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1Kinsmen Laboratory, Department of Psychiatry, 2Brain Research Centre, 3Centre for Molecular Medicine and Therapeutics and Department of Medical Genetics, 4Division of Neurology, Department of Medicine, University of British Columbia, Vancouver BC V6T 1Z3, Canada
Submitted 26 March 2004; accepted in final form 3 June 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate, and N-methyl-D-aspartate (NMDA). Glutamate receptor activation is necessary for normal sensorimotor control, as well as synaptogenesis and synaptic plasticity (Bliss and Collingridge 1993
; Dingledine et al. 1999), but excessive activity of these receptors can contribute to neuronal death in a variety of neuropathological processes, including ischemia, seizures, and neurodegenerative diseases such as amyotrophic lateral sclerosis, Parkinson disease and Huntington disease (DiFiglia 1990; Kochhar et al. 1988; Park et al. 1988; Rothstein et al. 1990, 1992; Simon et al. 1984; Srivastava et al. 1993; Turski et al. 1991). The NMDA-type glutamate receptor (NMDAR) is thought to play the critical role in induction of synaptic plasticity as well as cell death because of its voltage-dependent magnesium block, high calcium permeability, and slow deactivation and desensitization (Bliss and Collingridge 1993; Dingledine et al. 1999). NMDARs are composed of NR1 subunits together with NR2A, NR2B, NR2C, and/or NR2D. The NR2 subunit determines receptor-channel properties such as open probability, gating kinetics, Mg2+ block, and sensitivity to agonists and antagonists as well as interaction with signaling proteins (Chen et al. 1999a; Dingledine et al. 1999; Sans et al. 1999) so that excitotoxic potential may vary among NMDAR subtypes.
Huntington disease (HD) is an autosomal dominantly inherited neurodegenerative disease characterized by an extrapyramidal movement disorder, cognitive dysfunction, and behavioral changes (Kremer et al. 1992). Expansion of a CAG repeat to >35 in the HD gene, resulting in an expanded polyglutamine tract near the N terminus of the protein huntingtin (Huntington's Disease Collaborative Research Group 1993), causes selective degeneration of predominantly striatal projection neurons, and to a lesser extent, pyramidal neurons in the cerebral cortex, CA1 region of hippocampus, and certain neurons in other regions of the subcortex (Vonsattel and DiFiglia 1998; Vonsattel et al. 1985). Huntingtin is cleaved by a variety of proteases (Gafni et al. 2004; Hermel et al. 2004 and references therein), and the resulting N-terminal fragments containing the polyglutamine tract aggregate in the cytoplasm and nucleus of neurons where they may cause proteasome dysfunction and transcriptional dysregulation (Cha 2000; Goellner and Rechsteiner 2003). However, it is not clear how these processes can explain selective neurodegeneration.
Among the earliest changes associated with expression of mutant huntingtin in neurons are increased NMDAR-mediated current, disrupted calcium homeostasis, and enhanced vulnerability to NMDA-induced excitotoxicity (Hodgson et al. 1999; Panov et al. 2002; Zeron et al. 2002, 2004). Although huntingtin and NMDARs are expressed in most neurons of the brain, evidence suggests that these early changes are restricted to striatal projection neurons and may contribute to initial stages of neuronal dysfunction and/or trigger downstream pathways that result in late-stage cell death (Zeron et al. 2002, 2004).NR2B-type NMDARs are enriched in adult striatal projection neurons relative to other brain regions (Christie et al. 2000; Landwehrmeyer et al. 1995; Li L et al. 2003), and NR2B expression is particularly high in the subpopulation of striatal projection neurons that degenerate first in HD (Kuppenbender et al. 2000). Notably, expression of mutant huntingtin has been shown to enhance currents and excitotoxicity mediated by predominantly NR2B-type NMDARs (Chen et al. 1999b; Zeron et al. 2001, 2002), leading us to propose that a functional interaction between NR2B and mutant huntingtin may contribute to selective neuronal dysfunction and degeneration.
