Aop: 405

Title

Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [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

Organo-Phosphate Chemicals induced inhibition of AChE leading to impaired cognitive function

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
Organo-Phosphate Chemicals leading to impaired cognitive function

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

Authors

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

Saroj Kumar Amar and Kurt A. Gust*

U.S. Army Engineer Research and Development Center, Environmental Laboratory, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180 Sarojkumaramar@gmail.com, Saroj.K.Amar@erdc.dren.mil *Corresponding Author: Kurt.A.Gust@usace.army.mil

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
SAROJ AMAR   (email point of contact)

Contributors

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
  • SAROJ AMAR
  • Kurt A. Gust

Status

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: Not open for comment. Do not cite
This AOP was last modified on July 08, 2021 22:59
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
Acetylcholinesterase (AchE) Inhibition April 29, 2020 17:21
Acetylcholine accumulation in synapses June 26, 2020 13:06
Increased Cholinergic Signaling December 20, 2019 17:32
Decrease of neuronal network function May 28, 2018 11:36
Cognitive Function, Decreased August 09, 2018 11:55
AchE Inhibition leads to ACh Synaptic Accumulation December 19, 2019 15:57
Neuronal network function, Decreased leads to Cognitive Function, Decreased June 24, 2021 18:35
ACh Synaptic Accumulation leads to Increased Cholinergic Signaling December 20, 2019 09:16
Increased Cholinergic Signaling leads to Cognitive Function, Decreased June 24, 2021 18:36
Increased Cholinergic Signaling leads to Neuronal network function, Decreased June 24, 2021 18:34
Organophosphates November 29, 2016 21:20
Paraoxon June 24, 2021 18:47
Methyl parathion June 24, 2021 18:49
Ethyl Parathion June 24, 2021 18:52
N-methyl Carbamates October 07, 2019 14:19

Abstract

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

Organophosphate compounds (OP) are extensively used as pesticides/insecticides including paraoxon, ethyl-parathion, methyl-parathion, but have also been developed as warfare nerve agents such as soman, sarin, tabun and others. OP-induced cognitive deficits were observe not only among farm worker but also among environmentally exposed individual (Corral SA et al., 2017). The present adverse outcome pathways (AOP) describes the risk associated with a molecular initiating event (MIE) characteristic of OP exposure in which inhibition of acetylcholinesterase (AChE) activity causes a series of key events (KEs) that ultimately manifest as the adverse outcome (AO) of cognitive defects. The MIE of inhibited AChE triggers the KEs: accumulation of synaptic AChE (KE 1), increase of cholinergic signaling (KE 2), decrease of neuronal network function (KE 3), and decrease of cognitive function as an adverse outcome. The content of this AOP draws upon content from other AOPs in the AOPwiki page and has expanded interpretation in the interconnected network of AOPs linking the MIE of AChE inhibition to other AOs, including acute mortality.  The common threads between this and the other AOPs include common KEs of acetylcholine (ACh) accumulation at the synapses (KE 1), which results in (KE 2) excessive signaling from cholinergic neurons on a broad range of tissues throughout the body.  The MIE is engaged when OP binds to AChE causing an irreversible phosphorylation status of the enzyme. AChE is an enzyme responsible for controlling the level of the excitatory neurotransmitter, ACh, at neural synapses and neuromuscular junctions. AChE negatively regulates ACh via hydrolysis to acetic acid and choline (Wilson 2010). Inhibition of AChE (MIE) prevents degradation of ACh, which leads to (KE1) ACh accumulation at neural synapses and neuromuscular junctions in the central and peripheral nervous systems. (https://aopwiki.org/aops/16https://aopwiki.org/aops/312, Soreq and Seidman, 2001; Lushington 2006, Prado, 2017). ACh is generated in presynaptic neurons and released into the synaptic cleft where it can bind to both pre- and postsynaptic receptors. ACh availability is decreased when this neurotransmitter is degraded by AChE and by negative feedback loops controlled by muscarinic M2 receptors on the presynaptic neuron within the synapse (Soreq and Seidman, 2001). Affinity of ACh for metabotropic muscarinic receptors (mAChRs) and ionotropic nicotinic receptors (nAChRs), as well as rates of synaptic clearance (mediated through AChE activity) and local concentration of ACh in and outside the synapse, is critical for the control and specificity of cholinergic signaling (KE 2) (Sarter M et al., 2009, Picciotto MR, et al., 2012). Excessive accumulation of ACh at neural synapses (KE 1) and at neural-muscular junctions results in increased cholinergic signaling (KE 2) (AOP 16, https://aopwiki.org/aops/16). Endogenously released ACh regulates cognitive functions (AO), by acting as a neuromodulator and/or acting as a direct transmitter via nicotinic and muscarinic receptors in CNS by cholinergic signaling (KE 2) (Luchicchi A et al., 2014), which is evidence of direct relationship between KE2 and AO. The ability of a neuron to communicate is based on neural network formation (KE 3) that relies on functional synapse establishment by cholinergic neurons (KE 2) (Colón-Ramos, 2009) this is evidence that KE 2 leads to KE3. Muscarinic cholinergic activity influences sensory processing by facilitating or depressing neuronal responses to specific stimuli, and by modulating connection strength and neural synchronization: this results in the fine-tuning of cellular and network properties of neurons during developmental processes, the execution of attention tasks and perceptual learning (Colangelo C et al., 2019, Groleau et al., 2015). Damage or destruction of neurons during development when they are in the process of synapse formation, integration, and formation of neural networks, disrupts the organization and function of these networks (KE3), thereby setting the stage for subsequent impairment of learning and memory as sign of cognitive defects (AO), thus evident that KE 3 leads to AO. (AOP 13, https://aopwiki.org/aops/13).  Therefore, if exposure to OP occurs during neuronal differentiation and synaptogenesis processes, there is potential to initiate KE3, functional neuronal network damage, leading to the cognitive defects AO. Thus, this AOP provides a needed link to between chronic AChE inhibition and detrimental long-term impacts on cognitive function, which is relevant for understanding the impacts of long-term environmental and occupational OP pesticide exposures.

