Fast optical recordings of membrane potential changes from dendrites of pyramidal neurons

Understanding the biophysical properties of single neurons and how they pro cess information is fundamental to understanding how the brain works. A tec hnique that would allow recording of temporal and spatial dynamics of elect rical activity in neuronal processes with adequate resolution would facilit ate further research. Here, we report on the application of optical recordi ng of membrane potential transients at many sites on neuronal processes of vertebrate neurons in brain slices using intracellular voltage-sensitive dy es. We obtained evidence that 1) loading the neurons with voltage-sensitive dye using patch electrodes is possible without contamination of the extrac ellular environment; 2) brain slices do not show any autofluorescence at th e excitation/emission wavelengths used; 3) pharmacological effects of the d ye were completely reversible; 4) the level of photodynamic damage already allows meaningful measurements and could be reduced further; 5) the sensiti vity of the dye was comparable to that reported for invertebrate neurons; 6 ) the dye spread similar to 500 mu m into distal processes within 2 h incub ation period. This distance should increase with longer incubation; 7) the optically recorded action potential signals from basolateral dendrites (tha t are difficult or impossible to approach by patch electrodes) and apical d endrites show that both direct soma stimulation and synaptic stimulation tr iggered action potentials that originated near the soma. The spikes backpro pagated into both basolateral dendrites and apical processes; the propagati on was somewhat faster in the apical dendrites.