Development of an in vitro preparation for analysis of central mechanisms controlling mastication

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The purpose of this study was to develop an in vitro brainstem preparation isolated from adult animals, in which masticatory movements can be induced. In the first year of this study, we succeeded to induce rhythmical reciprocal activities in masseter and digatric muslces by repetitive stimulation of the pontine pyramidal tract in an in vitro brainstem preparation kept alive by O_2 supply with an artificial cerebraospinal fluid (ACSF) saturated with 95% 0_2-5% CO_2 via the vascular system and diffusion from the perfusing ACSF in the rcording chamber. However, the necessity of perfusion of the brainstem via the vascular system was the critical fault of this preparation. To overcome this drawback and to keep the in vitro brainstem alive by diffusion of 0_2 solely from the ACSF in the recording chamber, it was necessary to reduce the size of the brainstem prparation by isolating only the portion of the brainstem containing the minimum structure essential for the masticatory rhythm generation in the trigeminal motoneurons (V MNs). In the second year, accordingly, we studied (1) whether there is a single rhythm generator or separate genarators for the V, facial (VII) and hypoglossal (XII) MNs, and (2) the localization of the rhythm generator of the V MNs. Bath application of NMDA to the isolated brainstem of the newborn mouse induced rhythmical activities in V, VII and XII MNs with cycle times diferent from one another, and these rhythmical activities persisted after transection of the brainstem between the V and VII nerves, and between the VIl and XII nerves. The results indicate that separate rhythm generators for V VII and XII MNs are located segmentally at the V motor, VII and XII nuclei, respectively. This finding provides a basic data for developing an in vitro block preparation containing only the rhythm gnerator for the V MNs.