Epileptiform activity influences theta-burst induced LTP in the adult hippocampus: a role for lipid raft dynamics in early metaplasticity.

Non-epileptic seizures are identified as a common epileptogenic trigger. Early metaplasticity following seizures may contribute to epileptogenesis by abnormally altering synaptic strength and homeostatic plasticity. We now studied how in vitro epileptiform activity triggers early changes in CA1 long-term potentiation (LTP) induced by theta-burst stimulation (TBS) in rat hippocampal slices and the involvement of lipid rafts in these early metaplasticity events. Two forms of epileptiform activity (EA) were induced: 1) interictal-like EA triggered by withdrawal of Mg2+ and K+ elevation to 6mM in the superfusion medium or 2) ictal-like EA induced by bicuculine (10M) delivery. LTP induced 30 min post EA was impaired, an effect more pronounced after ictal-like EA. LTP recovered to control levels 60 min post interictal-like EA but was still impaired 60 min after ictal-like EA. The synaptic molecular events underlying this altered LTP were investigated 30 min post EA. Synaptosomes isolated from parallel slices showed enhanced AMPA GluA1 Ser831 phosphorylation but decreased Ser 845 phosphorylation and a marked decrease in GluA1/GluA2 ratio. A marked decrease in flotillin-1 and caveolin-1 levels concomitantly with a moderate increase in PSD-95 and marked increase in gephyrin levels. Altogether, EA differentially influences hippocampal CA1 LTP thorough regulation of GluA1/GluA2 levels and AMPA GluA1 phosphorylation suggesting altered LTP post-seizures may be a relevant target of antiepileptogenic therapies. In addition, this metaplasticity is also associated with marked alterations in classic and synaptic lipid raft markers, suggesting these may also constitute promising targets in epileptogenesis prevention.