Neural mechanisms of chromatic adaptation in L-type cone horizontal cells of the carp retina

When a background light is delivered, the responsiveness of horizontal cells to light stimulus initially lowers but subsequently recovers gradually as shown by the increase in response amplitude to test light. These changes of responsiveness are observed when white or close wavelengths are used for both the background and test lights. However, the response amplitude to blue-green test light was initially enhanced but decreased and reached a steady state after the onset of red background illumination. The mechanism causing such a change of test responses was studied in the luminosity-type cone horizontal cells. The initial response enhancement was accompanied by an increase of the slope of hyperpolarizing phase, while the subsequent decrease of response amplitude was caused by the advancement of the recovery phase. The advancement of the recovery phase was eliminated by gamma-aminobutyric acid (GABA) or Co2+ (50 microM), which blocks GABA-induced currents in cone terminals. Dopamine, which inhibits GABA release from horizontal cells, stimulated the advancement of the recovery phase. The time course of the hyperpolarizing phase was not affected by these agents. The enhancement and the subsequent decrease of test responses were also observed in 6-hydroxydopamine-treated retinae. The results suggest that the GABAergic feedback pathway from horizontal cells to cones advances the recovery phase of response to test light. It appears, therefore, that the feedback modifies the responsiveness of horizontal cells to light stimulus under a red background illumination. The mechanism enhancing response amplitude will be also discussed.