Developmental up-regulation and agonist-dependent down-regulation of GABAA receptor subunit mRNAs in chick cortical neurons.

We have used quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to analyze the expression of GABAA receptor subunit genes in cultured neurons from the chick embryo cerebral cortex. During maturation of the neurons between day 2 and day 8 in culture, levels of the alpha 1 subunit transcript (per ng total RNA) increased 3.8 +/- 0.3 fold, while those for the beta 2S and beta 4S subunits increased 2.4 +/- 0.4 and 1.8 +/- 0.2 fold, respectively. The accumulation of the beta 4 S subunit mRNA was more rapid than those encoding either the alpha 1 or beta 2S polypeptides. After 4 days in culture the beta 4S subunit transcript level reached 105 +/- 7.7% of that found after 8 days, while the corresponding amounts for the alpha 1 and beta 2S subunit mRNAs were 50 +/- 7.1% and 44 +/- 10.7%, respectively. On the other hand, no significant differences were observed in the level of either the gamma 1 or the gamma 2S subunit mRNA during development in vitro. Likewise, the ratios of the large/small splice variants (beta 2 = 0.16 +/- 0.02; beta 4 = 0.57 +/- 0.02; gamma 2 = 0.30 +/- 0.06) did not show detectable changes during this period. To study the down-regulation of the mRNAs, a single dose of 100 microM GABA was added to the culture medium. After 7 days of exposure to GABA, the levels of transcripts for the alpha 1, beta 2, beta 4, gamma 1, and gamma 2 subunits and their splice variants (where present) were all reduced by 47-65% compared to untreated controls.(ABSTRACT TRUNCATED AT 250 WORDS)