Pre-synaptic control by histamine H3 receptors of neurotransmitter release

Biomedical Aspects of Histamine: Current Perspectives

Volumn , Issue , 2011, Pages 339-368

  Osorio Espinoza, Angélica ^a   Ramos Jiménez, Judith ^b   Arias Montaño, José Antonio ^a  

^a DEPARTAMENTO DE FÍSICA   (Mexico)

^b Universidad de la Ciudad de México ^*  (Mexico)

Author keywords

brain; GABA; glutamate; H3 receptors; Histamine; neurotransmitter release

Indexed keywords

EID: 84900040025     PISSN: None     EISSN: None     Source Type: Book    
DOI: 10.1007/978-90-481-9349-3_15     Document Type: Chapter

References (116)

1. Alves-Rodrigues A, Timmerman H, Willems E, et al (1998) Pharmacological characterization of the histamine H3 receptor in the rat hippocampus. Brain Res 788:179-186
2. Arias-Montaño J-A, Florán B, Florán L, et al (2007) Dopamine D1 receptor facilitation of depolarisation-induced release of γ-Aminobutyric acid (GABA) in rat striatum is mediated predominantly via P/Q-Type calcium channels. Synapse 61:310-319
3. Arias-Montaño J-A, Florán B, García M, et al (2001) Histamine H3 receptor-mediated inhibition of depolarisation-induced, dopamine D1 receptor-Dependent release of [3H]-γ-Aminobutyric acid in rat striatum. Br J Pharmacol 133:165-171
4. Arrang J-M, Drutel G, Schwartz J-C (1995) Characterization of histamine H3 receptors regulating acetylcholine release in rat entorhinal cortex. Br J Pharmacol 114:1518-1522
5. Arrang J-M, Garbarg M, Schwartz J-C (1983) Auto-inhibition of brain histamine release mediated by a novel class (H3) of histamine receptor. Nature 302:832-837
6. Arrang JM, Garbarg M, Schwartz J-C (1985) Autoregulation of histamine release in brain by presynaptic H3-receptors. Neuroscience 15:553-562
7. Arrang J-M, Garbarg M, Schwartz J-C (1987) Autoinhibition of histamine synthesis mediated by presynaptic H3-receptors. Neuroscience 23:149-157
8. Arrang J-M, Morisset S, Gbahou S (2007) Constitutive activity of the histamine H3 receptor. Trends Pharmacol Sci 28:350-357
9. Bacciottini L, Passani MB, Gio Vannelli L, et al (2002) Endogenous histamine in the medial septumdiagonal band complex increases the release of acetylcholine from the hippocampus: A dualprobe microdialysis study in the freely moving rat. Eur J Neurosci 15:1669-1680
10. Bakker RA (2004) Histamine H3-receptor isoforms. Inflamm Res 53:509-516
11. Bakker RA, Flores-Lozada A, Van Marle A, et al (2006) Discovery of naturally occurring splice variants of the rat histamine H3 receptor that act as dominant-negative isoforms. Mol Pharmacol 69:1194-1206
12. Banks ML, Manvich DF, Bauzo RM, et al (2009) Effects of histamine H3 receptor activation on the behavioral-stimulant effects of methamphetamine and cocaine in mice and squirrel monkeys. Pharmacology 83:164-169
13. Barbier AJ, Aluisio L, Lord B, et al (2007) Pharmacological characterization of JNJ-28583867, a histamine H3 receptor antagonist and serotonin reuptake inhibitor. Eur J Pharmacol 576: 43-54
14. Bergquist F, Ruthven A, Ludwig M, et al (2006) Histaminergic and glycinergic modulation of GABA release in the vestibular nuclei of normal and labyrinthectomised rats. J Physiol 577:857-868
15. Bertrand PP, Bertrand RL (2010) Serotonin release and uptake in the gastrointestinal tract. Auton Neurosci 153:47-57
16. Blandina P, Giorgetti M, Bartolini L, et al (1996) Inhibition of cortical acetylcholine release and cognitive performance by histamine H3 receptor activation in rats. Br J Pharmacol 8:1656-1664
17. Bongers G, Bakker RA, Leurs R (2007a) Molecular aspects of the histamine H3 receptor. Biochem Pharmacol 73:1195-1204
