Pharmacology of Opiates and Their Derivatives

The opiates or opioids are exogenous substances which bind in a stereospecific and saturable manner to several subspecies of cell membrane receptors which are sites of action of endogenous ligands (enkephalins, B‐endorphine). The opiates, which have common structural features, are agonists or partial agonists, the potency of which generally correlates well with affinity for opiate receptors. Many of the actions of opiates are related to alteration of the release of endogenous neurotransmitters. Inhibition of intracelluar adenylate cyclase and altered transmembrane calcium fluxes also appear to be important consequences of opiate action. Most opiates share the pharmacological properties of morphine. The development of tolerance and physical dependence with chronic administration is a characteristic feature of all opiates limiting their clinical use and creating the liability for abuse. With established opiate tolerance, cessation of the drug is accompanied by a characteristic withdrawal syndrome which is suppressed by opiate administration and can also be precipitated by an opiate antagonist such as naloxone. Many of the differences between opiates are due to different pharmacokinetic properties, particularly oral bioavailability and elimination rate. The search for a non‐addictive opiate analgesic continues but is presently foiled by the inherent pharmacological properties of these drugs.