Opioid peptides in relation to antinociception.

Three families of endogenous opioid peptides, each derived from distinct precursor, as well as their localization, affinity and interaction with different subtypes of opioid receptors are described. The release of opioid peptides upon various stimulation procedures is also presented. The emphasis is made on the role of opioid peptides in analgesia by describing their antinociceptive potency and discussing the role of peptides deriving from the different precursors in conveying the acute pain stimuli and on the changes in activity of opioid peptide systems in chronic pain.