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Relationship: 2410


The title of the KER should clearly define the two KEs being considered and the sequential relationship between them (i.e., which is upstream and which is downstream). Consequently all KER titles take the form “upstream KE leads to downstream KE”.  More help

Increased Cholinergic Signaling leads to Neuronal network function, Decreased

Upstream event
Upstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help
Downstream event
Downstream event in the Key Event Relationship. On the KER page, clicking on the Event name under Upstream Relationship will bring the user to that individual KE page. More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

This table is automatically generated upon addition of a KER to an AOP. All of the AOPs that are linked to this KER will automatically be listed in this subsection. Clicking on the name of the AOP in the table will bring you to the individual page for that AOP. More help
AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Organo-Phosphate Chemicals induced inhibition of AChE leading to impaired cognitive function adjacent Moderate Moderate Brendan Ferreri-Hanberry (send email) Under development: Not open for comment. Do not cite

Taxonomic Applicability

Select one or more structured terms that help to define the biological applicability domain of the KER. In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. Authors can indicate the relevant taxa for this KER in this subsection. The process is similar to what is described for KEs (see pages 30-31 and 37-38 of User Handbook) More help
Term Scientific Term Evidence Link
human Homo sapiens High NCBI
rat Rattus norvegicus High NCBI
mouse Mus musculus Moderate NCBI
Drosophila melanogaster Drosophila melanogaster Moderate NCBI
zebra fish Danio rerio Moderate NCBI

Sex Applicability

Authors can indicate the relevant sex for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of the User Handbook). More help
Sex Evidence
Mixed High

Life Stage Applicability

Authors can indicate the relevant life stage for this KER in this subsection. The process is similar to what is described for KEs (see pages 31-32 of User Handbook). More help
Term Evidence
During brain development High

Key Event Relationship Description

Provide a brief, descriptive summation of the KER. While the title itself is fairly descriptive, this section can provide details that aren’t inherent in the description of the KEs themselves (see page 39 of the User Handbook). This description section can be viewed as providing the increased specificity in the nature of upstream perturbation (KEupstream) that leads to a particular downstream perturbation (KEdownstream), while allowing the KE descriptions to remain generalised so they can be linked to different AOPs. The description is also intended to provide a concise overview for readers who may want a brief summation, without needing to read through the detailed support for the relationship (covered below). Careful attention should be taken to avoid reference to other KEs that are not part of this KER, other KERs or other AOPs. This will ensure that the KER is modular and can be used by other AOPs. More help

Cholinergic signaling refers to the activation of receptors bound with acetylcholine. Receptors for acetylcholine is either acetylcholine or cholinergic receptors, which further classify into muscarinic and nicotinic ( Nicotinic cholinergic signaling is began early in development and spreads throughout the central nervous system via acetylcholine (ACh) and activating a range of ligand-gated ion channels (John D et al., 2015). Nicotinic cholinergic signaling clearly plays important roles in both during development in shaping the neural networks that form and in the adult where it modulates network function in numerous ongoing ways (John D et al., 2015). Recent study by Wang Y et al, (2021) showed that cholinergic signaling controls excitation and inhibition balance of neuronal network in brain. In thalamus neuronal networks are the target of extensive cholinergic projections from the basal forebrain. Upon activation, these cholinergic signals play important role in regulation of neuronal excitability and firing patterns of neuronal networks (Beierlein M et al., 2014).

Evidence Supporting this KER

Assembly and description of the scientific evidence supporting KERs in an AOP is an important step in the AOP development process that sets the stage for overall assessment of the AOP (see pages 49-56 of the User Handbook). To do this, biological plausibility, empirical support, and the current quantitative understanding of the KER are evaluated with regard to the predictive relationships/associations between defined pairs of KEs as a basis for considering WoE (page 55 of User Handbook). In addition, uncertainties and inconsistencies are considered. More help

