Delta opioid receptors: Overlooked outlier or the next big thing.

The delta opioid receptor (DOR) is an often-overlooked member of the opioid receptor family, lacking the euphoric and addictive effects of mu receptors and the dysphoric properties of kappa. Instead, the DOR functions to restore balance, showing minimal impact in acute pain but alleviating allodynia and hyperalgesia associated with chronic pain and producing anxiolytic and antidepressant-like behaviors. Though past clinical trials for osteoarthritis and depression were unsuccessful, emerging evidence supports DOR's therapeutic potential in migraine, substance use disorder, irritable bowel syndrome, and metabolic disorder. Advances in pharmacology, including structure-guided design and intracellular targeting, offer promising new directions for DOR-based drug development.