Status epilepticus induces increasing neuronal excitability and hypersynchrony as revealed by optical imaging

In the wake of acquired brain insults such as status epilepticus (SE), time-dependent neuronal network alterations may occur resulting in cortical hyperexcitability and enhanced synchrony merging into chronic epilepsy. To better understand the underlying processes, we performed electrophysiological and optical imaging studies on combined hippocampal-entorhinal cortex slices. These were prepared from rats 1, 4 and 8 weeks after electrically-induced SE. Non-invasive imaging using intrinsic optical signal changes allowed detailed analysis of onset and spread patterns of seizure-like events (SLE) since coverage of the entire preparation is possible. The latency to occurrence of first SLEs after omission of Mg(2+) from the artificial cerebrospinal fluid was significantly reduced at 4 and 8 weeks after SE compared with all other groups indicating increased brain excitability. Optical imaging displayed multiregional onset and discontiguous propagation of SLEs 8 weeks after SE. Such patterns indicate neuronal hypersynchrony and are not encountered in naïve rodents in which SLEs commonly begin in the entorhinal cortex and display contiguous spread to invade adjacent regions. The electrophysiological and optical findings of the current study indicate evolving fundamental brain plasticity changes after the detrimental event predisposing to chronic epilepsy. The current results should be incorporated in any strategies aiming at prevention of chronic epilepsy.