Abstracts

Rationale: While seizures are infrequent, long term intracranial recordings demonstrate far more frequent interictal discharges between seizures. Previous studies from our laboratory on human epileptic brain tissues have shown a ‘final common pathway' of specific genes that are upregulated at human epileptic foci. A large proportion of these genes belong to the mitogen-activated protein kinase (MAPK) and cAMP Response Element Binding Protein (CREB) pathways. Surprisingly, the expression levels of these genes do not correlate with seizure frequency, but correlate precisely with interictal spiking, suggesting that theses genes either produce or result from ongoing, electrical discharges. In order to understand interictal spike generation and their roles in epileptogenesis animal models and drugs that specifically modulate interictal spiking are needed. Methods: A neocortical spiking focus was induced in rats using a single tetanus toxin injection into somatosensory cortex. Following injection, six epidural electrodes (3 on each hemisphere) were implanted to monitor epileptic activities through long-term video/EEG recordings using an automated spike detection program. Regions of spiking and non-spiking cortex were analyzed for MAPK/CREB activation and downstream gene activations. A selective MEK inhibitor that blocks the MAPK pathway was tested on interictal spiking. Results: Progressive interictal spiking can be produced in localized brain regions using a single tetanus toxin injection. The system was optimized using a conical, rather than a beveled needle to concentrate the toxin in more superficial cortical layers. Long term EEG quantification was achieved using a spike detection algorithm that removes artifact channels and identifies spikes in each channel using a combination of two differed frequency filters and block scaling. In this model, focal spikes were produced with increasing frequency and amplitude over time. In addition, the spike field expanded over time, both anterior to and contralateral to the injection site. Virtually no seizures were detected. Interictal spiking was associated with a similar pattern of phosphoCREB and downstream gene inductions seen in human epileptic cortex, mostly in superficial cortical layers. The use of a selective MEK inhibitor, early on, blocked both CREB activation and also prevented the development of interictal spiking. Conclusions: This animal model recreates both the electrophysiology and molecular mechanisms of human neocortical epilepsy and therefore could be a valuable model for preclinical drug development. Our results show that interictal spiking is sufficient to induce persistent changes in the MAPK/CREB pathway that parallels observations in human epileptic neocortex. MAPK/CREB activation is required for the development of a chronic interictal state and therefore could represent a new anti-epileptogenic target.