CHROMATICITY OF SYNAPTIC INPUTS TO H1 HORIZONTAL CELLS IN CARP RETINA - ANALYSIS BY VOLTAGE-CLAMP AND SPECTRAL ADAPTATION

Cone photoreceptor inputs to H1 horizontal cells (H1 HCs) in carp retina were studied by measuring light-modulated currents (I(L)) to monochromatic stimuli (460, 533, 688 nm) under a voltage-clamp condition. By using double-barrelled micro-electrodes H1 HCs were voltage-clamped whilst perfusing with dopamine to uncouple the cells. The I(L) of the H1 HCs driven by each cone input was segregated by selective chromatic adaptation, and differences in the kinetics of the I(L) of the H1 HCs were revealed. Thus, all together, three types of I(L) were observed: (1) a 'fast outward' current to the long-wavelength stimulus; (2) a 'slow outward' current to the middle-wavelength stimulus; and (3) a 'delayed inward' current that followed the peak of 'slow outward' current to the short-wavelength stimulus. The reversal potentials of the three currents were estimated to be at least 20 mV more positive than the dark resting potential by extrapolation of the I(L)-V curve. These observations are consistent with the idea that the H1 HCs receive sign-inverting, conductance decreasing synaptic input(s) from at least one other cone mechanism, in addition to the main conventional EPSP type synaptic input from red-sensitive cones.