Most studies to date have reported that mutant huntingtin enhances current, calcium responses, and excitotoxicity induced by whole cell application of NMDA, reflecting NMDARs that either are purely nonsynaptic or else represent a pool of synaptic and nonsynaptic receptors (Cepeda et al. 2001; Chen et al. 1999b; Levine et al. 1999; Zeron et al. 2002, 2004). Those results are consistent with evidence indicating that NR1/NR2B-type receptors predominate at extrasynaptic sites, whereas most synaptic NMDARs are thought to contain NR2A (Li et al. 1998; Stocca and Vicini 1998; Tovar and Westbrook 1999). However, our recent data suggest that the amplitude of evoked NMDAR-mediated excitatory postsynoptic currents (EPSCs) is also increased at corticostriatal synapses in acute slices from the YAC72 HD mouse model compared with wild-type (FVB/N) littermates (Li L et al. 2003). Here, we present data to show that this effect is postsynaptic and that synaptic NMDARs found on medium spiny striatal neurons in our FVB/N mice are most likely predominantly triheteromers of NR1, NR2B, and NR2A. These results support the hypothesis that mutant huntingtin interacts with NR2B-containing NMDARs, which may help explain the selective degeneration of predominantly striatal projection neurons.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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FVB/N mice that were wild type (WT) or transgenic for the full-length human HD gene with 72 CAG repeats, introduced using the yeast artificial chromosome [YAC72, line 2511 (Hodgson et al. 1999)], were bred and maintained according to regulations of the Canadian Council on Animal Care. YAC72 mice used in these experiments were the progeny of homozygous YAC72 breeding pairs. Mice aged 21–31 days were used for all experiments except where mentioned. Acute corticostriatal slices were made as previously described (Joshi and Andrew 2001; Umemiya and Raymond 1997). Briefly, mice were anesthetized with halothane and decapitated, and the brain was rapidly removed into ice-cold, oxygenated (95%O2-5%CO2) artificial cerebrospinal fluid (ACSF) that contained (in mM) 125 NaCl, 2.5 KCl, 25 NaHCO3, 1.25 NaH2PO4, 1 MgCl2, 2 CaCl2, and 25 glucose, titrated to pH 7.3–7.4 with osmolality of 300–320 mosM. Coronal slices of 300-µm thickness were cut using a vibratome (Leica VT1000) and placed in oxygenated ACSF at room temperature for 
1 h before recording.
Electrophysiology
Slices were transferred into a recording chamber on the stage of a differential interference contrast microscope (Axioskop, Zeiss). The chamber was continuously perfused with oxygenated ACSF at room temperature at a rate of 2 ml/min. The whole cell patch-clamp technique was used to record from single striatal medium-sized spiny neurons (MSNs). MSNs were visualized in situ using a water-immersion objective lens (ACHROPLAN x 40 x) and were identified by location in the caudate or putamen nuclei, as well as by shape and size (ovoid cell body with 8- to 14-µm major axis) as described previously (Umemiya and Raymond 1997). Recording electrode resistance was 3–5 M
when filled with solution containing (in mM) 130 Cs-methanesulfonate, 5 CsCl, 4 NaCl, 1 MgCl2, 5 EGTA, 10 HEPES, 5 lidocaine N-ethyl chloride, 0.5 GTP, 10 Na2-phosphocreatine, and 5 MgATP, titrated to pH 7.3 with CsOH and the osmolality was 280–290 mosM. The access resistance was <15 M and was compensated 
70%.
