Event: 388

Key Event Title


Overactivation, NMDARs

Short name


Overactivation, NMDARs

Biological Context


Level of Biological Organization

Cell term


Cell term

Organ term


Key Event Components


Process Object Action
NMDA glutamate receptor activity NMDA selective glutamate receptor complex increased

Key Event Overview

AOPs Including This Key Event




Taxonomic Applicability


Term Scientific Term Evidence Link
human Homo sapiens High NCBI
mouse Mus musculus High NCBI
rat Rattus norvegicus High NCBI
zebrafish Danio rerio High NCBI

Life Stages


Sex Applicability


Key Event Description


Biological state: Please see MIE NMDARs, Binding of antagonist

Biological compartments: Please see MIE NMDARs, Binding of antagonist

General role in biology: Please see MIE NMDARs, Binding of antagonist

The above chapters belong to the AOP entitled: Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities since the general characteristic of the NMDA receptor biology is the same for both AOPs.

Additional text, specific for this AOP:

At resting membrane potentials, NMDA receptors are inactive. Depending on the specific impulse train received, the NMDA receptor activation triggers long term potentiation (LTP) or long-term depression (LTD) (Malenka and Bear, 2004; Luscher and Malenka, 2012). LTP (the opposing process to LTD) is the long-lasting increase of synaptic strength. For LTP induction both pre- and postsynaptic neurons need to be active at the same time because the postsynaptic neuron must be depolarized when glutamate is released from the presynaptic bouton to fully relieve the Mg2+ block of NMDARs that prevents ion flows through it. Sustained activation of AMPA or KA receptors by, for instance, a train of impulses arriving at a pre-synaptic terminal, depolarizes the post-synaptic cell, releasing Mg2+ inhibition and thus allowing NMDA receptor activation. Unlike GluA2-containing AMPA receptors, NMDA receptors are permeable to calcium ions as well as being permeable to other ions. Thus NMDA receptor activation leads to a calcium influx into the post-synaptic cells, a signal that is instrumental in the activation of a number of signalling cascades (Calcium-dependent processes are describe in Key Event Calcium influx, increased). Postsynaptic Ca2+ signals of different amplitudes and durations are able to induce either LPT or LTD.

Conversely to LTP, LTD is induced by repeated activation of the presynaptic neuron at low frequencies without postsynaptic activity (Luscher and Malenka, 2012). Therefore, under physiological conditions LTD is one of several processes that serves to selectively weaken specific synapses in order to make constructive use of synaptic strengthening caused by LTP. This is necessary because, if allowed to continue increasing in strength, synapses would ultimately reach a ceiling level of efficiency, which would inhibit the encoding of new information (Purves, 2008).

LTD is an activity-dependent reduction in the efficacy of neuronal synapses lasting hours or longer following a long patterned stimulus. It has also been found to occur in different types of neurons however, the most common neurotransmitter involved in LTD is L-glutamate that acts on the NMDARs, AMPAR, KARs and metabotropic glutamate receptors (mGluRs). It can result from strong synaptic stimulation (as occurs e.g. in the cerebellar Purkinje cells) or from persistent weak synaptic stimulation (as in the hippocampus) resulting mainly from a decrease in postsynaptic AMPA receptor density, although a decrease in presynaptic neurotransmitter release may also play a role. Moreover, cerebellar LTD has been hypothesized to be important for motor learning and hippocampal LTD may be important for the clearing of old memory traces (Nicholls et al., 2008; Mallere et al., 2010). The main molecular mechanism underlying-LTD is the phosphorylation of AMPA glutamate receptors and their synaptic elimination (Ogasawara et al., 2008).

It is now commonly understood in the field of spine morphology that long lasting NMDAR-dependent LTD causes dendritic spine shrinkage, reduces number of synaptic AMPA receptors (Calabrese et al., 2014), possibly leading to synaptic dysfunction, contributing to decreased neuronal network function and impairment of learning and memory processes.

Additional text, specific for the AOP “Acetylcholinesterase inhibition leading to neurodegeneration”:

              Seizures caused by cholinesterase dependent mechanisms result in an excess of glutamate release  that activates the NMDA receptors.  As a result, intracellular Ca2+ levels at the postsynaptic neuron can overload the calcium-control mechanisms, activating without control all the calcium-dependent enzymes (proteases, lipases…) (Deshpande et al., 2014; Garcia-Reyero et al., 2016). In cases of strong acetylcholinesterase inhibition of the CNS, the NMDAR overactivation initiated by cholinergic mechanisms can result, after the initial seizure activity (focal seizure), in the development of status epilepticus. This key event separates the initial toxicity, driven by cholinergic activity, from the secondary toxicity, which is cholinergic independent  (McDonough and Shih, 1997).

How It Is Measured or Detected


Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?

No OECD methods are available to measure the activation state of NMDA receptors.

