INSITU HYBRIDIZATION HISTOCHEMISTRY REVEALS A DIVERSITY OF GABA-A RECEPTOR SUBUNIT MESSENGER-RNAS IN NEURONS OF THE RAT SPINAL-CORD AND DORSAL-ROOT GANGLIA

The distribution and relative abundance of gene transcripts for diverse GABA(A) receptor subunits (alpha-1-3,5, beta-1-3, gamma-2) in neurons of the rat cervical spinal cord and dorsal root ganglia were determined by in situ hybridization histochemistry using S-35-labeled 60mer oligonucleotide probes. The receptor proteins (mapped by benzodiazepine receptor radioautography and immunohistochemistry with [H-3]flumazenil and a monoclonal antibody for the beta-2 + beta-3 subunits, respectively) were most abundant in the dorsal horn (layers II and III) and in layer X around the central canal. Although diverse receptor subunit mRNAs were detected (to varying degrees) in neurons throughout layers II-X of the spinal cord, motoneurons in layer IX were particularly strongly labeled. The gamma-2 mRNA was the most ubiquitous and abundant of the subunit variants investigated. The labeling of motoneurons in layer IX was particularly strong for alpha-2, moderate for beta-3 and gamma-2 and extremely weak for alpha-1 and alpha-3. In layers VII, VIII and X the beta-3 and gamma-2 transcripts were moderately expressed whereas the alpha-1 and beta-2 transcript levels differed markedly among the cells of these layers. Although the mRNAs of all subunit variants could be detected in layers IV-VI, only alpha-3, alpha-5, beta-3 and gamma-2 hybridization signals were observed in layers II and III. In the dorsal root ganglia, whereas alpha-2 transcripts were abundant in virtually all large sensory neurons and to a much lower degree in the small diameter cells, gamma-2 transcripts were confined to a subpopulation of large and small neurons. Furthermore, beta-2 and alpha-1 transcripts were even more restricted in their distribution. The findings provided a basis for the mediation of synaptic inhibition in the spinal cord by diverse GABA(A) receptors and further strong evidence for the long-established view that presynaptic inhibition of inter- and motoneurons, via axoaxonic synapses between GABAergic interneurons and primary afferent terminals, is mediated by GABA(A) receptors. The physiological roles and pharmacological implications of this receptor diversity have yet to be determined.