AMPAR- or NMDAR-mediated synaptic currents were evoked by local electrical stimulation of afferent cortical fibers using a glass pipette filled with ACSF (tip resistance: 5 M) that was placed 100–150 µm toward the cortex from the MSN recording electrode. The stimulating electrode was used to deliver current pulses (200 µs) at stimulation strengths ranging from 0.05 to 1.0 mA, and stimulation was delivered at intervals 1 min. In paired-pulse stimulation experiments, 0.5-mA stimuli were delivered at interstimulus intervals of 50, 100, 150, and 200 ms. Recordings were made in the presence of picrotoxin (50 µM) to block activation of 
-amino-butyric acid-A receptors. AMPAR- and NMDAR-mediated synaptic currents were recorded at holding potentials of –70 m and +60 mV, respectively. For the latter, 10 µM 6-cyano-7-nitrogui noxalene-2,3-dione (CNQX) was added to the external recording solution to block the AMPAR-mediated component. Data were acquired with a MultiClamp 700A. The CLAMPEX 8.1 software package interfaced to a Digidata 1322A acquisition board (Axon Instuments), and signals were filtered at 2 kHz and digitized at 10 kHz.
Drugs were applied by dissolving them to the desired final concentration in ACSF and by switching the perfusion from control ACSF to drug containing ACSF. All drugs were applied for 6–10 min unless otherwise indicated. Ifenprodil, Conantokin G (Con G), glycine, CNQX, picrotoxin, and APV were purchased from Sigma. Stock solutions of ifenprodil and picrotoxin were prepared in ethanol, and diluted 1,000-fold into the recording solution. The stock solution of CNQX was prepared in DMSO, and the final concentration of this solvent was <0.03%. All other drugs were dissolved in water. Stock solutions were stored at –20°C until just before use.
Results were calculated as means ± SE values. Data were tested for significance using a Student's t-test (significance was set at P < 0.05). The decay time constants were fit with a double-exponential equation: I(t) = If x exp(–t/
f) + Is x exp(–t/s), where If and Is are the amplittudes of the fast component and slow component, respectively, and f and s are the fast and slow time constants, respectively. Weighted time constants were calculated using the equation: w = [If/(If+ Is)] x f + [Is/(If + Is)] x s (Stocca and Vicini 1998).
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RESULTS |
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Previously we have shown that NMDAR-mediated EPSCs in MSNs recorded from corticostriatal slices in 100 µM glycine were significantly increased in YAC72 mice compared with WT mice (Li L et al. 2003). However, in those experiments the peak amplitude of NMDAR-mediated EPSCs recorded at +60 mV was estimated by measuring the current amplitude 75 ms after initiation of the EPSC to minimize the contribution from AMPARs. To determine the NMDAR EPSC peak amplitude more accurately, we recorded evoked EPSCs at +60 mV in the presence of the AMPAR competitive antagonist CNQX (10 µM). As well, to maximize the contribution from NR2B-containing NMDARs, we recorded EPSCs in 0.3 µM glycine because the EC50 for glycine is close to this value for the NR2B subtype, whereas it is in the range of 1–3 µM for NR1/NR2A (Kew et al. 1998; Priestley et al. 1995; Wafford et al. 1995). As shown in Fig. 1A, NMDAR-mediated EPSCs recorded from MSNs showed significantly increased mean peak amplitude in corticostriatal slices from YAC72 compared with WT mice. Mean cell capacitance was not significantly different for the two genotypes (3.5 ± 0.2 pF, n = 17 for YAC72 MSNs, and 4.1 ± 0.2 pF, n = 11 for WT MSNs; P > 0.05, unpaired t-test), and mean peak NMDAR-mediated EPSC density in response to 0.5-mA stimulatation strength was significantly larger for YAC72 than WT MSNs (134 ± 15 pA/pF vs. 90 ± 12 pA/pF; P < 0.05 by unpaired t-test). In contrast, the mean peak amplitude of AMPAR-mediated EPSCs was not significantly different for MSNs from WT and YAC72 mice at any stimulation strength, although there was a trend toward larger amplitudes in YAC72 mice (Fig. 1B).
