Postsynaptic GABAB Receptors Enhance Extrasynaptic GABAA Receptor Function in Dentate Gyrus Granule Cells

Ambient GABA in the brain tonically activates extrasynaptic GABA(A) receptors, and activity-dependent changes in ambient GABA concentration can also activate GABA(B) receptors. To investigate an interaction between postsynaptic GABA(B) and GABA(A) receptors, we recorded GABA(A) currents elicited by exogenous GABA (10 mu M) from dentate gyrus granule cells (DGGCs) in adult rat hippocampal slices. The GABA(B) receptor agonist baclofen (20 mu M) enhanced GABA(A) currents. This enhancement was blocked by the GABA(B) receptor antagonist CGP 55845 and intracellular solutions containing the GTP analog GDP-beta-s, indicating that baclofen was acting on postsynaptic GABA(B) receptors. Modulation of GABA(A) currents by postsynaptic GABA(B) receptors was not observed in CA1 pyramidal cells or layer 2/3 cortical pyramidal neurons. Baclofen reduced the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) but did not alter sIPSC amplitude or kinetics. Thus, GABA(A) receptors activated at synapses were not modulated by postsynaptic GABA(B) receptors. In contrast, tonic GAB(A) currents and currents activated by the GABA(A) receptor delta subunit-selective agonist THIP (10 mu M) were potentiated by baclofen. Our data indicate that postsynaptic GABA(B) receptors enhance the function of extrasynaptic GABA(A) receptors, including delta subunit-containing receptors that mediate tonic inhibition in DGGCs. The modulation of GABA(A) receptor function by postsynaptic GABA(B) receptors is a newly identified mechanism that will influence the inhibitory tone of DGGCs when GABA(B) and GABA(A) receptors are both activated.