GAT-1 and reversible GABA transport in Bergmann glia in slices

Although glial GABA uptake and release have been studied in vitro, GABA transporters (GATs) have not been characterized in glia in slices. Whole cell patch-clamp recordings were obtained from Bergmann glia in rat cerebellar slices to characterize carrier-mediated GABA influx and efflux. GABA induced inward currents at -70 mV that could be pharmacologically separated into GABA(A) receptor and GAT currents. In the presence of GABA(A/B/C) receptor blockers, mean GABA-induced currents measured -48 pA at -70 mV, were inwardly rectifying between -70 and +50 mV, were inhibited by external Na(+) removal, and were diminished by reduction of external Cl(-). Nontransportable blockers of GAT-1 (SKF89976-A and NNC-711) and a transportable blocker of all the GAT subtypes (nipecotic acid) reversibly reduced GABA-induced transport currents by 68 and 100%, respectively. A blocker of BGT-1 (betaine) had no effect. SKF89976-A and NNC-711 also suppressed baseline inward currents that likely result from tonic GAT activation by background GABA. The substrate agonists, nipecotic acid and beta-alanine but not betaine, induced voltage- and Na(+)-dependent currents. With Na(+) and GABA inside the patch pipette or intracellular GABA perfusion during the recording, SKF89976-A blocked baseline outward currents that activated at -60 mV and increased with more depolarized potentials. This carrier-mediated GABA efflux induced a local accumulation of extracellular GABA detected by GABA(A) receptor activation on the recorded cell. Overall, these results indicate that Bergmann glia express GAT-1 that are activated by ambient GABA. In addition, GAT-1 in glia can work in reverse and release sufficient GABA to activate nearby GABA receptors.