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An increase in amplitude of NMDAR-mediated EPSCs could be an artifact of electrode placement, resulting in more effective stimulation of cortical axons; or increased presynaptic release of glutamate because of mutant huntingtin expression, as suggested in another HD mouse model at certain stages (Cepeda et al. 2003). To examine these possibilities, we used two different approaches. First, we calculated the ratio of NMDAR- to AMPAR-mediated EPSC amplitudes for each neuron because a mutant huntingtin-induced increase in presynaptic glutamate release or differences in electrode placement should similarly affect the current amplitudes of both receptor subclasses. (In fact, NMDARs would be less affected than AMPARs by changes in glutamate release because they have a relatively higher affinity for glutamate than AMPARs and would therefore more likely be saturated.) As illustrated in Fig. 2A, the NMDAR/AMPAR peak EPSC ratios from YAC72 mice were significantly increased compared with those from WT mice at all stimulation strengths, ranging from 0.1 to 1 mA. These data suggest that an effect of mutant huntingtin on activity of postsynaptic NMDARs contributes to the enhancement of NMDAR-mediated EPSCs in YAC72 mice.
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). As represented in Fig. 2B, paired-pulse facilitation was observed at interstimulus intervals of 50 and 100 ms, whereas we found paired-pulse depression at interstimulus intervals of 150 and 200 ms, for both YAC72 and WT mice. There was no significant difference between the paired-pulse ratio at any inter-stimulus interval for YAC72 mice compared with WT mice. These data indicate that the presynaptic glutamate release probability is similar at corticostriatal synapses in YAC72 mice and WT mice, consistent with the idea that enhancement of the NMDAR-mediated EPSC amplitude in YAC72 mice was because of increased responsiveness of striatal MSN postsynaptic NMDARs. Effects of NR2B-selective antagonists on striatal NMDAR-mediated synaptic currents
Previous work indicates that predominantly NR1/NR2B-type NMDAR currents are potentiated in acutely dissociated adult striatal MSNs, as well as early postnatal cultured striatal MSNs, from the YAC72 transgenic mouse model of HD (Zeron et al. 2002, 2004) and that current mediated by NR1/NR2A-type NMDARs is unaffected by mutant huntingtin expression in transfected non-neuronal cells (Chen et al. 1999b). Enhancement of NR1/NR2B-type NMDAR-mediated current is associated with a significant increase in NMDA-induced apoptosis in cells co-transfected with mutant huntingtin and in postnatal cultured striatal neurons from YAC72 compared with WT mice (Zeron et al. 2001, 2002). However, these data were obtained by whole cell perfusion of NMDA, so that in neurons a combined population of synaptic and non-synaptic NMDARs was activated. In mature hippocampal and cortical cultured neurons, NR1/NR2B predominates at extrasynaptic sites, while NR2A-containing NMDARs are targeted mainly to synapses and comprise NR1/NR2A or the triheteromeric NR1/NR2A/NR2B (Barria and Malinow 2002; Li et al. 1998; Stocca and Vicini 1998; Tovar and Westbrook 1999), so we wondered whether the striatal synaptic NMDARs in our preparation contained NR2B.
To investigate the subunit composition of NMDARs at corticostriatal synapses in FVB/N mice, we began by testing the effects on NMDAR-mediated EPSCs of two NR2B selective antagonists—ifenprodil and Con G. Ifenprodil is a 400-fold more potent antagonist of NR1/NR2B than NR1/NR2A, inhibiting 70–80% of the peak NR1/NR2B current at concentrations of 3–10 µM (Williams 1993). As shown in Fig. 3A, 8 µM ifenprodil did not affect the NMDAR-mediated EPSC amplitude in MSNs from YAC72 and WT mice at age 21–31 days. However, in newborn mice (P4–P6), 8 µM ifenprodil inhibited the majority of NMDAR-mediated synaptic current for both genotypes (Fig. 3B; representative of n = 2 and 3 different neurons for WT and YAC72, respectively), consistent with the high expression of NR2B and near absence of NR2A during early postnatal development in the forebrain (Monyer et al. 1994). From these data we conclude that striatal NMDARs at mature synapses are unlikely to be composed of the pure NR1/NR2B subtype.
