IMPAIRMENT OF GABA(A) RECEPTOR FUNCTION BY N-METHYL-D-ASPARTATE-MEDIATED CALCIUM INFLUX IN ISOLATED CA1 PYRAMIDAL CELLS

Mechanisms of regulation of GABA(A) receptor function by intracellular calcium ([Ca2+](i)) were examined in cell somata and apical dendrites of pyramidal cells, acutely dissociated from the CA1 hippocampal subf ield of adult guinea-pigs. GABA(A) receptor-mediated currents were mea sured by whole-cell clamp recordings. N-methyl-D-aspartate receptor-me diated currents were used as conditioning source of calcium influx. Pe ak amplitudes of somatic GABA(A) whole-cell currents were reduced to a bout 15% of control values when net inward charge accumulation by N-me thyl-D-aspartate currents reached 1.85 nC. A similar decline of GABA(A ) currents was observed in dendritic recordings. The N-methyl-D-aspart ate-mediated reduction of somatic and dendritic GABA(A) currents was a ccompanied by a well correlated decrease in peak and chord conductance s. Pharmacological blockade of N-methyl-D-aspartate currents by 2-amin o-5-phosphonopentanoic acid prevented the N-methyl-D-aspartate-mediate d suppression of GABA(A) responses. The N-methyl-D-aspartate effect wa s mediated by the calcium component of N-methyl-D-aspartate receptor-m ediated currents as demonstrated by a lack of effect in the absence of extracellular calcium and faster N-methyl-D-aspartate-mediated suppre ssion of GABA(A) responses in lower intracellular 1,2-bis(2- aminophen oxy)ethane-N, N, N',N''-tetra-acetate. N-methyl-D-aspartate-mediated s uppression of GABA(A) currents was significantly less expressed when i ntracellular ATP was replaced by its analog adenosine 5'-O-(3-thiotrip hosphate) and when the specific phosphatase 2B inhibitor cypermethrin was added intracellularly. The reduction of GABA(A) responses persiste d after cessation of N-methyl-D-aspartate-mediated calcium influx, ind icating a long-term action of N-methyl-D-aspartate on GABA(A) response s. Voltage-activated calcium currents did not affect GABA(A) responses under the experimental conditions applied. In conclusion, the data pr esented show that calcium influxes through N-methyl-D-aspartate recept or channels result in long-term suppression of GABA(A) function in CA1 pyramidal cells. Intracellular mechanisms of N-methyl-D-aspartate-med iated reduction of GABA(A) receptor of GABA(A) conductances involve ac tivation of phosphatase 2B and consecutive dephosphorylation of the GA BA(A) receptor or a closely associated GABA(A) receptor-regulating enz yme. Possible mechanisms of such a distinct N-methyl-D-aspartate-depen dent calcium signaling pathway in the dephosphorylation-dependent supp ression or GABA(A) receptor function are discussed.