Axonless horizontal cells of the rabbit retina: synaptic connections and origin of the rod aftereffect.

An axonless horizontal cell (AHC) of the rabbit retina was penetrated with a microelectrode and stained with horseradish peroxidase after recording its light responses. The cell was then serially sectioned and its connections examined with the electron microscope. Physiologically, the cell exhibited cone-dominated responses and a minor rod influence known as rod aftereffect. Electron microscopy showed that this AHC was only connected to cones. Therefore, the rod aftereffect could only invade the cell through the gap junctions between the synaptic endings of rod and cone photoreceptors. In the synaptic invaginations of the cone pedicles contacting the cell, only one of the lateral elements was stained. This suggests that the two lateral elements of each cone-invaginating synapse belong to two different horizontal cells. By staining intracellularly adjacent AHCs, we showed that the two lateral processes may originate from two horizontal cells belonging to the same morphological type.