Calcium imaging for detection and estimation of spike activities in Aplysia neurons

We explored whether the calcium imaging can be applicable to detection of spike activity of Aplysia wide-spread neurons. Well-used procedures, the membrane permeable acetoxymethyl (AM) types and the retrograde labeling could not be used for loading the calcium sensitive dye into many neurons, however, impalement of each cell body with a microelectrode containing high concentration of the dye solution in turn could load the dye iontophoretically into many large neurons in a relatively short time and into several small neurons in a very short time. Spike activity just induced a fluorescent increase in the cell body while depolarization without spikes induced it in the axon hillock. In the cell body the slope of the fluorescent increase was almost relative to the spike frequency while the period of it almost corresponded to the firing duration. Therefore, the phase relationship of the rhythmic firing responses in plural neurons could be precisely detected. Moreover stable responses could be repeatedly obtained for a long time from small neurons unsuitable for a long time recording with microelectrodes. Removal of the extracellular Ca²⁺ completely suppressed the spike-induced increase in fluorescence in the cell body and application of nifedipine partly reduced it, suggesting contribution of L-type channels distributing in Aplysia wide-spread neurons.