Comparative developmental neurotoxicity of organophosphates in vivo: Transcriptional responses of pathways for brain cell development, cell signaling, cytotoxicity and neurotransmitter systems

Abstract

Organophosphates affect mammalian brain development through a variety of mechanisms beyond their shared property of cholinesterase inhibition. We used microarrays to characterize similarities and differences in transcriptional responses to chlorpyrifos and diazinon, assessing defined gene groupings for the pathways known to be associated with the mechanisms and/or outcomes of chlorpyrifos-induced developmental neurotoxicity. We exposed neonatal rats to daily doses of chlorpyrifos (1 mg/kg) or diazinon (1 or 2 mg/kg) on postnatal days 1-4 and evaluated gene expression profiles in brainstem and forebrain on day 5; these doses produce little or no cholinesterase inhibition. We evaluated pathways for general neural cell development, cell signaling, cytotoxicity and neurotransmitter systems, and identified significant differences for >60% of 252 genes. Chlorpyrifos elicited major transcriptional changes in genes involved in neural cell growth, development of glia and myelin, transcriptional factors involved in neural cell differentiation, cAMP-related cell signaling, apoptosis, oxidative stress, excitotoxicity, and development of neurotransmitter synthesis, storage and receptors for acetylcholine, serotonin, norepinephrine and dopamine. Diazinon had similar effects on many of the same processes but also showed major differences from chlorpyrifos. Our results buttress the idea that different organophosphates target multiple pathways involved in neural cell development but also that they deviate in key aspects that may contribute to disparate neurodevelopmental outcomes. Equally important, these pathways are compromised at exposures that are unrelated to biologically significant cholinesterase inhibition and its associated signs of systemic toxicity. The approach used here demonstrates how planned comparisons with microarrays can be used to screen for developmental neurotoxicity.

Targeting of neurotrophic factors, their receptors, and signaling pathways in the developmental neurotoxicity of organophosphates in vivo and in vitro

“…The results point to key divergences between chlorpyrifos and diazinon: just as we concluded earlier for the FGF family [79] the two organophosphates elicit differing alterations in neurotrophic factors that may ultimately contribute to disparities in the impact on neural pathways and behavior. Indeed, we already have substantial evidence for differing susceptibilities and damage to cholinergic and serotonergic pathways from neonatal chlorpyrifos exposure as compared to diazinon [75][76][77]81]. In turn, the fact that the two agents diverge in important ways reinforces the fact that the developmental neurotoxicity does not depend solely on the shared property of cholinesterase inhibition but rather reflects critical contributions of other mechanisms such as effects on neurotrophic factors [77,79].…”

“…Indeed, we already have substantial evidence for differing susceptibilities and damage to cholinergic and serotonergic pathways from neonatal chlorpyrifos exposure as compared to diazinon [75][76][77]81]. In turn, the fact that the two agents diverge in important ways reinforces the fact that the developmental neurotoxicity does not depend solely on the shared property of cholinesterase inhibition but rather reflects critical contributions of other mechanisms such as effects on neurotrophic factors [77,79]. Indeed, recent studies reinforce the separation of the effects observed here from those that depend on anticholinesterase effects: chronic, intermittent chlorpyrifos exposures in adults that are nonsymptomatic but well above the threshold for cholinesterase inhibition produce cognitive impairment in association with deficits in ntrk1, a subtype that was unaffected in our studies with lower exposures in the developing rat brain [90].…”

“…For ntf3, a deficit is likely to have devastating consequences: knockouts of this gene produce death within a few weeks of birth, whereas deletion of the ntf5 gene produces more subtle defects of neuronal structure and function [21,50]. Here, too, knockdowns of ntf3 expression gives outcomes similar to those seen with neonatal chlorpyrifos exposure, specifically involving shared effects on N-methyl-D-aspartate receptors that mediate excitatory neurotoxicity [39,77,92]. Because the organophosphates simultaneously target the neurotrophins as well as their receptors, their ultimate effects are likely to be more widespread than with knock-downs of any single gene [14,93].…”

“…The other values for that gene were used only to corroborate direction and magnitude of change. We also validated the readings on the arrays through the use of duplicate arrays for selected samples, as described previously [77,79].…”

Persistent cognitive alterations in rats after early postnatal exposure to low doses of the organophosphate pesticide, diazinon

“…Indeed, it is now clear that, in addition to cholinergic hyperstimulation consequent to inhibition of cholinesterase activity, OPs like chlorpyrifos (CPF) elicit their adverse effects on brain development through a family of mechanisms including direct effects on neural cell replication and differentiation [20,21]. Thus, although doses of CPF or other OPs high enough to cause substantial cholinesterase inhibition disrupt the pattern of genes that are essential to neurodevelopment [5,6], widespread changes in these genes are evident at much lower exposures that lie below the threshold for cholinesterase inhibition [23,24].…”