Rab27a Contributes to the Processing of Inflammatory Pain in Mice.

Tissue injury and inflammation may result in chronic pain, a severe debilitating disease that is associated with great impairment of quality of life. An increasing body of evidence indicates that members of the Rab family of small GTPases contribute to pain processing; however, their specific functions remain poorly understood. Here, we found using immunofluorescence staining and in situ hybridization that the small GTPase Rab27a is highly expressed in sensory neurons and in the superficial dorsal horn of the spinal cord of mice. Rab27a mutant mice, which carry a single-nucleotide missense mutation of Rab27a leading to the expression of a nonfunctional protein, show reduced mechanical hyperalgesia and spontaneous pain behavior in inflammatory pain models, while their responses to acute noxious mechanical and thermal stimuli is not affected. Our study uncovers a previously unrecognized function of Rab27a in the processing of persistent inflammatory pain in mice.

The H2S-producing enzyme CSE is dispensable for the processing of inflammatory and neuropathic pain.

Accumulating lines of evidence indicate that hydrogen sulfide (H2S) contributes to the processing of chronic pain. However, the sources of H2S production in the nociceptive system are poorly understood. Here we investigated the expression of the H2S releasing enzyme cystathionine γ-lyase (CSE) in the nociceptive system and characterized its role in chronic pain signaling using CSE deficient mice. We show that paw inflammation and peripheral nerve injury led to upregulation of CSE expression in dorsal root ganglia. However, conditional knockout mice lacking CSE in sensory neurons as well as global CSE knockout mice demonstrated normal pain behaviors in inflammatory and neuropathic pain models as compared to WT littermates. Thus, our results suggest that CSE is not critically involved in chronic pain signaling in mice and that sources different from CSE mediate the pain relevant effects of H2S.

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.

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.

Antioxidant activity of sestrin 2 controls neuropathic pain after peripheral nerve injury.

AIMS: Neuropathic pain is a chronic debilitating disease that is often unresponsive to currently available treatments. Emerging lines of evidence indicate that reactive oxygen species (ROS) are required for the development and maintenance of neuropathic pain. However, little is known about endogenous mechanisms that neutralize the pain-relevant effects of ROS. In the present study, we tested whether the stress-responsive antioxidant protein Sestrin 2 (Sesn2) blocks the ROS-induced neuropathic pain processing in vivo. RESULTS: We observed that Sesn2 mRNA and protein expression was up-regulated in peripheral nerves after spared nerve injury, a well-characterized model of neuropathic pain. Sesn2 knockout (Sesn2(-/-)) mice exhibited considerably increased late-phase neuropathic pain behavior, while their behavior in acute nociceptive and in inflammatory pain models remained unaffected. The exacerbated neuropathic pain behavior of Sesn2(-/-) mice was associated with elevated ROS levels and an enhanced activating transcription factor 3 up-regulation in sensory neurons, and it was reversed by the ROS scavenger N-tert-Butyl-α-phenylnitrone. In contrast, administration of the ROS donor tert-butyl hydroperoxide induced a prolonged pain behavior in naive Sesn2(-/-) mice. INNOVATION: We show that the antioxidant function of Sesn2 limits neuropathic pain processing in vivo. CONCLUSION: Sesn2 controls ROS-dependent neuropathic pain signaling after peripheral nerve injury and may, thus, provide a potential new target for the clinical management of chronic neuropathic pain conditions.