Endorphins, a term derived from endogenous morphine, are peptides synthesized in the brain that modulate pain perception and enhance subjective well-being. They are produced and stored within the brain's pituitary gland. These endogenous analgesics are frequently generated in the brain and adrenal medulla, particularly during physical exertion or orgasm, contributing to pain inhibition, alleviation of muscle cramps, and stress reduction.
History
Opioid peptides within the brain were initially identified in 1973 by John Hughes and Hans Kosterlitz at the University of Aberdeen. These researchers isolated "enkephalins" (derived from the Greek εγκέφαλος) from porcine brain tissue, specifically identifying met-enkephalin and leu-enkephalin. This discovery followed the identification of a receptor hypothesized to mediate the analgesic effects of morphine and other opioids, prompting Kosterlitz and Hughes to investigate endogenous opioid ligands. Contemporary research efforts were primarily directed toward developing an analgesic devoid of morphine's addictive properties or overdose potential.
Rabi Simantov and Solomon H. Snyder successfully isolated morphine-like peptides from calf brain tissue. Eric J. Simon, an independent discoverer of opioid receptors, subsequently designated these peptides as endorphins. The term was broadly applied to any peptide exhibiting morphine-like pharmacological activity. In 1976, Choh Hao Li and David Chung elucidated the sequences of α-, β-, and γ-endorphin, which were isolated from camel pituitary glands and characterized for their opioidergic activity. Li's research indicated that β-endorphin elicited potent analgesic effects. This finding was corroborated in 1977 by Wilhelm Feldberg and Derek George Smyth, who demonstrated β-endorphin's superior potency compared to morphine. Furthermore, they confirmed that naloxone, an opioid antagonist, reversed these effects.
Subsequent investigations have differentiated between enkephalins, endorphins, and endogenously synthesized true morphine, the latter being a non-peptide compound. Opioid peptides are categorized according to their larger precursor propeptide: endorphins originate from proopiomelanocortin (POMC), enkephalins from proenkephalin, and dynorphins and neoendorphins from prodynorphin.
Etymology
The term endorphin originates from the Greek word ἔνδον (Greek: éndon), signifying "within" (as in endogenous, ἐνδογενής; Greek: endogenes, meaning "proceeding from within"), combined with "morphine," which is derived from Morpheus (Ancient Greek: Μορφεύς, romanized: Morpheús), the Greek god of dreams. Consequently, "endorphin" represents a contraction of "endo(genous) (mo)rphin," where "morphin" denotes the archaic spelling of morphine.
Types
The endorphin family comprises three endogenous opioid peptides: α-endorphin, β-endorphin, and γ-endorphin. All endorphins are synthesized from the precursor protein proopiomelanocortin (POMC) and share a common met-enkephalin motif (Tyr-Gly-Gly-Phe-Met) at their N-terminus. Both α-endorphin and γ-endorphin are generated through the proteolytic cleavage of β-endorphin, specifically between the Thr(16)-Leu(17) residues and Leu(17)-Phe(18), respectively. Among these, α-endorphin possesses the shortest sequence, while β-endorphin exhibits the longest.
α-endorphin and γ-endorphin are predominantly localized in the anterior and intermediate lobes of the pituitary gland. Although β-endorphin is recognized for its opioid activity, α-endorphin and γ-endorphin exhibit negligible affinity for opiate receptors, consequently eliciting distinct physiological effects compared to β-endorphin. Certain research indicates that α-endorphin activity resembles that of psychostimulants, while γ-endorphin activity is comparable to that of neuroleptics.
Synthesis
The precursors for endorphins are predominantly synthesized within the pituitary gland. All three classes of endorphins are derived as fragments from the precursor protein proopiomelanocortin (POMC). Within the trans-Golgi network, POMC associates with carboxypeptidase E (CPE), a membrane-bound protein, which subsequently facilitates POMC's transport into nascent budding vesicles. Mammalian systems utilize proprotein convertase 1 (PC1) to cleave POMC into adrenocorticotropin (ACTH) and beta-lipotropin (β-LPH). Subsequently, β-LPH, a pituitary hormone exhibiting minimal opioid activity, undergoes further proteolytic fragmentation into various peptides, such as α-endorphin, β-endorphin, and γ-endorphin. Proprotein convertase 2 (PC2) mediates the cleavage of β-LPH into β-endorphin and γ-lipotropin. The generation of α-endorphin and γ-endorphin occurs via the proteolytic cleavage of β-endorphin.
Regulation
Noradrenaline has been demonstrated to enhance endorphin synthesis in inflamed tissues, thereby producing an analgesic effect; the stimulation of sympathetic nerves through electro-acupuncture is hypothesized to induce these pain-relieving outcomes.
Mechanism of Action
Endorphins are secreted by the pituitary gland, typically as a physiological response to pain, and exert their effects within both the central nervous system (CNS) and the peripheral nervous system (PNS). Within the PNS, β-endorphin constitutes the predominant endorphin released from the pituitary. Endorphins impede the transmission of pain signals by binding to μ-receptors on peripheral nerves, thereby preventing the release of the neurotransmitter substance P. The mechanism within the CNS operates similarly but involves the blockade of a distinct neurotransmitter, gamma-aminobutyric acid (GABA). Subsequently, the inhibition of GABA leads to an augmented production and release of dopamine, a neurotransmitter implicated in reward-based learning.
Functions
Endorphins are crucial in the body's inhibitory response to nociception. Studies have indicated that meditation, when practiced by trained individuals, can induce endorphin release. Furthermore, laughter has been shown to stimulate endorphin synthesis and increase an individual's pain threshold.
Vigorous aerobic exercise can stimulate endorphin production. The secretion of β-endorphin has been hypothesized to contribute to the phenomenon commonly referred to as "runner's high." Nevertheless, experimental inhibition of the mu opioid receptor has revealed that endorphins are not exclusively essential for the manifestation of exercise-induced euphoria. It has been proposed that other neurotransmitters, particularly endocannabinoids, are more probable contributors. Endorphins might partially mediate exercise-induced analgesia.
Neurobiological Effects of Physical Exercise
- Neurobiological effects of physical exercise
- Enkephalin
References
Endorphins within the U.S. National Library of Medicine's Medical Subject Headings (MeSH) database.
- Endorphins at the U.S. National Library of Medicine Medical Subject Headings (MeSH)