ABSTRACT
Opium is a mixture of alkaloids derived from extracts of the poppy seed. Opiates are naturally occurring alkaloids such as morphine or codeine. The term opioid has more utility in the field of pain and in addition to opiates includes many synthetic and semisynthetic compounds with mechanisms of action similar to that of morphine. Evidence supporting the existence of multiple opioid receptors was described in 1976 (1). Initial studies led to the description of three types of receptor, mu, delta, and kappa receptors, with subtypes of each receptor being more recently identified. Most opioid analgesic drugs demonstrate relatively selective agonist activity at the mu opioid receptor. More recently a novel receptor has been identified and named as “ORL1” (opioid receptor like). Despite its structural commonality with the other opioid receptors, the receptor appears to be pharmacologically distinct. Opioid receptors are widely expressed within the central and peripheral nervous systems and elsewhere. Discovery of these receptors led to the identification of endogenous opioid ligands within the brain. Three families of endogenous opioid peptides exist: the enkephalins, dynorphins, and beta-endorphins with each being derived from specific precursor polypeptides.
