Abstracts

Rationale: Dendrites are critical regulators of synaptic integration and overall neuronal excitability. In rodent models of acquired temporal lobe epilepsy (TLE), an episode of status epilepticus (SE) results in hippocampal dendritic injury with spine loss and homeostatic changes that alter the morphological and physiological properties of the remaining dendrites. These alterations are associated with significant long term morbidity, including the occurrence of unprovoked seizures and cognitive deficits. Aberrant mTOR activation has been reported following SE in rodents. Thus, we hypothesized that SE-induced mTOR pathway hyperactivation is a candidate mechanism underlying the dendritic pathology and memory deficits following SE. Therefore, we evaluated whether inhibition of SE-induced excessive mTOR signaling reversed these changes associated with SE.Methods: Male Sprague Dawley rats at postnatal day ~35 were treated with pilocarpine (Pilo) to induce 1 hr of SE. Vehicle sham-treated animals (controls) were processed in parallel. After 2 weeks, rapamycin (Rap; 6mg/kg) or vehicle was administered by intraperitoneal injection every other day for 1 week to suppress mTOR activation in sham (Sham + Rap or Sham + Veh) and pilocarpine treated rats (Pilo + Rap or Pilo + Veh). We used the Morris water maze (MWM) test to evaluate early hippocampal-dependent learning and memory deficits. Following behavioral tests we used Golgi staining to evaluate dendritic structure and immunohistochemistry to examine dendritic localization of Kv4.2 and HCN1 channels. Western blotting was used to evaluate mTOR pathway activation using downstream phosphorylation targets such as S6.Results: Testing in the Morris water maze (MWM) revealed that Pilo + Veh rats had a significantly longer latency to reach the hidden platform in the training phase and no preference for the quadrant that had housed the hidden platform in the probe trial compared to Sham + Veh or Sham + Rap (p < 0.01), which was reversed in the Pilo + Rap group. Golgi staining revealed a dramatic improvement in the structural appearance of hippocampal CA1 dendrites in the Pilo + Rap group compared to the Pilo + Veh. This correlated with a high density of Kv4.2 and HCN1 channels that co-localized with Map2 throughout the dendritic length in area CA1 in the groups treated with Rap. Western blotting confirmed mTOR hyperactivation in the Pilo + Veh group that was attenuated significantly with Rap treatment (p < 0.05).Conclusions: These findings suggest that acute mTOR hyperactivation plays a role in the early hippocampal-dependent memory dysfunction associated with SE. In parallel our findings suggest that mTOR dysregulation may contribute to the dendritic instability and aberrant localization of ion channels following SE.