GABA(A) BENZODIAZEPINE RECEPTOR-GAMMA(2) SUBUNIT GENE-EXPRESSION IN DEVELOPING NORMAL AND MUTANT MOUSE CEREBELLUM

Recent studies have identified several subunits (alpha, beta, gamma an d delta) of the gamma-aminobutyric acid(A) /benzodiazepine receptor; e ach consists of several variants. The gamma2 subunit appears to mediat e the interaction of the alpha and beta subunits making the receptor c apable of modulation by benzodiazepines. In the present studies, the e xpression of mRNA encoding the gamma2 subunit was examined in the cere bellum during development and in adult Purkinje cell degeneration, lur cher and reeler mutant mice. In the normal adult cerebellum, in situ h ybridization with [S-35]cRNA probes revealed a strong signal over the Purkinje cell layer and deep cerebellar nuclei, and a weaker signal ov er basket, stellate and granule cells. Labeling over Purkinje cells wa s detectable at birth, gradually becoming stronger and more punctate d uring postnatal weeks 1 and 2, as Purkinje cells formed a monolayer be tween the molecular and granule cell layers. Adult levels of grain den sity were reached by P20. The external germinal layer, which contained proliferating granule cells, was unlabeled throughout development; ho wever, weak labeling was detected over the internal granular layer at the end of postnatal week 1, as granule cells began their migration ac ross the molecular layer. During the second postnatal week, punctate l abeling became visible over the molecular layer in a distribution indi cative of basket and stellate cells. In adult Purkinje cell degenerati on and lurcher mutants, in which Purkinje cells have degenerated, no p unctate labeling characteristic of mature Purkinje cells was detected. In adult and developing reeler mutants, where all classes of cells ar e malpositioned throughout the cerebellum, the punctate hybridization signal was present and clearly associated with Purkinje cells in all c ortical regions. Our results suggest that developing Purkinje cells ex press the gamma2 gene at a time prior to receiving GABAergic inhibitor y input, and that the continued expression in the adult is not affecte d by the absence of afferents.