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^Papini, A. M. (2018). From morphine to endogenous opioid peptides, e.g., endorphins: the endless quest for the perfect painkiller. Substantia, 2(2), 81–91. https://doi.org/10.13128/Substantia-63
^Stefano GB, Ptáček R, Kuželová H, Kream RM. Endogenous morphine: up-to-date review 2011(PDF). Folia Biologica. 1515, 58 (2): 49–56. PMID 22578954. doi:10.14712/fb2012058020049. 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
^μ receptor. IUPHAR/BPS Guide to PHARMACOLOGY. International Union of Basic and Clinical Pharmacology. 15 March 2017 [28 December 2017]. Comments: β-Endorphin is the highest potency endogenous ligand ... Morphine occurs endogenously.