enkephalin--ala(2)-mephe(4)-gly(5)- and Peripheral-Nerve-Injuries

Mechanisms of acute pain transition to chronic pain are not fully understood. Here we demonstrate an active role of β-arrestin 2 (Arrb2) in regulating spinal cord NMDA receptor (NMDAR) function and the duration of pain. Intrathecal injection of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin produces paradoxical behavioural responses: early-phase analgesia and late-phase mechanical allodynia which requires NMDAR; both phases are prolonged in Arrb2 knockout (KO) mice. Spinal administration of NMDA induces GluN2B-dependent mechanical allodynia, which is prolonged in Arrb2-KO mice and conditional KO mice lacking Arrb2 in presynaptic terminals expressing Nav1.8. Loss of Arrb2 also results in prolongation of inflammatory pain and neuropathic pain and enhancement of GluN2B-mediated NMDA currents in spinal lamina IIo not lamina I neurons. Finally, spinal over-expression of Arrb2 reverses chronic neuropathic pain after nerve injury. Thus, spinal Arrb2 may serve as an intracellular gate for acute to chronic pain transition via desensitization of NMDAR.

Topics: Analgesics, Opioid; Animals; beta-Arrestin 2; Chronic Pain; Disease Models, Animal; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Humans; Hyperalgesia; Injections, Spinal; Male; Mice; Mice, Inbred ICR; Mice, Knockout; N-Methylaspartate; Neuralgia; Neurons; Peripheral Nerve Injuries; Receptors, N-Methyl-D-Aspartate; Receptors, Opioid, mu; Spinal Cord Dorsal Horn; Substantia Gelatinosa; Time Factors

Neuropathic pain, a distressing and debilitating disorder, is still poorly managed in clinic. Opioids, like morphine, remain the mainstay of prescribed medications in the treatment of this disorder, but their analgesic effects are highly unsatisfactory in part due to nerve injury-induced reduction of opioid receptors in the first-order sensory neurons of dorsal root ganglia. G9a is a repressor of gene expression. We found that nerve injury-induced increases in G9a and its catalyzed repressive marker H3K9m2 are responsible for epigenetic silencing of Oprm1, Oprk1, and Oprd1 genes in the injured dorsal root ganglia. Blocking these increases rescued dorsal root ganglia Oprm1, Oprk1, and Oprd1 gene expression and morphine or loperamide analgesia and prevented the development of morphine or loperamide-induced analgesic tolerance under neuropathic pain conditions. Conversely, mimicking these increases reduced the expression of three opioid receptors and promoted the mu opioid receptor-gated release of primary afferent neurotransmitters. Mechanistically, nerve injury-induced increases in the binding activity of G9a and H3K9me2 to the Oprm1 gene were associated with the reduced binding of cyclic AMP response element binding protein to the Oprm1 gene. These findings suggest that G9a participates in the nerve injury-induced reduction of the Oprm1 gene likely through G9a-triggered blockage in the access of cyclic AMP response element binding protein to this gene.

Topics: Animals; CREB-Binding Protein; Disease Models, Animal; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Functional Laterality; Ganglia, Spinal; Gene Expression Regulation; Histone-Lysine N-Methyltransferase; Loperamide; Membrane Potentials; Mice; Mice, Inbred C57BL; Microfilament Proteins; Narcotics; Nociceptin Receptor; Peripheral Nerve Injuries; Rats, Sprague-Dawley; Receptors, Opioid; Receptors, Opioid, delta; Receptors, Opioid, mu; Sensory Receptor Cells

