor response was reversed by the L-, but not the D-isomer, of arginine. Moreover, i.t. pre-treatment with Methylene blue, that interferes with NO production and function, effectively inhibited the expression of the pressor response to i.t. injection of carbachol. The 'anti-muscarinic' action of L-NAME was not due to a direct interaction with spinal muscarinic receptors, as L-NAME did not significantly displace [3H]methyl-scopolamine from spinal cord membranes in vitro. The results of this study support the hypothesis that spinal muscarinic cholinergic receptors participate in a sympathoexcitatory pathway that interacts either directly or indirectly with a local NO-generating system involved in the regulation of blood pressure.