Ontogeny of GABAA receptor subunit mRNAs in rat spinal cord and dorsal root ganglia

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Abstract
Relatively little is known about the development of GABAA receptor subunits and their gene expression in mammalian spinal cord. The expressions of mRNAs encoding 13 GABAA receptor subunits (α1–6, β1–3, γ1–3, and δ) in embryonic, postnatal, and adult rat spinal cord and dorsal root ganglia (DRG) cells were studied by in situ hybridization and reverse transcription‐polymerase chain reaction (RT ‐ PCR) analysis. Both techniques revealed the presence of all subunit mRNAs originally found in the rat brain, except for α6, which was not detectable, and δ, which was weakly detected only by RT‐PCR. Two anatomically distinctive sets of subunit mRNAs were found by in situ hybridization within the ventricular zone (VZ) and mantle zone (MZ). The trio of α4, β1, and γ1 subunit mRNAs emerged exclusively in neuroepithelial cells at embryonic day 13 (E13) and remained detectable in the VZ until E17. In the MZ, β3 subunit mRNA was first detected at E12, while α2, α3, α5, β2, γ2, and γ3 transcripts appeared at E13. Expressions of the subunit mRNAs in the MZ rapidly increased and expanded in a ventrodorsal sequence from motoneurons to dorsal horn neurons before reaching a peak in the late embryonic/early postnatal period. The mRNA expressions declined during postnatal development, by region‐selective depletion, with α4, α5, β1, β2, γ1, and γ3 subunit mRNAs becoming barely detectable. In contrast, α2, α3, β3, and γ2 transcripts persisted into adulthood with distinct anatomical distributions. RT‐PCR analysis revealed unique developmental patterns in the intensities of PCR products, most of which were in good agreement with developmental changes in the densities of hybridized mRNA signals. However, RT‐PCR amplified minute amounts of mRNAs for α1, α4, α5, β1, β2, γ1, γ3, and δ subunits in adults, which were not found in film autoradiograms, but could be detected in a few grain‐positive cells in emulsion‐dipped sections. DRG cells expressed α2, α3, α5, β2, β3, and γ2 subunit mRNAs during embryogenesis but only α2, β3, and γ2 subunit mRNAs were reliably detected in the adult. The transient expression of α4β1γ1 subunit mRNAs primarily in the VZ correlates with the period of neurogenesis, while the developmental changes of α2, α3, β3, and γ2 subunit mRNAs in the MZ parallels previously reported changes in GABA expression and in synapse formation and elimination. These results implicate a role of early‐expressed GABA and GABAA receptors in the development of rat spinal cord.