Olfactory bulb neurons express functional, entrainable circadian rhythms.

Circadian pacemakers drive many daily molecular, physiological and behavioural rhythms. We investigated whether the main olfactory bulb is a functional circadian pacemaker in rats. Long-term, multielectrode recordings revealed that individual, cultured bulb neurons expressed near 24-h oscillations in firing rate. Real-time recordings of Period1 gene activity showed that a population of cells within the bulb expressed synchronized rhythmicity starting on embryonic day 19. This rhythmicity was intrinsic to the mitral, and not the granule, cell layer, entrainable to physiological temperature cycles and temperature compensated in its period. However, removal of the olfactory bulbs had no effect on running wheel behaviour. These results indicate that individual mitral/tufted cells are competent circadian pacemakers which normally synchronize to each other. The daily rhythms in gene expression and firing rate intrinsic to the olfactory bulb are not required for circadian patterns of locomotion, indicating that they are involved in rhythms outside the canonical circadian system.