Subcellular distribution of AMPA and NMDA receptor subunit immunoreactivity in ventral posterior and reticular nuclei of rat and cat thalamus

Abstract

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) selective glutamate receptors mediate excitatory neurotransmission in the somatosensory thalamus, but morphological localization of the receptors at identified thalamic synapses has been lacking. The authors used electron microscopic immunocytochemistry to localize AMPA selective GluR 2/3 subunits (GluR2/3) and NMDA receptor subunit 1 (NMDAR1) in rat and cat ventral posterior lateral nucleus (VPL) and in the associated sector of the reticular nucleus (RTN). Light microscopy showed that GluR2/3 and NMDAR1 immunolabeled neurons are homogeneously distributed in both nuclei. The relationship between glutamate receptor labeled profiles and glutamate or gamma-aminobutyric acid (GABA) labeled synapses was revealed by combining preembedding and postembedding immunostaining at the electron microscopic level. GluR2/3 and NMDAR1 immunoreactivity was located in somata and in proximal and distal dendrites of VPL relay cells and of RTN cells. Immunoreactivity was concentrated in postsynaptic densities of glutamatergic synapses and absent from postsynaptic densities of GABAergic synapses. In the cat, GluR2/3 and NMDAR1 immunoreactivity was also localized in GABAergic interneurons, including their presynaptic dendrites (PSD). Of the GluR2/3 and NMDAR1 labeled thalamic synapses observed, 10-29% were lemniscal (RL) type synapses in VPL; 60-70% were corticothalamic (RS) type synapses in the VPL and RTN. In the cat, 7-19% were identified as PSD profiles, and more NMDAR1 labeled PSD were found in the VPL than in the RTN. The main findings were as follows: 1) AMPA selective GluR2/3 and NMDAR1 share similar distribution patterns in the rat and cat somatosensory thalamus, 2) both glutamate receptors are likely to be colocalized at postsynaptic densities of both RL and RS synapses, and 3) localization of the glutamate receptor proteins in GABAergic dendrites in the cat thalamus indicates that glutamatergic transmission to GABAergic neurons is also mediated by both NMDA and non-NMDA receptors.

Glutamate receptor functions in sensory relay in the thalamus

“…(Abbreviations used: APDC, 2R,4R-4-aminopyrrolidine-2,4-decarboxylate; CCG-1, (2S,19S,29S)-2-(2-carboxycyclopropyl) glycine; CHPG, 2-chloro-5-hydroxyphenylglycine; 4CPG, (S)-4-carboxyphenylglycine; CPPG, a-cyclopropyl-4-phosphonophenylglycine; DHPG, 3,5-dihydroxyphenylglycine; l-AP4, l-2-amino-4-phosphonobutyric acid; l-SOP, l-serine-O-phosphate; LY341495, 2S-2-amino-2 (1S,2S-2-carboxcyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid; LY354740, (1) of NMDA receptors and non-NMDA ionotropic receptors (Salt 1986;Crunelli et al 1987;Sillito et al 1990;Salt & Eaton 1991;, and it has become clear that this re ects a fast EPSP/C component mediated by AMPA receptors in conjunction with an NMDAreceptor-mediated synaptic component (Paulsen & Heggelund 1994;Turner et al 1994;Turner & Salt 1998;Kielland & Heggelund 2001). These in vivo and in vitro electrophysiological ndings are supported by ultrastructural evidence showing the localization of NMDA and AMPA receptor subunits postsynaptic to sensory terminals (Liu 1997;Mineff & Weinberg 2000). There seems to be little evidence for a contribution to sensory responses of additional glutamate receptors from in vitro physiological studies, even though experiments have been designed that have attempted to reveal roles for, for example, metabotropic glutamate receptors (Turner & Salt 1998).…”

“…This offers a multitude of potential signalling and modulatory possibilities for synaptically and extrasynaptically released glutamate (Kullmann 2000). Within the thalamus the distribution of a considerable number of the various glutamate receptors has been described in some detail (Martin et al 1992;Petralia et al 1996;Godwin et al 1996;Liu 1997;Jones et al 1998;Liu et al 1998;Mineff & Valtschanoff 1999;Neto et al 2000;Mineff & Weinberg 2000;Tamaru et al 2001;Bolea et al 2001). This paper seeks to provide a brief overview of some of the well-known synaptic roles of glutamate receptors in the sensory thalamic relay nuclei, and then on the basis of more recent work to indicate other more speculative synaptic roles and how this might relate to sensory transmission through the thalamus under physiological conditions.…”

Differential Effects of NMDA and AMPA Glutamate Receptors on Functional Magnetic Resonance Imaging Signals and Evoked Neuronal Activity during Forepaw Stimulation of the Rat

“…A considerable fraction of afferent neurotransmission is mediated by AMPA-R on the thalamic (Schwarz and Block, 1994) as well as on the cortical (Armstrong-James et al, 1993) level. Accordingly, AMPA-Rs are expressed in the ventrolateral posterior nucleus and the reticular nucleus of the thalamus (Liu, 1997;Ozawa et al, 1998;Mineff and Weinberg, 2000), as well as in the somatosensory cortex (Palomero-Gallagher and Zilles, 2004). It follows that any global blockade of AMPA-R will disrupt the thalamocortical input so that all neural responses (fast and slow) will be decreased in the somatosensory cortex.…”

“…Another possibility would be that AMPA-R blockade indirectly inhibited NMDA-Rmediated neurotransmission and subsequently the NMDA-Rmediated cerebrovascular response as described above. AMPA-Rs and NMDA-Rs are frequently colocalized in central postsynaptic membranes (Kharazia et al, 1996;Takumi et al, 1999) and in the thalamus (Liu, 1997). One unique feature of the NMDA-Rs compared with other ligand-gated ion channels is the dual dependence of function on agonist binding and membrane potential.…”

Differences in quantal amplitude reflect GluR4- subunit number at corticothalamic synapses on two populations of thalamic neurons

“…Corticothalamic terminals in RTN and VP were identified morphologically by using established criteria (16). Although called ST synapses in RTN and RS synapses in VP, they have identical characteristics: small size (0.2-0.5 m), spherical synaptic vesicles, few mitochondria, and asymmetrical membrane contacts.…”