DIFFERENTIAL EXPRESSION OF GABAA/BENZODIAZEPINE RECEPTOR-BETA-1, RECEPTOR-BETA-2, AND RECEPTOR-BETA-3 SUBUNIT MESSENGER-RNAS IN THE DEVELOPING MOUSE CEREBELLUM

Gamma aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian cerebellum. Cerebellar granule, Purkinje, and deep nuclear neurons are known to receive GABAergic afferents. Since GABA exerts its inhibitory effects via GABA receptors, it is of interest to determine the temporal relationship between the formation of GABAergic synapses and the expression of genes coding for the GABA receptor. In a previous study, we have examined the developmental expression of binding sites for [H-3]muscimol, which binds with high affinity to the beta subunits of the GABA(A)/benzodiazepine (GABA(A)/BZ) receptor. In the present study, [S-35]cRNA probes were used to examine the appearance and distribution of GABA(A)/BZ beta1, beta2, and beta3 subunit mRNAs in the developing C57BL/6 mouse cerebellum by in situ hybridization. In the adult cerebellum, the distribution of the three subunit mRNAs was clearly different, despite considerable overlap, and their temporal expression differed throughout postnatal development. The beta1 hybridization signal appeared within the cerebellar cortex during the second postnatal week as a discrete band at the interface of the molecular and granule cell layers. Grains were distributed diffusely over small densely staining cells surrounding the Purkinje cells; relatively few grains were visible over Purkinje cell bodies themselves. This distribution may reflect an association with Bergmann glia or basket cells. The beta2 and beta3 hybridization signals were present considerably earlier than that of the beta1 mRNA. The beta2 Signal was present at birth in the molecular/Purkinje cell layer; as development progressed, the signal became increasingly intense over both granule and Purkinje cells. At birth, the beta3 subunit mRNA was present in the external germinal and molecular layers, later becoming largely localized within the granule cell layer. Dense beta2 and beta3 cRNA probe labeling was present over the adult granule cell layer. Moderate levels Of beta2 Signal were seen over Purkinje cell bodies; considerably less labeling was observed with the beta3 probe. The adult distribution Of beta2 and beta3 cRNA probes showed good spatial correspondence with the known GABA(A) receptor beta subunit markers, [3H]-muscimol and the mAb 62-3G1 antibody, each being present within the granule cell layer. Our results indicate that the temporal expression of GABA(A)/BZ receptor beta subunit messages within a given cell type may be independently regulated, and that acquisition of the beta2 and beta3 mRNAs occurs before these cells become integrated into mature synaptic circuits.