SAFit2 ameliorates paclitaxel-induced neuropathic pain by reducing spinal gliosis and elevating pro-resolving lipid mediators.

BACKGROUND: Chemotherapy-induced neuropathic pain (CIPN) describes a pathological pain state that occurs dose-dependently as a side effect and can limit or even impede an effective cancer therapy. Unfortunately, current treatment possibilities for CIPN are remarkably confined and mostly inadequate as CIPN therapeutics themselves consist of low effectiveness and may induce severe side effects, pointing out CIPN as pathological entity with an emerging need for novel treatment targets. Here, we investigated whether the novel and highly specific FKBP51 inhibitor SAFit2 reduces paclitaxel-induced neuropathic pain. METHODS: In this study, we used a well-established multiple low-dose paclitaxel model to investigate analgesic and anti-inflammatory properties of SAFit2. For this purpose, the behavior of the mice was recorded over 14 days and the mouse tissue was then analyzed using biochemical methods. RESULTS: Here, we show that SAFit2 is capable to reduce paclitaxel-induced mechanical hypersensitivity in mice. In addition, we detected that SAFit2 shifts lipid levels in nervous tissue toward an anti-inflammatory and pro-resolving lipid profile that counteracts peripheral sensitization after paclitaxel treatment. Furthermore, SAFit2 reduced the activation of astrocytes and microglia in the spinal cord as well as the levels of pain-mediating chemokines. Its treatment also increased anti-inflammatory cytokines levels in neuronal tissues, ultimately leading to a resolution of neuroinflammation. CONCLUSIONS: In summary, SAFit2 shows antihyperalgesic properties as it ameliorates paclitaxel-induced neuropathic pain by reducing peripheral sensitization and resolving neuroinflammation. Therefore, we consider SAFit2 as a potential novel drug candidate for the treatment of paclitaxel-induced neuropathic pain.

SAFit2 reduces neuroinflammation and ameliorates nerve injury-induced neuropathic pain.

BACKGROUND: Neuropathic pain is experienced worldwide by patients suffering from nerve injuries, infectious or metabolic diseases or chemotherapy. However, the treatment options are still limited because of low efficacy and sometimes severe side effects. Recently, the deficiency of FKBP51 was shown to relieve chronic pain, revealing FKBP51 as a potential therapeutic target. However, a specific and potent FKBP51 inhibitor was not available until recently which hampered targeting of FKBP51. METHODS: In this study, we used the well-established and robust spared nerve injury model to analyze the effect of SAFit2 on nerve injury-induced neuropathic pain and to elucidate its pharmacodynamics profile. Therefore, the mice were treated with 10 mg/kg SAFit2 after surgery, the mice behavior was assessed over 21 days and biochemical analysis were performed after 14 and 21 days. Furthermore, the impact of SAFit2 on sensory neurons and macrophages was investigated in vitro. RESULTS: Here, we show that the FKBP51 inhibitor SAFit2 ameliorates nerve injury-induced neuropathic pain in vivo by reducing neuroinflammation. SAFit2 reduces the infiltration of immune cells into neuronal tissue and counteracts the increased NF-κB pathway activation which leads to reduced cytokine and chemokine levels in the DRGs and spinal cord. In addition, SAFit2 desensitizes the pain-relevant TRPV1 channel and subsequently reduces the release of pro-inflammatory neuropeptides from sensory neurons. CONCLUSIONS: SAFit2 ameliorates neuroinflammation and counteracts enhanced neuronal activity after nerve injury leading to an amelioration of nerve injury-induced neuropathic pain. Based on these findings, SAFit2 constitutes as a novel and promising drug candidate for the treatment of nerve injury-induced neuropathic pain.

The FKBP51 Inhibitor SAFit2 Restores the Pain-Relieving C16 Dihydroceramide after Nerve Injury.

