1521-0081/65/4/1–61$25.00 http://dx.doi.org/10.1124/pr.112.007138 PHARMACOLOGICAL REVIEWS Pharmacol Rev 65:1–61, October 2013 Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics ASSOCIATE EDITOR: DAVID R. SIBLEY Mu Opioids and Their Receptors: Evolution of a Concept Gavril W. Pasternak and Ying-Xian Pan Molecular Pharmacology and Chemistry Program and Department of Neurology, Memorial Sloan-Kettering Cancer Center, New York, New York Abstract .................................................................................... 3 I. Historical Overview......................................................................... 3 II. Opioids ..................................................................................... 5 A. Alkaloids ............................................................................... 5 1. Morphine Analogs ................................................................... 5 2. Morphinans. ......................................................................... 6 3. Benzomorphans. ................................................................... 6 4. Oripavines. ......................................................................... 6 5. Other. ............................................................................... 7 B. Opioid Peptides ......................................................................... 8 1. Endogenous Opioids ................................................................. 8 2. Synthetic Mu Peptides . .............................................................11 C. Mu Antagonists.........................................................................11 D. Endogenous Mu Alkaloids . .............................................................12 III. Mu Opiate Pharmacology ...................................................................12 A. Historical Overview . ...................................................................12 B. Bioassays ...............................................................................13 C. Analgesia ...............................................................................14 1. Sites of Action .......................................................................14 2. Synergy .............................................................................15 D. Opioid/Sigma1 Interactions . .............................................................15 E. Other Actions . .........................................................................16 F. Tolerance/Incomplete Cross-Tolerance/Dependence/Withdrawal. ..........................17 1. Adenyl Cyclase . ...................................................................18 2. N-Methyl-D-Aspartate Antagonists and Nitric-Oxide Synthase Inhibitors . 18 3. Enkephalin Systems .................................................................18 4. P-Glycoprotein .......................................................................18 5. Trafficking. .........................................................................19 6. Other. ...............................................................................19 G. Incomplete Cross-Tolerance .............................................................19 IV. Opioid Binding Sites: Early Studies . .......................................................20 A. Identification of Opioid Binding Sites . .................................................20 B. Localization of Binding Sites ............................................................21 1. Cellular Localization of Opioid Receptors . ...........................................21 2. Regional Localization of Opioid Receptors . ...........................................21 C. Development. .........................................................................22 D. Discrimination of Agonist and Antagonist Binding . ......................................22 1. Sodium Effect........................................................................23 This work was supported in part by the National Institutes of Health National Institute on Drug Abuse [Grants DA00220, DA02615, DA06241, DA07242] (to G.W.P.) and [Grants DA013997 and DA029244] (to X.Y.P.). Address correspondence to: Dr. Gavril Pasternak, Memorial Sloan-Kettering Cancer Center, 1275 York Ave, New York, NY 10065. E-mail: [email protected] dx.doi.org/10.1124/pr.112.007138 1 2 Pasternak and Pan 2. Divalent Cations and GTP ...........................................................24 3. Protein Modifying Reagents, Enzymes, and Temperature . ..........................24 V. Pharmacological Evidence for Multiple Mu Receptor Subtypes ...............................24 A. Receptor Binding........................................................................25 B. Pharmacology . .........................................................................25 1. Analgesia. .........................................................................25 2. Other Opioid Actions . .............................................................26 VI. Molecular Biology of Mu Receptors . .......................................................26 A. MOR-1. ...............................................................................27 B. Phylogeny and Evolution . .............................................................28 C. Single Nucleotide Polymorphisms .......................................................30 D. Binding Studies .........................................................................31 E. Regional Expression of MOR-1 in the Central Nervous System . ..........................31 1. mRNA...............................................................................31 2. Immunohistochemistry. .............................................................34 3. Developmental Expression of MOR-1 .................................................36 VII. The OPRM1 Gene . .........................................................................36 A. Chromosomal Mapping of OPRM1 .......................................................36 B. Promoters ..............................................................................36 1. Exon 1 Promoter. ...................................................................36 2. Exon 11 Promoter ...................................................................37 VIII. Alternative Splicing of the OPRM1 Gene . .................................................38 A. Full-length Variants and 39 Splicing .....................................................38 1. Rodent ..............................................................................38 2. Human..............................................................................39 B. Truncated MOR-1 Variants. .............................................................40 1. Exon 11-Associated 6TM Variants. .................................................40 2. Single Transmembrane Domain Variants. ...........................................42 C. Characterization of MOR-1 Splice Variants . ...........................................42 1. Regional Distribution of MOR-1 Variants. ...........................................42 2. Biochemical Characterization of MOR-1 Splice Variants ..............................44 3. Role of Alternative Splicing of MOR-1 in Opioid Analgesia . ..........................45 a. Full-length variants . .............................................................46 i. Antisense mapping ............................................................46 ii. Knockout models . .............................................................46 b. Truncated variants: 6TM . .......................................................47 c. Truncated variants: 1TM . .......................................................49 IX. The Future . ...............................................................................49 Acknowledgments. .........................................................................50 References . ...............................................................................50 ABBREVIATIONS: b-FNA, b-funaltrexamine; 6TM, truncated six transmembranereceptor1;7TM,full-lengthtraditionalseven transmembrane receptor; bp, base pair; BAM-22, bovine adrenal medulla 22 protein (Tyr-Gly-Gly-Phe-Met-Arg-Arg-Val-Gly-Arg-Pro-Glu- Trp-Trp-Met-Asp-Tyr-Gln-Lys-Arg-Tyr-Gly); CHO, Chinese hamster ovary; CTAP, H-D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2;CTOP, 2 4 5 2 5 H-D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2;DAMGO,[D-Ala ,MePhe ,Gly(ol) ]enkephalin; DPDPE, [D-Pen ,D-Pen ]enkephalin; DOR- 1, delta-opioid receptor; E, embryonic day; E1, exon 1 associated; E11, exon 11 associated; GPCR, G protein-coupled receptor; HEK, human embryonic kidney; IBNtxA, 39-iodobenzoyl-6b-naltrexamide; IL, interleukin; kb, kilobase pair; KOR-1, kappa1-opioid receptor; M6G, morphine-6b-glucuronide; MK-801, (5S,10R)-(1)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate or dizocilpine; MOR-1, the cloned mu1 receptor; NF-kB, nuclear factor-kB; NMD, nonsense-mediated mRNA degradation; norBNI, nor-binaltorphimine; NOS, nitric-oxide synthase; NRSE, neurorestrictive suppressor element; Pgp, P-glycoprotein; TM, truncated single transmembrane receptor; NMDA, N-methyl-D-aspartate; NSAID, nonsteroidal anti-inflammatory drugs; RT-PCR, reverse-transcription polymerase chain reaction; STAT, signal transducers and activators of transcription type; TAPS, Tyr-D-Arg-Phe-Sar; TAT, Tat peptide derived from the transactivator of transcription of human immunodeficiency virus; TRV130, [(3-methoxythiophen-2-yl)methyl]({2-[(9R)-9-(pyridin-2-yl)-6-