Gene and Protein Information
class A G protein-coupled receptor
Species	TM	AA	Chromosomal Location	Gene Symbol	Gene Name	Reference
Human	7	400	6q25.2	OPRM1	opioid receptor mu 1	184
Mouse	7	398	10 1.85 cM	Oprm1	opioid receptor, mu 1	114
Rat	7	398	1q11	Oprm1	opioid receptor, mu 1	27,47,161,183,194

Previous and Unofficial Names

Mu receptor | MOP | OP3 | MOPr | opioid receptor, mu 1 | opioid receptor

Database Links
Specialist databases
GPCRdb	oprm_human (Hs), oprm_mouse (Mm), oprm_rat (Rn)
Other databases
Alphafold	P35372 (Hs), P42866 (Mm), P33535 (Rn)
ChEMBL Target	CHEMBL233 (Hs), CHEMBL2858 (Mm), CHEMBL270 (Rn)
DrugBank Target	P35372 (Hs)
Ensembl Gene	ENSG00000112038 (Hs), ENSMUSG00000000766 (Mm), ENSRNOG00000018191 (Rn)
Entrez Gene	4988 (Hs), 18390 (Mm), 25601 (Rn)
Human Protein Atlas	ENSG00000112038 (Hs)
KEGG Gene	hsa:4988 (Hs), mmu:18390 (Mm), rno:25601 (Rn)
OMIM	600018 (Hs)
Pharos	P35372 (Hs)
RefSeq Nucleotide	NM_000914 (Hs), NM_001039652 (Mm), NM_013071 (Rn)
RefSeq Protein	NP_000905 (Hs), NP_001034741 (Mm), NP_037203 (Rn)
SynPHARM	3941 (in complex with β-FNA)
UniProtKB	P35372 (Hs), P42866 (Mm), P33535 (Rn)
Wikipedia	OPRM1 (Hs)

Selected 3D Structures
	Description:  Crystal structure of the mu-opioid receptor bound to a morphinan antagonist PDB Id:  4DKL Ligand:  β-FNA Resolution:  2.8Å Species:  Mouse References:  102
	Description:  Structure of the μ-opioid receptor–Gi protein complex PDB Id:  6DDF Ligand:  DAMGO Resolution:  3.5Å Species:  Mouse References:  86

Natural/Endogenous Ligands
dynorphin A-(1-13) {Sp: Human, Mouse, Rat}
dynorphin A {Sp: Human, Mouse, Rat}
dynorphin A-(1-8) {Sp: Human, Mouse, Rat}
dynorphin B {Sp: Human, Mouse, Rat}
endomorphin-1 {Sp: Human}
endomorphin-2 {Sp: Human}
β-endorphin {Sp: Human} , β-endorphin {Sp: Mouse} , β-endorphin {Sp: Rat}
[Leu]enkephalin {Sp: Human, Mouse, Rat}
[Met]enkephalin {Sp: Human, Mouse, Rat}
Comments: β-Endorphin is the highest potency endogenous ligand

Potential endogenous agonists

endomorphin-1, endomorphin-2

Principal endogenous agonists (Human)

β-endorphin (POMC, P01189), [Met]enkephalin (PENK, P01210), [Leu]enkephalin (PENK, P01210)

