The sensation of cold or heat depends on the activation of specific nerve endings in the skin. This involves heat- and cold-sensitive excitatory transient receptor potential (TRP) channels. However, we show here that the mechano-gated and highly temperature-sensitive potassium channels of the TREK/TRAAK family, which normally work as silencers of the excitatory channels, are also implicated. They are important for the definition of temperature thresholds and temperature ranges in which excitation of nociceptor takes place and for the intensity of excitation when it occurs. They are expressed with thermo-TRP channels in sensory neurons. TRAAK and TREK-1 channels control pain produced by mechanical stimulation and both heat and cold pain perception in mice. Expression of TRAAK alone or in association with TREK-1 controls heat responses of both capsaicin-sensitive and capsaicin-insensitive sensory neurons. Together TREK-1 and TRAAK channels are important regulators of nociceptor activation by cold, particularly in the nociceptor population that is not activated by menthol.

Cold hypersensitivity is the hallmark of oxaliplatin-induced neuropathy, which develops in nearly all patients under this chemotherapy. To date, pain management strategies have failed to alleviate these symptoms, hence development of adapted analgesics is needed. Here, we report that oxaliplatin exaggerates cold perception in mice as well as in patients. These symptoms are mediated by primary afferent sensory neurons expressing the thermoreceptor TRPM8. Mechanistically, oxaliplatin promotes over-excitability by drastically lowering the expression of distinct potassium channels (TREK1, TRAAK) and by increasing the expression of pro-excitatory channels such as the hyperpolarization-activated channels (HCNs). These findings are corroborated by the analysis of TREK1-TRAAK null mice and use of the specific HCN inhibitor ivabradine, which abolishes the oxaliplatin-induced cold hypersensibility. These results suggest that oxaliplatin exacerbates cold perception by modulating the transcription of distinct ionic conductances that together shape sensory neuron responses to cold. The translational and clinical implication of these findings would be that ivabradine may represent a tailored treatment for oxaliplatin-induced neuropathy.

Two-pore domain background K(+) channels (K2p or KCNK) produce hyperpolarizing currents that control cell membrane polarity and neuronal excitability throughout the nervous system. The TREK2 channel as well as the related TREK1 and TRAAK channels are mechanical-, thermal- and lipid-gated channels that share many regulatory properties. TREK2 is one of the major background channels expressed in rodent nociceptive neurons of the dorsal root ganglia that innervate the skin and deep body tissues, but its role in somatosensory perception and nociception has remained poorly understood. We now report that TREK2 is a regulatory channel that controls the perception of non aversive warm, between 40°C and 46°C, and moderate ambient cool temperatures, between 20°C and 25°C, in mice. TREK2 controls the firing activity of peripheral sensory C-fibers in response to changes in temperature. The role of TREK2 in thermosensation is different from that of TREK1 and TRAAK channels; rather, TREK2, TREK1, and TRAAK channels appear to have complementary roles in thermosensation. TREK2 is also involved in mechanical pain perception and in osmotic pain after sensitization by prostaglandin E2. TREK2 is involved in the cold allodynia that characterizes the neuropathy commonly associated with treatments with the anticancer drug oxaliplatin. These results suggest that positive modulation of the TREK2 channel may have beneficial analgesic effects in these neuropathic conditions.

Recent findings indicate that in addition to its hyperlipemic effect, a high fructose diet has a pro-oxidant effect in rats compared with a starch-based diet. Oligofructose (OFS) has already been shown to decrease plasma lipids in rats. We assessed the impact of fructose on oxidative stress by supplementing a high fructose diet with OFS. Rats were fed either a high fructose diet or a starch-based diet, with or without supplementation of 10 g/100 g oligofructose for 4 wk. Regardless of the type of carbohydrate, OFS in the diet produced an enlargement of the cecum and led to a significant increase in the SCFA cecum pool. Fructose feeding was associated with significantly higher insulin plasma concentrations (+63%) in the control groups, whereas insulin plasma concentrations did not differ in rats fed the fructose diet supplemented with OFS. Plasma leptin concentration was significantly lower (approximately 50%) in the OFS-supplemented fructose group compared with the other three groups. Fructose feeding in rats also significantly increased plasma (P < 0.001) and liver (P < 0.001) triglyceride (TG) concentrations and the addition of OFS prevented the TG accumulation induced by fructose in the liver (P < 0.05) and hyperlipemia (P < 0.05). OFS consumption prevented (P < 0.05) the lower plasma vitamin E/TG ratio in rats fed the fructose diet. Control rats fed the fructose diet had high plasma TBARS values compared with rats fed the starch diet, whereas TBARS values remained unchanged when rats were supplemented with OFS. Control rats fed the fructose diet had higher TBARS urine values and higher heart tissue susceptibility to peroxidation compared with rats fed the starch diet, and this effect was significantly reduced by OFS consumption. Further studies are required to identify the mechanisms underlying the protective effect of OFS against the pro-oxidant effect of fructose. However, the potential nutritional benefits of OFS supplementation in fructose-rich diets are suggested.

