The main route of information flow through the vertebrate retina is from the photoreceptors towards the
ganglion cells whose axons form the optic nerve. Bipolar cells of the frog have been so far reported to contact mostly amacrine cells and the majority of input to
ganglion cells comes from the amacrines. In this study,
ganglion cells of frogs from two species (Bufo marinus, Xenopus laevis) were filled retrogradely with
horseradish peroxidase. After visualization of the tracer, light-microscopic cross sections showed massive labeling of the somata in the
ganglion cell layer as well as their dendrites in the inner plexiform layer. In cross sections, bipolar output and
ganglion cell input synapses were counted in the electron microscope. Each synapse was assigned to one of the five equal sublayers (SLs) of the inner plexiform layer. In both species, bipolar cells were most often seen to form their characteristic synaptic dyads with two amacrine cells. In some cases, however, the dyads were directed to one amacrine and one
ganglion cell dendrite. This type of synapse was unevenly distributed within the inner plexiform layer with the highest occurrence in SL2 both in Bufo and Xenopus. In addition, SL4 contained also a high number of this type of synapse in Xenopus. In both species, we found no or few bipolar to
ganglion cell synapses in the marginal sublayers (SLs 1 and 5). In Xenopus, 22% of the bipolar cell output synapses went onto
ganglion cells, whereas in Bufo this was only 10%. We conclude that direct bipolar to
ganglion cell information transfer exists also in frogs although its occurrence is not as obvious and regular as in mammals. The characteristic distribution of these synapses, however, suggests that specific type of the bipolar and
ganglion cells participate in this process. These contacts may play a role in the formation of simple
ganglion cell receptive fields.