Uptake of 3 H‐glycine in the outer plexiform layer of the retina of the toad Bufo marinus

The uptake of 3 H‐glycine in the retina of the toad, Bufo marinus , was investigated by light and electron microscopical autoradiography. Uptake of 3 H‐glycine was very prominent in large cell bodies in the inner nuclear layer as well as in discrete clusters in both the outer plexiform layer (OPL) and the inner plexiform layer. This pattern is similar to that described for 3 H‐glycine‐aecumulating putative interplexiform cells in goldfish, frog, and Xenopus retinas. Electron microscopical autoradiography of the OPL revealed large, grain‐containing varicosities which had electron‐lucent cytoplasm and contained both small, agranular and large, dense‐core vesicles. The varicosities made extensive en passant and spine synapses in the OPL. Definitive identification of their postsynaptic targets was not achieved. However, autoradiographic analysis with 3 H‐GABA uptake as well as electrophysiological evidence suggests that axons but not cell bodies or dendrites of 3H‐GABA‐accumulating horizontal cells (H1 cells) are postsynaptic targets of the varicosities. The presence of dense‐core vesicles in the varicosities suggested co‐occurrence of glycine and a biogenic amine or neuropeptide. The indirect immunofluorescence technique was used to determine whether any such substances were present in the OPL of the toad retina. However, no specific labeling was found in the OPL for any of 19 substances tested. The extensive synaptic output provided by glycine‐accumulating varicosities in the toad OPL may indicate an important role of glycine in the synaptic function of the distal toad retina. We suggest that these varicosities derive from a presumably glycinergic interplexiform cell.