role of intracellular ions on the reverse GABA transport by the neuronal transporter GAT1 was studied using voltage-clamp and [ 3 H]GABA efflux determinations in Xenopus oocytes transfected with heterologous mRNA. Reverse transport was induced by intracellular GABA injections and measured in terms of the net outward current generated by the transporter. Changes in various intracellular ionic conditions affected the reverse current: higher concentrations of Na ϩ enhanced the ratio of outward over inward transport current, while a considerable decrease of the outward current and a parallel reduction of the transporter-mediated GABA efflux were observed after treatments causing a diminution of the intracellular Cl Ϫ concentration. Particularly interesting was the impairment of the reverse transport observed after depletion of internal Cl Ϫ generated by the activity of a coexpressed K ϩ -Cl Ϫ exporter KCC2. This finding suggests that reverse GABA transport may be physiologically regulated during early neuronal development, similarly to the functional alterations seen in GABA receptors caused by KCC2 activity. ␥-aminobutyric acid; KCC2; Xenopus oocytes NEUROTRANSMITTER TRANSPORTERS (NTTs), located on presynaptic terminals and surrounding glia cells in the central nervous system (CNS), are necessary for the reuptake of neurotransmitter molecules from the synaptic space. Their involvement in the termination of synaptic transmission, the prevention of neurotransmitter spread to neighboring synapses, and the maintenance of the neurotransmitter concentration below neurotoxic levels make them essential for an efficient signal transduction in the brain. Beside the uptake mode, NTTs can also work in reverse and they can release neurotransmitter in a nonvesicular, calcium-independent manner, as demonstrated, for example, for the ␥-aminobutyric acid (GABA) transporter GAT1 (7, 22, 23). GAT1 is thought to play an important role in physiologically regulating the level of tonic inhibition in the CNS, thereby contributing to the control of brain excitability. Failure of this function may result in pathological conditions such as epileptic seizures, possibly caused by GABA spillover through GABA transporters (31, 32).Under normal conditions, GAT1, a member of the SLC6 family, couples GABA uptake to that of two Na ϩ ions and one Cl Ϫ ion. Recently, on the basis of the atomic structure of the bacterial homologue LeuT (43), the putative external Cl Ϫ -binding site of GAT1 was identified (4, 12, 47): mutations of an uncharged serine residue in position 331 to a negatively charged amino acid converted GABA transport into a Cl Ϫ -independent process. Furthermore, it has been shown that a higher external Cl Ϫ concentration facilitates the binding of external Na ϩ ions necessary for the GAT1 transport cycle (13, 20, 24). These observations suggest that a negative charge is required for GABA translocation and that, in GAT1, which lacks an intrinsic negative charge, this is provided by Cl Ϫ binding. Indeed, Cl Ϫ -independent transporters of t...

The role of internal substrates in the biophysical properties of the GABA transporter GAT1 has been investigated electrophysiologically in Xenopus oocytes heterologously expressing the cotransporter. Increments in Cl(-) and/or Na(+) concentrations caused by intracellular injections did not produce significant effects on the pre-steady-state currents, while a positive shift of the charge-voltage (Q-V) and decay time constant (τ)-voltage (τ-V) curves, together with a slowing of τ at positive potentials, was observed following treatments producing cytosolic Cl(-) depletion. Activation of the reverse transport mode by injections of GABA caused a reduction in the displaced charge. In the absence of external Cl(-), a stronger reduction in the displaced charge, together with a significant increase in reverse transport current, was observed. Therefore, complementarity between pre-steady-state and transport currents, observed in the forward mode, is preserved in the reverse mode. All these findings can be qualitatively reproduced by a kinetic scheme in which, in the forward mode, the Cl(-) ion is released first, after the inward charge movement, while the two Na(+) ions can be released only after binding of external GABA. In the reverse mode, internal GABA must bind first to the empty transporter, followed by internal Na(+) and Cl(-).

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