SUSCEPTIBILITY OF DIFFERENT CELL-LAYERS OF THE ANTERIOR AND POSTERIOR PART OF THE PIRIFORM CORTEX TO ELECTRICAL-STIMULATION AND KINDLING - COMPARISON WITH THE BASOLATERAL AMYGDALA AND AREA TEMPESTAS

Several lines of evidence suggest that the piriform cortex functions as a generator in the development and propagation of forebrain (limbic type) seizures, particularly in the kindling model of epilepsy. It is, however, not clear where, within the rather large piriform cortex region, the generator resides, and how much tissue is involved. Highly sensitive loci to chemical or electrical stimulation have been described both in the deep anterior and posterior parts of the piriform cortex. Furthermore, data from piriform cortex slice preparations indicated that epileptiform potentials originate in deep structures, particularly the endopiriform nucleus that underlies the piriform cortex. In the present study, in rats, we implanted stimulation and recording electrodes in various rostrocaudal locations of the piriform cortex and endopiriform nucleus, including the ''area tempestas'', i.e. a structure in the anterior part of the piriform cortex previously proposed to be critically involved in the generation of convulsive seizures of limbic origin. Within the piriform cortex, electrodes were aimed at different cellular layers of this structure. For comparison, additional animals received electrodes in different parts of the basolateral amygdala. A total of 19 different locations was obtained in this way. The susceptibility of these locations to electrical stimulation was characterized by determining the threshold for induction of afterdischarges. The afterdischarge threshold was lowest in layer III of the posterior piriform cortex and some locations in the endopiriform nucleus, whereas amygdala and ''area tempestas'' displayed higher values. In several animals, particularly those with electrodes in layer III of the posterior piriform cortex, spontaneous spiking was seen in prestimulation recordings, whereas this was never observed in recordings from the amygdala. Subsequent kindling by repeated stimulation of the various locations demonstrated marked differences in afterdischarge threshold reduction and kindling rate. The most marked decreases in afterdischarge threshold were seen in locations within layer III of the piriform cortex, whereas several other locations, including the ''area tempestas'', exhibited only moderate decreases or no decrease at all. In contrast to previous observations with only few locations in the piriform cortex region, the posterior piriform cortex was not in general slower to kindle than the anterior piriform cortex, although some locations in the posterior piriform cortex exhibited significantly lower kindling rates than the amygdaia. The highest kindling rate was seen in the dorsal endopiriform nucleus. When the stepwise progression of kindling was studied, several locations in the piriform cortex stayed for long periods in the early and middle, i.e. focal stages of the kindling process, thus refuting previous suggestions that the piriform cortex generally proceeds rapidly through these stages. Behavioral characteristics of seizures kindled from the diverse sites did not differ. In conclusion, within the amygdala-piriform cortex region, the most sensitive cells to electrical induction of afterdischarge and reduction of afterdischarge threshold by repeated electrical stimulation reside in layer III of the posterior piriform cortex, while the anterior part of the piriform cortex, including the locus previously termed ''area tempestas'', is less susceptible in this regard. In contrast to the marked decrease in afterdischarge threshold, several locations in the posterior piriform cortex kindle only slowly, thus demonstrating a separation between decrease in seizure threshold and increase in seizure severity in response to repeated electrical stimulation.