Regulation of c-Fos gene expression in the rat olfactory bulb during olfactory learning.

An immunohistochemical method was used to study the expression of transcription factor c-Fos in the mid part of the main olfactory bulbs in 18-day-old rats after training to seek their mothers using an olfactory orientation cue (propionic acid) in a Y-maze. On the next day, rat pups demonstrated a significant preference for the propionic acid odor in behavioral tests, as compared with control animals trained to the skill without the olfactory orientation cue and pups familiarized with the odor without forming any association between it and any kind of reinforcement. The propionic acid odor evoked an insignificant increase in c-Fos expression, predominantly in the granular layer of the dorsomedial area of the olfactory bulb. Training in a maze lacking the odor signal evoked diffuse activation of c-Fos, in both the mitral and granular layers of all areas of the olfactory bulb. Training with the olfactory orientation cue was also accompanied by an increase in c-Fos expression to a level significantly exceeding that seen after odor-free training, in the granular layer of the dorsomedial area, i.e., the area in which the unreinforced odor acting alone induced insignificant activation of expression. These data provide evidence that, at the level of the regulation of neuronal transcription, associative learning is manifest as summation of the effects of a new neutral stimulus and excitation initiated by the motivational state. In the olfactory bulb, this summation is seen in areas on which non-specific centrifugal excitation and specific afferentation from the signal odor converge.