Neuropathic pain is a pathological pain state with a broad symptom scope that affects patients after nerve injuries, but it can also arise after infections or exposure to toxic substances. Current treatment possibilities are still limited because of the low efficacy and severe adverse effects of available therapeutics, highlighting an emerging need for novel analgesics and for a detailed understanding of the pathophysiological alterations in the onset and maintenance of neuropathic pain. Here, we show that the novel and highly specific FKBP51 inhibitor SAFit2 restores lipid signaling and metabolism in nervous tissue after nerve injury. More specifically, we identify that SAFit2 restores the levels of the C16 dihydroceramide, which significantly reduces the sensitization of the pain-mediating TRPV1 channel and subsequently the secretion of the pro-inflammatory neuropeptide CGRP in primary sensory neurons. Furthermore, we show that the C16 dihydroceramide is capable of reducing acute thermal hypersensitivity in a capsaicin mouse model. In conclusion, we report for the first time the C16 dihydroceramide as a novel and crucial lipid mediator in the context of neuropathic pain as it has analgesic properties, contributing to the pain-relieving properties of SAFit2.

Lysophospholipids Contribute to Oxaliplatin-Induced Acute Peripheral Pain.

Oxaliplatin, a platinum-based chemotherapeutic drug, which is used as first-line treatment for some types of colorectal carcinoma, causes peripheral neuropathic pain in patients. In addition, an acute peripheral pain syndrome develop in almost 90% of patients immediately after oxaliplatin treatment, which is poorly understood mechanistically but correlates with incidence and severity of the later-occurring neuropathy. Here we investigated the effects of acute oxaliplatin treatment in a murine model, showing that male and female mice develop mechanical hypersensitivity 24 h after oxaliplatin treatment. Interestingly, we found that the levels of several lipids were significantly altered in nervous tissue during oxaliplatin-induced acute pain. Specifically, the linoleic acid metabolite 9,10-EpOME (epoxide of linoleic acid) as well as the lysophospholipids lysophosphatidylcholine (LPC) 18:1 and LPC 16:0 were significantly increased 24 h after oxaliplatin treatment in sciatic nerve, DRGs, or spinal cord tissue as revealed by untargeted and targeted lipidomics. In contrast, inflammatory markers including cytokines and chemokines, ROS markers, and growth factors are unchanged in the respective nervous system tissues. Importantly, LPC 18:1 and LPC 16:0 can induce Ca2+ transients in primary sensory neurons, and we identify LPC 18:1 as a previously unknown endogenous activator of the ligand-gated calcium channels transient receptor potential V1 and M8 (transient receptor potential vanilloid 1 and transient receptor potential melastatin 8) in primary sensory neurons using both pharmacological inhibition and genetic knockout. Additionally, a peripheral LPC 18:1 injection was sufficient to induce mechanical hypersensitivity in naive mice. Hence, targeting signaling lipid pathways may ameliorate oxaliplatin-induced acute peripheral pain and the subsequent long-lasting neuropathy.SIGNIFICANCE STATEMENT The first-line cytostatic drug oxaliplatin can cause acute peripheral pain and chronic neuropathic pain. The former is causally connected with the chronic neuropathic pain, but its mechanisms are poorly understood. Here, we performed a broad unbiased analysis of cytokines, chemokines, growth factors, and ∼200 lipids in nervous system tissues 24 h after oxaliplatin treatment, which revealed a crucial role of lysophospholipids lysophosphatidylcholine (LPC) 18:1, LPC 16:0, and 9,10-EpOME in oxaliplatin-induced acute pain. We demonstrate for the first time that LPC 18:1 contributes to the activation of the ion channels transient receptor potential vanilloid 1 and transient receptor potential melastatin 8 in sensory neurons and causes mechanical hypersensitivity after peripheral injection in vivo These findings suggest that the LPC-mediated lipid signaling is involved in oxaliplatin-induced acute peripheral pain.

Oxaliplatin Causes Transient Changes in TRPM8 Channel Activity.

Oxaliplatin is a third-generation platinum-based anticancer drug that is widely used as first-line treatment for colorectal carcinoma. Patients treated with oxaliplatin develop an acute peripheral pain several hours after treatment, mostly characterized by cold allodynia as well as a long-term chronic neuropathy. These two phenomena seem to be causally connected. However, the underlying mechanisms that trigger the acute peripheral pain are still poorly understood. Here we show that the activity of the transient receptor potential melastatin 8 (TRPM8) channel but not the activity of any other member of the TRP channel family is transiently increased 1 h after oxaliplatin treatment and decreased 24 h after oxaliplatin treatment. Mechanistically, this is connected with activation of the phospholipase C (PLC) pathway and depletion of phosphatidylinositol 4,5-bisphosphate (PIP2) after oxaliplatin treatment. Inhibition of the PLC pathway can reverse the decreased TRPM8 activity as well as the decreased PIP2-concentrations after oxaliplatin treatment. In summary, these results point out transient changes in TRPM8 activity early after oxaliplatin treatment and a later occurring TRPM8 channel desensitization in primary sensory neurons. These mechanisms may explain the transient cold allodynia after oxaliplatin treatment and highlight an important role of TRPM8 in oxaliplatin-induced acute and neuropathic pain.

