Quantitative analysis of triphasic (H3) horizontal cell-cone connectivity in the cyprinid fish (roach) retina

Although horizontal cells encode chromatic information by means of a variety of spectrally opponent light-evoked response patterns, their synaptic connections with the different spectral classes of cone are not completely understood. In the cyprinid fish retina, where a hierarchical set of interactions between horizontal cells and cone types has been proposed, a particular type of horizontal cell generates light-evoked triphasic (red-hyperpolarizing/green depolarizing/blue-hyperpolarizing) responses. In the present study, we have studied the cone connectivity of these cells by intracellular recording and staining in the roach retina. The horizontal cells were first identified electrophysiologically using spectral stimuli, and then stained intracellularly with horseradish peroxidase. Light microscopy revealed that the cells had consistent H3-like morphologies. At an ultrastructural level, these horizontal cells were deduced to contact selectively blue-sensitive cones. Within the cone pedicles, the majority (approximately 80%) of the contacts were "central" to synaptic ribbons. Some 50% of the "lateral" processes were large and engulfed cone cytoplasm. Spinules were present within the contacted pedicles but not upon the dendrites of the stained horizontal cells, although previous work had suggested that horseradish peroxidase would not interfere with spinule dynamics. The results are discussed in terms of existing modes of horizontal cell-cone connectivity in cyprinid fish retinae.