18. Bongers G, Krueger KM, Miller TR, et al (2007b) A 80 amino acid deletion in the third intracellular loop of a naturally occurring human histamine H3 isoform confers pharmacological differences and constitutive activity. J Pharmacol Exp Ther 323:888-898
19. Bongers G, Sallmen T, Passani MB, et al (2007c) The Akt/GSK-3β axis as a new signaling pathway of the histamine H3 receptor. J Neurochem 103:248-258
20. Brown RE, Haas HL (1999) On the mechanism of histaminergic inhibition of glutamate release in the rat dentate gyrus. J Physiol 515:777-786
21. Brown RE, Reymann KG (1996) Histamine H3 receptor-mediated depression of synaptic transmission in the dentate gyrus of the rat in vitro. J Physiol 496:175-184
22. Celuch SM (1995) Possible participation of histamine H3 receptors in the modulation of noradrenaline release from rat spinal cord slices. Eur J Pharmacol 287:127-133
23. Clapham J, Kilpatrick GJ (1992) Histamine H3 receptors modulate the release of [3H]-Acetylcholine from slices of rat entorhinal cortex: Evidence for the possible existence of H3 receptor subtypes. Br J Pharmacol 107:919-923
24. Coge F, Guenin SP, Audinot V, et al (2001) Genomic organization and characterization of splice variants of the human histamine H3 receptor. Biochem J 355:279-288
25. Dai H, Fu Q, Shen Y, et al (2007) The histamine H3 receptor antagonist clobenpropit enhances GABA release to protect against NMDA-induced excitotoxicity through the cAMP/protein kinase A pathway in cultured cortical neurons. Eur J Pharmacol 563:117-123
26. Di Carlo G, Ghi P, OrsettiM(2000) Effect of R-(-)-α-methylhistamine and thioperamiDe on in vivo release of norepinephrine in the rat hippocampus. Prog Neuropsychopharmacol Biol Psychiatry 24:275-284
27. Doreulee N, Yanovsky Y, Flagmeyer I, et al (2001) Histamine H3 receptors depress synaptic transmission in the corticostriatal pathway. Neuropharmacology 40:106-113
28. Drutel G, Peitsaro N, Karlstedt K, et al (2001) Identification of rat H3 receptor isoforms with different brain expression and signaling properties. Mol Pharmacol 59:1-8
29. Du XJ, Dart AM (1993) Mechanisms of noradrenaline release in the anoxic heart of the rat. Cardiovasc Res 27:2011-2015
30. Endou M, Poli E, Levi R (1994) Histamine H3-receptor signaling in the heart: Possible involvement of Gi/Go proteins and N-Type Ca++ channels. J Pharmacol Exp Ther 269:221-229
31. EsbenshaDe TA, Browman KE, Bitner RS, et al (2008) The histamine H3 receptor: An attractive target for the treatment of cognitive disorders. Br J Pharmacol 154: 1166-1181
32. Fink K, Schlicker E, Neise A, et al (1990) Involvement of presynaptic H3 receptors in the inhibitory effect of histamine on serotonin release in the rat brain cortex. Naunyn Schmiedeberg's Arch Pharmacol 342:513-519
33. Fox GB, EsbenshaDe TA, Pan JB, et al (2005) Pharmacological properties of ABT-239 [4-(2-{2-[(2R)-2-Methylpyrrolidinyl]ethyl}-benzofuran-5-yl) benzonitrile]:II. Neurophysiological characterization and broad preclinical efficacy in cognition and schizophrenia of a potent and selective histamine H3 receptor antagonist. J Pharmacol Exp Ther 313: 176-190
34. Frieling T, Cooke HJ, Wood JD (1993) Histamine receptors on submucous neurons in guinea pig colon. Am J Physiol 264:G74-80
35. García M, Florán B, Arias-Montaño J-A, et al (1997) Histamine H3 receptor activation selectively inhibits dopamine D1 receptor-Dependent [3H]GABA release from depolarisation-stimulated slices of rat substantia nigra pars reticulata. Neuroscience 80:241-249
36. Garduño-Torres B, Treviño M, Gutiérrez R, et al (2007) Presynaptic histamine H3 receptors regulate glutamate, but not GABA release in rat thalamus. Neuropharmacology 52: 527-535
