Calcium action potential in ON-OFF transient amacrine cell of the carp retina

For accurate measurement of a reversal potential of a postsynaptic potential, it is essential to polarize a postsynaptic neuron uniformly at equipotential levels. So far as the conventional intracellular current injection is employed, uniform polarization cannot be achieved in such neurons as retinal amacrine cells which have extensive dendritic arborizations, and a reversal potential value is inevitably overestimated. In the present experiment, we employed a new technique; carp amacrine cells were polarized by a Ca2+-action potential produced in the cells themselves. To evoke the action potential, the retina was superfused with a Ringer solution containing tetraethylammonium chloride, and amacrine cells were depolarized either by intracellular or by extracellular electrical stimulation. The action potential appeared in a regenerative manner, and showed a refractoriness. In addition, Co2+ application suppressed the action potential, indicating its Ca2+-dependent nature. The Ca2+ action potential was more readily evoked or occurred even spontaneously in a solution containing high Ca2+, Ba2+ and some K+-channel blockers. It showed an overshoot and its duration was several seconds. During the overshoot, the transient light responses appeared in reversed, hyperpolarizing polarity, and their reversal potentials were measured at -10 mV. Based on the above results, physiological roles of the Ca2+-channel are discussed. Our technique is promising for wide application to neurons in other nervous systems if the superfusion technique is available for preparations.