Learning-stage dependent Fos expression in the rat brain during acquisition of an olfactory discrimination task.

By using Fos immunocytochemistry, we investigated the activation in olfactory-related areas at three stages (the first and fourth days of conditioning and complete acquisition) of an olfactory discrimination learning task. The trained rats (T) had to associate one odour of a pair with water-reward within a four-arm maze whereas pseudo-trained (P) rats were only submitted to the olfactory cues without any reinforcement. In the piriform cortex, both T and P rats exhibited a higher immunoreactivity on the first day, which seemed to indicate a novelty-related Fos expression in this area, but whatever the learning-stage, no significant difference in Fos expression between T and P rats was observed. In hippocampus, Fos expression was significantly different between T and P rats in CA1 and CA3 on the first and fourth days respectively. Thus we showed a differential activation of CA1 and CA3 subfields which might support a possible functional heterogeneity. In the orbitofrontal cortex, Fos immunoreactivity was significantly higher in T rats compared to P rats when mastery of the discrimination task was complete. In contrast, no learning-related Fos expression was found in infralimbic and prelimbic cortices. The present data suggest an early implication of the hippocampal formation and a later involvement of neocortical areas throughout different stages of a progressively acquired olfactory learning task.