A LEARNED ODOR DECREASES THE NUMBER OF FOS-IMMUNOPOSITIVE GRANULE CELLS IN THE OLFACTORY-BULB OF YOUNG-RATS

Olfactory stimulation evokes a column of activity within the olfactory bulb extending from the glomerular layer to the granule cell layer th at can be visualized with 2-deoxyglucose autoradiography, optical imag ing, Fos protein immunohistochemistry and c-fos mRNA in situ hybridiza tion. The Fos response to odors is typified by the activity of relativ ely few juxtaglomerular cells, which often occur in foci, and a large number of granule cells extending through much of the bulb. In this st udy, we characterized the granule cell response to an odor for which y oung rats had acquired a preference. Fos-like immunoreactive granule c ells were quantified by image analysis, and densely stained cells were counted in a region previously shown to be responsive to peppermint o dor. We found that odor-trained pups have about half the number of Fos -immunopositive superficial granule cells which respond to a learned o dor than do control pups. We then determined whether there was a corre lation between the juxtaglomerular cell response and the response of t he superficial granule cells deep to those glomerular layer cells. We found a positive correlation between the number of juxtaglomerular cel ls and the number of granule cells demonstrating Fos immunoreactivity in both control and trained pups, a relationship that changed with ear ly olfactory training.