Upstream eventAchE Inhibition
Increased Cholinergic Signaling
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Acetylcholinesterase inhibition leading to acute mortality||non-adjacent|
|Acetylcholinesterase Inhibition leading to Acute Mortality via Impaired Coordination & Movement||non-adjacent|
Life Stage Applicability
Key Event Relationship Description
AChE inhibition leading to increased cholinergic signaling manifests across a range of “cholinergic syndrome” symptoms appearing as organ-type-specific responses. In cases of acute cholinergic poisoning, certain signs are often measurable within just a few minutes after exposure to an AChE inhibitor.
One of the earliest and most frequent signs of cholinergic poisoning is constricted pupils (miosis) (Wadia, 1974), which is a manifestation mediated by muscarinic cholinergic receptors. Other manifestations observed in cases of cholinergic poisoning are collectively known as SLUDGE symptoms (Peter):
Other signs of cholinergic poisoning are mediated by nicotinic cholinergic signalling. These include (Costa):
Other signs of increased cholinergic signalling occurring in the lungs and heart include increased bronchial secretion, bronchoconstriction, bradycardia and tachycardia, hypotension and hypertension (Costa, Peter).
This KER is focussed on the signs of increased cholinergic signalling frequently described and/or measured in laboratory, field and clinical studies.
Evidence Supporting this KER
Extensive research provides evidence that AchE inhibition is associated with symptoms that are known to be mediated by increased cholinergic signalling. The interaction between increased acetylcholine and enhanced signalling via nicotinic and muscarinic receptors is well-established. Further, cholinergic neurons are known to innervate multiple physiological sites (reviewed in Costa, In Casarett and Doull's Toxicology, Lodish).
A study of 200 cases of suicidal ingestion of organophosphorus insecticides in India (1971-1973) documented the incidence of numerous neurological symptoms associated with cholinergic signalling. Miosis (constricted pupils) was nearly universal amongst the patients observed. The table below lists the “Neurological Findings Associated with Organophosphorus Insecticide Poisoning” (Wadia, 1974).
|Neurological signs||Patients (number)||Incidence (%)|
Pigs exposed to the AChE inhibitor, dichlorvos showed signs of cholinergic overstimulation, which included miosis, cyanosis, tremor, excess secretions and fasciculations. Measurement of AChE levels confirmed that dichlorvos treatment inhibited AChE activity. (Cui, 2013).
Mice exposed to AChE inhibitor, propoxur, showed overt signs of cholinergic signalling at the highest dose tested (5 mg/kg) - depression, tremor and salivation (Kobayashi, 1985)
In Japanese Quail, increased cholinergic signaling in the form of salivation and convulsions was seen in response to 2 different AchE inhibitors, dichlorvos and fenitrothion. (Kobayashi, 1983)
Sparrows exposed to lethal doses of the AChE inhibitor, fenthion, showed the following symptoms indicative of increased cholinergic signalling: body tremors, weakness, ataxia, loss of balance, partial paralysis (in the wings), full paralysis (of the legs), and salivation (Hunt, 1991)
Spontaneous movement in zebrafish embryos is regulated by cholinergic nicotinic receptors and is associated with developing primary and secondary motor neurons. These spontaneous movements were enhanced by treatment with dichlorvos, diazinon, and chlorpyrifos (Watson, 2014)
Uncertainties and Inconsistencies
Exposure to high levels of AchE inhibiting insecticides (organophosphates and carbamates) is considered a factor contributing to GWS, a collection of neurological symptoms experienced by soldiers after the Persian Gulf War. Symptoms included fatigue, mood-cognitive problems, musculoskeletal symptoms. Factor analysis indicated cognitive impairment, ataxia and arthro-myo-neuropathy in some veterans and these symptoms were interpreted to reflect exposure to centrally acting anti-AChEs (Soreq & Seidman, 2001, Haley, 1997, Golomb, 2008)
Quantitative Understanding of the Linkage
Signs of cholinergic toxicity/poisoning can show up within minutes of exposure in humans and mammals.
