naturally occurring environmental substances often mimic endogenous substances found in mammals and are capable of interacting with specific proteins , such as receptors , with a high degree of fidelity and selectivity . 
 narcotic alkaloids and amphibian skin secretions , introduced into human society through close association with plants and animals through folk medicine and religious divination practices , were incorporated into the armamentarium of the early pharmacopoeia . 
 these skin secretions contain a myriad of potent bioactive substances , including alkaloids , biogenic amines , peptides , enzymes , mucus , and toxins ( noxious compounds notwithstanding ) ; each class exhibits a broad range of characteristic properties . 
 one specific group of peptides , the opioids , containing the dermorphins ( dermal morphinelike substances ) and the deltorphins ( delta - selective opioids ) , display remarkable analgesic properties and include an amino acid with the rare ( in a mammalian context ) d - enantiomer in lieu of the normal l - isomer 
 . synthesis of numerous stereospecific analogues and conformational analyses of these peptides provided essential insights into the tertiary composition and microenvironment of the receptor " pocket " and the optimal interactions between receptor and ligand that trigger a biological response ; new advances in the synthesis and receptor - binding properties of the deltorphins are discussed in detail . 
 these receptor - specific opioid peptides act as more than mimics of endogenous opioids : their high selectivity for either the mu or delta receptor makes them formidable environmentally derived agents in the search for new antagonists for treating opiate addiction and in the treatment of a wide variety of human disorders.imagesp648-afigure 2.figure 3 .