Immunohistochemical localization of AMPA-type glutamate receptor subunits in the striatum of rhesus monkey

Abstract

Corticostriatal and thalamostriatal projections utilize glutamate as their neurotransmitter. Their influence on striatum is mediated, in part, by ionotropic AMPA-type glutamate receptors, which are heteromers composed of GluR1–4 subunits. While the cellular localization of AMPA-type subunits in the basal ganglia has been well characterized in rodents, the cellular localization of AMPA subunits in primate basal ganglia is not. We thus carried out immunohistochemical studies of GluR1–4 distribution in rhesus monkey basal ganglia in conjunction with characterization of each major neuron type. In striatum, about 65% of striatal neurons immunolabeled for GluR1, 75%–79% immunolabeled for GluR2 or GluR2/3, and only 2.5% possessed GluR4. All neurons the large size of cholinergic interneurons (mean diameter 26.1 μm) were moderately labeled for GluR1, while all neurons in the size range of parvalbuminergic interneurons (mean diameter 13.8 μm) were intensely rich in GluR1. Additionally, somewhat more than half of the neurons in the size range of projection neurons (mean diameter 11.6 μm) immunolabeled for GluR1, and about one third of these were very rich in GluR1. About half of the neurons the size of cholinergic interneurons were immunolabeled for GluR2, and the remainder of the neurons that were immunolabeled for GluR2 coincided with projection neurons in size and shape (GluR2 diameter = 10.7 μm), indicating that the vast majority of striatal projection neurons possess immunodectible GluR2. Similar results were observed with GluR2/3 immunolabeling. Half of the neurons the size of cholinergic interneurons immunolabeled for GluR4 and seemingly all neurons in the size range of parvalbuminergic interneurons possessed GluR4. These results indicate that AMPA receptor subunit combinations for striatal projection neurons in rhesus monkey are similar to those for the corresponding neuron types in rodents, and thus their AMPA responses to glutamate are likely to be similar to those demonstrated in rodents.

Introduction

The basal ganglia receive a prominent glutamatergic input from the cerebral cortex and from the thalamus (Somogyi et al., 1981, Gerfen, 1992). Corticostriatal and thalamostriatal terminals are known to make excitatory asymmetric synaptic contacts with the spines and dendrites of striatal projection neurons (Wilson, 1995, Reiner et al., 2003, Lei et al., 2004, Smith et al., 2004, Smith et al., 2001), as well as with the dendrites of parvalbuminergic and calretinergic striatal interneurons (Bennett & Bolam, 1993a, Bennett & Bolam, 1994, Kawaguchi, 1993, Kawaguchi et al., 1995). Consistent with these glutamatergic inputs, neurons in the striatum are rich in glutamate receptors (Chen et al., 1996, Götz et al., 1997, Deng et al., 2007). The response of basal ganglia neurons to excitatory input from cerebral cortex and thalamus has been shown to be mediated by three types of ionotropic glutamate receptors, the l-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type, the N-methyl-d-aspartate (NMDA) type, and the kainic acid (KA) type (Götz et al., 1997, Calabresi et al., 1998, Stefani et al., 1998). AMPA-type, NMDA-type, and KA-type receptors are multimeric transmembrane proteins consisting of combinations of any of several type-specific subunits that form ligand-gated ion channels permeable to cations such as Ca²⁺ and Na+ (Mayer and Armstrong, 2004).

AMPA receptors are the main glutamate receptor type that mediates the fast excitatory response of neurons to glutamatergic input, and they are formed from combinations of the glutamate receptor subunits GluR1–4 (Keinanen et al., 1990, Seeburg, 1993, Hollmann & Heinemann, 1994). The subunit composition of AMPA-type glutamate receptors influences their divalent ion permeability, rectification, sensitivity to polyamines, and kinetics (Seeburg, 1993, Hollmann & Heinemann, 1994, Jonas & Burnashev, 1995). Immunohistochemical and in situ hybridization studies in rodents have demonstrated that most basal ganglia neurons possess AMPA receptor subunits, with neuron type-specific differences in subunit composition (Tallaksen-Greene & Albin, 1994, Chen et al., 1996, Paquet & Smith, 1996, Kwok et al., 1997, Deng et al., 2007). For example, in rats medium-sized spiny GABAergic striatal projection neurons are enriched in GluR1, GluR2 and/or GluR3, whereas parvalbuminergic and cholinergic aspiny GABAergic striatal interneurons are enriched in GluR1 and/or GluR4 (Tallaksen-Greene & Albin, 1994, Bernard et al., 1996, Chen et al., 1996, Chen et al., 1998, Paquet & Smith, 1996, Kwok et al., 1997, Stefani et al., 1998, Deng et al., 2007). The differential expression of AMPA-type receptor subunits in projection neurons and interneurons may explain differences among these neuron types in their AMPA-mediated responses to glutamate or cortical excitation (Götz et al., 1997, Calabresi et al., 1998, Stefani et al., 1998, Vorobjev et al., 2000).

AMPA receptors have been identified in monkey (Martin et al., 1993a) and human basal ganglia (Meng et al., 1997, Tomiyama et al., 1997) by in situ hybridization histochemistry and immunohistochemistry, but detailed information on the types of neurons possessing the different AMPA subunits in monkey basal ganglia is not available. We thus used immunohistochemistry to characterize the size, shape and abundance of perikarya possessing GluR1-4 AMPA subunits in the striatum of rhesus monkey. Data on the size, shape and abundance of the various striatal neuron types allowed us to use AMPA subunit localization to clarify the AMPA subunits on specific basal ganglia neuron types.