all substance with opiate action are related chemically and have similarities in their three-dimensional structures. thus it was long suspected that opiates act upon the brain by binding to specific membrane receptors. investigators were able to show that the central nervous system does, indeed}have receptors for opiates. such receptors have been found not just in humans but in all other vertebrates tested. why would vertebrate brains have opiate receptors? only one answer seemed logical: because vertebrates brain themselves must produce opiates. this rather startling conclusion triggered a serchfor naturally occurring substances with opiate activity. many such internal opiates have now been isolated, and they have been given the name of endorphins, for endogenous morphine-like substance