GABAB receptor protein and mRNA distribution in rat spinal cord and dorsal root ganglia

The presence of metabotropic receptors for GABA, GABA(B), on primary afferent terminals in mammalian spinal cord has been previously reported. In this study we provide further evidence to support this in the rat and show that the GABA(B) receptor subunits GABA(B1) and GABA(B2) mRNA and the corresponding subunit proteins are present in the spinal cord and dorsal root ganglion. We also show that the predominant GABA(B1) receptor subunit mRNA present in the afferent fibre cell body appears to be the 1a form. in frozen sections of lumbar spinal cord and dorsal root ganglia (DRG) GABA(B) receptors were labelled with [H-3]CGP 62349 or the sections postfixed with paraformaldehyde and subjected to in situ hybridization using oligonucleotides designed to selectively hybridize with the mRNA for GABA(B(1a)), GABA(B(1b)) or GABA(B2). For immunocytochemistry (ICC), sections were obtained from rats anaesthetized and perfused-fixed with paraformaldehyde. The distribution of binding sites for [H-3]CGP 62349 mirrored that previously observed with [H-3]GABA at GABA(B) sites. The density of binding sites was high in the dorsal horn but much lower in the ventral regions. By contrast, the density of mRNA (pan) was more evenly distributed across the laminae of the spinal cord. The density of mRNA detected with the pan probe was high in the DRG and distributed over the neuron cell bodies. This would accord with GABA(B) receptor protein being formed in the sensory neurons and transported to the primary afferent terminals. Of the GABA(B1) mRNA in the DRG, = 90% was of the GABA(B(1a)) form and = 10% in the GABA(B(1b)) form. This would suggest that GABA(B(1a)) mRNA may be responsible for encoding presynaptic GABA(B) receptors on primary afferent terminals in a manner similar to that we have previously observed in the cerebellar cortex. GABA(B2) mRNA was also evenly distributed across the spinal cord laminae at densities equivalent to those of GABA(B1) in the dorsal horn. GABA(B2) mRNA was also detected to the same degree within the DRG. Immunocytochemical analysis revealed that GABA(B(1a)), GABA(B(1b)) and GABA(B2) were all present in the spinal cord. GABA(B(1a)) labelling appeared to be more dense than GABA(B(1b)) and within the superficial dorsal horn GABA(B(1a)) was present in the neuropil whereas GABA(B(1b)) was associated with cell bodies in this region. Both 1a and 1b immunoreactivity was expressed in motor neurons in lamina IX. GABA(B2) immunoreactivity was expressed throughout the spinal cord and was evident within the neuropil of the superficial laminae.