" Essentially, the phrase “increasing serotonin activity” can mean a number of different things. It can mean that your brain makes more serotonin or increases receptors for it, or that those receptors just become stickier to serotonin. It can also mean that the serotonin that’s made isn’t broken down as quickly or that the serotonin squirted into the synapse just hangs out for a while—giving it a longer opportunity to bind to the next neuron—instead of being quickly sucked back into the neuron. Changing any one of these factors can increase serotonin activity. "

― Alex Korb , The Upward Spiral: Using Neuroscience to Reverse the Course of Depression, One Small Change at a Time