A critical time window for the recruitment of bulbar newborn neurons by olfactory discrimination learning.

In the mammalian brain, the dentate gyrus and the olfactory bulb are regions where new neurons are continuously added. While the functional consequences of continuous hippocampal neurogenesis have been extensively studied, the role of olfactory adult-born neurons remains elusive. In particular, the involvement of these newborn neurons in odor processing is still a matter of debate. We demonstrate a critical impact of both the age of new neurons and the memory processes involved (learning vs recall) in the recruitment of newborn cells. Thus, odor stimulation preferentially recruited immature neurons over more mature ones (2 weeks old vs 5 and 9 weeks old), whereas associative learning based on odor discrimination preferentially recruited mature neurons (5-9 weeks old). Furthermore, while mature neurons were activated by this associative learning, they were not activated by long-term memory recall, indicating that the contribution of newborn neurons in olfactory functions depends also on the memory process involved. Our data thus show that newborn neurons are indeed involved in odor processing and that their recruitment is age- and memory process-dependent.