Synaptic organization involving receptor, horizontal and on- and off-center bipolar cells in the catfish retina.

HRP-injected bipolar cells were analyzed electron microscopically in catfish retina. The dendrites of the two types of bipolar cells (on-center and off-center) invaginated into the photoreceptor terminals in distinct patterns. Those from off-center bipolars ended as the central element of triads in such a way that every synaptic ridge examined was contacted by a single process along its apex. In contrast, labeled processes from on-center bipolars never made direct contact with the synaptic ridges of any photoreceptors. In addition, chemical synapses made by external horizontal cells onto photoreceptors were observed. The postsynaptic sites was always a telodendrial process of a photoreceptor.