A few weeks after receiving botox treatment for wrinkles, around half of patients find relief from depression. (See https://www.ncbi.nlm.nih.gov/pubmed/24910934.) The obvious conclusion is they're simply happier with how they look. But what if it's more than that?
#1 - Botox suppresses the release of acetylcholine.
#2 – Nicotinic acetylcholine receptors (nAChRs) respond to acetylcholine. https://en.wikipedia.org/wiki/Nicotinic_acetylcholine_receptor
#3 - nAChRs in the brain can become desensitized after repeated exposure to stimulants. http://web.as.uky.edu/Biology/faculty/cooper/Bio401G/nicotineDesen.pdf
#4 - Recent drug research on new antidepressants has been targeting nAChRs: http://www.hindawi.com/journals/tswj/2012/104105/
So here's the basic theory:
We in the west eat a large amount of choline-rich food, so we can easily synthesize lots of acetylcholine. The constant exposure to plentiful acetylcholine in conjunction with other internal and external stimulators of nAChRs can result in desensitization of the nicotinic acetylcholine receptors in our brains, dysfunction of which receptors is associated with depression. Botox allows a "reboot" of the system by cutting the acetylcholine supply for a while, and the nAChRs become more sensitive again.
And here's how to test it (beside getting botox treatments): Go on a low-choline diet (unless you're pregnant) for a 3-4 days, during which it would probably be good to also avoid anything that would mess with nAChRs (esp. nicotine, alcohol, and recreational drugs). Here’s a link to the choline content of foods. See if you feel different a couple weeks later. Don't do the low-choline diet for an extended period, though. We need choline, just maybe not quite as constantly as we get it.
Perhaps the non-constant supply of choline-rich animal products (which are relatively expensive) in the diet of people in poor countries is part of the reason why it is sometimes found that rates of depression are higher in richer countries. Also intriguing is the connection between gardening (soil is full of the bacteria that makes botox) and less severe depression.