Effects of GABA on horizontal cells in the tiger salamander retina

Application of gamma-aminobutyric acid (GABA) slows down the horizontal cell response time course (HCRRT) and induces membrane depolarization in horizontal cells (HCs) after synaptic inputs are blocked by Co2+. We present evidence that suggests both effects are probably mediated by GABAA receptors which open chloride channels in the HC membrane. In any given concentration of GABA, ranged from 0 to 100 microM, the HC membrane potential (VHC) in saturating light and in the presence of 100 microM Co2+ are identical. This result suggests that GABA in both light and 100 microM Co2+ opens the same amount of chloride channels (same gCl) so that VHC determined by chloride and leak conductances has the same value. Higher concentrations of Co2+ (> 300 microM) not only blocks synaptic transmission from photoreceptors to HCs, but also acts as an antagonist that suppresses the GABA-mediated depolarization in HCs.