• Neurodevelopmental Effects of Chlorpyrifos Depend on Age of Exposure and Maturational Stage: an In Vivo MR and Behavioral Study

      Mullins, Roger Jacob; Gullapalli, Rao P. (2015)
      Exposure to chlorpyrifos (CPF), an organophosphorus pesticide, is a serious worldwide health concern. Aside from issues of acute toxicity, a major focus of this concern is its harmful long-term effects on neurodevelopment in humans and animals. By employing MR techniques as well as standard animal behavioral tasks, we test the hypothesis that exposure to a sub-lethal dose of CPF causes neurodevelopmental deficits that are dependent on both the developmental stage of exposure and the extent of maturity after exposure. The scope of this hypothesis necessitates the examination of these neurodevelopmental effects in a guinea pig animal model over a range of neurodevelopmental stages, from gestation to maturation. In addition to studying normal development as a baseline, our investigation focused on the neurological deficits associated with CPF exposure in the prenatal and prepubertal periods, and examined the persistence of these effects into maturity. Our results demonstrated that normal development of the guinea pig brain was characterized by clear age-related increases in overall brain volume, white matter integrity, and microstructural complexity. Prenatal exposure to CPF resulted in decreases in forebrain volume, grey and white matter integrity, and spatial learning performance, with these effects being primarily associated with the integrity of the striatum and amygdalae. Prepubertal exposure to CPF yielded more complex results over development, which depends on the age of the animal at the time of examination. Significant spatial memory impairments and reductions in hippocampal myo-inositol were observed in mature animals a year after prepubertal exposure to CPF, while at the adolescent and young adult stages there were instead behavioral signs of anxiety, cytotoxic edema in the corpus callosum, striatum, and thalamus, and oxidative stress in the cerebral cortex. In the hippocampus there was a potentially neuroprotective glutamine response that may explain the lower severity of the effects in the young adult age compared to the more severely afflicted mature adults. These findings provide compelling evidence in support of the above neurodevelopmental CPF hypothesis. The key findings of these experiments are that lifelong deficits in brain function result from even a single exposure to CPF, vary greatly depending on the neurodevelopmental stage at time of exposure, and increase in severity as the animal matures. This knowledge of the complex neurodevelopmental effects of CPF aids future preventive measures, informs human translational research, and acts as a solid basis for further mechanistic animal studies.
    • A Structural, Functional, and Behavioral Evaluation of the Developmental Neurotoxicity of the Organophosphorus Insecticide Chlorpyrifos in Guinea Pigs: Mechanistic Implications

      Burke, Richard D.; Pereira, Edna F. R. (2016)
      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.