Distribution of Fos-positive neurons in cortical and subcortical structures after picrotoxin-induced convulsions varies with seizure type.

The distribution of Fos protein was mapped in rat brain following a single non-focal convulsive seizure. Single seizures were induced with intravenous picrotoxin in unhandled animals housed in isolation. Different convulsive behaviours occurred unpredictably. The least severe seizures were predominantly localised to the face, head and forelimbs, without loss of posture control (restricted seizures). The most extensive seizures affected all limbs and trunk, sometimes with falling (generalised seizures). There was a correlation between seizure behaviour and distribution of Fos induction. After restricted seizures, Fos was induced at highest levels in neocortex and piriform cortex and was prominent in entorhinal cortex, caudal-ventral caudate-putamen and amygdala. Regions of thalamus were consistently and lightly labelled, but Fos induction did not occur in hippocampus. After generalised seizures, there was Fos induction in cortex but less than after restricted seizures and, in three of four animals, also in dentate gyrus, hippocampus and subiculum. There was occasional or variable labelling of thalamus, basolateral amygdala and caudate-putamen. One animal with generalised seizures showed no hippocampal Fos induction. The findings indicate that picrotoxin induces seizures with at least two different patterns of neuronal involvement. The cortex, part of the caudate-putamen, amygdala and thalamus are involved in restricted seizures while the hippocampus, cortex and thalamus are involved in generalised seizures. The results do not support the view that generalised seizures are a progression from restricted forms. Cortical Fos involvement is entirely consistent with the participation of cortex in non-focal epilepsy. In these non-focal seizures, the dentate-hippocampus may be a source of excitation to cortex in the generalised group while the cortex appears to be the predominant site of excitation in the restricted group.