Gene and Protein Information
class A G protein-coupled receptor
Species	TM	AA	Chromosomal Location	Gene Symbol	Gene Name	Reference
Human	7	400	6q24-q25	OPRM1	opioid receptor mu 1	163
Mouse	7	398	10 A2	Oprm1	opioid receptor, mu 1	102
Rat	7	398	1p11	Oprm1	opioid receptor	24,43,145,162,172

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
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)
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:  90
	Description:  Structure of the μ-opioid receptor–Gi protein complex PDB Id:  6DDF Ligand:  DAMGO Resolution:  3.5Å Species:  Mouse References:  76

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 127 ⤷ pKd 9.2 (Kd 5.7x10-10 M) [127] carfentanil Cp Agonist 10.6 pKi 32 ⤷ pKi 10.6 (Ki 2.4x10-11 M) [32] Description: Binding affinity in guinea pig whole brain- displacement of [3H]DAGO binding. (-)-cyclazocine Hs Partial agonist 10.0 pKi 149 ⤷ pKi 10.0 [149] butorphanol Hs Partial agonist 9.9 pKi 45 ⤷ pKi 9.9 (Ki 1.2x10-10 M) [45] Description: Displacement of [3H]DAMGO from human μ opioid receptor expressed in CHO cells. sufentanil Hs Full agonist 9.9 pKi 158 ⤷ pKi 9.9 (Ki 1.38x10-10 M) [158] etonitazene Hs Full agonist 9.7 pKi 149 ⤷ pKi 9.7 [149] hydromorphone Hs Agonist 9.6 pKi 164 ⤷ pKi 9.6 (Ki 2.8x10-10 M) [164] ethylketocyclazocine Hs Full agonist 9.5 pKi 149 ⤷ pKi 9.5 [149] etorphine Hs Full agonist 9.5 pKi 149 ⤷ pKi 9.5 [149] fentanyl Rn Full agonist 9.4 pKi 127 ⤷ pKi 9.4 [127] DAMGO Hs Full agonist 9.3 pKi 55,149 ⤷ pKi 9.3 [55,149] loperamide Hs Agonist 9.3 pKi 25 ⤷ pKi 9.3 (Ki 5.3x10-10 M) [25] (-)-methadone Hs Full agonist 9.2 pKi 149 ⤷ pKi 9.2 [149] [Met]enkephalin {Sp: Human, Mouse, Rat} Rn Full agonist 9.2 pKi 127 ⤷ pKi 9.2 [127] fentanyl Hs Full agonist 9.2 pKi 149 ⤷ pKi 9.2 [149] cebranopadol Hs Agonist 9.1 pKi 85 ⤷ pKi 9.1 (Ki 7x10-10 M) [85] Description: Radioligand binding assay sufentanil Rn Full agonist 9.1 pKi 166 ⤷ pKi 9.1 (Ki 7.7x10-10 M) [166] eluxadoline Rn Agonist 9.1 pKi 17 ⤷ pKi 9.1 (Ki 9x10-10 M) [17] morphine Hs Full agonist 9.0 pKi 50,149 ⤷ pKi 9.0 [50,149] β-endorphin {Sp: Human} Rn Full agonist 9.0 pKi 127 ⤷ pKi 9.0 [127] PZM21 Hs Biased agonist 9.0 pKi 91 ⤷ pKi 9.0 (Ki 1.1x10-9 M) [91] dihydromorphine Hs Full agonist 8.8 pKi 149 ⤷ pKi 8.8 [149] dynorphin-(1-11) Hs Full agonist 8.8 pKi 149 ⤷ pKi 8.8 [149] normorphine Hs Full agonist 8.8 pKi 149 ⤷ pKi 8.8 [149] nalbuphine Hs Agonist 8.8 pKi 164 ⤷ pKi 8.8 (Ki 1.6x10-9 M) [164] buprenorphine Hs Partial agonist 8.8 pKi 149 ⤷ pKi 8.8 [149] eluxadoline Hs Agonist 8.8 pKi 17 ⤷ pKi 8.8 (Ki 1.7x10-9 M) [17] DADLE Hs Full agonist 8.7 pKi 149 ⤷ pKi 8.7 [149] DAMGO Rn Full agonist 8.7 pKi 127 ⤷ pKi 8.7 [127] hydrocodone Hs Agonist 8.7 pKi 115 ⤷ pKi 8.7 (Ki 2x10-9 M) [115] BU08028 Hs Partial agonist 8.7 pKi 71 ⤷ pKi 8.7 (Ki 2.14x10-9 M) [71] cebranopadol Rn Agonist 8.6 pKi 85 ⤷ pKi 8.6 (Ki 2.4x10-9 M) [85] Description: Radioligand binding assay dynorphin B {Sp: Human, Mouse, Rat} Hs Full agonist 8.5 pKi 149 ⤷ pKi 8.5 [149] endomorphin-2 {Sp: Human} Rn Full agonist 