The expression of three β subunit (β1, β2, and β3) mRNAs for γ-aminobutyric acidA receptor in the postnatal rat forebrain was examined by in situ hybridization h istochemistry with probes synthesized for the respective subunit mRNAs. The developmental expression of these subunit mRNAs conformed to one of three patterns. Pattern I was high expression of the mRNA at birth and a constant or increasing expression thereafter. In contrast, pattern II was no or very low expression of the mRNA at birth, with expression quickly increasing to reach the adult level in the early postnatal period. Pattern III was the transient expression of the subunit mRNA or else a marked decrease of its expression after a peak in the early postnatal period.
On the basis of this classification, the expression of β3 subunit mRNA followed pattern I in most regions of the forebrain, such as the isocortex, the olfactory bulb and some of its related areas, the hippocampal formation, the amygdala, the septum, the bed nucleus of the stria terminalis, the caudate-putamen, the nucleus accumbens, the globus pallidus, the ventral pallidum, and the hypothalamus. In some areas, such as the magnocellular preoptic nucleus, the thalamus, and the subthalamic nucleus, pattern III was seen for this subunit. However, none of the regions of the brain showed pattern II expression of β3 subunit mRNA. In contrast, the expression of β1 and β2 subunit mRNAs followed pattern II in most regions of the forebrain. These included the expression of β1 subunit mRNA in the isocortex, the olfactory bulb, the hippocampal formation, the amygdala, the septum, the bed nucleus of the stria terminalis, the thalamus, and the hypothalamus, and the expression of β2 subunit mRNA in the isocortex, the olfactory bulb and some of its related areas, the amygdala, the nucleus of the diagonal band, the caudate-putamen, the thalamus, and the hyothalamus. Pattern I was not found for β1 subunit mRNA, although it was seen in some areas for β2 subunit mRNA, such as the ventral pallidum, the globus pallidus, and the magnocellular preoptic nucleus. On the other hand, pattern III was followed by β1 subunit mRNA in the anterior olfactory nucleus, the olfactory tubercle, and the piriform cortex, and the same pattern for the β2 subunit was also found in the olfactory tubercle, the hippocampal formation, the septum, the bed nucleus of the stria terminalis, and the nucleus accumbens.
These results indicate that all three β subunit mRNAs are under developmental regulation, and that the different mRNAs are differentially regulated in various regions of the forebrain. In addition, the present study also showed that β3 subunit mRNA is highly expressed in both the neonatal and adult stages, indicating an important role for this subunit in brain development.