Pharmacological isolation of postsynaptic currents mediated by NR2A- and NR2B-containing NMDA receptors in the anterior cingulate cortex
© Wu et al; licensee BioMed Central Ltd. 2007
Received: 28 March 2007
Accepted: 30 April 2007
Published: 30 April 2007
NMDA receptors (NMDARs) are involved in excitatory synaptic transmission and plasticity associated with a variety of brain functions, from memory formation to chronic pain. Subunit-selective antagonists for NMDARs provide powerful tools to dissect NMDAR functions in neuronal activities. Recently developed antagonist for NR2A-containing receptors, NVP-AAM007, triggered debates on its selectivity and involvement of the NMDAR subunits in bi-directional synaptic plasticity. Here, we re-examined the pharmacological properties of NMDARs in the anterior cingulate cortex (ACC) using NVP-AAM007 as well as ifenprodil, a selective antagonist for NR2B-containing NMDARs. By alternating sequence of drug application and examining different concentrations of NVP-AAM007, we found that the presence of NVP-AAM007 did not significantly affect the effect of ifenprodil on NMDAR-mediated EPSCs. These results suggest that NVP-AAM007 shows great preference for NR2A subunit and could be used as a selective antagonist for NR2A-containing NMDARs in the ACC.
NMDA receptors (NMDARs) have pivotal roles in excitatory synaptic transmission and plasticity, and in various brain processes from memory formation to chronic pain [1, 2]. NMDARs are tetrameric complexes, which contain two NR1 and two NR2 subunits (NR2A-D). The type of NR2 subunits determines not only gating properties but also signaling pathways of NMDARs [3, 4]. Therefore, different subunit compositions confer NMDARs distinct roles in the regulation of neuronal functions. In consistence with this notion, NMDARs could undergo subunit-specific regulations under physiological or pathological conditions. For example, NR2A subunit gradually replaces NR2B in most brain areas during postnatal development , while NR2B but not NR2A is up-regulated in the anterior cingulate cortex (ACC) after peripheral inflammation .
Considering the distinct roles of NMDARs, dissection of their subtype-selective functions will promote our understanding of molecular mechanisms underlying physiological and pathological processes, such as memory and pain. Although pharmacological tools are powerful, subtype-selective antagonists for NMDARs are not well developed . Most selective antagonists are ifenprodil and its derivatives (e.g. Ro25-6981), which are more than 200-fold preference for NR1/NR2B than for NR1/NR2A [8, 9]. A relatively selective NR1/NR2A antagonist, NVP-AAM077 (NVP) was developed recently and found to have more than 100-fold preferential blockade of NR1/NR2A vs NR1/NR2B . Using these antagonists, recent studies have shown that NR2A-containg NMDARs are required for LTP, whereas NR2B NMDARs are required for LTD [11, 12]. However, the concept of subtype-dependent LTP and LTD was questioned by other studies that reported the lack of NMDA subtype receptor selectivity for bi-directional synaptic plasticity [13–18]. Moreover, some of these studies also argued that NVP is not sufficient to discriminate between NR2A- and NR2B-containing NMDARs, with less than 10-fold selectivity [19, 20].
NR2A and NR2B are highly expressed in the ACC, a forebrain area involved in emotion, memory and pain [21, 22]. Our recent results indicate that both NR2A and NR2B are required for the induction of cingulate LTP and LTD [17, 18]. Since previous debates of antagonist selectivity are based on results mostly obtained from hippocampal neurons and transfected cells, we wanted to re-examine the pharmacological properties of NMDARs with NVP and ifenprodil in the ACC. By testing antagonist effects with different application sequences and concentrations, we found that NVP at concentration of 0.4 μM and 0.1 μM is likely to be relatively selective for NR2A-containing NMDARs in ACC neurons.
Materials and methods
All adult C57BL/6 mice were purchased from Charles River and were maintained on a 12 h light/dark cycle with food and water provided ad libitum. The Animal Studies Committee at the University of Toronto approved all experimental protocols. Coronal brain slices (300 μm) containing the ACC from six- to eight-week-old C57BL/6 male mice were prepared using standard methods . Slices were transferred to a submerged recovery chamber with oxygenated (95 % O2 and 5 % CO2) artificial cerebrospinal fluid (ACSF) containing (in mM: 124 NaCl, 2.5 KCl, 2 CaCl2, 2 MgSO4, 25 NaHCO3, 1 NaH2PO4, 10 glucose) at room temperature for at least 1 h.
