The distribution and relative abundance of gene transcripts for diverse GABA_A receptor subunits (α_1–3,5, β_1–3, γ₂) in neurons of the rat cervical spinal cord and dorsal root ganglia were determined by in situ hybridization histochemistry using³⁵S-labeled 60mer oligonucleotide probes. The receptor proteins (mapped by benzodiazepine receptor radioautography and immunohistochemistry with [³H]flumazenil and a monoclonal antibody for the β₂ + β₃ 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 γ₂ mRNA was the most ubiquitous and abundant of the subunit variants investigated. The labeling of motoneurons in layer IX was particularly strong for α₂, moderate for β₃ and γ₂ and extremely weak for α₁ and α₃. In layers VII, VIII and X the β₃ and γ₂ transcripts were moderately expressed whereas the α₁ and β₂ 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 α₃, α₅, β₃ and γ₂ hybridization signals were observed in layers II and III. In the dorsal root ganglia, whereas α₂ transcripts were abundant in virtually all large sensory neurons and to a much lower degree in the small diameter cells, γ₂ transcripts were confined to a subpopulation of large and small neurons. Furthermore, β₂ and α₁ 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.