Pre- and postsynaptic localization of GABA(B) receptors in the basal ganglia in monkeys

GABAergic neurotransmission involves ionotropic GABA(A) and metabotropic GABA(B) receptor subtypes. Although fast inhibitory transmission through GABA(A) receptors activation is commonly found in the basal ganglia, the functions as well as the cellular and subcellular localization of GABA(B) receptors are still poorly known. Polyclonal antibodies that specifically recognize the GABA(B)R1 receptor subunit were produced and used for immunocytochemical localization of these receptors at the light and electron microscope levels in the monkey basal ganglia. Western blot analysis of monkey brain homogenates revealed that these antibodies reacted specifically with two native proteins corresponding to the size of the two splice variants GABA(B)R1a and GABA(B)R1b. Preadsorption of the purified antiserum with synthetic peptides demonstrated that these antibodies recognize specifically GABA(B)R1 receptors with no cross-reactivity with GABA(B)R2 receptors. Overall, the distribution of GABA(B)R1 immunoreactivity throughout the monkey brain correlates with previous GABA(B) ligand binding studies and in situ hybridization data as well as with recent immunocytochemical studies in rodents. GABA(B)R1-immunoreactive cell bodies were found in all basal ganglia nuclei but the intensity of immunostaining varied among neuronal populations in each nucleus. In the striatum, interneurons were more strongly stained than medium-sized projection neurons while in the substantia nigra, dopaminergic neurons of the pars compacta were much more intensely labeled than GABAergic neurons of the pars reticulata. In the subthalamic nucleus, clear immunonegative neuronal perikarya were intermingled with numerous GABA(B)R1-immunoreactive cells. Moderate GABA(B)R1 immunoreactivity was observed in neuronal perikarya and dendritic processes throughout the external and internal pallidal segments. At the electron microscope level, GABA(B)R1 immunoreactivity was commonly found in neuronal cell bodies and dendrites in every basal ganglia nuclei. Many dendritic spines also displayed GABA(B)R1 immunoreactivity in the striatum. In addition to strong postsynaptic labeling, GABA(B)R1-immunoreactive preterminal axonal segments and axon terminals were frequently encountered throughout the basal ganglia components. The majority of labeled terminals displayed the ultrastructural features of glutamatergic boutons and formed asymmetric synapses. In the striatum, GABA(B)R1-containing boutons resembled terminals of cortical origin, while in the globus pallidus and substantia nigra, subthalamic-like terminals were labeled. Overall, these findings demonstrate that GABA(B) receptors are widely distributed and located to subserve both pre- and postsynaptic roles in controlling synaptic transmission in the primate basal ganglia.