Intracellular recording from identified photoreceptors and horizontal cells of the Xenopus retina.

Intracellular recordings were made from rods, cones and horizontal cells of the Xenopus retina. The cells under study were identified by injection of the fluorescent dye, Lucifer yellow. Rod spectral sensitivity peaked near 524 nm, that of cones near 612 nm whereas horizontal cells reflected input from both these classes of photoreceptors. No intracellular recordings were made from blue-sensitive rods (lambda max = 445 nm) nor did this rod appear to provide an input to the horizontal cell. Under dark-adapted conditions, horizontal cells had a slow waveform, a Vmax less than or equal to 18 mV and were driven by 524 nm rods only. When light-adapted, horizontal cell responses were fast, Vmax was 30-40 mV and the responses reflected only 612 nm cone input. In the mesopic state rod and cone inputs to the horizontal cell interacted non-linearly: weak green backgrounds greatly enhanced the response to a superimposed red flash compared to the red flash response on a dark field. The length constant of the horizontal cell exceeded its dendritic arbor by 2-15 fold. All of the stained horizontal cells, however, possessed a long slender axon without a terminal but which emitted periodic short branches that appeared to contact receptors.