Although it is not suggested by this paper (which was brought to my attention by F1000 biology), I’d like to suggest this as a very, very naive hypothesis: That some part of the anti-depressant effect of SSRIs could be mediated by their ability to promote liver regeneration, as the above Science article suggests. The paper itself details how 5HT-2A and 2B receptors are upregulated during regeneration and how 2A & 2B antagonists slow regeneration.
There is some evidence for a connection between liver disease and depression. (Also, traditional chinese medicine views depression as a disease of the liver.) Without a doubt, the liver plays a well-established role in general detoxification.
Although I have no evidence to suspect it, I think it would be interesting to see if the SSRIs were leading to increased liver serotonin, resulting in liver regeneration and hence less depression.