The capacity of a neuron to interconnect is based on neural network formation that count on functional synapse establishment by cholinergic neuron (Anna Bal-Price et al., 2017; Colón-Ramos, 2009). Previous study reported that changes in the activity of cholinergic interneurons can play a vital role in motor control as well as social behavior (Martos et al., 2017). Thus modifications to the cholinergic system can lead to major dysfunction of neuronal network and the loss of cholinergic neuron from the forebrain can cause cognitive deficits associated with Parkinson’s and Alzheimer’s disease (Ahmed NY et al., 2019). Although the aberrant cholinergic signaling linked with several neurological disorder including schizophrenia but exact role is yet to elucidate (Ahmed NY et al., 2019). The ability of ACh to link the response of neuronal networks in brain makes cholinergic signaling a crucial mechanism underlying complex behaviors (Picciotto MR et al., 2012).

Biological Plausibility
Define, in free text, the biological rationale for a connection between KEupstream and KEdownstream. What are the structural or functional relationships between the KEs? For example, there is a functional relationship between an enzyme’s activity and the product of a reaction it catalyses. Supporting references should be included. However, it is recognised that there may be cases where the biological relationship between two KEs is very well established, to the extent that it is widely accepted and consistently supported by so much literature that it is unnecessary and impractical to cite the relevant primary literature. Citation of review articles or other secondary sources, like text books, may be reasonable in such cases. The primary intent is to provide scientifically credible support for the structural and/or functional relationship between the pair of KEs if one is known. The description of biological plausibility can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured (see page 40 of the User Handbook for further information).   More help

The cholinergic signaling system is deliberately located to exercise neuromodulatory effects on the excitatory and inhibitory balance. Thus endogenous cholinergic signaling regulate the excitatory and inhibitory balance via both nicotinic and muscarinic receptor (Lucas-Meunier E et al., 2009). The loss of cholinergic neurons has a profound effect on cholinergic signaling. Neurological diseases like Alzheimer’s connecting abnormal or loss of cholinergic signaling and excitatory and inhibitory imbalances. Activation of cholinergic receptors has a robust modulatory influence in the hippocampal network via activation of GABAergic interneurons (Jones and Yakel 1997). Activation of neuronal nicotinic ACh receptors excites interneurones which can inhibit large numbers of hippocampal excitatory and inhibitory neurons, thus neuronal nicotinic ACh receptors could participate in the cholinergic regulation of hippocampal neuronal activity (Jones S et al., 1997). Cholinergic signaling, during development is essential for physiological processes essential for the formation of the PNS and CNS including synaptogenesis (Dwyer, J. B et al, 2009; Rima, M et al., 2020). Neuronal network formation and function are established via the process of synaptogenesis. Cholinergic transmission can facilitate disparate actions through integration of postsynaptic signals (Calabresi et al., 2000). The developmental period of synaptogenesis is critical for the formation of the basic circuitry of the nervous system, although neurons are able to form new synapses throughout life (Rodier, 1995). Alterations in synaptic connectivity lead to refinement of neuronal networks during development (Cline and Haas, 2008).

Uncertainties and Inconsistencies
In addition to outlining the evidence supporting a particular linkage, it is also important to identify inconsistencies or uncertainties in the relationship. Additionally, while there are expected patterns of concordance that support a causal linkage between the KEs in the pair, it is also helpful to identify experimental details that may explain apparent deviations from the expected patterns of concordance. Identification of uncertainties and inconsistencies contribute to evaluation of the overall WoE supporting the AOPs that contain a given KER and to the identification of research gaps that warrant investigation (seep pages 41-42 of the User Handbook).Given that AOPs are intended to support regulatory applications, AOP developers should focus on those inconsistencies or gaps that would have a direct bearing or impact on the confidence in the KER and its use as a basis for inference or extrapolation in a regulatory setting. Uncertainties that may be of academic interest but would have little impact on regulatory application don’t need to be described. In general, this section details evidence that may raise questions regarding the overall validity and predictive utility of the KER (including consideration of both biological plausibility and empirical support). It also contributes along with several other elements to the overall evaluation of the WoE for the KER (see Section 4 of the User Handbook).  More help

The exact mechanism by which increase in cholinergic signaling lead to decrease in neuronal network function has not been fully elucidated.

