A mere decade ago, the idea that the adult brain could generate new cells was a hotly contested issue. It's easy to see why. Imagine buying a brand new computer, setting it up in your house, and then letting your kids randomly insert new resistors and capacitors into the motherboard while you were working on it! Forgetting for a moment how dangerous it would be to let kids play with a functioning electrical device, just think of how illogical it sounds to insert new components into something that was designed and built in a highly specified manner just in order to function properly in the first place. This was the conundrum facing neurobiologists. Why would we go through this exquisite and prolonged developmental process to establish functional neuronal circuits and then go and ''randomly'' introduce new cells into the brain throughout adulthood? New cells would be intrusive, they would have to compete with already established cells for both energy and synaptic connections, and scientists didn't (and some would argue still don't) have a good idea as to why we would require these new neurons in the first place.
Despite the fact that the idea of adult neurogenesis seemed to fly in the face of conventional wisdom, it has been established by a number of laboratories that new cells were indeed being created in the adult brain (Altman and Das, 1965;Kaplan and Hinds, 1977;Cameron et al., 1993;Kuhn et al., 1996). Indeed, in the past 20 years we are also beginning to appreciate that existing cells change constantly and that dendritic growth and shrinkage, spine turnover, and synaptic protein movement, are all common place in a healthy brain. As a result our view of the human brain as some sort of complex, but static, computer circuit board, is slowly changing and more and more scientists believe it is more appropriate to envision the brain as an organism that is a dynamic entity, constantly changing in response to the environment around us. The study of how the brain responds to the world around us is an enormous and convoluted Endeavour, however in this issue of Hippocampus, we present a number of research efforts that initially started as projects to examine how two factors, stress and exercise, could alter how the brain produces new cells in the dentate gyrus subfield of the hippocampus.
The issue leads off with two articles that examine how stress can differentially impact how exercise alters the processes of cell proliferation and survival (Kannangara et al., this issue; Snyder et al., this issue). The third article, by Leasure and Decker (this issue) shows that there are sex differences in the way rats respond to social isolation and that this can impede the ability of exercise to induce cell proliferation in these animals, highlighting a difference between rat and mouse animal models in this type of work.