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Previous studies suggested that reactive oxygen species (ROS) produced by NADPH oxidase 4 (Nox4) affect the processing of neuropathic pain. However, mechanisms underlying Nox4-dependent pain signaling are incompletely understood. In this study, we aimed to identify novel Nox4 downstream interactors in the nociceptive system. Mice lacking Nox4 specifically in sensory neurons were generated by crossing Advillin-Cre mice with Nox4fl/fl mice. Tissue-specific deletion of Nox4 in sensory neurons considerably reduced mechanical hypersensitivity and neuronal action potential firing after peripheral nerve injury. Using a proteomic approach, we detected various proteins that are regulated in a Nox4-dependent manner after injury, including the small calcium-binding protein S100A4. Immunofluorescence staining and Western blot experiments confirmed that S100A4 expression is massively up-regulated in peripheral nerves and dorsal root ganglia after injury. Furthermore, mice lacking S100A4 showed increased mechanical hypersensitivity after peripheral nerve injury and after delivery of a ROS donor. Our findings suggest that S100A4 expression is up-regulated after peripheral nerve injury in a Nox4-dependent manner and that deletion of S100A4 leads to an increased neuropathic pain hypersensitivity.

Background Cardiac accessory β-subunits are part of macromolecular Nav1.5 channel complexes modulating biophysical properties and contributing to arrhythmias. Recent studies demonstrated the structural interaction between β-subunits of Na+ (Nav1.5) and K+ (Kv4.3) channels. Here, we identified the dipeptidyl peptidase-like protein-10 (DPP10), which is known to modulate Kv4.3-current kinetics, as a new regulator of Nav1.5 channels. Methods We assessed DPP10 expression in the healthy and diseased human heart and we studied the functional effects of DPP10 on the Na+ current in isolated rat cardiomyocytes expressing DPP10 after adenoviral gene-transfer (DPP10ad). Results DPP10 mRNA and proteins were detected in human ventricle, with higher levels in patients with heart failure. In rat cardiomyocytes, DPP10ad significantly reduced upstroke velocity of action potentials indicating reduction in Na+-current density. DPP10 significantly shifted the voltage-dependent Na+ channel activation and inactivation curve to more positive potentials, resulting in greater availability of Na+ channels for activation, along with increasing window Na+ current. In addition, time-to-peak Na+ current was reduced, whereas time course of recovery from inactivation was significantly accelerated by DPP10ad. DPP10 co-immunoprecipitated with Nav1.5 channels in human ventricles, confirming their physical interaction. Conclusion We provide first evidence that DPP10 interacts with Nav1.5 channels, linking Na+- and K+-channel complexes in the heart. Our data suggest that increased ventricular DPP10 expression in heart failure might promote arrhythmias by decreasing peak Na+ current, while increasing window Na+ current and channel re-openings due to accelerated recovery from inactivation.

Previous studies suggest that adenosine A1 receptors (A1R) modulate the processing of pain. The aim of this study was to characterize the distribution of A1R in nociceptive tissues and to evaluate whether targeting A1R with the partial agonist capadenoson may reduce neuropathic pain in mice. The cellular distribution of A1R in dorsal root ganglia (DRG) and the spinal cord was analyzed using fluorescent in situ hybridization. In behavioral experiments, neuropathic pain was induced by spared nerve injury or intraperitoneal injection of paclitaxel, and tactile hypersensitivities were determined using a dynamic plantar aesthesiometer. Whole-cell patch-clamp recordings were performed to assess electrophysiological properties of dissociated DRG neurons. We found A1R to be expressed in populations of DRG neurons and dorsal horn neurons involved in the processing of pain. However, administration of capadenoson at established in vivo doses (0.03–1.0 mg/kg) did not alter mechanical hypersensitivity in the spared nerve injury and paclitaxel models of neuropathic pain, whereas the standard analgesic pregabalin significantly inhibited the pain behavior. Moreover, capadenoson failed to affect potassium currents in DRG neurons, in contrast to a full A1R agonist. Despite expression of A1R in nociceptive neurons, our data do not support the hypothesis that pharmacological intervention with partial A1R agonists might be a valuable approach for the treatment of neuropathic pain.

Cyclic nucleotide-gated (CNG) channels, which are directly activated by cAMP and cGMP, have long been known to play a key role in retinal and olfactory signal transduction. Emerging evidence indicates that CNG channels are also involved in signaling pathways important for pain processing. Here, we found that the expression of the channel subunits CNGA2, CNGA3, CNGA4 and CNGB1 in dorsal root ganglia, and of CNGA2 in the spinal cord, is transiently altered after peripheral nerve injury in mice. Specifically, we show using in situ hybridization and quantitative real-time RT-PCR that CNG channels containing the CNGB1b subunit are localized to populations of sensory neurons and predominantly excitatory interneurons in the spinal dorsal horn. In CNGB1 knockout (CNGB1−/−) mice, neuropathic pain behavior is considerably attenuated whereas inflammatory pain behavior is normal. Finally, we provide evidence to support CNGB1 as a downstream mediator of cAMP signaling in pain pathways. Altogether, our data suggest that CNGB1-positive CNG channels specifically contribute to neuropathic pain processing after peripheral nerve injury.