μ receptor | Opioid receptors | IUPHAR/BPS Guide to PHARMACOLOGY

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μ receptor

Target not currently curated in GtoImmuPdb

Target id: 319

Nomenclature: μ receptor

Family: Opioid receptors

Annotation status:  image of a green circle Annotated and expert reviewed. Please contact us if you can help with updates.  » Email us

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 1 165
Mouse 7 398 10 A2 Oprm1 opioid receptor, mu 1 103
Rat 7 398 1p11 Oprm1 opioid receptor, mu 1 24,43,147,164,174
Previous and Unofficial Names
Mu receptor | MOP | OP3 | MOPr | opioid receptor | opioid receptor, mu 1
Database Links
Specialist databases
GPCRDB oprm_human (Hs), oprm_mouse (Mm), oprm_rat (Rn)
Other databases
ChEMBL Target
DrugBank Target
Ensembl Gene
Entrez Gene
Human Protein Atlas
KEGG Gene
OMIM
RefSeq Nucleotide
RefSeq Protein
SynPHARM
UniProtKB
Wikipedia
Selected 3D Structures
Image of receptor 3D structure from RCSB PDB
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:  91
Image of receptor 3D structure from RCSB PDB
Description:  Structure of the μ-opioid receptor–Gi protein complex
PDB Id:  6DDF
Ligand:  DAMGO
Resolution:  3.5Å
Species:  Mouse
References:  77
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 129
pKd 9.2 (Kd 5.7x10-10 M) [129]
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 151
pKi 10.0 [151]
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 160
pKi 9.9 (Ki 1.38x10-10 M) [160]
etonitazene Hs Full agonist 9.7 pKi 151
pKi 9.7 [151]
hydromorphone Hs Agonist 9.6 pKi 166
pKi 9.6 (Ki 2.8x10-10 M) [166]
ethylketocyclazocine Hs Full agonist 9.5 pKi 151
pKi 9.5 [151]
etorphine Hs Full agonist 9.5 pKi 151
pKi 9.5 [151]
fentanyl Rn Full agonist 9.4 pKi 129
pKi 9.4 [129]
DAMGO Hs Full agonist 9.3 pKi 56,151
pKi 9.3 [56,151]
loperamide Hs Agonist 9.3 pKi 25
pKi 9.3 (Ki 5.3x10-10 M) [25]
(-)-methadone Hs Full agonist 9.2 pKi 151
pKi 9.2 [151]
[Met]enkephalin {Sp: Human, Mouse, Rat} Rn Full agonist 9.2 pKi 129
pKi 9.2 [129]
fentanyl Hs Full agonist 9.2 pKi 151
pKi 9.2 [151]
cebranopadol Hs Agonist 9.1 pKi 86
pKi 9.1 (Ki 7x10-10 M) [86]
Description: Radioligand binding assay
sufentanil Rn Full agonist 9.1 pKi 168
pKi 9.1 (Ki 7.7x10-10 M) [168]
eluxadoline Rn Agonist 9.1 pKi 17
pKi 9.1 (Ki 9x10-10 M) [17]
morphine Hs Full agonist 9.0 pKi 50,151
pKi 9.0 [50,151]
β-endorphin {Sp: Human} Rn Full agonist 9.0 pKi 129
pKi 9.0 [129]
PZM21 Hs Biased agonist 9.0 pKi 92
pKi 9.0 (Ki 1.1x10-9 M) [92]
dihydromorphine Hs Full agonist 8.8 pKi 151
pKi 8.8 [151]
dynorphin-(1-11) Hs Full agonist 8.8 pKi 151
pKi 8.8 [151]
normorphine Hs Full agonist 8.8 pKi 151
pKi 8.8 [151]
nalbuphine Hs Agonist 8.8 pKi 166
pKi 8.8 (Ki 1.6x10-9 M) [166]
buprenorphine Hs Partial agonist 8.8 pKi 151
pKi 8.8 [151]
eluxadoline Hs Agonist 8.8 pKi 17