37. Giannoni P, Passani MB, Nosi D, et al (2009) Heterogeneity of histaminergic neurons in the tuberomammillary nucleus of the rat. Eur J Neurosci 29:2363-2374
38. Gillard M, Van Der Perren C, Moguilevsky N, et al (2002) Binding characteristics of cetirizine and levocetirizine to human H1 histamine receptors: Contribution of Lys191 and Thr194. Mol Pharmacol 61:391-399
39. GiorgettiM, Bacciottini L, Bianchi L, et al (1997) GABAergic mechanism in histamine H3 receptor inhibition of K+-evoked release of acetylcholine from rat cortex in vivo. Inflamm Res 46(Suppl 1):S33-S34
40. Haas H, Panula P (2003) The role of histamine and the tuberomamillary nucleus in the nervous system. Nature Rev Neurosci 4:121-130
41. Hatta E, Yasuda K, Levi R (1997) Activation of histamine H3 receptors inhibits carrier-mediated norepinephrine release in a human model of protracted myocardial ischemia. J Pharmacol Exp Ther 283:494-500
42. Hew RWS, Hodgkinson CR, Hill SJ (1990) Characterization of histamine H3-receptors in guineapig ileum with H3-selective ligands. Br J Pharmacol 101:621-624
43. Ichinose M, Barnes PJ (1989) Histamine H3-receptors modulate nonadrenergic noncholinergic neural bronchoconstriction in guinea-pig in vivo. Eur J Pharmacol 174: 49-55
44. Imamura M, Lander HM, Levi R (1996) Activation of histamine H3-receptors inhibits carriermediated norepinephrine release during protracted myocardial ischemia. Comparison with adenosine A1-receptors and α2-Adrenoceptors. Circ Res 78:475-481
45. Imamura M, Poli E, Omoniyi AT, et al (1994) Unmasking of activated histamine H3-receptors in myocardial ischemia: Their role as regulators of exocytotic norepinephrine release. J Pharmacol Exp Ther 271:1259-1266
46. ImamuraM, Seyedi N, Lander HMet al (1995) Functional identification of histamine H3-receptors in the human heart. Circ Res 77:206-210
47. Ishikawa S, Sperelakis N (1987) A novel class (H3) of histamine receptors on perivascular nerve terminals. Nature 327:158-160
48. Jang I, Rhee J-S, Watanabe T, et al (2001) Histaminergic modulation of GABAergic transmission in rat ventromedial hypothalamic neurons. J Physiol 534:791-803
49. Jansen FP, Mochizuki T, Yamamoto Y, et al (1998) In vivo modulation of rat hypothalamic histamine release by the histamine H3 receptor ligands, immepip and clobenpropit. Effects of intrahypothalamic and peripheral application. Eur J Pharmacol 362:149-155
50. Jiang X, Chen A, Li H (2005) Histaminergic modulation of excitatory synaptic transmission in the rat basolateral amygdale. Neuroscience 131:691-703
51. Lamberty Y, Margineanu DG, Dassesse D, et al (2003) H3 agonist immepip markedly reduces cortical histamine release, but only weakly promotes sleep in the rat. Pharmacol Res 48: 193-198
52. Leineweber K, Heusch G, Schulz R (2007) Regulation and role of the presynaptic and myocardial Na+/H+ exchanger NHE1: Effects on the sympathetic nervous system in heart failure. Cardiovasc Drug Rev 25:123-131
53. Leurs R, Bakker RA, Timmeman H, et al (2005) The histamine H3 receptor: From gene cloning to H3 receptor drugs. Nature Rev Drug Discov 4:107-121
54. Leurs R, Brozius MM, Smit MJ, et al (1991) Effects of histamine H1-, H2-And H3-receptor selective drugs on the mechanical activity of guinea-pig small and large intestine. Br J Pharmacol 102:179-185
55. Leurs R, Smit MJ, Menge WM, et al (1994) Pharmacological characterization of the human histamine H2 receptor stably expressed in Chinese hamster ovary cells. Br J Pharmacol 112:847-854
56. Levi R, Seyedi N, Schaefer U, et al (2007) Histamine H3-receptor signaling in cardiac sympathetic nerves: Identification of a novel MAPK-PLA2-COX-PGE2-EP3R pathway. Biochem Pharmacol 73:1146-1156