Download all structure-activity data for this target as a CSV file

Agonists
Key to terms and symbols	View all chemical structures	Click column headers to sort
Ligand Sp. Action Value Parameter Reference [3H]DAMGO Rn Full agonist 9.2 pKd 143 ⤷ pKd 9.2 (Kd 5.7x10-10 M) [143] ohmefentanyl Rn Agonist 9.0 pKd 181 ⤷ pKd 9.0 (Kd 9.5x10-10 M) [181] Description: Binding affinity of [3H]ohmefentanyl carfentanil Cp Agonist 10.6 pKi 35 ⤷ pKi 10.6 (Ki 2.4x10-11 M) [35] Description: Binding affinity in guinea pig whole brain- displacement of [3H]DAGO binding. (-)-cyclazocine Hs Partial agonist 10.0 pKi 166 ⤷ pKi 10.0 [166] butorphanol Hs Partial agonist 9.9 pKi 49 ⤷ pKi 9.9 (Ki 1.2x10-10 M) [49] Description: Displacement of [3H]DAMGO from human μ opioid receptor expressed in CHO cells. sufentanil Hs Full agonist 9.9 pKi 177 ⤷ pKi 9.9 (Ki 1.38x10-10 M) [177] etonitazene Hs Full agonist 9.7 pKi 166 ⤷ pKi 9.7 [166] hydromorphone Hs Agonist 9.6 pKi 185 ⤷ pKi 9.6 (Ki 2.8x10-10 M) [185] ethylketocyclazocine Hs Full agonist 9.5 pKi 166 ⤷ pKi 9.5 [166] etorphine Hs Full agonist 9.5 pKi 166 ⤷ pKi 9.5 [166] fentanyl Rn Full agonist 9.4 pKi 143 ⤷ pKi 9.4 [143] DAMGO Hs Full agonist 9.3 pKi 63,166 ⤷ pKi 9.3 [63,166] DC-1-76.2 Mm Agonist 9.3 pKi 22 ⤷ pKi 9.3 (Ki 5x10-10 M) [22] Description: Binding affinity determined in a radioligand membrane binding assay, displacing [3H]naloxone loperamide Hs Agonist 9.3 pKi 28 ⤷ pKi 9.3 (Ki 5.3x10-10 M) [28] DC 128.1 Mm Agonist 9.3 pKi 22 ⤷ pKi 9.3 (Ki 5.6x10-10 M) [22] Description: Binding affinity determined in a radioligand membrane binding assay, displacing [3H]naloxone EWB-3-14 Hs Agonist 9.2 pKi 69 ⤷ pKi 9.2 (Ki 6x10-10 M) [69] Description: Displacement of [125I]Ioxycodone from human mu opioid receptor expressed in CHO cells EG-1-230 Hs Agonist 9.2 pKi 69 ⤷ pKi 9.2 (Ki 6x10-10 M) [69] Description: Displacement of [125I]Ioxycodone from human mu opioid receptor (-)-methadone Hs Full agonist 9.2 pKi 166 ⤷ pKi 9.2 [166] [Met]enkephalin {Sp: Human, Mouse, Rat} Rn Full agonist 9.2 pKi 143 ⤷ pKi 9.2 [143] fentanyl Hs Full agonist 9.2 pKi 166 ⤷ pKi 9.2 [166] cebranopadol Hs Agonist 9.1 pKi 97 ⤷ pKi 9.1 (Ki 7x10-10 M) [97] Description: Radioligand binding assay sufentanil Rn Full agonist 9.1 pKi 188 ⤷ pKi 9.1 (Ki 7.7x10-10 M) [188] eluxadoline Rn Agonist 9.1 pKi 18 ⤷ pKi 9.1 (Ki 9x10-10 M) [18] morphine Hs Full agonist 9.0 pKi 55,166 ⤷ pKi 9.0 [55,166] β-endorphin {Sp: Human} Rn Full agonist 9.0 pKi 143 ⤷ pKi 9.0 [143] PZM21 Hs Biased agonist 9.0 pKi 103 ⤷ pKi 9.0 (Ki 1.1x10-9 M) [103] bilorphin Hs Agonist 9.0 pKi 38 ⤷ pKi 9.0 (Ki 1.1x10-9 M) [38] dihydromorphine Hs Full agonist 8.8 pKi 166 ⤷ pKi 8.8 [166] dynorphin-(1-11) Hs Full agonist 8.8 pKi 166 ⤷ pKi 8.8 [166] normorphine Hs Full agonist 8.8 pKi 166 ⤷ pKi 8.8 [166] nalbuphine Hs Partial agonist 8.8 pKi 185 ⤷ pKi 8.8 (Ki 1.6x10-9 M) [185] buprenorphine Hs Partial agonist 8.8 pKi 166 ⤷ pKi 8.8 [166] eluxadoline Hs Agonist 8.8 pKi 18 ⤷ pKi 8.8 (Ki 1.7x10-9 M) [18] DC-1-90.2 Mm Agonist 8.7 pKi 22 ⤷ pKi 8.7 (Ki 1.83x10-9 M) [22] Description: Binding affinity determined in a radioligand membrane binding assay, displacing [3H]naloxone DC-1-76.1 Mm Agonist 8.7 pKi 22 ⤷ pKi 8.7 (Ki 1.91x10-9 M) [22] Description: Binding affinity determined in a radioligand membrane binding assay, displacing [3H]naloxone DADLE Hs Full agonist 8.7 pKi 166 ⤷ pKi 8.7 [166] DAMGO Rn Full agonist 8.7 pKi 143 ⤷ pKi 8.7 [143] hydrocodone Hs Agonist 8.7 pKi 128 ⤷ pKi 8.7 (Ki 2x10-9 M) [128] BU08028 Hs Partial agonist 8.7 pKi 81 ⤷ pKi 8.7 (Ki 2.14x10-9 M) [81] cebranopadol Rn Agonist 8.6 pKi 97 ⤷ pKi 8.6 (Ki 2.4x10-9 M) [97] Description: Radioligand binding assay dynorphin B {Sp: Human, Mouse, Rat} Hs Full agonist 8.5 pKi 166 ⤷ pKi 8.5 [166] endomorphin-2 {Sp: Human} Rn Full agonist 8.5 pKi 193 ⤷ pKi 8.5 (Ki 3.24x10-9 M) [193] (-)-pentazocine Hs Partial agonist 8.4 pKi 166 ⤷ pKi 8.4 [166] dynorphin A-(1-8) {Sp: Human, Mouse, Rat} Hs Full agonist 8.4 pKi 166 ⤷ pKi 8.4 [166] dynorphin A-(1-13) {Sp: Human, Mouse, Rat} Hs Full agonist 8.3 pKi 166 ⤷ pKi 8.3 [166] endomorphin-1 {Sp: Human} Hs Full agonist 8.3 pKi 57,193 ⤷ pKi 8.3 (Ki 5.01x10-9 M) [57,193] DSLET Hs Full agonist 8.2 pKi 166 ⤷ pKi 8.2 [166] PL017 Hs Full agonist 8.2 pKi 25,166 ⤷ pKi 8.2 [25,166] etonitazene Rn Full agonist 8.2 pKi 54 ⤷ pKi 8.2 (Ki 6.6x10-9 M) [54] Description: Binding affinity determined in radioligand displacement assay using rat brain and [3H]DAMGO as tracer [Leu]enkephalin {Sp: Human, Mouse, Rat} Hs Partial agonist 8.1 pKi 166 ⤷ pKi 8.1 [166] dynorphin A {Sp: Human, Mouse, Rat} Hs Full agonist 8.1 pKi 166 ⤷ pKi 8.1 [166] UFP-512 Hs Agonist 8.0 pKi 176 ⤷ pKi 8.0 [176] Description: Measuring displacement of [3H]-diprenorphine in vitro isotonitazene Rn Full agonist 7.9 pKi 174 ⤷ pKi 7.9 (Ki 1.12x10-8 M) [174] Description: Binding affinity in rat brain morphine Rn Partial agonist 7.9 pKi 143 ⤷ pKi 7.9 [143] BW373U86 Rn Agonist 7.8 pKi 24 ⤷ pKi 7.8 (Ki 1.5x10-8 M) [24] UFP-505 Hs Agonist 7.8 pKi 40-41 ⤷ pKi 7.8 [40-41] ADL5747 Hs Agonist 7.7 pKi 92 ⤷ pKi 7.7 (Ki 1.8x10-8 M) [92] SRI22141 Hs Agonist 7.7 pKi 94 ⤷ pKi 7.7 (Ki 1.97x10-8 M) [94] Description: Binding affinity brorphine