Activation of specific neuronal networks leads to different seizure onset types.

OBJECTIVE: Ictal events occurring in temporal lobe epilepsy patients and in experimental models mimicking this neurological disorder can be classified, based on their onset pattern, into low-voltage, fast versus hypersynchronous onset seizures. It has been suggested that the low-voltage, fast onset pattern is mainly contributed by interneuronal (γ-aminobutyric acidergic) signaling, whereas the hypersynchronous onset involves the activation of principal (glutamatergic) cells.
METHODS: Here, we tested this hypothesis using the optogenetic control of parvalbumin-positive or somatostatin-positive interneurons and of calmodulin-dependent, protein kinase-positive, principal cells in the mouse entorhinal cortex in the in vitro 4-aminopyridine model of epileptiform synchronization.
RESULTS: We found that during 4-aminopyridine application, both spontaneous seizure-like events and those induced by optogenetic activation of interneurons displayed low-voltage, fast onset patterns that were associated with a higher occurrence of ripples than of fast ripples. In contrast, seizures induced by the optogenetic activation of principal cells had a hypersynchronous onset pattern with fast ripple rates that were higher than those of ripples.
INTERPRETATION: Our results firmly establish that under a similar experimental condition (ie, bath application of 4-aminopyridine), the initiation of low-voltage, fast and of hypersynchronous onset seizures in the entorhinal cortex depends on the preponderant involvement of interneuronal and principal cell networks, respectively.