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1.  Pain without Nociceptors? Nav1.7-Independent Pain Mechanisms 
Cell Reports  2014;6(2):301-312.
Summary
Nav1.7, a peripheral neuron voltage-gated sodium channel, is essential for pain and olfaction in mice and humans. We examined the role of Nav1.7 as well as Nav1.3, Nav1.8, and Nav1.9 in different mouse models of chronic pain. Constriction-injury-dependent neuropathic pain is abolished when Nav1.7 is deleted in sensory neurons, unlike nerve-transection-related pain, which requires the deletion of Nav1.7 in sensory and sympathetic neurons for pain relief. Sympathetic sprouting that develops in parallel with nerve-transection pain depends on the presence of Nav1.7 in sympathetic neurons. Mechanical and cold allodynia required distinct sets of neurons and different repertoires of sodium channels depending on the nerve injury model. Surprisingly, pain induced by the chemotherapeutic agent oxaliplatin and cancer-induced bone pain do not require the presence of Nav1.7 sodium channels or Nav1.8-positive nociceptors. Thus, similar pain phenotypes arise through distinct cellular and molecular mechanisms. Therefore, rational analgesic drug therapy requires patient stratification in terms of mechanisms and not just phenotype.
Graphical Abstract
Highlights
•Phenotypically identical pain models have different underlying molecular mechanisms•Nav1.7 expression is required for sympathetic sprouting after neuronal damage•Oxaliplatin and cancer-induced bone pain are both Nav1.7-independent•Deleting Nav1.7 in adult mice reverses nerve damage-induced neuropathic pain
Wood and colleagues describe two pain syndromes that occur in the absence of Nav1.7, a sodium channel considered to be essential for pain perception and olfaction in humans. They provide evidence that pain phenotypes such as cold and mechanical allodynia can arise through distinct cell and molecular mechanisms after nerve injury in mouse peripheral sensory neurons. The existence of redundant mechanistically distinct peripheral pain mechanisms may help to explain recent difficulties with the development of new analgesic drugs.
doi:10.1016/j.celrep.2013.12.033
PMCID: PMC3969273  PMID: 24440715
2.  Neurological perspectives on voltage-gated sodium channels 
Brain  2012;135(9):2585-2612.
The activity of voltage-gated sodium channels has long been linked to disorders of neuronal excitability such as epilepsy and chronic pain. Recent genetic studies have now expanded the role of sodium channels in health and disease, to include autism, migraine, multiple sclerosis, cancer as well as muscle and immune system disorders. Transgenic mouse models have proved useful in understanding the physiological role of individual sodium channels, and there has been significant progress in the development of subtype selective inhibitors of sodium channels. This review will outline the functions and roles of specific sodium channels in electrical signalling and disease, focusing on neurological aspects. We also discuss recent advances in the development of selective sodium channel inhibitors.
doi:10.1093/brain/aws225
PMCID: PMC3437034  PMID: 22961543
ion channel; genetics; pain; epilepsy; SCN1A
3.  G Protein–Coupled Receptor Kinase 6 Acts as a Critical Regulator of Cytokine-Induced Hyperalgesia by Promoting Phosphatidylinositol 3-Kinase and Inhibiting p38 Signaling 
Molecular Medicine  2012;18(1):556-564.
The molecular mechanisms determining magnitude and duration of inflammatory pain are still unclear. We assessed the contribution of G protein–coupled receptor kinase (GRK)-6 to inflammatory hyperalgesia in mice. We showed that GRK6 is a critical regulator of severity and duration of cytokine-induced hyperalgesia. In GRK6−/− mice, a significantly lower dose (100 times lower) of intraplantar interleukin (IL)-1β was sufficient to induce hyperalgesia compared with wild-type (WT) mice. In addition, IL-1β hyperalgesia lasted much longer in GRK6−/− mice than in WT mice (8 d in GRK6−/− versus 6 h in WT mice). Tumor necrosis factor (TNF)-α–induced hyperalgesia was also enhanced and prolonged in GRK6−/− mice. In vitro, IL-1β–induced p38 phosphorylation in GRK6−/− dorsal root ganglion (DRG) neurons was increased compared with WT neurons. In contrast, IL-1β only induced activation of the phosphatidylinositol (PI) 3-kinase/Akt pathway in WT neurons, but not in GRK6−/− neurons. In vivo, p38 inhibition attenuated IL-1β– and TNF-α–induced hyperalgesia in both genotypes. Notably, however, whereas PI 3-kinase inhibition enhanced and prolonged hyperalgesia in WT mice, it did not have any effect in GRK6-deficient mice. The capacity of GRK6 to regulate pain responses was also apparent in carrageenan-induced hyperalgesia, since thermal and mechanical hypersensitivity was significantly prolonged in GRK6−/− mice. Finally, GRK6 expression was reduced in DRGs of mice with chronic neuropathic or inflammatory pain. Collectively, these findings underline the potential role of GRK6 in pathological pain. We propose the novel concept that GRK6 acts as a kinase that constrains neuronal responsiveness to IL-1β and TNF-α and cytokine-induced hyperalgesia via biased cytokine-induced p38 and PI 3-kinase/Akt activation.