Rn Agonist 7.6 pKi 174 ⤷ pKi 7.6 (Ki 2.42x10-8 M) [174] Description: Binding affinity for rat brain MOR, determined in a radiologand displacement assay ADL5859 Hs Agonist 7.5 pKi 91 ⤷ pKi 7.5 (Ki 3.2x10-8 M) [91] ICI-199441 Hs Agonist 7.3 pKi 88 ⤷ pKi 7.3 (Ki 5.3x10-8 M) [88] Description: Displacement of [3H]diprenorphine from human μ opioid receptor expressed in CHO-K1 cells SCH221510 Hs Agonist 7.2 pKi 175 ⤷ pKi 7.2 (Ki 6.5x10-8 M) [175] Description: Radioligand binding assay SR16835 Hs Agonist 7.1 pKi 167 ⤷ pKi 7.1 (Ki 7.99x10-8 M) [167] Description: Radioligand binding assay codeine Hs Full agonist 6.9 pKi 166 ⤷ pKi 6.9 [166] tapentadol Hs Agonist 6.8 pKi 169 ⤷ pKi 6.8 (Ki 1.6x10-7 M) [169] oxycodegol Hs Agonist 6.6 pKi 117 ⤷ pKi 6.6 (Ki 2.37x10-7 M) [117] Description: Competitive membrane binding assay measuring displacement [3H]naloxone from human μ receptors expressed by CHO cells. BW373U86 Hs Agonist 6.6 pKi 91 ⤷ pKi 6.6 (Ki 2.6x10-7 M) [91] HS665 Hs Agonist 6.3 pKi 156 ⤷ pKi 6.3 (Ki 5.42x10-7 M) [156] compound 3 [PMID: 23134120] Hs Agonist 6.1 pKi 156 ⤷ pKi 6.1 (Ki 8.26x10-7 M) [156] tramadol Hs Agonist 5.8 pKi 185 ⤷ pKi 5.8 (Ki 1.6x10-6 M) [185] Description: Displacement of the mu aginist peptide DAMGO from the mu receptor expressed in CHO cells. etonitazene Hs Full agonist 10.5 pEC50 54 ⤷ pEC50 10.5 (EC50 3x10-11 M) [54] Description: Potency determined as inhibition of cAMP accumulation in a Gi-cAMP assay isotonitazene Hs Full agonist 10.3 pEC50 174 ⤷ pEC50 10.3 (EC50 5x10-11 M) [174] Description: Potency determined in a cAMP accumulation assay in HEK cells expressing hMOR lofentanil Hs Agonist 9.9 pEC50 142 ⤷ pEC50 9.9 [142] Description: Determined using a NanoBiT based G protein dissociation assay SRI22141 Hs Agonist 9.7 pEC50 94 ⤷ pEC50 9.7 (EC50 2x10-10 M) [94] Description: Potency in a 35S-GTPγS assay atoxifent Hs Agonist 9.4 pEC50 178 ⤷ pEC50 9.4 (EC50 3.9x10-10 M) [178] mitragynine Hs Agonist 9.3 pEC50 21 ⤷ pEC50 9.3 (EC50 5x10-10 M) [21] Description: Determined as inhibition of forskolin-stimulated cAMP accumulation in HEK293T cells expressing human mu opioid receptor brorphine Hs Agonist 8.3 – 8.7 pEC50 80,174 ⤷ pEC50 8.7 (EC50 2.06x10-9 M) [174] Description: Potency determined in a cAMP accumulation assay in HEK cells expressing hMOR pEC50 8.3 (EC50 4.8x10-9 M) [80] Description: Potency in a GTPγS binding assay PZM21 Hs Biased agonist 8.3 pEC50 103 ⤷ pEC50 8.3 (EC50 4.6x10-9 M) [103] Description: In a Gi/o activation assay. 4F-MT-45 Hs Agonist 6.8 pEC50 8 ⤷ pEC50 6.8 (EC50 1.7x10-7 M) [8] Description: β-arrestin2 recruitment assay 2F-MT-45 Hs Agonist 6.7 pEC50 8 ⤷ pEC50 6.7 (EC50 2x10-7 M) [8] Description: β-arrestin2 recruitment assay mitragynine Mm Agonist 6.7 pEC50 171 ⤷ pEC50 6.7 (EC50 2.03x10-7 M) [171] Description: Determined as agonist-induced [35S]GTPγS binding in CHO cells expressing mouse mu opioid receptor 3F-MT-45 Hs Agonist 6.2 pEC50 8 ⤷ pEC50 6.2 (EC50 6.1x10-7 M) [8] Description: β-arrestin2 recruitment assay thebaine Hs Partial agonist 5.3 pEC50 129 ⤷ pEC50 5.3 (EC50 5.6x10-6 M) [129] Description: Measuring ligand-indiced activation of Gi signalling in a FRET-based assay MT-45 Hs Agonist 4.6 pEC50 8 ⤷ pEC50 4.6 (EC50 2.3x10-5 M) [8] Description: β-arrestin2 recruitment assay levorphanol Hs Agonist 9.9 pIC50 65 ⤷ pIC50 9.9 (IC50 1.3x10-10 M) [65] BU72 Cp Agonist 9.8 pIC50 71 ⤷ pIC50 9.8 (IC50 1.5x10-10 M) [71] Description: Measuring displacement of [3H]DAMGO from brain membranes isolated from Hartley guinea pigs. methadone Hs Agonist 8.4 pIC50 140 ⤷ pIC50 8.4 (IC50 4.1x10-9 M) [140] Description: Binding affinity against μ-opioid receptor (human) using [3H]DAMGO radioligand. pethidine Hs Agonist 6.5 pIC50 140 ⤷ pIC50 6.5 (IC50 3.15x10-7 M) [140] AR-M1000390 Hs Agonist 5.4 pIC50 4 ⤷ pIC50 5.4 (IC50 3.8x10-6 M) [4] [3H]PL017 Rn Agonist - - 66 ⤷ [66] U-47700 Rn Agonist - - 29 ⤷ [29]
View species-specific agonist tables
Agonist Comments
Discrimination of full or partial agonism is very dependent on the level of receptor expression and on the assay used to monitor agonist effects. Many agents may behave as full agonists or potent partial agonists in cell lines expressing cloned receptors in high concentration, but in other environments they may show only weak agonist activity. The identification of agonist activity in the table is largely based on the ability to stimulate GTPγ35S binding in cell lines expressing cloned human μ receptors. Agents giving 85% or greater stimulation than that given by DAMGO have been characterized as Full Agonists [166]. It is still unclear whether endomorphins are endogenous. Morphine occurs endogenously [139]. We have tagged the μ receptor as the primary drug target for hydrocodone based on this drug having the highest affinity at this receptor compared to the κ and δ receptors [128]. Similarly, we have tagged the μ receptor as the primary target of hydromorphone [185]. Methadone is selective for the μ receptor: comparable IC50s at the κ and δ receptors are 512 and 1090nM respectively [140]. [11C]carfentanil (PubChem CID 449698) binds the human μ receptor with a Ki of 0.07 nM, in a [3H]DAMGO displacement assay [67].

