Chronic GABA treatment downregulates the GABA _A receptor α₂ and α₃ subunit mRNAS as well as polypeptide expression in primary cultured cerebral cortical neurons

Abstract

Chronic GABA exposure of mammalian primary cultured cortical neurons results in a downregulation of the GABA-benzodiazepine receptor complex. In the present study, the mRNA levels, as well as polypeptide expression, for the GABA_A receptor α₂ and α₃ subunits in cultured embryonic mouse cerebral cortical neurons (7 day old) were examined using northern analysis and immunoblotting techniques following chronic GABA treatment. The α₁ subunit mRNA or polypeptide could not be detected in these neurons. The steady state levels of mRNA for the GABA_A receptor α₂ and α₃ subunits showed a decrease in comparison with untreated neurons. There was no change in the level of the β actin or poly(A)⁺ RNA under the same experimental conditions. This agonist-induced reduction in the GABA_A receptor α₂ and α₃ subunit mRNA was blocked by the concomitant exposure of neurons to R 5135, an antagonist of GABA_A receptor. The polypeptide expression for the GABA_A receptor α₂ and α₃ subunits in chronically GABA-treated neurons also showed a decline and this change was also blocked by the concomitant exposure of cells to GABA and R 5135. These results indicate that the chronic exposure of the GABA_A receptor complex to agonist downregulates the expression of the α subunits of the receptor complex, which may be related to an observed decreases in the number of binding sites and GABA-induced³⁶Cl-influx in the cortical neurons.