Directional functional coupling during limbic seizures in rats revealed by nonlinear Granger causality

Subject and Purpose - Temporal lobe epilepsy is increasingly recognized to involve widespread network alterations. In this study the temporal dynamics of directional interactions between the frontal cortex, hippocampus, thalamus and midbrain during limbic seizures are analyzed in rats. Methods - Local field potentials were recorded in Wistar rats expressing generalized limbic seizures, Racine's stage 4-5. These spontaneous seizures occur during chronic treatment with a CB1 (cannabinoid receptor type 1) antagonist SLV326. Time-frequency analysis, time-varying adapted nonlinear Granger causality and mutual information were applied. The Granger causality and mutual information values estimated from seizure episodes were compared to those of seizure-free periods. Results - During the seizure, two stages were detected: a high frequency (15-20 Hz) stage followed by a low frequency (2 Hz) stage. At seizure onset, a drop in coupling between all recorded sites was found. After the seizure onset, coupling restored to normal levels in the midbrain - hippocampus pair, but remained reduced in all other studied channel pairs for at least 10 s after the onset. The transition between stages and the seizure termination were characterized by couplings increase in some pairs and by decrease in others. Conclusion - Spontaneous generalized limbic seizures can be considered as a result of pathological reorganization of coupling architecture between different brain structures, developing in time, and providing transitions between seizure stages. These findings bring together views, considering seizures as an increase of couplings in the brain, and hypotheses, regarding seizures rather as a decoupled state.