GABA_A/benzodiazepine receptor γ₂ subunit gene expression in developing normal and mutant mouse cerebellum

Recent studies have identified several subunits (α, β, γ and δ) of the γ-aminobutyric acid_A/benzodiazepine receptor; each consists of several variants. The γ subunit appears to mediate the interaction of the α and β subunits making the receptor capable of modulation by benzodiazepines. In the present studies, the expression of mRNA encoding the γ₂ subunit was examined in the cerebellum during development and in adult Purkinje cell degeneration, lurcher and reeler mutant mice. In the normal adult cerebellum, in situ hybridization with [³⁵S]cRNA probes revealed a strong signal over the Purkinje cell layer and deep cerebellar nuclei, and a weaker signal over basket, stellate and granule cells. Labeling over Purkinje cells was detectable at birth, gradually becoming stronger and more punctate during postnatal weeks 1 and 2, as Purkinje cells formed a monolayer between the molecular and granule cell layers. Adult levels of grain density were reached by P20. The external germinal layer, which contained proliferating granule cells, was unlabeled throughout development; however, weak labeling was detected over the internal granular layer at the end of postnatal week 1, as granule cells began their migration across the molecular layer. During the second postnatal week, punctate labeling became visible over the molecular layer in a distribution indicative of basket and stellate cells. In adult Purkinje cell degeneration and lurcher mutants, in which Purkinje cells have degenerated, no punctate labeling characteristic of mature Purkinje cells was detected. In adult and developing reeler mutants, where all classes of cells are malpositioned throughout the cerebellum, the punctate hybridization signal was present and clearly associated with Purkinje cells in all cortical regions. Our results suggest that developing Purkinje cells express the γ₂ gene at a time prior to receiving GABAergic inhibitory input, and that the continued expression in the adult is not affected by the absence of afferents.