High susceptibility of the anterior and posterior piriform cortex to induction of convulsions by bicuculline.

Accumulating evidence suggests that the piriform cortex (PC) plays a critical role in the development of limbic motor seizures. In the anterior piriform cortex (aPC), a functionally defined, discrete epileptogenic site has been previously identified by unilateral microinjection of bicuculline in Sprague-Dawley (SD) rats and termed the 'area tempestas' (AT). Compared to this site in the aPC, more posterior PC sites, particularly a site in the transition zone between the posterior and aPC (central PC) exhibited a greater susceptibility to electrical stimulation. However, it is not known whether central and posterior sites in the PC differ from the aPC, including the AT, with regard to their sensitivity to bicuculline. In the present study, unilateral focal microinfusion of picomole quantities of bicuculline induced behavioural (focal and generalized) seizures in deep layers of all parts of the PC in two rat strains, Wistar and SD. The incidence of generalized seizures was higher in the AT of SD rats, but no such difference was seen in Wistar rats, arguing against the previous proposal that the rat AT is unique in its sensitivity to induction of seizures by bicuculline compared to other locations within or outside of the PC. Injection of biotin-dextran in PC seizure-sensitive sites in SD rats showed clear differences in anterograde and retrograde labelling between the different PC sites. Therefore, although it was possible to evoke generalized seizures from all parts of the PC, the anatomical connections of the bicuculline injection sites were qualitatively different. The results suggest that the deep layers of the entire PC are highly sensitive to seizure induction by bicuculline, thus substantiating the notion that the PC may be important in seizure generation and propagation.