Organophosphorus Insecticides as Developmental Neurotoxicants: Potential Mechanisms Contributing to Disruption of Synaptic Transmission and Cognition

Abstract

Acute toxicity of the organophosphorus insecticides (OPs) chlorpyrifos (CPF) and malathion (MLT) results from irreversible inhibition of acetylcholinesterase (AChE). However, prenatal exposures to CPF and MLT levels that do not cause substantial AChE inhibition have been associated with neurodevelopmental disorders in children, particularly boys. Preclinical studies have shown that developmental exposures of rodents to low levels of CPF also result in sexually dimorphic learning and memory impairments. The finding that cognitive deficits observed in male guinea pigs prenatally exposed to subacute doses of CPF correlate with an increase of GABAergic transmission in hippocampal CA1 pyramidal neurons. The first part of this study was designed to model in vitro this CPF-induced increase in GABAergic transmission. To this end, rat primary hippocampal cultures were exposed for 5-6 days to CPF (3-300 nM) or vehicle starting at different times after plating. Subsequently, excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were recorded from voltage-clamped neurons approximately one week after the end of the exposures. The IPSC frequency was significantly higher in neurons of cultures exposed to 30 nM CPF than in neurons of vehicle-exposed cultures. This in vitro model becomes unique for studies aimed at identifying mechanisms that underlie the developmental neurotoxicity of CPF. Previous studies reported that i) CPF interacts with the cannabinoid receptor type 1 (CB1R) both directly and indirectly, and ii) CB1R plays a regulatory role in synaptogenesis. To determine whether CB1Rs contribute to CPF-induced increase in GABAergic transmission, primary hippocampal cultures were exposed to CPF or vehicle with or without the CB1R antagonist AM4113 for 5-6 days.. Results presented here reveal that AM4113 mimicked the effect of CPF on GABAergic transmission and that there is no synergistic or additive effect between CPF and AM4113. These results suggest that CPF affects synaptogenesis by directly or indirectly decreasing the tonic activity of CB1Rs. The second part of this study was designed to demonstrate the developmental neurotoxicity of MLT in an animal model. Pregnant guinea pigs were exposed to a MLT dose regimen that induced no significant brain or blood AChE inhibition (20 mg/kg/day, ~gestation days 53 to 62). When tested in the Morris water maze, prepubertal male and female guinea pigs that had been prenatally exposed to MLT presented learning and memory deficits. To our knowledge this is the first demonstration of a cause-effect relationship between prenatal MLT exposures and postnatal cognitive deficits. The finding that spatial learning and memory deficits following prenatal MLT exposure resemble those following prenatal CPF exposure suggest that a common off-target mechanism may underlie the developmental neurotoxicity of both OPs.