The Mu₃ opiate receptor subtype

The mu₃ opiate receptor subtype has been characterized by various binding assays as opiate alkaloid selective (eg, morphine) and opioid peptide (eg, methionine enkephalin) insensitive. The binding is monophasic, saturable, and stereospecific, as well as naloxone reversible. This opiate receptor subtype has been found on human and invertebrate tissues, demonstrating that it has been conserved during evolution. Furthermore, in numerous reports, this receptor is coupled to constitutive nitric oxide release. In this regard, for example, morphine immune downregulating activities parallels those actions formerly attributed to nitric oxide. Thus, this opiate receptor represents an addition to mu receptor heterogeneity that offers an explanation for the difference in actions of opioid peptides and opiate alkaloids in physiological systems transcending analgesia.

Key words: nitric oxide, morphine, opiate receptor, mu₃ opiate receptor

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