Regulation of gamma-aminobutyric acid (GABA) transporters by extracellularGABA

gamma-Aminobutyric acid (GABA) transporters on neurons and glia at or near the synapse function to remove GABA from the synaptic cleft. Recent evidenc e suggests that GABA transporter function can be regulated, although the in itial triggers for such regulation are not known. One hypothesis is that tr ansporter function is modulated by extracellular GABA concentration, thus p roviding a feedback mechanism for the control of neurotransmitter levels at the synapse. To test this hypothesis, GABA uptake assays were performed on primary dissociated rat hippocampal cultures that endogenously express GAB A transporters and on mammalian cells stably expressing the cloned rat brai n GABA transporter GAT1, In both experimental systems, extracellular GABA i nduces chronic changes in GABA transport that occur in a dose-dependent and time-dependent manner. In addition to GABA, ACHC and nipecotic acid, both substrates of GAT1, up-regulate transport; GAT1 transport inhibitors that a re not transporter substrates down-regulate transport, These changes occur in the presence of blockers of both GABA(A) and GABA(B) receptors, occur in the presence of protein synthesis inhibitors, and are not influenced by in tracellular GABA, Surface biotinylation experiments reveal that the increas e in transport is correlated with an increase in surface transporter expres sion. This increase in surface expression is due, at least in part, to a sl owing of GAT1 internalization in the presence of extracellular GABA, These data suggest that the GABA transporter fine-tunes its function in response to extracellular GABA and would act to maintain a constant level of neurotr ansmitter at the synaptic cleft.