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65. Vidal, B.; Fieux, S.; Colom, M.; Billard, T.; Bouillot, C.; Barret, O.; Constantinescu, C.; Tamagnan, G.; Newman-Tancredi, A.; Zimmer, L. 18F-F13640 preclinical evaluation in rodent, cat and primate as a 5-HT1A receptor agonist for PET neuroimaging. *Brain Struct. Funct.* **2018**, *223*, 2973–2988. [CrossRef] [PubMed]

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68. Colom, M.; Vidal, B.; Fieux, S.; Redoute, J.; Costes, N.; Lavenne, F.; Mérida, I.; Irace, Z.; Iecker, T.; Bouillot, C.; et al. [18F]F13640, a 5-HT1A Receptor Radiopharmaceutical Sensitive to Brain Serotonin Fluctuations. *Front. Neurosci.* **2021**, *15*, 622423. [CrossRef]

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70. Rutz, S.; Riegert, C.; Rothmaier, A.K.; Buhot, M.C.; Cassel, J.C.; Jackisch, R. Presynaptic serotonergic modulation of 5-HT and acetylcholine release in the hippocampus and the cortex of 5-HT1B-receptor knockout mice. *Brain Res. Bull.* **2006**, *70*, 81–93. [CrossRef]

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82. Murrough, J.W.; Henry, S.; Hu, J.; Gallezot, J.-D.; Planeta-Wilson, B.; Neumaier, J.F.; Neumeister, A. Reduced ventral striatal/ventral pallidal serotonin1B receptor binding potential in major depressive disorder. *Psychopharmacology* **2011**, *213*, 547–553. [CrossRef] [PubMed]

83. Matuskey, D.; Bhagwagar, Z.; Planeta, B.; Pittman, B.; Gallezot, J.-D.; Chen, J.; Wanyiri, J.; Najafzadeh, S.; Ropchan, J.; Geha, P.; et al. Reductions in Brain 5-HT1B Receptor Availability in Primarily Cocaine-Dependent Humans. *Biol. Psychiatry* **2014**, *76*, 816–822. [CrossRef] [PubMed]

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85. Leysen, J.E. 5-HT2 receptors. *Curr. Drug Targets CNS Neurol. Disord.* **2004**, *3*, 11–26. [CrossRef]

86. Conn, P.J.; Sanders-Bush, E. Selective 5ht-2 antagonists inhibit serotonin stimulated phosphatidylinositol metabolism in cerebral cortex. *Neuropharmacology* **1984**, *23*, 993–996. [CrossRef]
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93. Gijsman, H.J.; Verkes, R.J.; Schouten-Verhagen, J.C.M.; Schoemaker, R.C.; Van Gerven, J.M.A.; De Rijk, R.H.; Van Kempen, G.M.J. A review of the role of serotonin receptors in psychiatric disorders. *Hum. Psychopharmacol.* **2000**, *15*, 397–415. [CrossRef]

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193. Patel, S.; Roberts, J.; Moorman, J.; Reavill, C. Localization of serotonin-4 receptors in the striatonigral pathway in rat brain. *Neuroscience* **1995**, *69*, 1159-1167. [CrossRef]

194. Pike, V.W.; Halldin, C.; Nobuhara, K.; Hiltunen, J.; Mulligan, R.S.; Swahn, C.G.; Karlsson, P.; Olsson, H.; Hume, S.P.; Hirani, E.; et al. Radioiodinated SB 207710 as a radioligand in vivo: Imaging of brain 5-HT4 receptors with SPET. *Eur. J. Nucl. Med. Mol. Imaging* **2003**, *30*, 1520-1528. [CrossRef] [PubMed]

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196. Kornum, B.R.; Lind, N.M.; Gillings, N.; Marner, L.; Andersen, F.; Knudsen, G.M. Evaluation of the novel 5-HT4 receptor PET ligand [11C]SB207145 in the Göttingen minipig. *J. Cereb. Blood Flow Metab.* **2009**, *29*, 186-196. [CrossRef] [PubMed]

