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A family of endogenous morphinelike peptides present within the central nervous system. The term endorphin is generic, referring to all the opioid peptides, while specific peptides are given individual names, such as the enkephalins and β-endorphin. Their discovery has greatly enhanced the understanding of the mechanism of action of opiate drugs and how the perception of pain is modulated within the central nervous system. See Opiates, Pain
Morphine, codeine, and their many synthetic and semisynthetic analogs are effective pain killers that act through specific recognition sites, or receptors, localized on the surface of neurons within selected brain regions. These receptors have been extensively characterized, and a number of different subtypes have been identified which vary in their specificity for various opiates and opioid peptides and in the actions they mediate. The existence of these highly specific receptors implied that morphine was mimicking endogenous compounds within the brain with morphinelike actions, which have since been termed endorphins. See Morphine alkaloids
The first endorphins to be isolated were the enkephalins, two pentapeptides differing only in their fifth amino acid, which is either methionine or leucine. Since the initial description of the enkephalins, a number of opioid peptides have been reported that all share either the structure of methionine (Met) enkephalin or leucine (Leu) enkephalin as their first five amino acids. The major genes for these peptides have been identified. β-Endorphin is perhaps the most interesting peptide; it is cogenerated with important, nonopioid hormones.
The enkephalins are distributed unevenly throughout the brain, with very high levels in the basal ganglia, the thalamus, and the periaqueductal gray. In addition, there are high concentrations of enkephalins in the adrenal medulla, where they are co-released with norepinephrine in response to stress, among other stimuli. The dynorphins and α-endorphin are located within the central nervous system with a distribution similar to that of the enkephalins. See Stress (psychology)
β-Endorphin has been identified in only a single group of cells within the hypothalamus. Its highest levels are in the pituitary gland. Within the pituitary, both ACTH and β-endorphin are derived from the same precursor protein and are located within the same cells. Stimuli that release ACTH, a stress hormone which in turn induces the adrenal gland to release steroids, also co-release β-endorphin at the same time. Thus, stressful stimuli that release ACTH and norepinephrine also release both β-endorphin from the pituitary and enkephalins from the adrenal into the blood. This is particularly intriguing in view of the decreased perception of pain reported under periods of stress, such as combat. See Endocrine system (vertebrate)
All the endorphins can modulate the intensity of pain despite the fact that they act through different classes of opiate receptors. However, the presence of high concentrations of endorphins in brain regions unrelated to pain perception clearly demonstrates that the full range of actions of these compounds within the brain is not yet fully understood. Furthermore, their systemic hormonal role remains uncertain. See Nervous system (vertebrate)