Centrifugal drive onto local inhibitory interneurons of the olfactory bulb.

The olfactory bulb is known to receive signals from sensory neurons and to convey them to higher processing centers. However, in addition to relaying sensory information to the cortex, the olfactory bulb is actively involved in sensory information processing. Hence, olfactory sensory inputs generate a reproducible spatial pattern of restricted activation in the glomerular layer that is subsequently transformed into highly distributed patterns by lateral interactions between output relay neurons and diverse types of local interneurons. Odor representation is thus highly dynamic and temporally orchestrated, right from the first central relay of the olfactory system. This major function of the olfactory bulb is subject to extensive local and extrinsic synaptic influences. The external (or centrifugal) inputs include the dense innervations preferentially targeting the granule cells of the olfactory bulb. The continuous arrival of newly generated neurons in the olfactory bulb of adults provides another source of plasticity influencing the olfactory circuitry. This review deals with the neuromodulation of granule cell activity and of the continuous recruitment of these cells throughout life.