Response-response Relationship
This subsection should be used to define sources of data that define the response-response relationships between the KEs. In particular, information regarding the general form of the relationship (e.g., linear, exponential, sigmoidal, threshold, etc.) should be captured if possible. If there are specific mathematical functions or computational models relevant to the KER in question that have been defined, those should also be cited and/or described where possible, along with information concerning the approximate range of certainty with which the state of the KEdownstream can be predicted based on the measured state of the KEupstream (i.e., can it be predicted within a factor of two, or within three orders of magnitude?). For example, a regression equation may reasonably describe the response-response relationship between the two KERs, but that relationship may have only been validated/tested in a single species under steady state exposure conditions. Those types of details would be useful to capture.  More help
This sub-section should be used to provide information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). This can be useful information both in terms of modelling the KER, as well as for analyzing the critical or dominant paths through an AOP network (e.g., identification of an AO that could kill an organism in a matter of hours will generally be of higher priority than other potential AOs that take weeks or months to develop). Identification of time-scale can also aid the assessment of temporal concordance. For example, for a KER that operates on a time-scale of days, measurement of both KEs after just hours of exposure in a short-term experiment could lead to incorrect conclusions regarding dose-response or temporal concordance if the time-scale of the upstream to downstream transition was not considered. More help
Known modulating factors
This sub-section presents information regarding modulating factors/variables known to alter the shape of the response-response function that describes the quantitative relationship between the two KEs (for example, an iodine deficient diet causes a significant increase in the slope of the relationship; a particular genotype doubles the sensitivity of KEdownstream to changes in KEupstream). Information on these known modulating factors should be listed in this subsection, along with relevant information regarding the manner in which the modulating factor can be expected to alter the relationship (if known). Note, this section should focus on those modulating factors for which solid evidence supported by relevant data and literature is available. It should NOT list all possible/plausible modulating factors. In this regard, it is useful to bear in mind that many risk assessments conducted through conventional apical guideline testing-based approaches generally consider few if any modulating factors. More help
Known Feedforward/Feedback loops influencing this KER
This subsection should define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits? In some cases where feedback processes are measurable and causally linked to the outcome, they should be represented as KEs. However, in most cases these features are expected to predominantly influence the shape of the response-response, time-course, behaviours between selected KEs. For example, if a feedback loop acts as compensatory mechanism that aims to restore homeostasis following initial perturbation of a KE, the feedback loop will directly shape the response-response relationship between the KERs. Given interest in formally identifying these positive or negative feedback, it is recommended that a graphical annotation (page 44) indicating a positive or negative feedback loop is involved in a particular upstream to downstream KE transition (KER) be added to the graphical representation, and that details be provided in this subsection of the KER description (see pages 44-45 of the User Handbook).  More help

Domain of Applicability

As for the KEs, there is also a free-text section of the KER description that the developer can use to explain his/her rationale for the structured terms selected with regard to taxonomic, life stage, or sex applicability, or provide a more generalizable or nuanced description of the applicability domain than may be feasible using standardized terms. More help

The main proof of evidence comes from in vivo studies in rodents. However, Rima, M et al., (2020) carried out a thorough spatiotemporal analysis of the cholinergic system in embryonic and larval zebrafish., cholinergic neurons in vertebrates found in the brain and spinal cord including the basal forebrain, brainstem and the habenula (Ahmed NY et al., 2019; Rima M et al., 2020). In rat spinal cord cholinergic propriospinal innervation analysis was done by Sherriff FE and Henderson Z. A (1994). Study by Eadaim et al. (2020) illustrated that in vivo reduction of cholinergic signaling induced synaptic homeostasis in Drosophila neurons


List of the literature that was cited for this KER description using the appropriate format. Ideally, the list of references should conform, to the extent possible, with the OECD Style Guide (OECD, 2015). More help

Ahmed NY, Knowles R, Dehorter N. New Insights into Cholinergic Neuron Diversity. Front Mol Neurosci. 2019 Aug 27; 12:204. doi: 10.3389/fnmol.2019.00204. PMID: 31551706; PMCID: PMC6736589.

