Activation of phosphatidylinositol 3-kinase and protein kinase B/Akt in dorsal root ganglia and spinal cord contributes to the neuropathic pain induced by spinal nerve ligation in rats

Mechanisms underlying fibronectin-induced up-regulation of P2X4R expression in microglia: distinct roles of PI3K-Akt and MEK-ERK signalling pathways

Abstract

Microglia

are resident immune cells in the central nervous system that become activated and produce pro-inflammatory and neurotrophic factors upon activation of various cell-surface receptors. The P2X₄ receptor (P2X₄R) is a sub-type of the purinergic ion-channel receptors expressed in microglia. P2X₄R expression is up-regulated under inflammatory or neurodegenerative conditions, and this up-regulation is implicated in disease pathology. However, the molecular mechanism underlying up-regulation of P2X₄R in microglia remains unknown. In the present study, we investigated the intracellular signal transduction pathway that promotes P2X₄R expression in microglia in response to fibronectin, an extracellular matrix protein that has previously been shown to stimulate P2X₄R expression. We found that in fibronectin-stimulated microglia, activation of phosphatidylinositol 3-kinase (PI3K)–Akt and mitogen-activated protein kinase kinase (MAPK kinase, MEK)–extracellular signal-regulated kinase (ERK) signalling cascades occurred divergently downstream of Src-family kinases (SFKs). Pharmacological interference of PI3K–Akt signalling inhibited fibronectin-induced P2X₄R gene expression. Activation of PI3K–Akt signalling resulted in a decrease in the protein level of the transcription factor p53 via mouse double minute 2 (MDM2), an effect that was prevented by MG-132, an inhibitor of the proteasome. In microglia pre-treated with MG-132, fibronectin failed to up-regulate P2X₄R expression. Conversely, an inhibitor of p53 caused increased expression of P2X₄R, implying a negative regulatory role of p53. On the other hand, inhibiting MEK–ERK signalling activated by fibronectin suppressed an increase in P2X₄R protein but interestingly did not affect the level of P2X₄R mRNA. We also found that fibronectin stimulation resulted in the activation of the translational factor eIF4E via MAPK-interacting protein kinase-1 (MNK1) in an MEK–ERK signalling-dependent manner, and an MNK1 inhibitor attenuated the increase in P2X₄R protein. Together, these results suggest that the PI3K–Akt and MEK–ERK signalling cascades have distinct roles in the up-regulation of P2X₄R expression in microglia at transcriptional and post-transcriptional levels, respectively.

Future treatment strategies for neuropathic pain

The prevalence of people suffering from chronic pain is extremely high and pain affects millions of people worldwide. As such, persistent pain represents a major health problem and an unmet clinical need. The reason for the high incidence of chronic pain patients is in a large part due to a paucity of effective pain control. An important reason for poor pain control is undoubtedly a deficit in our understanding of the underlying causes of chronic pain and as a consequence our arsenal of analgesic therapies is limited. However, there is considerable hope for the development of new classes of analgesic drugs by targeting novel processes contributing to clinically relevant pain. In this chapter we highlight a number of molecular species which are potential therapeutic targets for future neuropathic pain treatments. In particular, the roles of voltage-gated ion channels, neuroinflammation, protein kinases and neurotrophins are discussed in relation to the generation of neuropathic pain and how by targeting these molecules it may be possible to provide better pain control than is currently available.

(-)-Linalool attenuates allodynia in neuropathic pain induced by spinal nerve ligation in c57/bl6 mice

(-)-Linalool is a natural compound with anti-inflammatory and antinociceptive properties. The antinociceptive action of linalool has been reported in several models of inflammatory pain. However, its effects in neuropathic pain have not been investigated. Here, we used the spinal nerve ligation (SNL) model of neuropathic pain and studied the effects of acute and chronic administration of an established antinociceptive dose of linalool on mechanical and thermal sensitivity induced by the nerve injury in mice. Linalool did not affect pain behavior triggered by mechanical or thermal stimuli when administered as a single dose before SNL. However, mechanical allodynia was reduced transiently in neuropathic animals when linalool was administered for 7 consecutive days, while no changes were seen in the sensitivity to noxious radiant heat. We investigated the possible involvement of the PI3K/Akt pathway in linalool antinociceptive effect by western blot analysis. Linalool did not induce significant changes in Akt expression and phopshorylation though a trend toward an increased ratio of phosphorylated versus total Akt was observed in SNL animals treated with linalool, in comparison to SNL alone or sham. We then wondered whether linalool could modulate inflammatory processes and investigated spinal glia activation and IL-1beta contents following linalool treatment in SNL animals. The data suggest that mechanisms other than an action on inflammatory processes may mediate linalool ability to reduce mechanical allodynia in this model of neuropathic pain.

Intracellular signaling in primary sensory neurons and persistent pain

During evolution, living organisms develop a specialized apparatus called nociceptors to sense their environment and avoid hazardous situations. Intense stimulation of high threshold C- and Adelta-fibers of nociceptive primary sensory neurons will elicit pain, which is acute and protective under normal conditions. A further evolution of the early pain system results in the development of nociceptor sensitization under injury or disease conditions, leading to enhanced pain states. This sensitization in the peripheral nervous system is also called peripheral sensitization, as compared to its counterpart, central sensitization. Inflammatory mediators such as proinflammatory cytokines (TNF-alpha, IL-1beta), PGE(2), bradykinin, and NGF increase the sensitivity and excitability of nociceptors by enhancing the activity of pronociceptive receptors and ion channels (e.g., TRPV1 and Na(v)1.8). We will review the evidence demonstrating that activation of multiple intracellular signal pathways such as MAPK pathways in primary sensory neurons results in the induction and maintenance of peripheral sensitization and produces persistent pain. Targeting the critical signaling pathways in the periphery will tackle pain at the source.

Increase in reactive oxygen species and activation of Akt signaling pathway in neuropathic pain

Neuropathic pain occurs as a result of peripheral or central nervous system injury. Its pathophysiology involves mainly a central sensitization mechanism that may be correlated to many molecules acting in regions involved in pain processing, such as the spinal cord. It has been demonstrated that reactive oxygen species (ROS) and signaling molecules, such as the serine/threonine protein kinase Akt, are involved in neuropathic pain mechanisms. Thus, the aim of this study was to provide evidence of this relationship. Sciatic nerve transection (SNT) was used to induce neuropathic pain in rats. Western blot analysis of Akt and 4-hydroxy-2-nonenal (HNE)-Michael adducts, and measurement of hydrogen peroxide (H(2)O(2)) in the lumbosacral spinal cord were performed. The main findings were found seven days after SNT, when there was an increase in HNE-Michael adducts formation, total and p-Akt expression, and H(2)O(2) concentration. However, one and 15 days after SNT, H(2)O(2) concentration was raised in both sham (animals that were submitted to surgery without nerve injury) and SNT groups, showing the high sensibility of this ROS to nociceptive afferent stimuli, not only to neuropathic pain. p-Akt also increased in sham and SNT groups one day post injury, but at 3 and 7 days the increase occurred exclusively in SNT animals. Thus, there is crosstalk between intracellular signaling pathways and ROS, and these molecules can act as protective agents in acute pain situations or play a role in the development of chronic pain states.