P2X receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on P2X Receptors [6,29]) have a trimeric topology [25,28,42] with two putative TM domains, gating primarily Na⁺, K⁺ and Ca²⁺, exceptionally Cl^-. The Nomenclature Subcommittee has recommended that for P2X receptors, structural criteria should be the initial criteria for nomenclature where possible. X-ray crystallography indicates that functional P2X receptors are trimeric and three agonist molecules are required to bind to a single receptor in order to activate it [16-17,28,35]. Native receptors may occur as either homotrimers (e.g. P2X1 in smooth muscle) or heterotrimers (e.g. P2X2:P2X3 in the nodose ganglion [55], P2X1:P2X5 in mouse cortical astrocytes [33], and P2X2:P2X5 in mouse dorsal root ganglion, spinal cord and mid pons [7,47]. P2X2, P2X4 and P2X7 receptors have been shown to form functional homopolymers which, in turn, activate pores permeable to low molecular weight solutes [51]. The hemi-channel pannexin-1 has been implicated in the pore formation induced by P2X7 [46], but not P2X2 [5], receptor activation.

A317491 and RO3 also block the P2X2:P2X3 heteromultimer [14,24]. NF449, A317491 and RO3 are more than 10-fold selective for P2X1 and P2X3 receptors, respectively.

Agonists listed show selectivity within recombinant P2X receptors of ca. one order of magnitude. A804598, A839977, A740003 and A438079 are at least 10-fold selective for P2X7 receptors and show similar affinity across human and rodent receptors [9,11,18].

Several P2X receptors (particularly P2X1 and P2X3) may be inhibited by desensitisation using stable agonists (e.g. αΒ-meATP); suramin and PPADS are non-selective antagonists at rat and human P2X1-3,5 and hP2X4, but not rP2X4,6,7 [4], and can also inhibit ATPase activity [8]. Ip₅I is inactive at rP2X2, an antagonist at rP2X3 (pIC₅₀ 5.6) and enhances agonist responses at rP2X4 [31]. Antagonist potency of NF023 at recombinant P2X2, P2X3 and P2X5 is two orders of magnitude lower than that at P2X1 receptors [49]. The P2X7 receptor may be inhibited in a non-competitive manner by the protein kinase inhibitors KN62 and chelerythrine [48], while the p38 MAP kinase inhibitor GTPγS and the cyclic imide AZ11645373 show a species-dependent non-competitive action [10,39-40,50]. The pH-sensitive dye used in culture media, phenol red, is also reported to inhibit P2X1 and P2X3 containing channels [32]. Some recombinant P2X receptors expressed to high density bind [³⁵S]ATPγS and [³H]αβ-meATP, although the latter can also bind to 5′-nucleotidase [37]. [³H]A317491 and [³H]A804598 have been used as high affinity antagonist radioligands for P2X3 (and P2X2/3) and P2X7 receptors, respectively [11]. Several high affinity radioligands for the P2X7 receptor have been recently synthesized [12,34,52]. AF-219 has shown clinical efficacy in reducing refractory chronic cough [1].

* Key recommended reading is highlighted with an asterisk

Browne LE, Cao L, Broomhead HE, Bragg L, Wilkinson WJ, North RA. (2011) P2X receptor channels show threefold symmetry in ionic charge selectivity and unitary conductance. Nat. Neurosci., 14 (1): 17-8. [PMID:21170052]

Browne LE, Jiang LH, North RA. (2010) New structure enlivens interest in P2X receptors. Trends Pharmacol. Sci., 31 (5): 229-37. [PMID:20227116]

Burnstock G. (2008) Purinergic signalling and disorders of the central nervous system. Nat Rev Drug Discov, 7 (7): 575-90. [PMID:18591979]

Collingridge GL, Olsen RW, Peters J, Spedding M. (2009) A nomenclature for ligand-gated ion channels. Neuropharmacology, 56 (1): 2-5. [PMID:18655795]

* Di Virgilio F, Dal Ben D, Sarti AC, Giuliani AL, Falzoni S. (2017) The P2X7 Receptor in Infection and Inflammation. Immunity, 47 (1): 15-31. [PMID:28723547]

