Quantitative analysis of photoreceptor inputs to external horizontal cells in the goldfish retina.

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Abstract
The types of photoreceptors converging onto L-type external horizontal cells (LEHC) were identified and their interactions studied by analyzing responses recorded intracellularly in live immobilized goldfish under dark-adapted conditions. Apparently, LEHC receive inputs from red- and green-sensitive cones. The response waveform differed markedly when elicited by shorter (< 550 nm), as opposed to longer (> 600 nm), wavelength stimuli. On plotting response amplitude vs. log stimulus intensity, the slope of the curves increased as the stimulus wavelengths were decreased when the amplitude was measured at times > 100 ms after the onset of the flash. The spectral sensitivity of LEHC responses, measured at times > 100 ms after stimulus onset, was intermediate between those of red- and green-sensitive cones. The amplitude of responses to flashes of mixed wavelengths differed from those predicted by the principle of univariance. When brief red (706 nm) flashes were presented simultaneously with or within 600 ms after brief green (502 nm) flashes, the amplitudes of LEHC responses were significantly larger than predicted from linear summation of red and green inputs. Apparently, red- and green-sensitive cones converge on the LEHC but, in addition, thier inputs interact to enhance LEHC responses. The enhancement might occur, at least in part, presynaptically.