Reduced association between dendritic cells and corneal sub-basal nerve fibers in patients with fibromyalgia syndrome.

In our study, we aimed at investigating corneal langerhans cells (LC) in patients with fibromyalgia syndrome (FMS) and small fiber neuropathy (SFN) as potential contributors to corneal small fiber pathology. We enrolled women with FMS (n = 134) and SFN (n = 41) who underwent neurological examination, neurophysiology, prostaglandin analysis in tear fluid, and corneal confocal microscopy (CCM). Data were compared with those of 60 age-matched female controls. After screening for dry eye disease, corneal LC were counted and sub-classified as dendritic (dLC) and non-dendritic (ndLC) cells with or without nerve fiber association. We further analyzed corneal nerve fiber density (CNFD), length (CNFL), and branch density (CNBD). Neurological examination indicated deficits of small fiber function in patients with SFN. Nerve conduction studies were normal in all participants. Dry eye disease was more prevalent in FMS (17%) and SFN (28%) patients than in controls (5%). Tear fluid prostaglandin levels did not differ between FMS patients and controls. While corneal LC density in FMS and SFN patients was not different from controls, there were fewer dLC in association with nerve fibers in FMS and SFN patients than in controls (P < .01 each). Compared to controls, CNFL was lower in FMS and SFN patients (P < .05 each), CNFD was lower only in FMS patients (P < .05), and CNBD was lower only in SFN patients (P < .001). There was no difference in any CCM parameter between patients with and without dry eyes. Our data indicate changes in corneal innervation and LC distribution in FMS and SFN, potentially based on altered LC signaling.

Targeting CYP2J to reduce paclitaxel-induced peripheral neuropathic pain.

Chemotherapy-induced peripheral neuropathic pain (CIPNP) is a severe dose- and therapy-limiting side effect of widely used cytostatics that is particularly difficult to treat. Here, we report increased expression of the cytochrome-P450-epoxygenase CYP2J6 and increased concentrations of its linoleic acid metabolite 9,10-EpOME (9,10-epoxy-12Z-octadecenoic acid) in dorsal root ganglia (DRGs) of paclitaxel-treated mice as a model of CIPNP. The lipid sensitizes TRPV1 ion channels in primary sensory neurons and causes increased frequency of spontaneous excitatory postsynaptic currents in spinal cord nociceptive neurons, increased CGRP release from sciatic nerves and DRGs, and a reduction in mechanical and thermal pain hypersensitivity. In a drug repurposing screen targeting CYP2J2, the human ortholog of murine CYP2J6, we identified telmisartan, a widely used angiotensin II receptor antagonist, as a potent inhibitor. In a translational approach, administration of telmisartan reduces EpOME concentrations in DRGs and in plasma and reverses mechanical hypersensitivity in paclitaxel-treated mice. We therefore suggest inhibition of CYP2J isoforms with telmisartan as a treatment option for paclitaxel-induced neuropathic pain.

The leukotriene B4 receptors BLT1 and BLT2 form an antagonistic sensitizing system in peripheral sensory neurons.

