17 Activity Dependency and Aging in the Regulation of Adult Neurogenesis

Age and activity might be considered the two antagonistic key regulators of adult neurogenesis. Whereas adult neurogenesis declines with age, different kinds of “activities” positively regulate adult neurogenesis. An interaction between these two mechanisms exists. Aging influences aging, and activity affects aging processes. Aging is a principal determinant of life and as such cuts across all biological, psychological, and sociological research. The very essence of aging is difficult to conceptualize, because it is a uniquely omnipresent variable. Aging can thus only be addressed in an transdisciplinary approach, especially if the consequences of aging on complex brain functions are to be studied. In the context of neurogenesis research, “aging” has so far been largely equaled with the biology of long timescales. Implicit in this understanding is that age-dependent changes essentially reflect a unidirectional development in that everything builds on what has occurred before. In this sense, aging can also be seen as continued or lifelong development. This idea has limitations but is instructive with regard to adult neurogenesis because adult neurogenesis is neuronal development under the conditions of the adult brain. The age-related alterations of adult neurogenesis themselves have quantitative and qualitative components. So far, most research has focused on the quantitative aspects. But there can be little doubt that qualitative changes do not simply follow quantitative changes, for example, in cell or synapse numbers but emerge on a systems level and above, when an organism ages. The observation that adult neurogenesis is regulated by activity relates to this idea. From...