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Overactivation, NMDARs leads to Increased, Intracellular Calcium overload
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Binding of agonists to ionotropic glutamate receptors in adult brain causes excitotoxicity that mediates neuronal cell death, contributing to learning and memory impairment.||adjacent||Moderate||Anna Price (send email)||Open for citation & comment||WPHA/WNT Endorsed|
|Acetylcholinesterase Inhibition Leading to Neurodegeneration||adjacent||High||Karen Watanabe (send email)||Under development: Not open for comment. Do not cite|
Life Stage Applicability
Key Event Relationship Description
The NMDA receptor is distinct from the other glutamate receptors in two ways: first, it is both ligand-gated and voltage-dependent; second, it requires co-activation by two ligands: glutamate and either glycine or D-serine. Following membrane depolarization, the co-agonists, L-glutamate and glycine must bind to their respective sites on the receptor to open the channel. On activation, the NMDA receptor allows the influx of extracellular calcium ions into the postsynaptic neuron and neurotransmission occurs (reviewed in Higley and Sabatini, 2012). Calcium flux through NMDA receptors is also thought to be critical in synaptic plasticity, a cellular mechanism for learning and memory. Indeed, NMDA receptor–dependent synaptic potentiation (LTP) and depression (LTD) are two forms of activity-dependent long-term changes in synaptic efficacy that are believed to represent cellular correlates of learning and memory processes. The best characterized form of NMDA receptor-dependent LTP and LTD occurs between CA3 and CA1 pyramidal neurons of the hippocampus (Luscher and Malenka, 2012). It is now well established that modest activation of NMDARs leads to modest increases in postsynaptic calcium, triggering LTD, whereas much stronger activation of NMDARs leading to much larger increases in postsynaptic calcium are required to trigger LTP (Luscher and Malenka, 2012). The high-frequency stimulation causes a strong temporal summation of the excitatory postsynaptic potentials, and depolarization of the postsynaptic cell is sufficient to relieve the Mg2+ block of the NMDAR and allow a large amount of calcium to enter into the post-synaptic cells.
Evidence Supporting this KER
There is structural and functional mechanistic understanding supporting this relationship between KE1 and KE2.
The relationship between KE1 and KE2 is plausible as the expression of the functional NMDA receptors is commonly carried out or assessed by Ca2+ imaging method. Calcium imaging techniques have been extensively utilized in the literature to investigate the potential interactions between NMDA-evoked Ca2+ influx and NMDA receptor activation. Approximately 15% of the current through NMDA receptors is mediated by Ca2+ under physiological conditions (Higley and Sabatini, 2012).
It has been shown that less than five and, occasionally, only a single NMDA receptor opens under physiological conditions, causing a total Ca2+ influx of about 6000 ions into a spine head reaching a concentration of ∼10 µm (Higley and Sabatini, 2012). However, the majority of the ions are rapidly eliminated by binding to Ca2+ proteins, reaching ∼1 µM of free Ca2+ concentration (Higley and Sabatini, 2012).
It has been shown that in rat primary forebrain cultures the intracellular Ca2+ increases after activation of the NMDA receptor through administration of NMDA but this increase in Ca2+ is blocked when the cells are cultured under Ca2+ free conditions, demonstrating that the NMDA-evoked increase in intracellular Ca2+ derives from extracellular and not intracellular sources (Liu et al., 2013).
Indirect mechanism of domoic acid (DA) induced overactivation of NMDARs that result in Ca2+ overload: depolarization of the pre-synaptic cell activates the release of endogenous Ca2+ which mobilizes vesicles containing GLU to the membrane surface. Glutamate (GLU) is then released into the synaptic cleft by exocytosis where it is able to interact with cell surface receptors. Exogenous DA can interact within the synaptic cleft with each of the three ionotropic receptor subtypes including the kainate, AMPA, and NMDA receptors on cell membranes. Activation of the kainate and AMPA receptors results in release of Ca2+ via coupled ion channels, into the post-synaptic cell. DA is also able to bind to NMDA receptors that are linked to both Ca2+ and NA/K+ ion channels and results in a cellular influx of both Na+ and Ca2+. Unlike GLU, DA induces prolonged receptor activation causing a constant influx of cations into the cell and the appropriate chemical cues for desensitization are blocked. The excess intracellular Ca2+ causes disruption of cellular function, cell swelling and ultimately cell death (Lefebvre and Robertson,2010).
