Sensory systems must be adaptive to allow animals to cope with changing environments. For hundreds of years, neuroscientists thought that neural plasticity came from changes in the connections between neurons but that the total number of neurons remained fixed after embryonic development. However, in the last couple of decades, it has become clear that new neurons can be generated throughout adult life in a few mammalian brain regions, including the hippocampus and olfactory bulb (OB). How then do these new neurons manage to integrate into the existing brain circuitry in an adult animal?
Jenelle Wallace and Martin Wienisch in the Murthy lab investigated this question in the context of adult-born granule cells (abGCs) in the OB of mice. Previous studies had characterized the time course of cellular and synaptic maturation in ex vivo preparations, but how these abGCs are functionally integrated into the circuit to process odor information remained unknown. Wallace et al. (PDF) used multiphoton imaging to record the responses of individual abGC dendrites in vivo as a mouse sniffs odors. In a technically impressive feat, the authors were able to track review cells’ functional maturation for up to two months. They found that abGCs could respond to smells soon after they finish their migration to the OB and extend dendritic trees. Because their method of tracking individual cells provides more detailed information than previous population imaging strategies, they were able to discover that most cells are initially strongly responsive to many odorants and become more selective for particular odors as they mature; however, some cells exhibited the opposite pattern, hinting at interesting functional heterogeneity within the olfactory circuit.
Adult neurogenesis is thought to play a role in adapting neural circuits to changing environmental conditions. Therefore, Wallace et al. asked whether exposing mice to odors in their home cages (as a form of environmental enrichment) affects the functional maturation of abGCs. They found that enrichment prolonged the period over which abGCs are strongly responsive to odors, which could represent a mechanism for adult-born neurons to explore a greater space of inputs in a richer sensory environment.
This work offers support for the rapid integration of adult-born neurons into existing circuits, followed by experience-dependent refinement of their functional connectivity. Future work will be needed to elucidate how the functional maturation of abGCs allows them to contribute to sensory processing and whether this pattern of functional maturation applies across other brain regions and species.