57. Levi R, Smith NC (2000) Histamine H3-receptors: A new frontier in myocardial ischemia. J Pharmacol Exp Ther 292:825-830
58. Ligneau X, Perrin D, Landais L, et al (2007) BF2.649 [1-{3-[3-(4- Chlorophenyl) propoxy]propyl}piperidine, hydrochloride], a nonimidazole inverse agonist/antagonist at the human histamine H3 receptor: Preclinical pharmacology. J Pharmacol Exp Ther 320:365-375
59. Lovenberg TW, Roland BL, Wilson SJ, et al (1999) Cloning and functional expression of the human histamine H3 receptor. Mol Pharmacol 55:1101-1107
60. Lovenberg TW, Pyati J, Chang H, Wilson SJ, Erlander MG (2000) Cloning of rat histamine H3 receptor reveals distinct species pharmacological profiles. J Pharmacol Exp Ther 293:771-778
61. Marinissen MJ, Gutkind JS (2001) G-protein-coupled receptors and signaling networks: Emerging paradigms. Trends Pharmacol Sci 22:368-376
62. Mariottini C, Scartabelli T, Bongers G, et al (2009) Activation of the histaminergic H3 receptor induces phosphorylation of the Akt/GSK-3β pathway in cultured cortical neurons and protects against neurotoxic insults. J Neurochem 110:1469-1478
63. Mazenot C, Ribuot C, Durand A, et al (1999) In vivo demonstration of H3-histaminergic inhibition of cardiac sympathetic stimulation by R-α-methyl-histamine and its prodrug BP 2.94 in the dog. Br J Pharmacol 126:264-268
64. Medhurst AD, Atkins AR, Beresford IJ, et al (2007) GSK189254, a novel H3 receptor antagonist that binds to histamine H3 receptors in Alzheimer's disease brain and improves cognitive performance in preclinical models. J Pharmacol Exp Ther 321:1032-1045
65. Mochizuki T, Okakura-Mochizuki K, Horii A, et al (1994) Histaminergic modulation of hippocampal acetylcholine release in vivo. J Neurochem 62:2275-2282
66. Molina-Hernández A, Nuñez A, Arias-Montaño J-A (2000) Histamine H3-receptor activation inhibits dopamine synthesis in rat striatum. Neuroreport 17163-17166
67. Molina-Hernández A, Nuñez A, Sierra JJ, et al (2001) Histamine H3 receptor activation inhibits glutamate release from rat striatal synaptosomes. Neuropharmacology 41: 928-934
68. Moreno-Delgado D, Gómez-Ramírez J, Torrent-Moreno A, et al (2009) Different role of cAMP dependent protein kinase and CaMKII in H3 receptor regulation of histamine synthesis and release. Neuroscience 164:1244-1251
69. Morisset S, Rouleau A, Ligneau X, et al (2000) High constitutive activity of native H3 receptors regulates histamine neurons in brain. Nature 408: 860-864
70. Morisset S, Sasse A, Gbahou F, et al (2001) The rat H3 receptor: Gene organization and multiple isoforms. Biochem Biophys Res Commun 280:75-80
71. Morrey C, Estephan R, Abbott GW, et al (2008) Cardioprotective effect of histamine H3-receptor activation: Pivotal role of Gβγ-Dependent inhibition of voltage-operated Ca2+ channels. Pharmacol Exp Ther 326:871-878
72. Munzar P, Tanda G, Justinova Z, et al (2004) Histamine H3 receptor antagonists potentiate methamphetamine self-Administration and methamphetamine-induced accumbal dopamine release. Neuropsychopharmacology 29:705-717
73. Nemmara A, Delaunois A, Beckers JF, et al (1999) Modulatory effect of imetit, a histamine H3 receptor agonist, on C-fibers, cholinergic fibers and mast cells in rabbit lungs in vitro. Eur J Pharmacol 371: 23-30
74. Nowak P, Bortel A, Dabrowska J, et al (2008) Histamine H3 receptor ligands modulate L-Dopa-evoked behavioral responses and L-Dopa derived extracellular dopamine in dopaminedenervated rat striatum. Neurotox Res 13:231-240
75. Ohkubo T, Shibata M (1995) ATP-sensitive K+ channels mediate regulation of substance P release via the prejunctional histamine H3 receptor. Eur J Pharmacol 277:45-49
