Opioid

An opioid is any agent that binds to opioid receptors found principally in the central nervous system and gastrointestinal tract. There are four broad classes of opioids: endogenous opioid peptides, produced in the body; opium alkaloids, such as morphine (the prototypical opioid) and codeine; semi-synthetic opioids such as heroin and oxycodone; and fully synthetic opioids such as pethidine and methadone that have structures unrelated to the opium alkaloids.

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Although the term opiate is often used to refer to all opium-like drugs, it is more properly limited to the natural opium alkaloids and the semi-synthetics derived from them.

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Opioid receptor: Opioid receptors are a group of G-protein coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins and endorphins. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs)....

Central nervous system: The central nervous system (CNS) represents the largest part of the nervous system. Together with the peripheral nervous system, it has a fundamental role in the control of behaviour....

Gastrointestinal tract: The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and expels the remaining waste....