Background and purpose: Recent evidence indicates that carbon monoxide-releasing molecules (CO-RMs) exhibit potential anti-inflammatory properties. In the present study, we have investigated whether tricarbonyl dichloro ruthenium(II) dimer (CORM-2) can control the inflammatory response induced by cytokines in a human colonic epithelial cell line, Caco-2. Experimental approach: Caco-2 cells were preincubated with CORM-2 for 30 minutes and then stimulated with interleukin (IL)-1b, tumor necrosis factor-a and interferon-g for different times. Gene expression was analyzed by real-time PCR. Protein expression was investigated by Western blot and ELISA. Transcription factor activation was determined by the luciferase method. Key results: We have shown that CORM-2 significantly decreased the mRNA expression of nitric oxide synthase-2 (NOS-2) and the production of nitrite, in Caco-2 cells stimulated with cytokines. IL-8, IL-6 and metalloproteinase-7 (MMP-7) mRNA and protein were also significantly reduced by CORM-2. Time-course and small interfering RNA studies suggest that inhibition of IL-6 plays a role in the regulation of MMP-7 expression by CORM-2. These effects of CORM-2 can be dependent on the modulation of nuclear factor-kB (NF-kB), activator protein-1, CCAT/enhancer binding protein and the phosphorylated forms of NF-kB inhibitory protein-a, c-Jun N-terminal protein kinase 1/2, p38 and extracellular signal-regulated kinase 1/2. Conclusions and Implications: CORM-2 can regulate a number of genes relevant in intestinal inflammation and cancer progression. These findings provide new insights into the anti-inflammatory properties and potential applications of this class of compounds.

Recent findings indicate that a high fructose diet has a prooxidant effect in rats compared with a starch diet. Because honey is rich in fructose, the aim of this study was to assess the effect of substituting honey for refined carbohydrates on lipid metabolism and oxidative stress. Rats were fed for 2 wk purified diets containing 65 g/100 g carbohydrates as wheat starch or a combination of fructose and glucose or a honey-based diet prepared by substituting honey for refined carbohydrates (n = 9/group). The same amount of fructose was provided by the honey and fructose diets. The hypertriglyceridemic effect of fructose was not observed when fructose was provided by honey. Compared with those fed starch, fructose-fed rats had a lower plasma alpha-tocopherol level, higher plasma nitrite and nitrate (NOx) levels and were less protected from lipid peroxidation as indicated by heart homogenate TBARS concentration. Compared with those fed fructose, honey-fed rats had a higher plasma alpha-tocopherol level, a higher alpha-tocopherol/triacylglycerol ratio, lower plasma NOx concentrations and a lower susceptibility of heart to lipid peroxidation. Further studies are required to identify the mechanism underlying the antioxidant effect of honey but the data suggest a potential nutritional benefit of substituting honey for fructose in the diet.

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Inflammation is known to be responsible for the sensitization of peripheral sensory neurons, leading to spontaneous pain and invalidating pain hypersensitivity. Given its role in regulating neuronal excitability, the voltage-gated Nav1.9 channel is a potential target for the treatment of pathological pain, but its implication in inflammatory pain is yet not fully described. In the present study, we examined the role of the Nav1.9 channel in acute, subacute and chronic inflammatory pain using Nav1.9-null mice and Nav1.9 knock-down rats. In mice we found that, although the Nav1.9 channel does not contribute to basal pain thresholds, it plays an important role in heat pain hypersensitivity induced by subacute paw inflammation (intraplantar carrageenan) and chronic ankle inflammation (complete Freund's adjuvant-induced monoarthritis). We showed for the first time that Nav1.9 also contributes to mechanical hypersensitivity in both models, as assessed using von Frey and dynamic weight bearing tests. Consistently, antisense-based Nav1.9 gene silencing in rats reduced carrageenan-induced heat and mechanical pain hypersensitivity. While no changes in Nav1.9 mRNA levels were detected in dorsal root ganglia (DRGs) during subacute and chronic inflammation, a significant increase in Nav1.9 immunoreactivity was observed in ipsilateral DRGs 24 hours following carrageenan injection. This was correlated with an increase in Nav1.9 immunolabeling in nerve fibers surrounding the inflamed area. No change in Nav1.9 current density could be detected in the soma of retrolabeled DRG neurons innervating inflamed tissues, suggesting that newly produced channels may be non-functional at this level and rather contribute to the observed increase in axonal transport. Our results provide evidence that Nav1.9 plays a crucial role in the generation of heat and mechanical pain hypersensitivity, both in subacute and chronic inflammatory pain models, and bring new elements for the understanding of its regulation in those models.