Enhanced fos expression within the primary olfactory and limbic pathways induced by an aversive conditioned odor stimulus.

A central question in olfactory learning is how animals become tuned to odor stimuli that gain significance through conditioning. A leading view is that tuning to conditioned odor stimuli involves functional modifications within the primary olfactory pathways, themselves.(7) Here we studied this idea further by investigating responses within the olfactory system to an odor that had previously been paired with footshock in classical fear-conditioning trials in adult rats. Using the transcription factor Fos as a marker of odor-induced neuronal activation,(1,14) we found that in rats that had received forward pairings of odor and footshock during training, presentation of the conditioned odor stimulus, alone, produced an enhanced increase in levels of Fos in the main and accessory olfactory bulbs and anterior olfactory nucleus compared with that found in animals that had received backward presentations of the stimuli or of odor alone. These results demonstrate that Fos responses to an odor within the primary olfactory pathways can be modified through aversive conditioning, and are consistent with other evidence that olfactory conditioning can lead to functional changes within these pathways.(7)