Central Cholinergic Receptors in Cardiovascular and Antidiuretic Effects in Rats

1. Cardiovascular and antidiuretic responses to central cholinergic stimulation were investigated in conscious water loaded rats. 2. Pressor responses and antidiuresis were observed after injections of carbachol into the third ventricle. Intraventricular injection of nicotine produced bradycardia but no significant pressor or antidiuretic effects. 3. A prior injection of hexamethonium into the third ventricle abolished the effects of nicotine but did not attenuate the blood pressure increase or antidiuresis to carbachol. A prior injection of atropine into the third ventricle blocked both of these responses to carbachol, indicating the involvement of central periventricular muscarinic receptors in the central mediation of cardiovascular changes and antidiuretic hormone (ADH) release. 4. Analysis of blood levels of ADH after injection of carbachol into the third ventricle indicated the release of ADH in concentrations which were consistent with those seen to intravenous injections of ADH which produced equivalent antidiuresis, supporting the use of the on-line rat antidiuretic assay for measuring endogenous ADH release to central stimuli. 5. To test the possible involvement of catecholaminergic mechanisms in pressor and antidiuretic effect to cholineric receptor stimulation rats, carbachol was injected into the third ventricle of rats before and 10 min after central phentolamine treatment. Both pressor and antidiuretic effects of the carbachol injections were attenuated by phentolamine, indicating a role for a-adrenoreceptors in the mediation of both responses.