Dorsal raphe nucleus enkephalin peptide modulates behavioral preference

The endogenous opioid system is a powerful modulator of motivation and affect. The dorsal raphe nucleus (DRN) in the midbrain has been established as an important site of opioid action and is an integral hub in behavioral modulation. To investigate the functional significance of DRN opioid signaling in aversive and appetitive behaviors we disrupted preproenkephalin (Penk) in DRN using CRISPR-Cas9 technology in Penk-Cre mice. We found that CRISPR mediated knockdown of enkephalin peptide in the DRN (DRN^Penk) enhanced inflammation-induced mechanical sensitivity and odor avoidance. Additionally, loss of DRN^Penk diminished sucrose preference and engagement with a novel social stimulus. To further characterize the opioid system within the DRN, we performed Hiplex in situ hybridization of 12 genes in the same tissue. This revealed that DRN^Penk is largely separate from DRN serotonin cells and is instead distributed on glutamatergic and GABAergic cells. However, subtype-specific knockdown of DRN^Penk from glutamatergic and GABAergic cells was insufficient to replicate the behavioral effects of general DRN^Penk knockdown. This suggests that these neurons represent a novel population that mediate motivated behaviors distinctly from canonical DRN mechanisms.