How do ssris treat depression?

Placebo effect: Ssris- are selective serotonin reuptake inhibitors. They allow serotonin to stay present in the synapses of the brain for longer periods of time. Serotonin has been shown to be associated with mood and therefore allowing it to be present longer is believed to result in improved mood. However, most research indicates that the majority of improvement is likely placebo for mild/moderate depression.

Neurogenesis: Ssri antidepressants work by blocking reuptake of serotonin from synapses. Because they have a delayed onset of antidepressant effect (several weeks) relative to speed of blocking serotonin reuptake (several hours), any explanation has to account for this discrepancy. One hypothesis is that there is growth of new neurons in the hippocampus that is responsible for antidepressant effect.