dextro-Naloxone or levo-naloxone reverses the attenuation of morphine antinociception induced by lipopolysaccharide in the mouse spinal cord via a non-opioid mechanism.

Glial stimulation by intrathecal injection of lipopolysaccharide (LPS) attenuated the tail-flick inhibition produced by morphine given intrathecally in the spinal cord of the male CD-1 mice. The phenomenon has been defined as antianalgesia. The effects of dextro-naloxone or levo-naloxone on the attenuation of morphine-produced tail-flick inhibition induced by LPS were then studied. Pretreatment with dextro-naloxone or levo-naloxone reversed the attenuation of the morphine-produced tail-flick inhibition induced by LPS. Pretreatment with dextro-naloxone or levo-naloxone alone did not affect the morphine-produced tail-flick inhibition. It is concluded that dextro-naloxone and levo-naloxone block the LPS-induced antianalgesia against morphine antinociception via a non-opioid mechanism.

p38 mitogen-activated protein kinase inhibitor SB203580 reverses the antianalgesia induced by dextro-morphine or morphine in the mouse spinal cord.

We have previously demonstrated that intrathecal pretreatment with dextro-morphine or morphine attenuates the morphine-produced antinociception. The phenomenon has been defined as antianalgesia, which is mediated by a non-opioid receptor [Wu, H., Thompson, J., Sun, H., Terashvili, M., Tseng, L.F., 2005. Antianalgesia: stereo-selective action of dextro-morphine over levo-morphine on glia in the mouse spinal cord. J. Pharmacol. Exp. Ther. 314, 1101-1108]. To determine if p38 mitogen-activated protein kinase (MAPK) is involved in the antianalgesia, the effects of p38 MAPK inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580) on the attenuation of the morphine-produced tail-flick inhibition induced by dextro-morphine or morphine were studied in male CD-1 mice. Intrathecal pretreatment with SB203580 (24.2 nmol) reversed the attenuation of the morphine-produced tail-flick inhibition induced by dextro-morphine (33 fmol) or morphine (0.3 nmol) pretreatment. The finding indicates that the antianalgesia induced by dextro-morphine or morphine is mediated by the activation of p38 MAPK in the mouse spinal cord.

dextro- and levo-morphine attenuate opioid delta and kappa receptor agonist produced analgesia in mu-opioid receptor knockout mice.

We have demonstrated that the antianalgesia induced by dextro-morphine and levo-morphine is not mediated by the stimulation of mu-opioid receptors in male CD-1 mice. We now report that the dextro-morphine and levo-morphine attenuated antinociception produced by delta-opioid receptor agonist deltorphin II and kappa-opioid receptor agonist U50,488H given spinally in the male mu-opioid receptor knockout mice. The tail-flick response was used for the antinociceptive test. Intrathecal injection of levo-morphine (3 nmol) markedly inhibited the tail-flick response in wild type, partially in heterozygous, but not in homozygous mu-opioid receptor knockout mice. Intrathecal pretreatment with dextro-morphine (33 fmol) or levo-morphine (0.3 nmol) for 45 min also attenuated levo-morphine-produced antinociception in wide type mice. Intrathecal pretreatment with dextro-morphine (33 fmol) or levo-morphine (0.3 nmol) for 45 min attenuated the tail-flick inhibition produced by deltorphin II (12.8 nmol) and U50,488H (123.3 nmol) in wide type, heterozygous and homozygous mu-opioid receptor knockout mice. The findings provide additional evidence that mu-opioid receptors are not involved in the antianalgesia induced by dextro-morphine and levo-morphine.

Antianalgesia: stereoselective action of dextro-morphine over levo-morphine on glia in the mouse spinal cord.

