Corticothalamic feedback: a key to explain absence seizures.
Alain Destexhe

In: Computational Neuroscience in Epilepsy, Edited by Soltesz, I. and Staley, K., Elsevier, Amsterdam, pp. 184-214 (2008).

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Over the last years, decisive experimental data have been obtained concerning the biophysical mechanisms and ion channels properties important for seizure generation. Computational models have succeeded in proposing plausible mechanisms to explain the sudden emergence of hypersynchronized oscillations at ~3 Hz (or 5-10 Hz in some species), which are associated with “spike-and-wave” complexes in the electroencephalogram (EEG). The underlying mechanisms of such seizures involve thalamocortical loops, the particular oscillatory properties of thalamic neurons, and the particular biophysical properties of some receptor types (such as the GABAB receptor). Here, we overview these mechanisms step by step, starting from the genesis of hypersynchronized oscillations by thalamic circuits. We next consider how cortical circuits can generate spike-and-wave EEG patterns. These mechanisms are then merged together in thalamocortical loops, where we emphasize the central role played by the “feedback” projections from cortex to thalamus. If for some reason the corticothalamic feedback becomes too strong, thalamic circuits can switch to a slower and hypersynchronized oscillatory mode, which in turn entrains the whole thalamocortical system into hypersynchronized oscillations with spike-and-wave EEG patterns. We suggest that the key to explain absence seizures is this switching mechanism of thalamic circuits, induced by exceedingly strong corticothalamic feedback. Such a switch was identified in experiments in vitro, in which oscillatory properties could be controlled by stimulating corticothalamic fibers. According to this mechanism, absence seizures result from anomalously high cortical excitability with a physiologically intact thalamus.