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; Vicini et al. 1998). To determine whether pure NR1/NR2A and/or the triheteromeric NR1/NR2A/NR2B contribute to synaptic currents, we tested the effect of Con G on NMDAR EPSC amplitude from MSNs in YAC72 mice. At concentrations in the low micromolar range, Con G is selective for all NR2B-containing receptors, inhibiting currents mediated by NR1/NR2B as well as NR1/NR2B/NR2A but not affecting NR1/NR2A currents; because Con G is a competitive antagonist, efficacy of block depends on agonist concentration (Donevan and McCabe 2000). As illustrated in Fig. 3C 3 µM Con G applied for 5 min. inhibited the NMDAR EPSC amplitude by 40–50% with almost complete recovery after washing for 4 min (3 of 3 neurons tested showed similar extent of inhibition). These data are consistent with a major contribution from the triheteromeric NR1/NR2A/NR2B to NMDAR-mediated synaptic transmission in striatal MSNs. Glycine sensitivity and decay rate of striatal NMDAR EPSCs consistent with NR2B-containing receptors
Glycine is an essential co-agonist at NMDARs and the glycine-binding site is located on the NR1 subunit (Kuryatov et al. 1994). However, the glycine affinity of NMDARs varies with the NR2 subunit and is 10-fold lower (in the low micromolar range) for NR1/NR2A than for combinations of NR1 with the other NR2 subunits (Dingledine et al. 1999). Therefore we examined the effect of switching glycine concentration from 0.3 to 30 µM on the amplitude of striatal NMDAR-mediated synaptic currents to determine whether NR1/NR2A made an important contribution to synaptic transmission in corticostriatal slices from 21- to 31-day-old mice. In both YAC72 and WT mice, we found a significant, 20%, increase in peak amplitude of the NMDAR EPSC with the 100-fold increase in concentration of glycine (from 0.3 to 30 µM) (Fig. 4A and B). However, this small change in peak amplitude was far less than expected for receptors composed of NR1/NR2A, assuming that the glycine concentration contaminating the recording solution is <1 µM as suggested in previous reports (Johnson and Ascher 1987). The modest glycine effect is consistent with the involvement of NR2B-containing NMDARs that would be close to saturation at glycine concentrations in the high nanomolar range, although accurate measurement of the glycine EC50 of these synaptic NMDARs would require testing of a wider range of glycine concentrations and determination of actual glycine concentration in close proximity to the synaptic NMDARs. As a control, the mean amplitude of AMPAR-mediated EPSCs (774.5 ± 97.9 pA, n = 16 vs. 729.0 ± 97.7 pA, n = 17 in YAC72 mice; P = 0.74; 652.3 ± 75.3, n = 10 vs. 597.6 ± 108.6 pA, n = 13, in WT mice; P = 0.69) was not different when recorded in 0.3 µM glycine versus 30 µM glycine, respectively, in response to stimulation at 0.5 mA. Results for NMDAR EPSC amplitude were similar at other stimulation strengths as well (data not shown).
