A Structural, Functional, and Behavioral Evaluation of the Developmental Neurotoxicity of the Organophosphorus Insecticide Chlorpyrifos in Guinea Pigs: Mechanistic Implications

Abstract

Organophosphorus pesticides are among the most heavily used agricultural insecticides in the United States and throughout the world. Exposure of pregnant women to doses of chlorpyrifos, a prominent organophosphorus insecticide, that do not result in overt maternal intoxication has been associated with increased risk of attention deficit hyperactivity disorder, learning and memory impairment, and tremors in children. Learning and memory impairments are especially pronounced in boys and are readily reproduced in developmental rodent models of chlorpyrifos exposure. The cognitive deficits observed in these models are spatial in nature and can be mimicked by administration of GABAergic potentiating ligands. As such, the present study was designed to test the hypothesis that in-utero exposure to toxicologically relevant doses of CPF that do not elicit overt signs of maternal toxicity results in cognitive deficits, particularly in male offspring, that correlate with an increase of GABAergic transmission or reduction of glutamatergic transmission in hippocampal CA1 pyramidal neurons. To address this hypothesis we used a complementary multidisciplinary approach, including biochemical, behavioral, electrophysiological, and immunohistochemical assays, and a translationally relevant model - the guinea pig. Results here demonstrate that guinea pigs exposed prenatally to chlorpyrifos (25 mg/kg/day, GDs 53 to 62) presented in the Morris water maze (MWM) significant learning deficits, which were more pronounced among males. Electrophysiological experiments revealed that the frequency of spontaneous inhibitory spontaneous synaptic currents was significantly higher in the CA1 pyramidal neurons of chlorpyrifos-exposed animals. This increase was positively correlated with learning deficits in the MWM. Immunohistochemistry provided evidence of a selective increase in the microglia population in the CA1 field of chlorpyrifos-exposed animals. Taken together with the finding that expression of the pro-inflammatory cytokine TNF-? was also higher in the hippocampus of CPF-exposed animals, these results strongly indicated that prenatal exposure to chlorpyrifos was associated with a chronic neuroinflammatory process. Based on the results presented here a novel mechanism is proposed in which prenatal exposure to chlorpyrifos triggers a chronic neuroinflammatory process that contributes to enhancement of hippocampal GABAergic neurotransmission, which in turn increases the inhibitory tone to CA1 pyramidal neurons and this in turn contributes to the associated cognitive deficits.