Micro-electrode measurements and functional aspects of chloride activity in cyprinid fish retina: extracellular activity and intracellular activities of L- and C-type horizontal cells.

Extracellular Cl- activity and intracellular Cl- activities of luminosity and biphasic-chromaticity type horizontal cells were measured in freshly isolated, non-superfused roach retinae using double-barrelled Cl- -sensitive micro-electrodes. The extracellular Cl- activity in dark-adapted retinae was found to have a surprisingly wide range (54-143 mM), although in a given preparation it was extremely constant. The mean intracellular Cl- activities of both types of horizontal cell were identical (47 mM), and this value was significantly greater than that required for "passive distribution" i.e. Cl- equilibrium potentials were 11-12 mV more positive than respective membrane resting potentials in the dark. In the presence of 10 microM dopamine, however, the difference between the Cl- equilibrium potential and the membrane resting potential was abolished, consistent with the hypothesis that dopamine increases Cl- conductance, presumably at the interplexiform cell synapse onto horizontal cells. In turn, it is suggested that a functional consequence of this pathway is to modulate the input impedances of the horizontal cells, and hence their sensitivity to light.