Experiments were performed in a recording chamber on the stage of an Olympus BX51WI microscope (Tokyo, Japan) with infrared DIC optics for visualization of whole-cell patch clamp recording. Excitatory postsynaptic currents (EPSCs) were recorded from pyramidal neurons in layer II/III of the ACC with an Axon 200B amplifier (Molecular devices, CA) and the stimulations were delivered by a bipolar tungsten stimulating electrode placed in layer V. The recording pipettes (3–5 MΩ) were filled with the solution containing (mM): 145 CsMeSO3, 5 NaCl, 1 MgCl2, 0.2 EGTA, 10 HEPES, 2 Mg-ATP, 10 phosphocreatine, 0.1 Na3-GTP, 5 QX-314 (adjusted to pH 7.2 with CsOH). NMDA receptor-mediated EPSCs (NMDA EPSCs) were pharmacologically isolated in ACSF containing CNQX (20 μM), and picrotoxin (100 μM). Neurons were voltage clamped at -30 mV and NMDA EPSCs were evoked at 0.05 Hz. To study NVP- or ifenprodil-sensitive component in total NMDA EPSCs, NVP or ifenprodil was bath-applied for 10 min after obtaining stable baseline for 5 min. The NVP- or ifenprodil-sensitive component was calculated as the reduction of current in the corresponding drugs at the last one minute. NMDA EPSCs were fitted with single exponential function and decay time constant reflects the time decaying to 37% of NMDA EPSCs. Access resistance was 15–30 MΩ and was monitored throughout the experiment. All antagonists were applied through the perfusion solution. Data were discarded if access resistance changed more than 15% during an experiment. Results were analyzed by t-test to identify significant differences. All data are expressed as mean S.E.M. In all cases, P < 0.05 was considered statistically significant.
Results and discussion
We also examined the decay time constant of ifenprodil-sensitive and NVP-sensitive NMDA EPSCs (Figure 1C). Consistent with our previous reports [6, 18], ifenprodil-sensitive component showed typical slower kinetics (τ = 122.7 ± 8.9 ms) compared with NVP-sensitive NMDA EPSCs (τ = 100.0 ± 10.5 ms, P < 0.05, paired t-test), showing they are preferentially mediated by NR2B- and NR2A containing NMDARs, respectively.
We observed that NMDA EPSCs in different cells have various responses to ifenprodil and NVP. For example, in 17 cells tested, there are two cells showed around 50 % reduction when applied ifenprodil, whereas another 3 cells show less than 10 % of ifenprodil-sensitive component. To exclude the possibility that the insignificance is due to the variations among cells, we re-calculated the data by including the cells showing 10–50% ifenprodil-sensitive component in total NMDA EPSCs. No difference was found for ifenprodil-sensitive responses between the group treated with NVP followed by ifenprodil (18.6 ± 2.4 %, n = 7) and the group treated with ifenprodil followed by NVP (25.8 ± 2.2 %, n = 5, P = 0.08). Likewise, there is no difference in NVP-sensitive component between the two groups (65.4 ± 4.3 % vs 60.2 ± 3.9 %, P = 0.45). Taken together, these results suggest that NVP does not significantly affect NR2B component and could be used as a selective antagonist for NR2A-containing NMDARs in the ACC.
In the presence study, we used pharmacological tools to isolate postsynaptic currents mediated by NR2A- and NR2B-containing NMDARs, respectively. By alternating sequence of drug applications, we found that the presence of NVP did not significantly affect the effect of ifenprodil on NMDA EPSCs, suggesting that NVP is relatively selective for NR2A-containing NMDARs. There are several concerns related to our conclusions. First, it has been reported the regional difference of NMDARs in the ACC and hippocampus, such as NR2A/NR2B ratio, phosphorylated NR2A and NR2B  as well as regulations under chronic pain conditions . Therefore, the current results may not fully applied to other brain areas including hippocampus. Indeed, it has been reported that 0.4 μM NVP could block one third of Ro-6981-sensitive NMDA EPSCs in cultured hippocampal neurons . In the current study, due to the difficulty in washing out the drugs in slices, we cannot use the same way to calculate the possible effect of NVP on ifenprodil-sensitive component; Second, although it is difficult to reconcile the discrepancy for NVP and ifenprodil selectivity in different systems, the various reports may be due to complex subunit composition, spatial distributions of NMDARs, and their related scaffolding molecules [4, 7]. For instance, triheteromeric NMDA receptor, NR1/NR2A/NR2B, has been found in native tissues, particularly in the cortex [5, 25]. Moreover, it was shown that NR1/NR2A/NR2B is also highly sensitive to ifenprodil . Alternatively, it has been proposed that there are potential interactions for NR2A and NR2B , which may also account for the variability. Therefore, first perfusion of NVP or ifenprodil might affect the following effect of ifenprodil or NVP. Third, due to the limitation of conventional whole-cell recording, we observed the rundown of NMDA EPSCs during 25 minutes in some of neurons (3 out 7 neurons). Although the pooled data showed no significance in the reduction of NMDA EPSCs (12.9 ± 6.6 % of control, n = 7, P = 0.13, paired t-test), the result may confound the contribution of NVP- and ifenprodil-sensitive component. Last, we were aware the variability of percentage of NR2B component in ACC neurons. In all tested 29 neurons, we found 4 neurons have much smaller ifenprodil-sensitive current (<10% of total currents, Figure 4). Future experiments (e.g. single cell RT-PCR) are needed to test the possibility that whether these neurons express less NR2B subunits.
