Aop: 16


A descriptive title which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the AO. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Acetylcholinesterase inhibition leading to acute mortality

Short name
A short name should also be provided that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
AChE inhibition - acute mortality

Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.'  Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you.  Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help


List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help


(1) U.S. Environmental Protection Agency (Retired)

(2) National Health and Environmental Effects Research Laboratory, Office of Research and Development, Mid-Continent Ecology Division, US Environmental Protection Agency, Duluth, Minnesota, USA

(3) Research Computing Division, RTI International, Research Triangle Park, North Carolina, USA

(4) Physicians Committee for Responsible Medicine, Washington, DC, USA

  • Corresponding author for wiki entry (

Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate.  More help
Cataia Ives   (email point of contact)


List user names of all  authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact.  More help
  • Dan Villeneuve
  • Ginnie Hench
  • Cataia Ives
  • Kristie Sullivan
  • Cataia Ives


The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help
Author status OECD status OECD project SAAOP status
Under Development: Contributions and Comments Welcome Under Development 1.3 Included in OECD Work Plan
This AOP was last modified on April 05, 2021 18:16
The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page Revision Date/Time
Increased Mortality November 30, 2020 04:14
Acetylcholinesterase (AchE) Inhibition April 29, 2020 17:21
Acetylcholine accumulation in synapses June 26, 2020 13:06
Respiratory distress/arrest December 20, 2019 15:42
Increased Cholinergic Signaling December 20, 2019 17:32
Dysregulation of heart rate and vascular tone December 20, 2019 16:02
Decrease, Population trajectory September 26, 2017 11:33
AchE Inhibition leads to Increased Cholinergic Signaling December 20, 2019 11:07
AchE Inhibition leads to ACh Synaptic Accumulation December 19, 2019 15:57
ACh Synaptic Accumulation leads to Increased Cholinergic Signaling December 20, 2019 09:16
AchE Inhibition leads to Respiratory distress/arrest December 20, 2019 11:47
Increased Cholinergic Signaling leads to Respiratory distress/arrest December 20, 2019 09:51
AchE Inhibition leads to Cardiovascular dysregulation December 20, 2019 14:56
Increased Cholinergic Signaling leads to Cardiovascular dysregulation December 20, 2019 15:05
Cardiovascular dysregulation leads to Respiratory distress/arrest December 19, 2019 16:16
Respiratory distress/arrest leads to Increased Mortality December 20, 2019 10:26
AchE Inhibition leads to Increased Mortality December 20, 2019 17:33
Increased Mortality leads to Decrease, Population trajectory January 12, 2021 10:56
AchE Inhibition leads to Decrease, Population trajectory December 20, 2019 16:49


In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

The contents of this AOP page represent an interconnected network of AOPs linking the MIE of acetylcholinesterase inhibition to the AO of acute mortality. Both AOPs include the KE of acetylcholine accumulation at the synapses, which results in excessive signaling from cholinergic neurons on a broad range of tissues throughout the body. Respiratory failure is the predominant mechanism leading to acute mortality in humans (Satoh, 2006). While these two AOPs are represented on the basis of their most plausible linkages to acute mortality, other known symptoms of acetylcholinesterase inhibition mediated through actions on other receptors and tissues may also play a role (see Russom et al. 2014). Overall, there is strong evidence supporting the linkage of acetylcholinesterase inhibition and acetylcholine accumulation with acute mortality, but the precise contribution of the different organ-level effects across different species isn’t completely understood. This network of AOPs as a whole, including the indirect KERs depicted, supports the potential utility of in vitro or short-term in vivo measures of acetylcholinesterase inhibition for identifying chemicals with potential to cause acute mortality across a broad range of species. Caution is needed when interpreting the in vitro results, however, because well known chemical initiators of these AOPs are known to require metabolic activation, which can result in false negatives. In contrast, detoxification of these compounds is sometimes deficient in the young resulting in life stage differences in response to different chemicals that act through this mechanism. Toxicokinetics is also variable across species making this a major determinant of species sensitivity. At present, quantitative understanding is not sufficiently complete to accurately predict apical outcomes or potency from in vitro measurements alone, and the chemical-specific ADME and toxicokinetic considerations will be strong determinant of quantitative outcomes.

