Analysis of neural mechanisms of rhythm generation for sucking -induction of optical imaging method in in vitro preparations obtained from newborn mice.

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The recent development of in vitro CNS preparations of newborn mammals has provided important tools for physiological investigation of central rhythm generation of sucking as well as respiration and locomotion. The aim of this research was to develop an in vitro CNS preparation, which included neural circuits to generate rhythmic sucking-like activity, applicable to optical measuring of neuronal activity using voltage-sensitive or calcium-sensitive dyes. Experiments were performed on brainstem-spinal cord preparations isolated from newborn mice with or without the intact neural connection to the oro-facial structure. Neural activities were monitored from the trigeminal, facial and hypoglossal nerves with suction electrodes. Movements of face, tongue and jaw were recorded with a CCD camera system. The following results were obtained : (1) NMDA induced rhythmical burst activity in the trigeminal, facial and hypoglossal nerves ; (2) NMDA also induced rhythmic movements of the mandible, cheek and tongue ; (3) after complete transections of the brainstem between the trigeminal and facial nerves as well as at the pontomedullary junction, NMDA still induced the rhythmic activity in each of the trigeminal, facial and hypoglossal nerves. These results demonstrate that separate oscillators for rhythmic NMDA-induced activities in the trigeminal, facial and hypoglossal motoneurons are present at the respective levels of the brainstem, and that each set of these oscillators and their downstream motoneurons is included within a small block of an in vitro CNS preparation which is thin enough to record neuronal activities with optical imaging methods.