GAIN-CONTROL AND HYPERPOLARIZATION LEVEL IN CAT HORIZONTAL CELLS AS AFUNCTION OF LIGHT AND DARK-ADAPTATION

First a model is presented that accurately summarizes the dynamic prop erties of cat horizontal (H-) cells under photopic conditions as measu red in our previous work The model predicts that asymmetries in respon se to dark as compared to light flashes are flash-duration dependent. This somewhat surprising prediction is tested and confirmed in intrace llular recordings from the optically intact in vivo eye of the cat (Ex periment 1). The model implies that the gain of H-cells should be rela ted rather directly to the sustained (baseline) membrane potential, We performed three additional experiments to test this idea, Experiment 2 concerns response vs intensity (R-I-) curves for various flash-diame ters and background-sizes with background luminance varying over a 4 l og unit range. Results support the assumption of a rather strict coupl ing between flash sensitivity (gain) and the sustained level of hyperp olarization. In Experiment 3 we investigate this relation for both dar k and light flashes given on each of four background light levels. The results suggest that there are fixed minimum and maximum hyperpolariz ation levels, and that the baseline hyperpolarization for a given illu mination thus also sets the available range for dark and light flash-r esponses. The question then arises whether, or how this changes during dark adaptation, when the rod contribution to H-cell responses gradua lly increases, The fourth experiment therefore studies the relationshi p between gain and hyperpolarization level during prolonged dark-adapt ation. The results show that the rod contribution increases the polari zation range of H-cells, but that the gain and polarization level neve rtheless remain directly coupled, H-cell models relying on a close cou pling between polarization level and gain thus remain attractive optio ns. Copyright (C) 1996 Elsevier Science Ltd.