Dynorphinergic Mechanism Mediating Endomorphin-2-Induced Antianalgesia in the Mouse Spinal Cord

Abstract

We have previously demonstrated that both endomorphin-1 (EM-1) and endomorphin-2 (EM-2) at high doses (1.75-35 nmol) given intrathecally (i.t.) or intracerebroventricularly produce antinociception by stimulation of -opioid receptors. Now, we report that EM-2 at small doses (0.05-1.75 nmol), which injected alone did not produce antinociception, produces antianalgesia against opioid agonist-induced antinociception. The tail-flick (TF) response was used to test the antinociception in male CD-1 mice. Intrathecal pretreatment with EM-2 (0.02-1.75 nmol) 45 min before i.t. morphine (3.0 nmol) injection dose dependently attenuated morphine-induced TF inhibition. On the other hand, a similar dose of EM-1 (1.64 nmol) failed to produce any antianalgesic effect. The EM-2 (1.75 nmol)-produced anti-analgesia against morphine-induced TF inhibition was blocked by i.t. pretreatment with the -opioid antagonist naloxone or 3-methoxynaltrexone, but not ␦-opioid receptor antagonist naltrindole, -opioid receptor antagonist nor-binal- Endogenous opioid tetrapeptides endomorphin-1 (EM-1) and endomorphin-2 (EM-2) have been found to be highly selective for -opioid receptors (Zadina et al., 1997). Both EM-1 and EM-2 potently compete with -opioid receptor binding with no appreciable affinity with ␦-and -opioid receptors and selectively activate -opioid receptor-mediated G-proteins with [35 S]guanosine 5Ј-O-(3-thio)triphosphate binding (Goldberg et al., 1998;Narita et al., 1998Narita et al., , 2000Monory et al., 2000). The increases of the [ 35 S]guanosine 5Ј-O-(3-thio)triphosphate binding and antinociceptive effects induced by both EM-1 and EM-2 are selectively blocked by the pretreatment with selective -opioid receptor antagonist ␤-funaltrexamine or D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH 2 , indicating that the effects are mediated by the stimulation of -opioid receptors.However, recent studies indicate that the antinociceptive effects induced by EM-1 and EM-2 are mediated by the stimulation of different subtypes of -opioid receptors. Pretreatment with -opioid receptor antagonist 3-methoxynaltrexone selectively attenuated EM-2-but not EM-1-induced antinociception, whereas ␤-funaltrexamine inhibits both (Sakurada et al., 2000). There is an asymmetric cross-tolerance between EM-1 and EM-2, where mice made acutely tolerant to EM-1 are not cross-tolerant to EM-2, but mice made acutely tolerant to EM-2 cause partial cross-tolerance to EM-1 (Wu et al., 2001). Intrathecal pretreatment with antisense oligodeoxynucleotides against exon-8 of -opioid receptor clone inhibits the antinociception induced by EM-1

Paradoxical hyperalgesia induced by μ-opioid receptor agonist endomorphin-2, but not endomorphin-1, microinjected into the centromedial amygdala of the rat

“…The hyperalgesia produced by endomorphin-2 is blocked by the pretreatment with antiserum against dynorphin A(1-17), indicating that the effect is mediated by the release of dynorphn A(1-17). The finding is consistent with the previous findings that the analgesia and antianalgesia induced by endomorphin-2 from spinal cord or periaqueductal gray, respectively, are also mediated by the release of dynorphin A(1-17) Ohsawa et al, 2000Ohsawa et al, , 2001Sakurada et al, 2001;Wu et al, 2003). In a biochemical study, the release of dynorphin A(1-17) by endomorphin-2 from the rat spinal cord has been demonstrated .…”

“…The phenomenon has been defined as antianalgesia. The antianalgesia induced by endomorphin-2, but not endomorphin-1, is blocked by the pretreatment with 3-methoxynaltrexone (Terashvili et al, 2005;Wu et al, 2003). We found in the present study that the hyperalgesia induced by endomorphin-2 given into the centromedial amygdala was blocked by the pretreatment with 3-methoxynaltrexone.…”

Nonopioidergic Mechanism Mediating Morphine-Induced Antianalgesia in the Mouse Spinal Cord

“…Groups of mice were pretreated i.t. with the -opioid receptor antagonist 3-methoxynaltrexone (6.4 pmol), which antagonizes endomorphin-2, heroin, and morphine-6␤-glucuronide-produced analgesia but not morphine, 25 min (Brown et al, 1997;Sakurada et al, 2000); ␦-opioid receptor antagonist naltrindole (NTI, 11.1 or 22.3 nmol), 10 min (Mizoguchi et al, 1995;Wu et al, 2003); -opioid receptor antagonist nor-binaltorphimine (nor-BNI, 6.6 or 39.6 nmol), 24 h Ohsawa et al, 2001;Wu et al, 2003); nonselective opioid-receptor antagonist (Ϫ)-naloxone (0.03-27.5 pmol), 10 min; the nonopioid-receptor antagonist (ϩ)-naloxone (0.28 -55 pmol), 10 min; or NMDA receptor antagonist MK-801 (10 nmol), 20 min (Wu et al, 2003) before i.t. injection of morphine (0.3 nmol).…”

“…The antianalgesic effect is caused by the release of dynorphin A(1-17) through the stimulation of a subtype of -opioid receptors. The unique features of this endomorphin-2-induced antianalgesic action are that there is a lag period before dynorphin A(1-17) is released, and the antianalgesic action of endomorphin-2 corresponds with the time course of dynorphin release Wu et al, 2003). Furthermore, i.t.…”

Antianalgesia: Stereoselective Action ofdextro-Morphine overlevo-Morphine on Glia in the Mouse Spinal Cord

“…This antianalgesia is blocked by pretreatment with dextro-naloxone, indicating that the antianalgesia induced by levo-morphine is not mediated by the stimulation of conventional G protein-coupled -opioid receptors (Wu et al, 2004b). On the other hand, the antianalgesia induced by endogenous -opioid ligands endomorphin-1 and endomorphin-2 is blocked by levo-naloxone but not by dextro-naloxone, indicating that the antianalgesia induced by endomorphin-1 and endomorphin-2 is mediated by the desensitization of -opioid receptors by endomorphin-1 and endomorphin-2 pretreatment (Wu et al, 2003;Terashvili et al, 2005). Thus, levo-morphine has biphasic effects: it produces antinociception or analgesia, which is mediated by the stimulation of -opioid receptors, and also induces nonopioidergic antianalgesia.…”