Activity-regulated cytoskeleton-associated protein (arc) is an immediate early gene (IEG) whose mRNA is targeted to dendrites of activated synapses. The present study investigated the expression of three IEGs, arc and two transcription factors (NGFI-B and Nurr 1), in regions of the brain of rats that were subjected to a single afterdischarge-inducing electrical stimulation of the left amygdala and in the amygdala-kindled rats that developed consecutive generalized seizures. One hour after a single stimulus was administered to non-kindled rats, mRNA levels of arc and Nurr1 increased significantly in the ipsilateral piriform cortex and medial amygdaloid nucleus. mRNA levels of NGFI-B increased significantly in the left piriform cortex, basolateral amygdaloid nucleus and medial amygdaloid nucleus. In the amygdala-kindled rats, mRNAs of the three IEGs increased significantly in bilateral brain regions including the piriform cortex and dentate gyrus at 0.5-1 h after the last generalized kindled seizure, and returned near to basal levels by 6 h after. Photographic emulsion autoradiograms showed that the arc mRNA signals induced by the last generalized kindled seizures were associated with the cell bodies of the dentate granule cell layer and extended beyond the granule cell layer into the molecular layer, which contains the dendrites of granule cells. These results support that arc serves as an effector IEG whose mRNA is associated with synaptic reorganization in kindling.
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