A lack of serotonin is the chemical hypothesis for depression. If you give an SSRI (serontonin reuptake inhibitor) like Prozac or Zoloft, the levels of serotonin are increased and depression should resolve. There are problems with this model. In mild to moderate cases of depression the drugs barely outperform placebo. Fortunately SSRI's work much better for more severe forms of depression. If depression is solely a lack of serotonin, why does it take agonizing weeks for the antidepressants to begin working. If you pop a Prozac on day one, the brain is flooded with serotonin within hours without any reduction in symptoms.
Some researchers believe depression is not the result of a chemical imbalance but argue that the brain's cells are shrinking and degenerating as a result of mental stress.
In 2008 Italian researchers found that Prozac restored plasticity in the visual cortex of rats. The researchers were able to correct the symptoms of lazy eye.
The Japanese article you mentioned extends this hypothesis. Prozac reversed the maturation of cells in the hippocampus.
Although very speculative some researchers believe that the newfound youth and plasticity of cells caused by Prozac is more important in treating depression than raising the serotonin levels.
Here is a quote from Ronald Duman, professor of psychiatry and pharmacology at Yale. "The best way to think about depression is as a mild neurodegenerative disorder, Your brain cells atrophy, just like in other diseases [such as Alzheimer's and Parkinson's]. The only difference with depression is that it's reversible. The brain can recover."
It seems that Dr Duman is saying that the damage of Alzheimer's and Parkinson's is irreversible.
The above was summarized from this articlehttp://scienceblogs.com/cortex/2010/04/prozac_and_plasticity.php
I do have an interest in this topic since I had worked in geriatrics. We had many patients with Alzheimer's that were treated by geropsychiatrists with SSRI's. I never saw any significant improvement, and many had side-effects or an increase in abnormal behavior.