title: Paradoxical hyperalgesia induced by mu-opioid receptor agonist endomorphin-2, but not endomorphin-1, microinjected into the centromedial amygdala of the rat.
abstract: The effects of endomorphin-2 or endomorphin-1 microinjected into the centromedial amygdala on the thermally-induced tail-flick response were studied in male CD rats. Microinjection of endomorphin-2 (8.7-35.0 nmol) given into the centromedial amygdala time- and dose-dependently decreased the tail-flick latencies. On the other hand, endomorphin-1 (8-32.6 nmol) given into the same site did not cause any change of the tail-flick latency. However, endomorphin-1 (32.6 nmol) or endomorphin-2 (35.0 nmol) given into the basolateral site of amygdala did not affect the tail-flick latency. Pretreatment with the antiserum against dynorphin A(1-17) (200 microg) significantly reversed the decrease of the tail-flick latency induced by endomorphin-2. The decrease of the tail-flick latency induced by endomorphin-2 was also blocked by the endomorphin-2 selective micro-opioid receptor antagonist 3-methoxynaltrexone (6.4 pmol) and by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (30 nmol), but not by the kappa-opioid receptor antagonist nor-binaltorphimine (6.6 nmol). It is concluded that endomorphin-2, but not endomorphin-1, given into the centromedial amygdala stimulates a 3-methoxynaltrexone-sensitive mu-opioid receptor subtype to induce the release of dynorphin A(1-17), which then acts on the NMDA receptor, but not kappa-opioid receptor for producing hyperalgesia. This conclusion is further supported by the additional findings that dynorphin A(1-17) (2.3 nmol) given into the centromedial amygdala also caused the decrease of the tail-flick latency, which was similarly blocked by the NMDA receptor antagonist MK-801 (30 nmol), but not kappa-opioid receptor antagonist nor-binaltorphimine (6.6 nmol).
authors.affiliation: Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, WI 53226, USA.
authors.forename: Maia
authors.initials: M
authors.lastname: Terashvili
authors.forename: Hsiang-En
authors.initials: HE
authors.lastname: Wu
authors.forename: Emma
authors.initials: E
authors.lastname: Schwasinger
authors.forename: Leon F
authors.initials: LF
authors.lastname: Tseng
mesh_terms: D000679:Amygdala / Q000187:drug effects* / Q000473:pathology / Q000503:physiopathology; D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000633:toxicity; D000818:Animals; D016291:Dizocilpine Maleate / Q000008:administration & dosage / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004399:Dynorphins / Q000008:administration & dosage / Q000276:immunology / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000008:administration & dosage / Q000494:pharmacology; D006930:Hyperalgesia / Q000139:chemically induced / Q000503:physiopathology* / Q000517:prevention & control; D007106:Immune Sera / Q000494:pharmacology; D008297:Male; D008845:Microinjections; D009271:Naltrexone / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000008:administration & dosage / Q000494:pharmacology; D009842:Oligopeptides / Q000008:administration & dosage* / Q000633:toxicity; D010147:Pain Measurement / Q000379:methods; D011817:Rabbits; D051381:Rats; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000037:antagonists & inhibitors / Q000502:physiology; D013997:Time Factors
chemical_list: C107547:3-methoxynaltrexone; D000701:Analgesics, Opioid; D018691:Excitatory Amino Acid Antagonists; D007106:Immune Sera; D009292:Narcotic Antagonists; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; C051844:norbinaltorphimine; C105294:endomorphin 2; D009271:Naltrexone; D016291:Dizocilpine Maleate; D004399:Dynorphins