Regulation of c-fos expression in the rat olfactory bulbs during olfactory learning

The distribution of c-Fos-immunopositive neurons was examined in the mitral/tufted and granular cell layers in the medium part of the main olfactory bulbs of 18-day-old rats after they had been trained for propionic acid vapour-guided search for dam in the Y-maze. On the next day these pups exhibited a strong preference for the propionic acid odor as compared to the control pups trained for this task without the odor cue and odor-familiarized pups exposed to propionic acid as a novel neutral stimulus. Exposure to propionic acid produced a moderate activation of c-Fos expression, mainly in the granular layer of the dorsomedial part of the bulb. Training in the Y-maze devoid of odor cues resulted in diffuse increase in the number of c-Fos-positive neurons both in the mitral and granular cell layers in all parts of the olfactory bulb. Maze training with the odor cue produced activation of c-Fos expression (which significantly exceeded the non-odor Y-maze group) in the dorsomedial olfactory bulb. These data suggest that associative olfactory conditioning results in activation of c-Fos expression that combines the effect of diffuse motivational excitation and specific olfactory input to the neurons which process odor cues.