We have previously shown that the naturally occurring levo-morphine at a subanalgesic picomolar dose pretreated i.t. induces antianalgesia against levo-morphine-produced antinociception. We now report that the synthetic stereo-enantiomer dextro-morphine, even at an extremely low femtomolar dose, induces antianalgesia against levo-morphine-produced antinociception using the tail-flick (TF) test in male CD-1 mice. Intrathecal pretreatment with dextro-morphine (33 fmol) time-dependently attenuated the i.t. levo-morphine-produced TF inhibition for 4 h and returned to the preinjection control level at 24 h. Intrathecal pretreatment with dextro-morphine (0.3-33 fmol), which injected alone did not affect the baseline TF latency, dose-dependently attenuated the TF inhibition produced by i.t.-administered levo-morphine (3.0 nmol). The ED(50) value for dextro-morphine to induce antianalgesia was estimated to be 1.07 fmol, which is 71,000-fold more potent than the ED(50) value of levo-morphine, indicating the high stereoselective action of dextro-morphine over levo-morphine for the induction of antianalgesia. Like levo-morphine, the dextro-morphine-induced antianalgesia against levo-morphine-produced TF inhibition was dose-dependently blocked by the nonopioid dextro-naloxone and its stereo-enantiomer levo-naloxone, a nonselective mu-opioid receptor antagonist. The antianalgesia induced by levo-morphine and dextro-morphine is reversed by the pretreatment with the glial inhibitor propentofylline (3.3-65 nmol), indicating that the antianalgesia is mediated by glial stimulation. The findings strongly indicate that the antianalgesia induced by levo-morphine and dextro-morphine is mediated by the stimulation of a novel nonopioid receptor on glial cells.

Differential mechanisms of antianalgesia induced by endomorphin-1 and endomorphin-2 in the ventral periaqueductal gray of the rat.

The effects of pretreatment with endomorphin-1 (EM-1) and endomorphin-2 (EM-2) given into the ventral periaqueductal gray (vPAG) to induce antianalgesia against the tail-flick (TF) inhibition produced by morphine given into the vPAG were studied in rats. Pretreatment with EM-1 (3.5-28 nmol) given into vPAG for 45 min dose-dependently attenuated the TF inhibition produced by morphine (9 nmol) given into vPAG. Similarly, pretreatment with EM-2 (1.7-7.0 nmol) for 45 min also attenuated the TF inhibition induced by morphine; however, a high dose of EM-2 (14 nmol) did not attenuate the morphine-produced TF inhibition. The attenuation of morphine-produced TF inhibition induced by EM-2 or EM-1 pretreatment was blocked by pretreatment with mu-opioid antagonist (-)-naloxone (55 pmol) but not nonopioid (+)-naloxone (55 pmol). However, pretreatment with a morphine-6beta-glucuronide-sensitive mu-opioid receptor antagonist 3-methoxynaltrexone (6.4 pmol) selectively blocked EM-2- but not EM-1-induced antianalgesia. Pretreatment with dynorphin A(1-17) antiserum reversed only EM-2- but not EM-1-induced antianalgesia. Pretreatment with antiserum against beta-endorphin, [Met(5)]enkephalin, [Leu(5)]enkephalin, substance P or cholecystokinin, or with delta-opioid receptor antagonist naltrindole (2.2 nmol) or kappa-opioid receptor antagonist norbinaltorphimine (6.6 nmol) did not affect EM-2-induced antianalgesia. It is concluded that EM-2 selectively releases dynorphin A(1-17) by stimulation of a novel subtype of mu-opioid receptor, tentatively designated as mu(3) in the vPAG to induce antianalgesia, whereas the antianalgesia induced by EM-1 is mediated by the stimulation of another subtype of mu(1)- or mu(2)-opioid receptor.

Nonopioidergic mechanism mediating morphine-induced antianalgesia in the mouse spinal cord.

Intrathecal (i.t.) pretreatment with a low dose (0.3 nmol) of morphine causes an attenuation of i.t. morphine-produced analgesia; the phenomenon has been defined as morphine-induced antianalgesia. The opioid-produced analgesia was measured with the tail-flick (TF) test in male CD-1 mice. Intrathecal pretreatment with low dose (0.3 nmol) of morphine time dependently attenuated i.t. morphine-produced (3.0 nmol) TF inhibition and reached a maximal effect at 45 min. Intrathecal pretreatment with morphine (0.009-0.3 nmol) for 45 min also dose dependently attenuated morphine-produced TF inhibition. The i.t. morphine-induced antianalgesia was dose dependently blocked by the nonselective mu-opioid receptor antagonist (-)-naloxone and by its nonopioid enantiomer (+)-naloxone, but not by endomorphin-2-sensitive mu-opioid receptor antagonist 3-methoxynaltrexone. Blockade of delta-opioid receptors, kappa-opioid receptors, and N-methyl-D-aspartate (NMDA) receptors by i.t. pretreatment with naltrindole, nor-binaltorphimine, and (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), respectively, did not affect the i.t. morphine-induced antianalgesia. Intrathecal pretreatment with antiserum against dynorphin A(1-17), [Leu]-enkephalin, [Met]-enkephalin, beta-endorphin, cholecystokinin, or substance P also did not affect the i.t. morphine-induced antianalgesia. The i.t. morphine pretreatment also attenuated the TF inhibition produced by opioid muagonist [D-Ala2, N-Me-Phe4,Gly-ol5]-enkephalin, delta-agonist deltorphin II, and kappa-agonist U50,488H. It is concluded that low doses (0.009-0.3 nmol) of morphine given i.t. activate an antianalgesic system to attenuate opioid mu-, delta-, and kappa-agonist-produced analgesia. The morphine-induced antianalgesia is not mediated by the stimulation of opioid mu-, delta-, or kappa-receptors or NMDA receptors. Neuropeptides such as dynorphin A(1-17), [Leu]-enkephalin, [Met]-enkephalin, beta-endorphin, cholecystokinin, and substance P are not involved in this low-dose morphine-induced antianalgesia.

