Phosphodiesterase 2A localized in the spinal cord contributes to inflammatory pain processing.

BACKGROUND: Phosphodiesterase 2A (PDE2A) is an evolutionarily conserved enzyme that catalyzes the degradation of the cyclic nucleotides, cyclic adenosine monophosphate, and/or cyclic guanosine monophosphate. Recent studies reported the expression of PDE2A in the dorsal horn of the spinal cord, pointing to a potential contribution to the processing of pain. However, the functions of PDE2A in spinal pain processing in vivo remained elusive. METHODS: Immunohistochemistry, laser microdissection, and quantitative real-time reverse transcription polymerase chain reaction experiments were performed to characterize the localization and regulation of PDE2A protein and messenger RNA in the mouse spinal cord. Effects of the selective PDE2A inhibitor, BAY 60-7550 (Cayman Chemical, Ann Arbor, MI), in animal models of inflammatory pain (n = 6 to 10), neuropathic pain (n = 5 to 6), and after intrathecal injection of cyclic nucleotides (n = 6 to 8) were examined. Also, cyclic adenosine monophosphate and cyclic guanosine monophosphate levels in spinal cord tissues were measured by liquid chromatography tandem mass spectrometry. RESULTS: The authors here demonstrate that PDE2A is distinctly expressed in neurons of the superficial dorsal horn of the spinal cord, and that its spinal expression is upregulated in response to hind paw inflammation. Administration of the selective PDE2A inhibitor, BAY 60-7550, increased the nociceptive behavior of mice in animal models of inflammatory pain. Moreover, BAY 60-7550 increased the pain hypersensitivity induced by intrathecal delivery of cyclic adenosine monophosphate, but not of cyclic guanosine monophosphate, and it increased the cyclic adenosine monophosphate levels in spinal cord tissues. CONCLUSION: Our findings indicate that PDE2A contributes to the processing of inflammatory pain in the spinal cord.

Rab7-a novel redox target that modulates inflammatory pain processing.

Chronic pain is accompanied by production of reactive oxygen species (ROS) in various cells that are important for nociceptive processing. Recent data indicate that ROS can trigger specific redox-dependent signaling processes, but the molecular targets of ROS signaling in the nociceptive system remain largely elusive. Here, we performed a proteome screen for pain-dependent redox regulation using an OxICAT approach, thereby identifying the small GTPase Rab7 as a redox-modified target during inflammatory pain in mice. Prevention of Rab7 oxidation by replacement of the redox-sensing thiols modulates its GTPase activity. Immunofluorescence studies revealed Rab7 expression to be enriched in central terminals of sensory neurons. Knockout mice lacking Rab7 in sensory neurons showed normal responses to noxious thermal and mechanical stimuli; however, their pain behavior during inflammatory pain and in response to ROS donors was reduced. The data suggest that redox-dependent changes in Rab7 activity modulate inflammatory pain sensitivity.

Oxidant-induced activation of cGMP-dependent protein kinase Iα mediates neuropathic pain after peripheral nerve injury.

AIMS: Emerging lines of evidence indicate that oxidants such as hydrogen peroxide exert specific signaling functions during the processing of chronic pain. However, the mechanisms by which oxidants regulate pain processing in vivo remain poorly understood. Here, we investigated whether cyclic guanosine monophosphate (cGMP)-dependent protein kinase Iα (cGKIα), which can be activated by oxidants independently of cGMP, serves as a primary redox target during pain processing. RESULTS: After peripheral nerve injury, oxidant-induced cGKIα activation is increased in dorsal root ganglia of mice. Knock-in (KI) mice in which cGKIα cannot transduce oxidant signals demonstrated reduced neuropathic pain behaviors after peripheral nerve injury, and reduced pain behaviors after intrathecal delivery of oxidants. In contrast, acute nociceptive, inflammatory, and cGMP-induced pain behaviors were not impaired in these mice. INNOVATION: Studying cGKIα KI mice, we provide the first evidence that oxidants activate cGKIα in sensory neurons after peripheral nerve injury in vivo. CONCLUSION: Our results suggest that oxidant-induced activation of cGKIα specifically contributes to neuropathic pain processing, and that prevention of cGKIα redox activation could be a potential novel strategy to manage neuropathic pain.

Nox2-dependent signaling between macrophages and sensory neurons contributes to neuropathic pain hypersensitivity.

Emerging lines of evidence indicate that production of reactive oxygen species (ROS) at distinct sites of the nociceptive system contributes to the processing of neuropathic pain. However, the mechanisms underlying ROS production during neuropathic pain processing are not fully understood. We here detected the ROS-generating nicotinamide adenine dinucleotide phosphate oxidase isoform Nox2 in macrophages of dorsal root ganglia (DRG) in mice. In response to peripheral nerve injury, Nox2-positive macrophages were recruited to DRG, and ROS production was increased in a Nox2-dependent manner. Nox2-deficient mice displayed reduced neuropathic pain behavior after peripheral nerve injury, whereas their immediate responses to noxious stimuli were normal. Moreover, injury-induced upregulation of tumor necrosis factor α was absent, and activating transcription factor 3 induction was reduced in DRG of Nox2-deficient mice, suggesting an attenuated macrophage-neuron signaling. These data suggest that Nox2-dependent ROS production in macrophages recruited to DRG contributes to neuropathic pain hypersensitivity, underlining the observation that Nox-derived ROS exert specific functions during the processing of pain.