In humans signs of increased cholinergic signalling…
Pigs exposed to the AChE inhibitor, dichlorvos showed signs of cholinergic overstimulation within 5 minutes of treatment. AChE levels measured within 30 minutes of dichlorvos treatment showed significant inhibition, which continued to decrease over the course of the experiment (6 hours)(Cui, 2013).
Dichlorvos treatment (3-4 mg/kg) led to nervous system disruption within 7-15 minutes, whereas the response to fenitrothion treatment took longer. Quail treated with 250-350 mg/kg fenitrothion showed cholinergic signs 6-120 minutes post-treatment (Kobayashi, 1983)
A comparison of 5 organophosphate and 2 carbamate pesticides in rats showed dose-response data for a number of cholinergic signs (Moser 1995). The overall clinical picture of toxicity was similar but differences emerged in terms of specific signs, dose-response, and time-course.
Mice treated with propoxur demonstrated the following signs of increased cholinergic signalling within the timeframes noted in response to each tested dose.
|AChE Inhibitor||Dose||Time (post injection)||Cholinergic symptoms|
|Propoxur||2 mg/kg||60 minutes||no apparent toxic signs|
|o-sec-butylphenyl methylcarbamate (BPMC)||10 mg/kg||60 minutes||no apparent toxic signs|
|o-sec-butylphenyl methylcarbamate (BPMC)||50 mg/kg||15-40 minutes||depression|
|Propoxur||5 mg/kg||5-30 minutes||depression, tremor and salivation|
Mice treated with sublethal doses of BPMC (10 mg/kg) and propoxur (2 mg/kg) had increased acetylcholine and decreased acetylcholinesterase activity in the forebrain at 10 min post-treatment (Kobayashi, 1985).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Acetylcholine, the enzymes needed to generate it, and acetylcholine receptors have been described within metazoans in bilaterians (vertebrates, echinoderms, insects, nematodes, and annelids, etc.) and cnidarians (sea anemones, corals and hydrozoans). Acetylcholine receptors have not been described in placozoans, poriferans, and ctenophores, nor outside of metazoans. (Faltine-Gonzalez, 2018).
Costa. Toxic effects of pesticides. In Casarett and Doull's Toxicology: The Basic Science of Poisons. 9th ed. pp 1055-1106.
Golomb, BA, Acetylcholinesterase inhibitors and Gulf War illnesses, Proc Natl Acad Sci USA. 2008 Mar 18; 105(11):4295-300.
Haley, R. W., Kurt, T. L. & Hom, J. Is there a Gulf War Syndrome? Searching for syndromes by factor analysis of symptoms. J. Am. Med. Assoc. 277, 215–222 (1997).
Cui J, Li CS, He XH, Song YG. Protective effects of penehyclidine hydrochloride on acute lung injury caused by severe dichlorvos poisoning in swine. Chin Med J (Engl). 2013; 126(24):4764-70.
Hunt KA, Bird DM, Mineau P, Shutt L. 1991. Secondary poisoning hazard of fenthion to American kestrels. Arch Environ Contam Toxicol 21:84–90.
Kobayashi H, Yuyama A, Kajita T, Shimura K, Ohkawa T, Satoh K. 1985. Effects of insecticidal carbamates on brain acetylcholine content, acetylcholinesterase activity and behavior in mice. Toxicol Lett 29:153–159.
Kobayashi H, Yuyama A, Kudo M, Matsusaka N. 1983. Effects of organophosphorus compounds, O,O‐dimethyl‐o‐(2,2‐dichlorovinyl)phosphate (DDVP) and O,O‐dimethyl‐o‐(3‐methyl 4‐nitrophenyl)phosphorothioate (fenitrothion), on brain acetylcholine content and acetylcholinesterase activity in Japanese quail. Toxicology 28:219–227
Faltine-Gonzalez, DZ, Layden, MJ., The origin and evolution of acetylcholine signaling through AchRs in metazoans. bioRxiv 424804; doi: https://doi.org/10.1101/424804
Moser, VC. (1995). Comparisons of the acute effects of cholinesterase inhibitors using a neurobehavioral screening battery in rats. Neurotoxicol. Teratol. 17:617-625.