Effect of Developmental Exposure to Chlorpyrifos on the Expression of Neurotrophin Growth Factors and Cell-Specific Markers in Neonatal Rat Brain

Abstract

Chlorpyrifos (CPS), a known neurotoxicant, is a widely used agricultural organophosphorus insecticide. The effects of postnatal exposure to CPS on the expression of mRNA for two factors critical to brain development, nerve growth factor (NGF) and reelin, were investigated in the forebrain of rats. In addition, the expression of mRNA for the muscarinic acetylcholine receptor (mAChR) M(1) subtype and cell-specific markers for developing neurons (beta-III tubulin), astrocytes (glial fibrillary acidic protein, GFAP), and oligodendrocytes (myelin-associated glycoprotein, MAG) was also investigated. Oral administration of CPS (1.5 or 3.0 mg/kg) or the corn oil vehicle was performed daily from postnatal days (PNDs) 1 through 6. No signs of overt toxicity or of cholinergic hyperstimulation were observed after CPS administration. Body weight was significantly different from controls on PND7 in both males and females exposed to 3.0 mg/kg CPS. Quantitative PCR was performed on the forebrain. The expression of NGF, reelin, and M(1) mAChR mRNA was significantly reduced with both dosages of CPS in both sexes. beta-III Tubulin mRNA expression remained unchanged after exposure, whereas MAG mRNA expression was significantly decreased with both dosages of CPS in both sexes, suggesting effects on the developing oligodendrocytes. In contrast, GFAP mRNA levels were significantly increased with both dosages of CPS in both sexes, suggesting increased astrocyte reactivity. Our findings indicate that dosages of CPS which cause significant cholinesterase inhibition but do not exert overt toxicity can adversely affect the expression levels of critical genes involved in brain development during the early postnatal period in the rat.

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

“…3A). The largest effects were on mobp and brainstem mpz, although a small, significant decrease was also seen for forebrain mog; although there were comparable decreases in mag, these did not achieve statistical significance because of higher variability, but the direction and magnitude of change are consistent with earlier findings [13]. In contrast to the effects of CPF, neither treatment with 1 mg/kg DZN (Fig.…”

“…CPF produced a consistent pattern of suppression of myelin-related genes, with the strongest effects on mobp and mpz. In vitro studies or in vivo evaluations that use higher doses above the threshold for systemic toxicity or growth impairment have identified mag as an additional target [13,63] and here we saw a tendency toward that effect (decreased by CPF or by either dose of DZN), although it did not achieve statistical significance at our lower, subtoxic exposures. Notably, we did not see significant changes in mbp, in keeping with earlier work showing no change in its protein product, myelin basic protein, using this particular CPF regimen [38].…”

“…Interestingly, we saw a reduction in expression of the gene encoding glial fibrillary acidic protein, the prototypic astroglial biomarker [69], whereas other investigators using higher exposures that exceed the thresholds for biologically significant cholinesterase inhibition, signs of systemic toxicity and/or growth suppression, found increases in gfap [13,63]. Again, it is important to separate direct effects on glia from indirect effects related to reactive gliosis that occurs in response to neuronal cell injury [69].…”

“…This is not surprising, given that the brain regions are extremely heterogeneous, containing numerous types of cells and neurotransmitters, so that relatively large changes in mRNA expression in a small cluster of cells are likely to be diluted out by inclusion of larger, unaffected cell types or brain regions. Indeed, even when animals are treated with organophosphate doses above the threshold for outright signs of systemic toxicity or even lethality, the magnitude of gene expression changes in brain regions in vivo rarely exceeds 10-30% [13,28,30]. This provides yet another reason why the absolute fold-change is not necessarily a good determinant of what pathways are most critically affected by organophosphate exposure; rather, it is the coordinated pattern of changes in multiple genes in the pathway that is more important.…”

“…Profiling of gene transcription responses in these pathways represents a potentially valuable and informative approach to identification of common and disparate mechanisms of neural damage. To date, this strategy has been applied with only a handful of genes at a time and yet has yielded some promising results, including demonstrations of effects on the expression of factors involved in neural growth, glial cell development, myelination, apoptosis, muscarinic acetylcholine receptors (mAChRs) and acetylcholinesterase splice variants that typify neural damage and repair [13,27,50]. With gene microarray techniques, cell culture systems of tumor lines expressing neurohumoral characteristics further indicate a broader range of potential target pathways [61,63].…”