Group I metabotropic glutamate receptors in the monkey striatum: subsynaptic association with glutamatergic and dopaminergic afferents

Abstract

Group I metabotropic glutamate receptors (mGluRs) are involved in long-term synaptic plasticity and neuroprotection in the striatum, but the specific role(s) of mGluR1 and mGluR5 remain poorly understood. In this study, we used electron-microscopic immunocytochemistry to compare the pattern of subsynaptic and subcellular distribution of mGluR1a and mGluR5 in relation to putative glutamatergic and dopaminergic inputs to the monkey striatum. At the light-microscopic level, both group I mGluRs are expressed in the striatal neuropil. In addition, numerous perikarya of striatal output neurons are immunostained for mGluR5, but much less frequently for mGluR1a. At the electron-microscopic level, immunoreactivity for both receptor subtypes is primarily expressed postsynaptically in dendrites and spines, although presynaptic mGluR1a labeling of glutamatergic thalamostriatal boutons and, less frequently, dopaminergic and corticostriatal terminals is also seen. In contrast to mGluR1a, mGluR5 immunoreactivity is rarely encountered presynaptically. In postsynaptic elements, 40-70% of immunoreactivity for both receptor subtypes is expressed intracellularly, whereas 30-60% is apposed to the plasma membrane. More than 80% of the labeling apposed to the plasma membrane is extrasynaptic. The remaining 20% is located at the edges of putative glutamatergic synapses or in the active zone of symmetric synapses. In mGluR5-, but not mGluR1a-immunostained sections, approximately 70% of dopaminergic symmetric synapses are labeled perisynaptically. These data emphasize the differential pattern of subsynaptic localization of the two group I mGluRs and provide various presynaptic and postsynaptic sites whereby mGluR1 and mGluR5 could mediate different, but complementary, effects on glutamatergic and dopaminergic transmission in the primate striatum.

Figure 2.

Histogram comparing the relative distribution of mGluR1a and mGluR5 immunoperoxidase labeling in different neuronal compartments and glia in the monkey striatum. Data are expressed as the mean percentage (±SD) of labeling from three monkeys. Note the substantial difference in the relative abundance of mGluR1a- versus mGluR5-containing terminals. Total number (n) of labeled elements examined from three monkeys: mGluR1a, 367; mGluR5, 476. Dend., Dendrites; U., unmyelinated; Term., terminals. Error bars indicate SD.

Figure 3.

Intracellular and plasma membrane-apposed group I mGluR immunogold labeling in the monkey striatum. A, Relative proportion of intracellular versus plasma membrane-apposed immunogold particles for group I mGluRs in different neuronal compartments of striatal neurons. Mean values (±SD) from three animals (n) were used. A two-factor ANOVA did not reveal any significant difference between the distribution of mGluR1a and mGluR5 in the different neuronal compartments. The numbers of gold particles (g) and neuronal elements (e) examined in three animals were as follows: mGluR1a spines, 665 (g) and 408 (e); mGluR5 spines, 891 (g) and 405 (e); mGluR1a dendrites, 1966 (g) and 744 (e); mGluR5 dendrites, 5526 (g) and 843 (e); mGluR1a soma, 3579 (g) and 71 (e); and mGluR5 soma, 6111 (g) and 25 (e). Error bars indicate SD. B, Electron micrograph showing examples of mGluR1a-containing dendrites (den) and spines (Sp). Some intracellular and plasma membrane-apposed gold particles are indicated by small arrows and arrowheads, respectively. den, Dendrites; Sp, dendritic spines. Scale bar, 0.25 μm.

Figure 1.

Immunoperoxidase labeling of group I mGluRs in the monkey striatum. A, B, Light and electron micrographs of mGluR1a immunostaining are shown. A, A dense mGluR1a immunoreactive neuropil composed of small punctate elements is depicted. B, Dark DAB reaction product is observed in small dendrites (den), dendritic spines (Sp), unmyelinated axons (ax), and axon terminals (t). Note the mGluR1a-positive axon terminal (t1) forming an asymmetric axodendritic synapse (double arrows). The synaptic specialization of another labeled bouton (t2) cannot be determined. C, D, Light and electron micrographs of mGluR5 immunostaining. C, Immunoreactive cell bodies (arrowheads) and proximal dendrites (small arrows) of medium-sized projection neurons embedded into a dense meshwork of labeled processes. D, mGluR5-immunoreactive spines (Sp), dendrites (den), and axons (ax) are depicted. Note the dense mGluR5 immunoreactivity associated with microtubules (arrowheads). Scale bars: (in A) A, C, 25 μm; (in B) B, D, 0.5 μm.

Figure 8.

