Cholinergic mechanisms in the ventrolateral medullary depressor area

Cardiovascular actions of cholinergic agonists in the ventrolateral medullary depressor area (VLDA) were investigated. Microinjections (0.2-1.6 nmol/site) of an M2 muscarinic receptor agonist (cis-methyldioxolane; CD) into the VLDA decreased blood pressure (33-71 mm Hg) and heart rate (11-62 beats/min). Microinjections of AFDX-116 (a specific blocker for M2 receptors), but not those of pirenzepine (a specific blocker for M1 receptors), prevented the depressor responses induced by CD. Unilateral microinjections of CD (0.3 nmol) into the VLDA also elicited depressor and bradycardic effects which were blocked by microinjections of bicuculline methiodide (200 pmol) into the ipsilateral ventrolateral medullary pressor area (VLPA). Bilateral vagotomy did not alter the depressor and bradycardic actions of CD in the VLDA. McN-A343 (2-3 nmol/site), an M1 receptor agonist, failed to evoke any response when microinjected into the VLDA. These results indicate that muscarinic receptors of the M2 type are present in the VLDA. Activation of the latter by cholinergic agonists results in depressor and bradycardic responses. These responses are mediated via the VLPA.