Role of medullary cholinoceptors in baroreflex bradycardia

Cholinergic receptors present in three medullary nuclei namely, the nucleus tractus solitarii (NTS), nucleus ambiguous (AMB) and the dorsal motor nucleus of the vagus nerve (DMV) have been studied with regard to their role in regulation of heart rate (HR), blood pressure (BP) and baroreceptor reflex activation induced bradycardia in cats. Microinjection of carbachol into NTS was without effect while administration of carbachol or pilocarpine into AMB and DMV elicited dose related decrease in HR without affecting BP. These effects were completely antagonized by ethylbenztropine. Bilateral muscarinic cholinoceptor blockade of either AMB or DMV, with ethylbenztropine, produced a partial inhibition of the baroreflex bradycardia while intracisternal ethylbenztropine completely abolished this reflex response. Involvement of muscarinic cholinoceptors of AMB or DMV in baroreflex mediated adjustments of HR is therefore suggested.