Fig. 5. Substrate-induced currents. (A) Inward currents induced by 100 µmol l^-1 D-aspartate onto Xenopus oocytes expressing AeaEAAT for the duration indicated by the black bar. A holding potential of -60 mV was used. Similar inward currents were produced by application of L-Glu, L-Asp and L-Cys (data not shown). (B) Reversal of current upon exposing AeaEAAT-expressing oocytes to a sodium-free buffer containing a high potassium concentration (98 mmol l^-1), at a holding potential of -60 mV. This represents the transport of endogenous excitatory amino acids out of the oocyte, and was not observed in water-injected oocytes (data not shown). (C) AeaEAAT-mediated substrate-activated currents. These current–voltage relationships are the difference between substrate-activated currents and currents in the buffer alone, obtained by off-line subtraction of the steady state portion of voltage-jump curves. Reversal potentials for the four amino acids (L-Glu, L-and D-Asp and L-Cys) applied at 100 µmol l^-1 each are approximately +37 mV. (D) Sodium dependence of substrate-activated currents. Replacement of extracellular sodium with equimolar choline completely abolishes the D-aspartate (100 µmol l^-1)-activated current.