Neurogenesis and neuronal recruitment occur in many vertebrates, including humans. Most of the new neurons die before reaching their destination. Those which survive migrate to various brain regions, replace older ones and connect to existing circuits. Evidence suggests that this replacement is related to acquisition of new information. Therefore, neuronal replacement can be seen as a form of brain plasticity that enables organisms to adjust to environmental changes. However, direct evidence of a causal link between replacement and learning remains elusive. Our hypothesis is that increased neuronal recruitment is associated with increase in memory load. Moreover, since neuronal recruitment is part of a turnover process, we assume that the same conditions that favor survival of some neurons induce the death of others. I present studies that investigated the effect of various behaviors and environmental conditions (food-hoarding, social change, reproductive cycle) on neuronal recruitment and survival in adult avian brains, and discuss how these phenomena relate to the life of animals. I offer a frame and rationale for comparing neuronal replacement in the adult brain, in order to uncover the pressures, rules, and mechanisms that govern its constant rejuvenation. The review emphasizes the importance of using various approaches (behavioral, anatomical, cellular and hormonal) in neuroethological research, and the need to study natural populations, in order to fully understand how neurogenesis and neuronal replacement contribute to life of animals. Finally, the review indicates to future directions and ends with the hope that a better understanding of adult neuronal replacement will lead to medical applications.
Bibliographical noteFunding Information:
Thanks are due also to graduate students from the Zoology Department at Tel-Aviv University, who performed the experiments described in the review: Einat Adar, Shay Barkan, Dina Lipkind, Adina Mishal, and Meital Pnini. Their studies were supported by several generous funding agencies: The Institute for Psychobiology in Israel, The Israel Science foundation (Grant #481/04), and The Open University Research Fund. I would also like to thank Gila Haimovic for editing the manuscript. And finally, I am grateful to the organizers of the 9th International Symposium on Avian Endocrinology who invited me to participate in this interesting event in Leuven, Belgium in July 2008.
- Adult brain
- Environmental changes
- Neuronal replacement