Unique Functional Properties of Mature Adult-Born Neurons in the Mouse Olfactory Bulb.

The rodent olfactory bulb (OB) is continuously supplied with adult-born cells maturing into GABAergic neurons. Using in vivo ratiometric Ca imaging to readout ongoing and sensory-driven activity, we asked whether mature adult-born cells (mABCs) in the glomerular layer of the bulb become functionally identical to resident GABAergic (Res) neurons. In awake head-restrained mice the two cell populations differed significantly in terms of ongoing spontaneous activity, with 24% of mABCs contributing to a strongly active cell cluster, absent among Res cells. Odor-evoked responses of mABCs were sparse, less reliable, and had smaller amplitudes compared with Res cells. The opposite was seen under anesthesia, with response reliability increasing and response size of mABCs becoming larger than that of Res cells. Furthermore, ongoing activity of mABCs showed increased sensitivity to ketamine/xylazine and was selectively blocked by the antagonist of serotonin receptors methysergide. These functional features of mABCs clearly distinguish them from other OB interneurons.