Inhibition of the swallowing reflex by local application of serotonergic agents into the nucleus of the solitary tract.

Swallowing is a medullary polysynaptic reflex organized by an interneuronal network localized mainly within the nucleus of the solitary tract (NST). The existence of several putative neurotransmitters within the NST has been well demonstrated. The presence of serotonin (5-HT) in nerve terminals and fibers has been particularly well-documented. This study was therefore designed to determine the role of 5-HT in the swallowing reflex. The effects of serotonergic agents were investigated in the rat, on rhythmic swallowing elicited by long repetitive stimulation of the superior laryngeal nerve (SLN). The agents were microinjected by pressure application, within the swallowing region of the NST. Microinjections of 5-HT (0.3-5 nmol, 30-50 nl) significantly decreased the number and the amplitude of swallows elicited by stimulation of the ipsilateral SLN without changing the swallowing reflex induced by contralateral SLN stimulation. The decrease induced by 5-HT microinjections was dose-related. No significant modification of swallowing was induced by control injections of the vehicle within the active sites. Moreover, the effect of 5-HT microinjections was significantly antagonized by pretreatment with metitepine (0.4 nmol) applied locally in the NST and microinjections of quipazine (2.5 nmol) also decreased the number of swallows. It can therefore be concluded that the present findings suggest the existence within the NST of a serotonergic inhibition of the swallowing reflex elicited by laryngeal afferents.