There are three families of opioid peptides: the endorphins, the enkephalins, and the dynorphins. There are also at least three kinds of receptors. During the 1970s naloxone was thought to be the key to searching for opioid peptides and investigating their properties, but it now appears that naloxone is unsatisfactory for this purpose. More selective and powerful antagonists have become available, and much of the research using naloxone needs repeating with the new antagonists.
The opioid peptides are widespread throughout the body and probably have a role not only in pain perception but also in other sensory pathways and also in autonomic and motor control. They may affect the immune system, though this is not yet established.
The relevance of these substances to acupuncture is also uncertain. To quote the recent words of one authority (John M.H. Rees, Senior Lecturer in Pharmacology at Manchester University Medical School): “Studies showing changes in blood and CSF opioid peptide concentrations during acupuncture are erratic and … one wonders [about] the relevance of changes in these peptides in any event. Perhaps it is unfair to be pessimistic, but the mechanism of acupuncture must remain unclear for the next decade, hopefully not for the millennium.”