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Distribution of the major γ‐aminobutyric acidA receptor subunits in the basal ganglia and associated limbic brain areas of the adult rat

Within the basal ganglia, γ‐aminobutyric acid (GABA) exerts a fundamental role as neurotransmitter of local circuit and projection neurons. Its fast hyperpolarizing action is mediated through GABAA receptors. These ligand‐gated chloride channels are assembled from five subunits, which derive from multiple genes. Using immunocytochemistry, we investigated the distribution of 12 major GABAA receptor subunits (α1–5, β1–3, γ1–3, and δ) in the basal ganglia and associated limbic brain areas of the rat. Immunoreactivity for an additional subunit (subunit α6) was not observed. The striatum, the nucleus accumbens, and the olfactory tubercle displayed strong, diffuse staining for the subunits α2, α4, β3, and δ presumably located on dendrites of the principal medium spiny neurons. Subunit α1‐, β2‐, and γ2‐immunoreactivities were apparently mostly restricted to interneurons of these areas. In contrast, the globus pallidus, the entopeduncular nucleus, the ventral pallidum, the subthalamic nucleus, and the substantia nigra pars reticulata revealed dense networks of presumable dendrites of resident projection neurons, which were darkly labeled for subunit α1‐, β2‐, and γ2‐immunoreactivities. The globus pallidus, ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata, all areas receiving innervations from the striatum, displayed strong subunit γ1‐immunoreactivity compared to other brain areas. In the substantia nigra pars compacta and in the ventral tegmental area, numerous presumptive dopaminergic neurons were labeled for subunits α3, γ3, and/or δ. This highly heterogeneous distribution of individual GABAA receptor subunits suggests the existence of differently assembled, and presumably also functionally different, GABAA receptors within individual nuclei of the basal ganglia and associated limbic brain areas. J. Comp. Neurol. 433:526–549, 2001. © 2001 Wiley‐Liss, Inc.