Morphine peripheral analgesia depends on activation of the PI3Kγ/AKT/nNOS/NO/K _ATP signaling pathway

Abstract

Morphine is one of the most prescribed and effective drugs used for the treatment of acute and chronic pain conditions. In addition to its central effects, morphine can also produce peripheral analgesia. However, the mechanisms underlying this peripheral action of morphine have not yet been fully elucidated. Here, we show that the peripheral antinociceptive effect of morphine is lost in neuronal nitric-oxide synthase null mice and that morphine induces the production of nitric oxide in primary nociceptive neurons. The activation of the nitric-oxide pathway by morphine was dependent on an initial stimulation of PI3Kγ/AKT protein kinase B (AKT) and culminated in increased activation of K ATP channels. In the latter, this intracellular signaling pathway might cause a hyperpolarization of nociceptive neurons, and it is fundamental for the direct blockade of inflammatory pain by morphine. This understanding offers new targets for analgesic drug development.M orphine is one of the most prescribed and effective drugs used for treatment of postoperatory and acute severe pain. Nevertheless, its use is frequently limited by undesirable side effects including respiratory depression, tolerance, and addiction. The discovery that morphine can also produce peripheral analgesia in the setting of inflammatory pain opened the possibility of developing peripheral restricted opioids devoid of central side effects (1).Morphine peripheral analgesia was discovered by its direct effect on already established inflammatory hypernociception induced by prostaglandin E 2 (PGE 2 ) injected in rat hind paws (1). Therefore, in contrast to aspirin-like drugs whose analgesic mechanism depends on prevention of nociceptor sensitization by inhibiting synthesis of prostaglandins, opioids are able to directly block ongoing nociceptor sensitization. However, the molecular mechanisms triggered by morphine to promote this action have not been fully elucidated. The present study reports on a series of experiments using behavioral, biochemical, and electrophysiological approaches to address this issue. The following major findings are reported herein: (i) the activation of peripheral opioid receptors in primary nociceptive neurons by morphine triggers a cascade of intracellular signaling events initiated by PI3Kγ/Protein kinase B (AKT); (ii) this is accompanied by activation of neuronal nitric oxide synthase (nNOS) and nitric oxide (NO) production, which (iii) induces an increase in K ATP channel currents; and (iv) it causes a hyperpolarization of nociceptive neurons. Results and DiscussionBased on the evidence that cAMP was the key intracellular second messenger involved in PGE 2 -induced nociceptor sensitization (2) and that opioid-receptor activation in vitro was coupled to adenylyl-cyclase inhibition (3), it was initially suggested that these drugs counteracted inflammatory hypernociception directly through inhibition of PGE 2 -induced adenylyl-cyclase activation (1, 4). Subsequent in vitro studies, which confirmed the ability of opioids to inhibit ad...

G Protein-Coupled Receptor Kinase 6 Acts as a Critical Regulator of Cytokine-Induced Hyperalgesia by Promoting Phosphatidylinositol 3-Kinase and Inhibiting p38 Signaling

“…Interestingly, recent evidence shows that PI 3-kinase also mediates a negative feedback loop in preventing neuronal hyperexcitability in the Drosophila neuromuscular junction (30). In mice, it was shown that PI 3-kinase inhibition blocks capsaicin-and NGF-induced increases in pain sensitivity (hyperalgesia) (31), whereas the morphine-induced reduction in pain sensitivity (analgesia) is mediated via PI 3-kinase (32). Our present data demonstrate that PI 3-kinase inhibition increases and prolongs cytokine-induced hyperalgesia in WT mice.…”

“…Our present data demonstrate that PI 3-kinase inhibition increases and prolongs cytokine-induced hyperalgesia in WT mice. The differential effects of PI 3-kinase activity in determining pain sensitivity may depend on the fact that the isoform of PI 3-kinase that is activated in response to NGF is of a different subtype than the PI 3-kinase activated by cytokines or morphine, PI3Kγ (PI 3-kinase γ) (32 Figures 3A, B).…”

Modulation of Peripheral Sensory Neurons by the Immune System: Implications for Pain Therapy

“…Colocalization of GIRK channels and -opioid receptors was shown on sensory nerve endings in the epidermis (Khodorova et al, 2003), but no direct evidence of functional coupling or modulation of GIRK channels in DRG neurons has been provided so far. However, morphine was suggested to hyperpolarize DRG neurons by activation of K ATP currents via the nitric oxide pathway (Cunha et al, 2010). Furthermore, -opioid receptor-mediated hyperpolarization, increased K ϩ currents, and decreased firing rates of action potentials were shown in neu-862 rons from neonatal rats (Takeda et al, 2004).…”

Analgesic roles of peripheral intrinsic met-enkephalin and dynorphin A in long-lasting inflammatory pain induced by complete Freund's adjuvant in rats

“…The involvement of peripheral opioids in inflammatory pain regulation has been well demonstrated (1)(2)(3)(4). Under inflammatory conditions, opioid peptides are released from local immunocytes upon the stimulation of corticotrophin-releasing factor (CRF) and a number of cytokines, including interleukin (IL)-1β (5-7).…”