A sound strategy for improving the clinical efficacy of opioids involves exploiting positive interactions with drugs directed at other targets in pain pathways. The current study investigated the role of dopamine receptor D2 (D2R) in modulation of spinal dorsal horn excitability to noxious input, and interactions therein with μ-opioid receptor (MOR) in an animal model of neuropathic pain induced by spinal nerve ligation (SNL). C-fiber-evoked field potentials in the spinal dorsal horn were depressed concentration dependently by spinal superfusion with the D2R agonist quinpirole both in nerve-injured and sham-operated (control) rats. However, quinpirole-induced depression was significant at 10 μmol/L after SNL but only at 100 μmol/L in control rats. This quinpirole effect was completely abolished by MOR antagonist CTOP at subclinical concentration (1 μmol/L) in nerve-injured rats, but was unaltered in sham-operated rats. Nine days after SNL, D2R was upregulated to both presynaptic and postsynaptic locations in dorsal horn neurons, as revealed by double confocal immunofluorescence stainings for synaptophysin and PSD-95. In addition, D2R/MOR co-localization was increased after SNL. Co-administration of 1 μmol/L quinpirole, insufficient per se to alter evoked potentials, dramatically enhanced inhibition of evoked potentials by MOR agonist DAMGO, reducing the IC50 value of DAMGO by 2 orders of magnitude. The present data provide evidence of profound functional and subcellular changes in D2R-mediated modulation of noxious input after nerve injury, including positive interactions with spinal MOR. These results suggest D2R co-stimulation as a potential avenue to improve MOR analgesia in sustained pain states involving peripheral nerve injury.

Topics: Analgesics, Opioid; Animals; Disease Models, Animal; Disks Large Homolog 4 Protein; Dopamine Agonists; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Evoked Potentials; Intracellular Signaling Peptides and Proteins; Male; Membrane Proteins; Nerve Fibers, Unmyelinated; Peripheral Nerve Injuries; Quinpirole; Rats; Rats, Sprague-Dawley; Receptors, Dopamine D2; Receptors, Opioid, mu; Somatostatin; Synapses; Synaptophysin; Up-Regulation

Activation of opioid receptors on peripheral sensory neurons has the potential for safe pain control, as it lacks centrally mediated side effects. While this approach often only partially suppressed neuropathic pain in animal models, opioids were mostly applied to animal paws although neuropathy was induced at the nerve trunk. Here we aimed to identify the most relevant peripheral site of opioid action for efficient antinociception in neuropathy. On days 2 and 14 following a chronic constriction injury (CCI) of the sciatic nerve in mice, we evaluated dose and time relationships of the effects of μ-, δ-, and κ-opioid receptor agonists injected either at the CCI site or intraplantarly (i.pl.) into the lesioned nerve-innervated paw, on spontaneous paw lifting and heat and mechanical hypersensitivity (using Hargreaves and von Frey tests, respectively). We found that neither agonist diminished spontaneous paw lifting, despite the application site. Heat hypersensitivity was partially attenuated by i.pl. μ-receptor agonist only, while it was improved by all three agonists applied at the CCI site. Mechanical hypersensitivity was slightly diminished by all agonists administered i.pl., whereas it was completely blocked by all opioids injected at the CCI site. These antinociceptive effects were opioid receptor type-selective and site-specific. Thus, opioids might not be effective against spontaneous pain, but they improve heat and mechanical hypersensitivity in neuropathy. Importantly, efficient alleviation of hypersensitivity is governed by peripheral opioid receptors at the injured nerve trunk rather than at its peripheral terminals. Identifying the primary action site of analgesics is important for the development of adequate pain therapies.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics, Opioid; Animals; Behavior, Animal; Constriction, Pathologic; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Foot; Hot Temperature; Hyperalgesia; Male; Mice; Mice, Inbred C57BL; Nerve Endings; Neuralgia; Pain Measurement; Peripheral Nerve Injuries; Peripheral Nerves; Physical Stimulation; Receptors, Opioid

