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Quantitative changes in alpha 1 and alpha 5 gamma-aminobutyric acid type A receptor subunit mRNAs and proteins after a single treatment of cerebellar granule neurons with N-methyl-D-aspartate.

It was previously demonstrated that daily administration of N-methyl-D-aspartate (NMDA) to primary cultures of cerebellar granule neurons for 5 days in vitro mediates an increase in the relative content of mRNAs encoding selected subunits of the gamma-aminobutyric acid (GABA)A receptor. This analysis was extended using a competitive polymerase chain reaction assay with internal standards to quantitate changes that occur in the absolute amounts of selected GABAA receptor subunit mRNAs in cerebellar granule neurons in vitro after the single administration of nontoxic doses of either NMDA or glutamate. For these studies, we focused on the alpha 1, alpha 5, and alpha 6 receptor subunit mRNAs and examined their absolute contents in cultures maintained in low KCl (12.5 mM), maintained in low KCl and treated with NMDA (10 microM) for 24 hr, or maintained in high KCl (25 mM). The absolute amounts of each mRNA varied in these paradigms; whereas the alpha 1 and alpha 5 receptor subunit mRNAs increased in response to NMDA-selective glutamate receptor stimulation, the alpha 6 receptor subunit mRNA did not. The time course of the alpha 1 and alpha 5 mRNA increases, dose dependence, and effects of glutamate in the presence or absence of MK-801 were also analyzed. Treatment of cultures maintained in 12.5 mM KCl with 5 microM glutamate resulted in comparable changes in the alpha 1 and alpha 5 receptor subunit mRNA contents, and a somewhat smaller increase in the alpha 6 mRNA content was observed. Using corresponding GABAA receptor subunit-specific antibodies, it was shown that the observed mRNA changes are accompanied by increased expression of alpha 1- and alpha 5-like receptor subunit immunoreactivity. Collectively, these data demonstrate that signal transduction mechanisms triggered by NMDA-selective glutamate receptor stimulation differentially modulate the levels of selected GABAA receptor subunit mRNAs and the corresponding proteins they encode.