THE NUMBER AND DISTRIBUTION OF BIPOLAR TO GANGLION-CELL SYNAPSES IN THE INNER PLEXIFORM LAYER OF THE ANURAN RETINA

The main route of information flow through the vertebrate retina is fr om the photoreceptors towards the ganglion cells whose axons form the optic nerve. Bipolar cells of the frog have been so far reported to co ntact mostly amacrine cells and the majority of input to ganglion cell s comes from the amacrines. In this study, ganglion cells of frogs fro m 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 th e ganglion cell layer as well as their dendrites in the inner plexifor m layer. In cross sections, bipolar output and ganglion cell input syn apses were counted in the electron microscope. Each synapse was assign ed to one of the five equal sublayers (SLs) of the inner plexiform lay er. 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 in ner plexiform layer with the highest occurrence in SL2 both in Bufo an d 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 Xen opus, 22% of the bipolar cell output synapses went onto ganglion cells , whereas in Bufo this was only 10%. We conclude that direct bipolar t o ganglion cell information transfer exists also in frogs although its occurrence is not as obvious and regular as in mammals. The character istic distribution of these synapses, however, suggests that specific type of the bipolar and ganglion cells participate in this process. Th ese contacts may play a role in the formation of simple ganglion cell receptive fields.