From ASAP Science: What's going on inside the brain of a depressed person?
Recent thinking suggests that rather than a shortage of serotonin, a lack of synaptogenesis (the growth of new synapses, or nerve contacts) and neurogenesis (the generation and migration of new neurons) could cause depression.
The main group of medications to treat depression, SSRIs, might promote synaptogenesis and neurogenesis by turning on genes that make ITGB3 as well as other proteins that are involved in these processes. ITGB3 stands for integrin beta-3.
If the neurogenesis and synaptogenesis hypothesis holds, a drug that specifically targeted miR-221 or miR-222 could bring sunnier days to those suffering from depression. The miRs are two microRNA molecules.
From DW: What helps relieve depression, according to Professor Malek Bajbouj, Berlin's Charité Hospital:
Unraveling the Mystery of How Antidepression Drugs Work. Scientific American, 2013.