Abstracts

Rationale: Patients with mesial temporal lobe epilepsy (MTLE) often suffer an initial insult to the brain several years before spontaneous, recurrent seizures start. This insult is thought to initiate a series of structural, molecular, and metabolic changes resulting in epilepsy. To study this process, we used diffusion tensor imaging (DTI) to probe the underlying tissue microstructure. Previously, we examined the long-term structural changes associated with the glutamine synthetase inhibitor, methionine sulfoximine (MSO) model of MTLE. Here, we seek to examine structural changes in the early stages of epileptogenesis in the MSO model. Methods: Male Sprague Dawley rats (370-430g) were unilaterally implanted with a 28-day osmotic pump that continuously infused either MSO (n=11) or phosphate buffered saline (PBS; n=9) in the right entorhinal cortex at a rate of 0.25 μL/hr. Immediately after surgery, rats were attached to a video-intracranial electroencephalography (EEG) recording system and monitored for 4 days before being perfused with 4% paraformaldehyde in PBS. Brains were extracted and fixed for three weeks. Prior to scanning, brains were washed with PBS and placed in a MRI-compatible tube filled with Fluorinert. For both anatomical and DTI images, 34 coronal slices of 100 μm were acquired at 9.4T to cover the whole brain. The individual rat brains were registered to a reference rat brain to generate averaged anatomy and fractional anisotropy (FA) maps. Difference maps (MSO minus PBS) were generated and then masked to only show statistically significant (p<0.05) differences. Results: All of the MSO and none of the PBS rats exhibited seizures. Reflective of this early stage of epileptogenesis, seizures were predominately Racine scale stage 1 seizures, with no stage 5 seizures. Rats on average had a total of 12-13 seizures. Significant differences in FA were seen bilaterally throughout the brain. Increases in FA were seen bilaterally in the thalamic structures, substantia nigra, and posterior cortical amygdaloid nucleus, and contralaterally in the piriform cortex. Bilateral decreases in FA were also seen in the primary somatosensory cortex, cingulate cortex and hippocampus. The averaged anatomical images exhibited very few differences, with the most significant differences in the upper cortex and injection site. Conclusions: Short-term, injection of MSO in the entorhinal cortex results in significant bilateral structural alterations that presumably persist through the chronic stages of MTLE. The underlying cellular and molecular correlates to these changes are currently being investigated by immunohistochemistry. These structural changes may provide new early biomarkers to identify risk factors for severe seizures. This work was supported by National Institutes of Health (NIH) grants NS058674 and NS070824 (to TE), by CTSA grant number UL1 RR024139 from the National Center for Research Resources (NCRR), a component of the NIH, and NIH Roadmap for Medical Research. Its contents are solely the responsibility of the authors and do not necessarily represent the official view of the NCRR or NIH.