Abstracts

Rationale: We have previously demonstrated that specific dysregulation in the direct cortical input to area CA1 of the hippocampus (the temporoammonic [TA] pathway) but not in cortical input to the dentate gyrus or internal hippocampal circuits (the Schaffer collateral pathway) was evident in an animal model of epilepsy (Ang et al. 2006). If this aberrantly powerful TA pathway participated directly in seizure generation, we hypothesized that the degree of dysregulation should then correlate with epilepsy severity in epileptic animals. We therefore assessed seizure frequency as a measure of epilepsy severity in a number of rats, measured the efficacy of the TA pathway in these same animals, and conducted a correlation analysis to test this hypothesis.Methods: Epilepsy was produced in rats using pilocarpine injection as previously described (Ang et al. 2006). The pilocarpine-induced status epilepticus (SE) was terminated (with either diazepam or pentobarbital) after 30-60 min to increase variability in epilepsy severity. Rats were video-monitored for 56-104 hrs to determine seizure frequency. Animals were then sacrificed, and horizontal hippocampal-entorhinal cortical slices (350 µm) were prepared. Slices were stained with the voltage-sensitive dye (VSD) JPW 3031. Trains of four stimuli (200 µs each, 100-200 µA, 100 Hz) were applied to the TA pathway by a bipolar stimulation electrode. VSD signals were imaged using an 80 x 80 CCD camera (1 kHz frame rate) during epi-illumination by a 150 W Xenon lamp. Voltage changes in the tissue were expressed as relative changes in fluorescence (ΔF/F). VSD signals from two regions of interest in stratum lacunosum-moleculare (SLM) and stratum radiatum (SR) of area CA1 were compared and the power of the TA pathway expressed as a ratio of VSD_SR /VSD_SLM.Results: Seizure frequency in the epileptic animals ranged from 0.0001 to 12.7 seizures per day. As in earlier studies, all slices from epileptic animals (n=8) displayed excitatory TA input to CA1 with activation of all layers. In control slices from untreated naïve animals (n=8) stimulation of the TA pathway induced excitation of SLM followed by strong inhibition in SR resulting in a net inhibitory activation of CA1. All epileptic animals had abnormally powerful TA inputs. In addition, the efficacy of TA input in slices from epileptic animals correlated strongly and exponentially with seizure frequency: The VSD_SR /VSD_SLM ratio increased rapidly as the number of seizures per day increased, and reached a plateau in animals with high seizure frequencies.Conclusions: Our data demonstrates that the abnormal excitation generated by TA input to hippocampal area CA1 correlates with epilepsy severity (reflected as seizure frequency). Although this data is correlative, this finding lends support to the hypothesis that TA pathway abnormality may contribute directly to seizure generation (and epilepsy severity) in temporal lobe epilepsy. (Source of founding: NINDS R37 NS32403 to D.A.C.)