Prenatal alteration and distribution of the GABA(B1) and GABA(B2) receptor subunit mRNAs during rat central nervous system development.

The prenatal developmental expression changes and distribution of the gamma-aminobutyric acid (GABA)(B1) and GABA(B2) receptor subunit were investigated using in situ hybridization and RNase protection assay (RPA). We defined a different expression pattern of GABA(B1) subunit mRNA to that of GABA(B2) subunit. GABA(B1) subunit mRNA signals were moderately expressed in the cerebral cortex neuroepithelium of discrete brain regions on gestational day (GD) 11.5 and 12.5 and were highly expressed in the brain and spinal cord on GD 13.5 and 15.5. However, GABA(B2) subunit mRNAs were not detected on GD 11.5 and 12.5 and were first weakly detected on GD 13.5. On GD 15.5, 17.5, and 19.5, these subunit mRNAs were found in the retina, hippocampus, cerebral cortex, spinal cord, and cerebellum area. On GD 19.5 and 21.5, these subunits mRNA signals increased in the cerebral cortex, hippocampus, thalamus, and cerebellum, but decreased in the spinal cord, spinal ganglion, and midbrain, reaching similar levels to that of the adult brain. On GD 21.5, these subunit mRNAs were similarly expressed in almost all brain areas with a higher expression level of GABA(B1) subunit mRNA than GABA(B2) subunit mRNA. Our results found that GABA(B1) and GABA(B2) subunit mRNAs showed different expression patterns, with the GABA(B1) subunit expressed earlier and higher. We suggest that GABA(B1) and GABA(B2) subunits might have a role in the fetal brain during the gestational period for pre- and post-synaptogenesis, proliferation, differentiation, and neuronal maturation, and GABA(B1) subunit may be more important than GABA(B2) subunit during rat prenatal development.