Immunohistochemical localization of subtype 4a metabotropic glutamate receptors in the rat and mouse basal ganglia

Abstract

Recent studies suggest that metabotropic glutamate receptors (mGluRs) may play a significant role in regulating basal ganglia functions. In this study, we investigated the localization of mGluR4a protein in the mouse and rat basal ganglia. Polyclonal antibodies that specifically react with the metabotropic glutamate receptor subtype mGluR4a were produced and characterized by Western blot analysis. These antibodies recognized a native protein in wild‐type mouse brain with a molecular weight similar to the molecular weight of the band from a mGluR4a‐transfected cell line. The immunoreactivity was absent in brains of knockout mice deficient in mGluR4. mGluR4a immunoreactivity was most intense in the molecular layer of the cerebellum. We also found a striking mGluR4a immunoreactivity in globus pallidus, and moderate staining in substantia nigra pars reticulata and entopeduncular nucleus. Moderate to low mGluR4a immunoreactivity was present in striatum and other brain regions, including hippocampus, neocortex, and thalamus. Double labeling with mGluR4a antibodies and antibodies to either a dendritic marker or a marker of presynaptic terminals suggest a localization of mGluR4a on presynaptic terminals. Immunocytochemistry at electron microscopy level confirmed these results, revealing that in the globus pallidus, mGluR4a is mainly localized in presynaptic sites in axonal elements. Finally, quinolinic acid lesion of striatal projection neurons decreased mGluR4a immunoreactivity in globus pallidus, suggesting a localization of mGluR4a on striatopallidal terminals. These data support the hypothesis that mGluR4a serves as a presynaptic heteroreceptor in the globus pallidus, where it may play an important role in regulating g‐amino‐n‐butyric acid (GABA) release from striatopallidal terminals. J. Comp. Neurol. 407:33–46, 1999. © 1999 Wiley‐Liss, Inc.

Group III Metabotropic Glutamate Receptors Maintain Tonic Inhibition of Excitatory Synaptic Input to Hypocretin/Orexin Neurons

“…This view is consistent with our studies of electrically evoked postsynaptic potentials that were decreased in amplitude by activation of mGluRs, coupled with the absence of any modulation of postsynaptic responses to ionotropic glutamate receptor agonists. That mGluRs are located on presynaptic terminals is consistent both with previous electrophysiological (Schrader and Tasker, 1997;Evans et al, 2000;Awatramani and Slaughter, 2001) and immunocytochemical (Bradley et al, 1996(Bradley et al, , 1999Shigemoto et al, 1997) studies of other neuronal types.…”

“…Anatomical and electrophysiological studies have shown that group III mGluRs can be located presynaptically or postsynaptically on different neurons (Martin et al, 1997;Shigemoto et al, 1997;Bradley et al, 1999;Taverna and Pennartz, 2003). To examine the site of action of the metabotropic group III agonist L-AP-4 in hypocretin neurons, we further studied its effect on the amplitude and frequency of miniature EPSCs (mEPSCs).…”

“…Activation of these mGluRs activates G i /G o -proteins, causing a decrease in cAMP, and modulates synaptic activity by altering phosphorylation of ion channels, receptors, and second-messenger systems (Nakanishi, 1994;Cartmell and Schoepp, 2000;Schoepp, 2001). Group III mGluRs have been localized immunocytochemically to presynaptic axons in some brain regions and to postsynaptic cells in other brain regions (Bradley et al, 1996(Bradley et al, , 1999Shigemoto et al, 1997). In some brain areas, group III mGluRs selectively inhibit excitatory synaptic activity (Awad-Granko et al, 2001).…”

Group I Metabotropic Glutamate Receptors in the Monkey Striatum: Subsynaptic Association with Glutamatergic and Dopaminergic Afferents

“…are linked to the activation of phospholipase C and generally mediate postsynaptic excitatory effects, whereas group II (mGluR2 and -3) and group III (mGluR4, receptors are negatively coupled to adenylyl cyclase and generally mediate presynaptic inhibitory influences on neurotransmitter release (Nakanishi, 1994;Conn and Pin, 1997) The basal ganglia are enriched in the three groups of mGluRs, but current knowledge of their synaptic localization and functions remains limited. In the striatum, group I mGluRs are primarily expressed in projection neurons and subpopulations of interneurons, whereas group II and III mGluRs are mostly localized presynaptically on glutamatergic terminals (Testa et al, 1994(Testa et al, , 1995(Testa et al, , 1998Kinoshita et al, 1998;Bradley et al, 1999;Kosinki et al, 1999;Tamaru et al, 2001;Corti et al, 2002). In addition to their preponderance in postsynaptic striatal neurons, activation of group I mGluRs also mediates presynaptic effects on GABA and dopamine release in the rat striatum (Verma and Moghaddam, 1998;Bruton et al, 1999;Battaglia et al, 2001).…”

Molecular Determinants of High Affinity Binding to Group III Metabotropic Glutamate Receptors

“…1). With the exception of mGluR6, which is localized postsynaptically in cells of the retina, the other Group III mGluRs show primarily a presynaptic localization (3,4), although at some synapses these receptors may also be present postsynaptically. Presynaptic Group III mGluRs act as autoreceptors to inhibit the release of L-glutamate from glutamatergic nerve terminals (4 -6), and ␥-aminobutyric acid release via glutamate spillover onto GABAergic synapses expressing mGluRs (7).…”