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; Flint et al. 1997; Vicini et al. 1998). Here we measured the decay time constants of NMDAR EPSCs recorded from striatal MSNs in the corticostriatal slice from WT and YAC72 mice. No significant differences were found in decay time constants between WT and YAC72 mice with 0.3 µM glycine or 30 µM glycine (Table 1). Although decay time constants tended to be slower in 30 µM glycine compared with those measured in 0.3 µM glycine (Table 1), the difference was small, consistent with previous studies in corticostriatal slices (Chapman et al. 2003). The decay time constant (600 ms) of the NMDAR EPSC evoked by 0.5-mA stimulation in both YAC72 and WT mice was dramatically slower than that expected for pure NR1/NR2A, which is in the range of 50–150 ms (Chen et al. 1999a; Flint et al. 1997; Vicini et al. 1998). Moreover, the decay time constant of the NMDAR EPSC evoked by 0.5-mA stimulation recorded from P4 to P6 animals, which should reflect predominantly NR1/NR2B-type NMDARs, was 587 ± 44 ms (n = 5), not significantly different from that found in older animals (see Table 1). Taken together with results of pharmacological experiments, these data provide evidence to support the idea that NR1/NR2A/NR2B predominates at striatal MSN synapses with cortical afferents, and receptors containing NR2B are therefore potentiated in YAC72 mice.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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). The data presented here clearly indicate that postsynaptic NMDARs on dendritic spines of striatal MSNs are potentiated while synaptic AMPAR currents are not significantly altered in this mouse model and that presynaptic glutamate release is similar at corticostriatal synapses in YAC72 and WT mice at this early stage, well before symptoms develop (Hodgson et al. 1999). However, we have not excluded the possibility that changes in presynaptic glutamate release may occur just prior to or concurrent with clinical manifestations of disease as suggested in other mice models of HD (Cepeda et al. 2003; Li H et al. 2003).
In previous studies, the magnitude of enhancement of whole cell NMDA-evoked current in striatal neurons from YAC72 mice compared with WT mice was in the range of 30–60% (Cepeda et al. 2001; Zeron 2002, 2004). At near maximal stimulation strengths, for which a large proportion of available synapses were activated and mean NMDAR peak currents recorded at the soma were in the range of 300–500 pA, the NMDAR EPSC amplitude was 30–40% larger for YAC72 mice. The largest increase in evoked NMDAR EPSC amplitude (77%) for YAC72 compared with WT mice was observed for the minimal stimulation required to produce a reliable response (0.1 mA). At this minimal stimulation level, only a small proportion of the total synapses available would have been activated, and NMDAR-mediated current amplitudes were quite small (mean was 106 and 188 pA for WT and YAC72, respectively). It is likely that this latter value (77% increase) is more accurate because as more synapses are activated and the NMDAR currents become larger, the true peak current amplitude is no longer being measured accurately due to inability to adequately voltage clamp the dendrites, resulting in shunting of the rapid current changes.
Recent studies suggest that an altered interaction between the NMDAR-associated scaffolding protein PSD-95 and huntingtin containing the polyglutamine expansion results in increased targeting of PSD-95 to the plasma membrane, association with NMDARs, and cellular sensitivity to NMDA-induced excitotoxicity (Song et al. 2003; Sun et al. 2001). Although our data demonstrate increased postsynaptic NMDAR current, independent of any alteration in glutamate release probability, the mechanism remains unclear. Increased evoked current could be a result of larger numbers of synapses, increased numbers of receptors per synapse, or enhanced function of individual receptors due to increased open channel probability or single-channel conductance. Previous work from our lab suggests that mutant huntingtin does not alter NMDAR channel open probability or single-channel conductance (Chen et al. 1999b), and we have data to indicate increased NMDAR surface expression in cultured striatal MSNs from YAC72 compared with WT mice (M. Fan, L. Zhang, and L. Raymond, unpublished data). PSD-95 has not been shown to alter directly NMDAR surface expression, although recent studies indicate it may alter channel gating (Li B et al. 2003; Rutter et al. 2002). Notably, the increase in NMDAR current found for synaptic receptors is similar to the enhancement previously reported for non-synaptic and recombinant NMDARs co-expressed with mutant huntingtin in the absence of PSD-95 (Cepeda et al. 2001; Chen et al. 1999b; Zeron et al. 2002, 2004). Therefore the effect we found for synaptic NMDAR current in YAC72 striatal MSNs may be independent of PSD-95. Instead, surface expression of NMDARs or activity of individual receptors might be upregulated by other mechanisms, such as serine or tyrosine phosphorylation, or altered interaction with the cytoskeleton and/or proteins involved in regulating receptor trafficking. Further experiments are required to fully investigate the mechanisms underlying enhanced synaptic NMDAR current in YAC72 striatal MSNs.
