Development of an optical recording system for neuronal network activities of invertebrates

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An optical recording system that can be used in electrophysiological experiments using the whole animal preparation of invertebrates has been developed to visualize the change in intracellular calcium concentration associated with the synaptic activity. The system consists of an epifluorescence device for exciting Fura-2 that is iontophoretically injected into a cell, a dissecting microscope with the working distance of 187mm, a Peltier-cooled CCD video camera, and a personal computer with newly developed programs for image analysis. To test the performance of this recording system, nerve cells in the terminal abdominal ganglion of crayfish were loaded with the dye and observed from the dorsal side with the CCD exposure time of 16sec. The whole cell structure having the soma, dendrites and axon could be visualized in situ together with the electrode tip under the dissecting microscope by either 380nm or 340nm excitation. When the sensory nerve bundle was stimulated repeatedly at 1Hz during the exposure time, a decrease in the fluorescence of the dye to excitation at 380nm was detected in a specific dendritic region of the cell. However, any significant change in the ratio of fluorescence by 380nm to that by 340nm excitation during synaptic activation could not be detected probably due to the relatively weak strength of the latter. It is concluded that although the sensitivity and time resolution are not enough to quantify the dynamics of intracellular calcium ions, the presently developed system can be used to localize the functional synaptic region within a cell by visualizing the change in intracellular calcium concentration due to synaptic activity together with the whole cell structure.