Analysis of rhythmical jaw movements produced by taste stimulation in rats

Taste stimulation of the mouth induces various oral movements. Sucrose or salt solution induces rhythmical jaw movements (RJM) or tongue protrusion as an ingestive behavior. Bitter taste induces a gaping or tongue retraction as an aversive behavior. There is no report that describes the precise pattern of jaw movements induced by taste stimulation. The purpose of the present study is to clarify the pattern of the taste-induced RJM with electromyographic activities of the masticatory muscles in the anesthetized rat. When water was injected into the mouth of the rat, an RJM was induced. In this type of RJM, the lower jaw swung right and left side in each open-close cycle alternately. The anterior digastric muscle was active in every opening phase, while activities of the jaw closing muscles were generally weak. The RJM induced by water was opening-dominant movements. Sucrose or salt solution induced a similar pattern of RJM to water-induced RJM. When acetic acid was injected, amplitude of the alternate lateral jaw movement was significantly larger than that in the water-induced RJM. The activity of the superficial temporalis muscle was large in those closing phases with ipsilateral side movement, while it was small in the closing phases with contralateral movement. The pattern of quinine-induced RJM was characterized by small lateral, large open-close and large antero-posterior movements. Tastes which are reported to induce ingestive behavior such as sweet or salty don't alter the pattern of RJM; however, the tastes which induce aversive behavior make the pattern of RJM different from the water-induced RJM.