Differential Subcellular Localization of mGluR1a and mGluR5 in the Rat and Monkey Substantia Nigra

Abstract

Neurons in the rat substantia nigra (SN) are enriched in group I metabotropic glutamate receptor (mGluR) subtypes and respond to group I mGluR activation. To better understand the mechanisms by which mGluR1 and mGluR5 mediate these effects, the goal of this study was to elucidate the subsynaptic localization of these two receptor subtypes in the rat and monkey substantia nigra. At the light microscope level, neurons of the SN pars reticulata (SNr) displayed moderate to strong immunoreactivity for both mGluR1a and mGluR5 in rats and monkeys. However, mGluR1a labeling was much stronger in monkey than in rat SN pars compacta (SNc) neurons, whereas a moderate level of mGluR5 immunoreactivity was found in both species. At the electron microscope level, the immunoreactivity for both group I mGluR subtypes was primarily expressed postsynaptically, although light mGluR1a labeling was occasionally seen in axon terminals in the rat SNr. Immunogold studies revealed a striking difference in the subcellular distribution of mGluR1a and mGluR5 immunoreactivity in SNr and SNc neurons. Although the bulk of mGluR1a was attached to the plasma membrane, Ͼ80% of mGluR5 immunoreactivity was intracellular. Plasma membrane-bound immunoreactivity for group I mGluRs in both SNc and SNr neurons was mostly extrasynaptic or in the main body of symmetric, putative GABAergic synapses. On the other hand, asymmetric synapses either were nonimmunoreactive or displayed perisynaptic labeling. These data raise important questions about the trafficking, internalization, and potential functions of group I mGluRs at extrasynaptic sites or symmetric synapses in the substantia nigra. Key words: metabotropic; glutamate; receptor internalization; receptor trafficking; dopamine neurons; Parkinson's disease; immunogold methodThe substantia nigra pars reticulata (SNr) and the internal segment of the globus pallidus are the main output nuclei of the basal ganglia, whereas the SN pars compacta (SNc) is a group of dopaminergic neurons that project to the striatum. Both the SNr and SNc receive glutamatergic innervation from the subthalamic nucleus), cerebral cortex, and pedunculopontine tegmental nucleus (Wichmann and DeLong, 1998).The effects of glutamate in the CNS are mediated by activation of ionotropic and metabotropic receptors. Ionotropic receptors are ligand-gated cation channels that mediate fast excitatory neurotransmission, whereas metabotropic glutamate receptors (mGluRs) belong to a family of G-protein-coupled receptors that mediate modulatory effects of synaptic transmission by activation of a number of intracellular metabolic pathways. The family of mGluRs is subdivided into three groups (Conn and Pin, 1997). The group I mGluRs consist of mGluR1, mGluR5, and all their splice variants. These receptors are coupled to phosphoinositol hydrolysis and usually induce slow depolarization. On the other hand, group II (mGluR2,3) and group III (mGluR4,6,7,8) mGluRs are negatively coupled to adenylate cyclase and often induce presynaptic inhibition of trans...

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