Sensitization of the heat-activated ion channel transient receptor potential vanilloid 1 (TRPV1) through lipids is a fundamental mechanism during inflammation-induced peripheral sensitization. Leukotriene B4 is a proinflammatory lipid mediator whose role in peripheral nociceptive sensitization is not well understood to date. Two major G-protein-coupled receptors for leukotriene B4 have been identified: the high-affinity receptor BLT1 and the low-affinity receptor BLT2. Transcriptional screening for the expression G-protein-coupled receptors in murine dorsal root ganglia showed that both receptors were among the highest expressed in dorsal root ganglia. Calcium imaging revealed a sensitization of TRPV1-mediated calcium increases in a relative narrow concentration range for leukotriene B4 (100-200 nm). Selective antagonists and neurons from knock-out mice demonstrated a BLT1-dependent sensitization of TRPV1-mediated calcium increases. Accordingly, leukotriene B4-induced thermal hyperalgesia was mediated through BLT1 and TRPV1 as shown using the respective knock-out mice. Importantly, higher leukotriene B4 concentrations (>0.5 µm) and BLT2 agonists abolished sensitization of the TRPV1-mediated calcium increases. Also, BLT2 activation inhibited protein kinase C- and protein kinase A-mediated sensitization processes through the phosphatase calcineurin. Consequently, a selective BLT2-receptor agonist increased thermal and mechanical withdrawal thresholds during zymosan-induced inflammation. In accordance with these data, immunohistochemical analysis showed that both leukotriene B4 receptors were expressed in peripheral sensory neurons. Thus, the data show that the two leukotriene B4 receptors have opposing roles in the sensitization of peripheral sensory neurons forming a self-restricting system.

The oxidized linoleic acid metabolite 12,13-DiHOME mediates thermal hyperalgesia during inflammatory pain.

Eicosanoids play a crucial role in inflammatory pain. However, there is very little knowledge about the contribution of oxidized linoleic acid metabolites in inflammatory pain and peripheral sensitization. Here, we identify 12,13-dihydroxy-9Z-octadecenoic acid (12,13-DiHOME), a cytochrome P450-derived linoleic acid metabolite, as crucial mediator of thermal hyperalgesia during inflammatory pain. We found 12,13-DiHOME in increased concentrations in peripheral nervous tissue during acute zymosan- and complete Freund's Adjuvant-induced inflammatory pain. 12,13-DiHOME causes calcium transients in sensory neurons and sensitizes the transient receptor potential vanilloid 1 (TRPV1)-mediated intracellular calcium increases via protein kinase C, subsequently leading to enhanced TRPV1-dependent CGRP-release from sensory neurons. Peripheral injection of 12,13-DiHOME in vivo causes TRPV1-dependent thermal pain hypersensitivity. Finally, application of the soluble epoxide hydrolase (sEH)-inhibitor TPPU reduces 12,13-DiHOME concentrations in nervous tissue and reduces zymosan- and CFA-induced thermal hyperalgesia in vivo. In conclusion, we identify a novel role for the lipid mediator 12,13-DiHOME in mediating thermal hyperalgesia during inflammatory pain and propose a novel mechanism that may explain the antihyperalgesic effects of sEH inhibitors in vivo.

Phenotyping the function of TRPV1-expressing sensory neurons by targeted axonal silencing.

Specific somatosensations may be processed by different subsets of primary afferents. C-fibers expressing heat-sensitive TRPV1 channels are proposed, for example, to be heat but not mechanical pain detectors. To phenotype in rats the sensory function of TRPV1(+) afferents, we rapidly and selectively silenced only their activity, by introducing the membrane-impermeant sodium channel blocker QX-314 into these axons via the TRPV1 channel pore. Using tandem mass spectrometry we show that upon activation with capsaicin, QX-314 selectively accumulates in the cytosol only of TRPV1-expressing cells, and not in control cells. Exposure to QX-314 and capsaicin induces in small DRG neurons a robust sodium current block within 30 s. In sciatic nerves, application of extracellular QX-314 with capsaicin persistently reduces C-fiber but not A-fiber compound action potentials and this effect does not occur in TRPV1(-/-) mice. Behavioral phenotyping after selectively silencing TRPV1(+) sciatic nerve axons by perineural injections of QX-314 and capsaicin reveals deficits in heat and mechanical pressure but not pinprick or light touch perception. The response to intraplantar capsaicin is substantially reduced, as expected. During inflammation, silencing TRPV1(+) axons abolishes heat, mechanical, and cold hyperalgesia but tactile and cold allodynia remain following peripheral nerve injury. These results indicate that TRPV1-expressing sensory neurons process particular thermal and mechanical somatosensations, and that the sensory channels activated by mechanical and cold stimuli to produce pain in naive/inflamed rats differ from those in animals after peripheral nerve injury.

Inhibition of TRPM8 function by prostacyclin receptor agonists requires coupling to Gq/11 proteins.

