GABAA receptors in the primate basal ganglia: An autoradiographic and a light and electron microscopic immunohistochemical study of the ?1 and ?2,3 subunits in the baboon brain

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Abstract
The distribution of gamma‐aminobutyric acidA (GABAA) receptors was investigated in the basal ganglia in the baboon brain by using receptor autoradiography and the immunohistochemical localisation of the α1 and β2,3 subunits of the GABAA receptor by light and electron microscopy. In the caudate‐putamen, the α1 subunit was distributed in high densities in the matrix compartment, and the β2,3 subunits were more homogeneously distributed; the globus pallidus showed lower levels of the α1 and β2,3 subunits. Four types of α1 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, α1 and β2,3 subunit immunoreactivity was localised in the neuropil of the striatum in both symmetrical and asymmetrical synaptic contacts. In the globus pallidus, α1 and β2,3 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 GABAA receptors varies between the striosome and the matrix compartments in the striatum and that there is receptor subunit homogeneity in the globus pallidus. J. Comp. Neurol. 397:297–325, 1998.