GABA A receptors: immunocytochemical distribution of 13 subunits in the adult rat brain

Abstract

GABA A receptors are ligand-operated chloride channels assembled from five subunits in a heteropentameric manner. Using immunocytochemistry, we investigated the distribution of GABA A receptor subunits deriving from 13 different genes (α1–α6, β1–β3, γ1–γ3 and δ) in the adult rat brain. Subunit α1-, β1-, β2-, β3- and γ2-immunoreactivities were found throughout the brain, although differences in their distribution were observed. Subunit α2-, α3-, α4-, α5-, α6-, γ1- and δ-immunoreactivities were more confined to certain brain areas. Thus, α2-subunit-immunoreactivity was preferentially located in forebrain areas and the cerebellum. Subunit α6-immunoreactivity was only present in granule cells of the cerebellum and the cochlear nucleus, and subunit γ1-immunoreactivity was preferentially located in the central and medial amygdaloid nuclei, in pallidal areas, the substantia nigra pars reticulata and the inferior olive. The α5-subunit-immunoreactivity was strongest in Ammon’s horn, the olfactory bulb and hypothalamus. In contrast, α4-subunit-immunoreactivity was detected in the thalamus, dentate gyrus, olfactory tubercle and basal ganglia. Subunit α3-immunoreactivity was observed in the glomerular and external plexiform layers of the olfactory bulb, in the inner layers of the cerebral cortex, the reticular thalamic nucleus, the zonal and superficial layers of the superior colliculus, the amygdala and cranial nerve nuclei. Only faint subunit γ3-immunoreactivity was detected in most areas; it was darkest in midbrain and pontine nuclei. Subunit δ-immunoreactivity was frequently co-distributed with α4 subunit-immunoreactivity, e.g. in the thalamus, striatum, outer layers of the cortex and dentate molecular layer. Striking examples of complementary distribution of certain subunit-immunoreactivities were observed. Thus, subunit α2-, α4-, β1-, β3- and δ-immunoreactivities were considerably more concentrated in the neostriatum than in the pallidum and entopeduncular nucleus. In contrast, labeling for the α1-, β2-, γ1- and γ2-subunits prevailed in the pallidum compared to the striatum. With the exception of the reticular thalamic nucleus, which was prominently stained for subunits α3, β1, β3 and γ2, most thalamic nuclei were rich in α1-, α4-, β2- and δ-immunoreactivities. Whereas the dorsal lateral geniculate nucleus was strongly immunoreactive for subunits α4, β2 and δ, the ventral lateral geniculate nucleus was predominantly labeled for subunits α2, α3, β1, β3 and γ2; subunit α1- and α5-immunoreactivities were about equally distributed in both areas. In most hypothalamic areas, immunoreactivities for subunits α1, α2, β1, β2 and β3 were observed. In the supraoptic nucleus, staining of conspicuous dendritic networks with subunit α1, α2, β2, and γ2 antibodies was contrasted by perykarya labeled for α5-, β1- and δ-immunoreactivities. Among all brain regions, the median emminence was most heavily labeled for subunit β2-immunoreactivity. In most pontine and cranial nerve nuclei and in the medulla, only subunit α1-, β2- and γ2-immunoreactivities were strong, whereas the inferior olive was significantly labeled only for subunits β1, γ1 and γ2. In this study, a highly heterogeneous distribution of 13 different GABA A receptor subunit-immunoreactivities was observed. This distribution and the apparently typical patterns of co-distribution of these GABA A receptor subunits support the assumption of multiple, differently assembled GABA A receptor subtypes and their heterogeneous distribution within the adult rat brain.