Kyoh-Ichi Takahashi

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.

Reversal potentials of color opponent responses in horizontal cells of the carp retina

Reversal potentials of color opponent responses were measured in H2 and H3 horizontal cells of the carp using Ca2+ action potential method. All responses reversed around -5 mV, suggesting that the ionic mechanisms of light responses are common among all types of horizontal cells. Glutamate-induced depolarizations in H2 and H3 cells had the same reversal potentials as those of light responses, consistent with the idea that these cells receive glutamate-mediated inputs from the cones. Simultaneous recordings showed that depolarization of H1 cells by Ca2+ action potential was transmitted to H2 cells in sign-inverted manner through a negative feedback interposed between these cells.

Subtype of excitatory amino acid receptor in cone horizontal cells of the carp retina as specified by reversal potential measurement technique

Effects of agonists of the excitatory amino acid (EAA) transmitters were examined in carp cone horizontal cells where glutamate (Glu) or aspartate (Asp) is believed to act as the transmitter released from the photoreceptors. Bath application of kainic (KA), quisqualic (QA) and N-methyl-D-aspartic (NMDA) acids produced little effect on cone cells, indicating that their effects act directly on the horizontal cells. KA and QA (100 microM for both) produced depolarizations in the horizontal cells. Their reversal potentials were measured by our novel technique which was developed to overcome a serious experimental disadvantage due to electrical coupling between horizontal cells. The retina was perfused with a modified Ringer solution which contained high-Ca2+,Ba2+, and some K+-channel blockers. A Ca2+ action potential having an overshoot was evoked in the horizontal cells when they were depolarized by application of the EAA. During the action potential, perfect potential uniformity was achieved throughout electrically coupled cells. Responses induced by KA and QA during the overshoot appeared in reversed polarities to those elicited at the resting state. Their reversal potentials were then estimated to be similar at around -6mV, and this value coincided with that of the Glu- or Asp-induced responses. On the other hand, effects of NMDA were diverse even though applied in the order of mM; some cells were hyperpolarized, but the others were little affected. These observations indicate that the EAA receptor of carp horizontal cells is KA/QA (non-NMDA) type.