8.5 pKi 171 ⤷ pKi 8.5 (Ki 3.24x10-9 M) [171] (-)-pentazocine Hs Partial agonist 8.4 pKi 149 ⤷ pKi 8.4 [149] dynorphin A-(1-8) {Sp: Human, Mouse, Rat} Hs Full agonist 8.4 pKi 149 ⤷ pKi 8.4 [149] dynorphin A-(1-13) {Sp: Human, Mouse, Rat} Hs Full agonist 8.3 pKi 149 ⤷ pKi 8.3 [149] endomorphin-1 {Sp: Human} Hs Full agonist 8.3 pKi 52,171 ⤷ pKi 8.3 (Ki 5.01x10-9 M) [52,171] DSLET Hs Full agonist 8.2 pKi 149 ⤷ pKi 8.2 [149] PL017 Hs Full agonist 8.2 pKi 22,149 ⤷ pKi 8.2 [22,149] [Leu]enkephalin {Sp: Human, Mouse, Rat} Hs Partial agonist 8.1 pKi 149 ⤷ pKi 8.1 [149] dynorphin A {Sp: Human, Mouse, Rat} Hs Full agonist 8.1 pKi 149 ⤷ pKi 8.1 [149] UFP-512 Hs Agonist 8.0 pKi 157 ⤷ pKi 8.0 [157] Description: Measuring displacement of [3H]-diprenorphine in vitro morphine Rn Partial agonist 7.9 pKi 127 ⤷ pKi 7.9 [127] BW373U86 Rn Agonist 7.8 pKi 21 ⤷ pKi 7.8 (Ki 1.5x10-8 M) [21] UFP-505 Hs Agonist 7.8 pKi 36-37 ⤷ pKi 7.8 [36-37] ADL5747 Hs Agonist 7.7 pKi 81 ⤷ pKi 7.7 (Ki 1.8x10-8 M) [81] ADL5859 Hs Agonist 7.5 pKi 80 ⤷ pKi 7.5 (Ki 3.2x10-8 M) [80] SCH221510 Hs Agonist 7.2 pKi 156 ⤷ pKi 7.2 (Ki 6.5x10-8 M) [156] Description: Radioligand binding assay SR16835 Hs Agonist 7.1 pKi 150 ⤷ pKi 7.1 (Ki 7.99x10-8 M) [150] Description: Radioligand binding assay codeine Hs Full agonist 6.9 pKi 149 ⤷ pKi 6.9 [149] tapentadol Hs Agonist 6.8 pKi 152 ⤷ pKi 6.8 (Ki 1.6x10-7 M) [152] BW373U86 Hs Agonist 6.6 pKi 80 ⤷ pKi 6.6 (Ki 2.6x10-7 M) [80] HS665 Hs Agonist 6.3 pKi 140 ⤷ pKi 6.3 (Ki 5.42x10-7 M) [140] compound 3 [PMID: 23134120] Hs Agonist 6.1 pKi 140 ⤷ pKi 6.1 (Ki 8.26x10-7 M) [140] tramadol Hs Agonist 5.8 pKi 164 ⤷ pKi 5.8 (Ki 1.6x10-6 M) [164] Description: Displacement of the mu aginist peptide DAMGO from the mu receptor expressed in CHO cells. PZM21 Hs Biased agonist 8.3 pEC50 91 ⤷ pEC50 8.3 (EC50 4.6x10-9 M) [91] Description: In a Gi/o activation assay. levorphanol Hs Agonist 9.9 pIC50 57 ⤷ pIC50 9.9 (IC50 1.3x10-10 M) [57] BU72 Cp Agonist 9.8 pIC50 62 ⤷ pIC50 9.8 (IC50 1.5x10-10 M) [62] Description: Measuring displacement of [3H]DAMGO from brain membranes isolated from Hartley guinea pigs. methadone Hs Agonist 8.4 pIC50 126 ⤷ pIC50 8.4 (IC50 4.1x10-9 M) [126] Description: Binding affinity against μ-opioid receptor (human) using [3H]DAMGO radioligand. pethidine Hs Agonist 6.5 pIC50 126 ⤷ pIC50 6.5 (IC50 3.15x10-7 M) [126] AR-M1000390 Hs Agonist 5.4 pIC50 4 ⤷ pIC50 5.4 (IC50 3.8x10-6 M) [4] [3H]PL017 Rn Agonist - - 58 ⤷ [58] U-47700 Rn Agonist - - 26 ⤷ [26]
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 [149]. It is still unclear whether endomorphins are endogenous. Morphine occurs endogenously [125]. 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 [115]. Similarly, we have tagged the μ receptor as the primary target of hydromorphone [164]. Methadone is selective for the μ receptor: comparable IC50s at the κ and δ receptors are 512 and 1090nM respectively [126]. [11C]carfentanil (PubChem CID 449698) binds the human μ receptor with a Ki of 0.07 nM, in a [3H]DAMGO displacement assay [59].