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

Events:

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
KE 10 Acetylcholine accumulation in synapses ACh Synaptic Accumulation
KE 39 Increased Cholinergic Signaling Increased Cholinergic Signaling
KE 386 Decrease of neuronal network function Neuronal network function, Decreased
AO 402 Cognitive Function, Decreased Cognitive Function, Decreased

Relationships Between Two Key Events (Including MIEs and AOs)

This 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

Stressors

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
Name Evidence Term
Organophosphates High
Paraoxon High
Methyl parathion Moderate
Ethyl Parathion Moderate
N-methyl Carbamates Moderate

Life Stage Applicability

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

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
Term Scientific Term Evidence Link
Homo sapiens Homo sapiens Moderate NCBI
Rattus norvegicus Rattus norvegicus High NCBI
Mus musculus Mus musculus High NCBI

Sex Applicability

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

Acute and chronic exposure to organophosphate, act by inhibiting cholinesterases which is widely associated with cognitive and motor impairments that can be observed even several months post-intoxication (Roldán-Tapia et al., 2006). Previous studies reported that the cognitive deficits were frequently observed not only in agricultural workers that directly manipulate pesticides but also individuals indirectly exposed during environmental applications (Corral SA et al., 2017). The multifactorial nature of pesticide-related cognitive impairment is consistent with experimental results showing distinct roles for pesticide exposure, duration of exposure and age of exposed individuals. The extent to which each of the aforementioned factors contributes to cognitive deficits remains under-explored (Aloizou AM et al., 2020).  AChE inhibition is initiated by electrostatic interaction at the anionic site of the enzyme and binding with the serine hydroxyl radicals at the esteratic site of AChE (Wilson 2010; Fukuto 1990).  This irreversible binding between AChE and OP pesticides is due to phosphorylation of enzyme. ACh is crucial for a number of important task including normal function of CNS, learning and memory, cognitive function as well as emotional and behavioral functions (Kilgard and Merzenich, 1998), reward (Leslie et al., 2013) and attention (Klinkenberg et al., 2011;  Picciotto et al., 2012). This AChE inhibition (MIE) leads directly to the KE1 where ACh has unmitigated accumulation in neuronal synapses.  This KER is directly supported the given observations demonstrating that AChE catalyzes degradation of ACh into choline and acetate (Wilson 2010). For goal-directed behavior, an appropriate levels of ACh are require to stimulate relevant sensory information (Sarter et al., 2009). An increase in cholinergic tone (KE 2) appears to be sufficient to induce depression-like symptoms in humans (Piccioto et al., 2012) and increasing ACh levels (KE 1) increases symptoms of depression (Overstreet, 1993).  ACh can induce heterogeneous effects in different brain areas that appear to have opposite behavioral consequences depending on the specific anatomical location (Piccioto et al., 2012).  Regardless, the predominant response of excessive accumulation of ACh at neural synapses (KE 1) and at neural-muscular junctions results in increased cholinergic signaling (KE 2) (AOP 16, https://aopwiki.org/aops/16). The ability of Ach (KE1) to induce synaptic plasticity through actions on pre- and post-synaptic nAChRs and mAChRs is likely to modulate learning and memory – symptoms of cognitive defects (AO), including memory of stressful events (Piccioto et al., 2012, Nijholt et al., 2004), and a role for ACh in regulation of hippocampal excitability (responsible for cognitive task) through presynaptic release of glutamate and GABA has also been well-characterized (Alkondon et al., 1997). It is clear that ACh (KE1), released from the cholinergic inputs (KE 2) of the basal forebrain, striatal, and the pontomesencephalic (PM) areas, plays an important role in supporting neurocognitive (AO) and motivational functions of the prefrontal cortical, hippocampal, and ventral tegmental projections to the striatum (Berman JA et al., 2007, Cragg, 2006Sarter et al., 2005; Wonnacott et al., 2005). The integrated-information processing and communication role of neurons is dependent to neural network formation (KE 3) that count on functional synapse formation by cholinergic neurons (KE 2) (Colón-Ramos, 2009) which is influential in KE 2 leading to KE3.  Still, the most difficult remaining gap for  neuroscientific investigation of OP effects center on connecting the impacts on neuronal network function (KE3) to (AO) cognition, including learning and memory. It is unknown which alterations in neuronal circuits are essential to change motor behavior as per learning and memory test record (Mayford et al., 2012), meaning that there is no clear understanding about how this KE and AO are connected (AOP 13, https://aopwiki.org/aops/13). It’s difficult to establish the relationship between alteration of neural network function and cognitive deficits due to complexity of synaptic interactions in even the simplest brain circuit. Linking of neurophysiological assessments to learning and memory processes have been made across simple monosynaptic connections and largely focused on the hippocampus (AOP 13, https://aopwiki.org/aops/13). There is very limited information on the degree of quantitative change in neural network function (KE 3) required to alter cognitive behaviors (AO). This is a result of the diversity of methods for measuring both neuronal network function and learning and memory deficits, which hamper cross-study analysis (AOP 13, https://aopwiki.org/aops/13). In humans, the hippocampus is involved in recollection of an event’s rich spatial-temporal contexts and distinguished from simple semantic memory, which is memory of a list of facts (https://aopwiki.org/events/402, Burgess et al., 2002).

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 different species (https://aopwiki.org/aops/16).

Life Stage

Evidence

Child

High

Adult

Moderate

Taxonomic Applicability

Though AChE enzyme can be traced in all vertebrate and invertebrate but the activity among taxa differs (https://aopwiki.org/aops/16)

Sex Applicability

Study with 44 children for scanning the OP pesticide exposure risk witnessed differences for sex of the child, with male levels higher than female levels (Loewenherz C et al., 1997).

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
  • MIE: Inhibition, AChE: AChE is a serine hydrolase that terminates the action of the neurotransmitter ACh by hydrolyzing it into acetic acid and choline. (McHardy SF et al., 2017) The AChE inhibitors (pesticides) bind to the enzyme and interfere with the breakdown of ACh, leading to the deposition of ACh (KE1) in the nerve synapses and causing disrupted neurotransmission (Thapa S et al.,2017, https://aopwiki.org/aops/16https://aopwiki.org/aops/312). Previous studies with vertebrate and invertebrate validate the dependence of AChE activity to the dose of OP and increasing inhibition of AChE in dose dependent manner with OP as reported in fish, birds, nematodes, rodents and mollusk (https://aopwiki.org/events/12).
  • Key Event 1: Accumulation, synaptic ACh: ACh is stored in nerve endings at cholinergic synapses in the central and peripheral nervous systems (Soreq and Seidman, 2001; Lushington 2006). OP anticholinesterases potentially have a mechanism of toxicity in common, that is, phosphorylation of AChE causing accumulation of ACh (KE 1), overstimulation of cholinergic receptors, and consequent clinical signs of cholinergic toxicity. However, some OP pesticides appear capable of altering noncholinergic neurochemical processes. These additional actions may contribute to qualitative and quantitative differences in toxicity sometimes noted in the presence of similar levels of AChE inhibition induced by different OP pesticides (Pope CN., 1999). Epidemiological studies have reported statistically significant correlations between prenatal subacute exposures to OP insecticides and neurological deficits that range from cognitive impairments to tremors in childhood (Burke RD et al., 2017). Excessive accumulation of ACh at neural synapses (KE 1) and at neural-muscular junctions results in increased cholinergic signaling (KE 2) (AOP 16, https://aopwiki.org/aops/16,AOP 312 https://aopwiki.org/aops/312).
  • Key Event 2: Increase, Cholinergic signaling: Acetylcholine is a neurotransmitter and neuromodulator that can exert either excitatory or inhibitory effects, depending on the receptor it binds to. ACh facilitates central and peripheral functions as well as somatic and autonomic functions. Excessive accumulation of acetylcholine at neural synapses and at neural-muscular junctions results in increased cholinergic signaling (https://aopwiki.org/relationships/456). The complexity of CNS cholinergic circuits and signaling mechanisms produces a system in which origins and end results may be easier to conclude than intervening intermediate steps. It is well reported that ACh, releases from the cholinergic inputs of the basal forebrain and striatal and from pontomesencephalic (PM) areas is supporting neurocognitive and motivational functions (Cragg, 2006; Sarter et al., 2005). Endogenously released ACh regulates cognitive functions (AO), by acting as a neuromodulator or acting as a direct transmitter via nicotinic and muscarinic receptors in CNS by cholinergic signaling  (KE 2) (Luchicchi A et al., 2014), which is evidence of direct relationship between KE2 and AO. The capability of a neuron to communicate is centered on neural network formation (KE 3) that depend on functional synapse formation through cholinergic neurons (KE 2) (Colón-Ramos, 2009) this is evidence that KE 2 leads to KE3. The capacity of a neuron to communicate is dependent to neural network formation (KE 3) that depend on functional synapse formation by cholinergic neurons (KE 2) (Colón-Ramos, 2009) this is evidence that KE 2 leads to KE3.
  • Key Event 3 Decrease, neuronal network function: Exposure to the potential developmental toxicants and OP during neuronal differentiation and synaptogenesis will increase the risk of functional neuronal network damage (KE3) leading to cognitive defects (AO), (https://aopwiki.org/aops/13).  Moreover, it is well accepted that alterations in synaptic transmission and plasticity contribute to (AO) deficits in cognitive function (AOP 13, https://aopwiki.org/aops/13). Damage or destruction of neurons during development when they are in the process of synapse formation, integration, and formation of neural networks, disrupts the organization and function of these networks (KE3), thereby setting the stage for subsequent impairment of learning and memory as sign of cognitive defects (AO), thus evident that KE 3 leads to AO. (AOP 13, https://aopwiki.org/aops/13) Neuronal network formation and function are established via the process of synaptogenesis. The initial period of synaptogenesis is important for the formation of the basic circuitry of the nervous system, though neurons can form new synapses throughout life (Rodier, 1995). Proper neuronal communication is dependent to brain electrical activity and synapse formation.  The main roles of synapses are responsible for the regulation of intercellular communication in nervous system as well as the information flow among neural networks. The connectivity and functionality of neural networks depends on where and when synapses are formed (Colón-Ramos, 2009).  So, the decreased synapse formation during the process of synaptogenesis is vital and resulting to the decrease of neural network formation.

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
  • The overall weight of evidence supporting the indirect relationship between AChE inhibition and cognitive defects is very strong and there are many physiological activities associated with ACh neurotransmission that are plausibly linked with organism survival.
  • Based on the current information assembled for this AOP, the essentiality of the key events downstream of ACh accumulation is less clear. While there are several key events that correspond with well-known symptoms of AChE inhibition, it is presently unclear which of these the major driver of cognitive defects are across different species. Given the abundance of literature on ACh signaling and adverse effects associated with AChE inhibition, this is an area of the AOP that warrants further development.
  • Biological Plausibility
  • ACh is a critical neurotransmitter localized to neuronal synapses. Biological plausibility to support the relationship between AChE inhibition and accumulation of ACh is rooted in evidence demonstrating that AChE catalyzes degradation of ACh into choline and acetate. Therefore, inhibition of the AChE leads to ACh accumulation.
  • Biological plausibility for ACh accumulation at the synapse leading to nervous system dysfunction is rooted in the well-established understanding of ACh’s function as a neurotransmitter and neuromodulator. By acting upstream of a range of cellular and physiological functions, it is biologically plausible that accumulation of ACh at neurological synapses will lead to systemic dysfunctions, which are often freely evident and assessable in clinical and research settings.
  • Neuronal network formation and function are established via the process of synaptogenesis. The initial period of synaptogenesis is vital for the formation of the elementary circuitry of the nervous system, while neurons can form new synapses throughout their life (Rodier, 1995). The brain electrical activity dependence on synapse formation is critical for proper neuronal communication (https://aopwiki.org/wiki/index.php/Relationship:358).
  • Learning-induced enhancement in neuronal excitability, a measurement of neural network function, has also been shown in hippocampal neurons following classical conditioning in several experimental approaches (Saar and Barkai, 2003). Previous study with Morris water maze (MWM) test to investigate spatial learning and memory in laboratory rats also indicated  that the interruption between neuronal networks rather than the brain damage of certain regions is accountable for the impairment of MWM presentation. Functional integrated neural networks that involve the coordination action of different brain regions are consequently important for spatial learning and MWM performance (https://aopwiki.org/wiki/index.php/Relationship:359, D'Hooge and De Deyn, 2001).
  • Moreover, it is well accepted that alterations in synaptic transmission and plasticity contribute to deficits in cognitive function
  • Concordance of dose-response relationships:
  • Striatal AChE activity and extracellular ACh levels were measured in rats intracerebrally perfused with paraoxon (0, 0.03, 0.1, 1, 10 or 100 μM, 1.5 μl/min for 45 min) (https://aopwiki.org/relationships/11, Ray, 2009). In that study, ACh was below the limit of detection at the low dose of paraoxon (0.1 uM), but was transiently elevated (0.5–1.5 hr) with 10 μM paraoxon. Concentration-dependent AchE inhibition was noted but reached a plateau of about 70% at 1 μM and higher concentrations (Ray, 2009). The association among AChE inhibition and ACh accumulation at the synapse can be detected within 30 minutes after application of  AChE inhibitor (Ray, 2009).
  • The main proof of evidence comes from in vivo studies in rodents. Though, Colón-Ramos (2009) has recently showed that the initial developmental events that during the course of synaptogenesis in invertebrates, indicating the significance of this process in neural network formation and function.
