Optical imaging of respiratory burst activity in newborn rat medullary block preparations

We report on the optical imaging of excitation propagation induced by electrical stimulation of the nucleus tractus solitarius (NTS) area and subsequent inspiratory burst activity in the ventrolateral medulla (VLM) of a medullary block preparation. A medullary block preparation with a thickness of 1.0-1.4 mm was made from brainstems isolated from 0- to 4-day-old rats and stained with a fluorescent voltage-sensitive dye, RH795. Neuronal responses in the VLM evoked by electrical stimulation were recorded as a fluorescence change using an optical recording apparatus with a 128 x 128 photodiode array and a maximum time resolution of 0.6 ms. Motoneuronal activity was simultaneously recorded at the contralateral hypoglossal nerve roots. Neuronal excitation evoked by stimulation of the NTS area propagated to the VLM through the intermediate reticular zone (IRt). In contrast, caudal VLM stimulation induced excitation which propagated to the rostral VLM without any detectable excitation propagation in the IRt toward the NTS area from the VLM. NTS stimulation also induced an inspiratory burst activity in the hypoglossal nerve root activity with a 150-200 ms delay. Fluorescence changes corresponding to the inspiratory burst activity were observed in the VLM which coincided with the area in which the localization of many respiratory neurons had been demonstrated in previous studies using whole-brainstem preparations. The present results show the feasibility of using optical recordings for the analysis of respiratory neuron activity as well as for analysis of the transmission pathway of afferent and/or efferent information in the medulla.