Identified retinal axons occupy postsynaptic positions in the optic tectum of Bufo marinus

Horseradish peroxidase was applied to the proximal stump of the cut optic nerve, and the anterogradely transported tracer was visualised by the cobalt-intensified diaminobenzidine method. The optic fibre receiving laminae of the optic tectum were systematically searched for those synapses in which the tracer-filled profiles occupied postsynaptic positions. Optic terminals were rarely but regularly postsynaptic to unlabelled, small clear vesicle-containing profiles, mostly in lamina A. Quantitative analysis showed that only in 0.95% of the synapses formed by optic fibres could this arrangement be substantiated. These results suggest that presynaptic modulatory mechanisms may play a role in optic information processing.