Triheteromeric NR1/NR2A/NR2B Receptors Constitute the Major N-Methyl-d-aspartate Receptor Population in Adult Hippocampal Synapses

Abstract

NMDA receptors (NMDARs), fundamental to learning and memory and implicated in certain neurological disorders, are heterotetrameric complexes composed of two NR1 and two NR2 subunits. The function of synaptic NMDARs in postnatal principal forebrain neurons is typically attributed to diheteromeric NR1/NR2A and NR1/NR2B receptors, despite compelling evidence for triheteromeric NR1/NR2A/NR2B receptors. In synapses, the properties of triheteromeric NMDARs could thus far not be distinguished from those of mixtures of diheteromeric NMDARs. To find a signature of NR1/NR2A/NR2B receptors, we have employed two gene-targeted mouse lines, expressing either NR1/NR2A or NR1/NR2B receptors without NR1/NR2A/ NR2B receptors, and compared their synaptic properties with those of wild type. In acute hippocampal slices of mutants older than 4 weeks we found a distinct voltage dependence of NMDA R-mediated excitatory postsynaptic current (NMDA EPSC) decay time for the two diheteromeric NMDARs. In wild-type mice, NMDA EPSCs unveiled the NR1/NR2A characteristic for this voltage-dependent deactivation exclusively, indicating that the contribution of NR1/NR2B receptors to evoked NMDA EPSCs is negligible in adult CA3-to-CA1 synapses. The presence of NR1/NR2A/NR2B receptors was obvious from properties that could not be explained by a mixture of diheteromeric NR1/NR2A and NR1/NR2B receptors or by the presence of NR1/NR2A receptors alone. The decay time for NMDA EPSCs in wild type was slower than that for NR1/NR2A receptors, and the sensitivity of NMDA EPSCs to NR2B-directed NMDAR antagonists was 50%. Thus, NR2B is prominent in adult hippocampal synapses as an integral part of NR1/NR2A/NR2B receptors.Among ionotropic glutamate receptors, the function of NMDA receptors (NMDARs) 2 is unique because of the requirement of simultaneous agonist/co-agonist binding and because of a highly Ca 2ϩ -permeable pore, which is controlled in a voltage-dependent manner. The voltage-dependent Mg 2ϩ block enables NMDARs to detect presynaptic glutamate release and postsynaptic depolarization through somatic or dendritic spikes. NMDARs are therefore coincidence detectors of preand postsynaptic activity. Their function is related to the induction of synaptic plasticity, various forms of learning and memory, and several pathophysiological processes (1-3).Two glycine/serine-binding NR1 (GluN1) and two glutamate-binding NR2 (GluN2) subunits presumably form heterotetrameric NR1/NR2 complexes in which the NR1 subunits assemble diagonally to each other (4, 5). NR1 shows continuous and ubiquitous expression in the brain, whereas expression of the four NR2 subunit genes, NR2A-D, is temporally and spatially regulated (6, 7). The expression of NR2A and NR2C starts after birth in the forebrain and cerebellum, respectively, whereas NR2B and NR2D are present already during embryonic development. In the hippocampus, NR2C immunoreactivity cannot be detected throughout development (6,8), and NR2D expression decreases postnatally (6) and appears to be confined to extrasynaptic ...

Synaptic NMDA receptors in basolateral amygdala principal neurons are triheteromeric proteins: physiological role of GluN2B subunits

“…The most parsimonious explanation of these data is that postsynaptic receptors at BLA excitatory synapses are largely GluN1/GluN2A/GluN2B heterotrimers. Interestingly, a similar conclusion has recently been reached for excitatory synapses on mature hippocampal pyramidal neurons (Rauner and Kohr 2011), where the properties of NMDA receptor-mediated EPSCs share the same kinetic and pharmacological properties as those in BLA neurons .…”

“…Both GluN2A and GluN2B subunits are present in the basolateral amygdala (BLA) (Farb and LeDoux 1997;Lopez de Armentia and Sah 2003), and pharmacological studies have suggested that amygdala-dependent learning requires the activation of GluN1/GluN2B heterodimers (Rodrigues et al 2001;Walker and Davis 2008). However, although GluN2B subunits are present at synapses, whether they assemble as heterodimers or heterotrimers is not clear Rauner and Kohr 2011). In this study, we investigated the subunit composition of synaptic NMDA receptors at glutamatergic synapses on principal neurons in the BLA.…”

Structural basis of subunit selectivity for competitive NMDA receptor antagonists with preference for GluN2A over GluN2B subunits

“…By contrast, some highly subunit-selective negative allosteric modulators are less effective inhibitors of triheteromeric GluN1/2A/2B receptors and therefore primarily inhibit either GluN1/2A or GluN1/2B receptors [e.g., TCN-201 or ifenprodil, respectively (20,45)]. Both subunit selectivity and mechanism of action are therefore important considerations for the application of a pharmacological tool compound in neurophysiological studies, because many central synapses in the adult hippocampus and cortex contain a mixed population of GluN1/2A, GluN1/2B, and triheteromeric GluN1/2A/2B receptors (31).…”

“…3C). However, a competitive antagonist with 100-fold selectivity for GluN1/2A over GluN1/2B is still expected to produce robust inhibition of triheteromeric GluN1/2A/2B receptors, which are the primary GluN2B-containing NMDA receptors in the adult hippocampus and cortex (31)(32)(33).…”

NMDA Receptor as a Newly Identified Member of the Metabotropic Glutamate Receptor Family: Clinical Implications for Neurodegenerative Diseases

“…Among these, NMDAR has been the center of attention predominantly due to its unique features: 1) it allows Ca 2+ influx upon activation, in addition to other cation-mediated currents, and 2) it requires additional conditions for activation (specifically, the Mg 2+ block has to be released for opening) (MacDermott et al, 1986;McBain and Mayer, 1994;Regehr and Tank, 1990). The combination of 2 duplicated subunits (generally, GluN1 subunits with co-activator, glycine binding sites, and regulatory GluN2 subunits with glutamate binding sites) consist of NMDARs, and the subunit composition (particularly among GluN2 isoforms) is known to determine ion conductance through the receptor and be developmentally regulated (Al-Hallaq et al, 2007;Collingridge et al, 2009;Monyer et al, 1994;Rauner and Kohr, 2011;Sheng et al, 1994;Stocca and Vicini, 1998). The mature human brain is known to express mainly GluN1 and GluN2A (Petralia et al, 1994).…”