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

The types of photoreceptors converging onto L-type external horizontal cells (LEHCs) were identified, and their interactions studied, by analyzing responses recorded intracellularly in live, immobilized goldfish under dark-adapted conditions. Our observations indicate that LEHCs receive inputs from red-sensitive and green-sensitive cones, in that: (1) the response waveform differed markedly when elicited by shorter (less than 550 n m)-, as opposed to longer (greater than 600 nm)-, wavelength stimuli; (2) 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 greater than 100 msec after the onset of the flash; (3) the spectral sensitivity of LEHC responses, measured at times greater than 100 msec after stimulus onset, was intermediate between those of red-sensitive cones and green-sensitive cones; and (4) 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 msec after brief, green (502 nm) flashes, the amplitudes of LEHC responses were significantly larger than predicted from linear summation of red and green inputs. Our observations thus indicate not only that red-sensitive and green-sensitive cones converge on the LEHC, but in addition, that their inputs interact to enhance LEHC responses. The enhancement might occur, at least in part, presynaptically.