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Event: 10

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Acetylcholine accumulation in synapses

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
ACh Synaptic Accumulation

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help
Cell term
eukaryotic cell

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
acetylcholine increased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
AChE inhibition - acute mortality KeyEvent Cataia Ives (send email) Under Development: Contributions and Comments Welcome Under Development
AChE Inhibition Leading to Neurodegeneration KeyEvent Allie Always (send email) Under development: Not open for comment. Do not cite
AChE inhibition - acute mortality via predation KeyEvent Allie Always (send email) Under development: Not open for comment. Do not cite


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
zebra fish Danio rerio High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
All life stages High

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Unspecific High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help
  • Acetylcholine is a neurotransmitter that is stored in nerve endings at cholinergic synapses in the central and peripheral nervous systems (Soreq and Seidman, 2001; Lushington 2006).

  • Acetylcholine can bind multiple types of nicotinic and muscarinic receptors. The downstream consequences of those events are tissue and receptor-specific.

  • Acetylcholine is released into the synaptic cleft when stimulation of the nerve occurs, and then binds to a receptor protein; either muscarinic (metabotropic) or nicotinic (ionotropic). The binding to the receptor results in changes in the flow of ions across the cell, thereby signaling activity (Fukuto 1990; Mileson et al 1998; Soreq and Seidman 2001; Lushington 2006).  

    • Inhibition of acetylcholine binding at the serine site via AChE inhibition results in an accumulation of acetylcholine in synapses associated with muscarinic and nicotinic receptors, resulting in unregulated excitation at neuromuscular junctions of skeletal muscle; pre-ganglionic neurotransmitters and post-ganglionic nerve endings of the autonomic nervous system; and neurotransmitters in the brain or central nervous system (CNS). 

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?
  • Several techniques are available to measure acetylcholine levels, including the Hestrin method (Augustinsson 1957, Hestrin 1949, Stone 1955), molecular probes or assays, microdialysis techniques (Zapata, 2009, Russom, 2014) or by liquid chromatography - tandem mass spectrometer LC-MS/MS (Gómez-Canela et al., 2017).

  • Hestrin’s method involves a colorimetric measurement of esterase activity. The rate of hydrolysis of acetylcholine with hydroxylamine to form hydroxamic acid is measured to determine the amount of acetylcholine:


This method is performed at alkaline pH in water and is applicable over a wide range of ester concentrations (Hestrin 1949).

  • Hydrolysis of acetylcholine by acetylcholinesterase in the synaptic cleft is fast, so concentration in the extracellular fluid is low (0.1-6 nM). Brain microdialysate studies quantify nanomolar concentrations of acetylcholine in extracellular fluid using chromatographic mass spectrometric techniques (Nirogi 2009). Choice of analytical method should provide detection limits below the lowest concentration expected in the dialysate and requiring the smallest sample volume. High-pressure liquid chromatography coupled to electrochemical detection (HPLC-EC) is based on enzymatic conversion of acetylcholine into choline and acetate by acetylcholinesterase, and subsequent oxidation of choline by choline oxidase to betaine and hydrogen peroxide, which can be oxidized on a platinum electrode. This method permits detection of dialysate acetylcholine concentrations in the 5-10 nM range (Zapata, 2009). Other microdialysis techniques for quantification of acetylcholine are liquid chromatography mass spectrometry (Nirogi 2009) and pyrolysis-gas chromatography (Szilagyi 1968).

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help
  • Acetylcholine and cholinergic receptors are found in invertebrate and vertebrate species. Specific examples from the literature documenting acetylcholine accumulation include:  Penaeid prawn exposed to sublethal exposure of methylparathion and malathion showed significantly increased ACh levels, in nervous tissue (Reddy 1990).

  • Brain tissue of tadpoles exposed to single sublethal concentrations methyl parathion for 24 h showed an increase in acetylcholine levels (Nayeemunnisa and Yasmeen 1986).  

  • Acute (48h) sublethal exposure to methyl parathion resulted in increased AChE levels in brain tissue in fish (Oreochromis mossambicus) (Rao and Rao, 1984). Researchers found a significant increase in acetylcholine at all time points measured (12-48hr) with acetylcholine levels increasing from 33-83% as compared to controls over the same time span. 

  • A study of male quail (Coturnix japonica) exposed to lethal concentrations of two OP pesticides (i.e., DDVP or fenitrothion), found significant increases in total and free acetylcholine (Kobayashi et al., 1983).

  • Mice singly injected with propoxur displayed changes in cholinergic parameters in the brain: increased brain ACh content, decreased AChE activity, and high-affinity choline uptake into synaptosomes (Kobayashi 1988). 

