Fos induction following systemic kainic acid: early expression in hippocampus and later widespread expression correlated with seizure.

We determined the distribution of Fos protein expression in a model of generalised epilepsy caused by excessive neuronal excitation. Fos immunoreactivity was mapped in forebrain in unrestrained rats, previously prepared with an indwelling venous catheter, after the intravenous administration of kainic acid (10 mg/kg). We determined cerebral activation following various periods of exposure to kainic acid by using intravenous administration of pentobarbitone to prevent further activation. Within a few minutes, kainic acid caused episodes of staring, sniffing, wet dog shakes, nodding and chewing. Fos induction occurred initially and simultaneously in hippocampus, subiculum, septum and entorhinal cortex as early as 9.5 min after kainate injection. After up to 40 min of staring, sniffing, wet dog shakes, nodding and chewing, Fos induction was not further increased above levels present within the first 9.5 min. After 56 +/- 6 min a motor convulsion occurred, initially affecting the jaw, head and tail and variably extending to the forelimbs, trunk or hindlimbs. Following the convulsive event, additional Fos was expressed in hippocampus, thalamus, caudate-putamen and other subcortical structures and in the cerebral cortex. Fos induction was sometimes asymmetric in entorhinal, visual, piriform, cingulum, parietal and frontal cortices and in amygdala and dorsal endopiriform area. Electroencephalographic recordings after a few minutes exposure to kainic acid revealed an increased amplitude of fast frequencies in hippocampus which appeared to correlate with Fos induction in this structure. The findings are generally consistent with the reported distribution and slow development of kainic acid-induced seizure activity using electrophysiological and deoxyglucose methods. However, the Fos distribution suggests that (i) hippocampal, possibly dentate, activation precedes significant activation elsewhere, (ii) extensive involvement of other cerebral structures and cerebral cortex occurs simultaneously and correlates with motor seizures and (iii) brain structures can be recruited asymmetrically.