  • Because the adult hippocampus is involved in learning and memory, it is a brain region of remarkable plasticity (Johnston et al., 2009). Use-dependent synaptic plasticity is critical during brain development for synaptogenesis and fine-tuning of synaptic connectivity (Johnston et al., 2009).
  • Temporal concordance among the key events and adverse effect:
  • 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 decrease cognitive function. The literature includes many studies linking increases in ACh in brain tissues after exposure to an OP or carbamate pesticide with increased AChE inhibition in various taxa. As previous studies with crustacea (Reddy et al., 1990); tadpoles (Nayeemunnisa and Yasmeen, 1986); fish (Rao and Rao 1984; Verma et al., 1981); birds (Kobayashi et al., 1983); and rodents (Kobayashi et al., 1988) revealed.
  • ACh is a neurotransmitter and neuromodulator that can exert either excitatory or inhibitory effects, depending on the receptor it binds to (Picciotto, 2012). ACh mediates central and peripheral functions, including somatic and autonomic functions (Picciotto, 2012). Excessive accumulation of ACh at neural synapses and at neural-muscular junctions results in increased cholinergic signaling. Clinical manifestations of an acute exposure of humans to OP insecticides include a well-defined cholinergic crisis that develops as a result of the irreversible inhibition of AChE, the enzyme that hydrolyzes the neurotransmitter ACh  (Burke RD et al., 2017).
  • ACh is considered to be the most important neurotransmitter involved in the regulation of cognitive functions. Once releasing from the presynaptic neuron, ACh accumulated into the synaptic cleft, followed by binding to the ACh receptors on the postsynaptic membrane, and the signal from the nerve was transmitted during the process (Schliebs R and Arendt T, 2011, Gold P. E., 2003, Wang XC, 2018). Study showed the cholinergic overstimulation once pigs exposed to dichlorvos (the AChE inhibitor), symptoms may include miosis, cyanosis, tremor, excess secretions and fasciculations. Estimation of AChE levels established that dichlorvos treatment inhibited AChE activity. (Cui, 2013).
  • There is strong empirical evidence linking the key events, beginning with the molecular initiating event; AChE inhibition, followed by an increase in the ACh at synapses of muscarinic and nicotinic receptors, and subsequent physiological and biochemical response resulting in cholinergic activity (https://aopwiki.org/aops/16) , Picciotto MR et al, 2012.
  • Consistency:
  • (https://aopwiki.org/relationships/11) Previous study showed female ICR (Institute of Cancer Research) mice exposed to either fenobucarb or propoxur,  reported a major increase in ACh in brain tissue 10 minutes after injection, simultaneously major elevation in AChE inhibition (Kobayashi et al., 1985). Sub lethal exposure to methyl parathion conclude that AChE levels in brain tissue in fish (Oreochromis mossambicus) were highly  inhibited during 12-48 hrs, with inhibition increasing from 36-62% as in comparison to controls over the time elapse (Rao and Rao, 1984). The researchers found a significant increase in ACh at all-time courses measured (12-48hr) with ACh levels increasing from 33-83% as compared to controls over the same time span (Rao and Rao, 1984). A study of quail (Coturnix japonica) exposed to lethal concentrations of two OP pesticides (i.e., DDVP or fenitrothion), found significant increases in total and free ACh, and major inhibition of AChE in-comparison  to controls (Kobayashi et al., 1983). Measurements (in vitro) of AChE inhibition, ACh and electrophysiological responses on the pedal ganglion of the gastropod Aplysia californica, were found to be dose-dependent, with increase