Background (optional)

This optional subsection should be used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development. The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help


Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help
Key Events (KE)
This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page.  More help
Adverse Outcomes (AO)
An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP.  More help
Sequence Type Event ID Title Short name
1 MIE 12 Acetylcholinesterase (AchE) Inhibition AchE Inhibition
2 KE 10 Acetylcholine accumulation in synapses ACh Synaptic Accumulation
3 KE 39 Increased Cholinergic Signaling Increased Cholinergic Signaling
4 KE 445 Respiratory distress/arrest Respiratory distress/arrest
5 KE 1703 Dysregulation of heart rate and vascular tone Cardiovascular dysregulation
6 AO 351 Increased Mortality Increased Mortality
7 AO 360 Decrease, Population trajectory Decrease, Population trajectory

Relationships Between Two Key Events (Including MIEs and AOs)

TESTINGThis table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button.  Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the user will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.).  Click ‘Create [adjacent/non-adjacent] relationship.’  The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and WoE summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help


The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help
Life stage Evidence
All life stages High

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. 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

Sex Applicability

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

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help
Life Stage Applicability

The key molecular target is the AChE enzyme, which appears to be available in all life stages of vertebrate and invertebrate species, although studies have found that AChE activity increases as the organism develops.

    • AChE can be inhibited by stressors from early in development throughout aging.  AChE inhibition has been measured in rat fetuses when the dam was dosed with chlorpyrifos (Lassiter et al., 1999), and also in aged rats up to 2 years of age treated with either carbaryl or methomyl (Moser et al., 2015).

    • In many species, sensitivity to stressors is greater in the young.  There are several factors that influence these age-related differences.  Intake from food and water is higher on a body weight basis, and in children, certain behaviors (crawling, hand-to-mouth) can also increase intake.  More importantly, considerable evidence shows that immature detoxification in the young account for much of the age differences. AChE inhibitors are metabolized or detoxified through a number of pathways, including hydrolysis by or binding to various esterases, and microsomal metabolism.  This leads to life-stage differences that are highly dependent on the stressor and the individual kinetic profile of each (Moser, 2011)

    • AChE in developing precocial birds were found to level off at the embryo life stage, while in altricial species the AChE activity increased as the bird developed until it reached a steady state at adulthood (Grue et al., 1981).

    • A study examining the acetylcholine and cholinesterase levels in developing brains of Sprague-Dawley rats found that acetylcholine synthesis was significantly lower in neonate and juveniles when compared to the adult levels, and the neonate cholinesterase levels were significantly lower than adult levels of activity (Karanth and Pope 2003). When exposed to either parathion or chlorpyrifos, the researchers found differences in peak inhibition of cholinesterase with neonates seeing the greatest impact early in the exposure (4-24 hrs), while juveniles and adults saw the most severe impacts at 96 hrs of exposure (Karanth and Pope 2003). Exposure to pesticides did not seem to impact acetylcholine production in the rats regardless of life stage, but researchers thought the pesticides may be altering other biochemical process (e.g., choline acetyltransferase) which might ultimately impact the measures of acetylcholine (Karanth and Pope 2003).

    • In Drosophila, changes in AChE activity in the developing organism were observed, with egg stages displaying the lowest activity, and maximum activity at the pupae life stage (Parkash and Kaur 1982).

    • In mammals and birds, studies have determined that skeletal muscles of immature birds and mammals contain both butyrylcholinesterase and AChE, with butyrylcholinesterase decreasing and AChE increasing as the animal develops (Tsim et al. 1988; Berman et al, 1987).

    • Another study found that changes in AChE within the developing pig brain were dependent on the area of the brain and life stage of the animal, with significant decreases in activity within the pons and hippocampus from birth to 36 months, and no significant change in activity in the cerebellum, where activity increased up to four months of age, leveling off thereafter (Adejumo and Egbunike, 2004).

    • Evidence exists that immature life stages in mammals and birds may be more sensitive to OP pesticides (see Grue et al., 1997; Grue et al., 1983; Grue; 1981), although this may be related to the amount of pesticide ingested in relation to body size (Ludke et al, 1975).

    • Researchers reported that frog (Bufo arenarum Hensel) embryos were more tolerant to parathion exposure than frog larvae, and associated this with the ability of the embryo AChE to recover to baseline levels faster than the larval life stage (Anguiano et al, 1994).

