Fluoxetine is a Selective Serotonin Reuptake Inhibitor (SSRI) often prescribed for the treatment of anxiety and depression. Many of its effects are thought to be mediated by the dentate gyrus, but the mechanism by which some patients respond to treatment and some do not remains poorly understood. In this study we have characterized a previously-unknown component of the behavioral response to fluoxetine in the dentate gyrus, using a combination of genomic, behavioral, and imaging techniques. We have found that different components of the opioid system are involved in the treatment efficacy of fluoxetine in the dentate gyrus. Specifically, we have identified a population of anatomically and transcriptionally distinct mature granule cells that are defined by their high levels of proenkephalin expression after fluoxetine treatment. Furthermore, we have shown that the delta opioid receptor is partly mediating some of the behavioral effects of fluoxetine in a neurogenesis-independent manner. These results open an interesting new avenue for research into the involvement of the opioid system in the behavioral response to SSRIs.