Glutamate transport across the plasma membrane of neurons and glia is powered by the transmembrane electrochemical gradients for sodium, potassium, and pH, but there is controversy over the number of Na ϩ cotransported with glutamate. The stoichiometry of glutamate transporters is important because it determines a lower limit to the extracellular glutamate concentration, [glu] o , in both normal and pathological conditions. We used whole-cell clamping to study the stoichiometry of the glial transporter GLT-1, the most abundant glutamate transporter in the brain, expressed under control of the Tet-On system in a Chinese hamster ovary (CHO) cell line selected for low endogenous glutamate transport. After the induction of GLT-1 expression with doxycycline, glutamate evoked a Na

The role of cystine‐glutamate exchange in controlling the extracellular glutamate concentration in the central nervous system was examined by whole‐cell clamping neurons in rat brain slices, and using their glutamate receptors as sensors of extracellular glutamate concentration. Applying cystine to cerebellar slices generated a membrane current in Purkinje cells which was abolished by glutamate receptor blockers. Similar cystine‐evoked currents were seen in pyramidal cells of frontal cortex slices. Control experiments on non‐N‐methyl‐D‐aspartate (non‐NMDA) receptors in enzymatically isolated Purkinje cells showed that cystine did not produce a current in slice Purkinje cells by directly activating glutamate receptors, nor by potentiating the action of background levels of glutamate on receptors. Experiments on isolated salamander Müller cells showed that cystine did not block Na+‐dependent GLAST glutamate transporters (homologous to the transporters in the Bergmann glia ensheathing the Purkinje cells), nor did it block the current produced by EAAT4 and EAAC1 glutamate transporters in Purkinje cells. Thus the cystine‐evoked current in Purkinje cells is not due to a rise in extracellular glutamate concentration caused by block of Na+‐dependent uptake. The dependence of cystine‐evoked current on cystine concentration in slice Purkinje cells could be fitted by a Michaelis‐Menten relation with a Km of 250 μM. The Km predicted from this for cystine activating glutamate efflux is less than 140 μM, because of the non‐linear dependence on glutamate concentration of the Purkinje cell current. The current evoked by 1 mM cystine was little affected by removal of extracellular chloride or addition of 1 mM furosemide (frusemide), but was potentiated by 1 mM 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS). These data suggest that external cystine generates a current in slice Purkinje cells by activating cystine‐glutamate exchange in cells of the slice, releasing glutamate which activates non‐NMDA receptors in the Purkinje cell membrane.

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