GABA TRANSPORTERS CONTROL GABAergic NEUROTRANSMISSION IN THE MOUSE SUBPLATE

The subplate is a transient layer between the cortical plate and intermediate zone in the developing cortex. Thalamo-cortical axons form temporary synapses on subplate neurons (SPns) before invading the cortical plate. Neuronal activity within the subplate is of critical importance for the development of neocortical circuits and architecture. Although both glutamatergic and GABAergic inputs on SPns were reported, short-term plasticity of GABAergic transmission has not been investigated yet. GABAergic postsynaptic currents (GPSCs) were recorded from SPns in coronal neocortical slices prepared from postnatal day 3-4 mice using whole-cell patch-clamp technique. Evoked GPSCs (eGPSCs) elicited by electrical paired-pulse stimulation demonstrated paired-pulse depression at all interstimulus intervals tested. Baclofen, a specific GABA(B) receptor (GABA(B)R) agonist, reduced eGPSC amplitudes and increased paired-pulse ratio (PPR), suggesting presynaptic location of functional GABA(B)Rs. Baclofen-induced effects were alleviated by (2S)-3-[[(1S)-1-(3,4-dichlorophenyl) ethyl] amino-2-hydroxypropyl](phenylmethyl) phosphinic acid (CGP55845), a selective GABA(B)R blocker. Moreover, CGP55845 increased eGPSC amplitudes and decreased PPR even under control conditions, indicating that GABA(B)Rs are tonically activated by ambient GABA. Because extracellular GABA concentration is mainly regulated by GABA transporters (GATs), we asked whether GATs release GABA. 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene) amino]oxy]ethyl]-3-pyridinecarboxylic acid (NNC-711) (10 mu M), a selective GAT-1 blocker, increased eGPSC decay time, decreased eGPSC amplitudes and PPR. The two last effects but not the first one were blocked by CGP55845, indicating that GAT-1 blockade causes an elevation of extracellular GABA concentration and in turn activation of extrasynaptic GABA(A)Rs and presynaptic GABA(B)Rs. 1-[2-[tris(4-methoxyphenyl)methoxy]ethyl]-(S)-3-piperidinecarboxylic acid (SNAP-5114), a specific GAT-2/3 blocker, failed to affect eGPSC kinetics. However, in contrast to NNC-711 SNAP-5114 increased eGPSC amplitudes and decreased PPR. In the presence of SNAP-5114 CGP55845 did not influence GABAergic transmission, indicating that GABA(B)Rs are not activated any longer. We conclude that in the subplate GAT-2/3 operates in reverse mode. GABA released via GAT-2/3 activates presynaptic GABA(B)Rs on GABAergic synapses and tonically inhibits GABAergic inputs on SPns. (C) 2015 IBRO. Published by Elsevier Ltd. All rights reserved.