Taxonomic Applicability

Although AChE enzyme is found in all vertebrate and invertebrate species, the activity within taxa varies.

    • An examination of the STRING (V9.0, which is a database of known and predicted protein interactions, finds that the AChE enzyme is well conserved across vertebrate and invertebrate species.

    • A pattern of species sensitivity for OPs using terrestrial species found birds to be highly sensitive, mammals moderately sensitive, and fish and amphibians of lower sensitivity (Wallace 1992).

    • Taxonomic differences as they relate to key events within the AOP may be due to a number of factors. For instance, the organism’s habitat (e.g., sediment, water, and grass), diet, body size, behavior, mobility, and skin/exoskeleton permeability could all affect the level of exposure and the ability of the chemical to reach the molecular target. A complicating factor when identifying relative species sensitivities are differences in routes of exposure which may impact the time it takes for the substance to reach the molecular target, and impacts to adsorption, distribution, metabolism, and elimination (ADME) within the organism.

    • Phase I metabolism of phosphorothioates and phosphorodithioates by mixed function oxidases results in the more toxic oxon, which is the form that binds to AChE. Differences in species sensitivity may be impacted by the rate at which this reaction occurs, as would any mechanism related to detoxification of the oxon form. For instance, in mammals the oxon form undergoes an ester detoxification pathway which is not present in insect species, resulting in insects having a higher susceptibility to OPs than mammals (Ecobichon 2001). Similarly for procarbamates, invertebrates are able to convert the substance to the active form, while vertebrate species lack this metabolic mechanism (Stenersen 2004).

    • Studies have provided evidence that differences in the AChE enzyme across taxa may better explain differences in species sensitivity. Evidence suggests that the relative activity of AChE is linked to the hydrophobic and electronic configuration of the enzyme, which directly impacts the speed and/or efficiency that a substance can bind to the enzyme, and could impact the ability/speed of the enzyme to reactivate to its normal state (Wallace, 1992; Wallace and Kemp 1991). For instance, taxonomic differences in the electronic and steric properties of the esteratic site, nucleophilic strength of the enzyme center, the distance between the anionic and esteratic sites, and the electronic/steric properties of the anionic site may all impact the relative binding efficiency of the enzyme to the pesticide (O’Brien 1963; Monserrat and Bianchini 2001; Wallace, 1992; Wallace and Kemp 1991).

    • Baseline levels of the enzyme can significantly vary depending on species, strains, age class, sex, season, reproductive and nutritional status, and disease state (Cowman and Mazanti, 2000; Hill 1988; Rattner and Fairbrother 1991).

    • There are specific differences in the genes that code for the cholinergic AChE enzyme. In vertebrate species, AChE is encoded by a single gene (Ace) resulting in a conserved enzyme across the taxonomic group (Lu et al., 2012; Taylor 2011). AChE is encoded in the Diptera suborder Brachycera (e.g., Drosophila, common house fly) by the gene Ace2, while in other insects both an Ace1 and Ace2 gene encode AChE (Lu et al., 2012). The Ace1 gene produces an AChE with a cysteine residue, which is not found in vertebrate AChE, or in the AChE from the Ace2 gene form. Acetylcholinesterase from Ace1 is associated with neurotransmissions within the insect, while AChE from the Ace2 gene is responsible for non-cholinergic activity such as embryonic development, growth, and reproduction (Lu et al., 2012).

    • Comparisons of susceptibility of Xenopus laevis and human forms of AChE to OP and carbamate pesticides found that the enzyme in frog embryos has a much higher resistance to these pesticides than the human form of the enzyme (Shapira et al., 1998).

    • Acetylcholinesterase is found in presynaptic membranes of motor neurons in the spinal cord, cranial nerves within skeletal muscle, and preganglionic sympathetic and postganglionic parasympathetic neurons of vertebrates (Mileson et al., 1998). In invertebrates, AChE appears to be associated with sensory, brain, and other muscle activity (Fulton and Key, 2001; Habig and Di Giulio 1991; Mileson et al. 1998; Ware and Whitacre 2004).

