In vivo glutamate decline associated with kainic acid-induced status epilepticus

Abstract

Neurophysiological, biochemical, and anatomical evidence implicates glutamatergic mechanisms in epileptic seizures. Until recently, however, longitudinal characterization of in vivo glutamate dynamics was not possible. Here, we present data using in vivo magnetic resonance spectroscopy (MRS) optimized for the detection of glutamate to identify changes that evolve following kainic acid (KA)-induced status epilepticus. Wild-type male Wistar rats underwent whole-brain MR imaging and single-voxel MRS on a clinical 3 T scanner equipped with a high-strength insert gradient coil. Scanning took place before and then 3 days, 28-32 days, and 42-50 days after induction of status epilepticus. Analyses compared 5 seizure (Sz), 5 no-seizure (NoSz; received KA but did not exhibit seizures), and 6 control (Con) animals. This longitudinal study demonstrated reduced glutamate levels in vivo in the dorsal hippocampus 3 days and 1 month following status epilepticus in Sz animals compared with Con animals. Additionally, previous results were replicated: in the Sz group, computed T2 was higher in the ventral hippocampus and limbic cortex 3 days after seizure activity compared with baseline but resolved in both regions at the 1 month scan, suggesting a transient edema. Three days following seizure activity, N-acetylaspartate (NAA) declined and lactate increased in the dorsal hippocampus of the Sz group compared with the Con and NoSz group; both metabolites approached baseline levels by the third scan. Taken together, these results support the conclusion that seizure activity following KA infusion causes loss of glutamatergic neurons.

Figure 1

top: manually drawn regions of interest (ROIs) on late-echo axial slices for computation of T2. bottom: Percent computed T2 increase in the 3 regions, 3 days after kainic acid-induced status epilepticus. * p≤.05 between Sz group and Con group, and Sz group and NoSz group.

Figure 2

Late-echo (top panel) and computed T2 (bottom panel) MR images of an axial brain slice of a single rat. Closed arrows point to the ventral hippocampus, open arrows to the limbic cortex. Note the regions of hyperintensity on computed T2 images at 3 days, and the presence of fluid-filled cysts in the ventral hippocampus at 28 and 42 days following status epilepticus.

Figure 3

top: Early-echo MR images of an axial brain slice of a single seizure rat demonstrating enlargement of ventricular volume. bottom: Thresholded ventricles on T2 computed images from the same animal.

Figure 4

Quantified ventricular volume of each seizure animal at the 3 time points following kainic acid- induced status epilepticus.

Figure 5

Voxel placement in the dorsal hippocampus and cerebellum including reproducibility of voxel placement across the first 3 scan time points.

Figure 6

Sample spectra from a single seizure animal (dashed, red) and a single control animal (solid, blue) for voxels in the dorsal hippocampus (top) and cerebellum (bottom) across the first 3 scan sessions.

Figure 7

Water-referenced metabolite levels in the dorsal hippocampus at baseline (MRS 1), 3 days (MRS 2), and 28–32 days (MRS 3) after kainic acid (Sz+NoSz) or water (Con) injection as quantified using peak integration. * p<.05 between Sz and Con, # p<.05 between Sz and NoSz.

Figure 8

Water-referenced metabolite levels in the cerebellum at baseline (MRS 1), 3 days (MRS 2), and 28–32 days (MRS 3) after kainic acid (Sz+NoSz) or water (Con) injection as quantified using peak integration. * p<.05 between Sz and Con, # p<.05 between Sz and NoSz, + p<.05 between Con and NoSz.

Figure 9

Correlations (non parametric Spearman Rank) between ventricular volume and NAA or glutamate, and NAA and glutamate at the third scan.