Sleep oscillations developing into seizures in corticothalamic systems
The aim of this article is to discuss the neuronal substrates of sleep oscillations leading to seizures consisting of spike-wave (SW) complexes at 2-4 Hz, mimicking those seen in absence epilepsy, or SW and polyspike-wave (PSW) complexes at 1.5-2.5 Hz, often associated with fast runs at 10-15 Hz, as in the Lennox-Gastaut syndrome. Extracellular recordings were done in permanently implanted animals during the natural waking-sleep cycle. Single and dual simultaneous recordings from cortical neurons, cortical and thalamic neurons, or cortical neurons and glial cells were performed in cats under ketamine-xylazine anesthesia. (a) The minimal substrate of SW seizures is the neocortex because such seizures may occur in thalamectomized animals, in which spindles are absent. In intact-brain animals, SW seizures are initiated in neocortex and spread to the thalamus after a few seconds. The majority of thalamocortical (TC) neurons are steadily hyperpolarized throughout the cortical SW seizures. (b) In the Lennox-Gastaut syndrome, the paroxysmal depolarizing shifts (PDSs) associated with the EEG "spike" of SW/PSW complexes contain an important inhibitory component, whereas the hyperpolarization during the EEG "wave" component is not due to gamma-aminobutryic acid (GABA)ergic inhibitory postsynaptic potentials (IPSPs) but is ascribed to a mixture of disfacilitation and K+ currents. As is also the case with seizures consisting of pure SW complexes, the majority of TC neurons are hyperpolarized during the cortical paroxysms and disinhibited after the cessation of cortical seizures. Seizures with SW complexes and of the Lennox-Gastaut type preferentially evolve from sleep oscillations. They are initiated in neocortex and spread to the thalamus after a few seconds. The majority of TC neurons are inhibited during these seizures.