Antagonists
Key to terms and symbols	View all chemical structures	Click column headers to sort
Ligand Sp. Action Value Parameter Reference [3H]diprenorphine Mm Antagonist 10.1 pKd 143 ⤷ pKd 10.1 (Kd 7.94x10-11 M) [143] [3H]naloxone Rn Antagonist 8.6 – 9.0 pKd 131 ⤷ pKd 8.6 – 9.0 (Kd 2.8x10-9 – 1x10-9 M) [131] Description: CHO and BHK cell lines stably expressing the rat μ receptor naloxonazine Mm Antagonist 10.3 pKi 143 ⤷ pKi 10.3 [143] samidorphan Hs Antagonist 10.3 pKi 186 ⤷ pKi 10.3 (Ki 5.2x10-11 M) [186] diprenorphine Mm Antagonist 10.1 pKi 143 ⤷ pKi 10.1 [143] quadazocine Hs Antagonist 10.0 pKi 166 ⤷ pKi 10.0 [166] (-)-bremazocine Hs Antagonist 9.7 pKi 166 ⤷ pKi 9.7 [166] CTOP Hs Antagonist 9.7 pKi 59,143 ⤷ pKi 9.7 [59,143] nalmefene Hs Antagonist 9.5 pKi 166 ⤷ pKi 9.5 [166] β-FNA Hs Antagonist 9.5 pKi 166 ⤷ pKi 9.5 [166] β-FNA Mm Antagonist 9.5 pKi 143 ⤷ pKi 9.5 [143] NFP Hs Antagonist 9.4 pKi 196 ⤷ pKi 9.4 (Ki 3.6x10-10 M) [196] Description: In a competitive radioligand membrane binding assay measuring displacement of [3H]naloxone by NFP from μ receptor expressed in CHO cells. naltrexone Hs Antagonist 9.1 – 9.7 pKi 79,166 ⤷ pKi 9.1 – 9.7 (Ki 7.1x10-10 – 1.99x10-10 M) [79,166] GSK1521498 Hs Antagonist 9.4 pKi 79 ⤷ pKi 9.4 (Ki 4.17x10-10 M) [79] alvimopan Hs Antagonist 9.3 pKi 90 ⤷ pKi 9.3 (Ki 4.7x10-10 M) peripheral [90] diprenorphine Hs Antagonist 9.1 pKi 166 ⤷ pKi 9.1 [166] levallorphan Hs Antagonist 8.8 – 9.3 pKi 101 ⤷ pKi 8.8 – 9.3 (Ki 1.69x10-9 – 4.8x10-10 M) [101] Description: Competition binding assay- the calculated Ki varies depending on the radioligand used. naloxone Mm Antagonist 9.0 pKi 143 ⤷ pKi 9.0 [143] naldemedine Hs Antagonist 8.9 pKi 73 ⤷ pKi 8.9 (Ki 1.13x10-9 M) [73] nalorphine Hs Antagonist 8.9 pKi 166 ⤷ pKi 8.9 [166] naloxone Hs Antagonist 8.9 pKi 166 ⤷ pKi 8.9 [166] BNTX Hs Antagonist 8.8 pKi 166 ⤷ pKi 8.8 [166] AT-076 Hs Antagonist 8.8 pKi 166,195 ⤷ pKi 8.8 (Ki 1.67x10-9 M) [166,195] Description: Radioligand binding assay methylnaltrexone Hs Antagonist 8.7 pKi 185 ⤷ pKi 8.7 (Ki 2x10-9 M) [185] Description: Displacement of [3H]DAMGO from human μ opioid receptors expressed in CHO cells CTAP Hs Antagonist 8.6 pKi 25,166 ⤷ pKi 8.6 [25,166] naloxone benzoylhydrazone Hs Antagonist 8.2 – 8.7 pKi 166 ⤷ pKi 8.7 [166] pKi 8.2 [166] naltrindole Hs Antagonist 8.2 pKi 166 ⤷ pKi 8.2 [166] naltriben Hs Antagonist 7.9 pKi 166 ⤷ pKi 7.9 [166] nor-binaltorphimine Hs Antagonist 7.7 pKi 166 ⤷ pKi 7.7 [166] aticaprant Hs Antagonist 7.6 pKi 145 ⤷ pKi 7.6 (Ki 2.4x10-8 M) [145] zyklophin Hs Antagonist 5.2 pKi 133 ⤷ pKi 5.2 (Ki 5.88x10-6 M) [133] navacaprant Hs Antagonist 7.0 pIC50 58 ⤷ pIC50 7.0 (IC50 1.1x10-7 M) [58] [3H]CTOP Hs Antagonist - - 165 ⤷ [165]
View species-specific antagonist tables
Antagonist Comments
β-FNA is an electrophilic affinity label. The pKi reflects both the reversible and irreversible binding components. CTOP is a good somatostatin receptor (sst receptor) agonist in addition to having antagonist activity at μ opioid receptors; it should never be used in studies of μ receptor function in situations where sst receptors may be involved. CTAP does not activate sst receptors [33]. The μ receptor is tagged as the primary target for the drug levallorphan, since the drug is mainly used for its antagonistic actions as an antidote to opioid overdose. Note that this drug also acts as a partial agonist at the κ receptor.

Allosteric Modulators
Key to terms and symbols	View all chemical structures	Click column headers to sort
Ligand Sp. Action Value Parameter Reference BMS-986123 Hs Neutral 6.0 pKB 19 ⤷ pKB 6.0 (KB 1x10-6 M) [19] BMS-986121 Hs Positive 5.7 pKB 19 ⤷ pKB 5.7 (KB 2x10-6 M) [19] BMS-986124 Hs Neutral 5.7 pKB 19 ⤷ pKB 5.7 (KB 2x10-6 M) [19] BMS-986122 Hs Positive 5.3 pKB 19 ⤷ pKB 5.3 (KB 5x10-6 M) [19] BMS-986187 Rn Positive 7.2 pKi 98 ⤷ pKi 7.2 (Ki 6.3x10-8 M) In the presence of 10 μM BMS-986187, the affinity of the μ-OR agonist DAMGO was enhanced 11-fold from 724 nM to 63 nM. [98] BMS-986121 Hs Positive 6.0 pEC50 19 ⤷ pEC50 6.0 (EC50 1x10-6 M) [19] BMS-986122 Hs Positive 5.5 pEC50 19 ⤷ pEC50 5.5 (EC50 3x10-6 M) [19]
View species-specific allosteric modulator tables
Allosteric Modulator Comments
BMS-986123 and BMS-986124 are Silent Allosteric Modulators (SAMs). These compounds neither potentiate nor inhibit the actions of an orthosteric agonist, however they block the actions of the Positive Allosteric Modulators (PAMs). A number of proteins such as G protein-coupled receptor kinases, β-arrestins and G proteins clearly regulate μ opioid receptor affinities and function. In addition sodium and guanyl nucleotides can modify the functional μ receptor complex and G protein interaction. Also, μ receptors are reported to form heterodimers with other receptors of the OP family or with non-opioid G protein-coupled receptors. Heterodimerisation may alter μ receptor function and/or trafficking [51,56,134].

Other Binding Ligands
Key to terms and symbols		Click column headers to sort
Ligand Sp. Action Value Parameter Reference compound 16 [PMID: 31498617] Hs Binding 5.4 pKi 141 ⤷ pKi 5.4 (Ki 3.73x10-6 M) [141] Description: Receptor binding in a radioligand displacement assay using [3H]DAMGO as tracer. EG-1-203 N/A - 9.0 pEC50 60 ⤷ pEC50 9.0 (EC50 9.5x10-10 M) [60] Description: Agonist activity at human MOR, determined in a forskolin-stimulated cAMP accumulation luminescence assay

Primary Transduction Mechanisms
Transducer	Effector/Response
Gi/Go family	Adenylyl cyclase stimulation Adenylyl cyclase inhibition Phospholipase C stimulation Potassium channel Calcium channel Phospholipase A2 stimulation Phospholipase D stimulation Other - See Comments
Comments:  The following systems have also been reported to be activated following Gi/Go activation via the μ receptor: Epidermal growth factor receptor transactivation and subsequent mitogen activated protein kinase ERK [12,96] Jun N-terminal kinase (JNK) expression and activity [45,77,153] Signal transducer and activator of transcription 3 (STAT3) [192] Focal adhesion kinase [104] Nuclear Ca2+/calmodulin translocation [179] Phosphatidylinositol-3 kinase expression and activity [77,126].
References:  6,17,20,23,36,48,68,83,116,119,125,130,137-138,144,170,190

Secondary Transduction Mechanisms
Transducer	Effector/Response
Gq/G11 family	Phospholipase C stimulation
Comments:  G16 couples to the μ opioid receptor and activates PLC.
References:  70,93

Tissue Distribution
Skin: dermal and epidermal nerve fibers. Species:  Human Technique:  Immunohistochemistry. References:  157
Immune cells: CEM x174 T/B lymphocytes, Raji B cells, CD4+, monocytes/macrophages, neutrophils. Species:  Human Technique:  RT-PCR. References:  34
CNS: caudate putamen, nucleus accumbens, endopiriform nucleus, fundus striati, habenula, amygdaloid nuclei, thalamus, hypothalamus, zona incerta, ventral tegmental area, interpeduncular nucleus, central gray, dentate gyrus, substantia nigra, the superior colliculus. Species:  Mouse Technique:  Radioligand binding. References:  82
Pregnant uterus. Species:  Mouse Technique:  in situ hybridisation. References:  197
CNS: Olfactory bulb, striatal patches and subcallosal streak, medial septum, piriform and cingulate cortex, entorhinal cortex, bed nucleus stria terminalis, medial preoptic area, globus and ventral pallidum, thalamic nuclei, lateral hypothalamus, mammillary nuclei , hippocampus, amygdaloid nuclei, ventral and lateral periaqueductal grey, ventral tegmental area and substantia nigra pars compacta, superior and inferior colliculi, interpeduncular nuclei, locus ceruleus, parabrachial nuclei, median raphe, nucleus of the solitary tract, spinal cord (dorsal root ganglia and layers I and II). Species:  Rat Technique:  in situ hybridisation. References:  161
CNS: olfactory bulb. Species:  Rat Technique:  immunocytochemistry. References:  150
CNS: cerebral cortex, striatum, hippocampus, locus coeruleus, superficial laminae of the dorsal horn. Species:  Rat Technique:  Immunohistochemistry. References:  5
Gastrointestinal tract. Species:  Rat Technique:  Immunohistochemistry. References:  7
CNS: superficial layers of the dorsal horn. Species:  Rat Technique:  immunocytochemistry. References:  31-32
CNS: striatum, medial habenular nucleus, medial terminal nucleus of the accessory optic tract, interpeduncular nucleus, median raphe nucleus, parabrachial nuclei, locus coeruleus, ambiguous nucleus, nucleus of the solitary tract, and laminae I and II of the medullary and spinal dorsal horns, cerebral cortex, amygdala, thalamus, and hypothalamus. Species:  Rat Technique:  Immunohistochemistry. References:  42
CNS: striatum, layers I and III of the cortex, the pyramidal cell layer of the hippocampal formation, specific nuclei of the thalamus, the pars reticulata of the substantia nigra, the interpeduncular nucleus, and the locus coeruleus. Species:  Rat Technique:  Radioligand binding. References:  160
Accessory optic tract. Species:  Rat Technique:  Immunohistochemistry. References:  44
CNS: anterior cingulate cortex, neocortex, amygdala, hippocampus, ventral dentate gyrus, presubiculum, nucleus accumbens, caudate putamen, thalamus, habenula, interpeduncular nucleus, pars compacta of the substantia nigra, superior and inferior colliculi, raphe nuclei. Species:  Rat Technique:  Radioligand binding. References:  108
CNS: caudate putamen. Species:  Rat Technique:  immunocytochemistry. References:  78,180
CNS: superficial layers of the spinal cord dorsal horn, nucleus caudalis of the spinal tract of the trigeminal, nucleus of the solitary tract, nucleus ambiguous, locus coeruleus, interpeduncular nucleus, lateral habenular nucleus, caudate-putamen, nucleus accumbens, ventral tegmental area, thalamus, hypothalamus, amygdaloid nuclei, nucleus accumbens, cerebral cortex, septum and diagonal band, preoptic area, medial thalamic and habenular nuclei, locus coeruleus, nucleus ambiguous, trigeminal nucleus caudalis, spinal cord substantia gelatinosa zones. Species:  Rat Technique:  Immunohistochemistry. References:  122
CNS: Purkinje cells and granular and molecular layers of the fetal, neonatal and adult cerebellum. Species:  Rat Technique:  Immunohistochemistry. References:  123
Ear: cochleae. Species:  Rat Technique:  RT-PCR. References:  76,135
CNS: nucleus accumbens (plasma membranes: extrasynaptic neuronal > glial) Species:  Rat Technique:  immunocytochemistry. References:  158
CNS: nucleus accumbens (plasma membranes of GABAergic neurons). Species:  Rat Technique:  immunocytochemistry. References:  159
CNS: locus coeruleus (noradrenergic perikarya and dendrites). Species:  Rat Technique:  immunocytochemistry. References:  172-173
CNS: accessory olfactory bulb, anterior olfactory nuclei, striatal patches of the nucleus accumbens and caudate-putamen, endopiriform nucleus, claustrum, diagonal band of Broca, globus pallidus, ventral pallidum, bed nucleus of stria terminalis, most thalamic nuclei, medial and posteriocortical medial amygdala, lateral, dorsomedial, posterior and mammillary nuclei of the hypothalamus, presubiculum, subiculum, rostral interpeduncular nucleus, median raphe, inferior colliculus, parabrachial nucleus, locus coeruleus, central grey, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis, nucleus cuneatus, dorsal motor nucleus of vagus. Species:  Rat Technique:  In situ hybridisation and radioligand binding. References:  107
CNS: thalamus, striatum, hypothalamus and pons-medulla > hippocampus and midbrain > cerebral cortex and cerebellum. Species:  Rat Technique:  Northern blotting. References:  115
CNS: olfactory bulb, caudate-putamen, nucleus accumbens, lateral and medial septum, diagonal band of Broca, bed nucleus of the stria terminalis, most thalamic nuclei, hippocampus, amygdala, medial preoptic area, superior and inferior colliculi, central gray, dorsal and median raphe, raphe magnus, locus coeruleus, parabrachial nucleus, pontine and medullary reticular nuclei, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis and cuneatus, dorsal motor nucleus of vagus, spinal cord, dorsal root ganglia. Species:  Rat Technique:  in situ hybridisation. References:  106
CNS: superficial layers (laminae I and II) of the dorsal horn of the spinal cord. Species:  Rat Technique:  Radioligand binding. References:  14
CNS: superficial layers of the medullary and spinal dorsal horns. Colocalisation with substance P. Species:  Rat Technique:  immunocytochemistry. References:  43,95
CNS: accessory olfactory bulb, striatal patches and streaks, amygdaloid nuclei, ventral hippocampal subiculum and dentate gyrus, numerous thalamic nuclei, geniculate bodies, central grey, superior and inferior colliculi, solitary and pontine nuclei and substantia nigra. Species:  Rat Technique:  Radioligand binding. References:  151
CNS: thalamus, striosomes of the caudate-putamen, globus pallidus, cerebral cortex. Species:  Rat Technique:  in situ hybridisation. References:  39
CNS: thalamic, brainstem and reticular core nuclei (highest in the habenular and thalamic nuclei). Species:  Rat Technique:  in situ hybridisation. References:  52
Tissue Distribution Comments
μ opioid receptors are widely distributed with dense labelling throughout the fore, mid and hindbrain regions in the CNS. Quantitatively, the μ receptor is the most highly expressed of all the opioid receptors. Although the early studies used non-selective ligands such as [3H]dihydromorphine, characterisation of the distribution of the μ opioid receptor has been aided by the availability of [3H]DAMGO, a highly selective opioid agonist that has been the ligand of choice for labelling μ opioid receptors for over 20 years. Immunohistochemistry has largely confirmed receptor autoradiography. For a review of μ opioid receptor expression in the rat see [105].