BACKGROUND AND PURPOSE: The TRPM8 ion channel is involved in innocuous cold sensing and has a potent anti-inflammatory action. Its activation by lower temperature or chemical agonists such as menthol and icilin induces analgesic effects, reversing hypersensitivity and reducing chronic pain. On the other hand, prostacyclin (PGI2) enhances pain and inflammation by activating the IP receptors. Due to the critical roles of TRPM8 and IP receptors in the regulation of inflammatory pain, and considering their overlapping expression pattern, we analysed the functional interaction between human TRPM8 and IP receptors. EXPERIMENTAL APPROACH: We transiently expressed human TRPM8 channels and IP receptors in HEK293T cells and carried out intracellular calcium and cAMP measurements. Additionally, we cultured neurons from the dorsal root ganglia (DRGs) of mice and determined the increase in intracellular calcium triggered by the TRPM8 agonist, icilin, in the presence of the IP receptor agonist cicaprost, the IP receptor antagonist Cay10441, and the Gq/11 inhibitor YM254890. KEY RESULTS: Activation of IP receptors by selective agonists (cicaprost, beraprost, and iloprost) inhibited TRPM8 channel function, independently of the Gs-cAMP pathway. The potent inhibition of TRPM8 channels by IP receptor agonists involved Gq/11 coupling. These effects were also observed in neurons isolated from murine DRGs. CONCLUSIONS AND IMPLICATIONS: Our results demonstrate an unusual signalling pathway of IP receptors by coupling to Gq/11 proteins to inhibit TRPM8 channel function. This pathway may contribute to a better understanding of the role of TRPM8 channels and IP receptors in regulating pain and inflammation.

Ocular surface changes in mice with streptozotocin-induced diabetes and diabetic polyneuropathy.

PURPOSE: Diabetes mellitus (DM) is a leading risk factor for corneal neuropathy and dry eye disease (DED). Another common consequence of DM is diabetic peripheral polyneuropathy (DPN). Both complications affect around 50 % of the DM patients but the relationship between DM, DED and DPN remains unclear. METHODS: In this study, we examined mice with early onset of DM and PN after streptozotocin (STZ)-induced diabetes (DPN). We compared the early morphological changes of the sciatic nerve, dorsal root and trigeminal ganglia with the changes in the ocular surface, including tear proteomic and we also investigated respective changes in the gene expressions and morphological alterations in the eye tissues involved in tear production. RESULTS: The lacrimal gland, conjunctival goblet cells and cornea showed morphological changes along with alterations in tear proteins without any obvious signs of ocular surface inflammation. The gene expression for respectively altered tear proteins i.e., of Clusterin in cornea, Car6, Adh3a1, and Eef1a1 in eyelids, and Pigr in the lacrimal gland also showed significant changes compared to control mice. In the trigeminal ganglia like in the dorsal root ganglia neuronal cells showed swollen mitochondria and, in the latter, there was a significant increase of NADPH oxidases and MMP9 suggestive of oxidative and neuronal stress. In the dorsal root ganglia and the sciatic nerve, there was an upregulation of a number of pro-inflammatory cytokines and pain-mediating chemokines. CONCLUSION: The early ocular changes in DM Mice only affect the lacrimal gland. Which, is reflected in the tear film composition of DPN mice. Due to the high protein concentration in tear fluid in humans, proteomic analysis in addition to noninvasive investigation of goblet cells and cornea can serve as a tools for the early diagnosis of DPN, DED in clinical practice. Early treatment could delay or even prevent the ocular complications of DM such as DED and PN.

Development of a Potent and Selective G2A (GPR132) Agonist.

G protein-coupled receptor G2A was postulated to be a promising target for the development of new therapeutics in neuropathic pain, acute myeloid leukemia, and inflammation. However, there is still a lack of potent, selective, and drug-like G2A agonists to be used as a chemical tool or as the starting matter for the development of drugs. In this work, we present the discovery and structure-activity relationship elucidation of a new potent and selective G2A agonist scaffold. Systematic optimization resulted in (3-(pyridin-3-ylmethoxy)benzoyl)-d-phenylalanine (T-10418) exhibiting higher potency than the reference and natural ligand 9-HODE and high selectivity among G protein-coupled receptors. With its favorable activity, a clean selectivity profile, excellent solubility, and high metabolic stability, T-10418 qualifies as a pharmacological tool to investigate the effects of G2A activation.

