Abstracts

RATIONALE: The piriform cortex (PC) and the perirhinal cortex have been suggested as an essential substrate for the development and propagation of forebrain (limbic type) seizures and for the development and expression of secondary generalized motor seizures in amygdala kindling, respectively. For further characterization of the critical regions involved in the progression of amygdala kindling, we studied the effect of selective bilateral excitotoxic lesions of different parts of the PC and of the perirhinal cortex before onset of amygdala kindling in rats. METHODS: Neuronal lesions of the anterior, central or posterior PC, or of a large part of the perirhinal cortex were obtained by bilateral microinjections of ibotenic acid (0.8 mg in 0.2 ml saline) in anesthetized female Wistar rats (n=6-10)using stereotaxic methods. Controls received the same amount of saline. All rats also received a biploar stimulation/recording electrode into the right basolateral amygdala. Two weeks later the rats were kindled by daily stimulation of the amygdala, and the afterdischarge threshold before and after kindling and the kindling parameters were recorded. After the experiments, the brains of all rats were histologically analyzed for size and location of lesions and electrode. RESULTS: The lesions had a volume of 0.5-1 mm3 and usually comprised most of layers II/III and the adjacent endopiriform nucleus of the anterior, central or posterior PC, or all layers of the perirhinal cortex. Bilateral lesions of the central PC, but not of the other regions were able to significantly decrease the kindling rate, particularly the transition from stage 3 to stage 4/5. The extent of the lesion within the central PC and the kindling rate were significantly correlated. The cumulative afterdischarge to reach stage 4/5 was significantly longer in rats with lesions of the central PC. CONCLUSIONS: The results indicate that the central piriform cortex is part of an epileptic network which is important for limbic epileptogenesis by amygdala kindling. Other parts of the piriform cortex and particularly the perirhinal cortex seem not to be critically involved in this process.