pKi 8.8 (Ki 1.7x10-9 M) [17]
DADLE Hs Full agonist 8.7 pKi 151
pKi 8.7 [151]
DAMGO Rn Full agonist 8.7 pKi 129
pKi 8.7 [129]
hydrocodone Hs Agonist 8.7 pKi 116
pKi 8.7 (Ki 2x10-9 M) [116]
BU08028 Hs Partial agonist 8.7 pKi 72
pKi 8.7 (Ki 2.14x10-9 M) [72]
cebranopadol Rn Agonist 8.6 pKi 86
pKi 8.6 (Ki 2.4x10-9 M) [86]
Description: Radioligand binding assay
dynorphin B {Sp: Human, Mouse, Rat} Hs Full agonist 8.5 pKi 151
pKi 8.5 [151]
endomorphin-2 {Sp: Human} Rn Full agonist 8.5 pKi 173
pKi 8.5 (Ki 3.24x10-9 M) [173]
(-)-pentazocine Hs Partial agonist 8.4 pKi 151
pKi 8.4 [151]
dynorphin A-(1-8) {Sp: Human, Mouse, Rat} Hs Full agonist 8.4 pKi 151
pKi 8.4 [151]
dynorphin A-(1-13) {Sp: Human, Mouse, Rat} Hs Full agonist 8.3 pKi 151
pKi 8.3 [151]
endomorphin-1 {Sp: Human} Hs Full agonist 8.3 pKi 52,173
pKi 8.3 (Ki 5.01x10-9 M) [52,173]
DSLET Hs Full agonist 8.2 pKi 151
pKi 8.2 [151]
PL017 Hs Full agonist 8.2 pKi 22,151
pKi 8.2 [22,151]
[Leu]enkephalin {Sp: Human, Mouse, Rat} Hs Partial agonist 8.1 pKi 151
pKi 8.1 [151]
dynorphin A {Sp: Human, Mouse, Rat} Hs Full agonist 8.1 pKi 151
pKi 8.1 [151]
UFP-512 Hs Agonist 8.0 pKi 159
pKi 8.0 [159]
Description: Measuring displacement of [3H]-diprenorphine in vitro
morphine Rn Partial agonist 7.9 pKi 129
pKi 7.9 [129]
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 82
pKi 7.7 (Ki 1.8x10-8 M) [82]
ADL5859 Hs Agonist 7.5 pKi 81
pKi 7.5 (Ki 3.2x10-8 M) [81]
SCH221510 Hs Agonist 7.2 pKi 158
pKi 7.2 (Ki 6.5x10-8 M) [158]
Description: Radioligand binding assay
SR16835 Hs Agonist 7.1 pKi 152
pKi 7.1 (Ki 7.99x10-8 M) [152]
Description: Radioligand binding assay
codeine Hs Full agonist 6.9 pKi 151
pKi 6.9 [151]
tapentadol Hs Agonist 6.8 pKi 154
pKi 6.8 (Ki 1.6x10-7 M) [154]
BW373U86 Hs Agonist 6.6 pKi 81
pKi 6.6 (Ki 2.6x10-7 M) [81]
HS665 Hs Agonist 6.3 pKi 142
pKi 6.3 (Ki 5.42x10-7 M) [142]
compound 3 [PMID: 23134120] Hs Agonist 6.1 pKi 142
pKi 6.1 (Ki 8.26x10-7 M) [142]
tramadol Hs Agonist 5.8 pKi 166
pKi 5.8 (Ki 1.6x10-6 M) [166]
Description: Displacement of the mu aginist peptide DAMGO from the mu receptor expressed in CHO cells.
PZM21 Hs Biased agonist 8.3 pEC50 92
pEC50 8.3 (EC50 4.6x10-9 M) [92]
Description: In a Gi/o activation assay.
levorphanol Hs Agonist 9.9 pIC50 58
pIC50 9.9 (IC50 1.3x10-10 M) [58]
BU72 Cp Agonist 9.8 pIC50 63
pIC50 9.8 (IC50 1.5x10-10 M) [63]
Description: Measuring displacement of [3H]DAMGO from brain membranes isolated from Hartley guinea pigs.
methadone Hs Agonist 8.4 pIC50 127
pIC50 8.4 (IC50 4.1x10-9 M) [127]
Description: Binding affinity against μ-opioid receptor (human) using [3H]DAMGO radioligand.
pethidine Hs Agonist 6.5 pIC50 127
pIC50 6.5 (IC50 3.15x10-7 M) [127]
AR-M1000390 Hs Agonist 5.4 pIC50 4
pIC50 5.4 (IC50 3.8x10-6 M) [4]
[3H]PL017 Rn Agonist - - 59
[59]
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 [151].

