Synaptic connections linking cones and horizontal cells in the retina of the pikeperch (Stizostedion vitreum)

Abstract

Cones and horizontal cells of the pikeperch retina were studied with morphological and physiological techniques. Gap junctions were observed between cone pedicles and basal processes emitted by neighboring cones. Intracellular recordings showed that the light-evoked hyperpolarizing cone response was enhanced by light falling upon neighboring receptors within a radius of 50μm. We suggest that the network of gap junctions between cones mediates the summative lateral interaction described. Three sub-classes of horizontal cells (H₁, H₂, H₃) send dendrites to cones; H₁ and H₂ cells appear to appear to contact twin cones, exclusively or preferntially, whereas H₃ cells appear to synapse only with single cones. Horizontal cells of the same sub-class are joined by gap junctions between dendrites or at the lateral faces of perikarya. These unions extend over several μm² and as seen in transmission electron microscopy consist of patches of close apposition alternating with areas of membrane separation, folding and occasional zonulae adherentes. Freeze-fracture profiles of horizontal gap junctions show localized areas of dense particle aggregation on the P-face and pits on the E-face flanked by regions of unspecialized membrane. These morphological findings provide support for the known spatial and color-coding properties of pikeperch horizontal cells.