  • AChE levels and acetylcholine synthesis in rat striatum were compared in controls and animals exposed to 0.3 and 1 times the maximum tolerated dose. Acetylcholine was present in significantly less concentrations than in the adult rats (Karanth, 2003).


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help
  • Augustinsson, K.B. 1957. In: Glick,D.(Ed.); Methods of Biochemical Analysis, Interscience Publishers, Inc., New York, NY.

  • Gómez-Canela, C., D. Tornero-Cañadas, E. Prats, B. Piña, R. Tauler and D. Raldúa (2018), "Comprehensive characterization of neurochemicals in three zebrafish chemical models of human acute organophosphorus poisoning using liquid chromatography-tandem mass spectrometry”, Analytical and Bioanalytical Chemistry 410(6): 1735-1748. DOI: 10.1007/s00216-017-0827-3.
  • Fukuto TR.  1990. Mechanism of action of organophosphorus and carbamate insecticides.  Environ Health Perspect 87:245-254.    

  • Hestrin, S. (1949). The Reaction of Acetylcholine and Other Carboxylic Acid Derivatives with Hydroxylamine, and its Analytical Application. J. Biol. Chem. 180(1): 249-61.

  • Karanth, S., Pope, C. 2003. Age-related effects of chlorpyrifos and parathion on acetylcholine synthesis in rat striatum. Neurotoixol. Teratol. 25(5): 599-606.  

  • Kobayashi H, Yuyama A, Kudo M, Matsusaka N. 1983. Effects of organophosphorus compounds, O,O‐dimethyl‐o‐(2,2‐dichlorovinyl)phosphate (DDVP) and O,O‐dimethyl‐o‐(3‐methyl 4‐nitrophenyl)phosphorothioate (fenitrothion), on brain acetylcholine content and acetylcholinesterase activity in Japanese quail. Toxicology 28:219–227.

  • Kobayashi, H., Yuyama, A., Ohkawa, T., and Kajita, T. 1988. Effect of Single or Chronic Injection with a Carbamate, Propoxur, on the Brain Cholinergic System and Behavior of Mice. Jpn.J.Pharmacol. 47[1], 21-27.

  • Lushington GH, Guo J-X, Hurley MM.  2006. Acetylcholinesterase: Molecular modeling with the whole toolkit.  Curr Topics Medic Chem 6:57-73.

  • Mileson, BE, Chambers JE, Chen WL, Dettbarn W, Ehrich M, Eldefrawi AT, Gaylor DW, Hamernik K, Hodgson E, Karczmar AG, Padilla S, Pope CN, Richardson RJ, Saunders DR, Sheets LP, Sultatos LG, Wallace KB.  1998. Common mechanism of toxicity: A case study of organophosphorus pesticides. Toxicol Sci 41:8-20.

  • Molecular Probes. (2004). Amplex Red Acetylcholine/Acetylcholinesterase Assay Kit (A12217). Retrieved from:

  • Nayeemunnisa, Yasmeen N. 1986. On the presence of calmodulin in the brain of control and methyl parathion‐exposed developing tadpoles of frog, Rana cyanophlictis. Curr Sci (Bangalore) 55:546–548.

  • Nirogi, R., Mudigonda, K., Kandikere, V. Ponnamaneni, R. (2010). Quantification of Acetylcholine, an Essential Neurotransmitter, in Brain Microdialysis Samples by Liquid Chromatography Mass Spectrometry. Biomed Chromatogr. 24(1), 39-48. 

  • Rao KSP, Rao KVR. 1984. Impact of methyl parathion toxicity and eserine inhibition on acetylcholinesterase activity in tissues of the teleost (Tilapia mossambica)—A correlative study. Toxicol Lett 22:351–356.

  • Reddy MS, Jayaprada P, Rao KVR. 1990. Impact of methyl parathion and malathion on cholinergic and non‐cholinergic enzyme systems of penaeid prawn, Metapenaeus monoceros. Biochem Int 22:769–780.

  • Sogob MA, Vilanova E.  2002. Enzymes involved in the detoxification of organophosphorus, carbamate and pyrethroid insecticides through hydrolysis.  Toxicol  Lett 128:215-228.

  • Szilagyi, P.I.A., Schmidt, D.E., Green, J.P. (1968). Microanalytical determination of acetylcholine, other choline esters, and choline by pyrolysis-gas chromatography. Analytical Chemistry. 40(13), 2009-2013. 

  • Zapata, A., V.I. Chefer, T.S. Shippenberg, and L. Denoroy. 2009. Detection and quantification of neurotransmitters in dialysates. Curr. Protoc. Neurosci. Chapter 7:Unit 7.4.1-30.