Upstream eventAchE Inhibition
ACh Synaptic Accumulation
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Acetylcholinesterase inhibition leading to acute mortality||adjacent||High||Moderate|
|Acetylcholinesterase Inhibition Leading to Neurodegeneration||adjacent||High||Moderate|
|Acetylcholinesterase Inhibition leading to Acute Mortality via Impaired Coordination & Movement||adjacent|
|Metapenaeus monoceros||Metapenaeus monoceros||High||NCBI|
|Philosamia ricini||Samia ricini||High||NCBI|
|Rana cyanophlyetis||Euphlyctis cyanophlyctis||Moderate||NCBI|
|Tilapia mossambica||Oreochromis mossambicus||High||NCBI|
Life Stage Applicability
Key Event Relationship Description
AChE is an enzyme responsible for controlling the level of acetylcholine available at neural synapses and neuromuscular junctions. AChE negatively regulates acetylcholine via hydrolysis to acetic acid and choline (Wilson 2010). Inhibition of AChE prevents degradation of acetylcholine which leads to acetylcholine accumulation at neural synapses and neuromuscular junctions in the central and peripheral nervous systems. (Soreq and Seidman, 2001; Lushington 2006, Prado, 2017).
- See KEGG Reaction R01026
Evidence Supporting this KER
- Acetylcholine is a critical neurotransmitter localized to neuronal synapses. Biological plausibility to support the relationship between AChE inhibition and accumulation of acetylcholine is rooted in evidence demonstrating that AChE catalyzes degradation of acetylcholine into choline and acetate. Therefore inhibition of the AChE leads to acetylcholine accumulation.
- In a study where female ICR mice were exposed to either the fenobucarb or propoxur, authors reported a significant increase in acetylcholine in brain tissue 10 minutes after injection, with a concurrent significant increase in AChE inhibition (Kobayashi et al., 1985).
- An acute (48h) sublethal exposure to methyl parathion found that AChE levels in brain tissue in fish (Oreochromis mossambicus) were significantly inhibited at all measured durations ranging from 12-48 hrs with inhibition increasing from 36-62% as compared to controls over the time span (Rao and Rao, 1984). The researchers found a significant increase in acetylcholine at all time courses measured (12-48hr) with acetylcholine 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 acetylcholine, and significant inhibition of AChE as compared to controls (Kobayashi et al., 1983).
- Measurements (in vitro) of AChE inhibition, acetylcholine 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 acetylcholine, 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 acetylcholine concentrations, and enhanced contractile responses in jejunum muscle (Kobayashi et al., 1994).
- At sublethal concentrations ( 56% of the LD50), researchers found a statistically significant (18%) increase in the amount of acetylcholine in brain tissue of Charles River rats exposed to disulfoton for 3 days, with measured AChE inhibition of 68% as compared 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 acetylcholine, and significant impacts to motor activity (nocturnal rearing response) (Karanth et al., 2006).
- Tadpoles (20 d) were exposed to single sublethal concentration of the methyl parathion for 24 h. Analysis of brain tissue found a significant inhibition in AChE activity and a concurrent increase in acetylcholine levels, as compared to controls (Nayeemunnisa and Yasmeen 1986).
- Study of fourth instar Ailanthus silkworm exposed to malathion for 5 days found increased mortality, decreased AChE, and increases in acetylcholine as compared to controls (Pant and Katiyar 1983).
Uncertainties and Inconsistencies
- No known qualitative inconsistencies or uncertainties associated with this relationship.
Quantitative Understanding of the Linkage
The general kinetic equation is:
Where AX is the substrate, either acetylcholine or an inhibitor of AChE (e.g., OP or carbamate);
AChE-AX is the enzyme-substrate complex;
AChE-A is the acylated, carbamylated or phosphorylated enzyme;
X is the leaving group (e.g., choline);
AChE is the free enzyme; and
A is acetic acid, phosphate (P(=O)(=O)(R2)or methylamine.
In a normally functioning enzyme system k1 is the rate-limiting step for hydrolysis of acetylcholine, but k3 is the rate limiting step when AChE is inhibited by carbamates or OPs (Wilson 2010).
Some rate constants for OPs and carbamates have been published for use in PBPK models (Knaak et al., 2004, 2008)
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). Acetylcholine 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 relationship between AChE inhibition and ACh accumulation at the synapse can be observed within 30 minutes after application of a AChE inhibitor (Ray, 2009).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The literature includes many studies linking increases in acetylcholine in brain tissues after exposure to an OP or carbamate pesticide with increased AChE inhibition in various taxa. Examples include 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)
- Wilson, B.W. 2010. Cholinesterases. IN: Kreiger, R. (Ed.). Hayes’ Handbook of Pesticide Toxicology. Third Edition, Volume 2. Elsevier, Amsterdam, The Netherlands. pp. 1457-1478.
- Soreq, H. and S. Seidman. 2001. Acetylcholinesterase – new roles for an old actor. Nat. Rev. Neurosci. 2: 294-302.