76. Ohkubo T, Shibata M, Inoue M, et al (1995) Regulation of substance P release mediated via prejunctional histamine H3 receptors. Eur J Pharmacol 273:83-88
77. Osorio-Espinoza A, Garcia-Ramirez M, Ramos-Jimenez J, et al (2009) Modulation by presynaptic histamine H3 receptors of glutamatergic transmission in rat globus pallidus. In: Abstracts of the British pharmacological society summer meeting 2009, Edinburgh, UK. http://www.pa2online.org/abstracts/ Vol7Issue2abst004P.pdf. Retrieved November 18 2009
78. Passani MB, Cangioli I, Baldi E, et al (2001) Histamine H3 receptor-mediated impairment of contextual fear conditioning and in-vivo inhibition of cholinergic transmission in the rat basolateral amygdala. Eur J Neurosci 14:1522-1532
79. Pillot C, Héron A, Cochois V, et al (2002) A detailed mapping of the histamine H3 receptor and its gene transcripts in rat brain. Neuroscience 114:173-193
80. Poli E, Coruzzi G, Bertaccini G (1991) Histamine H3 receptors regulate acetylcholine release from the guinea pig ileum myenteric plexus. Life Sci 48:PL63-PL68
81. Prast H, Tran MH, Fischer H, et al (1999) Histaminergic neurons modulate acetylcholine release in the ventral striatum: Role of H3 histamine receptors. Naunyn Schmiedeberg's Arch Pharmacol 360:558-564
82. Racke K, Reimann A, Schworer H, et al (1996) Regulation of 5-HT release from enterochromaffin cells. Behav Brain Res 73:83-87
83. Rouleau A, Heron A, Cochois V, et al (2004) Cloning and expression of the mouse histamine H3 receptor: Evidence for multiple isoforms. J Neurochem 90:1331-1338
84. Ryu JH, Yanai K, Iwata R, et al (1994) Heterogenous distribution of histamine H3, dopamine D1 and D2 receptors in rat brain. Neuroreport 5:621-624
85. Sahlholm K, Nilsson J, Marcellino D, et al (2007) The human histamine H3 receptor couples to GIRK channels in Xenopus oocytes. Eur J Pharmacol 567:206-210
86. Sander K, Kottke T, Stark H (2008) Histamine H3 receptor antagonists go to clinics. Biol Pharm Bull 31:2163-2181
87. Savitz J, Lucki I, Drevets WC (2009) 5-HT1A receptor function in major depressive disorders. Prog Neurobiol 88:17-31
88. Schlicker E, Behling A, Lümmen G, et al (1992) Histamine H3A receptor-mediated inhibition of noradrenaline release in the mouse brain cortex. Naunyn-Schmiedeberg's Arch Pharmacol 345:489-493
89. Schlicker E, Betz R, Göthert M (1988) Histamine H3-receptor-mediated inhibition of serotonin release in the rat brain cortex. Naunyn-Schmiedeberg's Arch Pharmacol 337:588-590
90. Schlicker E, Fink K, Detzner M, et al (1993) Histamine inhibits dopamine release in the mouse striatum via presynaptic H3 receptors. J Neural Transm Gen Sect 93:1-10
91. Schlicker E, Fink K, Hinterhaner M, et al (1989) Inhibition of noradrenaline release in the rat brain cortex via presynaptic H3 receptors. Naunyn-Schmiedeberg's Arch Pharmacol 340:633-638
92. Schlicker E, Gfthert M (1998) Interactions between the presynaptic α2-Autoreceptor and presynaptic inhibitory heteroreceptors on noradrenergic neurons. Brain Res Bull 47:129-132
93. Schlicker E, Kathmann M, Detzner M, et al (1994) H3 receptor-mediated inhibition of noradrenaline release: An investigation into the involvement of Ca2+ and K+ ions, G protein and adenylate cyclase. Naunyn Schmiedebergs Arch Pharmacol 350:34-41
94. Schlicker E, Schunack W, Göthert M (1990) Histamine H3 receptor-mediated inhibition of noradrenaline release in pig retina discs. Naunyn-Schmiedeberg's Arch Pharmacol 342:497-501
95. Schlicker E, Werthwein S, Zentner J (1999) Histamine H3 receptor-mediated inhibition of noradrenaline release in the human brain. Fundam Clin Pharmacol 13:120-122
