Figure 4.8. The current responses resulting from activation of the GABA transporter on horizontal cells, and GABAa receptors on Müller cells. A. GABA-induced inward currents of skate horizontal cells are mediated by an electrogenic carrier mechanism, and as shown in B, the resultant I-V curve shows no sign of a reversal potential. C. The GABA-induced currents of skate Müller cells result from activation of GABAA receptors. The voltagedependence of the response to GABA exhibits a reversal potential (at ~0 mV) as the holding potential is stepped from -100 mV to +80 mV. The increase in the current responses to brief 10 mV depolarizations superimposed on each trace reflects the GABA-induced increase in membrane conductance. D. Dependence of the reversal potential on [Cl-] follows the Nernst equation (Malchow etal., 1989; Malchow and Ripps, 1990). (Copyright 1989,1990 National Academy of Sciences, U.S.A., reprinted with permission.)

Fig. 4.9A, low concentrations of GABA induce current responses that are significantly enhanced when sodium ions are eliminated from the bathing solution. Thus, inhibiting GABA transport by removing extracellular Na+ allows GABA to act solely on the GABAa receptor, resulting in currents of larger amplitude. But as the GABA concentration is increased, the current contributed by the GABA transporter is proportionately reduced until, with GABA concentrations >2 ^M, removing Na+ no longer enhances the

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