Cellular-level pathological changes in the kainate mouse model of temporal lobe epilepsy (TLE) have been well-characterized immunohistochemically (IHC) and include neuronal injury followed by granule cell dispersion. Non-invasive tracking of these changes by MRI/MRS might reveal early biomarkers for epilepsy, leading to an improved understanding of epileptogenesis and potentially opening up the possibility of early interventions. Here we investigate the possibility to track granule cell dispersion and neuronal injury using diffusion imaging and ¹H-spectroscopy with quantitative validation with IHC.

Mouse model of temporal lobe epilepsy: Unilateral hippocampal injection of kainic acid (KA) was used to induce epileptogenesis in C57BL/6N mice (n=8). Saline-injected animals (n=5) served as controls.

The animals underwent MR scans before and 1, 4, 8, 16 and 31 days after KA/saline injection. Subsequent immunohistochemistry (IHC) was used to characterize pathological changes.

MR System: 7T small animal system (Bruker BioSpec) equipped with a CryoProbe (Bruker, Ettlingen, Germany).

Diffusion imaging: spin-echo DTI-EPI, 30 diffusion directions, b=1000s/mm², TE=33ms, TR=2.5s, 3 segments, (1.4x interpolated) resolution 58x58x400μm³, acquisition time 24min prolonged by respiratory triggering. A global optimization tractography approach¹ was also used to reconstruct microstructural features from the diffusion data.

¹H-spectroscopy: PRESS, TE=20ms, TR=2.5s, NA=400, voxel 2x1.4x1.4mm³ placed in the septal part of the hippocampus, quantification of spectra with LCModel (Provencher, Canada).

During epileptogenesis, DTI and tractography revealed a strong increase in dorso-ventral diffusivity in the ipsilateral dentate gyrus (Fig.1,a-c). This effect may be caused by a hypertrophy of radial glial cells and sprouting of mossy fibers within the dispersed granule cell layer (Fig.1,d).

Subsequently the total dispersed volume was assessed by counting voxels with elevated dorso-ventral diffusivity. The dispersed volume quantified by IHC correlated with the volume quantified by DTI at 31 days; interestingly a strong correlation could also be identified with the DTI quantification at earlier time points (Fig.1, bottom).

¹H-Spectroscopy revealed very early and persistent decreases of N-acetylaspartate (NAA) and glutamate (Glu) concentrations in the ipsilateral HC (Fig.2, top). The reduction of NAA and Glu at day 1 correlated well with the extent of the microgliosis in CA1 identified by IHC, and therefore may reflect the neuronal injury after KA injection (Fig.2, bottom).

The reduction of Glu (also NAA, not shown here) concentrations also correlated strongly with the later development of a dispersed granule cell layer (Fig.3).

Neuronal cell death caused by an injury and the development of a dispersed granule cell layer are markers to identify TLE with IHC. The reduction of NAA and Glu at day 1 after KA injection found by MRS and the development of a dispersed volume identified by DTI at 8 days correlated with the results from IHC performed in the chronic phase and can thus be used as early markers of epileptogenicity in this mouse model of TLE.