Glufosinate (GLF) is the methylphosphinate analog of glutamate that directly can activate NMDARs (Lantz et al., 2014, Matsumura et al., 2001, Faro et al., 2013) (as described in KE: NMDARs, Binding of agonist). It is well established in the existing literature that activation of NMDARs leads to the intra-cellular Ca2+ overload and based on this assumption it can be suggested that an exposure to GLF leads to increased intra-cellular calcium levels.
Uncertainties and Inconsistencies
A case of a 59-yr-old woman who ingested a herbicide containing glufosinate was suffering from severe intoxication of this herbicide, however, she did not develop convulsions, which experimentally occurs in rats treated with glufosinate (Koyama et al., 1994) and is described in other human cases (Watanabe and Sano 1998).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Data not available
Berman FW, LePage KT, Murray TF., Domoic acid neurotoxicity in cultured cerebellar granule neurons is controlled preferentially by the NMDA receptor Ca(2+) influx pathway. Brain Res., 2002, 924: 20-29.
Faro LR, Ferreira Nunes BV, Alfonso M, Ferreira VM, Durán R., Role of glutamate receptors and nitric oxide on the effects of glufosinate ammonium, an organophosphate pesticide, on in vivo dopamine release in rat striatum. Toxicology., 2013, Sep 15, 311: 154-61.
Giordano G, White CC, McConnachie LA, Fernandez C, Kavanagh TJ, Costa LG., Neurotoxicity of domoic Acid in cerebellar granule neurons in a genetic model of glutathione deficiency. Mol Pharmacol. 2006., 70: 2116-2126.
Giordano G, White CC, Mohar I, Kavanagh TJ, Costa LG., Glutathione levels modulate domoic acid-induced apoptosis in mouse cerebellar granule cells. Toxicol Sci., 2007, 100: 433-444.
Higley MJ, Sabatini BL., Calcium signalling in dendritic spines. Cold Spring Harb Perspect Biol., 2012, 4: a005686.
Koyama K, Andou Y, Saruki K, Matsuo H., Delayed and severe toxicities of a herbicide containing glufosinate and a surfactant. Vet Hum Toxicol., 1994, 36: 17-8.
Lantz Stephen R , Cina M. Mack , Kathleen Wallace, Ellen F. Key , Timothy J. Shafer , John E. Casida., Glufosinate binds N-methyl-D aspartate receptors and increases neuronal network activity in vitro. NeuroToxicology, 2014, 45: 38–47.
Lefebvre KA, Robertson A. Domoic acid and human exposure risks: a review. Toxicon. 2010 Aug 15;56(2):218-30.
Liu F, Patterson TA, Sadovova N, Zhang X, Liu S, Zou X, Hanig JP, Paule MG, Slikker W Jr, Wang C., Ketamine-induced neuronal damage and altered N-methyl-D-aspartate receptor function in rat primary forebrain culture. Toxicol Sci., 2013, 131: 548-557.
Luscher C. and Robert C. Malenka. NMDA Receptor-Dependent Long-Term Potentiation and Long-Term Depression (LTP/LTD). Cold Spring Harb Perspect Biol., 2012;4:a005710.
Matsumura N1, Takeuchi C, Hishikawa K, Fujii T, Nakaki T., Glufosinate ammonium induces convulsion through N-methyl-D-aspartate receptors in mice. Neurosci Lett., 2001, 304(1-2): 123-5.
Qiu S, Currás-Collazo MC., Histopathological and molecular changes produced by hippocampal microinjection of domoic acid. Neurotoxicol Teratol., 2006, 28: 354-362.
Watanabe T1, Sano T., Neurological effects of glufosinate poisoning with a brief review. Hum Exp Toxicol. 1998, 17: 35-9.