A magnetic resonance imaging study of early brain injury in a rat model of acute DFP intoxication

Abstract

Current treatments for seizures induced by organophosphates do not protect sufficiently against progressive neurodegeneration or delayed cognitive impairment. Developing more effective therapeutic approaches has been challenging because the pathogenesis of these delayed consequences is poorly defined. Using magnetic resonance imaging (MRI), we previously reported brain lesions that persist for months in a rat model of acute intoxication with the OP, diisopropylfluorophosphate (DFP). However, the early spatiotemporal progression of these lesions remains unknown. To address this data gap, we used in vivo MRI to longitudinally monitor brain lesions during the first 3 d following acute DFP intoxication. Adult male Sprague Dawley rats acutely intoxicated with DFP (4mg/kg, sc) were MR imaged at 6, 12, 18, 24, 48, 72h post-DFP, and their brains then taken for correlative histology to assess neurodegeneration using FluoroJade C (FJC) staining. Acute DFP intoxication elicited moderate-to-severe seizure activity. T2-weighted (T2w) anatomic imaging revealed prominent lesions within the thalamus, piriform cortex, cerebral cortex, hippocampus, corpus striatum, and substantia nigra that corresponded to neurodegeneration, evident as bands of FJC positive cells. Semi-quantitative assessment of lesion severity demonstrated significant regional variation in the onset and progression of injury, and suggested that lesion severity may be modulated by isoflurane anesthesia. These results imply that the timing of therapeutic intervention for attenuating brain injury following OP intoxication may be regionally dependent, and that longitudinal assessment of OP-induced damage by MRI may be a powerful tool for assessing therapeutic response.

Keywords: In vivo imaging; Neuropathology; Organophosphate; Seizure; T2-weighted MRI.

Figure 1. T2w anatomic MR images depicting the spatiotemporal progression of brain lesions following acute DFP intoxication

Lesions were characterized by localized hyperintensity compared to neighboring tissue, and a vehicle control reference scan. Group 1 and Group 2 images each represent a single animal imaged at 6, 18, 48 and 72 h, or 12, 24 and 72 h respectively. Note that Group 1 begins MR imaging 6 h prior to Group 2. Across time points, images were selected to match bregma to ≤ 250 µm. Arrows indicate the positions of brain regions displaying hyperintense lesions at one or more time points following intoxication. Sagittal-view atlas reference was adapted from Watson and Paxinos, The Rat Atlas in Stereotactic Coordinates (Paxinos, 2007).

Figure 2. T2w anatomic MR images depicting brain regions with sporadic lesions across study animals

Images of lesions in the septal area, caudate putamen, globus pallidus, and substantia nigra collected from Group 1 animals. The images displaying the substantia nigra and caudate putamen lesions are from the same animal at two separate time points; the other images are each from separate animals.

Figure 3. Lesions detected by T2w MRI at 72 h post DFP intoxication correspond to regions of ongoing neurodegeneration as assessed by FluoroJade C (FJC) staining

The vehicle control animal shows no T2w hyperintensity or FJC positive neurons in the hippocampus or piriform cortex. In contrast, DFP intoxication produced hyperintensity in the hippocampus and piriform cortex that shows spatial registry to bands of FJC positive cells. Magnification: 100×.

Figure 4. Mixed-model regression from multiple brain regions demonstrate that the lesion severity changes over time in a brain-region and imaging group-dependent manner

Each progression curve is modeled on data from six animals from Group 1 (solid lines) or six animals from Group 2 (dashed lines). 95% confidence bands are included around the curves for each imaging group. Group 1 animals had significantly lower lesion severity scores for the thalamus and piriform cortex than Group 2 animals over the entire study period (denoted with an “*”). Note: Group 1 animals received anesthesia 6 h earlier than Group 2 animals.