Neuropathic pain alters opioid self-administration in rats. The brain regions altered in the presence of neuropathic pain mediating these differences have not been identified, but likely involve ascending pain pathways interacting with the limbic system. The amygdala is a brain region that integrates noxious stimulation with limbic activity.. μ-Opioid receptors were blocked in the amygdala using the irreversible antagonist, β-funaltrexamine, and the antiallodynic and reinforcing effects of heroin were determined in spinal nerve-ligated rats. In addition, the effect of β-funaltrexamine was determined on heroin self-administration in sham-operated rats.. β-Funaltrexamine decreased functional activity of μ-opioid receptors by 60 ± 5% (mean ± SD). Irreversible inhibition of μ-opioid receptors in the amygdala significantly attenuated the ability of doses of heroin up to 100 μg/kg to reverse hypersensitivity after spinal nerve ligation. Heroin intake by self-administration in spinal nerve-ligated rats was increased from 5.0 ± 0.3 to 9.9 ± 2.1 infusions/h after administration of 2.5 nmol of β-funaltrexamine in the lateral amygdala, while having no effect in sham-operated animals (5.8 ± 1.6 before, 6.7 ± 0.9 after). The antiallodynic effects of 60 μg/kg heroin were decreased up to 4 days, but self-administration was affected for up to 14 days.. μ-Opioid receptors in the lateral amygdala partially meditate heroin's antiallodynic effects and self-administration after peripheral nerve injury. The lack of effect of β-funaltrexamine on heroin self-administration in sham-operated subjects suggests that opioids maintain self-administration through a distinct mechanism in the presence of pain.

Topics: Amygdala; Analgesics, Opioid; Animals; Behavior, Animal; Brain; Conditioning, Operant; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Guanosine 5'-O-(3-Thiotriphosphate); Heroin; Hyperalgesia; Infusions, Intravenous; Male; Naltrexone; Narcotic Antagonists; Peripheral Nerve Injuries; Rats; Rats, Inbred F344; Reinforcement, Psychology; Self Administration; Spinal Nerves

Previous studies in rats have shown that spinal morphine loses potency and efficacy to suppress an acute nociceptive stimulus applied to the tail or the paw following injury to peripheral nerves by tight ligation of the L5/L6 spinal nerves. Additionally, intrathecal (i.th.) morphine is ineffective in suppressing tactile allodynia at fully antinociceptive doses in these animals. The molecular basis for this loss of morphine potency and efficacy in nerve injury states is not known. One possible explanation for this phenomenon is a generalized, multi-segmental loss of opioid mu (mu) receptors in the dorsal horn of the spinal cord after nerve injury. This hypothesis was tested here by determining whether nerve injury produces (a) a decrease in mu receptors in the lumbar spinal cord; (b) a decrease in the affinity of ligand-receptor interaction, (c) a decrease in the fraction of high-affinity state of the mu receptors and (d) a reduced ability of morphine to activate G-proteins via mu receptors. Lumbar spinal cord tissues were examined 7 days after the nerve injury, a time when stable allodynia was observed. At this point, no differences were observed in the receptor density or affinity of [3H]DAMGO (mu selective agonist) or [3H]CTAP (mu selective antagonist) in the dorsal quadrant of lumbar spinal cord ipsilateral to nerve injury. Additionally, no change in morphine's potency and efficacy in activating G-proteins was observed. In contrast, staining for mu opioid receptors using mu-selective antibodies revealed a discrete loss of mu opioid receptors localized ipsilateral to the nerve injury and specific for sections taken at the L6 level. At these spinal segments, mu opioid receptors were decreased in laminae I and II. The data indicate that the loss of mu opioid receptors are highly localized and may contribute to the loss of morphine activity involving input at these spinal segments (e.g., foot-flick response). On the other hand, the lack of a generalized loss of opioid mu receptors across spinal segments makes it unlikely that this is the primary cause for the loss of potency and efficacy of mu opioids to suppress multi-segmental reflexes, such as the tail-flick response.

Topics: Animals; Binding, Competitive; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; GTP-Binding Proteins; Guanosine 5'-O-(3-Thiotriphosphate); Male; Morphine; Narcotic Antagonists; Narcotics; Nociceptors; Pain; Peptide Fragments; Peptides; Peripheral Nerve Injuries; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Somatostatin; Spinal Cord; Tritium