Expression Datasets
Show »« Hide Log average relative transcript abundance in mouse tissues measured by qPCR from Regard, J.B., Sato, I.T., and Coughlin, S.R. (2008). Anatomical profiling of G protein-coupled receptor expression. Cell, 135(3): 561-71. [PMID:18984166] [Raw data: website] There should be a chart of expression data here, you may need to enable JavaScript!

Functional Assays
Measurement of intracellular cAMP levels in SH-SY5Y cells endogenously expressing the μ receptor. Species:  Human Tissue:  SH-SY5Y cells. Response measured:  Inhibition of cAMP accumulation. References:  191
Measurement of [35S]GTPγS binding. Species:  Rat Tissue:  Brain slices. Response measured:  [35S]GTPγS binding. References:  152

Physiological Functions
Constriction of the pupil. Species:  Human Tissue:  Pupil. References:  124
DAMGO increases the conductance of an inwardly rectifying potassium conductance and hyperpolarises locus coeruleus neurons. Species:  Rat Tissue:  Brain. References:  130
μ receptor agonsts reduce bith early (GABAA receptor-mediated) and late (GABAB receptor-mediated) inhibitory postsynaptic currents in the dentate gyrus of hippocampal slices. Species:  Rat Tissue:  Hippocampal slices. References:  187
Morphine inhibits N- and P/Q-type Ca2+ channels in the nucleus traxtus solitarius of the rat. Species:  Rat Tissue:  Brain. References:  144
Morphine is responsible for modulating the Ca2+ currents in the mouse periaqueductal grey neurons. Species:  Mouse Tissue:  Periaqueductal grey neurons. References:  36
Morphine inhibits interpheron (IFN)-γ promotor activity in activated mouse T cells, which is mediated through two distinct cAMP-dependent pathways, the NF-κB signalling pathway and the ERK1/2, p38 MAPK, AP-1/NFAT pathway. Species:  Mouse Tissue:  T cells. References:  182
Body temperature regulation: μ receptor activation induces hypothermia, blocked by selective μ receptor antagonists. The effect is centrally mediated, involving both oxidative metabolism and heat exchange. Species:  Rat Tissue:  In vivo. References:  64

Physiological Consequences of Altering Gene Expression
Analgesia: Untreated μ receptor knockout mice display shorter latencies on tail flick and hot plate tests for spinal and supraspinal nociceptive responses than wild-type mice, which support the role for endogenous opioid-peptide interactions with the μ receptor in normal nociceptive processing. Interestingly, analgesia produced by the δ opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) in hot plate and tail flick tests is dramatically reduced in μ opioid receptor knockout mice in a gene-dose-dependent fashion, suggesting that DPDPE may require μ opioid receptor occupancies for full efficacy. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  99,109,148,154-155
Analgesia: Loss of μ opioid receptors prevents the plasma membrane translocation of δ opioid receptors in the dorsal horn of the spinal cord caused by chronic inflammatory pain induced by intraplantar injection of Freund's adjuvant. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  120-121
Addictions; drug-induced reinforcement: Opioid self-administration is abolished in μ receptor knockout mice. On the contrary, morphine is aversive in the μ opioid deficient mice by interaction with κ opioid receptors. In addition, μ opioid receptors may play a role in mediating various addictive agents such as ethanol, cocaine, nicotine and cannabinoid. Ethanol consumption is decreased in μ opioid knockout mice, and the animals exhibit less ethanol reward in a conditioned place preference paradigm. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  9-10,13,37,53,61-62,72
Addictions; locomotor activity: Cocaine-induced locomotor activity but not sensitisation is abolished in μ receptor knockout mice, while wild-type and heterozygous μ receptor mice display reduced cocaine conditioned place-preference, confirming a central role of μ receptors in drug reward but opposing effects in locomotor sensitisation. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  189
Emotional responsivity: μ opioid receptors may play a role in the modification of emotional responses to novelty, anxiety and depression. μ receptor knockout mice show less anxiety in the elevated plus maze and emergence tests, reduced response to novel stimuli in the novelty test and less depressive-like behaviour in the forced swim test. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  46
Attachment behaviour: Pups of μ receptor knockout mice emit fewer ultrasonic vocalisations when removed from their mothers. It indicates a role for μ opioid receptors in diseases characterised by deficits in attachment behaviour, such as autism or reactive attachment disorder. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  118
Modulation of neurotransmitter systems, dopamine: Administration of apomorphine increases the locomotor activity of μ receptor knockout mice more than wild-type mice, which may be related to the increased binding sites of the dopamine D2 receptor in the caudate putamen of receptor deficient mice. A tonically active μ opioid system modulates the basal dopamine neurotransmission in the nucleus accumbens (NAc). Microdialysis studies have revealed significant decreases in the dopamine fraction in μ opioid receptor knckout mice. μ opioid receptor knockout mice show diminished food-anticipatory activity which is dependent on μ-regulated dopaminergic activity. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  26,164
Modulation of neurotransmitter systems, acetylcholine: Muscarinic M1 receptor mRNA and protein levels are reduced in various bran regions when compared to the wild-type. In μ opioid receptor deficient mice an up-regulation of acetylcholinesterase activity and compensatory down-regulation of M2 muscarinic receptors in the striatal caudate putamen and nucleus accumbens have been reported, which can be associated with the enhanced tremors after administration of acetylcholinesterase inhibitors. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  163
Modulation of neurotransmitter systems, glutamate, somatostatin: An increase in glutamate and somatostatin binding was observed in μ receptor knockout mice, which may contribute to the enhanced excitability in these mice, showing an accelerated kindling development induced by the convulsant drug pentylenetetrazol. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  118
Learning and memory: Several studies have demonstrated that the loss of μ opioid receptors decreases LTP in the dentate gyrus of the hippocampus, suggesting the possibility that the lack of μ opioid receptors may acccompany a change in learning and memory. μ opioid receptor knockout mice show a significant spatial memory impairment compared to wild-type in the Morris water maze. They also exhibit an impairment in the ultimate level of spatial learning, suggesting that the μ opioid receptor may play a positive role in learning and memory by increasing LTP in CA3 neurons. On the other hand, the learning deficit induced in pentylenetetrazol kindling id absent in μ opioid receptor knockout mice. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  74-75,110
Immune responses: In μ receptor knockout mice chronic morphine administration cannot induce lymphoid organ atrophy, nor diminish the ratio of CD4+ CD8+ cells in the thymus nor reduce natural killer activity. Morphine modulation of macrophage phagocytosis and macrophage secretion of TNFα is not observed in μ receptor knockout animals. In contrast, morphine reduction of splenic and thymic cell number and mitogen-induced proliferation are unaffected, as is morphine inhibition of Il-1 and Il-6 secretion by macrophages. Morphine treatment promotes T(H2) differentiation through a μ opioid dependent mechanism. Developing T cells are responsive to the chemotactic effect of μ opioid agonists, an effect not seen in μ opioid knockout mice. Deficiency of μ receptor exacerbates experimental colitis whereas administration of the μ receptor agonist DAMGO reduces inflammation in wild-type mice. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  50,113,136,146-147
Other physiological functions: Sexual function in male homozygotes is affected, as shown by reducing mating activity and a decrease in sperm count and motility. Morphine-induced inhibition of gastrointestinal transit is abolished in μ receptor knockout mice, and basal GI motility is lower as compared to heterozygous and wild-type animals. μ opioid receptor knockout mice develop insulin resistance more rapidly than wild-type mice indicating a role for μ in controlling insulin resistance. Species:  Mouse Tissue:  Technique:  Gene targeting in embryonic stem cells. References:  30,162

