Age dependence of organophosphate and carbamate neurotoxicity in the postnatal rat: extrapolation to the human

One important aspect of risk assessment for the organophosphate and carbamate pesticides is to determine whether their neurotoxicity occurs at lower dose levels in human infants compared to adults. Because these compounds probably exert their neurotoxic effects through the inhibition of acetylcholinesterase (AChE), the above question can be narrowed to whether the cholinesterase inhibition and neurotoxicity they produce is age-dependent, both in terms of the effects produced and potency. The rat is the animal model system most commonly used to address these issues. This paper first discusses the adequacy of the postnatal rat to serve as a model for neurodevelopment in the postnatal human, concluding that the two species share numerous pathways of postnatal neurodevelopment, and that the rat in the third postnatal week is the neurodevelopmental equivalent of the newborn human. Then, studies are discussed in which young and adult rats were dosed by identical routes with organophosphates or carbamates. Four pesticides were tested in rat pups in their third postnatal week: aldicarb, chlorpyrifos, malathion, and methamidophos. The first three, but not methamidophos, caused neurotoxicity at dose levels that ranged from 1.8- to 5.1-fold lower (mean 2.6-fold lower) in the 2- to 3-week-old rat compared to the adult. This estimate in the rat, based on a limited data set of three organophosphates and a single carbamate, probably represents the minimum difference in the neurotoxicity of an untested cholinesterase-inhibiting pesticide that should be expected between the human neonate and adult. For the organophosphates, the greater sensitivity of postnatal rats, and, by analogy, that expected for human neonates, is correlated with generally lower levels of the enzymes involved in organophosphate deactivation.