Anna Bal-Price, Pamela J. Lein, Kimberly P. Keil, Sunjay Sethi, Timothy Shafer, Marta Barenys, Ellen Fritsche, Magdalini Sachana, M.E. (Bette) Meek. Developing and applying the adverse outcome pathway concept for understanding and predicting neurotoxicity, NeuroToxicology, Volume 59, 2017, Pages 240-255, ISSN 0161-813X,

Beierlein M. Synaptic mechanisms underlying cholinergic control of thalamic reticular nucleus neurons. J Physiol. 2014 Oct 1; 592 (19):4137-45. doi: 10.1113/jphysiol.2014.277376. Epub 2014 Jun 27. PMID: 24973413; PMCID: PMC4215766.

Calabresi, P., Centonze, D., Gubellini, P., Pisani, A., and Bernardi, G. (2000). Acetylcholine-mediated modulation of striatal function. Trends Neurosci. 23, 120–126. doi: 10.1016/s0166-2236(99)01501-5.

Cline H, Haas K. (2008). The regulation of dendritic arbor development and plasticity by glutamatergic synaptic input: A review of the synaptotrophic hypothesis. J Physiol 586: 1509-1517.

Colón-Ramos DA. (2009). Synapse formation in developing neural circuits. Curr Top Dev Biol. 87: 53-79.

Dwyer, J. B., McQuown, S. C. & Leslie, F. M. The dynamic effects of nicotine on the developing brain. Pharmacol. Ther. 122, 125–139 (2009).

Eadaim A, Hahm ET, Justice ED, Tsunoda S. Cholinergic Synaptic Homeostasis Is Tuned by an NFAT-Mediated α7 nAChR-Kv4/Shal Coupled Regulatory System. Cell Rep. 2020 Sep 8; 32(10):108119. doi: 10.1016/j.celrep.2020.108119. PMID: 32905767; PMCID: PMC7521586.

John D, Berg DK. Long-lasting changes in neural networks to compensate for altered nicotinic input. Biochem Pharmacol. 2015 Oct 15; 97 (4):418-424. doi: 10.1016/j.bcp.2015.07.020. Epub 2015 Jul 20.

Jones S, Yakel JL. 1997. Functional nicotinic ACh receptors on interneurones in the rat hippocampus. J Physiol. 504:603—610.

Lucas-Meunier E, Monier C, Amar M, Baux G, Fregnac Y, Fossier P. Involvement of nicotinic and muscarinic receptors in the endogenous cholinergic modulation of the balance between excitation and inhibition in the young rat visual cortex. Cereb Cortex. 2009 doi:10.1093/cercor/bhn258.

Martos, Y. V., Braz, B. Y., Beccaria, J. P., Murer, M. G., and Belforte, J. E. (2017). Compulsive social behavior emerges after selective ablation of striatal cholinergic interneurons. J. Neurosci. 37, 2849–2858. doi: 10.1523/jneurosci.3460-16.2017

Picciotto MR, Higley MJ, Mineur YS. Acetylcholine as a neuromodulator: cholinergic signaling shapes nervous system function and behavior. Neuron. 2012; 76(1):116-129. doi:10.1016/j.neuron.2012.08.036.

Rima, M., Lattouf, Y., Abi Younes, M. et al. Dynamic regulation of the cholinergic system in the spinal central nervous system. Sci Rep 10, 15338 (2020).

Rodier PM. (1995). Developing brain as a target of toxicity. Environ. Health Perspect. 103: 73-76.

Sherriff FE, Henderson Z. A cholinergic propriospinal innervation of the rat spinal cord. Brain Res. 1994 Jan 14; 634(1):150-4. doi: 10.1016/0006-8993(94)90268-2. PMID: 8156385.

Wang Y, Tan B, Wang Y, Chen Z. Cholinergic Signaling, Neural Excitability, and Epilepsy. Molecules. 2021; 26(8):2258. Published 2021 Apr 13. doi:10.3390/molecules26082258