Selective serotonin reuptake inhibitors (SSRIs) are commonly used antidepressants; however, their mechanism of action is elusive. The authors here show that activation of serotonin 5-HT1B receptors on cholecystokinin-expressing interneurons (CCK neurons) in the dentate gyrus underlies the effects of acute SSRI treatment on depressive-like behaviours in mice, whereas 5-HT2A receptors mediate the effects of chronic SSRI treatment. Chemogenetic inhibition of CCK neurons had an acute antidepressant effect, suggesting that this neuronal population might be a viable target for the development of improved antidepressants.