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 127 ⤷ pKd 10.1 (Kd 7.94x10-11 M) [127] [3H]naloxone Rn Antagonist 8.6 – 9.0 pKd 117 ⤷ pKd 8.6 – 9.0 (Kd 2.8x10-9 – 1x10-9 M) [117] Description: CHO and BHK cell lines stably expressing the rat μ receptor naloxonazine Mm Antagonist 10.3 pKi 127 ⤷ pKi 10.3 [127] diprenorphine Mm Antagonist 10.1 pKi 127 ⤷ pKi 10.1 [127] quadazocine Hs Antagonist 10.0 pKi 149 ⤷ pKi 10.0 [149] (-)-bremazocine Hs Antagonist 9.7 pKi 149 ⤷ pKi 9.7 [149] CTOP Hs Antagonist 9.7 pKi 127 ⤷ pKi 9.7 [127] nalmefene Hs Antagonist 9.5 pKi 149 ⤷ pKi 9.5 [149] β-FNA Hs Antagonist 9.5 pKi 149 ⤷ pKi 9.5 [149] β-FNA Mm Antagonist 9.5 pKi 127 ⤷ pKi 9.5 [127] NFP Hs Antagonist 9.4 pKi 174 ⤷ pKi 9.4 (Ki 3.6x10-10 M) [174] 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 70,149 ⤷ pKi 9.1 – 9.7 (Ki 7.1x10-10 – 1.99x10-10 M) [70,149] GSK1521498 Hs Antagonist 9.4 pKi 70 ⤷ pKi 9.4 (Ki 4.17x10-10 M) [70] alvimopan Hs Antagonist 9.3 pKi 79 ⤷ pKi 9.3 (Ki 4.7x10-10 M) [79] diprenorphine Hs Antagonist 9.1 pKi 149 ⤷ pKi 9.1 [149] levallorphan Hs Antagonist 8.8 – 9.3 pKi 89 ⤷ pKi 8.8 – 9.3 (Ki 1.69x10-9 – 4.8x10-10 M) [89] Description: Competition binding assay- the calculated Ki varies depending on the radioligand used. naloxone Mm Antagonist 9.0 pKi 127 ⤷ pKi 9.0 [127] naldemedine Hs Antagonist 8.9 pKi 64 ⤷ pKi 8.9 (Ki 1.13x10-9 M) [64] nalorphine Hs Antagonist 8.9 pKi 149 ⤷ pKi 8.9 [149] naloxone Hs Antagonist 8.9 pKi 149 ⤷ pKi 8.9 [149] BNTX Hs Antagonist 8.8 pKi 149 ⤷ pKi 8.8 [149] AT-076 Hs Antagonist 8.8 pKi 149,173 ⤷ pKi 8.8 (Ki 1.67x10-9 M) [149,173] Description: Radioligand binding assay methylnaltrexone Hs Antagonist 8.7 pKi 164 ⤷ pKi 8.7 (Ki 2x10-9 M) [164] Description: Displacement of [3H]DAMGO from human μ opioid receptors expressed in CHO cells CTAP Hs Antagonist 8.6 pKi 22,149 ⤷ pKi 8.6 [22,149] naloxone benzoylhydrazone Hs Antagonist 8.2 – 8.7 pKi 149 ⤷ pKi 8.7 [149] pKi 8.2 [149] naltrindole Hs Antagonist 8.2 pKi 149 ⤷ pKi 8.2 [149] naltriben Hs Antagonist 7.9 pKi 149 ⤷ pKi 7.9 [149] nor-binaltorphimine Hs Antagonist 7.7 pKi 149 ⤷ pKi 7.7 [149] LY2456302 Hs Antagonist 7.6 pKi 129 ⤷ pKi 7.6 (Ki 2.4x10-8 M) [129] zyklophin Hs Antagonist 5.2 pKi 119 ⤷ pKi 5.2 (Ki 5.88x10-6 M) [119]
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 [30]. 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 18 ⤷ pKB 6.0 (KB 1x10-6 M) [18] BMS-986121 Hs Positive 5.7 pKB 18 ⤷ pKB 5.7 (KB 2x10-6 M) [18] BMS-986124 Hs Neutral 5.7 pKB 18 ⤷ pKB 5.7 (KB 2x10-6 M) [18] BMS-986122 Hs Positive 5.3 pKB 18 ⤷ pKB 5.3 (KB 5x10-6 M) [18] BMS-986187 Rn Positive 7.2 pKi 86 ⤷ 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. [86] BMS-986121 Hs Positive 6.0 pEC50 18 ⤷ pEC50 6.0 (EC50 1x10-6 M) [18] BMS-986122 Hs Positive 5.5 pEC50 18 ⤷ pEC50 5.5 (EC50 3x10-6 M) [18]
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 [47,51,120].

