Voltage clamp study of electrophysiologically-identified horizontal cells in carp retina.

Passive membrane properties and electromotive force of light modulated currents of L-, R/G-type and rod-driven horizontal cells were studied by voltage-clamp using double-barrelled micro-electrodes whilst perfusing with 5 microM dopamine to uncouple the gap junctions. Input impedances of horizontal cells in darkness were 31 +/- 1.4 M omega (mean +/- SE, n = 63); the resting potentials were -37 +/- 1.3 mV. Current-voltage relationships had regions of both inward and outward rectification and a region of negative resistance was commonly observed. Reversal potentials of light modulated currents were estimated on average to be -7 +/- 4 mV (n = 14), which is consistent with the involvement of K+ and Na+ and/or Ca2+ gradients. Importantly in R/G cells both depolarizing and hyperpolarizing components of the response had essentially the same reversal potential.