Exposure to Organophosphates Reduces the Expression of Neurotrophic Factors in Neonatal Rat Brain Regions: Similarities and Differences in the Effects of Chlorpyrifos and Diazinon on the Fibroblast Growth Factor Superfamily

Abstract

BackgroundThe fibroblast growth factor (FGF) superfamily of neurotrophic factors plays critical roles in neural cell development, brain assembly, and recovery from neuronal injury.ObjectivesWe administered two organophosphate pesticides, chlorpyrifos and diazinon, to neonatal rats on postnatal days 1–4, using doses below the threshold for systemic toxicity or growth impairment, and spanning the threshold for barely detectable cholinesterase inhibition: 1 mg/kg/day chlorpyrifos and 1 or 2 mg/kg/day diazinon.MethodsUsing microarrays, we then examined the regional expression of mRNAs encoding the FGFs and their receptors (FGFRs) in the forebrain and brain stem.ResultsChlorpyrifos and diazinon both markedly suppressed fgf20 expression in the forebrain and fgf2 in the brain stem, while elevating brain stem fgfr4 and evoking a small deficit in brain stem fgf22. However, they differed in that the effects on fgf2 and fgfr4 were significantly larger for diazinon, and the two agents also showed dissimilar, smaller effects on fgf11, fgf14, and fgfr1.ConclusionsThe fact that there are similarities but also notable disparities in the responses to chlorpyrifos and diazinon, and that robust effects were seen even at doses that do not inhibit cholinesterase, supports the idea that organophosphates differ in their propensity to elicit developmental neurotoxicity, unrelated to their anticholinesterase activity. Effects on neurotrophic factors provide a mechanistic link between organophosphate injury to developing neurons and the eventual, adverse neurodevelopmental outcomes.

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

“…Importantly for our work, the fgf and wnt families are coexpressed in adjacent locations within the developing brain and deletion of genes from either group produces parallel defects in forebrain development [9,38,66,69,70,100]. As in our earlier work [79], we utilized microarrays to examine the key members of each gene family, comparing similarities and differences in the effects of chlorpyrifos and diazinon: if the involvement of neurotrophic mechanisms is unrelated to the inhibition of cholinesterase, then there are likely to be significant disparities between the two agents. We utilized doses that straddle the threshold for barely-detectable cholinesterase inhibition, too low to elicit any signs of cholinergic hyperstimulation [81,84], and our assessments were conducted in the brainstem and forebrain, regions that differ both in anatomical attributes as well as in maturational timetables.…”

“…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].…”

“…The effects of chlorpyrifos or diazinon exposure on expression of the genes encoding the FGFs and their receptors in neonatal rat brain regions have been presented previously [79], so the present study focused on the other neurotrophic factors and their receptors, along with the wnt and fzd families and their associated signaling pathways. In control rats, values for the genes encoding the neurotrophic factors and their receptors were significantly higher overall in the brainstem than in the forebrain (main effect of region, p < 0.0001), representing 5 out of 10 genes showing preferential expression in that region, as opposed to only 1 gene more highly expressed in the forebrain (Table 1, first 10 genes).…”

“…Exposure of neonatal rats to doses of chlorpyrifos sufficient to cause cholinesterase inhibition and somatic growth impairment produced only modest reductions in NGF or BDNF [7,8]. In contrast, several key constituents of the FGF family showed deficits at much lower chlorpyrifos or diazinon exposures, specifically involving the members known to be involved in the proliferation, differentiation and survival of the neuronal phenotypes and brain regions targeted by the organophosphates [79]. These findings point to the likely involvement of a specific set of neurotrophic factors as targets underlying organophosphate developmental neurotoxicity.…”

“…The roles of neurotrophic factors in brain development extend to cell proliferation, migration, differentiation, survival, synaptogenesis and myelination [63], with the various factors dictating regional, cellular as well as temporal specification during critical windows of development [19]. Because many of the factors have overlapping functions, it is important to note that changes in expression or function of any one neurotrophic factor may not necessarily lead to gross morphological deficits but rather to more subtle defects, akin to those seen with the organophosphate pesticides [7,79]. A number of recent studies have focused on three sets of trophic factors, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and the superfamily of fibroblast growth factors (FGFs).…”