Properties of glutamate channels in solitary horizontal cells of the goldfish retina

Ca2+ regulation by the Na(+)-Ca2+ exchanger in retinal horizontal cells depolarized by L-glutamate

This study is concerned with regulation of the intracellular Ca2+ concentration ([Ca2+]i) of horizontal cells isolated from cyprinid fish retinae, with the main emphasis on the role of the (Na+)-Ca2+ exchanger. An inward current was blocked by Ca2+ (4 mM) during prolonged (> 1 h) depolarization by L-glutamate (100 microM) in the whole-cell voltage-clamp configuration, suggesting the persistent activation of voltage-gated Ca2+ channels. This (Co2+)-sensitive current was absent when extracellular Na+ was replaced by Li+ to suppress (Na+)-Ca2+ exchange. Measurement of [Ca2+]i using the Fura-2 ratiometric method gave the following results. (1) L-Glutamate (100 microM) caused [Ca2+]i to increase from the resting level of 75.4+/-36.8 nM (mean +/-S.D., n = 11) to the maximum level (2.2+/-1.4 microM, n = 11) within 15 s and then to decrease to a steady level of 0.59+/-0.23 microM (n = 11). (2) Nifedipine (100 microM) lowered the L-glutamate-induced steady [Ca2+]i level, which was still higher than the resting level. (3) L-Glutamate caused [Ca2+]i to increase even after blockading the voltage-gated Ca2+ channels by nifedipine or by clamping the membrane voltage at -55 mV. (4) (Na+)-free superfusate elevated the L-glutamate-induced steady [Ca2+]i level. (5) The time course of the [Ca2+]i decrease from the L-glutamate-induced steady level to the resting level was prolonged in the (Na+)-free superfusate. These results suggest that the (Na+)-Ca2+ exchanger extrudes intracellular Ca2+ to maintain a low [Ca2+]i level by counteracting the continuous Ca2+ influx through the voltage-gated Ca2+ channels and glutamate-gated channels when horizontal cells in situ are tonically depolarized by L-glutamate released from the photoreceptors. The (Na+)-Ca2+ exchange current isolated by a voltage-clamp experiment depends exponentially on the membrane potential.

Horizontal Cells in the Monkey Retina: Cone connections and dendritic network

Horizontal cells of the macaque monkey retina were quantified and the number of cones converging onto an individual horizontal cell as well as the number of horizontal cells contacting a single cone were determined. This was done by combining data from individual horizontal cells stained by the Golgi method with the results of immunocytochemical staining described in the preceding paper (Röhrenbeck et al., 1989). The observation (Boycott et al., 1987) that all horizontal cells contact all cones in their dendritic field irrespective of cone type was confirmed. The particular cones contacted by the terminal aggregates of each horizontal cell were found. The dendritic fields of H1 and H2 cells increase with increasing eccentricity; close to the fovea H1 cells are smaller than H2 cells, at 6 mm eccentricity they are about the same size and in peripheral retina H1 cells are much larger than H2 cells. The density gradients of the two cell types balance their denritic field changes so that throughout the retina each and every cone synapses with 3 - 5 horizontal cells of each type. Horizontal cells of both cat (Wässle et al., 1978) and monkey retina follow the general rule that all cones in the dendritic fields are contacted, their perikarya form a regular mosaic and the boundaries of their dendritic fields are marked by the perikarya of their homologous neighbours.