Leukocyte-derived opioid peptides and inhibition of pain.

In peripheral inflamed tissue interactions between leukocyte-derived opioid peptides and opioid receptors on sensory neurons lead to potent, clinically relevant inhibition of pain. Opioid receptors are present on peripheral terminals of sensory neurons and are upregulated in inflammation. Their endogenous ligands, opioid peptides, are synthesized in circulating immune cells, which migrate to injured tissues directed by chemokines and adhesion molecules. Under stressful stimuli or in response to releasing agents (e.g., corticotropin-releasing factor, cytokines, catecholamines) leukocytes can secrete opioids. These peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of excitatory neuropeptides. These effects occur without central opioid side effects such as depression of breathing, clouding of consciousness, or addiction. Future research should elucidate the selective targeting of opioid peptide-containing immune cells to sites of painful tissue injury and the augmentation of opioid peptide and receptor synthesis.