Developmental expression of GABAA receptor subunit and GAD genes in mouse somatosensory barrel cortex

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Abstract
In situ hybridization histochemistry with radioactive cRNA probes was used to study patterns of gene expression for α1, α2, α4, α5, β1, β2, and γ2 subunit mRNAs of type A gamma aminobutyric acid (GABAA) receptors and for 67‐kDa glutamic acid decarboxylase (GAD67) mRNA in mouse barrel cortex during the period (postnatal days 1‐12; P1‐P12) when thalamocortical innervation of layer IV barrels is occurring. The α1, β2, and γ2 subunit mRNAs increased substantially with age, especially in layers V and VI, and throughout the period studied, invariably had the same laminar‐specific patterns of expression. All three mRNAs were highly expressed in the dense cortical plate at P1. In layer IV after differentiation of barrels, they were expressed in cells of both barrel walls and hollows but especially in the walls. The α2, α4, α5, and β1 subunit mRNAs were expressed at lower levels and had different laminar patterns of distribution; α2 and α4 showed switches between layers over time; α5 was invariably associated with the subplate or its derivative, β1 with layer IV. Levels of α2 mRNA did not change over time; α4 and β1 mRNAs increased and α5 decreased. GAD67 mRNA was highest in layer I at P1 and progressively increased in other layers. These results suggest that postnatal development of GABAA receptors is mainly directed at the production of receptors assembled from α1, β2, and γ2 subunits, with β1 contributing in layer IV. Other subunits may be associated with receptors involved in trophic actions of GABA during development and may give GABAA receptor‐mediated responses in the developing cortex their particular physiological profile. J. Comp. Neurol. 383:199‐219, 1997.