Human Neuronal γ-Aminobutyric AcidAReceptors: Coordinated Subunit mRNA Expression and Functional Correlates in Individual Dentate Granule Cells

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Abstract
γ-Aminobutyric acidA receptors (GABARs) are heteromeric proteins composed of multiple subunits. Numerous subunit subtypes are expressed in individual neurons, which assemble in specific preferred GABAR configurations. Little is known, however, about the coordination of subunit expression within individual neurons or the impact this may have on GABAR function. To investigate this, it is necessary to profile quantitatively the expression of multiple subunit mRNAs within individual cells. In this study, single-cell antisense RNA amplification was used to examine the expression of 14 different GABAR subunit mRNAs simultaneously in individual human dentate granule cells (DGCs) harvested during hippocampectomy for intractable epilepsy. α4, β2, and δ-mRNA levels were tightly correlated within individual DGCs, indicating that these subunits are expressed coordinately. Levels of α3- and β2-mRNAs, as well as ε- and β1-mRNAs, also were strongly correlated. No other subunit correlations were identified. Coordinated expression could not be explained by the chromosomal clustering of GABAR genes and was observed in control and epileptic rats as well as in humans, suggesting that it was not species-specific or secondary to epileptogenesis. Benzodiazepine augmentation of GABA-evoked currents also was examined to determine whether levels of subunit mRNA expression correlated with receptor pharmacology. This analysis delineated two distinct cell populations that differed in clonazepam modulation and patterns of α-subunit expression. Clonazepam augmentation correlated positively with the relative expression of α1- and γ2-mRNAs and negatively with α4- and δ-mRNAs. These data demonstrate that specific GABAR subunit mRNAs exhibit coordinated control of expression in individual DGCs, which has significant impact on inhibitory function.