Comparative molecular neuroanatomy of cloned GABAAreceptor subunits in the rat CNS

Abstract

γ‐Aminobutyric acid_A (GABA_A) receptors in the mammalian central nervous system (CNS) are members of a family of ligand‐gated ion channels consisting of heterooligomeric glycoprotein complexes in synaptic and extrasynaptic membranes. Although molecular cloning studies have identified 5 subunits (with ∼40% amino acid homology) and isoforms thereof (∼70% homology), namely α1–6, β1–4, γ1–3, δ, and ρ, the subunit composition and stoichiometry of native receptors are not known. The regional distribution and cellular expression of GABA_A receptor messenger RNAs (mRNAs) in the rat CNS have now been investigated by in situ hybridization histochemistry with subunit‐specific ^35S‐labelled oligonucleotide probes on adjacent cryostat sections. Whereas α1, β2, and γ2 transcripts were the most abundant and ubiquitous in the rat brain—correlating with the radioautographic distribution of GABA_A receptors revealed by an ionophore ligand—others had a more restricted expression while often being abundant. For example, α2 transcripts were found only in the olfactory bulb, cerebral cortex, caudate putamen, hippocampal formation, and certain lower brain stem nuclei; α3 only in the olfactory bulb and cerebral cortex; α5 in the hippocampal formation; and α6 only in cerebellar granule cells. In addition, β1, β3, γ1, and δ mRNAs were also uniquely expressed in restricted brain regions. Moreover, in the spinal cord, α1–3, β2,3, and γ2 mRNAs were differently expressed in Rexed layers 2–9, with α2, β3, and γ2 transcripts most prominent in motoneurons of layer 9. Although differential protein trafficking could lead to the incorporation of some subunits into somatic membranes and others into dendritic membranes, some tentative conclusions as to the probable composition of native proteins in various regions of the CNS may be drawn. For example, according to the strength of the hybridization signal the following subunits might be expected to play a prominent role in GABAergic neurotransmission in the corresponding regions: α1, β2 (1,3), γ2 in olfactory bulb mitral cells; α2, β3, γ2, δ in caudate putamen; α1,3 (2,5), β2,3, γ2 in cerebral cortex; α1, β2, γ1,2 in pallidum; α1,2,5, β1–3, γ2 in hippocampus; α1,5, β3, γ2, δ in dentate gyrus; α1, β2, γ1,2 in substantia nigra zona reticularis; α1, β2, γ2 in cerebellar Purkinje cells; α1,6, β2,3, γ2, δ in granule cells; α1,2, β2,3, γ2 in vestibular and facial nuclei; and finally α2, β3, γ2 in motoneurons of Rexed layer 9 in the spinal cord.