Developmental neurotoxicity of the pesticide, chlorpyrifos: Mechanisms and consequences

Neonates are more sensitive to acute chlorpyrifos toxicity than adults, despite recovering from cholinesterase inhibition faster. We exposed neonatal rats doses of chlorpyrifos on postnatal (PN) days 1--4 or PN11--14, which produce no signs of overt toxicity (i.e. mortality or weight loss). Nevertheless, chlorpyrifos on PN1--4 inhibited DNA synthesis with no effect on RNA or protein synthesis. This targeting of an event unique to mitosis may play a role in delayed cell loss. However a further involvement of programmed cell death could not be verified using relatively crude markers, c- fos and p53 expression. Changes in cell number do not necessarily produce behavioral dysfunction, but may trigger alterations in neuronal function. Therefore, we examined chlorpyrifos' effects on cholinergic synaptogenesis and neural activity using choline acetyltransferase activity and hemicholinium-3 binding. Chlorpyrifos produced a deficit in both markers suggesting impaired cholinergic synaptic function. However, effects extended to other transmitter systems. We saw increases in both norepinephrine and dopamine turnover, particularly in the cerebellum, a region nearly devoid of cholinergic innervation. To determine whether chlorpyrifos could directly elicit catecholamine release, we preloaded rat brain synaptosomes with radiolabeled norepinephrine and exposed them to chlorpyrifos in vitro. Chlorpyrifos causes a release of norepinephrine, via mechanisms unrelated to cholinergic stimulation. Finally, to determine the functional consequences of developmental chlorpyrifos exposure, simple behaviors based on coordination skills and locomotion were assessed. Exposure on PN1--4 produced deficits that were gender-selective, initially affecting females by delaying the development of reflex righting and negative geotaxis. Later on, males developed deficits in open field activities. Gender-dependent effects on cholinesterase activity were detected with the early treatment paradigm, but could not totally account for the behavioral deficits. Treatment on PN11--14 did not impair locomotion and instead caused hyperactivity in rearing and grooming, effects that were not gender selective. These studies indicate that developmental exposure to chlorpyrifos produces central nervous system deficits at both the cellular and functional levels and that mechanisms other than cholinesterase inhibition participate in the effects. Re-evaluating the safety of this compound should encompass endpoints that are unique to development rather than simple assessment of cholinesterase inhibition.