In one experiment, Jacobs and collaborators administered the antidepressant drug Prozac to rats for three weeks and found a 70 percent increase in neurogenesis in the hippocampus. Jacobs further developed the idea when he spent a sabbatical working with neuroscientist Fred Gage of the Salk Institute in San Diego. Gage, who is his co-author on the papers that describe the new theory of depression, has shown that exercise and a stimulating environment both factors that might relate to depression promote neurogenesis in rats. As Jacobs originally discussed in the May issue of Molecular Psychiatry, several other lines of reasoning support his hypothesis. Recent research by Gould has shown, for example, that stress, which is a common trigger for depression, also is a potent suppressor of neurogenesis.
Stress promotes the release of steroids, which inhibit new cell development in the hippocampus.
Using MRI scans, other researchers had also shown that the size of the hippocampus is reduced in patients with depression. Also, patients who suffer from a form of epilepsy that severely impairs the hippocampus have a high incidence of depression. The true test, he said, would be to observe directly the birth of new neurons in people with depression and to compare the observations to those of healthy people. Unfortun-ately, such technology does not exist, although Jacobs expressed confidence that it will be developed. In the meantime, there are further animal tests that could give the theory additional backing.