It is still unclear whether endomorphins are endogenous. Morphine occurs endogenously [126].

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 [116]. Similarly, we have tagged the μ receptor as the primary target of hydromorphone [166].

Methadone is selective for the μ receptor: comparable IC50s at the κ and δ receptors are 512 and 1090nM respectively [127].

[11C]carfentanil (PubChem CID 449698) binds the human μ receptor with a Ki of 0.07 nM, in a [3H]DAMGO displacement assay [60].
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 129
pKd 10.1 (Kd 7.94x10-11 M) [129]
[3H]naloxone Rn Antagonist 8.6 – 9.0 pKd 118
pKd 8.6 – 9.0 (Kd 2.8x10-9 – 1x10-9 M) [118]
Description: CHO and BHK cell lines stably expressing the rat μ receptor
naloxonazine Mm Antagonist 10.3 pKi 129
pKi 10.3 [129]
diprenorphine Mm Antagonist 10.1 pKi 129
pKi 10.1 [129]
quadazocine Hs Antagonist 10.0 pKi 151
pKi 10.0 [151]
(-)-bremazocine Hs Antagonist 9.7 pKi 151
pKi 9.7 [151]
CTOP Hs Antagonist 9.7 pKi 129
pKi 9.7 [129]
nalmefene Hs Antagonist 9.5 pKi 151
pKi 9.5 [151]
β-FNA Hs Antagonist 9.5 pKi 151
pKi 9.5 [151]
β-FNA Mm Antagonist 9.5 pKi 129
pKi 9.5 [129]
NFP Hs Antagonist 9.4 pKi 176
pKi 9.4 (Ki 3.6x10-10 M) [176]
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 71,151
pKi 9.1 – 9.7 (Ki 7.1x10-10 – 1.99x10-10 M) [71,151]
GSK1521498 Hs Antagonist 9.4 pKi 71
pKi 9.4 (Ki 4.17x10-10 M) [71]
alvimopan Hs Antagonist 9.3 pKi 80
pKi 9.3 (Ki 4.7x10-10 M) [80]
diprenorphine Hs Antagonist 9.1 pKi 151
pKi 9.1 [151]
levallorphan Hs Antagonist 8.8 – 9.3 pKi 90
pKi 8.8 – 9.3 (Ki 1.69x10-9 – 4.8x10-10 M) [90]
Description: Competition binding assay- the calculated Ki varies depending on the radioligand used.
naloxone Mm Antagonist 9.0 pKi 129
pKi 9.0 [129]
naldemedine Hs Antagonist 8.9 pKi 65
pKi 8.9 (Ki 1.13x10-9 M) [65]
nalorphine Hs Antagonist 8.9 pKi 151
pKi 8.9 [151]
naloxone Hs Antagonist 8.9 pKi 151
pKi 8.9 [151]
BNTX Hs Antagonist 8.8 pKi 151
pKi 8.8 [151]
AT-076 Hs Antagonist 8.8 pKi 151,175
pKi 8.8 (Ki 1.67x10-9 M) [151,175]
Description: Radioligand binding assay
methylnaltrexone Hs Antagonist 8.7 pKi 166
pKi 8.7 (Ki 2x10-9 M) [166]
Description: Displacement of [3H]DAMGO from human μ opioid receptors expressed in CHO cells
CTAP Hs Antagonist 8.6 pKi 22,151
pKi 8.6 [22,151]
naloxone benzoylhydrazone Hs Antagonist 8.2 – 8.7 pKi 151
pKi 8.7 [151]
pKi 8.2 [151]
naltrindole Hs Antagonist 8.2 pKi 151
pKi 8.2 [151]
naltriben Hs Antagonist 7.9 pKi 151
pKi 7.9 [151]
nor-binaltorphimine Hs Antagonist 7.7 pKi 151
pKi 7.7 [151]
LY2456302 Hs Antagonist 7.6 pKi 131
pKi 7.6 (Ki 2.4x10-8 M) [131]
zyklophin Hs Antagonist 5.2 pKi 120
pKi 5.2 (Ki 5.88x10-6 M) [120]
BTRX-335140 Hs Antagonist 7.0 pIC50 53
pIC50 7.0 (IC50 1.1x10-7 M) [53]
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 87
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. [87]
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 [47,51,121].
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 128
pKi 5.4 (Ki 3.73x10-6 M) [128]
