Long-term imaging reveals dynamic changes in the neuronal composition of the glomerular layer

The mammalian olfactory bulb (OB) contains a rich and highly heterogeneous network of local interneurons (INs). These INs undergo continuous turnover in the adult OB in a process known as "adult neurogenesis." Although the overall magnitude of adult neurogenesis has been estimated, the detailed dynamics of the different subpopulations remains largely unknown. Here we present a novel preparation that enables long-term in vivo time-lapse imaging in the mouseOBthrough a chronic cranial window in a virtually unlimited number of sessions. Using this preparation, we followed the turnover of a specific neuronal population in the OB, the dopaminergic (DA) neurons, for as long as 9 months. By following the same population over long periods of time, we found clear addition and loss of DA neurons in the glomerular layer. Both cell additionandloss increased over time.ThenumbersofnewDAcells were consistentlyandsignificantly higher than lostDAcells, suggesting a net increase in the size of this particular population with age. Over a 9monthperiod of adult life, the net addition ofDAneurons reached_13%.Our data argue that the fine composition of the bulbar IN network changes throughout adulthood rather than simply being replenished.