This work is supported by grants from the Canadian Institutes of Health Research, the EJLB-CIHR Michael Smith Chair in Neurosciences and Mental Health, and the Canada Research Chair to M. Z. L.-J.W. is supported by postdoctoral fellowships from the Canadian Institutes of Health Research and Fragile X Research Foundation of Canada. We are grateful to Dr. Y. Auberson of Novartis Pharmaceuticals for the gift of NVP-AAM077.
- Collingridge GL, Bliss TV: Memories of NMDA receptors and LTP. Trends Neurosci 1995, 18: 54–56. 10.1016/0166-2236(95)93868-XPubMedView Article
- Zhuo M: Glutamate receptors and persistent pain: targeting forebrain NR2B subunits. Drug Discov Today 2002, 7: 259–267. 10.1016/S1359-6446(01)02138-9PubMedView Article
- Cull-Candy S, Brickley S, Farrant M: NMDA receptor subunits: diversity, development and disease. Curr Opin Neurobiol 2001, 11: 327–335. 10.1016/S0959-4388(00)00215-4PubMedView Article
- Sheng M, Kim MJ: Postsynaptic signaling and plasticity mechanisms. Science 2002, 298: 776–780. 10.1126/science.1075333PubMedView Article
- Sheng M, Cummings J, Roldan LA, Jan YN, Jan LY: Changing subunit composition of heteromeric NMDA receptors during development of rat cortex. Nature 1994, 368: 144–147. 10.1038/368144a0PubMedView Article
- Wu LJ, Toyoda H, Zhao MG, Lee YS, Tang J, Ko SW, Jia YH, Shum FW, Zerbinatti CV, Bu G, Wei F, Xu TL, Muglia LJ, Chen ZF, Auberson YP, Kaang BK, Zhuo M: Upregulation of forebrain NMDA NR2B receptors contributes to behavioral sensitization after inflammation. J Neurosci 2005, 25: 11107–11116. 10.1523/JNEUROSCI.1678-05.2005PubMedView Article
- Kohr G: NMDA receptor function: subunit composition versus spatial distribution. Cell Tissue Res 2006, 326: 439–446. 10.1007/s00441-006-0273-6PubMedView Article
- Fischer G, Mutel V, Trube G, Malherbe P, Kew JN, Mohacsi E, Heitz MP, Kemp JA: Ro 25–6981, a highly potent and selective blocker of N-methyl-D-aspartate receptors containing the NR2B subunit. Characterization in vitro. J Pharmacol Exp Ther 1997, 283: 1285–1292.PubMed
- Williams K: Ifenprodil discriminates subtypes of the N-methyl-D-aspartate receptor: selectivity and mechanisms at recombinant heteromeric receptors. Mol Pharmacol 1993, 44: 851–859.PubMed
- Auberson YP, Allgeier H, Bischoff S, Lingenhoehl K, Moretti R, Schmutz M: 5-Phosphonomethylquinoxalinediones as competitive NMDA receptor antagonists with a preference for the human 1A/2A, rather than 1A/2B receptor composition. Bioorg Med Chem Lett 2002, 12: 1099–1102. 10.1016/S0960-894X(02)00074-4PubMedView Article
- Massey PV, Johnson BE, Moult PR, Auberson YP, Brown MW, Molnar E, Collingridge GL, Bashir ZI: Differential roles of NR2A and NR2B-containing NMDA receptors in cortical long-term potentiation and long-term depression. J Neurosci 2004, 24: 7821–7828. 10.1523/JNEUROSCI.1697-04.2004PubMedView Article
- Liu L, Wong TP, Pozza MF, Lingenhoehl K, Wang Y, Sheng M, Auberson YP, Wang YT: Role of NMDA receptor subtypes in governing the direction of hippocampal synaptic plasticity. Science 2004, 304: 1021–1024. 10.1126/science.1096615PubMedView Article
- Morishita W, Lu W, Smith GB, Nicoll RA, Bear MF, Malenka RC: Activation of NR2B-containing NMDA receptors is not required for NMDA receptor-dependent long-term depression. Neuropharmacology 2007, 52: 71–76. 10.1016/j.neuropharm.2006.07.005PubMedView Article
