Role of Spinal Cord Akt-mTOR Signaling Pathways in Postoperative Hyperalgesia Induced by Plantar Incision in Mice.

Poor postoperative pain (POP) control increases perioperative morbidity, prolongs hospitalization days, and causes chronic pain. However, the specific mechanism(s) underlying POP is unclear and the identification of optimal perioperative treatment remains elusive. Akt and mammalian target of rapamycin (mTOR) are expressed in the spinal cord, dorsal root ganglion, and sensory axons. In this study, we explored the role of Akt and mTOR in pain-related behaviors induced by plantar incision in mice. Plantar incision activated spinal Akt and mTOR in a dose-dependent manner. Pre-treatment with Akt inhibitors intrathecally prevented the activation of mTOR dose-dependently. In addition, blocking the Akt-mTOR signaling cascade attenuated pain-related behaviors and spinal Fos protein expression induced by plantar incision. Our observations demonstrate that Akt-mTOR might be a potential therapeutic target for the treatment of POP.

PI3K/Akt Pathway: A Potential Therapeutic Target for Chronic Pain.

Chronic pain is among the most disabling and costly disorders, with prevalence ranging from 10% to 55%. However, current therapeutic strategies for chronic pain are unsatisfactory due to our poor understanding of its mechanisms. Thus, novel therapeutic targets need to be found in order to improve these patients' quality of life. PI3K and its downstream Akt are widely expressed in the spinal cord, particularly in the laminae I-IV of the dorsal horn, where nociceptive C and Aδ fibers of primary afferents principally terminate. Recent studies have demonstrated their critical roles in the development and maintenance of chronic pain. In this review, we summarized the roles and mechanisms of PI3K/Akt pathway in the progression of chronic pain through sciatic nerve injury, diabetic neuropathy, spinal cord injury, bone cancer, opioid tolerance, or opioid-induced hyperalgesia.

Minocycline attenuates bone cancer pain in rats by inhibiting NF-κB in spinal astrocytes.

AIM: To investigate the mechanisms underlying the anti-nociceptive effect of minocycline on bone cancer pain (BCP) in rats. METHODS: A rat model of BCP was established by inoculating Walker 256 mammary carcinoma cells into tibial medullary canal. Two weeks later, the rats were injected with minocycline (50, 100 µg, intrathecally; or 40, 80 mg/kg, ip) twice daily for 3 consecutive days. Mechanical paw withdrawal threshold (PWT) was used to assess pain behavior. After the rats were euthanized, spinal cords were harvested for immunoblotting analyses. The effects of minocycline on NF-κB activation were also examined in primary rat astrocytes stimulated with IL-1ß in vitro. RESULTS: BCP rats had marked bone destruction, and showed mechanical tactile allodynia on d 7 and d 14 after the operation. Intrathecal injection of minocycline (100 µg) or intraperitoneal injection of minocycline (80 mg/kg) reversed BCP-induced mechanical tactile allodynia. Furthermore, intraperitoneal injection of minocycline (80 mg/kg) reversed BCP-induced upregulation of GFAP (astrocyte marker) and PSD95 in spinal cord. Moreover, intraperitoneal injection of minocycline (80 mg/kg) reversed BCP-induced upregulation of NF-κB, p-IKKα and IκBα in spinal cord. In IL-1ß-stimulated primary rat astrocytes, pretreatment with minocycline (75, 100 µmol/L) significantly inhibited the translocation of NF-κB to nucleus. CONCLUSION: Minocycline effectively alleviates BCP by inhibiting the NF-κB signaling pathway in spinal astrocytes.

Chemokines and Their Receptors: Potential Therapeutic Targets for Bone Cancer Pain.

Bone cancer pain (BCP) is still an intractable problem currently because the analgesic pharmacological intervention remains insufficient. Thus, the development of novel therapeutic target is critical for the treatment of BCP. Emerging evidence demonstrated that some chemokines and their receptors contribute to the induction and maintenance of BCP. In this article, we reviewed the current evidence for the role of different chemokines and their receptors (e.g. CXCL12/CXCR4, CXCL1/CXCR2, CCL2/CCR2, CCL5/CCR5, CX3CL1/CX3CR1 and CXCL10/CXCR3) in mediating BCP. By extensively understanding the involvement of chemokines and their receptors in BCP, novel therapeutic targets may be revealed for the treatment of BCP.

Activation of spinal chemokine receptor CXCR3 mediates bone cancer pain through an Akt-ERK crosstalk pathway in rats.

Previously, we showed that activation of the spinal CXCL9, 10/CXCR3 pathway mediated bone cancer pain (BCP) in rats. However, the cellular mechanism involved is poorly understood. Here, we found that the activated CXCR3 was co-localized with either neurons, microglia, and astrocytes in the spinal cord, or non-peptidergic-, peptidergic-, and A-type neurons in the dorsal root ganglion. The inoculation of Walker-256 mammary gland carcinoma cells into the rat's tibia induced a time-dependent phosphorylation of Akt and extracellular signal-regulated kinase (ERK1/2) in the spinal cord, and CXCR3 was necessary for the phosphorylation of Akt and ERK 1/2. Meanwhile, CXCR3 was co-localized with either pAkt or pERK1/2. Blockage of either Akt or ERK1/2 prevented or reversed the mechanical allodynia in BCP rats. Furthermore, there was cross-activation between PI3K/Akt and Raf/MEK/ERK pathway under the BCP condition. Our results demonstrated that the activation of spinal chemokine receptor CXCR3 mediated BCP through Akt and ERK 1/2 kinase, and also indicated a crosstalk between PI3K/Akt and Raf/MEK/ERK signaling pathways under the BCP condition.