* Di Virgilio F, Schmalzing G, Markwardt F. (2018) The Elusive P2X7 Macropore. Trends Cell Biol., 28 (5): 392-404. [PMID:29439897]

Donnelly-Roberts D, McGaraughty S, Shieh CC, Honore P, Jarvis MF. (2008) Painful purinergic receptors. J. Pharmacol. Exp. Ther., 324 (2): 409-15. [PMID:18042830]

Evans RJ. (2010) Structural interpretation of P2X receptor mutagenesis studies on drug action. Br. J. Pharmacol., 161 (5): 961-71. [PMID:20977449]

Guile SD, Alcaraz L, Birkinshaw TN, Bowers KC, Ebden MR, Furber M, Stocks MJ. (2009) Antagonists of the P2X(7) receptor. From lead identification to drug development. J. Med. Chem., 52 (10): 3123-41. [PMID:19191585]

* Habermacher C, Dunning K, Chataigneau T, Grutter T. (2016) Molecular structure and function of P2X receptors. Neuropharmacology, 104: 18-30. [PMID:26231831]

Hu H, Hoylaerts MF. (2010) The P2X1 ion channel in platelet function. Platelets, 21 (3): 153-66. [PMID:20201633]

* Jacobson KA, Müller CE. (2016) Medicinal chemistry of adenosine, P2Y and P2X receptors. Neuropharmacology, 104: 31-49. [PMID:26686393]

Jarvis MF. (2010) The neural-glial purinergic receptor ensemble in chronic pain states. Trends Neurosci., 33 (1): 48-57. [PMID:19914722]

Jarvis MF, Khakh BS. (2009) ATP-gated P2X cation-channels. Neuropharmacology, 56 (1): 208-15. [PMID:18657557]

Kaczmarek-Hájek K, Lörinczi E, Hausmann R, Nicke A. (2012) Molecular and functional properties of P2X receptors--recent progress and persisting challenges. Purinergic Signal., 8 (3): 375-417. [PMID:22547202]

* Khakh BS, Burnstock G, Kennedy C, King BF, North RA, Séguéla P, Voigt M, Humphrey PP. (2001) International union of pharmacology. XXIV. Current status of the nomenclature and properties of P2X receptors and their subunits. Pharmacol. Rev., 53 (1): 107-18. [PMID:11171941]

Khakh BS, North RA. (2012) Neuromodulation by extracellular ATP and P2X receptors in the CNS. Neuron, 76 (1): 51-69. [PMID:23040806]

* North RA. (2016) P2X receptors. Philos. Trans. R. Soc. Lond., B, Biol. Sci., 371 (1700). [PMID:27377721]

North RA, Jarvis MF. (2013) P2X receptors as drug targets. Mol. Pharmacol., 83 (4): 759-69. [PMID:23253448]

Pankratov Y, Lalo U, Krishtal OA, Verkhratsky A. (2009) P2X receptors and synaptic plasticity. Neuroscience, 158 (1): 137-48. [PMID:18495357]

Skaper SD, Debetto P, Giusti P. (2010) The P2X7 purinergic receptor: from physiology to neurological disorders. FASEB J., 24 (2): 337-45. [PMID:19812374]

* Stokes L, Layhadi JA, Bibic L, Dhuna K, Fountain SJ. (2017) P2X4 Receptor Function in the Nervous System and Current Breakthroughs in Pharmacology. Front Pharmacol, 8: 291. [PMID:28588493]

Surprenant A, North RA. (2009) Signaling at purinergic P2X receptors. Annu. Rev. Physiol., 71: 333-59. [PMID:18851707]

Young MT. (2010) P2X receptors: dawn of the post-structure era. Trends Biochem. Sci., 35 (2): 83-90. [PMID:19836961]

Zemková H, Balík A, Jindrichová M, Vávra V. (2008) Molecular structure of purinergic P2X receptors and their expression in the hypothalamus and pituitary. Physiol Res, 57 Suppl 3: S23-38. [PMID:18481917]

1. Abdulqawi R, Dockry R, Holt K, Layton G, McCarthy BG, Ford AP, Smith JA. (2015) P2X3 receptor antagonist (AF-219) in refractory chronic cough: a randomised, double-blind, placebo-controlled phase 2 study. Lancet, 385 (9974): 1198-205. [PMID:25467586]