5,6-EET is released upon neuronal activity and induces mechanical pain hypersensitivity via TRPA1 on central afferent terminals.

Epoxyeicosatrienoic acids (EETs) are cytochrome P450-epoxygenase-derived metabolites of arachidonic acid that act as endogenous signaling molecules in multiple biological systems. Here we have investigated the specific contribution of 5,6-EET to transient receptor potential (TRP) channel activation in nociceptor neurons and its consequence for nociceptive processing. We found that, during capsaicin-induced nociception, 5,6-EET levels increased in dorsal root ganglia (DRGs) and the dorsal spinal cord, and 5,6-EET is released from activated sensory neurons in vitro. 5,6-EET potently induced a calcium flux (100 nm) in cultured DRG neurons that was completely abolished when TRPA1 was deleted or inhibited. In spinal cord slices, 5,6-EET dose dependently enhanced the frequency, but not the amplitude, of spontaneous EPSCs (sEPSCs) in lamina II neurons that also responded to mustard oil (allyl isothiocyanate), indicating a presynaptic action. Furthermore, 5,6-EET-induced enhancement of sEPSC frequency was abolished in TRPA1-null mice, suggesting that 5,6-EET presynaptically facilitated spinal cord synaptic transmission by TRPA1. Finally, in vivo intrathecal injection of 5,6-EET caused mechanical allodynia in wild-type but not TRPA1-null mice. We conclude that 5,6-EET is synthesized on the acute activation of nociceptors and can produce mechanical hypersensitivity via TRPA1 at central afferent terminals in the spinal cord.

Rethinking the use of NSAIDs in early acute pain.

Non-steroidal anti-inflammatory drugs (NSAIDs) are the most widely used analgesics to treat inflammatory pain. Despite their efficacy, recent studies show that NSAID use in early acute pain can prolong pain and inflammation and delay their resolution. We suggest using analgesics without inflammation-related properties in early acute pain instead of NSAIDs.

Structure-Based Discovery of a New Selectivity-Enabling Motif for the FK506-Binding Protein 51.

In recent years, the selective inhibition of FKBP51 has emerged as a possible treatment for chronic pain, obesity-induced diabetes, or depression. All currently known advanced FKBP51-selective inhibitors, including the widely used SAFit2, contain a cyclohexyl residue as a key motif for enabling selectivity over the closest homologue and anti-target FKBP52. During a structure-based SAR exploration, we surprisingly discovered thiophenes as highly efficient cyclohexyl replacement moieties that retain the strong selectivity of SAFit-type inhibitors for FKBP51 over FKBP52. Cocrystal structures revealed that the thiophene-containing moieties enable selectivity by stabilizing a flipped-out conformation of Phe67 of FKBP51. Our best compound, 19b, potently binds to FKBP51 biochemically as well as in mammalian cells, desensitize TRPV1 in primary sensory neurons, and has an acceptable PK profile in mice, suggesting its use as a novel tool compound for studying FKBP51 in animal models of neuropathic pain.

Overdose of the histamine H3 inverse agonist pitolisant increases thermal pain thresholds.

OBJECTIVE AND DESIGN: Pitolisant (BF2.649) is a selective inverse agonist for the histamine H(3) receptor and was developed for the treatment of excessive daytime sleepiness in Parkinson disease, narcolepsy, and schizophrenia. Since H(3)-ligands can decrease inflammatory pain, we tested Pitolisant in inflammatory and neuropathic pain models. MATERIALS AND TREATMENTS: Behavioral effects of pitolisant and the structural different H(3) receptor inverse agonists ciproxifan and ST-889 were tested in zymosan-induced inflammation and the spared nerve injury model for neuropathic pain. METHODS: Responses to mechanical and thermal stimuli were determined. Calcium imaging was performed with primary neuronal cultures of dorsal root ganglions. RESULTS: Clinically relevant doses of pitolisant (10 mg/kg) had no relevant effect on mechanical or thermal pain thresholds in all animal models. Higher doses (50 mg/kg) dramatically increased thermal but not mechanical pain thresholds. Neither ciproxifan nor ST-889 altered thermal pain thresholds. In peripheral sensory neurons high concentrations of pitolisant (30-500 µM), but not ciproxifan, partially inhibited calcium increases induced by capsaicin, a selective activator of transient receptor potential vanilloid receptor 1 (TRPV1). High doses of pitolisant induced a strong hypothermia. CONCLUSION: The data show a dramatic effect of high dosages of pitolisant on the thermosensory system, which appears to be H(3) receptor-independent.

