Subcellular Localization of GABA B Receptor Subunits in Rat Globus Pallidus LEI CHEN, 1,2 JUSTIN BOYES, 1 WING-HO YUNG, 2 AND J. PAUL BOLAM 1 * 1 Medical Research Council Anatomical Neuropharmacology Unit, Department of Pharmacology, Oxford OX1 3TH, United Kingdom 2 Department of Physiology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, Hong Kong, People’s Republic of China ABSTRACT The inhibitory amino acid -aminobutyric acid (GABA) is the major neurotransmitter in the globus pallidus. Although electrophysiological studies have indicated that functional GABA B receptors exist in rat globus pallidus, the subcellular localization of GABA B receptor subunits and their spatial relationship to glutamatergic and GABAergic synapses are un- known. Here, we use pre-embedding immunogold labeling to study the subcellular localiza- tion of GABA B receptor subunits, GABA B1 and GABA B2 , in globus pallidus neurons and identified populations of afferent terminals. Immunolabeling for GABA B1 and GABA B2 was observed throughout the globus pallidus, with GABA B1 more strongly expressed in perikarya and GABA B2 mainly expressed in the neuropil. Electron microscopic analysis revealed that the majority of GABA B1 labeling was localized within the cytoplasm, whereas most of GABA B2 labeling was associated with the plasma membrane. At the subcellular level, both the GABA B1 and GABA B2 immunogold labeling was localized at pre- and postsynaptic sites. At asymmetric, putative excitatory, synapses, GABA B1 and GABA B2 immunogold labeling was found at perisynaptic sites of both pre- and postsynaptic specializations. Double immu- nolabeling, using the vesicular glutamate transporter 2 (VGLUT2), revealed the glutama- tergic nature of most immunogold-labeled asymmetric synapses. At symmetric, putative GABAergic, synapses, including those formed by anterogradely labeled striatopallidal termi- nals, GABA B1 and GABA B2 immunogold labeling was found in the main body of both pre- and postsynaptic specializations. These results demonstrate the existence of presynaptic GABA B auto- and heteroreceptors and postsynaptic GABA B receptors, which may be involved in modulating synaptic transmission in the globus pallidus. J. Comp. Neurol. 474:340 –352, 2004. © 2004 Wiley-Liss, Inc. Indexing terms: basal ganglia; synapses, striatopallidal; pre-embedding immunogold; VGLUT2 The globus pallidus (the external segment of the globus pallidus in primates) occupies a critical position in the indirect pathway of the basal ganglia and, as such, plays an important role in the expression of basal ganglia behav- iour. Morphological and physiological studies indicate that the globus pallidus receives inhibitory -aminobutyric acid (GABA)ergic inputs from the striatum and excitatory glu- tamatergic inputs from subthalamic nucleus, neocortex, and thalamus (Smith et al., 1990b, 1998; Kita, 1993; Par- ent and Hazrati, 1995; Nambu et al., 2000). In turn, the globus pallidus is in a position to influence the activity of the whole basal ganglia through its GABAergic projec- tions to the striatum, substantia nigra, entopeduncular nucleus, and subthalamic nucleus (Smith et al., 1990a; Bolam and Smith, 1992; Kita and Kitai, 1994; Bevan et al., 1998; Smith et al., 1998; Bolam et al., 2000). It has been suggested that abnormal hypoactivity and decreased Grant sponsor: Medical Research Council UK; Research Grants Council of Hong Kong; Grant number: CUHK 4080/00M. *Correspondence to: J. Paul Bolam, MRC Anatomical Neuropharmacol- ogy Unit, Department of Pharmacology, Mansfield Road, Oxford OX1 3TH, UK. E-mail: paul.bolam@pharm.ox.ac.uk Received 1 October 2003; Revised 5 December 2003; Accepted 9 Decem- ber 2003 DOI 10.1002/cne.20143 Published online in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 474:340 –352 (2004) © 2004 WILEY-LISS, INC.