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There is actually substantial critique against the simple "chemical hypothesis" that depression is caused by low levels of serotonin. See for instance:
http://www.mindhacks.com/blog/2005/05/is_depression_a_brai.html
http://www.mindhacks.com/blog/2005/11/depression_and_the_l.html
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nrn/journal/v6/n3/full/nrn1629_fs.html
Coincidentally, depression is essentially defined as a condition of lowered interest and pleasure. If you are depressed then you lose interest in things that used to interest you and you have a difficult time deriving pleasure from formerly pleasing activities. Therefore it's no surprise if there is some anecdotal evidence that drugs targetting the dopamine system can help alleviate some symptoms of depression. After all, the hallmarks of the excited dopamine system are heightened feelings of interest and pleasure.
So why is it that antidepressant drugs target the dopamine system? I'm not sure, but it may be because the dopamine system isn't a good thing to mess with. Another hallmark of exciting the dopamine system is that it induces strong feelings of wanting, craving-- so any drug that excites it is likely to be highly addictive. You can get rats to compulsively drink a bitter salt solution that they otherwise hate if you stimulate their dopamine system, even while they exhibit behaviors normally indicative of displeasure. And of course in the human case, there are any number of drugs that affect the dopamine system that are notoriously addictive and thus damaging in the long run. So it's strong stuff.
Of course, that's not all. Excessive levels of dopamine can lead to paranoid, delusional, and otherwise warped thinking-- see schizophrenia for an extreme case. Another potential danger is that regular use of a dopamine agonist could possibly lead to lowered biological production of dopamine in the brain. Cessation of the drug, combined with lowered levels of internally produced dopamine, could then cause a sharp drop in dopamine levels. But dopamine levels that are too low are also bad. For extreme examples of what can go wrong with levels of dopamine that are too low, see Huntington's Disease and Parkinson's Disease. (Again, not coincidentally, Huntington's and Parkinson's are both often associated with depression.)
http://www.mindhacks.com/blog/2005/05/is_depression_a_brai.html
http://www.mindhacks.com/blog/2005/11/depression_and_the_l.html
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nrn/journal/v6/n3/full/nrn1629_fs.html
Coincidentally, depression is essentially defined as a condition of lowered interest and pleasure. If you are depressed then you lose interest in things that used to interest you and you have a difficult time deriving pleasure from formerly pleasing activities. Therefore it's no surprise if there is some anecdotal evidence that drugs targetting the dopamine system can help alleviate some symptoms of depression. After all, the hallmarks of the excited dopamine system are heightened feelings of interest and pleasure.
So why is it that antidepressant drugs target the dopamine system? I'm not sure, but it may be because the dopamine system isn't a good thing to mess with. Another hallmark of exciting the dopamine system is that it induces strong feelings of wanting, craving-- so any drug that excites it is likely to be highly addictive. You can get rats to compulsively drink a bitter salt solution that they otherwise hate if you stimulate their dopamine system, even while they exhibit behaviors normally indicative of displeasure. And of course in the human case, there are any number of drugs that affect the dopamine system that are notoriously addictive and thus damaging in the long run. So it's strong stuff.
Of course, that's not all. Excessive levels of dopamine can lead to paranoid, delusional, and otherwise warped thinking-- see schizophrenia for an extreme case. Another potential danger is that regular use of a dopamine agonist could possibly lead to lowered biological production of dopamine in the brain. Cessation of the drug, combined with lowered levels of internally produced dopamine, could then cause a sharp drop in dopamine levels. But dopamine levels that are too low are also bad. For extreme examples of what can go wrong with levels of dopamine that are too low, see Huntington's Disease and Parkinson's Disease. (Again, not coincidentally, Huntington's and Parkinson's are both often associated with depression.)