Piriform cortex ictogenicity in vitro

The piriform cortex is recognized to play critical roles in focal ictogenesis, both in animal models and in humans. We review here the contribution of in vitro studies performed on rodent brain tissue that were aimed at understanding the ictogenic properties of the piriform cortex and the contiguous olfactory areas. During in vitro experiments, epileptiform events can be easily generated in the piriform area by diverse pro-convulsive drugs (4-aminopyridine, bicuculline, picrotoxin) or by electrical stimulation. Simultaneous intracellular and field potential recordings performed on in vitro preparations, which include brain slices of rats and mice and the isolated brains of guinea pigs, demonstrated that both the piriform cortex proper and the endopiriform nucleus (also considered part of the piriform area) generate interictal spikes, high-frequency oscillations and seizure-like activities that mimic focal discharges. These findings were confirmed both by optical recordings of intrinsic signals coupled with brain activity and by fast imaging of optical signals generated by voltage-sensitive dyes. Overall, these studies demonstrated that epileptiform discharges effectively propagate from the piriform structures to the limbic regions, supporting the conditions for secondarily generalized ictogenesis.