Effects of intracellular calcium on GABAA receptors in mouse cortical neurons

Using the patch-clamp technique, we studied the effect of intracellular Ca2+ on Cl- current gated by type A gamma-aminobutyric acid receptors (GABA(A)) in mouse cortical neurons, When the rapid Ca2+ chelator 1,3-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was in the pipette solution, the GABA-activated Cl- current amplitude decreased over time to 49 +/- 7% of control. In contrast, equimolar replacement of BAPTA with ethylenebis(oxonitrilo)tetraacetate (EGTA) caused a 60 +/- 10% increase in GABA current. An increased intracellular Ca2+ concentration caused a transient augmentation of the GABA current. This effect of Ca2+ was concentration dependent (10 nM to 34 mu M) Ca2+ increased the amplitude of the current by enhancing the maximal response to GABA rather than by changing the affinity of the receptor to GABA (EC50 = 5 +/- 0.4 mu M vs. 7 +/- 0.3 mu M). Both calmodulin (CaM) and a CaM kinase II inhibitor (200 mu M) blocked the potentiating effect of Ca2+ suggesting that it was mediated by activation of CaM kinase II. We found that regulation of GABA(A) receptors by intracellular Ca2+ in cortical neurons has important physiological implications since the potentiating effect of increasing the intracellular Ca2+ on responses to GABA was mimicked by activating excitatory receptors with 100 mu M N-methyl-D-aspartate (NMDA). These findings suggest that modulation of GABA(A) receptor activity by glutamate may be brought about via changes in intracellular Ca2+.