THE EFFECTS OF LOWERED EXTRACELLULAR-SODIUM ON GAMMA-AMINOBUTYRIC-ACID (GABA)-INDUCED CURRENTS OF MULLER (GLIAL) CELLS OF THE SKATE RETINA

1. The effects of external sodium on GABA-induced chloride currents we re examined with whole-cell voltage-clamp recordings obtained from enz ymatically dissociated solitary Muller cells in culture. Our goal was to determine whether a sodium-dependent GABA uptake mechanism influenc es the GABA(a)-mediated responses of skate Muller cells. 2. At low con centrations of GABA (0.01 to 0.5 muM), removal of sodium from the exte rnal solution resulted in a marked increase in the ligand-gated curren ts mediated by activation of GABA(a) receptors. The enhancement by low ered sodium was greatest at hyperpolarizing potentials and decreased p rogressively as the cell was depolarized. 3. The reversal potential fo r the GABA-induced response was not significantly altered by the remov al of sodium, suggesting that sodium ions did not directly contribute to the GABA(a)-mediated current. 4, Lowering external sodium had no ef fect on the currents induced by the GABA(a)-agonist muscimol, consiste nt with its much lower affinity for the GABA transport carrier. 5. App lication of the GABA uptake blocker nipecotic acid also abolished the effects of lowered sodium. 6. These findings suggest that the effects of lowered external sodium resulted from a decrease in the uptake of G ABA into the Muller cells, thus raising the effective concentration of GABA acting upon the GABA(a) receptors.