Endomorphins

Definition
Endomorphin (EM)-1 and EM-2 are opioid tetrapeptides located in the central nervous system and immune tissues with high selectivity and affinity for the µ-opioid receptor 1.

Related Peptides
Opioid peptides and their G-protein-coupled receptors (d, ? and µ) are located in the central nervous system and peripheral tissues. The opioid system has been studied to determine the intrinsic mechanism of modulation of pain and to develop uniquely effective pain-control substances with minimal abuse potential and side effects. Two types of endogenous opioid peptides exist, one containing Try-Gly-Gly-Phe as the message domain (enkephalins, endorphins, dynorphins) and the other containing the Tyr-Pro-Phe/Trp sequence (endomorphins-1 and -2) 2.

Discovery
In 1997, Zadina et al., isolated Endomorphin 1 (EM1) and endomorphin 2 (EM2) from bovine brain, and reported them to be tetrapeptides having the highest specificity and affinity for the µ receptor of any endogenous substance so far described 3.

Structural Characteristics
Opioidmimetics and opioid peptides containing the amino acid sequence of the message domain of endomorphins (EMs), Tyr-Pro-Phe/Trp, have been found to exhibit unique binding activity. Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) have high µ­ receptor affinity and remarkable selectivity 2. The proper spatial orientation and conformational restriction of the third aromatic ring is supposed to be crucial for the interaction of EMs with MOR (µ­ opioid receptor) 4.

Mode of Action
The endomorphins have the highest specificity and affinity to the µ receptor among all endogenous substance so far described. EM1 is more effective than the µ- selective analogue DAMGO in vitro and produces potent and prolonged analgesia in mice. EM2 (H-Tyr-Pro-Phe-Phe-NH2) also has a high affinity and selectivity to the µ receptor 2. The µ-opioid receptors are G protein-coupled receptors that play a pivotal role in the analgesic effects of opioid receptor agonists used clinically. Endomorphin-induced antinociception is mediated by spinal µ-opioid receptors 5.

Functions
Endomorphins have been implicated in a broad range of physiological functions including antinociceptive, cardiovascular, respiratory, digestive, rewarding, and endocrine responses 5. EM 1 and EM2 have significant naloxone-sensitive, vasodepressor activity 6. They modulate phagocytosis, chemotaxis and superoxide anion production by microglia 7. The analogues designed based on endomorphins may have therapeutic potential 8.

References
1. Coventry TL, Jessop DS, Finn DP, Crabb MD, Kinoshita H, Harbuz MS (2001). Endomorphins and activation of the hypothalamo-pituitary-adrenal axis. J Endocrinol., 169(1):185-193.
2. Okada Y, Tsuda Y, Bryant SD, Lazarus LH (2002). Endomorphins and related opioid peptides. Vitam Horm., 65:257-279.
3. Zadina JE, Hackler L, Ge LJ, Kastin AJ (1997). A potent and selective endogenous agonist for the mu-opiate receptor. Nature, 386(6624):499–502
4. Yu Y, Shao X, Cui Y, Liu HM, Wang CL, Fan YZ, Liu J, Dong SL, Cui YX, Wang R (2007).Structure-activity study on the spatial arrangement of the third aromatic ring of endomorphins 1 and 2 using an atypical constrained C terminus. ChemMedChem., 2(3):309-317.
5. Xie H, Woods JH, Traynor JR, Ko MC (2008). The Spinal Antinociceptive Effects of Endomorphins in Rats: Behavioral and G Protein Functional Studies. Anesth Analg., 106(6):1873-1881.
6. Champion HC, Zadina JE, Kastin AJ, Hackler L, Ge LJ, Kadowitz PJ (1997).. The Endogenous Mu-Opioid Receptor Agonists Endomorphins 1 and 2 Have Novel Hypotensive Activity in the Rabbit. Biochemi Biophysl Res Commun., 235(3) 567-570
7. Azuma Y, Ohura K, Wang PL, Shinohara M (2001). Endomorphins 1 and 2 modulate chemotaxis, phagocytosis and superoxide anion production by microglia. J Neuroimmunol., 119(1):51-56.
8. Huo XF, Ren WH, Wu N, Wang R (1998).The design and synthesis of endomorphins and their analogues. Chinese Science Bulletin., 46(13):1096-1099.