Dynorphinergic mechanism mediating endomorphin-2-induced antianalgesia in the mouse spinal cord.

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 mu-opioid receptors. Now, we report that EM-2 at small doses (0.05-1.75 nmol), which injected alone did not produce antinociception, produces anti-analgesia 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 mu-opioid antagonist naloxone or 3-methoxynaltrexone, but not delta-opioid receptor antagonist naltrindole, kappa-opioid receptor antagonist nor-binaltorphimine, or N-methyl-d-aspartate (NMDA) receptor antagonist MK-801. The EM-2-induced antianalgesic effect against morphine-induced TF inhibition was blocked by i.t. pretreatment with antiserum against dynorphin A(1-17), but not beta-endorphin, [Met]-enkephalin, [Leu]-enkephalin, or cholecystokinin antiserum (200 microg each). The i.t. EM-2 pretreatment also attenuated the TF inhibition induced by other mu-opioid agonists, [d-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin, EM-1 and EM-2, delta-opioid agonist deltorphin II, and kappa-opioid agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methane-sulfonate hydrate (U50,488H). It is concluded that EM-2 at subanalgesic doses presumably stimulates a subtype of mu-opioid receptor and subsequently induces the release of dynorphin A(1-17) to produce antianalgesic effects against mu-, delta-, or kappa-agonists-induced antinociception. The EM-2-induced antianalgesia is not mediated by the release of [Met]-enkephalin, [Leu]-enkephalin, beta-endorphin, or cholecystokinin, nor does it involve kappa- or delta-opioid or NMDA receptors in the spinal cord.

Antinociceptive properties of oxymorphazole in the mouse.

Oxymorphazole (17-methyl-6,7-dehydro-3,14-dihydroxy-4,5 alpha-epoxy-6,7:3',4'-pyrazolomorphinan), a hydrophilic opioid, given intracerebroventricularly (2.5-50 nmol) or intrathecally (0.3-5 nmol) dose-dependently produced tail-flick inhibition in male CD-1 mice. However, oxymorphazole given subcutaneously even at high doses (10-80 mg/kg) produced weak tail-flick inhibition. Oxymorphazole given intraperitoneally (0.1 to 10 mg/kg) dose-dependently inhibited abdominal constriction response induced by intraperitoneally injection of 0.6% acetic acid. Oxymorphazole given intracerebroventricularly (25 nmol) or intrathecally (5 nmol) induced tail-flick inhibition was blocked by pretreatment with the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Orn-Thr-Pen-Thr-NH2, but not kappa-opioid receptor antagonist nor-binaltrophimine. The delta-opioid receptor antagonist, naltrindole, blocked the tail-flick inhibition induced by oxymorphazole given intrathecally but not intracerebroventricularly. The inhibition of the abdominal constriction response by oxymorphazole given intraperitoneally was blocked by intraperitoneally pretreatment with naloxone, but not naltrindole or nor-binaltrophimine. Thus, oxymorphazole given systemically produces antinociception only with the abdominal constriction test, but not the tail-flick test, suggesting that it produces the antinociception at the peripheral sites when administered systemically. The oxymorphazole-induced antinociception is mainly mediated by the stimulation of mu-opioid receptors when given either centrally or systemically and also the delta-opioid receptors when given intrathecally. The lack of central antinociceptive effect of oxymorphazole given systemically may have interesting clinical implications.