Evidence that GABA(A) receptor subunit mRNA expression during development is regulated by GABA(A) receptor stimulation

The expression of six mRNA species (alpha 2, alpha 3, alpha 5, beta 2, beta 3, and gamma 2) encoding for GABA(A) receptor subunits was followed in cultured early postnatal cortical neurons by in situ hybridization histochemistry, In untreated control cultures it was found that these subunit mRNA expression profiles closely follow those seen during development in vivo. alpha 3, alpha 5, and beta 3 subunit expression declined, alpha 2 expression increased, whereas beta 2 and gamma 2 subunit mRNA expression remained relatively constant. To test the hypothesis that GABA(A) receptor stimulation regulates these expression profiles, we tested the effect of a GABA(A) receptor positive modulator, allopregnanolone, and a GABA(A) receptor noncompetitive antagonist, tert-butylbicyclophosphorothionate (TBPS). It was found that allopregnanolone augmented the rate at which the alpha 3, alpha 5, or beta 3 subunit mRNA expression declined and prevented the increase in alpha 2 subunit mRNA expression, As well, allopregnanolone down-regulated beta 2 subunit mRNA expression. TBPS, on the other hand, up-regulated alpha 3, alpha 5, beta 2, and beta 3 subunit mRNA expression, It also down-regulated the expression of alpha 2 subunit mRNA. Both allopregnanolone and TBPS had no effect on gamma 2 subunit mRNA expression. These results imply that the developmental switchover of GABA receptor subunit mRNA expression is regulated by GABA(A) receptor activity.