2. AstraZeneca. AZ11657312. Accessed on 12/09/2014. Modified on 12/09/2014. astrazeneca.com, http://openinnovation.astrazeneca.com/what-we-offer/compound/az11657312/

3. Bhattacharya A, Wang Q, Ao H, Shoblock JR, Lord B, Aluisio L, Fraser I, Nepomuceno D, Neff RA, Welty N et al.. (2013) Pharmacological characterization of a novel centrally permeable P2X7 receptor antagonist: JNJ-47965567. Br. J. Pharmacol., 170 (3): 624-40. [PMID:23889535]

4. Buell G, Lewis C, Collo G, North RA, Surprenant A. (1996) An antagonist-insensitive P2X receptor expressed in epithelia and brain. EMBO J., 15 (1): 55-62. [PMID:8598206]

5. Chaumont S, Khakh BS. (2008) Patch-clamp coordinated spectroscopy shows P2X2 receptor permeability dynamics require cytosolic domain rearrangements but not Panx-1 channels. Proc. Natl. Acad. Sci. U.S.A., 105 (33): 12063-8. [PMID:18689682]

6. Collingridge GL, Olsen RW, Peters J, Spedding M. (2009) A nomenclature for ligand-gated ion channels. Neuropharmacology, 56 (1): 2-5. [PMID:18655795]

7. Compan V, Ulmann L, Stelmashenko O, Chemin J, Chaumont S, Rassendren F. (2012) P2X2 and P2X5 subunits define a new heteromeric receptor with P2X7-like properties. J. Neurosci., 32 (12): 4284-96. [PMID:22442090]

8. Crack BE, Beukers MW, McKechnie KC, Ijzerman AP, Leff P. (1994) Pharmacological analysis of ecto-ATPase inhibition: evidence for combined enzyme inhibition and receptor antagonism in P2X-purinoceptor ligands. Br. J. Pharmacol., 113 (4): 1432-8. [PMID:7889301]

9. Donnelly-Roberts DL, Jarvis MF. (2007) Discovery of P2X7 receptor-selective antagonists offers new insights into P2X7 receptor function and indicates a role in chronic pain states. Br. J. Pharmacol., 151 (5): 571-9. [PMID:17471177]

10. Donnelly-Roberts DL, Namovic MT, Faltynek CR, Jarvis MF. (2004) Mitogen-activated protein kinase and caspase signaling pathways are required for P2X7 receptor (P2X7R)-induced pore formation in human THP-1 cells. J. Pharmacol. Exp. Ther., 308 (3): 1053-61. [PMID:14634045]

11. Donnelly-Roberts DL, Namovic MT, Han P, Jarvis MF. (2009) Mammalian P2X7 receptor pharmacology: comparison of recombinant mouse, rat and human P2X7 receptors. Br. J. Pharmacol., 157 (7): 1203-14. [PMID:19558545]

12. Fantoni ER, Dal Ben D, Falzoni S, Di Virgilio F, Lovestone S, Gee A. (2017) Design, synthesis and evaluation in an LPS rodent model of neuroinflammation of a novel ¹⁸F-labelled PET tracer targeting P2X7. EJNMMI Res, 7 (1): 31. [PMID:28374288]

13. Ferrari D, Pizzirani C, Adinolfi E, Forchap S, Sitta B, Turchet L, Falzoni S, Minelli M, Baricordi R, Di Virgilio F. (2004) The antibiotic polymyxin B modulates P2X7 receptor function. J. Immunol., 173 (7): 4652-60. [PMID:15383600]

14. Ford AP, Gever JR, Nunn PA, Zhong Y, Cefalu JS, Dillon MP, Cockayne DA. (2006) Purinoceptors as therapeutic targets for lower urinary tract dysfunction. Br. J. Pharmacol., 147 Suppl 2: S132-43. [PMID:16465177]

15. Gargett CE, Wiley JS. (1997) The isoquinoline derivative KN-62 a potent antagonist of the P2Z-receptor of human lymphocytes. Br. J. Pharmacol., 120 (8): 1483-90. [PMID:9113369]

