Immature Hippocampus: Epileptogenic Properties
This program is based on the premise that recurring seizures in early-life contribute to cognitive deficits in children with intractable epilepsy. Animal models of early-onset epilepsy have advanced our understanding of the effects seizures have on brain development. Several laboratories have shown that infantile recurring seizures can produce deficits in learning and memory. These effects are not explained by the death of neurons. However, dendritic abnormalities in hippocampal pyramidal cells - including a reduction in dendritic branching - have been reported. Consistent with this observation are recent findings of decreased expression of dendritic glutamatergic postsynaptic proteins. Our group has recently shown that these effects are reproduced in an in vitro model of developmental epilepsy, where epileptiform activity is induced in hippocampal slice cultures by treatment with a GABAa receptor antagonist.
During the course of these studies evidence accumulated that the primary deficit produced by epileptiform activity was not on glutamatergic synaptic transmission but on dendrites. Further studies confirmed this and showed that chronic disinhibition actually prevents the growth of dendrites and in a NMDA receptor dependent manner. Using our in vitro model, additional studies explored molecular mechanisms responsible for the suppression of dendrite growth. Results showed that activity through a signaling cascade previously implicated in dendrogenesis, the MAPK-CREB signaling pathway, is “shut-off” by chronic network hyperexcitability. Our most recent results have confirmed 3 critical aspects of our in vitro data in an in vivo model. Based on these results we hypothesize that recurrent seizures in early-life suppress the growth of dendrites and the “shut-off” in signaling to CREB contributes to dendrite growth suppression.
To test these hypotheses four experiments are planned. Using the Flurothyl Model we plan to directly demonstrate that recurrent seizures suppress the growth of dendrites. Dendrite reconstructions at selected developmental time points will attempt to show that developmental increases in animals that have experienced seizures lag behind those observed in controls. The functional consequences of growth suppression will be examined electrophysiologically. Other experiments will assess the NMDA receptor dependency of dendrite growth suppression as well as seizure-induced learning deficits. Additional studies in vivo will set out to show that the shut-off in CREB signaling is an antecedent of and therefore potentially causative in growth suppression. Transfections of slice culture neurons with the constitutively active form of CREB are aimed at directly implicating a shut-off in CREB signaling in dendrite growth suppression. By identifying the cellular and molecular events responsible for dendrite growth suppression, novel therapies may be developed to prevent learning deficits produced by recurring seizures.
Relevance of the project to IDDRC mission:
Commonly children with intractable epilepsy are cognitively impaired. The goals of this program are to understand the mechanisms underlying this developmental disability so new treatments can be developed.