doi:10.2119/molmed.2011.00398
PMCID: PMC3388142  PMID: 22331028
4.  Temporal control of gene deletion in sensory ganglia using a tamoxifen-inducible Advillin-Cre-ERT2 recombinase mouse 
Molecular Pain  2011;7:100.
Background
Tissue-specific gene deletion has proved informative in the analysis of pain pathways. Advillin has been shown to be a pan-neuronal marker of spinal and cranial sensory ganglia. We generated BAC transgenic mice using the Advillin promoter to drive a tamoxifen-inducible CreERT2 recombinase construct in order to be able to delete genes in adult animals. We used a floxed stop ROSA26LacZ reporter mouse to examine functional Cre expression, and analysed the behaviour of mice expressing Cre recombinase.
Results
We used recombineering to introduce a CreERT2 cassette in place of exon 2 of the Advillin gene into a BAC clone (RPCI23-424F19) containing the 5' region of the Advillin gene. Transgenic mice were generated using pronuclear injection. The resulting AvCreERT2 transgenic mice showed a highly specific expression pattern of Cre activity after tamoxifen induction. Recombinase activity was confined to sensory neurons and no expression was found in other organs. Less than 1% of neurons showed Cre expression in the absence of tamoxifen treatment. Five-day intraperitoneal treatment with tamoxifen (2 mg per day) induced Cre recombination events in ≈90% of neurons in dorsal root and cranial ganglia. Cell counts of dorsal root ganglia (DRG) from transgenic animals with or without tamoxifen treatment showed no neuronal cell loss. Sensory neurons in culture showed ≈70% induction after 3 days treatment with tamoxifen. Behavioural tests showed no differences between wildtype, AvCreERT2 and tamoxifen-treated animals in terms of motor function, responses to light touch and noxious pressure, thermal thresholds as well as responses to inflammatory agents.
Conclusions
Our results suggest that the inducible pan-DRG AvCreERT2 deleter mouse strain is a useful tool for studying the role of individual genes in adult sensory neuron function. The pain phenotype of the Cre-induced animal is normal; therefore any alterations in pain processing can be unambiguously attributed to loss of the targeted gene.
doi:10.1186/1744-8069-7-100
PMCID: PMC3260248  PMID: 22188729
AvCreERT2; pain and nociception; tamoxifen inducible; ROSA26 LacZ reporter; behaviour; DRG
5.  Distinct Nav1.7-dependent pain sensations require different sets of sensory and sympathetic neurons 
Nature Communications  2012;3:791-.
Human acute and inflammatory pain requires the expression of voltage-gated sodium channel Nav1.7 but its significance for neuropathic pain is unknown. Here we show that Nav1.7 expression in different sets of mouse sensory and sympathetic neurons underlies distinct types of pain sensation. Ablating Nav1.7 gene (SCN9A) expression in all sensory neurons using Advillin-Cre abolishes mechanical pain, inflammatory pain and reflex withdrawal responses to heat. In contrast, heat-evoked pain is retained when SCN9A is deleted only in Nav1.8-positive nociceptors. Surprisingly, responses to the hotplate test, as well as neuropathic pain, are unaffected when SCN9A is deleted in all sensory neurons. However, deleting SCN9A in both sensory and sympathetic neurons abolishes these pain sensations and recapitulates the pain-free phenotype seen in humans with SCN9A loss-of-function mutations. These observations demonstrate an important role for Nav1.7 in sympathetic neurons in neuropathic pain, and provide possible insights into the mechanisms that underlie gain-of-function Nav1.7-dependent pain conditions.
Sodium channel Nav1.7 is essential for acute human pain but its role in chronic neuropathic pain is unclear. Minett and colleagues show that Nav1.7 expression specifically in sympathetic neurons, rather than sensory neurons, is required for the development of chronic neuropathic pain after injury.
doi:10.1038/ncomms1795
PMCID: PMC3337979  PMID: 22531176

Results 1-5 (5)