Ontogeny of GABA(B) receptor subunit expression and function in the rat spinal cord

Little is known about the chronology of expression, cellular localization and function of GABA(B) subunits in the developing rat spinal cord. In the present study, in situ hybridization, immunohistochemistry and quantitative RT-PCR analysis were used to examine this issue. At embryonic day 18, in situ hybridization reveals that all three transcripts, GABA(B(1a)), GABA(B(1b)), and GABA(B(2)), are present throughout the gray matter. At postnatal day (PN) 2, while overall expression appears to decrease, it becomes more highly concentrated in motoneurons of the ventral horn. By PN 7, distinct subpopulations of cells expressing the transcripts become heavily expressed in motoneurons. Immunohistochemical analysis revealed that, unlike mRNA, GABA(B(1)) protein is more highly concentrated in the dorsal horn as compared to the motoneurons. Analysis using RT-PCR demonstrates that in spinal cord GABA(B(1a)) mRNA expression remains constant throughout development, GABA(B(1b)) increases from PN 2 to adult, and GABA(B(2)) decreases from PN 2 to adult. The distribution of functional GABA(B) receptors, as measured by baclofen-stimulated [35S]GTPgammaS binding, in the spinal cord during development generally follows the distribution of subunit expression, being widely distributed throughout the gray matter in embryonic spinal cord slices and becoming more concentrated in the dorsal horn during postnatal development, similar to the distribution of subunit proteins as measured by immunohistochemistry. These findings suggest that spinal cord GABA(B(1a)), GABA(B(1b)), and GABA(B(2)) transcripts are differentially regulated during development with the chronology of this expression suggesting that GABA(B) receptor subunits, in addition to forming functional GABA(B) receptors, may have a trophic function or participate in synaptogenesis.