The measurement of the activation or the inhibition of NMDA receptors is done indirectly by recording the individual ion channels that are selective to Na+, K+ and Ca2+ by the patch clamp technique. This method relies on lack of measurable ion flux when NMDA ion channel is closed, whereas constant channel specific conductance is recorded at the open state of the receptor (Blanke and VanDongen, 2009). Furthermore, this method is based on the prediction that activation or inhibition of an ion channel results from an increase in the probability of being in the open or closed state, respectively (Ogdon and Stanfield, 2009; Zhao et al., 2009).

The whole-cell patch clamp recording techniques have also been used to study synaptically-evoked NMDA receptor-mediated excitatory or inhibitory postsynaptic currents (EPSCs and IPSCs, respectively) in brain slices and neuronal cells, allowing the evaluation of the activated or inhibited state of the receptor.

Microelectrode array (MEA) recordings are used to measure mainly spontaneous network activity of cultured neurons (Keefer et al., 2001, Gramowski et al., 2000 and Gopal, 2003; Johnstone et al., 2010). However, using specific agonists and antagonists of a receptor, including NMDAR, MEA technology can be used to measure evoked activity, including glutamatergic receptor function (Lantz et al., 2014). For example it has been shown that MEA-coupled neuronal cortical networks are very sensitive to pharmacological manipulation of the excitatory ionotropic glutamatergic transmission (Frega et al., 2012). MEAs can also be applied in higher throughput platforms to facilitate screening of numerous chemical compounds (McConnell et al., 2012).

Excessive excitability can be also measured directly by evaluating the level of the extracellular glutamate using the enzyme-based microelectrode arrays. This technology is capable of detecting glutamate in vivo, to assess the effectiveness of hyperexcitability modulators on glutamate release in brain slices. Using glutamate oxidase coated ceramic MEAs coupled with constant voltage amperometry, it is possible to measure resting glutamate levels and synaptic overflow of glutamate after K(+) stimulation in brain slices (Quintero et al., 2011).

Neuronal network function can be also measured using optical detection of neuronal spikes both in vivo and in vitro (Wilt et al., 2013).

Drebrin immunocytochemistry: drebrin, a major actin-filament-binding protein localized in mature dendritic spines is a target of calpain mediated proteolysis under excitotoxic conditions induced by the overactivation of NMDARs. In cultured rodent neurons, degradation of drebrin was confirmed by the detection of proteolytic fragments, as well as a reduction in the amount of full-length drebrin. The NMDA-induced degradation of drebrin in mature neurons occurres concomitantly with a loss of f-actin. Biochemical analyses using purified drebrin and calpain revealed that calpain degraded drebrin directly in vitro. These findings suggest that calpain-mediated degradation of drebrin is mediated by excitotoxicity, regardless of whether they are acute or chronic. Drebrin (A and E) regulates the synaptic clustering of NMDARs. Therefore, degradation of drebrin can be used as a readout for excitotoxicity induced by NMDAR overactivation. Degradation of drebrin can be evaluated quantitatively by Western blot analysis (mRNA evel) or by immunocytochemistry (at protein level) (Chimura et al., 2015: Sekino et al., 20069.

NMDAR overactivation-induced long lasting LTD can be measured by the dendritic spine shrinkage by quantification of cofilin and phospho-cofilin in neurons expressing eGFP and combined with immunocytochemical techniques (Calabrese et al., 2014).

Domain of Applicability


It is important to note that in invertebrates the glutamatergic synaptic transmission has an inhibitory and not an excitatory role like in vertebrates. This type of neurotransmission is mediated by glutamate-gated chloride channels that are members of the ‘cys-loop’ ligand-gated anion channel superfamily found only in invertebrates. The subunits of glutamate-activated chloride channel have been isolated from C. elegans and from Drosophila (Blanke and VanDongen, 2009).



Blanke ML, VanDongen AMJ., Activation Mechanisms of the NMDA Receptor. In: Van Dongen AM, editor. Biology of the NMDA Receptor. Boca Raton (FL): CRC Press; 2009, Chapter 13. Available from: http://www.ncbi.nlm.nih.gov/books/NBK5274/.

Calabrese B, Saffin JM, Halpain S. Activity-dependent dendritic spine shrinkage and growth involve downregulation of cofilin via distinct mechanisms. PLoS One., 2014, 16;9(4):e94787.

Chimura T., Launey T., Yoshida N.,Calpain-Mediated Degradation of Drebrin by Excitotoxicity In vitro and In vivo PLOS ONE, 2015, |DOI:10.1371/journal.pone.0125119.

Deshpande, L. S., D. S. Carter, K. F. Phillips, R. E. Blair and R. J. DeLorenzo (2014), "Development of status epilepticus, sustained calcium elevations and neuronal injury in a rat survival model of lethal paraoxon intoxication”, NeuroToxicology 44: 17-26. DOI: 10.1016/j.neuro.2014.04.006.

Frega M, Pasquale V, Tedesco M, Marcoli M, Contestabile A, Nanni M, Bonzano L, Maura G, Chiappalone M., Cortical cultures coupled to micro-electrode arrays: a novel approach to perform in vitro excitotoxicity testing. Neurotoxicol Teratol. 2012: 34(1):116-27.