96. Schömig A (1990) Catecholamines in myocardial ischemia. Systemic and cardiac release. Circulation 82(Suppl II):13-22
97. Seyedi N, Mackins CJ, Machida T, et al (2005) Histamine H3-receptor-induced attenuation of norepinephrine exocytosis: A decreased protein kinase A activity mediates a reduction in intracellular calcium. J Pharmacol Exp Ther 312:272-280
98. Silver RB, Mackins CJ, Smith NC, et al (2001) Coupling of histamine H3 receptors to neuronal Na+/H+ exchange: A novel protective mechanism in myocardial ischemia. Proc Natl Acad Sci USA 98:2855-2859
99. Silver RB, Poonwasi KS, Seyedi N, et al (2002) Decreased intracellular calcium mediates the histamine H3-receptor-induced attenuation of norepinephrine exocytosis from cardiac sympathetic nerve endings. Proc Natl Acad Sci USA 99:501-506
100. Smits RPJM, Mulder AH (1991) Inhibitory effects of histamine on the release of serotonin and noradrenaline from rat brain slices. Neurochem Int 18:215-220
101. Strakhova MI, Fox GB, Carr TL, et al (2008) Cloning and characterization of the monkey histamine H3 receptor isoforms. Eur J Pharmacol 601:8-15
102. Takeshita Y, Watanabe T, Sakata T, et al (1998) Histamine modulates high-voltage-Activated calcium channels in neurons dissociated from the rat tuberomammillary nucleus. Neuroscience 87:797-805
103. Tanaka H, Kannari K, Maeda T, et al (1999) Role of serotonergic neurons in L-DOPA-Derived extracellular dopamine in the striatum of 6-OHDA-lesioned rats. Neuroreport 10:631-634
104. Tardivel-Lacombe J, Morisset S, Gbahou F, et al (2001) Chromosomal mapping and organization of the human histamine H3 receptor gene. Neuroreport 12:321-324
105. Tardivel-Lacombe J, Rouleau A, Heron A, et al (2000) Cloning and cerebral expression of the guinea pig histamine H3 receptor: Evidence for two isoforms. Neuroreport 11:755-759
106. Taylor SJ, Kilpatrick GJ (1992) Characterization of histamine-H3 receptors controlling nonadrenergic non-cholinergic contractions of the guinea-pig isolated ileum. Br J Pharmacol 105:667-674
107. Tedford HW, Zamponi GW (2006) Direct G protein modulation of Cav2 calcium channels. Pharmacol Rev 58:837-862
108. Threlfell S, Cragg SJ, Kallo I, et al (2004) Histamine H3 receptors inhibit serotonin release in substantia nigra pars reticulata. J Neurosci 24:8704-871
109. Tibbs GR, Barrie AP, Van Mieghem FJ, et al (1989) Repetitive action potentials in isolated nerve terminals in the presence of 4-Aminopyridine: Effects on cytosolic free Ca2+ and glutamate release. J Neurochem 53:1693-1699
110. Timm J, Marr I, Werthwein S, et al (1998) H2 receptor-mediated facilitation and H3 receptormediated inhibition of noradrenaline release in the guinea-pig brain. Naunyn-Schmiedeberg's Arch Pharmacol 357:232-239
111. Trzeciakowski JP (1987) Inhibition of guinea pig ileum contractions mediated by a class of histamine receptor resembling the H3 subtype. J Pharmacol Exp Ther 243:874-880
112. Van Der Werf JF, Bast A, Bijloo GJ, et al (1987) HA autoreceptor assay with superfused slices of rat brain cortex and electrical stimulation. Eur J Pharmacol 138:199-206
113. Wellendorph P, Goodman MW, Burstein ES, et al (2002) Molecular cloning and pharmacology of functionally distinct isoforms of the human histamine H3 receptor. Neuropharmacology 42:929-940
114. Welty N, Shoblock J (2009) The effects of thioperamiDe on extracellular levels of glutamate and GABA in the rat prefrontal cortex. Psychopharmacology 208:265-277
115. Westerink BH, Cremers TI, De Vries JB, et al (2002) Evidence for activation of histamine H3 autoreceptors during handling stress in the prefrontal cortex of the rat. Synapse 43:238-243
116. Wise RA (2004) Dopamine, learning and motivation. Nature Rev Neurosci 5:483-494