Activation of spinal phosphatidylinositol 3-kinase/protein kinase B mediates pain behavior induced by plantar incision in mice.

The etiology of postoperative pain may be different from antigen-induced inflammatory pain and neuropathic pain. However, central neural plasticity plays a key role in incision pain. It is also known that phosphatidylinositol 3-kinase (PI3K) and protein kinase B/Akt (PKB/Akt) are widely expressed in laminae I-IV of the spinal horn and play a critical role in spinal central sensitization. In the present study, we explored the role of PI3K and Akt in incision pain behaviors. Plantar incision induced a time-dependent activation of spinal PI3K-p110γ and Akt, while activated Akt and PI3K-p110γ were localized in spinal neurons or microglias, but not in astrocytes. Pre-treatment with PI3K inhibitors, wortmannin or LY294002 prevented the activation of Akt brought on by plantar incision in a dose-dependent manner. In addition, inhibition of spinal PI3K signaling pathway prevented pain behaviors (dose-dependent) and spinal Fos protein expression caused by plantar incision. These data demonstrated that PI3K signaling mediated pain behaviors caused by plantar incision in mice.

Stimulation of the pedunculopontine tegmental nucleus may affect renal function by melanocortinergic signaling.

Deep brain stimulation of the pedunculopontine tegmental nucleus (PPTg) has been reported to improve gait disturbance in animal models of Parkinsonism and among patients with Parkinson's disease. Evidence suggests that neurons in the PPTg are involved in the control of the sympathetic outflow to the kidneys, and sympathetic regulation is a major component of central melanocortin action. Our recent studies using transneuronal labeling pseudorabies virus (PRV)-614 and melanocortin-4 receptor (MC4R)-green fluorescent protein (GFP) transgenic mice supported the melanocortinergic nature of the middle and caudal PPTg (mPPTg and cPPTg). Because PRV-614/MC4R-GFP double-labeled neurons in the mPPTg and cPPTg were detected, we propose a hypothesis that deep brain stimulation of the PPTg may influence renal function by the melanocortinergic pathway.

Phosphatidylinositol 3-kinase mediates pain behaviors induced by activation of peripheral ephrinBs/EphBs signaling in mice.

EphBs receptors and their ephrinBs ligands are present in the adult brain and peripheral tissue and play a critical role in modulating multiple aspects of physiology and pathophysiology. Our recent evidence has shown that ephrinBs acted as a sensitizer to participate in peripheral sensitization and hyperalgesia induced by activation of peripheral ephrinBs/EphBs signaling. In the present study, we explored the role of phosphatidylinositol 3-kinase (PI3K) in ephrinB1-Fc-induced pain behaviors. Intraplantar injection of ephrinB1-Fc produced a time- and dose-dependent increase of PI3K-p110gamma expression and of phosphorylation of AKT in skin of injection site. Pre-treatment with PI3K inhibitor wortmannin or LY294002 prevented activation of peripheral AKT by ephrinB1-Fc. The activated AKT expressed in peripheral nerve terminals and DRG peptide-containing and small non-peptide-containing neurons. Inhibition of peripheral PI3K signaling dose-dependently prevented and reversed pain behaviors and spinal Fos protein expression induced by intraplantar injection of ephrinB1-Fc. Furthermore, pre-treatment with PI3K inhibitor wortmannin or LY294002 prevented ephrinB1-Fc-induced ERK activation in a dose-dependent manner. These data demonstrated that PI3K and PI3K crosstalk to ERK signaling mediated pain behaviors induced by activation of peripheral ephrinBs/EphBs signaling in mice.

Activation of peripheral ephrinBs/EphBs signaling induces hyperalgesia through a MAPKs-mediated mechanism in mice.

EphBs receptors and ephrinBs ligands are present in the adult brain and peripheral tissue and play a critical role in modulating multiple aspects of physiology and pathophysiology. Ours and other studies have demonstrated that spinal ephrinBs/EphBs signaling was involved in the modulation of nociceptive information and central sensitization. However, the role of ephrinBs/EphBs signaling in peripheral sensitization is poorly understood. This study shows that intraplantar (i.pl.) injection of ephrinB1-Fc produces a dose- and time-dependent thermal and mechanical hyperalgesia and the increase of spinal Fos protein expression in mice, which can be partially prevented by pre-treatment with EphB1-Fc. EphrinB1-Fc-induced hyperalgesia is accompanied with the NMDA receptor-mediated increase of expression in peripheral and spinal phosphorylated mitogen-activated protein kinases (phospho-MAPKs) including p-p38, pERK and pJNK, and also is prevented or reversed by the inhibition of peripheral and spinal MAPKs. Furthermore, in formalin inflammation pain model, pre-inhibition of EphBs receptors by the injection of EphB1-Fc reduces pain behavior, which is accompanied by the decreased expression of peripheral p-p38, pERK and pJNK. These data provide evidence that ephrinBs may act as a prominent contributor to peripheral sensitization, and demonstrate that activation of peripheral ephrinBs/EphBs system induces hyperalgesia through a MAPKs-mediated mechanism.