Garcia-Reyero, N., L. Escalon, E. Prats, M. Faria, A. M. V. M. Soares and D. Raldúa (2016), "Targeted Gene Expression in Zebrafish Exposed to Chlorpyrifos-Oxon Confirms Phenotype-Specific Mechanisms Leading to Adverse Outcomes”, Bulletin of Environmental Contamination and Toxicology 96(6): 707-713. DOI: 10.1007/s00128-016-1798-3.

Gopal K., Neurotoxic effects of mercury on auditory cortex networks growing on microelectrode arrays: a preliminary analysis. Neurotoxicol Teratol., 2003, 25: 69-76.

Gramowski A, Schiffmann D, Gross GW., Quantification of acute neurotoxic effects of trimethyltin using neuronal networks cultures on microelectrode arrays. Neurotoxicology, 2000, 21: 331-342.

Johnstone AFM, Gross GW, Weiss D, Schroeder O, Shafer TJ.,Use of microelectrode arrays for neurotoxicity testing in the 21st century Neurotoxicology, 2000, 31: 331-350.

Keefer E, Norton S, Boyle N, Talesa V, Gross G., Acute toxicity screening of novel AChE inhibitors using neuronal networks on microelectrode arrays. Neurotoxicology, 2001, 22: 3-12.

Lantz SR, Mack CM, Wallace K, Key EF, Shafer TJ, Casida JE. Glufosinate binds N-methyl-D-aspartate receptors and increases neuronal network activity in vitro. Neurotoxicology. 2014, 45:38-47.

Luscher C. and Malenka R.C., NMDA Receptor-Dependent Long-Term Potentiation and Long-Term Depression (LTP/LTD). Cold Spring Harb Perspect Biol., 2012, 4:a005710.

Malenka RC, Bear MF., LTP and LTD: An embarrassment of riches. Neuron, 2004, 44: 5–21.

Malleret G, Alarcon JM, Martel G, Takizawa S, Vronskaya S, Yin D, Chen IZ, Kandel ER, Shumyatsky GP., Bidirectional regulation of hippocampal long-term synaptic plasticity and its influence on opposing forms of memory". J Neurosci., 2010, 30 (10): 3813–25.

McConnell ER, McClain MA, Ross J, LeFew WR, Shafer TJ., Evaluation of multi-well microelectrode arrays for neurotoxicity screening using a chemical training set Neurotoxicology, 2012, 33: 1048-1057.

McDonough, J. H., Jr. and T. M. Shih (1997), "Neuropharmacological mechanisms of nerve agent-induced seizure and neuropathology”, Neurosci Biobehav Rev 21(5): 559-579.

Nicholls RE, Alarcon JM, Malleret G, Carroll RC, Grody M, Vronskaya S, Kandel ER., Transgenic mice lacking NMDAR-dependent LTD exhibit deficits in behavioral flexibility". Neuron, 2008, 58 (1): 104–17.

Ogasawara H, Doi T, Kawato M. Systems biology perspectives on cerebellar long-term depression. Neurosignals, 2008, 16 (4): 300–17.

Ogdon D, Stanfield P., Patch clamp techniques for single channel and whole-cell recording. Chapter 4, pages 53-78, (http://www.utdallas.edu/~tres/microelectrode/microelectrodes_ch04.pdf).

Paradiso MA, Bear MF, Connors BW., Neuroscience: exploring the brain. 2007, Hagerstwon, MD: Lippincott Williams & Wilkins. p. 718. ISBN 0-7817-6003-8.

Purves D., Neuroscience (4th ed.). Sunderland, Mass: Sinauer., 2008, pp. 197–200. ISBN 0-87893-697-1.

Sekino Y, Tanaka S, Hanamura K, Yamazaki H, Sasagawa Y, Xue Y, Hayashi K, Shirao T., Activation of N-methyl-D-aspartate receptor induces a shift of drebrin distribution: disappearance from dendritic spines and appearance in dendritic shafts. Mol Cell Neurosci. 2006, 31(3):493-504.

Quintero JE, Pomerleau F, Huettl P, Johnson KW, Offord J, Gerhardt GA. 2011. Methodology for rapid measures of glutamate release in rat brain slices using ceramic-based microelectrode arrays: basic characterization and drug pharmacology. Brain Res.2011, 1401:1-9.

Wilt BA, Fitzgerald JE, Schnitzer MJ., Photon shot noise limits on optical detection of neuronal spikes and estimation of spike timing. Biophys J. 2013, 8; 104(1):51-62.

Zhao Y, Inayat S, Dikin DA, Singer JH, Ruoff RS, Troy JB., Patch clamp techniques: review of the current state of art and potential contributions from nanoengineering. Proc. IMechE 222, Part N: J. Nanoengineering and Nanosystems, 2009, 149. DOI: 10.1243/17403499JNN149.