Presynaptic mGluR1a in putative glutamatergic terminals from the thalamus and cerebral cortex. A, Shown is a mGluR1a-containing terminal forming an asymmetric axospinous synapse (arrowhead). The peroxidase deposit is indicated by a large arrow. B, C, Depicted are thalamostriatal (B) and corticostriatal (C) BDA-labeled boutons (asterisks in B and C) that express mGluR1a immunoreactivity (gold labeling) in the postcommissural putamen. In these micrographs, BDA has been revealed with DAB, whereas mGluR1a immunoreactivity is localized with gold particles. u. sp, Unlabeled spine; den, dendrite. Scale bars: (in A) B, 0.5 μm; C, 0.8 μm. D, Histogram that compares the relative abundance of thalamostriatal and corticostriatal terminals that express mGluR1a immunoreactivity in the monkey putamen. Note the preponderance of presynaptic mGluR1a labeling in thalamostriatal boutons. Term., Terminals; Thalamostr., thalamostriatal; Corticostr., corticostriatal.

Figure 4.

Extrasynaptic and synaptic plasma membrane-apposed group I mGluR immunogold labeling in the monkey striatum. A, Relative proportion of extrasynaptic versus synaptic plasma membrane-apposed gold particles for group I mGluRs in different compartments of striatal neurons. Mean values (±SD) from three animals (n) were used. The distribution of gold-particle labeling for either receptor subtype in spines was significantly different (p < 0.05; ANOVA with post hoc Newman-Keul) from that in dendrites and soma. The total numbers of gold particles (g) and neuronal elements (e) examined in three animals were as follows: mGluR1a spines, 419 (g) and 263 (e); mGluR5 spines, 465 (g) and 287 (e); mGluR1a dendrites, 892 (g) and 634 (e); mGluR5 dendrites, 1593 (g) and 843 (e); mGluR1a soma, 342 (g) and 70 (e); and mGluR5 soma, 1042 (g) and 25 (e). Error bars indicate SD. B, Distance between extrasynaptic plasma membrane-apposed gold-particle labeling for mGluR5 and the closest edge of asymmetric postsynaptic specializations. Serial sections of asymmetric synapses established by anterogradely labeled boutons from the cortex and thalamus were used in this analysis. The total number of gold particles and synapses examined is indicated in parentheses. The axospinous synapses are established by cortical terminals, whereas axodendritic synapses involve thalamostriatal boutons. Note that the majority of gold particles are located <0.5 μm away from either population of putative glutamatergic synapses.

Figure 5.

Distribution of synaptic (perisynaptic vs active-zone) plasma membrane-apposed group I mGluR immunogold labeling in dendrites and spines of striatal neurons. A, Relative distribution of gold particles in relation to asymmetric (putative glutamatergic) and symmetric (putative GABAergic) synapses. Numbers of gold particles (g) and elements (e) examined were as follows: mGluR1a spines, 97 (g) and 81 (e); mGluR5 spines, 85 (g) and 71(e); mGluR1a dendrites, 40 (g) and 32 (e); and mGluR5 dendrites, 101 (g) and 62 (e). B, An example of perisynaptic gold-particle labeling for mGluR1a (arrow) at an axospinous asymmetric synapse (arrowheads). C, An example of gold-particle labeling for mGluR5 (arrows) apposed to the active zone of an axodendritic symmetric synapse (arrowheads). Sp, Spines; den, dendrites; t, axon terminals. Scale bar: (in B) B, C, 0.25 μm.

Figure 6.

Serial ultrathin sections through perisynaptic mGluR5 immunoreactivity (arrows) at an asymmetric axospinous synapse (arrowheads) established by an anterogradely labeled cortical bouton. Note the high degree of specificity of the perisynaptic immunogold labeling in these sections. Sp, Dendritic spines. Scale bar: (in A), A-D, 0.25 μm.

Figure 7.

Postsynaptic mGluR5 immunoreactivity at dopaminergic synapses in the monkey striatum. A, Two perisynaptic gold particles (arrows) are associated with an en passant axosomatic symmetric synapse (arrowheads) established by an axonal process that displays the ultrastructural features of a dopaminergic nigrostriatal terminal. B, C, Examples of perisynaptic labeling for mGluR5 (arrows) at dopaminergic synapses. In B, the TH-positive axonal process (TH+) forms an en passant-type synaptic contact (arrowhead) with a dendrite (den), whereas in C, the TH-positive axon terminal (TH+) is in contact with a spine neck (Sp). Scale bars: A, 0.25 μm; (in B), B, C, 0.25 μm.

Figure 9.

Presynaptic group I mGluR immunoreactivity in dopaminergic terminals (asterisks) in the monkey putamen. In these micrographs, TH is localized with the amorphous DAB reaction product, whereas mGluR1a is labeled with gold particles. A TH-negative bouton that expresses mGluR1a immunoreactivity (Te) is shown in B, and unlabeled terminals (Ut) for both TH and mGluR1a are indicated. den, Dendrites; Sp, dendritic spines. Scale bar: (in A), A, B, 250 nm.