197. Marner, L.; Gillings, N.; Comley, R.A.; Baaré, W.F.C.; Rabiner, E.A.; Wilson, A.A.; Houle, S.; Hasselbalch, S.G.; Svarer, C.; Gunn, R.N.; et al. Kinetic modeling of 11C-SB207145 binding to 5-HT4 receptors in the human brain in vivo. *J. Nucl. Med.* **2009**, *50*, 900-908. [CrossRef] [PubMed]

198. Marner, L.; Gillings, N.; Madsen, K.; Erritzoe, D.; Baaré, W.F.C.; Svarer, C.; Hasselbalch, S.G.; Knudsen, G.M. Brain imaging of serotonin 4 receptors in humans with [11C]SB207145-PET. *Neuroimage* **2010**, *50*, 855-861. [CrossRef] [PubMed]

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200. Haahr, M.E.; Fisher, P.M.; Jensen, C.G.; Frokjaer, V.G.; McMahon, B.; Madsen, K.; Baaré, W.F.C.; Lehel, S.; Norremolle, A.; Rabiner, E.A.; et al. Central 5-HT4 receptor binding as biomarker of serotonergic tonus in humans: A [11C]SB207145 PET study. *Mol. Psychiatry* **2014**, *19*, 427-432. [CrossRef]

201. Perfalk, E.; da Cunha-Bang, S.; Holst, K.K.; Keller, S.; Svarer, C.; Knudsen, G.M.; Frokjaer, V.G. Testosterone levels in healthy men correlate negatively with serotonin 4 receptor binding. *Psychoneuroendocrinology* **2017**, *81*, 22-28. [CrossRef]

202. Jakobsen, G.R.; Fisher, P.M.; Dyssegaard, A.; McMahon, B.; Holst, K.K.; Lehel, S.; Svarer, C.; Jensen, P.S.; Knudsen, G.M.; Frokjaer, V.G. Brain serotonin 4 receptor binding is associated with the cortisol awakening response. *Psychoneuroendocrinology* **2016**, *67*, 124-132. [CrossRef] [PubMed]

203. Deen, M.; Hansen, H.D.; Hougaard, A.; Nørgaard, M.; Eiberg, H.; Lehel, S.; Ashina, M.; Knudsen, G.M. High brain serotonin levels in migraine between attacks: A 5-HT4 receptor binding PET study. *NeuroImage Clin.* **2018**, *18*, 97-102. [CrossRef] [PubMed]

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205. Terry, G.E.; Hirvonen, J.; Liow, J.S.; Zoghbi, S.S.; Gladding, R.; Tauscher, J.T.; Schaus, J.M.; Phebus, L.; Felder, C.C.; Morse, C.L.; et al. Imaging and quantitation of cannabinoid CB1 receptors in human and monkey brains using 18F-labeled inverse agonist radioligands. *J. Nucl. Med.* **2010**, *51*, 112-120. [CrossRef]
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184. Hegde, S.S.; Eglen, R.M. Peripheral 5-HT 4 receptors. *FASEB J.* **1996**, *10*, 1398-1407. [CrossRef] [PubMed]

185. Bockaert, J.; Claeysen, S.; Compan, V.; Dumuis, A. 5-HT4 receptors, a place in the sun: Act two. *Curr. Opin. Pharmacol.* **2011**, *11*, 87-93. [CrossRef] [PubMed]

186. Hagena, H.; Manahan-Vaughan, D. The serotonergic 5-HT4 receptor: A unique modulator of hippocampal synaptic information processing and cognition. *Neurobiol. Learn. Mem.* **2017**, *138*, 145-153. [CrossRef]

187. Claeysen, S.; Bockaert, J.; Giannoni, P. Serotonin: A New Hope in Alzheimer's Disease? *ACS Chem. Neurosci.* **2015**, *6*, 940-943. [CrossRef]

