The cellular origin of an unusual type of S-potential: an intracellular horseradish peroxidase study in a cyprinid fish retina.

L2-type S-potentials are mainly blue/green-sensitive hyperpolarizing responses with a red-sensitive depolarizing component which is either weak or absent. They were first described in the retina of the roach, a cyprinid fish, by Djamgoz (1978, 1984) and Djamgoz & Ruddock (1978, 1979a). The cellular origin of these responses has been determined and characterized by intracellular recording, horseradish peroxidase staining, and light and electron microscopy. They were found to arise in horizontal cells with H2-like morphologies on average (Stell & Lightfoot, 1975). The dendrites of these cells contacted green- and blue-sensitive cone pedicles within which both lateral and central contacts were made at ribbon synapses. The laterally-positioned dendrites had incompletely formed spinules associated with them. A number of similarities between these units and the biphasic, chromaticity (Cb)-type S-potentials have been outlined and it is suggested that L2 units are essentially Cb-units with a weak depolarizing component. In turn, it is suggested that the depolarizing component is reduced as a consequence of the relatively dark-adapted states of the retinae. It is concluded that the negative feed-back pathway that subserves the generation of depolarizing (Cb-type) S-potentials is weak or absent in dark-adapted retinae and that spinules may be the site of this feed-back interaction.