16. Gonzales EB, Kawate T, Gouaux E. (2009) Pore architecture and ion sites in acid-sensing ion channels and P2X receptors. Nature, 460 (7255): 599-604. [PMID:19641589]

17. Hattori M, Gouaux E. (2012) Molecular mechanism of ATP binding and ion channel activation in P2X receptors. Nature, 485 (7397): 207-12. [PMID:22535247]

18. Honore P, Donnelly-Roberts D, Namovic M, Zhong C, Wade C, Chandran P, Zhu C, Carroll W, Perez-Medrano A, Iwakura Y et al.. (2009) The antihyperalgesic activity of a selective P2X7 receptor antagonist, A-839977, is lost in IL-1alphabeta knockout mice. Behav. Brain Res., 204 (1): 77-81. [PMID:19464323]

19. Honore P, Donnelly-Roberts D, Namovic MT, Hsieh G, Zhu CZ, Mikusa JP, Hernandez G, Zhong C, Gauvin DM, Chandran P et al.. (2006) A-740003 [N-(1-{[(cyanoimino)(5-quinolinylamino) methyl]amino}-2,2-dimethylpropyl)-2-(3,4-dimethoxyphenyl)acetamide], a novel and selective P2X7 receptor antagonist, dose-dependently reduces neuropathic pain in the rat. J. Pharmacol. Exp. Ther., 319 (3): 1376-85. [PMID:16982702]

20. Jacobson KA, Ivanov AA, de Castro S, Harden TK, Ko H. (2009) Development of selective agonists and antagonists of P2Y receptors. Purinergic Signal., 5 (1): 75-89. [PMID:18600475]

21. Jacobson KA, Jarvis MF, Williams M. (2002) Purine and pyrimidine (P2) receptors as drug targets. J. Med. Chem., 45 (19): 4057-93. [PMID:12213051]

22. Jacobson KA, Kim YC, Wildman SS, Mohanram A, Harden TK, Boyer JL, King BF, Burnstock G. (1998) A pyridoxine cyclic phosphate and its 6-azoaryl derivative selectively potentiate and antagonize activation of P2X1 receptors. J. Med. Chem., 41 (13): 2201-6. [PMID:9632352]

23. Jacobson KA, Müller CE. (2016) Medicinal chemistry of adenosine, P2Y and P2X receptors. Neuropharmacology, 104: 31-49. [PMID:26686393]

24. Jarvis MF, Burgard EC, McGaraughty S, Honore P, Lynch K, Brennan TJ, Subieta A, Van Biesen T, Cartmell J, Bianchi B et al.. (2002) A-317491, a novel potent and selective non-nucleotide antagonist of P2X3 and P2X2/3 receptors, reduces chronic inflammatory and neuropathic pain in the rat. Proc. Natl. Acad. Sci. U.S.A., 99 (26): 17179-84. [PMID:12482951]

25. Jiang LH, Kim M, Spelta V, Bo X, Surprenant A, North RA. (2003) Subunit arrangement in P2X receptors. J. Neurosci., 23 (26): 8903-10. [PMID:14523092]

26. Jiang LH, Mackenzie AB, North RA, Surprenant A. (2000) Brilliant blue G selectively blocks ATP-gated rat P2X(7) receptors. Mol. Pharmacol., 58 (1): 82-8. [PMID:10860929]

27. Kassack MU, Braun K, Ganso M, Ullmann H, Nickel P, Böing B, Müller G, Lambrecht G. (2004) Structure-activity relationships of analogues of NF449 confirm NF449 as the most potent and selective known P2X1 receptor antagonist. Eur J Med Chem, 39 (4): 345-57. [PMID:15072843]

28. Kawate T, Michel JC, Birdsong WT, Gouaux E. (2009) Crystal structure of the ATP-gated P2X(4) ion channel in the closed state. Nature, 460 (7255): 592-8. [PMID:19641588]

29. Khakh BS, Burnstock G, Kennedy C, King BF, North RA, Séguéla P, Voigt M, Humphrey PP. (2001) International union of pharmacology. XXIV. Current status of the nomenclature and properties of P2X receptors and their subunits. Pharmacol. Rev., 53 (1): 107-18. [PMID:11171941]