Description: Receptor binding in a radioligand displacement assay using [3H]DAMGO as tracer.
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 [11,85]
  • Jun N-terminal kinase (JNK) expression and activity [41,69,139]
  • Signal transducer and activator of transcription 3 (STAT3) [172]
  • Focal adhesion kinase [93]
  • Nuclear Ca2+/calmodulin translocation [161]
  • Phosphatidylinositol-3 kinase expression and activity [69,114].
References:  6,16,19-20,33,44,61,74,105,107,113,117,124-125,130,155,170
Secondary Transduction Mechanisms
Transducer Effector/Response
Gq/G11 family Phospholipase C stimulation
Comments:  G16 couples to the μ opioid receptor and activates PLC.
References:  62,83
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:  143
Pregnant uterus.
Species:  Mouse
Technique:  in situ hybridisation.
References:  177
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:  73
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:  146
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:  39,84
CNS: caudate putamen.
Species:  Rat
Technique:  immunocytochemistry.
References:  70,162
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:  110
CNS: Purkinje cells and granular and molecular layers of the fetal, neonatal and adult cerebellum.
Species:  Rat
Technique:  Immunohistochemistry.
References:  111
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:  97
CNS: nucleus accumbens (plasma membranes: extrasynaptic neuronal > glial)
Species:  Rat
Technique:  immunocytochemistry.
References:  144
CNS: nucleus accumbens (plasma membranes of GABAergic neurons).
Species:  Rat
Technique:  immunocytochemistry.
References:  145
CNS: locus coeruleus (noradrenergic perikarya and dendrites).
Species:  Rat
Technique:  immunocytochemistry.
References:  156-157
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:  137
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:  96
CNS: thalamus, striatum, hypothalamus and pons-medulla > hippocampus and midbrain > cerebral cortex and cerebellum.
Species:  Rat
Technique:  Northern blotting.
References:  104
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:  95
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:  68,122
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:  147
CNS: olfactory bulb.
Species:  Rat
Technique:  immunocytochemistry.
References:  136
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 [94].
Expression Datasets

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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]

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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:  171
Measurement of [35S]GTPγS binding.
Species:  Rat
Tissue:  Brain slices.
Response measured:  [35S]GTPγS binding.
References:  138
Physiological Functions
Constriction of the pupil.
Species:  Human
Tissue:  Pupil.
References:  112
DAMGO increases the conductance of an inwardly rectifying potassium conductance and hyperpolarises locus coeruleus neurons.
Species:  Rat
Tissue:  Brain.
References:  117