enkephalin--ala(2)-mephe(4)-gly(5)- and Chronic-Pain

Microglia participates in the modulation of pain signaling. The activation of microglia is suggested to play an important role in affective disorders that are related to a dysfunction of the mesocorticolimbic system (MCLS) and are commonly associated with chronic pain. Moreover, there is evidence that mu-opioid receptors (MORs), expressed in the MCLS, are involved in neuroinflammatory events, although the way by which they do it remains to be elucidated. In this study, we propose that MOR pharmacological activation within the MCLS activates and triggers the local release of proinflammatory cytokines and this pattern of activation is impacted by the presence of systemic inflammatory pain. To test this hypothesis, we used in vivo microdialysis coupled with flow cytometry to measure cytokines release in the nucleus accumbens and immunofluorescence of IBA1 in areas of the MCLS on a rat model of inflammatory pain. Interestingly, the treatment with DAMGO, a MOR agonist locally in the nucleus accumbens, triggered the release of the IL1α, IL1β, and IL6 proinflammatory cytokines. Furthermore, MOR pharmacological activation in the ventral tegmental area (VTA) modified the levels of IBA1-positive cells in the VTA, prefrontal cortex, the nucleus accumbens and the amygdala in a dose-dependent way, without impacting mechanical nociception. Additionally, MOR blockade in the VTA prevents DAMGO-induced effects. Finally, we observed that systemic inflammatory pain altered the IBA1 immunostaining derived from MOR activation in the MSCLS. Altogether, our results indicate that the microglia-MOR relationship could be pivotal to unravel some inflammatory pain-induced comorbidities related to MCLS dysfunction.

Topics: Animals; Calcium-Binding Proteins; Chronic Pain; Disease Models, Animal; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Female; Male; Microfilament Proteins; Microglia; Neuroinflammatory Diseases; Nucleus Accumbens; Prefrontal Cortex; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Ventral Tegmental Area

Of painful conditions, somatic pain of acute nociceptive origin can be effectively managed clinically, while neuropathic pain of chronic neuropathy origin is difficult to control. For molecules involved in pain sensation, substance P (SP) is algesic, exacerbating painful sensation, while its amino-terminal fragment, heptapeptide SP

Topics: Amides; Analgesics; Analgesics, Opioid; Animals; Chronic Pain; Disease Models, Animal; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Male; Mice, Inbred ICR; Neuralgia; Pain Threshold; Peptide Fragments; Receptors, Opioid, mu; Substance P

Chronic pain caused by spinal cord injury (SCI) is notoriously resistant to treatment, particularly by opioids. After SCI, DRG neurons show hyperactivity and chronic depolarization of resting membrane potential (RMP) that is maintained by cAMP signaling through PKA and EPAC. Importantly, SCI also reduces the negative regulation by Gαi of adenylyl cyclase and its production of cAMP, independent of alterations in G protein-coupled receptors and/or G proteins. Opioid reduction of pain depends on coupling of opioid receptors to Gαi/o family members. Combining high-content imaging and cluster analysis, we show that in male rats SCI decreases opioid responsiveness

Topics: Animals; Cells, Cultured; Chronic Pain; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Ganglia, Spinal; HEK293 Cells; Humans; Male; Membrane Potentials; Nociceptors; Proto-Oncogene Proteins c-raf; Rats, Sprague-Dawley; Receptors, Opioid, mu; Signal Transduction; Spinal Cord Injuries

Morphine tolerance remains a challenge in the management of chronic pain in the clinic. As shown in our previous study, the dopamine D2 receptor (D2DR) expressed in spinal cord neurons might be involved in morphine tolerance, but the underlying mechanisms remain to be elucidated. In the present study, selective spinal D2DR blockade attenuated morphine tolerance in mice by inhibiting phosphatidylinositol 3 kinase (PI3K)/serine-threonine kinase (Akt)-mitogen activated protein kinase (MAPK) signaling in a μ opioid receptor (MOR)-dependent manner. Levo-corydalmine (l-CDL), which exhibited micromolar affinity for D2DR in D2/CHO-K1 cell lines in this report and effectively alleviated bone cancer pain in our previous study, attenuated morphine tolerance in rats with chronic bone cancer pain at nonanalgesic doses. Furthermore, the intrathecal administration of l-CDL obviously attenuated morphine tolerance, and the effect was reversed by a D2DR agonist in mice. Spinal D2DR inhibition and l-CDL also inhibited tolerance induced by the MOR agonist DAMGO. l-CDL and a D2DR small interfering RNA (siRNA) decreased the increase in levels of phosphorylated Akt and MAPK in the spinal cord; these changes were abolished by a PI3K inhibitor. In addition, the activated Akt and MAPK proteins in mice exhibiting morphine tolerance were inhibited by a MOR antagonist. Intrathecal administration of a PI3K inhibitor also attenuated DAMGO-induced tolerance. Based on these results, l-CDL antagonized spinal D2DR to attenuate morphine tolerance by inhibiting PI3K/Akt-dependent MAPK phosphorylation through MOR. These findings provide insights into a more versatile treatment for morphine tolerance.

Topics: Analgesics, Opioid; Animals; Berberine; Cell Line, Tumor; CHO Cells; Chronic Pain; Cricetinae; Cricetulus; Dopamine Antagonists; Drug Tolerance; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Female; Male; MAP Kinase Signaling System; Mice; Morphine; Phosphatidylinositol 3-Kinases; Proto-Oncogene Proteins c-akt; Rats; Rats, Sprague-Dawley; Receptors, Dopamine D2; Receptors, Opioid, mu

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

The primary afferent nociceptor was used as a model system to study mechanisms of pain induced by chronic opioid administration. Repeated intradermal injection of the selective mu-opioid receptor (MOR) agonist DAMGO induced mechanical hyperalgesia and marked prolongation of prostaglandin E2 (PGE2) hyperalgesia, a key feature of hyperalgesic priming. However, in contrast to prior studies of priming induced by receptor-mediated (i.e., TNFα, NGF, or IL-6 receptor) or direct activation of protein kinase Cε (PKCε), the pronociceptive effects of PGE2 in DAMGO-treated rats demonstrated the following: (1) rapid induction (4 h compared with 3 d); (2) protein kinase A (PKA), rather than PKCε, dependence; (3) prolongation of hyperalgesia induced by an activator of PKA, 8-bromo cAMP; (4) failure to be reversed by a protein translation inhibitor; (5) priming in females as well as in males; and (6) lack of dependence on the isolectin B4-positive nociceptor. These studies demonstrate a novel form of hyperalgesic priming induced by repeated administration of an agonist at the Gi-protein-coupled MOR to the peripheral terminal of the nociceptor. Significance statement: The current study demonstrates the molecular mechanisms involved in the sensitization of nociceptors produced by repeated activation of mu-opioid receptors and contributes to our understanding of the painful condition observed in patients submitted to chronic use of opioids.

Topics: Analgesics, Opioid; Animals; Chronic Pain; Cyclic AMP-Dependent Protein Kinases; Dinoprostone; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Female; Glycoproteins; Hyperalgesia; Lectins; Male; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Versicans