30. Khakh BS, Proctor WR, Dunwiddie TV, Labarca C, Lester HA. (1999) Allosteric control of gating and kinetics at P2X(4) receptor channels. J. Neurosci., 19 (17): 7289-99. [PMID:10460235]

31. King BF, Liu M, Pintor J, Gualix J, Miras-Portugal MT, Burnstock G. (1999) Diinosine pentaphosphate (IP5I) is a potent antagonist at recombinant rat P2X1 receptors. Br. J. Pharmacol., 128 (5): 981-8. [PMID:10556935]

32. King BF, Liu M, Townsend-Nicholson A, Pfister J, Padilla F, Ford AP, Gever JR, Oglesby IB, Schorge S, Burnstock G. (2005) Antagonism of ATP responses at P2X receptor subtypes by the pH indicator dye, Phenol red. Br. J. Pharmacol., 145 (3): 313-22. [PMID:15778739]

33. Lalo U, Pankratov Y, Wichert SP, Rossner MJ, North RA, Kirchhoff F, Verkhratsky A. (2008) P2X1 and P2X5 subunits form the functional P2X receptor in mouse cortical astrocytes. J. Neurosci., 28 (21): 5473-80. [PMID:18495881]

34. Lord B, Ameriks MK, Wang Q, Fourgeaud L, Vliegen M, Verluyten W, Haspeslagh P, Carruthers NI, Lovenberg TW, Bonaventure P et al.. (2015) A novel radioligand for the ATP-gated ion channel P2X7: [3H] JNJ-54232334. Eur. J. Pharmacol., 765: 551-9. [PMID:26386289]

35. Mansoor SE, Lü W, Oosterheert W, Shekhar M, Tajkhorshid E, Gouaux E. (2016) X-ray structures define human P2X(3) receptor gating cycle and antagonist action. Nature, 538 (7623): 66-71. [PMID:27626375]

36. Michel AD, Chambers LJ, Walter DS. (2008) Negative and positive allosteric modulators of the P2X(7) receptor. Br. J. Pharmacol., 153 (4): 737-50. [PMID:18071294]

37. Michel AD, Chau NM, Fan TP, Frost EE, Humphrey PP. (1995) Evidence that [3H]-alpha,beta-methylene ATP may label an endothelial-derived cell line 5'-nucleotidase with high affinity. Br. J. Pharmacol., 115 (5): 767-74. [PMID:8548175]

38. Michel AD, Clay WC, Ng SW, Roman S, Thompson K, Condreay JP, Hall M, Holbrook J, Livermore D, Senger S. (2008) Identification of regions of the P2X(7) receptor that contribute to human and rat species differences in antagonist effects. Br. J. Pharmacol., 155 (5): 738-51. [PMID:18660826]

39. Michel AD, Ng SW, Roman S, Clay WC, Dean DK, Walter DS. (2009) Mechanism of action of species-selective P2X(7) receptor antagonists. Br. J. Pharmacol., 156 (8): 1312-25. [PMID:19309360]

40. Michel AD, Thompson KM, Simon J, Boyfield I, Fonfria E, Humphrey PP. (2006) Species and response dependent differences in the effects of MAPK inhibitors on P2X(7) receptor function. Br. J. Pharmacol., 149 (7): 948-57. [PMID:17031382]

41. Michel AD, Xing M, Thompson KM, Jones CA, Humphrey PP. (2006) Decavanadate, a P2X receptor antagonist, and its use to study ligand interactions with P2X7 receptors. Eur. J. Pharmacol., 534 (1-3): 19-29. [PMID:16487507]

42. Nicke A, Bäumert HG, Rettinger J, Eichele A, Lambrecht G, Mutschler E, Schmalzing G. (1998) P2X1 and P2X3 receptors form stable trimers: a novel structural motif of ligand-gated ion channels. EMBO J., 17 (11): 3016-28. [PMID:9606184]

43. North RA, Jarvis MF. (2013) P2X receptors as drug targets. Mol. Pharmacol., 83 (4): 759-69. [PMID:23253448]

44. Nörenberg W, Hempel C, Urban N, Sobottka H, Illes P, Schaefer M. (2011) Clemastine potentiates the human P2X7 receptor by sensitizing it to lower ATP concentrations. J. Biol. Chem., 286 (13): 11067-81. [PMID:21262970]

