Role of GABA transporter 3 in GABAergic synaptic transmission at striatal output neurons

Striatal GABAergic signaling has been shown to be essential for basal ganglia output and proper motor performance. In the mouse neostriatum GABA transporter 1 (GAT-1) was previously found to assist in the clearance of GABA from the extracellular space and influence both phasic and tonic GABAergic inhibition of medium-sized striatal output neurons (SONs). It currently remains unknown whether GAT subtypes other than GAT-1 participate in the modulation of GABAergic transmission in this brain structure. In this study, we aimed at assessing the role of GAT-3 in the mouse neostriatum. To this end, we recorded GABAergic inhibitory postsynaptic currents (IPSCs) from SONs in brain slices at different developmental stages (postnatal days (P) 7-9, 12-14, and 28-34) using the whole-cell patch-clamp technique. When applied under control conditions, SNAP-5114 (40 microM), a specific GAT-3 blocker, did not affect miniature or evoked IPSCs (m/eIPSCs) and produced no significant effect on tonic GABAA receptor-mediated conductances in SONs. However, in the presence of NO-711 (10 microM), a specific GAT-1 blocker, SNAP-5114 reduced mIPSC frequencies without affecting mIPSC amplitudes or kinetics. In addition, SNAP-5114 reduced the mean amplitude of eIPSCs and increased the paired-pulse ratio. These effects were entirely abolished by CGP55845 (1 microM), a specific GABAB receptor blocker, indicating that they were mediated by presynaptic GABAB receptors. Similar results were obtained from all age groups. We conclude that GAT-3 is functionally expressed in the mouse neostriatum. Whereas an acute block of GAT-3 under resting conditions is fully compensated for by GAT-1, GAT-3 might provide an additional uptake capacity when neuronal activity and GABA release are increased.