Repeated exposures to diisopropylfluorophosphate result in structural disruptions of myelinated axons and persistent impairments of axonal transport in the brains of rats

Organophosphates (OPs) are found in hundreds of valuable agricultural, industrial, and commercial compounds; however, they have also been associated with a variety of harmful effects in humans. The acute toxicity of OPs is attributed to the inhibition of the enzyme acetylcholinesterase (AChE); however, this mechanism may not account for all of the deleterious neurologic effects of OPs, especially at doses that produce no overt signs of acute toxicity. In this study, the effects of two weeks of daily subcutaneous exposure to the OP-nerve agent diisopropylfluorophosphate (DFP) in doses ranging from 0.125-0.500 mg/kg on whole brain volume, white matter, and gray matter integrity were evaluated in post mortem tissues using histology and magnetic resonance imaging (MRI) methods. The effects of DFP on axonal transport in the brains of living rats were evaluated using a manganese-enhanced MRI (MEMRI) method. DFP was associated with dose-dependent impairments in red blood cell and brain AChE (down to 29 and 18% of control, respectively at the highest dose), 24 h after the last injection. However, there were no visible signs of cholinergic toxicity noted in any portion of the study. Moreover, histological and MRI analysis of post mortem brains did not reveal any pronounced alterations of whole brain, white matter, or gray matter volumes associated with DFP. Electron microscopy did reveal a DFP-related increase in structural disruptions of myelinated axons (i.e., decompactions) in the fimbria region on the corpus callosum. MEMRI indicated that DFP was also associated with dose-dependent decreases in axonal transport in the brains of living rats, an effect that was also present after a 30-day (DFP-free) washout period, when AChE was not significantly inhibited. These results indicate that repeated exposures to the nerve agent, DFP at doses that are below the threshold for acute toxicity, can result in alterations in myelin structure and persistent decreases in axonal transport in the rodent brain. These observations could explain some of the long-term neurological deficits that have been observed in humans who have been repeatedly exposed to OPs.