Any of a group of proteins occurring in the brain and having pain-relieving properties typical of opium and related opiates. Discovered in the 1970s, they include enkephalin, beta-endorphin, and dynorphin. Each is distributed in characteristic patterns throughout the nervous system. Endorphins are released in response to pain or sustained exertion (causing, e.g., the “runner's high”). They are also believed to have a role in appetite control, release of pituitary sex hormones, and shock. There is strong evidence that they are connected with “pleasure centres” in the brain, and they seem to be activated by acupuncture. Knowledge of their behaviour has implications for treating addictions and chronic pain.
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The term "endorphin" implies a pharmacological activity (analogous to the activity of the corticosteroid category of biochemicals) as opposed to a specific chemical formulation. It consists of two parts: endo- and -orphin; these are short forms of the words endogenous and morphine, intended to mean "a morphine like substance originating from within the body."
The term endorphin rush has been adopted in popular speech to refer to feelings of exhilaration brought on by pain, danger, or other forms of stress, supposedly due to the influence of endorphins. However, this term does not occur in the medical literature.
Beta-endorphin has the highest affinity for the μ1-opioid receptor, slightly lower affinity for the μ2- and δ-opioid receptors and low affinity for the κ1-opioid receptors. μ-receptors are the main receptor through which morphine acts. Classically, μ-receptors are presynaptic, and inhibit neurotransmitter release; through this mechanism, they inhibit the release of the inhibitory neurotransmitter GABA, and disinhibit the dopamine pathways, causing more dopamine to be released. By hijacking this process, exogenous opioids cause inappropriate dopamine release, and lead to aberrant synaptic plasticity which causes addiction. Opioid receptors have many other and more important roles in the brain and periphery however, modulating pain, cardiac, gastric and vascular function as well as possibly panic and satiation, and receptors are often found at postsynaptic locations as well as presynaptically.
Scientists debate whether specific activities release measurable levels of endorphins. Much of the current data comes from animal models which may not be relevant to humans. The studies that do involve humans often measure endorphin plasma levels, which do not necessarily correlate with levels in the CNS. Other studies use a blanket opioid antagonist (usually naloxone) to indirectly measure the release of endorphins by observing the changes that occur when any endorphin activity that might be present is blocked.
However, some scientists question the mechanisms at work, their research possibly demonstrating the high comes from completing a challenge rather than as a result of exertion. Studies in the early 1980s cast doubt on the relationship between endorphins and the runner's high for several reasons:
A study in 2004 by Georgia Tech found that runner's high might be caused by the release of another naturally produced chemical, the endocannabinoid anandamide. Anandamide is similar to the active chemical, THC, found in marijuana. The authors suggest that the body produces this chemical to deal with prolonged stress and pain from strenuous exercise, similar to the original theory involving endorphins. However, the release of anandamide was not reported with the cognitive effects of the runner’s high; this suggests that anandamide release may not be significantly related to runner's high.
In 2008, researchers in Germany reported that the myth of the runner's high was in fact true. Using PET scans combined with recently available chemicals that reveal endorphins in the brain, they were able to compare runners’ brains before and after a run. The runners the researchers recruited were told that the opioid receptors in their brains were being studied, and did not realize that their endorphin levels were being studied in regard to the runner's high.
The participants were scanned and received psychological tests before and after a two-hour run. Data received from the study showed endorphins were produced during the exercise and were attaching themselves to areas of the brain associated with emotions (limbic and prefrontal areas).
Correlation between ovarian steroidogenesis and [beta]-endorphin in the Lizard Uromastyx acanthinura: Immunohistochemical approach
Sep 01, 2009; Abstract: In Mammals, opioid peptides are involved in various physiological processes including the reproductive function. The...