    • In plants, the function of AChE is not well understood, but levels of acetylcholine appear to be involved in the regulation of membrane permeability, and the ability of a leaf to unroll (Tretyn and Kendrick 1991).

Sex Applicability

No studies were located reporting significant differences in AChE activity between male and female organisms.

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence.The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications.  More help
  • There are numerous studies which have shown the blockage or reversibility of downstream events following the administration of pharmacological agents that either bypass acetylcholinesterase activity by directly activating acetylcholine receptors or that act as direct agonists or antagonists of the various types of acetylcholine receptors. Additionally, evidence from experiments with AChR morpholino antisense oligonucleotides have provided additional evidence for an essential role of acetylcholine receptor activation in mediating some of the downstream key events.
  • Based on the current information assembled for this AOP, the essentiality of the key events downstream of acetylcholine accumulation is less clear. While there are several key events that correspond with well known symptoms of acetylcholinesterase inhibition (as characterized through a number of nonadjacent KERs), it is presently unclear which of these are the major driver of lethality across different species. In humans, addressing respiratory failure is routinely used to prevent death from poisoning with chemicals that act on acetylcholinesterase. Data from ecological species suggest that other failure points could be equally important. Given the abundance of literature on acetylcholine signaling and adverse effects associated with acetylcholinesterase inhibition, this is an area of the AOP that warrants further development.

Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together.  Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook.  More help
  • There is strong evidence that the initial inhibition of the AChE enzyme is required prior to triggering key events that lead to the adverse outcome of mortality (See US EPA 2006; Grue and Shipley 1984).
  • There is strong empirical evidence linking the key events, beginning with the molecular initiating event; AChE inhibition, followed by an increase in the acetylcholine at synapses of muscarinic and nicotinic receptors, and subsequent physiological and biochemical response resulting in cholinergic activity resulting in the death of the organism.
  • Strong evidence based on measured AChE inhibition and statistically-derived acute endpoints (e.g., LC/LD50) demonstrate a correlation of increase in enzyme inhibition and increased lethality. The open literature has many studies reporting these effects across invertebrate and vertebrate species with examples presented below.
  • The overall weight of evidence supporting the indirect relationship between AChE inhibition and mortality is very strong and there are many physiological activities associated with acetylcholine neurotransmission that are plausibly linked with organism survival. However, there remain significant gaps in the current AOP description regarding which specific intermediate events are primarily responsible for the toxicity observed. It is likely that it is a combination of these physiological responses rather than any one alone, and that the key driver is also species dependent.

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help

Considerations for Potential Applications of the AOP (optional)

At their discretion, the developer may include in this section discussion of the potential applications of an AOP to support regulatory decision-making. This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale.To edit the “Considerations for Potential Applications of the AOP” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Considerations for Potential Applications of the AOP” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page or 'Update and continue' to continue editing AOP text sections.  The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help


List the bibliographic references to original papers, books or other documents used to support the AOP. More help
  • Adejumo, D.O. and G.N. Egbunike. 2004. Changes in acetylcholinesterase activities in the developing and aging pig brain and hypophyses. Int. J. Agric. Rural. Dev. 5: 46-53.

  • Anguiano, O.L., C.M. Montagna, M. Chifflet de Llamas, L. Gauna, and A.M. Pechen de D'Angelo. 1994. Comparative toxicity of parathion in early embryos and larvae of the toad, Bufo arenarum Hensel. Bull. Environ. Contam. Toxicol. 52(5): 649-655.

  • Berman, H.A., M.M. Decker, and J. Sangmee. 1987. Reciprocal regulation of acetylcholinesterase and butyrylcholinesterase in mammalian skeletal muscle. Dev. Biol. 120(1): 154-161.

  • Cowman, D.F. and L.E. Mazanti. 2000. Ecotoxicology of “new generation” pesticides to amphibians. In: D.W. Sparling, G. Linder, and C.A. Bishop (Eds.), Ecotoxicology of Amphibians and Reptiles, pp 233-268, SETAC Press, Pensacola, FL.

  • Ecobichon, D.J. 2001. Toxic effects of pesticides. In: C.D. Klaassen (Ed.), Casarett and Doull’s Toxicology: The Basic Science of Poisons; Sixth Edition. (pp. 763-810). McGraw-Hill, New York, NY.

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