Student Research: Jenna Fisher

Behavioral Effects of Early Postnatal Low-level Chlorpyrifos-oxon Exposure in Paraoxonae (PON1) Knockout Mice
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Chlorpyrifos is one of the most widely used organophosphorate insecticides n the U.S. The active metabolite, chlorpyrifos-oxon (CPO), elicits toxicity in the brain through inhibition of the enzyme acetylcholinesterase, and is detoxified primarily by paraoxonase (PON1). In addition to the overtly toxic effects caused by the accumulation of the neurotransmitter acetylcholine, there is increasing evidence that sub-toxic doses of chlorpyrifos can affect brain development in the young animal. Effects that could occur in the absence of obvious signs of acute toxicity are of increasing concern since children could potentially be exposed at low levels during critical times of development. The major detoxifying enzyme, PON1, is not fully developed in neonates, which may intensify the effect of CPO. In this study, PON1 knockout mice exposed to CPO during early postnatal development were assessed for changes in neurobehavioral function using a battery of tests.

Subcutaneous injections of various doses of CPO, causing 10% (0.15 mg/kg), 20% (0.18 mg/kg) inhibition of acetylcholinesterase in the brain of postnatal day four (PND4) pups, or control doses of vehicle, were administered daily to mouse pups starting on PND4 through weaning at PND21. Pups were observed daily for both the appearance of developmental landmarks and to ensure that there were no signs of acute toxicity. Early reflexes were assessed by evaluating reflex righting, cliff avoidane and negative geotaxis. After weaning, mice were tested for neuromotor function on a rotorod and activity levels were assessed using an open field test. Sensory-motor reflexes were tested with an acoustic startle and pre-pulse inhibition on PND22 and again on PND50. Learning and memory were assessed using a fear conditioning test, the Morris water task and a water radial arm maze test. No significant differences in reflexes, motor coordination or activity levels were revealed, although some trends were observed that might become significant after testing of all experimental cohorts. The intermediate dose group showed deficits in Morris maze retention at one week and high dose mice had more errors and required more time to find a platform on day two of the radial arm maze. Additional mice will need to be tested to determine if any of the trends apparent in these first two cohorts might become statistically significant. With the possible exception of reflex righting and startle amplitude, these results suggest that additional research of neonatal exposure to CPO should focus on assessments of cerebella and spatial learning and memory.