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 μ-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 μ-opioid antagonist naloxone or 3-methoxynaltrexone, but not δ-opioid receptor antagonist naltrindole, κ-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 β-endorphin, [Met]-enkephalin, [Leu]-enkephalin, or cholecystokinin antiserum (200 μg each). The i.t. EM-2 pretreatment also attenuated the TF inhibition induced by other μ-opioid agonists, [d-Ala²,N-Me-Phe⁴,Gly-ol⁵]-enkephalin, EM-1 and EM-2, δ-opioid agonist deltorphin II, and κ-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 μ-opioid receptor and subsequently induces the release of dynorphin A(1-17) to produce antianalgesic effects against μ-, δ-, or κ-agonists-induced antinociception. The EM-2-induced antianalgesia is not mediated by the release of [Met]-enkephalin, [Leu]-enkephalin, β-endorphin, or cholecystokinin, nor does it involve κ- or δ-opioid or NMDA receptors in the spinal cord.