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Department of Physiology, Keio University School of Medicine, Tokyo, Japan.

1. In the carp retina perfused with a solution containing high-Ca2+, Ba2+ and some K+-channel blockers, the horizontal cell produced a regenerative Ca2+ action potential when the cell was depolarized by bath application of L-glutamate (Glu) or L-aspartate (Asp). The action potential was triggered also by a transretinal electrical stimulation which evoked an e.p.s.p. in the horizontal cell. In this solution, some cells produced the action potential spontaneously. 2. The action potential had an overshoot of about 20 mV which lasted for several seconds or even minutes. It had a threshold and showed refractoriness. In addition, it was insensitive to tetrodotoxin, but was blocked by Co2+. These observations revealed, in horizontal cells in situ, the presence of a voltage-dependent Ca2+ channel similar to that found in dissociated cells. It is supposed that, in a physiological environment, the Ca2+ channel is prevented from becoming regenerative probably because it is counteracted by K+ channel activities. 3. Simultaneous recordings from two separate horizontal cells showed full synchronization of the Ca2+ action potentials whose amplitudes were identical. The potential uniformity thus formed in the S-space (Naka & Rushton, 1967) enabled us to measure reversal potentials of horizontal cell responses irrespective of the electrical coupling between the cells. 4. During an overshoot of the Ca2+ action potential, an electrically evoked e.p.s.p. as well as a light response appeared with polarities reversed to those elicited at the resting state. Their reversal potentials could be estimated within a very narrow range between -5 and -10 mV. At this range, both Glu- and Asp-induced potentials reversed the polarity, too. 5. These observations suggest that the ionic mechanisms are identical in the three kinds of horizontal cell response: light response, e.p.s.p. and amino acid-induced potentials.

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