Optical recording of spontaneous respiratory neuron activity in the rat brain stem

We report on the optical imaging of spontaneous respiratory neuron bursts i n the ventrolateral medulla (VLM) of medullary slices or brain stem-spinal cord preparations. A medullary slice with a thickness of 1.0-1.4 mm or brai n stem-spinal cord from 0- to 4-d-old rats was stained with fluorescent vol tage-sensitive dye, RH795. Optical signals were recorded as a fluorescence change by using an optical recording apparatus with a 128x128 photodiode ar ray and a maximum time resolution of 0.6 ms. Motoneuronal activity was simu ltaneously recorded at the hypoglossal nerve roots or fourth cervical ventr al roots. Fluorescence changes corresponding to the spontaneous inspiratory burst activity were detected in the hypoglossal nucleus and VLM in slice p reparations, and in a limited area extending rostrocaudally in the VLM of t he brain stem-spinal cord preparation. These measurements did not require s ignal averaging by multiple trials. Results suggest that inspiratory neuron s are localized in more compact form at the level of the nucleus ambiguus t han at the more rostral VLM, and that peak activity during the inspiratory phase propagates from the caudal to the rostral VLM. In 60% of brain stem-s pinal cord preparations, weak and scattered fluorescence changes preceding the inspiratory burst activity were detected more predominantly in the rost ral part of the VLM. The present findings show the feasibility of optical r ecordings for the in vitro analysis of spontaneous respiratory neuron activ ity in the medulla.