Optical analysis of neural activities in the nucleus solitarius of the vagus nerve

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Using an intrinsic optical imaging technique, we examined neural responses in the rat brainstem evoked by vagal stimulation. First we searched for the optimal depth of the focal plane from which the largest response area could be obtained : the image of the response typically reached its maximum extent around a depth of 700μm from the surface of the cortex at the obex. Subsequently, changing the intensity or frequency of the stimulating current, we made the optical imaging together with measurements of blood pressure. Blood pressure fell with vagal stimulation, and the degree of the fall was constant under the same condition of the stimulation. However, even to repeated applications of the same vagal stimulus, considerable variations were often observed in the size/extent, configuration, and orientation of the intrinsic optical response area between trials. In general, the response area extended forward the rostro-caudal direction from the commissural nucleus of tractus solitarius, depending upon the increase in the intensity or frequency of the stimulus. Furthermore, it has been considered that the frequency- dependent variation of the response area was observed in the nucleus of tractus solitarius.