Cl- homeodynamics in gap junction-coupled astrocytic networks on activation of GABAergic synapses

The electrophysiological properties and functional role of GABAergic signal transmission from neurons to the gap junction-coupled astrocytic network are still unclear. GABA-induced astrocytic Cl- flux has been hypothesized to affect the driving force for GABAergic transmission by modulating [Cl- ](o). Thus, revealing the properties of GABA-mediated astrocytic responses will deepen our understanding of GABAergic signal transmission. Here, we analysed the Cl- dynamics of neurons and astrocytes in CA1 hippocampal GABAergic tripartite synapses, using Cl- imaging during GABA application, and whole cell recordings from interneuron-astrocyte pairs in the stratum lacunosum-moleculare. Astrocytic [Cl-](i) was adjusted to physiological conditions (40 mM). Although GABA application evoked bidirectional Cl- flux via GABA(A) receptors and mouse GABA transporter 4 (mGAT4) in CA1 astrocytes, a train of interneuron firing induced only GABA(A) receptor-mediated inward currents in an adjacent astrocyte. A GAT1 inhibitor increased the interneuron firing-induced currents and induced bicuculline-insensitive, mGAT4 inhibitor-sensitive currents, suggesting that synaptic spillover of GABA predominantly induced the astrocytic Cl- efflux because GABA(A) receptors are localized near the synaptic clefts. This GABA-induced Cl- efflux was accompanied by Cl- siphoning via the gap junctions of the astrocytic network because gap junction inhibitors significantly reduced the interneuron firing-induced currents. Thus, Cl- efflux from astrocytes is homeostatically maintained within astrocytic networks. A gap junction inhibitor enhanced the activity-dependent depolarizing shifts of reversal potential of neuronal IPSCs evoked by repetitive stimulation to GABAergic synapses. These results suggest that Cl- conductance within the astrocytic network may contribute to maintaining GABAergic synaptic transmission by regulating [Cl-](o).