Our results suggest that in the mature FVB/N mouse brain, postsynaptic NMDARs in the synapse between cortical glutamatergic afferents and striatal MSNs contain NR2B with a majority of NR1/NR2A/NR2B NMDARs. In support of this idea, our data demonstrate that synaptic NMDAR current in mature striatal MSNs is insensitive to inhibition by 8 µM ifenprodil but shows 50% inhibition by 3 µM Con G. Ifenprodil and its derivatives block 80% of current mediated by NR1/NR2B, but almost none by NR1/NR2A/NR2B or NR1/NR2A, in transfected non-neuronal cells (Brimecombe et al. 1997; Vicini et al. 1998). On the other hand, Con G is a competitive antagonist at the NMDA/glutamate site for recombinant NMDARs containing NR2B (IC50 <1 µM for NR1/NR2B and 5 µM for NR1/NR2A/NR2B) but does not inhibit NR1-1a/NR2A (Donevan and McCabe 2000; Klein et al. 2001), and NR1-1a is the major splice variant expressed in striatal GABAergic projection neurons (Kuppenbender et al. 1999). Moreover, we found that the NMDAR EPSC amplitude is near maximal in 0.3 µM glycine, whereas NR1/NR2A current is less than half-maximal in this glycine concentration (Kew et al. 1998; Priestley et al. 1995). Finally, the NMDAR EPSC shows a decay time constant that is not significantly different from that found for NR1/NR2B, consistent with results for NR1-1a/NR2A/NR2B in transfected non-neuronal cells (Vicini et al. 1998). Our results, using a combination of electrophysiological and pharmacological approaches, are in general agreement with biochemical evidence from rat striatum suggesting that the majority of synaptic NMDARs are composed of NR1/NR2A/NR2B (Dunah and Standaert 2003).
Although our data strongly suggest a predominantly triheteromeric composition of striatal MSN synaptic NMDARs, it must be noted that a number of variables other than subunit composition can contribute to altering NMDAR gating properties and sensitivity to agonists/antagonists. Therefore it can be difficult to make definite conclusions about subunit composition by assessing synaptic NMDAR pharmacology, glycine sensitivity, or decay time kinetics and then making comparisons with data from recordings of recombinant NMDARs in non-neuronal cells. For example, the properties of triheteromeric NMDAR complexes are not well defined and are difficult to study in heterologous systems transfected with NR1, NR2A, and NR2B because whole cell current may represent mixed populations of di- and tri-heteromeric NMDARs (Dingledine et al. 1999). However, more precise information can be gleaned from studying recombinant NMDARs at the single-channel level, where one study indicated that NR1/NR2A/NR2B is insensitive to NR2B-selective antagonists like ifenprodil (Brimecombe et al. 1997), consistent with our data showing lack of NMDAR EPSC inhibition with low micromolar concentrations of ifenprodil in mature striatal MSNs. On the other hand, data from a different study in mature cultured cortical neurons suggest that these triheteromers exhibit high sensitivity to ifenprodil inhibition and low affinity for Gly (Kew et al. 1998), so the precise pharmacology of NR1/NR2A/NR2B-type NMDARs remains unclear. Furthermore, in addition to NR2 subunit composition, the identity of the NR1 splice variant can alter NMDAR decay time kinetics, such that NMDARs containing NR2B and NR1 with the N1-cassette (e.g., NR1-1b) show markedly faster decay time constants while remaining sensitive to low micromolar ifenprodil analogue (Rumbaugh et al. 2000). However, striatal MSNs contain the NR1-1a splice variant and specifically lack any NR1 with the N1-cassette (Kuppenbender et al. 1999), and are also insensitive to ifenprodil, so it is unlikely that receptors composed of NR1-1b combined with NR2B can explain the intermediate decay time constant (faster than for NR1-1a/NR2B but slower than for NR1-1a/NR2A) that we found for synaptic NMDARs. Finally, interactions with PSD-95 can change apparent EC50's for glutamate and glycine (Rutter et al. 2002), as well as desensitization (Li B et al. 2003), likely due to changes in gating, effects that may result from PSD-95's function to co-localize a variety of protein kinases and phosphatases with NMDARs.
