1521-0081/72/1/50–79$35.00 https://doi.org/10.1124/pr.118.016311 PHARMACOLOGICAL REVIEWS Pharmacol Rev 72:50–79, January 2020 Copyright © 2019 by The American Society for Pharmacology and Experimental Therapeutics ASSOCIATE EDITOR: CHARLES P. FRANCE Imidazoline Receptor System: The Past, the Present, and the Future Pascal Bousquet, Alan Hudson, Jesús A. García-Sevilla, and Jun-Xu Li Faculty of Medicine, University of Strasbourg, Strasbourg, France (P.B.); Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada (A.H.); Laboratory of Neuropharmacology, University Research Institute on Health Sciences, University of the Balearic Islands, Palma de Malllorca, Spain (J.A.G.-S.); and Department of Pharmacology and Toxicology, University at Buffalo, Buffalo, New York (J.-X.L.) Abstract .................................................................................... 51 I. Introduction . ...............................................................................51 II. History of Imidazoline Receptors .............................................................52 III. Endogenous Imidazoline Receptor Ligands . .................................................54 A. Clonidine-Displacing Substance . .......................................................54 B. Harmane . ...............................................................................55 C. Imidazole-4-Acetic Acid-Ribotide . .......................................................56 Downloaded from D. Agmatine . ...............................................................................57 IV. Imidazoline Subtype 1 Receptors.............................................................57 A. Definition. ...............................................................................57 B. Specific Binding Properties, Selective Ligands, and Tissue and Subcellular Localization..............................................................................58 by guest on September 23, 2021 1. Selective Ligands . ...................................................................58 2. Tissue Distribution ...................................................................58 3. Subcellular Distribution . .............................................................59 4. Second-Generation Imidazoline Subtype 1 Receptor Ligands . ..........................59 5. Selective Imidazoline Subtype 1 Receptor Ligands with High Affinity as Pharmacological Probes. .............................................................60 6. Receptor–Receptor Interactions .......................................................60 7. Transduction Mechanisms ............................................................61 8. Attempts to Clone Imidazoline Subtype 1 Receptors . ................................62 C. Cellular Effects . .........................................................................62 1. Imidazoline Subtype 1 Receptors and Apoptosis, Cell Viability, Growth, and Proliferation . .........................................................................62 2. Imidazoline Subtype 1 Receptors and Insulin and Adiponectin .........................62 3. Imidazoline Subtype 1 Receptors and Neurons . ......................................62 D. Imidazoline Subtype 1 Receptors and In Vivo Effects .....................................63 1. Effects on Blood Pressure and Heart Rate . ...........................................63 2. Antiarrhythmic Effects. .............................................................63 3. Effects in Congestive Heart Failure . .................................................63 4. Effects on Glucose and Lipid Metabolism and Interest in Metabolic Syndrome . ........63 V. Imidazoline Subtype 2 Receptors.............................................................64 A. Definition, Distribution, and Composition.................................................64 B. Synthetic Imidazoline Subtype 2 Receptor Ligands . ......................................65 C. Neuropharmacology of Imidazoline Subtype 2 Receptor Ligands. ..........................66 1. Pain. ...............................................................................66 Address correspondence to: Jun-Xu Li, Department of Pharmacology and Toxicology, Jacobs School of Medicine and Biomedical Sciences, University at Buffalo, 955 Main St., Buffalo, NY 14203. E-mail: [email protected] P.B. and A.H. contributed equally to this work. J.A.G.-S. and J.-X.L. contributed equally to this work. https://doi.org/10.1124/pr.118.016311. 