Impairment of GABAA receptor function by N-methyl-D-aspartate-mediated calcium influx in isolated CA1 pyramidal cells.
Mechanisms of regulation of GABAA receptor function by intracellular calcium ([Ca2+]i) were examined in cell somata and apical dendrites of pyramidal cells, acutely dissociated from the CA1 hippocampal subfield of adult guinea-pigs. GABAA receptor-mediated currents were measured by whole-cell clamp recordings. N-methyl-D-aspartate receptor-mediated currents were used as conditioning source of calcium influx. Peak amplitudes of somatic GABAA whole-cell currents were reduced to about 15% of control values when net inward charge accumulation by N-methyl-D-aspartate currents reached 1.85 nC. A similar decline of GABAA currents was observed in dendritic recordings. The N-methyl-D-aspartate-mediated reduction of somatic and dendritic GABAA currents was accompanied by a well correlated decrease in peak and chord conductances. Pharmacological blockade of N-methyl-D-aspartate currents by 2-amino-5-phosphonopentanoic acid prevented the N-methyl-D-aspartate-mediated suppression of GABAA responses. The N-methyl-D-aspartate effect was mediated by the calcium component of N-methyl-D-aspartate receptor-mediated currents as demonstrated by a lack of effect in the absence of extracellular calcium and faster N-methyl-D-aspartate-mediated suppression of GABAA responses in lower intracellular 1,2-bis(2-aminophenoxy)ethane-N,N,N',N"-tetra-acetate. N-methyl-D-aspartate-mediated suppression of GABAA currents was significantly less expressed when intracellular ATP was replaced by its analog adenosine 5'-O-(3-thiotriphosphate) and when the specific phosphatase 2B inhibitor cypermethrin was added intracellularly. The reduction of GABAA responses persisted after cessation of N-methyl-D-aspartate-mediated calcium influx, indicating a long-term action of N-methyl-D-aspartate on GABAA responses. Voltage-activated calcium currents did not affect GABAA responses under the experimental conditions applied. In conclusion, the data presented show that calcium influxes through N-methyl-D-aspartate receptor channels result in long-term suppression of GABAA receptor function in CA1 pyramidal cells. Intracellular mechanisms of N-methyl-D-aspartate-mediated reduction of GABAA conductances involve activation of phosphatase 2B and consecutive dephosphorylation of the GABAA receptor or a closely associated GABAA receptor-regulating enzyme. Possible mechanisms of such a distinct N-methyl-D-aspartate-dependent calcium signalling pathway in the dephosphorylation-dependent suppression or GABAA receptor function are discussed.
Stelzer
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A
Department of Pharmacology, State University of New York, Brooklyn 11203.
Shi
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H
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology; D002147:Calmodulin / Q000502:physiology; D018118:Chloride Channels / Q000187:drug effects / Q000502:physiology; D003712:Dendrites / Q000187:drug effects / Q000502:physiology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D058787:GABA-A Receptor Antagonists; D006168:Guinea Pigs; D006624:Hippocampus / Q000166:cytology; D007424:Intracellular Fluid; D016202:N-Methylaspartate / Q000494:pharmacology*; D009419:Nerve Tissue Proteins / Q000187:drug effects / Q000502:physiology*; D010749:Phosphoprotein Phosphatases / Q000502:physiology; D010766:Phosphorylation / Q000187:drug effects; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D017966:Pyramidal Cells / Q000187:drug effects* / Q000378:metabolism; D011963:Receptors, GABA-A / Q000502:physiology; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects*; D015398:Signal Transduction
D015220:Calcium Channels; D002147:Calmodulin; D018118:Chloride Channels; D058787:GABA-A Receptor Antagonists; D009419:Nerve Tissue Proteins; D011963:Receptors, GABA-A; D016194:Receptors, N-Methyl-D-Aspartate; C022571:adenosine 5'-O-(3-thiotriphosphate); D004533:Egtazic Acid; D016202:N-Methylaspartate; D000255:Adenosine Triphosphate; D010749:Phosphoprotein Phosphatases; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium