What does the abbreviation MOR mean?

MOR stands for Mu-Opioid Receptor. The receptor is also referred to as OP3 in older pharmacological literature and MOP in some IUPHAR documents. The encoding gene is OPRM1.

What gene encodes the mu-opioid receptor?

The OPRM1 gene, located on the long arm of chromosome 6 (6q25.2) in humans.

Is MOR the only opioid receptor that 7-OH activates?

No. 7-OH and mitragynine show measurable activity at multiple opioid receptor subtypes in published assays, but MOR is the principal receptor of interest based on its higher affinity and the magnitude of pharmacological effects observed.

What is biased agonism at MOR?

Biased agonism describes a ligand that preferentially activates one downstream receptor pathway over another. At MOR, this typically means preferential G-protein signaling over β-arrestin recruitment - a profile reported for the kratom alkaloids in published in vitro assays.

How does buprenorphine relate to 7-OH at MOR?

Both are characterized as partial agonists at MOR. Buprenorphine is the canonical clinical example of a MOR partial agonist; 7-OH is described in published research as a partial agonist with a G-protein-biased profile.

Receptor Pharmacology

Mu-Opioid Receptor (MOR)

At-a-glance comparison

Spec	Value
Receptor name	Mu-opioid receptor (MOR; also OP3, MOP)
Encoding gene	OPRM1 (chromosome 6q25.2 in humans)
Receptor superfamily	G-protein-coupled receptor (GPCR), Class A (rhodopsin-like)
Primary signaling	Inhibitory G-protein (Gαi/o); β-arrestin recruitment as alternative pathway
Endogenous ligands	β-Endorphin, enkephalins (also engage delta), endomorphin-1 and endomorphin-2
Reference exogenous agonists	Morphine, fentanyl, methadone (full agonists); buprenorphine (partial agonist)
Reference antagonists	Naloxone, naltrexone
Kratom alkaloid activity (in vitro)	Mitragynine and 7-OH: partial agonists with G-protein-biased signaling profiles

What is the mu-opioid receptor?

The mu-opioid receptor is a cell-surface G-protein-coupled receptor encoded by the gene OPRM1. It belongs to the Class A (rhodopsin-like) GPCR family and is one of the four classical opioid-receptor subtypes - alongside the delta-opioid receptor (DOR; OPRD1), kappa-opioid receptor (KOR; OPRK1), and nociceptin/orphanin FQ peptide receptor (NOP; OPRL1). MOR is widely distributed in the central nervous system, with notable density in regions involved in pain processing, reward, and autonomic function.

Endogenous activation of MOR comes from peptide ligands including β-endorphin and the enkephalins. Pharmacologically, MOR is the principal site of action of morphine and most clinically used opioid analgesics, as well as the focus of contemporary research into biased and partial agonist ligands.

How MOR signals

When activated, MOR couples primarily to inhibitory heterotrimeric G-proteins (Gαi/o), which reduce adenylate cyclase activity, decrease intracellular cyclic AMP, modulate ion channels (closing voltage-gated calcium channels and opening inwardly-rectifying potassium channels), and ultimately reduce neuronal excitability.

After activation, MOR also recruits β-arrestin, a cytoplasmic scaffolding protein that mediates receptor desensitization and internalization, and that initiates an alternative signaling cascade. The relative balance between G-protein activation and β-arrestin recruitment is the foundation of biased agonism - a pharmacological concept that is central to current research interest in 7-hydroxymitragynine and mitragynine.

MOR partial agonism vs full agonism

Pharmacologically, MOR ligands are categorized as full agonists, partial agonists, or antagonists. Full agonists such as morphine produce maximal receptor activation (Emax = 100% in functional assays). Partial agonists produce a submaximal functional response regardless of dose; their Emax is below that of a full agonist. Antagonists bind without activating the receptor and block agonist binding.

Buprenorphine is the canonical partial MOR agonist in clinical use. The kratom alkaloids 7-OH and mitragynine are characterized in published research as partial MOR agonists. The full-vs-partial distinction has implications for receptor pharmacology that are explored in the dedicated Partial Agonist glossary entry.

Common questions about mu-opioid receptor (mor)

What does the abbreviation MOR mean?: MOR stands for Mu-Opioid Receptor. The receptor is also referred to as OP3 in older pharmacological literature and MOP in some IUPHAR documents. The encoding gene is OPRM1.
What gene encodes the mu-opioid receptor?: The OPRM1 gene, located on the long arm of chromosome 6 (6q25.2) in humans.
Is MOR the only opioid receptor that 7-OH activates?: No. 7-OH and mitragynine show measurable activity at multiple opioid receptor subtypes in published assays, but MOR is the principal receptor of interest based on its higher affinity and the magnitude of pharmacological effects observed.
What is biased agonism at MOR?: Biased agonism describes a ligand that preferentially activates one downstream receptor pathway over another. At MOR, this typically means preferential G-protein signaling over β-arrestin recruitment - a profile reported for the kratom alkaloids in published in vitro assays.
How does buprenorphine relate to 7-OH at MOR?: Both are characterized as partial agonists at MOR. Buprenorphine is the canonical clinical example of a MOR partial agonist; 7-OH is described in published research as a partial agonist with a G-protein-biased profile.

References

Pasternak GW, Pan YX. (2013). Mu opioids and their receptors: evolution of a concept. Pharmacological Reviews.
Váradi A, Marrone GF, Palmer TC, et al. (2016). Mitragynine/Corynantheidine pseudoindoxyls as opioid analgesics with mu agonism and delta antagonism. Journal of Medicinal Chemistry. PMID 27513560.
Kruegel AC, Gassaway MM, Kapoor A, et al. (2016). Synthetic and receptor signaling explorations of the Mitragyna alkaloids. JACS.
Hassan Z, Muzaimi M, Navaratnam V, et al. (2013). From kratom to mitragynine and its derivatives. Neuroscience and Biobehavioral Reviews. PMID 23206666.

Important safety information:

Products containing 7-hydroxymitragynine (7-OH) are sold for adult use only (21+). These statements have not been evaluated by the U.S. Food and Drug Administration. Products are not intended to diagnose, treat, cure, or prevent any disease. The FDA has raised safety concerns regarding concentrated 7-OH products; consult a qualified healthcare professional before use. Do not operate vehicles or machinery after use. Keep out of reach of children and pets. Laws vary by state, buyers are responsible for knowing applicable law.