Analysis of neural mechanisms of transformation from sucking to mastication in in vitro brain preparations.

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Feeding behavior of mammals changes from sucking to mastication, coinciding with the teeth eruption. In adult mammals, it is thought that the neurons involved in the rhythm generation of mastication locate in the lower brainstem and distributed along rostro-caudal axis. The purpose of this study was to investigate the neural organization of the rhythm generation of sucking in in vitro preprarations. Rhythmical sucking-like activity (RSA) is known to be induced in the hypoglossal nerve (XIIn) in an isolated brainstem-spinal cord preparation from newborn rats with bath application of N-methyl-D-aspartate (NMDA). Since natural sucking consists of well-coordinated rhythmical activities of the tongue, face, and jaw muscles, it is assumed that the trigeminal (V) and facial (VII) motoneurons would show rhythmical activities during the RSA.In this study, I tried (1 ) to induce rhythmical activities in these nerves during NMDA application, and (2) if they are induced, to locate the structures essential for generation of the NMDA-induced rhythmical activities. Experiments were performed on brainstem-spinal cord preparations isolated from newborn mice with or without the orofacial structure. Neural activities were recorded from each nerve with suction electrodes. Movements of the tongue and jaw were recorded with a CCD camera system. It was found that (1) bath application of NMDA induced RSA in the V, VII, and XII ns, (2) it also induced rhythmical sucking-like movements of the jaw and tongue, and (3) after complete transections of the brainstem between the V and VII ns as well as the pontomedullary junction, NMDA still induced the RSA in the V, VII, and XII ns, These results demonstrate that separate rhythm generators for the NMDA-induced activities are located at the respective levels in the brainstem, and that the reorganization of the CPG(s) itself would be necessary for the conversion from sucking to mastication.