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Aims: The present study assessed the functions of the transcription factor hypoxia-inducible factor (HIF) in sensory neurons in models of acute, inflammatory, ischemic, and neuropathic pain. The alpha subunit, HIF1α, was specifically deleted in neurons of the dorsal root ganglia by mating HIF1αfl/fl mice with SNScre mice. Results: SNS-HIF1α−/− mice were more sensitive to noxious heat and cold pain stimulation than were HIF1αfl/fl control mice. They also showed heightened first-phase nociceptive responses in the formalin and capsaicin tests with increased numbers of cFos-positive neurons in the dorsal horn, and intensified hyperalgesia in early phases after paw inflammation and hind limb ischemia/reperfusion. The behavioral cold and heat pain hypersensitivity was explained by increased calcium fluxes after transient receptor potential channel activation in primary sensory neurons of SNS-HIF1α−/− mice and lowered electrical activation thresholds of sensory fibers. SNS-HIF1α−/− mice however, developed less neuropathic pain after sciatic nerve injury, which was associated with an abrogation of HIF1-mediated gene up-regulation. Innovation: The results suggest that HIF1α is protective in terms of acute heat and cold pain but in case of ongoing activation in injured neurons, it may promote the development of neuropathic pain. Conclusion: The duality of HIF1 in pain regulation may have an impact on the side effects of drugs targeting HIF1, which are being developed, for example, as anticancer agents. Specifically, in patients with cancer neuropathy, however, temporary HIF1 inhibition might provide a welcome combination of growth and pain reduction. Antioxid. Redox Signal. 20, 2555-2571. [ABSTRACT FROM AUTHOR]

The biolipid sphingosine-1-phosphate (S1P) is an essential modulator of innate immunity, cell migration, and wound healing. It is released locally upon acute tissue injury from endothelial cells and activated thrombocytes and, therefore, may give rise to acute post-traumatic pain sensation via a yet elusive molecular mechanism. We have used an interdisciplinary approach to address this question, and we find that intradermal injection of S1P induced significant licking and flinching behavior in wild-type mice and a dose-dependent flare reaction in human skin as a sign of acute activation of nociceptive nerve terminals. Notably, S1P evoked a small excitatory ionic current that resulted in nociceptor depolarization and action potential firing. This ionic current was preserved in "cation-free" solution and blocked by the nonspecific Cl- channel inhibitor niflumic acid and by preincubation with the G-protein inhibitor GDP-β-S. Notably, S1P3 receptor was detected in virtually all neurons in human and mouse DRG. In line with this finding, S1P-induced neuronal responses and spontaneous pain behavior in vivo were substantially reduced in S1P3-/- mice, whereas in control S1P1 floxed (S1P1fl/fl) mice and mice with a nociceptor-specific deletion of S1P1-/- receptor (SNS-S1P1-/-), neither the S1P-induced responses in vitro nor the S1P-evoked pain-like behavior was altered. Therefore, these findings indicate that S1P evokes significant nociception via G-protein-dependent activation of an excitatory Cl- conductance that is largely mediated by S1P3 receptors present in nociceptors, and point to these receptors as valuable therapeutic targets for post-traumatic pain. [ABSTRACT FROM AUTHOR]

To provide a tool to investigate the mechanisms inducing and maintaining cancer-related pain and hyperalgesia, a soft tissue tumor/ metastasis model was developed that is applicable in C57BL/6J wild-type and transgenic mice. We show that the experimental tumor/induced heat hyperalgesia and nociceptor sensitization were prevented by systemic treatment with the tumor necrosis factor α (TNFα) antagonist etanercept. In naive mice, exogenous TNF α evoked heat hyperalgesia in vivo and sensitized nociceptive nerve fibers to heat in vitro. TNFα enhanced the expression of the nociceptor-specific heat transducer ion channel transient receptor potential vanilloid 1 (TRPV1) and increased the amplitudes of capsaicin and heat-activated ionic currents via p38/MAP (mitogen -activated protein) kinase and PKC (protein kinase C). Deletion of the tumor necrosis factor receptor type 2 (TNFR2) gene attenuated heat hyperalgesia and prevented TRPV1 upregulation in tumor-bearing mice, whereas TNFR1 gene deletion played a minor role. We propose endogenous TNFα as a key player in cancer-related heat hyperalgesia and nociceptor sensitization that generates TRPV1 upregulation and sensitization via TNFR2. [ABSTRACT FROM AUTHOR]

Abstract: Peripheral nerve injuries result in pronounced alterations in dorsal root ganglia, which can lead to the development of neuropathic pain. Although the polymodal mechanosensitive transient receptor potential ankyrin 1 (TRPA1) ion channel is emerging as a relevant target for potential analgesic therapies, preclinical studies do not provide unequivocal mechanistic insight into its relevance for neuropathic pain pathogenesis. By using a transgenic mouse model with a conditional depletion of the interleukin-6 (IL-6) signal transducer gp130 in Nav1.8 expressing neurons (SNS-gp130-/-), we provide a mechanistic regulatory link between IL-6/gp130 and TRPA1 in the spared nerve injury (SNI) model. Spared nerve injury mice developed profound mechanical hypersensitivity as indicated by decreased withdrawal thresholds in the von Frey behavioral test in vivo, as well as a significant increase in mechanosensitivity of unmyelinated nociceptive primary afferents in ex vivo skin-nerve recordings. In contrast to wild type and control gp130fl/fl animals, SNS-gp130-/- mice did not develop mechanical hypersensitivity after SNI and exhibited low levels of Trpa1 mRNA in sensory neurons, which were partially restored by adenoviral gp130 re-expression in vitro. Importantly, uninjured but not injured neurons developed increased responsiveness to the TRPA1 agonist cinnamaldehyde, and neurons derived from SNS-gp130-/- mice after SNI were significantly less responsive to cinnamaldehyde. Our study shows for the first time that TRPA1 upregulation is attributed specifically to uninjured neurons in the SNI model, and this depended on the IL-6 signal transducer gp130. We provide a solution to the enigma of TRPA1 regulation after nerve injury and stress its significance as an important target for neuropathic pain disorders. [ABSTRACT FROM AUTHOR]