Effect of catecholamines on the swallowing reflex after pressure microinjections into the lateral solitary complex of the medulla oblongata.

The present study was carried out to elucidate the influence of catecholamines on swallowing, a polysynaptic reflex organized by an interneuronal network localized mainly within the lateral solitary complex (LSC) of the medulla oblongata. The effects of catecholaminergic agents were investigated in the rat, on rhythmic swallowing elicited by repetitive stimulation of the superior laryngeal nerve (SLN). Catecholaminergic agents were microinjected by pressure application, through multibarrelled glass micropipettes, into the LSC including the tractus solitarius, the swallowing region of the nucleus of the solitary tract and the adjacent reticular formation. Microinjections of noradrenaline (NA, 0.1-5 nmol, 50 nl) induced a significant decrease of the number and the amplitude of the rhythmic swallows elicited by stimulation of the ipsilateral SLN. This inhibitory effect was dose-related. Microinjections of clonidine (2.5 nmol, 50 nl), dopamine (0.25-2.5 nmol, 50 nl) and apomorphine (0.5 nmol, 50 nl), also inhibited swallowing. No significant modification of swallowing was induced by control injections of the vehicle (50 nl) within the active sites. Moreover the NA-induced inhibition of swallowing, was significantly antagonized by pretreatment with the alpha-adrenergic blocker phentolamine applied locally in the LSC. Furthermore neither blood pressure, nor respiratory rhythm were consistently modified by the catecholaminergic microinjections, indicating that the catecholamine-induced inhibition of swallowing was not a secondary side effect originating from alteration of these functions. It can therefore be concluded that the present results demonstrate the existence within the LSC of a catecholaminergic inhibition of the swallowing reflex. This inhibitory effect likely arises from activation of specific catecholaminergic receptors and affects the swallowing structures localized within the LSC, i.e., the laryngeal swallowing afferents running in the solitary tract and/or the swallowing interneurons within the nucleus of the solitary tract.