Expression of c-fos mRNA following seizures evoked from an epileptogenic site in the deep prepiriform cortex: regional distribution in brain as shown by in situ hybridization.

Using in situ hybridization histochemistry for the detection of c-fos mRNA, we examined the pathways activated by seizures evoked by a focal application of bicuculline into a highly discrete epileptogenic site in the deep prepiriform cortex, the area tempestas (AT). Thirty minutes after the initiation of limbic motor seizures evoked by bicuculline in AT, a marked increase in c-fos mRNA was detected in the hippocampal formation, amygdala, olfactory bulb, piriform cortex, and entorhinal cortex. The increase of c-fos mRNA was strictly dependent upon the infusion of the drug in AT. Infusions of bicuculline in the same dose outside the AT did not increase c-fos mRNA levels. The extent to which the mapping pattern of c-fos mRNA expression was specific to limbic seizures was evaluated by examining another focally evoked seizure model involving the application of bicuculline bilaterally into the inferior colliculus. The absence of any detectable c-fos induction in the limbic system after explosive running-bouncing clonic seizures evoked by bicuculline injected into the inferior colliculus indicates that the pattern of activation that we found in the AT-evoked seizure model is not common to all forms of convulsive activity. Furthermore these observations suggest that the pattern of activation we have observed is seizure dependent and not stress induced. Our results indicate that c-fos mRNA expression is useful in the functional mapping of pathways involved in seizure propagation and that the anatomic pattern of activation is selectively related to the type of seizure evoked.