Subcellular localization of GABA_B receptor subunits in rat globus pallidus

Abstract

The inhibitory amino acid gamma-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 unknown. Here, we use pre-embedding immunogold labeling to study the subcellular localization 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 immunolabeling, using the vesicular glutamate transporter 2 (VGLUT2), revealed the glutamatergic nature of most immunogold-labeled asymmetric synapses. At symmetric, putative GABAergic, synapses, including those formed by anterogradely labeled striatopallidal terminals, 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.

Synaptically Released GABA Activates Both Pre- and Postsynaptic GABAB Receptors in the Rat Globus Pallidus

“…GABA B receptors are localized on the GABAergic and glutamatergic presynaptic terminals as well as on postsynaptic elements in the rat GP (Chen et al 2004). We examined whether synaptically released GABA can activate presynaptic GABA B receptors and control GABA release.…”

“…Similarly, stimulation of the IC activates the local collaterals and intra-pallidal collaterals of Str-entopeduncular/substantia nigra axons (Kita and Kitai 1994;Wu et al 2000). Receptor localization studies suggest both striatal and local collateral synapses can evoke GABA B responses (Chen et al 2004). …”

“…This was in contrast with evoking saturated-slow IPSPs that require a repetitive stimulation of ϳ50 pulses at 50 Hz. The possible reasons for the difference include that GABA B autoreceptors are largely localized at the main body of the synaptic specializations that are closest to GABA release site, while postsynaptic GABA B receptors are distributed more distantly (Chen et al 2004). Different mechanisms between pre-and postsynaptic actions may also be involved.…”

“…Although previous studies focused mainly on the ionotropic GABA A receptor-mediated actions, accumulating evidence suggests that metabotropic GABA B receptors may also be active in the GP. The expression of both the GABA B R1 and GABA B R2 subunits in the GP has been revealed by immunohistochemical and in situ hybridization studies in rat, monkey, and humans (Charara et al , 2004Chen et al 2004;Smith et al 2000;Waldvogel et al 2004). Electron microscopic analysis showed both pre-and postsynaptic expressions of GABA B receptors in the GP (Chen et al 2004).…”

“…The expression of both the GABA B R1 and GABA B R2 subunits in the GP has been revealed by immunohistochemical and in situ hybridization studies in rat, monkey, and humans (Charara et al , 2004Chen et al 2004;Smith et al 2000;Waldvogel et al 2004). Electron microscopic analysis showed both pre-and postsynaptic expressions of GABA B receptors in the GP (Chen et al 2004). In other brain areas, activation of postsynaptic GABA B receptors induces a slow hyperpolarization through the opening of potassium conductances and activation of receptors located at presynaptic terminals inhibits neurotransmitter release (Misgeld et al 1995).…”