Virtually every form of sensory stimulation has been shown to ease depression, from aromatherapy to acupuncture to exercise. What makes you think that neurotransmitters are the root cause of the problem? To me the fact that no matter how you mess with the neurotransmitters it eases depression shows that this is a function of sensory novelty, as opposed to some neurotransmitter imbalance being the root cause.
"There are certainly many theories, and depression is a complicated disease."
What makes you think it's a single disease?