Accumulation of c-fos mRNA in rat hippocampus during acquisition of a brightness discrimination.

Training rats to attain a foot-shock-motivated brightness discrimination in a Y-maze results in an early and transient increase of hippocampal c-fos mRNA levels. Maximal accumulation was observed immediately after training, returning to basal levels during the following 2 h. A similar increase was obtained when rats were subjected to a pseudotraining with an equal number of runs, but with random pairing of the choice of bright and dark alleys with foot shock. It is suggested that induction of hippocampal c-fos mRNA expression is a necessary, but not sufficient, prerequisite for the formation of long-term memory trace. This early gene expression seems rather to correspond to an initial stage induced by complex stimulus presentation of both the training and the pseudotraining procedure. The subsequent late synthesis or processing of target proteins finally contributing to the formation of a permanent trace requires the action of further convergent signals to principal cells, probably mediating reward or emotional influences.