Because NR2B expression is highest at birth, whereas NR2A is not expressed until around P7 (Monyer et al. 1994), NMDARs at the earliest synapses in the forebrain are composed of NR1/NR2B, consistent with our data showing robust inhibition of NMDAR EPSCs by ifenprodil in corticostriatal slices from P4 to P6 mice. As NR2A levels increase during neuronal maturation, synaptic NR1/NR2B receptors are replaced by NR2A-containing NMDARs (Barria and Malinow 2002; Li et al. 1998; Philpot et al. 2001; Stocca and Vicini 1998; Tovar and Westbrook 1999), which may be pure NR1/NR2A or the triheteromer NR1/NR2A/NR2B (Luo et al. 1997; Sheng et al. 1994). The timing of this switch in subunit composition is determined, at least in part, by the expression levels of NR2A (Barria and Malinow 2002; Philpot et al. 2001), suggesting that increased or decreased expression levels of NR2A compared with NR2B may be tightly correlated with the proportions of synaptic NR1/NR2A versus NR1/NR2A/NR2B. Previously we have shown that NR2B is enriched relative to NR2A in the FVB/N mouse striatum compared with cortex and hippocampus (Li L et al. 2003), which would be consistent with our current data suggesting that NR1/NR2A/NR2B-type NMDARs predominate at striatal MSN synapses with cortical glutamatergic afferents.
In HD, striatal MSNs in the neostriatum degenerate first in the medial segment, and degeneration progresses toward the lateral aspects of this region (Vonsattel and DiFiglia 1998; Vonsattel et al. 1985). It is therefore interesting that mRNA and binding studies indicate high NR2B expression in MSNs throughout the neostriatum but that NR2A expression increases dramatically in a medial to lateral gradient (Christie et al. 2000; Kuppenbender et al. 2000). Consistent with these latter results, recordings of NMDAR EPSCs in MSNs of the rat neostriatum indicate that NR1/NR2B predominates in the medial aspects and NR2A content increases in the lateral regions at striatal MSN synapses (Chapman et al. 2003). Although we did not attempt to rigorously compare recordings from medial and lateral segments of the putamen, and most of our recordings were made from the lateral aspect, none of the neurons tested in slices from either mouse genotype showed >10% inhibition of EPSC amplitude by 8 µM ifenprodil, and EPSC decay rate as well as glycine sensitivity showed no apparent regional variability. However, further more careful study is required to accurately determine whether NMDAR subunit composition varies in a medial to lateral gradient in the neostriatum of FVB/N mice.
In summary, we have demonstrated increased amplitude of NMDAR-mediated evoked EPSCs at the corticostriatal-MSN synapse in a mouse model of HD compared with WT littermate controls. The effect is present early in life and is due to potentiation of postsynaptic NR2B-containing NMDAR current. The NR2B to NR2A ratio is higher in the striatum than other brain regions, and within the striatum, enkephalin-containing MSNs are the most vulnerable to degeneration in HD and also have the highest NR2B:NR2A ratios (Kuppenbender et al. 2000). Therefore our results may help explain the selective neuronal degeneration found in HD, and suggest that therapeutic trials of NR2B-selective antagonists in the presymptomatic or very early stages of HD may be an effective strategy.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint reprint requests and other correspondence: L. A. Raymond, Dept. of Psychiatry, University of British Columbia, 4N3-2255 Wesbrook Mall, Vancouver, BC V6T 1Z3, Canada (E-mail: lynnr{at}interchange.ubc.ca).
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REFERENCES |
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