Evidence in support of a photoreceptoral origin for the "light-peak substance".

The light peak of the DC ERG was recorded in in vitro preparations of chick neural retina-RPE-choroid. Either the pharmacological agents Co2+, Mg2+, and low Ca2+, known to block calcium-dependent synaptic transmission or 2-amino-4-phosphonobutyric acid (APB), which blocks transmission in the ON pathway, were added to the solution perfusing the retina. These agents suppressed the b-wave of the ERG, thus indicating a severe suppression of synaptic transmission in the neural retina. The retina was also perfused with the OFF pathway inhibitor cis-2,3-piperidine dicarboxylic acid (PDA). None of these agents, however, significantly depressed the light peak, supporting the hypothesis that the light peak is generated by a photoreceptor-RPE interaction.