Comparative immunohistochemical localisation of GABA B1a, GABA B1b and GABA B2 subunits in rat brain, spinal cord and dorsal root ganglion

Abstract

GABA B receptors are G-protein-coupled receptors mediating the slow onset and prolonged synaptic actions of GABA in the CNS. The recent cloning of two genes, GABA B1 and GABA B2, has revealed a novel requirement for GABA B receptor signalling. Studies have demonstrated that the two receptor subunits associate as a GABA B1/GABA B2 heterodimer to form a functional GABA B receptor. In this study we have developed polyclonal antisera specific to two splice variants of the GABA B1 subunit, GABA B1a and GABA B1b, as well as an antiserum to the GABA B2 subunit. Using affinity-purified antibodies derived from these antisera we have mapped out the distribution profile of each subunit in rat brain, spinal cord and dorsal root ganglion. In brain the highest areas of GABA B1a, GABA B1b and GABA B2 subunit expression were found in neocortex, hippocampus, thalamus, cerebellum and habenula. In spinal cord, GABA B1 and GABA B2 subunits were expressed in the superficial layers of the dorsal horn, as well as in motor neurones in the deeper layers of the ventral horn. GABA B receptor subunit immunoreactivity in dorsal root ganglion suggested that expression of GABA B1b was restricted to the large diameter neurones, in contrast to GABA B1a and GABA B2 subunits which were expressed in both large and small diameter neurones. Although expression levels of GABA B1 and GABA B2 subunits varied we found no areas in which GABA B1 was expressed in the absence of GABA B2. This suggests that most, if not all, GABA B1 immunoreactivity may represent functional GABA B receptors. Although our data are in general agreement with functional studies, some discrepancies in GABA B1 subunit expression occurred with respect to other immunohistochemical studies. Overall our data suggest that GABA B receptors are widely expressed throughout the brain and spinal cord, and that GABA B1a and GABA B1b subunits can associate with GABA B2 to form both pre- and post-synaptic receptors.