45. Nörenberg W, Sobottka H, Hempel C, Plötz T, Fischer W, Schmalzing G, Schaefer M. (2012) Positive allosteric modulation by ivermectin of human but not murine P2X7 receptors. Br. J. Pharmacol., 167 (1): 48-66. [PMID:22506590]

46. Pelegrin P, Surprenant A. (2009) The P2X(7) receptor-pannexin connection to dye uptake and IL-1beta release. Purinergic Signal., 5 (2): 129-37. [PMID:19212823]

47. Saul A, Hausmann R, Kless A, Nicke A. (2013) Heteromeric assembly of P2X subunits. Front Cell Neurosci, 7: 250. [PMID:24391538]

48. Shemon AN, Sluyter R, Conigrave AD, Wiley JS. (2004) Chelerythrine and other benzophenanthridine alkaloids block the human P2X7 receptor. Br. J. Pharmacol., 142 (6): 1015-9. [PMID:15210579]

49. Soto F, Lambrecht G, Nickel P, Stühmer W, Busch AE. (1999) Antagonistic properties of the suramin analogue NF023 at heterologously expressed P2X receptors. Neuropharmacology, 38 (1): 141-9. [PMID:10193905]

50. Stokes L, Jiang LH, Alcaraz L, Bent J, Bowers K, Fagura M, Furber M, Mortimore M, Lawson M, Theaker J et al.. (2006) Characterization of a selective and potent antagonist of human P2X(7) receptors, AZ11645373. Br. J. Pharmacol., 149 (7): 880-7. [PMID:17031385]

51. Surprenant A, North RA. (2009) Signaling at purinergic P2X receptors. Annu. Rev. Physiol., 71: 333-59. [PMID:18851707]

52. Territo PR, Meyer JA, Peters JS, Riley AA, McCarthy BP, Gao M, Wang M, Green MA, Zheng QH, Hutchins GD. (2017) Characterization of ¹¹C-GSK1482160 for Targeting the P2X7 Receptor as a Biomarker for Neuroinflammation. J. Nucl. Med., 58 (3): 458-465. [PMID:27765863]

53. Tomasinsig L, Pizzirani C, Skerlavaj B, Pellegatti P, Gulinelli S, Tossi A, Di Virgilio F, Zanetti M. (2008) The human cathelicidin LL-37 modulates the activities of the P2X7 receptor in a structure-dependent manner. J. Biol. Chem., 283 (45): 30471-81. [PMID:18765670]

54. Virginio C, Robertson G, Surprenant A, North RA. (1998) Trinitrophenyl-substituted nucleotides are potent antagonists selective for P2X1, P2X3, and heteromeric P2X2/3 receptors. Mol. Pharmacol., 53 (6): 969-73. [PMID:9614197]

55. Vulchanova L, Riedl MS, Shuster SJ, Buell G, Surprenant A, North RA, Elde R. (1997) Immunohistochemical study of the P2X2 and P2X3 receptor subunits in rat and monkey sensory neurons and their central terminals. Neuropharmacology, 36 (9): 1229-42. [PMID:9364478]

Subcommittee members: Charles Kennedy (Chairperson) Strathclyde Institute of Pharmacy and Biomedical Sciences University of Strathclyde 161 Cathedral Street Glasgow, G4 0RE UK Francesco Di Virgilio Department of Morphology, Surgery and Experimental Medicine University of Ferrara Italy Simonetta Falzoni Department of Morphology, Surgery and Experimental Medicine University of Ferrara Italy Michael F. Jarvis AbbVie 1 North Waukegan Road North Chicago, IL 60064 USA Brian King Faculty of Life Sciences University College London London UK John A. Peters Room CB 08 Carnelley Building School of Life Sciences The University of Dundee Dundee DD1 5EH Scotland UK

Other contributors: Richard J. Evans Cell Physiology & Pharmacology University of Leicester UK Baljit S. Khakh Department of Physiology David Geffen School of Medicine University of California Los Angeles Los Angeles CA 90095-1751 USA Patrizia Pellegatti Department of Experimental and Diagnostic Medicine University of Ferrara