50 A Review of Imidazoline Receptors 51 2. Discriminative Stimulus Effects.......................................................68 3. Neuroprotection . ...................................................................69 4. Body Temperature. ...................................................................70 D. Pharmacological and Neurobiological Mechanisms . ......................................70 VI. Imidazoline Subtype 3 Receptors.............................................................72 VII. Summary and Conclusions ...................................................................73 References...................................................................................73 Abstract——Imidazoline receptors historically re- receptors associate with several distinct proteins, but the ferred to a family of nonadrenergic binding sites that identities of these proteins remain elusive. I2 receptor recognize compounds with an imidazoline moiety, agonists have demonstrated various centrally mediated although this has proven to be an oversimplification. effects including antinociception and neuroprotection. A For example, none of the proposed endogenous new I2 receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol- ligands for imidazoline receptors contain an imida- 1yl) quinazoline], demonstrated clear analgesic activity zoline moiety but they are diverse in their chemical in a recently completed phase II clinical trial and structure. Three receptor subtypes (I1,I2,andI3) holds great promise as a novel I2 receptor–based first- have been proposed and the understanding of each in-class nonopioid analgesic. The understanding of has seen differing progress over the decades. I1 I3 receptors is relatively limited. Existing data receptors partially mediate the central hypoten- suggest that I3 receptors may represent a binding sive effects of clonidine-like drugs. Moxonidine and site at the Kir6.2-subtype ATP-sensitive potassium rilmenidine have better therapeutic profiles (fewer side channels in pancreatic b-cells and may be involved effects) than clonidine as antihypertensive drugs, thought in insulin secretion. Despite the elusive nature of to be due to their higher I1/a2-adrenoceptor selectivity. their molecular identities, recent progress on drug Newer I1 receptor agonists such as LNP599 [3-chloro-2- discovery targeting imidazoline receptors (I1 and methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)- I2) demonstrates the exciting potential of these amine hydrochloride] have little to no activity on compounds to elicit neuroprotection and to treat a2-adrenoceptors and demonstrate promising therapeutic various disorders such as hypertension, metabolic potential for hypertension and metabolic syndrome. I2 syndrome, and chronic pain. I. Introduction the fifth edition of the Guide to Receptors and Channels Although the imidazoline receptor concept was pro- (Alexander et al., 2011), imidazoline receptors are only posed decades ago and the field has been consistently briefly described under the entry on a2-adrenoceptors. evolving, many pharmacologists have not yet heard However, there has recently been renewed interest in about this receptor. This is not surprising because, to increasing understanding and pharmacological study of some extent, this concept has not received unanimous this system due to exciting new developments regarding acceptance in the biomedical community. For example, the therapeutic potential of imidazoline receptor ligands. in the 12th edition of Goodman & Gilman’s The Pharma- This review provides a comprehensive update on the cological Basis of Therapeutics (Brunton et al., 2011), there historical and current status of imidazoline receptor is only one sentence mentioning imidazoline receptors. In research and offers our perspective on future directions. ABBREVIATIONS: 2-BFI, 2-(2-benzofuranyl)-2-imidazoline; AMPA, a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; AMPK, 59 adenosine monophosphate-activated protein kinase; BU216, 3-[4,5-dihydroimidaz-2-yl]-quinoline hydrochloride; BU224, 2-[4,5-dihy- droimidaz-2-yl]-quinoline hydrochloride; BU226, 2-[4,5-dihydroimidaz-2-yl]-isoquinoline hydrochloride; CCI, chronic constriction injury; CDS, clonidine-displacing substance; CFA, complete Freund’s adjuvant; CNS, central nervous system; CR4056, 2-phenyl-6-(1H-imidazol-1yl) qui- nazoline; DSP-4, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine; ERK, extracellular signal-regulated kinase; HPLC, high-performance liquid chromatography; IAA-RP, imidazole-4-acetic acid-ribotide; IRAS, imidazoline receptor antisera-selected; KATP, ATP-sensitive potassium; KU14-R, 2-(2-ethyl-2,3-dihydro-2-benzofuranyl)-1H-imidazole; LNP509, cis-/trans-dicyclopropylmethyl-(4,5-dimethyl-4,5-dihydro-3H-pyrrol- 2-yl)-amine; LNP599, 3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride; LNP630, (2,6-dichloro-phenyl)- (4-methyl-4,5-dihydro-1H-imidazol-2-yl)-amine; LNP906, 2-(5-azido-2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline; LNP911, (2-(2-chloro-4-