GABA(A) receptors in the primate basal ganglia: an autoradiographic and a light and electron microscopic immunohistochemical study of the alpha1 and beta2,3 subunits in the baboon brain.

The distribution of gamma-aminobutyric acid(A) (GABA(A)) receptors was investigated in the basal ganglia in the baboon brain by using receptor autoradiography and the immunohistochemical localisation of the alpha1 and beta2,3 subunits of the GABA(A) receptor by light and electron microscopy. In the caudate-putamen, the alpha1 subunit was distributed in high densities in the matrix compartment, and the beta2,3 subunits were more homogeneously distributed; the globus pallidus showed lower levels of the alpha1 and beta2,3 subunits. Four types of alpha1 subunit immunoreactive neurons were identified in the baboon striatum: the most numerous (75%) were type 1 medium-sized aspiny neurons; type 2 (2%) were large aspiny neurons with an indented nuclear membrane located in the ventral striatum; type 3 neurons were the least numerous (1%) and were comprised of large neurons in the ventromedial regions of the striatum; and type 4 (22%) neurons were medium to large aspiny neurons located in striosomes. At the ultrastructural level, alpha1 and beta2,3 subunit immunoreactivity was localised in the neuropil of the striatum in both symmetrical and asymmetrical synaptic contacts. In the globus pallidus, alpha1 and beta2,3 subunits were localised on large neurons and were found in three types of synaptic terminals: type 1 terminals were small and established symmetrical synapses; type 2 terminals were large; and type 3 terminals formed small synaptic terminals with subjunctional dense bodies. These results show that the subunit composition of GABA(A) receptors varies between the striosome and the matrix compartments in the striatum and that there is receptor subunit homogeneity in the globus pallidus.