Inhibitory influence of monoamines and brainstem monoaminergic regions on the medullary swallowing reflex.

Swallowing is a polysynaptic reflex organized by a neuronal network localized mainly within the lateral region of the nucleus tractus solitarius (NTS). Monoamine microinjections (serotonin, noradrenaline; 1 nmol, 50 nl) within the swallowing region of the NTS produced a significant decrease in the number of swallows elicited by repetitive stimulation of the superior laryngeal nerve. Stimulation, with concentric bipolar electrodes, of several brainstem structures overlapping monoaminergic regions such as the nucleus raphe magnus, the nucleus raphe pallidus, the lemniscus medialis, the ventrolateral reticular formation, the locus coeruleus and the nucleus commissuralis, induced an inhibition of the laryngeal-initiated swallowing. These results indicate the existence, within the NTS, of a monoaminergic inhibition of the swallowing reflex which might originate from monoaminergic brainstem regions.