188. Lezoualc'h, F. 5-HT4 receptor and Alzheimer's disease: The amyloid connection. *Exp. Neurol.* **2007**, *205*, 325-329. [CrossRef]

189. Jean, A.; Conductier, G.; Manrique, C.; Bouras, C.; Berta, P.; Hen, R.; Charnay, Y.; Bockaert, J.; Compan, V. Anorexia induced by activation of serotonin 5-HT4 receptors is mediated by increases in CART in the nucleus accumbens. *Proc. Natl. Acad. Sci. USA* **2007**, *104*, 16335-16340. [CrossRef]

190. Lucas, G.; Rymar, V.V.; Du, J.; Mnie-Filali, O.; Bisgaard, C.; Manta, S.; Lambas-Senas, L.; Wiborg, O.; Haddjeri, N.; Piñeyro, G.; et al. Serotonin4 (5-HT4) Receptor Agonists Are Putative Antidepressants with a Rapid Onset of Action. *Neuron* **2007**, *55*, 712-725. [CrossRef]

191. Kaumann, A.J.; Gaster, L.M.; King, F.D.; Brown, A.M. Blockade of human atrial 5-HT4 receptors by SB 207710, a selective and high affinity 5-HT4 receptor antagonist. *Naunyn Schmiedebergs Arch. Pharmacol.* **1994**, *349*, 546-548. [CrossRef]

192. Varnäs, K.; Halldin, C.; Pike, V.W.; Hall, H. Distribution of 5-HT4 receptors in the postmortem human brain—An autoradiographic study using [125I]SB 207710. *Eur. Neuropsychopharmacol.* **2003**, *13*, 228-234. [CrossRef]

193. Patel, S.; Roberts, J.; Moorman, J.; Reavill, C. Localization of serotonin-4 receptors in the striatonigral pathway in rat brain. *Neuroscience* **1995**, *69*, 1159-1167. [CrossRef]

194. Pike, V.W.; Halldin, C.; Nobuhara, K.; Hiltunen, J.; Mulligan, R.S.; Swahn, C.G.; Karlsson, P.; Olsson, H.; Hume, S.P.; Hirani, E.; et al. Radioiodinated SB 207710 as a radioligand in vivo: Imaging of brain 5-HT4 receptors with SPET. *Eur. J. Nucl. Med. Mol. Imaging* **2003**, *30*, 1520-1528. [CrossRef] [PubMed]

195. Gee, A.D.; Martarello, L.; Passchier, J.; Wishart, M.; Parker, C.; Matthews, J.; Comley, R.; Hopper, R.; Gunn, R. Synthesis and evaluation of [11C]SB207145 as the first in vivo serotonin 5-HT4 receptor radioligand for PET imaging in man. *Curr. Radiopharm.* **2008**, *1*, 110-114. [CrossRef]

196. Kornum, B.R.; Lind, N.M.; Gillings, N.; Marner, L.; Andersen, F.; Knudsen, G.M. Evaluation of the novel 5-HT4 receptor PET ligand [11C]SB207145 in the Göttingen minipig. *J. Cereb. Blood Flow Metab.* **2009**, *29*, 186-196. [CrossRef] [PubMed]

197. Marner, L.; Gillings, N.; Comley, R.A.; Baaré, W.F.C.; Rabiner, E.A.; Wilson, A.A.; Houle, S.; Hasselbalch, S.G.; Svarer, C.; Gunn, R.N.; et al. Kinetic modeling of 11C-SB207145 binding to 5-HT4 receptors in the human brain in vivo. *J. Nucl. Med.* **2009**, *50*, 900-908. [CrossRef] [PubMed]

