Optical monitoring of excitatory synaptic activity between cultured hippocampal neurons by a multi-site Ca2+ fluorometry.

The fluorescence intensities from cultured hippocampal neurons loaded with a Ca2+-indicator (fura-2), showed spontaneous, periodical oscillations, which were synchronized among the cells. Tetrodotoxin or 2-aminophosphonovalerate, a glutamate-receptor antagonist, blocked the oscillation. The rising phase of the fluorescence was accompanied by a burst of inward currents, as monitored by a patch electrode. It is thus suggested that the fluorescence elevation represents an increase in intracellular Ca2+ concentration accompanied by excitation of neurons which formed synaptic connections between each other. This method for detection of neuronal activity including synaptic excitation escapes from the limitations of the conventional technique employing microelectrodes.