Among the factors known to influence the toxicity of many insecticides is an animal's age at the time of exposure. Both mammalian and avian species have demonstrated age-dependence in their sensitivity to organophosphate pesticides (OPs). In general, younger animals tend to be more sensitive than their adult counterparts, with differences being dependent on the species and the OP under study. This sensitivity appears to be due to under-developed protection mechanisms that function in adult animals but are reduced or absent in the young.
The variable effects of anti-cholinesterase (anti-ChE) pesticides on wildlife can make it difficult to predict relative sensitivities to such chemicals among species. Schafer and coworkers (1972, 1973, 1979 and 1983) have shown substantial variation in susceptibility among avian species exposed to these organophosphorus (OP) and carbamate (CB) compounds. Avian species, in general, tend to be more sensitive than mammals to such compounds. Efforts to explain varying sensitivity to anti-ChEs include physiological, biochemical and molecular approaches (Walker, 1983; Kemp and Wallace, 1990).
Carbamates and OPs exert their toxicity by binding to and inhibiting acetylcholinesterase (AChE), the enzyme that hydrolyzes the neurotransmitter, acetylcholine (ACh). This inhibition leads to a build-up of ACh in neuro-muscular and neuro-neuronal synapses, causing an initial overstimulation, followed by desensitization paralysis in the post-synaptic tissue. Death is usually due to respiratory failure as a result of constricted airways (from contracted tracheal smooth muscle and over stimulation of mucosal secretory glands), decreased ventilation capability (due to paralysis of intercostal skeletal muscles - birds have no diaphragm) and direct suppression of respiratory centers in the central nervous system. Variation in sensitivity to anti-ChEs can be due to differences in either the sensitivity of a species' AChE to inhibition by a particular anti-ChE, or that species' ability to inactivate, sequester or excrete the anti-ChE before it reaches critical AChE targets.
A number of studies have shown that AChE sensitivity to an inhibitor, at the molecular level, varies across class, as well as species, lines. The sensitivity of AChE to an inhibitor is a function of the enzyme's affinity for the inhibitor. Effective inhibitors are those which mimic ACh best, fitting into the AChE active site and providing the best molecular target for the enzyme's hydrolytic action. Good inhibitors are, therefore, those for which AChE has a high affinity. The higher the affinity, the lower the inhibitor concentration necessary to inhibit the enzyme.
There are a variety of mechanisms responsible for removing anti-ChEs from the body prior to their inhibition of AChE.
These include, in order of their occurrence after compound absorption from the gut, the following:1) The ability of the plasma to degrade or sequester the active, ChE-inhibiting form of the chemical via inhibition of plasma esterases, degradation by plasma OP hydrolases (OPs only) and non-specific hydrolysis by plasma proteins.
2) The rates at which the liver activates (OPs primarily) and degrades (both OPs and carbamates) the chemical and subsequently excretes it through the bile.
3) The rate at which the brain activates (OPs primarily) and degrades the chemical.
4) The rate at which the kidney performs the similar functions as the liver and excretes parent compound and the metabolites into the urine.
As carbamate-type compounds enter the body in an already activated form, the most important of the above processes are those that serve a degradative process, keeping carbamate levels low so they do not threaten brain AChE activity. Plasma and hepatic degradation, therefore, are crucial to an animal's defense against the carbamate.
Recently, studies investigating the mechanism of age-dependent sensitivity to the OP, diazinon, in European starlings (Sturnus vulgarus) have shown that butyrylcholinesterase (BChE) in peripheral tissues is critical in protecting adult starlings from diazinon toxicity due to its high affinity for the active oxon, diazoxon (Leopold et al 1994, Leopold 1996). Selective inhibition of BChE with the preferential inhibitor, iso-OMPA, was obtained such that AChE was unaffected and CaE was minimally inhibited. In the absence of BChE, diazinon acute toxicity was increased dramatically, with a shift in LD50 values from approximately 1200 mg/kg to less that 200 mg/kg. Similar studies with nestling starlings, which have less than 10% of the BChE activity of adults, showed only a halving of LD50 values from 10 to 5 mg/kg (Parker et al. 1996). These authors showed that the probable reason for BChE's importance is its high affinity for diazoxon, the activated anti-ChE form of diazinon. The diazoxon concentration necessary to inhibit (in vitro) 50 percent of an enzyme's activity is known as the IC50, IC indicating inhibitory concentration. The diazoxon IC50 for BChE was 100-fold less than that necessary to inhibit AChE. BChE's higher affinity allowed the enzyme to act as an anti-ChE sponge or buffer, reacting with the oxon at sufficiently low concentrations to ensure that levels were not reached that would inhibit the physiologically crucial AChE. These studies suggest that, in birds, the availability of BChE and its sensitivity to an anti-ChE inhibitor can be determinants in the differential sensitivity to that inhibitor. Understanding species-specific affinities of plasma esterases for particular anti-ChE compounds (and thus their potential to protect AChE) may provide one means to predict relative anti-ChE toxicity among species.
Mammals
Mortensen, S.R., M.J. Hooper and S. Padilla. 1998. Age-related sensitivity to cholinesterase-inhibiting insecticides: Is brain acetylcholinesterase from young animals intrinsically more sensitive to inhibition? Toxicology 125(1):13-19.
Mortensen, S.R., S. Brimijoin, M.J. Hooper and S. Padilla. 1998. Comparison of the in vitro sensitivity of rat tissue acetylcholinesterase to chlorpyrifos-oxon: What do IC50 values represent? Toxicol. and Appl. Pharm. 148:46-49.
Mortensen, S.R., S.M. Chanda, M.J. Hooper and S. Padilla. 1996. Maturational differences in chlopyrifos oxonase activity may contribute to age-related sensitivity to chlorpyrifos. J. Biochem. Toxicol. 11(6):279-287.
Birds
McInnes, P.F., D.E. Anderson, D. J. Hoff, M.J. Hooper and L.L. Kinkel. 1996. Monitoring exposure of nestling songbirds to agricultural application of an organophosphate insecticide using cholinesterase activity. Envtl. Tox. and Chem. 15(4):544-552.
Gard, N.W. and M.J. Hooper. 1993. Age-dependent changes in brain and plasma cholinesterase activity of eastern bluebirds and European starlings. J. Wildlife Disease. 29:1-9.