Primary Transduction Mechanisms
Transducer	Effector/Response
Gi/Go family	Adenylate cyclase stimulation Adenylate 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 [11,84] Jun N-terminal kinase (JNK) expression and activity [41,68,137] Signal transducer and activator of transcription 3 (STAT3) [170] Focal adhesion kinase [92] Nuclear Ca2+/calmodulin translocation [159] Phosphatidylinositol-3 kinase expression and activity [68,113].
References:  6,16,19-20,33,44,60,73,104,106,112,116,123-124,128,153,168

Secondary Transduction Mechanisms
Transducer	Effector/Response
Gq/G11 family	Phospholipase C stimulation
Comments:  G16 couples to the μ opioid receptor and activates PLC.
References:  61,82

Tissue Distribution
Immune cells: CEM x174 T/B lymphocytes, Raji B cells, CD4+, monocytes/macrophages, neutrophils. Species:  Human Technique:  RT-PCR. References:  31
Skin: dermal and epidermal nerve fibers. Species:  Human Technique:  Immunohistochemistry. References:  141
Pregnant uterus. Species:  Mouse Technique:  in situ hybridisation. References:  175
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:  72
Gastrointestinal tract. Species:  Rat Technique:  Immunohistochemistry. References:  7
CNS: superficial layers of the dorsal horn. Species:  Rat Technique:  immunocytochemistry. References:  28-29
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:  38
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:  144
Accessory optic tract. Species:  Rat Technique:  Immunohistochemistry. References:  40
CNS: superficial layers of the medullary and spinal dorsal horns. Colocalisation with substance P. Species:  Rat Technique:  immunocytochemistry. References:  39,83
CNS: caudate putamen. Species:  Rat Technique:  immunocytochemistry. References:  69,160
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:  109
CNS: Purkinje cells and granular and molecular layers of the fetal, neonatal and adult cerebellum. Species:  Rat Technique:  Immunohistochemistry. References:  110
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:  96
CNS: nucleus accumbens (plasma membranes: extrasynaptic neuronal > glial) Species:  Rat Technique:  immunocytochemistry. References:  142
CNS: nucleus accumbens (plasma membranes of GABAergic neurons). Species:  Rat Technique:  immunocytochemistry. References:  143
CNS: locus coeruleus (noradrenergic perikarya and dendrites). Species:  Rat Technique:  immunocytochemistry. References:  154-155
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:  135
CNS: thalamus, striosomes of the caudate-putamen, globus pallidus, cerebral cortex. Species:  Rat Technique:  in situ hybridisation. References:  35
CNS: thalamic, brainstem and reticular core nuclei (highest in the habenular and thalamic nuclei). Species:  Rat Technique:  in situ hybridisation. References:  48
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:  95
CNS: thalamus, striatum, hypothalamus and pons-medulla > hippocampus and midbrain > cerebral cortex and cerebellum. Species:  Rat Technique:  Northern blotting. References:  103
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:  94
CNS: superficial layers (laminae I and II) of the dorsal horn of the spinal cord. Species:  Rat Technique:  Radioligand binding. References:  13
Ear: cochleae. Species:  Rat Technique:  RT-PCR. References:  67,121
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:  145
CNS: olfactory bulb. Species:  Rat Technique:  immunocytochemistry. References:  134
CNS: cerebral cortex, striatum, hippocampus, locus coeruleus, superficial laminae of the dorsal horn. Species:  Rat Technique:  Immunohistochemistry. References:  5
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 [93].

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:  169
Measurement of [35S]GTPγS binding. Species:  Rat Tissue:  Brain slices. Response measured:  [35S]GTPγS binding. References:  136

Physiological Functions
Constriction of the pupil. Species:  Human Tissue:  Pupil. References:  111
DAMGO increases the conductance of an inwardly rectifying potassium conductance and hyperpolarises locus coeruleus neurons. Species:  Rat Tissue:  Brain. References:  116
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:  165
Morphine inhibits N- and P/Q-type Ca2+ channels in the nucleus traxtus solitarius of the rat. Species:  Rat Tissue:  Brain. References:  128
Morphine is responsible for modulating the Ca2+ currents in the mouse periaqueductal grey neurons. Species:  Mouse Tissue:  Periaqueductal grey neurons. References:  33
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:  161
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:  56

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:  87,97,132,138-139
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: