COMPARATIVE MOLECULAR NEUROANATOMY OF CLONED GABA-A RECEPTOR SUBUNITS IN THE RAT CNS

Gamma-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 approximately 40% amino acid homology) and isoforms thereof (approximately 70% homology), namely alpha1-6, beta1-4, gamma1-3, delta, and rho, 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 S-35-labelled oligonucleotide probes on adjacent cryostat sections. Whereas alpha1, beta2, and gamma2 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, alpha2 transcripts were found only in the olfactory bulb, cerebral cortex, caudate putamen, hippocampal formation. and certain lower brain stem nuclei; alpha3 only in the olfactory bulb and cerebral cortex; alpha5 in the hippocampal formation; and alpha6 only in cerebellar granule cells. In addition, beta1, beta3, gamma1, and delta mRNAs were also uniquely expressed in restricted brain regions. Moreover, in the spinal cord, alpha1-3, beta2,3, and gamma2 mRNAs were differently expressed in Rexed layers 2-9, with alpha2, beta3, and gamma2 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: alpha1, beta2 (1,3), gamma2 in olfactory bulb mitral cells; alpha2, beta3, gamma2, delta in caudate putamen; alpha1,3 (2,5), beta2,3, gamma2 in cerebral cortex; alpha1, beta2, gamma1,2 in pallidum; alpha1,2,5, beta1-3, gamma2 in hippocampus; alpha1,5, beta3, gamma2, delta in dentate gyrus; alpha1, beta2, gamma1,2 in substantia nigra zona reticularis; alpha1, beta2, gamma2 in cerebellar Purkinje cells; alpha1,6, beta2,3, gamma2, delta in granule cells; alpha1,2, beta2,3, gamma2 in vestibular and facial nuclei; and finally alpha2, beta3, gamma2 in motoneurons of Rexed layer 9 in the spinal cord.