title: 5-hydroxytryptamine (HT)1A receptors and the tail-flick response. II. High efficacy 5-HT1A agonists attenuate morphine-induced antinociception in mice in a competitive-like manner.
abstract: This study examined the influence of s.c. administration of 5-hydroxytryptamine (HT)1A agonists upon the antinociceptive action of s.c. injected morphine in tail-flick tests to noxious heat and pressure. The selective 5-HT1A agonist, (+-)-8-hydroxy-diprolaminotetralin HBr (8-OH-DPAT), dose-dependently antagonized morphine-induced antinociception (MIA) without affecting the latency to respond when applied alone. In the presence of increasing doses of 8-OH-DPAT (0.16-0.63 mg/kg), the morphine dose-response curve was shifted progressively in parallel to the right and the maximal effect of morphine was not altered; Schild analysis yielded a slope of close to -1.0. 8-OH-DPAT both prevented and reversed the action of morphine. The action of 8-OH-DPAT was reversible (at 24 hr). In distinction, 8-OH-DPAT neither blocked morphine-induced Straub tail nor precipitated withdrawal in morphine-dependent animals; thus, it lacked opioid-antagonist properties. The antagonism of MIA by 8-OH-DPAT was mimicked by additional drugs acting as high efficacy 5-HT1A agonists: lisuride, 5-methoxy-N,N-dimethyltryptamine hydrogen oxalate, RU 24969 [methoxy-3-(1,2,3.6-tetrahydropyridin-4-yl)-1H-indole] and d-lysergic acid diethylamide. In contrast, the 5-HT1B/1C agonist, TFMPP m-trifluromethylphenylpiperazine HCl, and the 5-HT1C/2 agonist, DOI (+-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCI, were ineffective. The putative selective 5-HT1A antagonists, BMY 7378 [(8-[-[4-(2-,ethoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4]- decane-7,9-dione-2-HCL] and spiperone, did not reduce MIA. Indeed, BMY 7378 blocked the ability of 8-OH-DPAT to antagonize MIA. Under the present conditions, agonists and antagonists at adrenergic and dopaminergic receptors did not attenuate MIA. These data show that, over a certain range of doses, the systemic administration of 8-OH-DPAT and other high efficacy 5-HT1A agonists functionally antagonizes the antinociceptive action of systemically applied morphine in a competitive-like manner. It is suggested that 5-HT1A receptors play an important role in the modulation of opioidergic antinociceptive mechanisms.
authors.affiliation: Neurobiology Division, Fondax, Groupe de Recherche Servier, Puteaux, France.
authors.forename: M J
authors.initials: MJ
authors.lastname: Millan
authors.forename: F C
authors.initials: FC
authors.lastname: Colpaert
mesh_terms: D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D000698:Analgesia; D000818:Animals; D015259:Dopamine Agents / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D006358:Hot Temperature; D008297:Male; D051379:Mice; D009020:Morphine / Q000494:pharmacology*; D011312:Pressure; D011985:Receptors, Serotonin / Q000187:drug effects*; D013566:Sympathomimetics / Q000494:pharmacology*; D013764:Tetrahydronaphthalenes / Q000494:pharmacology*
chemical_list: D015259:Dopamine Agents; D011985:Receptors, Serotonin; D013566:Sympathomimetics; D013764:Tetrahydronaphthalenes; D009020:Morphine; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin