Neural Mechanisms of Swallowing Inhibition Following Noxious Orofacial Stimulation

Abstract

The number of water-induced swallows was decreased following capsaicin injection into the facial (whisker pad) skin, masseter or lingual muscle. The capsaicin-induced inhibitory effect on the swallowing reflex was depressed after intrathecal administration of mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor. The inhibitory effect on swallowing following capsaicin injection into the lingual muscle was diminished by paratrigeminal nucleus (Pa5) lesioning. Many phosphorylated extracellular signal-regulated kinase-like immunoreactive neurons in the nucleus tractus solitarii (NTS) showed gamma-aminobutyric acid (GABA) immunoreactivity and capsaicin-induced inhibition of the swallowing reflex was diminished by local microinjection of the GABAA receptor antagonist into the NTS.The present findings suggest that facial skin-NTS, masseter muscle-NTS, lingual muscle-NTS and lingual muscle-Pa5-NTS pathways are involved in the swallowing inhibition by facial, masseter and lingual pain, and that the activation of GABAergic NTS neurons may be involved in inhibition of the swallowing reflex.