From Nature:
The way in which antidepressants exert their effects on brain cells has been revealed by two separate teams of researchers working independently of each other.
Antidepressants work by preventing neurons in the brain from importing certain chemicals, such as dopamine and serotonin, which are used to pass messages from cell to cell. The route by which these chemicals are imported depends on passageways in the outer membrane of the cells called transporter proteins, and it is on these passageways that the antidepressants exert their influence. But how exactly they hold up the process has remained a mystery since the drugs were discovered 45 years ago, says Les Iversen, a pharmacologist at the University of Oxford, UK.
More here.