|Vertebrate endogenous opioids neuropeptide|
Opioid peptides are peptides that bind to opioid receptors in the brain; opiates and opioids mimic the effect of these peptides. Such peptides may be produced by the body itself, for example endorphins. The effects of these peptides vary, but they all resemble those of opiates. Brain opioid peptide systems are known to play an important role in motivation, emotion, attachment behaviour, the response to stress and pain, and the control of food intake.
Opioid-like peptides may also be absorbed from partially digested food (casomorphins, exorphins, and rubiscolins). The opioid food peptides have lengths of typically 4–8 amino acids. The body's own opioids are generally much longer.
Opioid peptides are released by post-translational proteolytic cleavage of precursor proteins. The precursors consist of the following components: a signal sequence that precedes a conserved region of about 50 residues; a variable-length region; and the sequence of the neuropeptides themselves. Sequence analysis reveals that the conserved N-terminal region of the precursors contains 6 cysteines, which are probably involved in disulfide bond formation. It is speculated that this region might be important for neuropeptide processing.
Endogenous opioids produced by the body 
- The nucleotide sequence of the human gene for proopiomelanocortin (POMC) was characterized in 1980. The POMC gene codes for endogenous opioids such as β-endorphin and gamma-endorphin. The peptides with opioid activity that are derived from proopiomelanocortin comprise the class of endogenous opioid peptides called "endorphins".
- The human gene for the enkephalins was isolated and its sequence described in 1982.
- The human gene for dynorphins (originally called the "Enkephalin B" gene because of sequence similarity to the enkephalin gene) was isolated and its sequence described in 1983.
- The PNOC gene encoding prepronociceptin, which is cleaved into nociceptin and potentially two additional neuropeptides.
- Adrenorphin, amidorphin, and leumorphin were discovered in the 1980s.
- The endomorphins were discovered in the 1990s.
- Opiorphin and spinorphin, enkephalinase inhibitors (i.e., prevent the metabolism of enkephalins).
- Hemorphins, hemoglobin-derived opioid peptides, including hemorphin-4, valorphin, and spinorphin, among others.
Opioid food peptides
- Casomorphin (from casein found in milk of mammals, including cows)
- Gluten exorphin (from gluten found in wheat, rye, barley)
- Gliadorphin/gluteomorphin (from gluten found in wheat, rye, barley)
- Soymorphin-5 (from soybean)
- Rubiscolin (from spinach)
Amphibian opioid peptides
Synthetic opioid peptides
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Positive evolutionary pressure has apparently preserved the ability to synthesize chemically authentic morphine, albeit in homeopathic concentrations, throughout animal phyla. ... The apparently serendipitous finding of an opiate alkaloid-sensitive, opioid peptide-insensitive, µ3 opiate receptor subtype expressed by invertebrate immunocytes, human blood monocytes, macrophage cell lines, and human blood granulocytes provided compelling validating evidence for an autonomous role of endogenous morphine as a biologically important cellular signalling molecule (Stefano et al., 1993; Cruciani et al., 1994; Stefano and Scharrer, 1994; Makman et al., 1995). ... Human white blood cells have the ability to make and release morphine