ODOR-INDUCED INCREASES IN C-FOS MESSENGER-RNA EXPRESSION REVEAL AN ANATOMICAL UNIT FOR ODOR PROCESSING IN OLFACTORY-BULB

Expression of the immediate-early gene c-fos was used to evaluate the coordinate activation of olfactory bulb neurons by brief exposure to s pecific odors in the alert rat. In situ hybridization to c-fos mRNA wa s compared to regional increases in 2-deoxy-D-[C-14]glucose incorporat ion in an adjacent section analysis. Levels of c-fos mRNA in olfactory bulb were high in rats recently removed from their home cage but were low in animals placed in a relatively odor-free chamber for 30 min. P resentation of specific odors to alert rats for as little as 5 min inc reased c-fos mRNA in radially distributed neuronal ensembles that span ned the lamina of the main olfactory bulb. The complementary RNA (cRNA )-labeled neuronal collectives consisted of cells in the glomerular la yer that precisely defined the borders of individual glomeruli and und erlying tufted, mitral, and granule cells. The activated fields were m uch broader in the granule cell layer than in the overlying glomerular layer and thus exhibited a flask-like, as opposed to a columnar, cont our. The bulbar distribution of cRNA-labeled cell arrays differed with different odors and, in the glomerular layer, corresponded to focal r egions of high 2-deoxy-D-[C-14]glucose uptake. Administration of the n oncompetitive N-methyl-D-aspartate receptor antagonist MK801 did not a ttenuate the odor induction of c-fos but, instead, increased c-fos mRN A levels throughout the bulb. We propose that the neuronal ensembles e xpressing increased c-fos mRNA with odor stimulation represent princip al functional units of sensory processing in the main olfactory bulb o f the behaving rat.