198. Marner, L.; Gillings, N.; Madsen, K.; Erritzoe, D.; Baaré, W.F.C.; Svarer, C.; Hasselbalch, S.G.; Knudsen, G.M. Brain imaging of serotonin 4 receptors in humans with [11C]SB207145-PET. *Neuroimage* **2010**, *50*, 855-861. [CrossRef] [PubMed]

199. Madsen, K.; Marner, L.; Haahr, M.; Gillings, N.; Knudsen, G.M. Mass dose effects and in vivo affinity in brain PET receptor studies—A study of cerebral 5-HT 4 receptor binding with [11C]SB207145. *Nucl. Med. Biol.* **2011**, *38*, 1085-1091. [CrossRef]

200. Haahr, M.E.; Fisher, P.M.; Jensen, C.G.; Frokjaer, V.G.; McMahon, B.; Madsen, K.; Baaré, W.F.C.; Lehel, S.; Norremolle, A.; Rabiner, E.A.; et al. Central 5-HT4 receptor binding as biomarker of serotonergic tonus in humans: A [11C]SB207145 PET study. *Mol. Psychiatry* **2014**, *19*, 427-432. [CrossRef]

201. Perfalk, E.; da Cunha-Bang, S.; Holst, K.K.; Keller, S.; Svarer, C.; Knudsen, G.M.; Frokjaer, V.G. Testosterone levels in healthy men correlate negatively with serotonin 4 receptor binding. *Psychoneuroendocrinology* **2017**, *81*, 22-28. [CrossRef]

202. Jakobsen, G.R.; Fisher, P.M.; Dysisgaard, A.; McMahon, B.; Holst, K.K.; Lehel, S.; Svarer, C.; Jensen, P.S.; Knudsen, G.M.; Frokjaer, V.G. Brain serotonin 4 receptor binding is associated with the cortisol awakening response. *Psychoneuroendocrinology* **2016**, *67*, 124-132. [CrossRef] [PubMed]

203. Deen, M.; Hansen, H.D.; Hougaard, A.; Nørgaard, M.; Eiberg, H.; Lehel, S.; Ashina, M.; Knudsen, G.M. High brain serotonin levels in migraine between attacks: A 5-HT4 receptor binding PET study. *NeuroImage Clin.* **2018**, *18*, 97-102. [CrossRef] [PubMed]

204. Xu, R.; Hong, J.; Morse, C.L.; Pike, V.W. Synthesis, structure-affinity relationships, and radiolabeling of selective high-affinity 5-HT4 receptor ligands as prospective imaging probes for positron emission tomography. *J. Med. Chem.* **2010**, *53*, 7035-7047. [CrossRef] [PubMed]

205. Terry, G.E.; Hirvonen, J.; Liow, J.S.; Zoghbi, S.S.; Gladding, R.; Tauscher, J.T.; Schaus, J.M.; Phebus, L.; Felder, C.C.; Morse, C.L.; et al. Imaging and quantitation of cannabinoid CB1 receptors in human and monkey brains using 18F-labeled inverse agonist radioligands. *J. Nucl. Med.* **2010**, *51*, 112-120. [CrossRef]
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209. Tavares, A.A.S.; Caillé, F.; Barret, O.; Papin, C.; Lee, H.; Morley, T.J.; Fowles, K.; Holden, D.; Seibyl, J.P.; Alagille, D.; et al. In vivo evaluation of 18F-MNI698: An 18F-labeled radiotracer for imaging of serotonin 4 receptors in brain. *J. Nucl. Med.* **2014**, *55*, 858–864. [CrossRef]

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207. Caillé, F.; Morley, T.J.; Tavares, A.A.S.; Papin, C.; Twardy, N.M.; Alagille, D.; Lee, H.S.; Baldwin, R.M.; Seibyl, J.P.; Barret, O.; et al. Synthesis and biological evaluation of positron emission tomography radiotracers targeting serotonin 4 receptors in brain: [18F]MNI-698 and [18F]MNI-699. *Bioorg. Med. Chem. Lett.* **2013**, *23*, 6243–6247. [CrossRef]

