Abstract: | Opioid peptides are endogenous substances present in central nervous system and various tissues whose actions are mediated by opiate receptors. They belong to two different classes: short peptides like the two pentapeptides enkephalin and substances of higher molecular weight like beta-endorphin. It appears that these various peptides play a messenger role between cells, either as neurotransmitters in the case of enkephalins or as hormones in the case of beta-endorphin. |