Expression of Fos in the piriform cortex after acquisition of olfactory learning: an immunohistochemical study in the rat.

The piriform cortex (PCx), the main area of the primary olfactory cortex, is assumed to play a role in olfactory memory. Involvement of this paleocortex in mnesic processes was investigated by using Fos immunocytochemistry after acquisition of a two-odor discrimination task. Trained rats had to associate one odor of a pair with water reward while pseudo-trained rats were randomly rewarded. We further used non-trained rats and home cage control animals to determine the effect of manipulation and basal Fos level respectively. Except in control rats, Fos immunoreactivity was mainly distributed in brain areas involved in olfactory processing, learning and arousal. The trained, pseudo-trained, and non-trained rats showed a high Fos labeling in the entire PCx. However, quantitative analysis demonstrated a statistically higher Fos immunoreactivity in the anterior PCx in comparison with the posterior PCx for these rats. Furthermore, behavioral data allowed us to distinguish two groups of trained rats according to the number of days required to acquire the task. Rats with slow acquisition showed a higher Fos immunoreactivity in the whole PCx in comparison with the rats exhibiting a fast acquisition. Our findings support the assumption of a PCx rostro-caudal heterogeneity which could sustain differential information processing.