in dose resulting in increased AChE inhibition, increased levels of ACh, and a decrease in the electrophysiological response (Oyama et al., 1989). Wister rats injected with a sublethal concentration of dichlorvos found a significant decrease in AChE activity, increased ACh concentrations, and enhanced contractile responses in jejunum muscle. At sublethal concentrations (56% of the LD50), researchers found a significant (18%) increase in the amount of ACh in brain tissue of Charles River rats exposed to disulfoton for 3 days and resulted in AChE inhibition of 68% with respect to controls (Stavinoha et al., 1969). An acute sublethal exposure of chlorpyrifos to Sprague-Dawley rats found significant dose and time related effects including increased inhibition of AChE, increased levels of ACh, and significant influences to motor activity  (Karanth et al., 2006). Tadpoles of 20 days were treated with single sub lethal dose of the methyl parathion for 24 hrs and  analysis of brain tissue found a significant inhibition in AChE activity and a concurrent increase in ACh levels, as compared to controls (https://aopwiki.org/relationships/11, Nayeemunnisa and Yasmeen 1986). Study of fourth instar Ailanthus silkworm exposed to malathion for 5 days found increased mortality, decreased AChE, and increases in ACh as compared to controls (Pant and Katiyar 1983).
  • The relationship between excess ACh at synapses and nervous system dysfunction has been reviewed in Molecular Cell Biology, 4th Edition (https://aopwiki.org/relationships/456, Lodish, 2000). ACh is a neurotransmitter in most vertebrate and invertebrate species, but the mechanism of activity may differ. For example in insects, ACh acts as a neurotransmitter between sensory neurons and the central nervous system but glutamate acts as a neurotransmitter between motor neurons and skeletal muscles (https://aopwiki.org/relationships/456, Stenersen, 2004). 8-14 weeks old male quail were exposed to a single dose of either dichlorvos or fenitrothion by subcutaneous injection and brain tissue showed an 80% reduction of AChE, and a simultaneous major increase in ACh as compared to controls. With maximum doses, mortality was headed by symptoms including vigorous tremors, lacrimation, salivation, ataxia, and respiratory distress (Kobayashi, 1985). Previous study with male and female starlings of three age groups (5 days to >1 year) showed that the LD50 for nestlings was around half of the LD50 for adult birds exposed to dicrotophos. Simultaneously all birds exposed, showing impaired coordination and tremors. AChE inhibition increased in dose dependent manner for all three-age groups. There is no sex differences in LC50 or AChE inhibition were reported (Grue and Shipley, 1984).  Asian stinging catfish (Heteropneustes fossilis) exposed for 40 days to sublethal concentrations of oxydemeton-methyl, had a >71% inhibition of AChe in the brain and a concurrent increase of ACh in brain (>200%) and muscles (>188%), with fish displaying violent body movements (tremors) followed by loss of equilibrium (Verma 1981). 
  • Single injection of methylmercury (8 mg/kg by gavage) at gestational day 15. Offsprings examined at the age of 14, 21, and 60 days showed a reduction in the number of muscarinic receptors at 14 and 21 days and a decline in avoidance latency at 60 days, demonstrating learning and memory deficits (Zanoli et al., 1994), (https://aopwiki.org/relationships/359, Rice, 1992).
  • Uncertainties, inconsistencies, and data gaps:
  • No known qualitative inconsistencies or uncertainties associated between AChE inhibition and ACh accumulation at the synapse as well as ACh accumulation at the synapse to cholinergic signaling.
  • The exact mechanism by which a change in cell number, reduced dendritric arborization and synaptogenesis may lead to decreased neuronal network function has not been fully elucidated.

