A pharmacological analysis of neurally induced inhibition of carotid body chemoreceptor activity in cats

Experiments were performed to determine the mechanism by which centrifugal impulses in the carotid sinus nerve (CSN) reduce the frequency of impulse traffic in afferent chemoreceptor fibers from the carotid body in cats. Recordings of chemoreceptor activity were made from single or few fiber preparations dissected off the CSN, while the remainder of the CSN was stimulated electrically to produce neurally induced inhibition of chemoreceptor activity. Various drugs were injected either intravenously or directly into the arterial blood supply to the carotid body. We found that catecholamines (dopamine, norepinephrine and epinephrine) inhibited spontaneous chemoreceptor activity, and that alpha adrenergic antagonists abolished both this inhibition and that produced by electrical stimulation of the CSN in the same preparation. Atropine, but not nicotinic antagonists of acetylcholine, consistently blocked neurally induced inhibition but not that produced by catecholamines. Muscarinic agonists had no effect on spontaneous chemoreceptor activity. We conclude that centrifugal activity in the CSN causes release of endogenous catecholamines in the carotid body, and that these catecholamines mediate neurally induced inhibition of chemoreceptor activity. Our results are not compatible with the proposal by others that neurally induced inhibition of chemoreceptor activity is due to the vasomotor effects of acetylcholine.