208. Tavares, A.A.S.; Caillé, F.; Barret, O.; Papin, C.; Lee, H.; Morley, T.J.; Fowles, K.; Holden, D.; Seibyl, J.P.; Alagille, D.; et al. Whole-body biodistribution and dosimetry estimates of a novel radiotracer for imaging of serotonin 4 receptors in brain: [18F]MNI-698. *Nucl. Med. Biol.* **2014**, *41*, 432–439. [CrossRef]

209. Tavares, A.A.S.; Caillé, F.; Barret, O.; Papin, C.; Lee, H.; Morley, T.J.; Fowles, K.; Holden, D.; Seibyl, J.P.; Alagille, D.; et al. In vivo evaluation of 18F-MNI698: An 18F-labeled radiotracer for imaging of serotonin 4 receptors in brain. *J. Nucl. Med.* **2014**, *55*, 858–864. [CrossRef]

210. Fresneau, N.; Dumas, N.; Tournier, B.B.; Fossey, C.; Ballandonne, C.; Lesnard, A.; Millet, P.; Charnay, Y.; Cailly, T.; Bouillon, J.P.; et al. Design of a serotonin 4 receptor radiotracer with decreased lipophilicity for single photon emission computed tomography. *Eur. J. Med. Chem.* **2015**, *94*, 386–396. [CrossRef]

211. Monsma, F.J.; Shen, Y.; Ward, R.P.; Hamblin, M.W.; Sibley, D.R. Cloning and expression of a novel serotonin receptor with high affinity for tricyclic psychotropic drugs. *Mol. Pharmacol.* **1993**, *43*, 320–327.

212. Ruat, M.; Traifort, E.; Leurs, R.; Tardivel-Lacombe, J.; Diaz, J.; Arrang, J.M.; Schwartz, J.C. Molecular cloning, characterization, and localization of a high-affinity serotonin receptor (5-HT7) activating cAMP formation. *Proc. Natl. Acad. Sci. USA* **1993**, *90*, 8547–8551. [CrossRef]

213. Kohen, R.; Metcalf, M.A.; Khan, N.; Druck, T.; Huebner, K.; Lachowicz, J.E.; Meltzer, H.Y.; Sibley, D.R.; Roth, B.L.; Hamblin, M.W. Cloning, characterization, and chromosomal localization of a human 5-HT6 serotonin receptor. *J. Neurochem.* **1996**, *66*, 47–56. [CrossRef] [PubMed]

214. Yoshioka, M.; Matsumoto, M.; Togashi, H.; Mori, K.; Saito, H. Central distribution and function of 5-HT6 receptor subtype in the rat brain. *Life Sci.* **1998**, *62*, 1473–1477. [CrossRef]

215. Ward, R.P.; Hamblin, M.W.; Lachowicz, J.E.; Hoffman, B.J.; Sibley, D.R.; Dorsa, D.M. Localization of serotonin subtype 6 receptor messenger RNA in the rat brain by in situ hybridization histochemistry. *Neuroscience* **1995**, *64*, 1105–1111. [CrossRef]

216. Gérard, C.; Martres, M.P.; Lefèvre, K.; Miquel, M.C.; Vergé, D.; Lanfumey, L.; Doucet, E.; Hamon, M.; El Mestikawy, S. Immun localization of serotonin 5-HT6 receptor-like material in the rat central nervous system. *Brain Res.* **1997**, *746*, 207–219. [CrossRef]

217. Roberts, J.C.; Reavill, C.; East, S.Z.; Harrison, P.J.; Patel, S.; Routledge, C.; Leslie, R.A. The distribution of 5-HT6 receptors in rat brain: An autoradiographic binding study using the radiolabelled 5-HT6 receptor antagonist [125I]SB-258585. *Brain Res.* **2002**, *934*, 49–57. [CrossRef]

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