Distribution of the major gamma-iminobutyric acid(A) receptor subunits in the basal ganglia and associated limbic brain areas of the adult rat

Within the basal ganglia, gamma -aminobutyric acid (GABA) exerts a fundamen tal role as neurotransmitter of local circuit and projection neurons. Its f ast hyperpolarizing action is mediated through GABA(A) receptors. These lig and-gated chloride channels are assembled from five subunits, which derive from multiple genes. Using immunocytochemistry, we investigated the distrib ution of 12 major GABA(A) receptor subunits (alpha1-5, beta1-3, gamma1-3, a nd delta) in the basal ganglia and associated limbic brain areas of the rat . Immunoreactivity for an additional subunit (subunit alpha6) was not obser ved. The striatum, the nucleus accumbens, and the olfactory tubercle displa yed strong, diffuse staining for the subunits alpha2, alpha4, beta3, and de lta presumably located on dendrites of the principal medium spiny neurons. Subunit alpha1-, beta2-, and gamma2-immunoreactivities were apparently most ly restricted to interneurons of these areas. In contrast, the globus palli dus, the entopeduncular nucleus, the ventral pallidum, the subthalamic nucl eus, and the substantia nigra pars reticulata revealed dense networks of pr esumable dendrites of resident pi-ejection neurons, which were darkly label ed for subunit alpha1-, beta2-, and gamma2-immunoreactivities. Tile globus pallidus, ventral pallidum, entopeduncular nucleus, and substantia nigra pa rs reticulata, all areas receiving innervations from the striatum, displaye d strong subunit gamma1-immunoreactivity compared to other brain areas. In the substantia nigra pars compacta and in the ventral tegmental area, numer ous presumptive dopaminergic neurons were labeled for subunits alpha3, gamm a3, and/or delta. This highly heterogeneous distribution of individual GABA (A) receptor subunits suggests the existence of differently assembled, and presumably also functionally different, GABA(A) receptors within individual nuclei of the basal ganglia and associated limbic brain areas. (C) 2001 Wi ley-Liss, Inc.