The direct relationship of alterations in neural network function and specific cognitive deficits is difficult to ascertain given the many forms that learning and memory can take and the complexity of synaptic interactions in even the simplest brain circuit. Linking of neurophysiological assessments to learning and memory processes have been made across simple monosynaptic connections and largely focused on the hippocampus. Changes in synaptic function have been noticed even in the lack of any behavioral losses. (https://aopwiki.org/relationships/359).

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

Exposure to organophosphate (OP) pesticides, which inhibit acetylcholinesterase, increases the risk of neurological disorders (Voorhees, J.R. et al., 2019). Recent study suggested that organophosphate pesticides may cause cognitive impairment. Mild cognitive impairment are dominating with symptoms like decreased attention or vigilance, narrowed information processing speed and memory impairment (Zhang HY et al., 2021). Mild cognitive impairment is largely being ignored for a long periods of time. Though, it might have huge impact on patients' life and work, and even progress to irreversible neurodegenerative disorder (Zhang HY et al., 2021). On cognitive tasks of learning and memory, male TgF344-AD rats displayed chlorpyrifos- an OP pesticide dependent deficits that were not seen in WT males or females of either genotype (Voorhees, J.R. et al., 2019). Previous study suggested that lower cognitive performances with huge decline in performances in vine workers is linked with pesticides exposure (Audrey Blanc-Lapierre et al., 2013). Thus this AOP attempt to establish the quantitative relationship between organophosphate pesticides and cognitive defects. Understanding the underlying mechanisms, this AOP can provide new means to avoid or neutralize the pesticide exposure risk. The stepwise relationships between consecutive key events is as follow. Existing key event relationship number 11, define inhibition of acetylcholinesterase (MIE) leads to synaptic accumulation of acetylcholine (KE1). Simultaneously key event relationship number 456 explained synaptic accumulation of acetylcholine (KE1) increases cholinergic signaling (KE2). Although the exact mechanism for increase cholinergic signaling lead to decreased neuronal network function (KE 3) has not been fully elucidated. But it’s well-known that the ability of a neuron to communicate is based on neural network formation that relies on functional synapse establishment by cholinergic neuron (Colón-Ramos, 2009). The direct relationship of alterations in neural network function (KE 3) and specific cognitive deficits (AO) is difficult to ascertain given the many forms that learning and memory can take and the complexity of synaptic interactions in even the simplest brain circuit (https://aopwiki.org/relationships/359). Though the AOP 13 advocate that, damage of neurons during development when they are in the process of formation of neural networks (KE3), setting the stage for subsequent impairment of learning and memory as sign of cognitive defects (AO). Thus above evidence supporting the development of this AOP.

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

Although the present AOP may require supplementary conditions to fully establish the neurotoxicity potential of OP pesticide and their mode of action. This AOP is an attempt to establish the mechanism of organophosphorus (OP) pesticides induced cognitive defects via cholinergic signaling. It can also be applied to risk assessment in predictive modeling of OP pesticide toxicity.

References

List the bibliographic references to original papers, books or other documents used to support the AOP. More help

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