- Lushington, G.H., J-X. Guo, and M.M. Hurley. 2006. Acetylcholinesterase: Molecular modeling with the whole toolkit. Curr. Topics Medic. Chem. 6: 57-73.
- Reddy, M. S., Jayaprada, P., and Rao, K. V. R. 1990. Impact of Methylparathion and Malathion on Cholinergic and Non-Cholinergic Enzyme Systems of Penaeid Prawn, Metapenaeus monoceros. Biochem.Int. 22, 769-780.
- Nayeemunnisa and Yasmeen, N. 1986. On the Presence of Calmodulin in the Brain of Control and Methyl Parathion-Exposed Developing Tadpoles of Frog, Rana cyanophlictis. Curr.Sci.(Bangalore) 55, 546-548.
- Rao, K. S. P. and Rao, K. V. R. 1984. Impact of Methyl Parathion Toxicity and Eserine Inhibition on Acetylcholinesterase Activity in Tissues of the Teleost (Tilapia mossambica) - a Correlative Study. Toxicol.Lett. 22, 351-356.
- Verma, S. R., Tonk, I. P., Gupta, A. K., and Dalela, R. C. 1981. In Vivo Enzymatic Alterations in Certain Tissues of Saccobranchus fossilis Following Exposure to Four Toxic Substances. Environ.Pollut.A. 26, 121-127.
- Kobayashi, H., Yuyama, A., Ohkawa, T., and Kajita, T. 1988. Effect of Single or Chronic Injection with a Carbamate, Propoxur, on the Brain Cholinergic System and Behavior of Mice. Jpn.J.Pharmacol. 47, 21-27.
- Kobayashi, H., Yuyama, A., Kudo, M., and Matsusaka, N. 1983. Effects of Organophosphorus Compounds, O,O-Dimethyl O-(2,2-Dichlorovinyl)Phosphate (DDVP) and O,O-Dimethyl O-(3-Methyl 4-Nitrophenyl)Phosphorothioate (Fenitrothion), on Brain Acetylcholine Content and Acetylcholinesterase Activity in Japanese Quail. Toxicology 28, 219-227.
- Kobayashi,H., A. Yuyama, T. Kajita, K. Shimura, T. Ohkawa, and K. Satoh. 1985. Effects of Insecticidal Carbamates on Brain Acetylcholine Content, Acetylcholinesterase Activity and Behavior in Mice. Toxicol. Lett.29(2-3): 153-159.
- Oyama, Y., N. Hori, M.L. Evans, C.N. Allen, and D.O. Carpenter. 1989. Electrophysiological estimation of the actions of acetylcholinesterase inhibitors on acetylcholine receptor and cholinesterase in physically isolated Aplysia neurons. Br. J. Pharmacol. 96:573-582.
- Kobayashi, H., Sato, I., Akatsu, Y., Fujii, S. I., Suzuki, T., Matsusaka, N., and Yuyama, A. 1994. Effects of Single or Repeated Administration of a Carbamate, Propoxur, and an Organophosphate, DDVP, on Jejunal Cholinergic Activities and Contractile Responses in Rats. J.Appl.Toxicol. 14, 185-190.
- Stavinoha, W. B., Ryan, L. C., and Smith, P. W. 1969. Biochemical Effects of an Organophosphorus Cholinesterase Inhibitor on the Rat Brain. Ann.N.Y.Acad.Sci. 160, 378-382.
- Karanth, S., Liu, J., Mirajkar, N., and Pope, C. 2006. Effects of Acute Chlorpyrifos Exposure on In Vivo Acetylcholine Accumulation in Rat Striatum. Toxicol.Appl.Pharmacol. 216, 150-156.
- Pant, Radha, and S. K. Katiyar. 1983. “Effect of Malathion and Acetylcholine on the Developing Larvae Of Philosamia Ricini (Lepidoptera: Saturniidae).” Journal of Biosciences 5 (1): 89–95. https://doi.org/10.1007/BF02702598.
- Ray, A., J. Liu, S. Karanth, Y. Gao, S. Brimijoin, and C. Pope. 2009. “CHOLINESTERASE INHIBITION AND ACETYLCHOLINE ACCUMULATION FOLLOWING INTRACEREBRAL ADMINISTRATION OF PARAOXON IN RATS.” Toxicology and Applied Pharmacology 236 (3): 341–47. https://doi.org/10.1016/j.taap.2009.02.022.
- Siva Prasada Rao, K., and K. V. Ramana Rao. 1984. “Impact of Methyl Parathion Toxicity and Eserine Inhibition on Acetylcholinesterase Activity in Tissues of the Teleost (Tilapia Mossambica)--a Correlative Study.” Toxicology Letters 22 (3): 351–56. https://doi.org/10.1016/0378-4274(84)90113-9.
- Prado, MAM, Marchot, P, Silman, I. Preface: Cholinergic Mechanisms. J Neurochem. 2017 Aug;142 Suppl 2:3-6. doi: 10.1111/jnc.14027.