I'm aware of this research article that explores the relationship between motion and serotonin systems: 5-HT and motor control: a hypothesis.
Based on the article above, it seems to me that a "very active" individual would have serotonergic system firing rapidly throughout the day. Each step or motion would cause activity - Serotonin release in synaptic cleft, then reuptake.
I'm interested in what happens when such "very active" individual suddenly cannot perform as much activity every day due to a life change (sedentary job, bed bound in hospital, etc). What happens to the serotonin reuptake when the activity level is dramatically reduced for an extended period of time?
In particular, I'm interested if "very active" people have a "very active" serotonin reuptake system that clears Serotonin rapidly. Can this be a cause of depression or reduced mood when an activity level is reduced?