title: ST91 [2-(2,6-diethylphenylamino)-2-imidazoline hydrochloride]-mediated spinal antinociception and synergy with opioids persists in the absence of functional alpha-2A- or alpha-2C-adrenergic receptors.
abstract: Agonists acting at alpha2-adrenergic receptors (alpha2ARs) produce antinociception and synergize with opioids. The alpha2ARs are divided into three subtypes, alpha(2A)AR, alpha(2B)AR, and alpha(2C)AR. Most alpha2AR agonists require alpha(2A)AR activation to produce antinociception and opioid synergy. The same subtype also mediates the side effect of sedation, which limits the clinical utility of these compounds. Identification of a non-alpha(2A)AR-mediated antinociceptive agent would enhance the therapeutic utility of alpha2AR agonists in pain management. Previous studies have suggested that the alpha2AR agonist ST91 [2-(2,6-diethylphenylamino)-2-imidazoline hydrochloride] has a nonsedating, non-alpha(2A)AR mechanism of action. We examined the pharmacology of spinal ST91 and its interaction with the delta-opioid agonist deltorphin II (Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide) in mice lacking either functional alpha(2A)ARs or alpha(2C)ARs. All drugs were administered by direct lumbar puncture, and drug interactions were evaluated using isobolographic analysis. In contrast to the majority of alpha2AR agonists, ST91 potency was only moderately reduced (3-fold) in the absence of the alpha(2A)AR. Studies with the alpha2AR subtype-preferring antagonists BRL-44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole maleate) and prazosin [[4-(4-amino-6,7-dimethoxy-quinazolin-2-yl) piperazin-1-yl]-(2-furyl)methanone] and the pan-alpha2AR antagonist SKF-86466 (6-chloro-2,3,4,5-tetrahydro-3-methyl-1-H-3-benzazepine) suggest a shift from alpha(2A)AR to the other alpha2AR subtype(s) in the absence of alpha(2A)AR. Antinociceptive synergy with deltorphin II was preserved in the absence of either alpha(2A)AR or alpha(2C)AR. In conclusion, ST91 activates both alpha(2A)AR and non-alpha(2A)AR subtypes to produce spinal antinociception and opioid synergy. This study confirms that the spinal pharmacology of ST91 differs from that of other alpha2AR agonists and extends those data to include spinal synergy with opioid agonists. The unique profile of ST91 may be advantageous in pain management.
authors.affiliation: Faculty of Dentistry, McGill Centre for Research on Pain, 3640 University Street, Montreal, Quebec H3A 2B2, Canada. laura.s.stone@mcgill.ca
authors.forename: Laura S
authors.initials: LS
authors.lastname: Stone
authors.forename: Kelley F
authors.initials: KF
authors.lastname: Kitto
authors.forename: James C
authors.initials: JC
authors.lastname: Eisenach
authors.forename: Carolyn A
authors.initials: CA
authors.lastname: Fairbanks
authors.forename: George L
authors.initials: GL
authors.lastname: Wilcox
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000494:pharmacology / Q000627:therapeutic use*; D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology / Q000627:therapeutic use*; D000818:Animals; D001416:Back Pain / Q000188:drug therapy*; D001522:Behavior, Animal / Q000187:drug effects; D003000:Clonidine / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology / Q000627:therapeutic use; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004359:Drug Therapy, Combination; D051379:Mice; D017354:Point Mutation; D018341:Receptors, Adrenergic, alpha-2 / Q000235:genetics; D013116:Spinal Cord / Q000187:drug effects*; D013373:Substance P / Q000494:pharmacology
chemical_list: C547769:Adra2a protein, mouse; C547775:Adra2c protein, mouse; D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D000701:Analgesics, Opioid; D018341:Receptors, Adrenergic, alpha-2; D013373:Substance P; C011266:ST 91; D003000:Clonidine