An interesting, and apparently well-researched post from io9 on the status of research related to depression that illustrate how little we actually know. Though the title is little too linkbaity for my tastes.
The corollary to the chemical imbalance theory, which implies that raising brain serotonin levels alleviates depression, has also been hard to prove. In fact, the serotonin-depleting drug reserpine was itself shown to be an effective anti-depressant in the 1950s, the same decade in which other studies claimed that reserpine caused depression-like symptoms. At the time, few psychiatrists acknowledged these conflicting reports, as the studies muddled a beautiful, though incorrect, theory. Tianeptine is another drug that decreases serotonin levels while also serving as a bona-fide anti-depressant. Tianeptine does just the opposite of SSRIs – it enhances serotonin reuptake. Wellbutrin is a third anti-depressant that doesn’t increase serotonin levels. You get the picture.
If you prefer your data to be derived more accurately, but less relevantly, from rodents, you might consider a recent meta-analysis carried out by psychologist Paul Andrews (McMaster University, Ontario) and colleagues. This meta-analysis revealed that, in rodents, elevated serotonin was usually associated with depression. While Andrews argues that depression is therefore a disorder of too much serotonin, it’s clear from decades of conflicting literature that any chemical imbalance theory, serotonin low or high, will be exceedingly difficult to prove.
Well worth a read.