Abstracts

Rationale: Sprouting and reorganization of hippocampal projections have been demonstrated in human and experimental Temporal Lobe Epilepsy (TLE). Sprouting across hippocampal lamellae has been shown in the mossy fiber projection with new recurrent collaterals into the Dentate Gyrus and the CA3 region using anatomical techniques. Similarly, epileptic sprouting has also been demonstrated in the CA1 axons with new recurrent collaterals into CA1 and across additional lamellae for its projection to Subiculum. We have hypothesized that these anatomical phenomena result in translamellar hyperexcitability (Cavazos and Cross, 2006). We examined this hypothesis with field extracellular physiological techniques in the Kainic Acid (KA) model. Methods: In-vivo evoked field potentials were recorded from fixed subicular columns 1 wk, 3 wks, or several months after saline (control) or KA injections that induced convulsive status epilepticus (KA groups). SD male rats were monitored to assess the development of spontaneous seizures. Stereotactic recordings were obtained under urethane anethesia using tungsten recording electrodes (0.5M?) and Platinum-Iridium bipolar stimulating (intertip distance 0.5mm) electrodes inserted 6.3mm posterior from bregma and 2.5 or 4.5mm lateral from midline, respectively. To optimize the CA1-Subiculum evoked field potential (fEPSP), electrodes were inserted at 20-25 degrees from horizontal plane. Depth of insertions were 4.5mm (recording) and 7.0 0.24mm (stimulation) from the dorsal surface of the brain. After recording spontaneous activity, 0.5ms square wave pulses of 1V were triggered at 2Hz to evoke fEPSPs in Subiculum. fEPSPs were averaged at least for 10 events at each stimulating depth. To determine the functional-topographical organization of the CA1 projection to the Subiculum, we moved the stimulation electrode every 100?m along dorsal-ventral axis of CA1 pyramidal layer. Animals were prepared for histology to assess location of electrodes and presence of sprouting. Results: Frequent spontaneous fEPSPs were seen in the Subiculum several months, but not at 1 wk after KA. They were very rare in controls. The dorso-ventral extent of CA1 stimulation that evoked fEPSP in Subiculum was 4.4mm in controls vs 5.9 mm in KA rats. Evoked fEPSPs in Subiculum consisted of three components that evolve in morphology at different depths of CA1 stimulation. There were progressive alterations in the amplitude, area under the curve, slope and duration of fEPSPs in KA groups as compared to controls. For example, a progressive increase in the maximal duration of Subicular fEPSPs was noted (20.7 ms in controls vs 109.4 ms in KA rats). Conclusions: Progressive physiological alterations in the hippocampal formation were observed in experimental TLE. There was an increase functional interconnectivity across hippocampal lamellae at the CA1 projection to Subiculum in the KA rat model correlating with progressive increases in sprouting, cellular hyperexcitability and spontaneous seizures. Funding: VA Merit (JEC); AHA 0865151F (GT)