Evidence for the identification of synaptic transmitters released by photoreceptors of the toad retina.

1. When toad retinae were incubated with veratrine, kainic acid, and L‐alpha‐aminoadipic acid, photoreceptor cells survived and most other neurones died. This preparation of ‘isolated’ photoreceptor cells accumulated radioactive molecules from the incubation medium and metabolized these into labelled compounds. When a preparation was placed on a filter and superfused, radioactive molecules which were released into the superfusion fluid could be collected and later analysed. Several procedures were used for inducing the release of possible transmitter compounds. Each released compound was chemically identified. 2. Three compounds, aspartic acid, glutamic acid, and N‐acetyl histidine, were released when the potassium concentration was increased in media that lacked calcium and contained cobalt. 3. The release of these compounds was further increased when cobalt was removed and calcium returned to the extracellular medium. 4. Two additional compounds, putrescine and cadavarine, were also released during depolarization when calcium was present. 5. The efflux of each of the compounds listed in Section 2 was also increased by homo‐ and hetero‐exchange. For at least aspartate, exchange was sodium‐dependent. 6. The post‐synaptic effect of released compounds was tested by their ability to increase the efflux of [3H]GABA from ‘isolated’ horizontal cells. 0 . 1 mM‐L‐aspartate, or L‐glutamate produced an increase in GABA efflux. N‐acetyl histidine, putrescine, and cadavarine were ineffective. 7. Isolated photoreceptors and intact retinae were incubated with [3H]aspartate, or [3H]putrescine. Subsequent histology and autoradiography demonstrated that both compounds were selectively accumulated by cones.