Phenotypes, Alleles and Disease Models

Mouse data from MGI

Show »« Hide Allele Composition & genetic background Accession Phenotype Id Phenotype Reference Oprm1tm1Jlw Oprm1tm1Jlw/Oprm1tm1Jlw involves: 129P2/OlaHsd MGI:97441  MP:0009748 abnormal behavioral response to addictive substance PMID: 18207746  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0009278 abnormal bone marrow cell physiology PMID: 9126934  Oprm1+|Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1+ involves: Black Swiss MGI:97441  MP:0009278 abnormal bone marrow cell physiology PMID: 9126934  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd * C57BL/6 * DBA/2 MGI:97441  MP:0000188 abnormal circulating glucose level PMID: 16505249  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0006410 abnormal common myeloid progenitor cell morphology PMID: 9126934  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd * BALB/c * C57BL/6 MGI:97441  MP:0002912 abnormal excitatory postsynaptic potential PMID: 10727705  Oprm1tm1Yxp Oprm1tm1Yxp/Oprm1tm1Yxp involves: 129/Sv * C57BL/6 MGI:97441  MP:0006001 abnormal intestinal transit time PMID: 19273844  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0003959 abnormal lean body mass PMID: 19221053  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd * BALB/cJ * C57BL/6J MGI:97441  MP:0003633 abnormal nervous system physiology PMID: 15893889  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0002803 abnormal operant conditional behavior PMID: 15355329  Oprm1+|Oprm1tm1Jep Oprm1tm1Jep/Oprm1+ B6.129S-Oprm1 MGI:97441  MP:0002803 abnormal operant conditional behavior PMID: 15355329  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff 129S2/SvPas MGI:97441  MP:0001970 abnormal pain threshold PMID: 10835636  8893006  Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1tm1Uhl involves: 129S7/SvEvBrd * C57BL/6J MGI:97441  MP:0001970 abnormal pain threshold PMID: 9037090  Oprd1tm1Jep|Oprm1tm1Jep Oprd1tm1Jep/Oprd1tm1Jep,Oprm1tm1Jep/Oprm1tm1Jep,Oprm1tm1Jep/Oprm1tm1Jep involves: 129S/SvEv * 129S2/SvPas MGI:97438  MGI:97441  MP:0008872 abnormal physiological response to xenobiotic PMID: 17544222  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd MGI:97441  MP:0001463 abnormal spatial learning PMID: 14499482  Oprm1tm2.1Loh Oprm1tm2.1Loh/Oprm1tm2.1Loh Not Specified MGI:97441  MP:0001968 abnormal touch/ nociception PMID: 12525693  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0002675 asthenozoospermia PMID: 9126934  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff 129S2/SvPas MGI:97441  MP:0009776 decreased behavioral withdrawal response PMID: 10835636  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd MGI:97441  MP:0001982 decreased chemically-elicited antinociception PMID: 9555078  Oprm1+|Oprm1tm1Loh Oprm1tm1Loh/Oprm1+ involves: 129P2/OlaHsd MGI:97441  MP:0001982 decreased chemically-elicited antinociception PMID: 9555078  Oprm1tm1Yxp Oprm1tm1Yxp/Oprm1tm1Yxp involves: 129/Sv * C57BL/6 MGI:97441  MP:0001982 decreased chemically-elicited antinociception PMID: 19273844  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0001982 decreased chemically-elicited antinociception PMID: 9126934  Oprm1tm1Jabl Oprm1tm1Jabl/Oprm1tm1Jabl C57BL/6-Oprm1 MGI:97441  MP:0001982 decreased chemically-elicited antinociception PMID: 19528658  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0001935 decreased litter size PMID: 9126934  Oprm1+|Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1+ either: B6.129S7-Oprm1 or (involves: 129S7/SvEvBrd) MGI:97441  MP:0008874 decreased physiological sensitivity to xenobiotic PMID: 11377918  Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1tm1Uhl involves: 129S7/SvEvBrd * C57BL/6J MGI:97441  MP:0009767 decreased sensitivity to xenobiotic induced morbidity/mortality PMID: 11377918  Oprm1+|Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1+ either: B6.129S7-Oprm1 or (involves: 129S7/SvEvBrd) MGI:97441  MP:0009767 decreased sensitivity to xenobiotic induced morbidity/mortality PMID: 11377918  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd MGI:97441  MP:0009767 decreased sensitivity to xenobiotic induced morbidity/mortality PMID: 9555078  Oprm1+|Oprm1tm1Loh Oprm1tm1Loh/Oprm1+ involves: 129P2/OlaHsd MGI:97441  MP:0009767 decreased sensitivity to xenobiotic induced morbidity/mortality PMID: 9555078  Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1tm1Uhl involves: 129S7/SvEvBrd * C57BL/6J MGI:97441  MP:0003998 decreased thermal nociceptive threshold PMID: 9037090  Oprm1tm1Jabl Oprm1tm1Jabl/Oprm1tm1Jabl C57BL/6-Oprm1 MGI:97441  MP:0003998 decreased thermal nociceptive threshold PMID: 19528658  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0009749 enhanced behavioral response to addictive substance PMID: 15355329  Oprm1+|Oprm1tm1Jep Oprm1tm1Jep/Oprm1+ B6.129S-Oprm1 MGI:97441  MP:0009749 enhanced behavioral response to addictive substance PMID: 15355329  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep involves: 129S/SvEv MGI:97441  MP:0009779 enhanced behavioral response to anesthetic PMID: 11032994  Oprm1+|Oprm1tm1Jep Oprm1tm1Jep/Oprm1+ involves: 129S/SvEv MGI:97441  MP:0009779 enhanced behavioral response to anesthetic PMID: 11032994  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd MGI:97441  MP:0009754 enhanced behavioral response to cocaine PMID: 12781916  Oprm1+|Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1+ either: B6.129S7-Oprm1 or (involves: 129S7/SvEvBrd) MGI:97441  MP:0009713 enhanced conditioned place preference behavior PMID: 11377918  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0009171 enlarged pancreatic islets PMID: 19221053  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff 129S2/SvPas MGI:97441  MP:0001402 hypoactivity PMID: 8893006  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0001402 hypoactivity PMID: 9126934  Oprm1+|Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1+ involves: Black Swiss MGI:97441  MP:0001402 hypoactivity PMID: 9126934  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0002578 impaired ability to fire action potentials PMID: 15926936  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep involves: 129S/SvEv MGI:97441  MP:0009778 impaired behavioral response to anesthetic PMID: 10195199  11032994  Oprm1+|Oprm1tm1Jep Oprm1tm1Jep/Oprm1+ involves: 129S/SvEv MGI:97441  MP:0009778 impaired behavioral response to anesthetic PMID: 10195199  11032994  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff involves: 129S2/SvPas MGI:97441  MP:0009778 impaired behavioral response to anesthetic PMID: 10195199  Oprd1tm1Jep|Oprm1tm1Jep Oprd1tm1Jep/Oprd1tm1Jep,Oprm1tm1Jep/Oprm1tm1Jep,Oprm1tm1Jep/Oprm1tm1Jep involves: 129S/SvEv * 129S2/SvPas MGI:97438  MGI:97441  MP:0009778 impaired behavioral response to anesthetic PMID: 17544222  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff 129S2/SvPas MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 8893006  Oprm1tm1Uhl Oprm1tm1Uhl/Oprm1tm1Uhl involves: 129S7/SvEvBrd * C57BL/6J MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 11377918  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 12781916  9555078  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 9126934  Oprm1+|Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1+ involves: Black Swiss MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 9126934  Oprm1tm1Jabl Oprm1tm1Jabl/Oprm1tm1Jabl C57BL/6-Oprm1 MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 19528658  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep involves: 129S/SvEv MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 10195199  11032994  Oprm1+|Oprm1tm1Jep Oprm1tm1Jep/Oprm1+ involves: 129S/SvEv MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 10195199  11032994  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff involves: 129S2/SvPas MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 10195199  Oprd1tm1Jep|Oprm1tm1Jep Oprd1tm1Jep/Oprd1tm1Jep,Oprm1tm1Jep/Oprm1tm1Jep,Oprm1tm1Jep/Oprm1tm1Jep involves: 129S/SvEv * 129S2/SvPas MGI:97438  MGI:97441  MP:0009757 impaired behavioral response to morphine PMID: 17544222  Oprm1tm1Jabl Oprm1tm1Jabl/Oprm1tm1Jabl C57BL/6-Oprm1 MGI:97441  MP:0009712 impaired conditioned place preference behavior PMID: 19528658  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0005292 improved glucose tolerance PMID: 19221053  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0001260 increased body weight PMID: 19221053  Oprm1tm1Jlw Oprm1tm1Jlw/Oprm1tm1Jlw involves: 129P2/OlaHsd MGI:97441  MP:0001981 increased chemically-elicited antinociception PMID: 18207746  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0002079 increased circulating insulin level PMID: 19221053  Oprm1tm1Kff Oprm1tm1Kff/Oprm1tm1Kff 129S2/SvPas MGI:97441  MP:0003063 increased coping response PMID: 10835636  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0003135 increased erythroid progenitor cell number PMID: 9126934  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0003058 increased insulin secretion PMID: 19221053  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0009108 increased pancreas weight PMID: 19221053  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0005458 increased percent body fat PMID: 19221053  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0009336 increased splenocyte proliferation PMID: 9126934  Oprm1+|Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1+ involves: Black Swiss MGI:97441  MP:0009336 increased splenocyte proliferation PMID: 9126934  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0010024 increased total body fat amount PMID: 19221053  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0002574 increased vertical activity PMID: 9126934  Oprm1+|Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1+ involves: Black Swiss MGI:97441  MP:0002574 increased vertical activity PMID: 9126934  Oprm1tm1Jep Oprm1tm1Jep/Oprm1tm1Jep B6.129S-Oprm1 MGI:97441  MP:0008911 induced hyperactivity PMID: 15355329  Oprm1+|Oprm1tm1Jep Oprm1tm1Jep/Oprm1+ B6.129S-Oprm1 MGI:97441  MP:0008911 induced hyperactivity PMID: 15355329  Oprm1tm2Loh Oprm1tm2Loh/Oprm1tm2Loh Not Specified MGI:97441  MP:0002169 no abnormal phenotype detected PMID: 12525693  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0002687 oligozoospermia PMID: 9126934  Oprm1tm1Loh Oprm1tm1Loh/Oprm1tm1Loh involves: 129P2/OlaHsd * BALB/c * C57BL/6 MGI:97441  MP:0001473 reduced long term potentiation PMID: 10727705  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0001922 reduced male fertility PMID: 9126934  Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1tm1Lyu involves: Black Swiss MGI:97441  MP:0001380 reduced male mating frequency PMID: 9126934  Oprm1+|Oprm1tm1Lyu Oprm1tm1Lyu/Oprm1+ involves: Black Swiss MGI:97441  MP:0001380 reduced male mating frequency PMID: 9126934

Biologically Significant Variants
Type:  Splice variants Species:  Mouse Description:  Several splice variant forms of the μ receptor (formerly MOR-1) have been identified. These variant forms were designated MOR-1A through MOR-1X; some of the variants express truncated forms of the receptor. The B, C and D variants differ in the amino acid composition at the C-terminus. The distribution of the protein expressed from the B, C and D variant forms has been studied by immunohistochemistry in the rat brain. They show a different distribution in the brain and spinal cord. When compared to the μ receptor, MOR-1D is deferentially desensitised in response to opioid agonists. References:  1-3,15,83,85,89,112,127,132,149-150,168,198
Type:  Single nucleotide polymorphism Species:  Human Description:  An Asn40 -> Asp polymorphism has been found in high abundance in the caucasian and asian population. There are studies showing functional differences of the variant to wild-type receptor in vitro and in vivo. In addition, there are several reports showing association of this polymorphism with addiction and idiopathic epilepsy. Amino acid change:  N40D References:  11,16,87,100,111
Type:  Single nucleotide polymorphism Species:  Human Description:  A rare Ser268 -> Pro polymorphism has been identified in the human receptor gene. The variant receptor possesses a marked reduction in coupling efficiency and is less desensitised upon agonist exposure. Amino acid change:  S268P References:  84
Biologically Significant Variant Comments
There are many additional polymorphisms of the μ receptor which are either without function or their functional significance is presently unknown.