Hypertension following intrathecal injection of cholinergic agonists in conscious rats: role of endogenous acetylcholine.

The spinal cord is capable of initiating a significant and long-lasting pressor response following intrathecal injection of cholinergic agonists in freely moving rats. The magnitude of the pressor response to the cholinesterase inhibitor, neostigmine, was greatest when the site of injection was restricted to the thoracic level. Intrathecal (i.t.) injection of neostigmine (1-10 micrograms) elicited a dose-related increase in mean arterial pressure of up to 45 mm Hg which remained elevated for almost 2 h. Significant inhibition of acetylcholinesterase was localized to the spinal cord, with the thoracic region exhibiting the greatest degree of inhibition. Also, depletion of spinal acetylcholine levels following i.t. injection of hemicholinium-3 (HC-3) resulted in a significant reduction in the magnitude of the neostigmine-induced pressor response. Carbachol, a direct-acting cholinergic receptor agonist also increased mean arterial pressure following i.t. injection. However, the pressor response to carbachol was not reduced following HC-3. For both agonists, cardiovascular changes were accompanied by significant behavioral changes characterized by tremor, scratching, tail biting and chewing. The appearances of these behaviors following neostigmine injection were reduced in frequency and intensity in HC-3-pretreated animals. These findings demonstrate the ability of spinal cholinergic neurons to mediate a significant hypertensive response. The presence of marked behavioral changes accompanying the cardiovascular response suggests the possibility that cholinergic neurons may be part of an ascending spinal system.