- Bartlett TE, Bannister NJ, Collett VJ, Dargan SL, Massey PV, Bortolotto ZA, Fitzjohn SM, Bashir ZI, Collingridge GL, Lodge D: Differential roles of NR2A and NR2B-containing NMDA receptors in LTP and LTD in the CA1 region of two-week old rat hippocampus. Neuropharmacology 2007, 52: 60–70. 10.1016/j.neuropharm.2006.07.013PubMedView Article
- Berberich S, Punnakkal P, Jensen V, Pawlak V, Seeburg PH, Hvalby O, Kohr G: Lack of NMDA receptor subtype selectivity for hippocampal long-term potentiation. J Neurosci 2005, 25: 6907–6910. 10.1523/JNEUROSCI.1905-05.2005PubMedView Article
- Weitlauf C, Honse Y, Auberson YP, Mishina M, Lovinger DM, Winder DG: Activation of NR2A-containing NMDA receptors is not obligatory for NMDA receptor-dependent long-term potentiation. J Neurosci 2005, 25: 8386–8390. 10.1523/JNEUROSCI.2388-05.2005PubMedView Article
- Toyoda H, Zhao MG, Zhuo M: Roles of NMDA receptor NR2A and NR2B subtypes for long-term depression in the anterior cingulate cortex. Eur J Neurosci 2005, 22: 485–494. 10.1111/j.1460-9568.2005.04236.xPubMedView Article
- Zhao MG, Toyoda H, Lee YS, Wu LJ, Ko SW, Zhang XH, Jia Y, Shum F, Xu H, Li BM, Kaang BK, Zhuo M: Roles of NMDA NR2B subtype receptor in prefrontal long-term potentiation and contextual fear memory. Neuron 2005, 47: 859–872. 10.1016/j.neuron.2005.08.014PubMedView Article
- Frizelle PA, Chen PE, Wyllie DJ: Equilibrium constants for (R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquino xalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) acting at recombinant NR1/NR2A and NR1/NR2B N-methyl-D-aspartate receptors: Implications for studies of synaptic transmission. Mol Pharmacol 2006, 70: 1022–1032. 10.1124/mol.106.024042PubMedView Article
- Feng B, Tse HW, Skifter DA, Morley R, Jane DE, Monaghan DT: Structure-activity analysis of a novel NR2C/NR2D-preferring NMDA receptor antagonist: 1-(phenanthrene-2-carbonyl) piperazine-2,3-dicarboxylic acid. Br J Pharmacol 2004, 141: 508–516. 10.1038/sj.bjp.0705644PubMed CentralPubMedView Article
- Zhuo M: Molecular mechanisms of pain in the anterior cingulate cortex. J Neurosci Res 2006, 84: 927–933. 10.1002/jnr.21003PubMedView Article
- Wiltgen BJ, Brown RA, Talton LE, Silva AJ: New circuits for old memories: the role of the neocortex in consolidation. Neuron 2004, 44: 101–108. 10.1016/j.neuron.2004.09.015PubMedView Article
- Wu LJ, Zhao MG, Toyoda H, Ko SW, Zhuo M: Kainate receptor-mediated synaptic transmission in the adult anterior cingulate cortex. J Neurophysiol 2005, 94: 1805–1813. 10.1152/jn.00091.2005PubMedView Article
- Gerkin RC, Lau PM, Nauen DW, Wang YT, Bi GQ: Modular Competition Driven by NMDA Receptor Subtypes in Spike-Timing-Dependent Plasticity. J Neurophysiol 2007, 97: 2851–2862. 10.1152/jn.00860.2006PubMedView Article
- Luo J, Wang Y, Yasuda RP, Dunah AW, Wolfe BB: The majority of N-methyl-D-aspartate receptor complexes in adult rat cerebral cortex contain at least three different subunits (NR1/NR2A/NR2B). Mol Pharmacol 1997, 51: 79–86.PubMed
- Hatton CJ, Paoletti P: Modulation of triheteromeric NMDA receptors by N-terminal domain ligands. Neuron 2005, 46: 261–274. 10.1016/j.neuron.2005.03.005PubMedView Article
- Mallon AP, Auberson YP, Stone TW: Selective subunit antagonists suggest an inhibitory relationship between NR2B and NR2A-subunit containing N-methyl-D: -aspartate receptors in hippocampal slices. Exp Brain Res 2005, 162: 374–383. 10.1007/s00221-004-2193-6PubMedView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.