摘要: Publisher Summary This chapter presents a brief historical account of the discoveries opiate receptors and their ligands. It was direct biochemical demonstration stereospecific binding sites that led to discovery endogenous opioid peptides initiated era explosive developments in neuropeptide research. With development vitro assay, it became possible demonstrate wide distribution these receptor throughout central nervous system (CNS) which suggested physiological functions. Moreover, were shown exist all vertebrate species some invertebrates. Their survival evolution so many bears further witness importance organisms possess them, is not likely be result ability bind plant alkaloids. There sufficient circumstantial evidence permit hypothesis CNS endorphins are neurotransmitters or modulators may play important roles pain modulation, addiction, various aspects emotion behavior.