Drug Repurposing to Target Neuroinflammation and Sensory Neuron-Dependent Pain.

Around 20% of the American population have chronic pain and estimates in other Western countries report similar numbers. This represents a major challenge for global health care systems. Additional problems for the treatment of chronic and persistent pain are the comparably low efficacy of existing therapies, the failure to translate effects observed in preclinical pain models to human patients and related setbacks in clinical trials from previous attempts to develop novel analgesics. Drug repurposing offers an alternative approach to identify novel analgesics as it can bypass various steps of classical drug development. In recent years, several approved drugs were attributed analgesic properties. Here, we review available data and discuss recent findings suggesting that the approved drugs minocycline, fingolimod, pioglitazone, nilotinib, telmisartan, and others, which were originally developed for the treatment of different pathologies, can have analgesic, antihyperalgesic, or neuroprotective effects in preclinical and clinical models of inflammatory or neuropathic pain. For our analysis, we subdivide the drugs into substances that can target neuroinflammation or substances that can act on peripheral sensory neurons, and highlight the proposed mechanisms. Finally, we discuss the merits and challenges of drug repurposing for the development of novel analgesics.

Oxidized linoleic acid metabolites maintain mechanical and thermal hypersensitivity during sub-chronic inflammatory pain.

Inflammatory pain serves as a protective defense mechanism which becomes pathological when it turns into chronic inflammatory pain. This transition is mediated by a variety of peripheral mediators that sensitize nociceptors and increase pain perception in sensory neurons. Besides cytokines, chemokines and growth factors, accumulating evidence shows that oxidized lipids, such as eicosanoids and oxidized linoleic acid metabolites, contribute to this sensitization process. Most notably, the oxidized linoleic acid metabolite and partial TRPV1 agonist 9-HODE (hydroxyoctadecadienoic acid) was shown to be involved in this sensitization process. However, it is still unknown how some of the oxidized linoleic acid metabolites are synthesized in the inflammatory environment and in which phase of inflammation they become relevant. Here we show that the concentrations of oxidized linoleic acid metabolites, especially 9-HODE and 13-HODE, are significantly increased in inflamed paw tissue and the corresponding dorsal root ganglia in the sub-chronic phase of inflammation. Surprisingly, classical inflammatory lipid markers, such as prostaglandins were at basal levels in this phase of inflammation. Moreover, we revealed the cell type specific synthesis pathways of oxidized linoleic acid metabolites in primary macrophages, primary neutrophils and dorsal root ganglia. Finally, we show that blocking the most elevated metabolites 9-HODE and 13-HODE at the site of inflammation in the sub-chronic phase of inflammation, leads to a significant relief of mechanical and thermal hypersensitivity in vivo. In summary, these data offer an approach to specifically target oxidized linoleic acid metabolites in the transition of acute inflammatory pain to chronic inflammatory pain.

Slack Potassium Channels Modulate TRPA1-Mediated Nociception in Sensory Neurons.

The transient receptor potential (TRP) ankyrin type 1 (TRPA1) channel is highly expressed in a subset of sensory neurons where it acts as an essential detector of painful stimuli. However, the mechanisms that control the activity of sensory neurons upon TRPA1 activation remain poorly understood. Here, using in situ hybridization and immunostaining, we found TRPA1 to be extensively co-localized with the potassium channel Slack (KNa1.1, Slo2.2, or Kcnt1) in sensory neurons. Mice lacking Slack globally (Slack-/-) or conditionally in sensory neurons (SNS-Slack-/-) demonstrated increased pain behavior after intraplantar injection of the TRPA1 activator allyl isothiocyanate. By contrast, pain behavior induced by the TRP vanilloid 1 (TRPV1) activator capsaicin was normal in Slack-deficient mice. Patch-clamp recordings in sensory neurons and in a HEK cell line transfected with TRPA1 and Slack revealed that Slack-dependent potassium currents (IKS) are modulated in a TRPA1-dependent manner. Taken together, our findings highlight Slack as a modulator of TRPA1-mediated, but not TRPV1-mediated, activation of sensory neurons.