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.

Painful diabetic neuropathy is one of the most common complications of insulin-dependent diabetes in man. Conflicting results have been obtained in experimentally diabetic animals subjected to pain stimuli. This work aimed to systematically study the response of rats made diabetic (hyperglycemia > or = 14 mM) by injection of streptozocin (STZ) (75 mg/kg, i.p.), to various pain stimuli: mechanical, thermal (warm and cold) and chemical. The time course of the scores was followed for 4 weeks simultaneously with the clinical symptoms (weight, body and skin temperature, motility) and hyperglycemia. A decrease in reaction thresholds to noxious heat stimuli (44 degrees C and 46 degrees C) and to non-painful thermal (cold: 10 degrees C, and warm: 38-42 degrees C) and mechanical stimulation (paw pressure) was observed. This can be considered as evidence for hyperalgesia and allodynia, respectively. These troubles appeared gradually and required at least 2 weeks of diabetes to reach statistical significance. Four weeks after the induction of diabetes, the scores obtained in diabetic rats injected with formalin were greater than those in normal rats, indicating hyperalgesia. Variation in sensitivity to pain occurred at the same time as arrested weight increase, fall in skin temperature, some amyotrophy measured in terms of hind-paw volume, and the usual polyuria-polydipsia syndrome. Spontaneous motor activity of the rats was lowered. This model is thus of interest as the observed reactions to noxious and non-noxious stimuli correspond to hyperalgesia and allodynia, symptoms encountered in painful diabetic neuropathy in man. Operating conditions for this model are discussed.

Molecular mechanisms underlying diabetes-induced painful neuropathy are poorly understood. We have demonstrated, in rats with streptozotocin-induced diabetes, that mechanical hyperalgesia, a common symptom of diabetic neuropathy, was correlated with an early increase in extracellular signal-regulated protein kinase (ERK), p38, and c-Jun N-terminal kinase (JNK) phosphorylation in the spinal cord and dorsal root ganglion at 3 weeks after induction of diabetes. This change was specific to hyperalgesia because nonhyperalgesic rats failed to have such an increase. Immunoblot analysis showed no variation of protein levels, suggesting a post-translational regulation of the corresponding kinases. In diabetic hyperalgesic rats, immunocytochemistry revealed that all phosphorylated mitogen-activated protein kinases (MAPKs) colocalized with both the neuronal (NeuN) and microglial (OX42) cell-specific markers but not with the astrocyte marker [glial fibrillary acidic protein (GFAP)] in the superficial dorsal horn-laminae of the spinal cord. In these same rats, a 7-day administration [5 g/rat/day, intrathecal (i.t.)] of 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580), and anthra(1,9-cd)pyrazol-6(2H)-one (SP600125), which inhibited MAPK kinase, p38, and JNK, respectively, suppressed mechanical hyperalgesia, and decreased phosphorylation of the kinases. To characterize the cellular events upstream of MAPKs, we have examined the role of the NMDA receptor known to be implicated in pain hypersensitivity. The prolonged blockade of this receptor during 7 days by (5R,10S)-(ϩ)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]-cyclohepten-5-10-imine hydrogen maleate (MK801; 5 g/rat/day, i.t.), a noncompetitive NMDA receptor antagonist, reversed hyperalgesia developed by diabetic rats and blocked phosphorylation of all MAPKs. These results demonstrate for the first time that NMDA receptor-dependent phosphorylation of MAPKs in spinal cord neurons and microglia contribute to the establishment and longterm maintenance of painful diabetic hyperalgesia and that these kinases represent potential targets for pain therapy.Sensitive peripheral neuropathies represent a common and debilitating complication of diabetes (types 1 and 2) and affect an increasing proportion of diabetic patients as the disease progresses. Even though antidepressant and antiepileptic agents have been shown to be partially effective, clinical studies have reported the difficulty of managing pain caused by these neuropathies (Sindrup and Jensen, 1999), which may be due to changes in peripheral nerve or to neuronal processing (e.g., central sensitization in the spinal cord) (Woolf and Salter, 2000). Whereas the pathophysiological and neuroanatomical changes associated with the development of neuropathic pain are well documented, the molecular and cellular mechanisms underlying its initiation and maintenance remain poorly understood.The mitogen-activated protein kinase (MAPK...

The streptozocin-induced diabetic rat has been put forward as a model of chronic pain with signs of hyperalgesia and allodynia that may reflect signs observed in diabetic humans. The aim of this work was to assess, in streptozocin-induced diabetic rats, the pharmacological activity to several analgesic drugs known to be effective (clomipramine, amitriptyline, desipramine, clonidine, lidocaine), ineffective (aspirin), or with a doubtful effectiveness (morphine) in human painful diabetic neuropathy. The animals were submitted to a mechanical pain test (paw pressure) and the ability of the drugs to reverse diabetes-induced hyperalgesia was tested. The tested antidepressants (0.125-8 mg/kg, i.v.) were slightly effective in diabetic rats; amitriptyline and clomipramine induced a weak effect, whereas desipramine was more active, suggesting noradrenergic specificity. This was confirmed by the effectiveness of clonidine (50, 100, 150 micrograms/kg, s.c.). Lidocaine (1-9 mg/kg, i.v.) had prolonged efficacy on mechanical hyperalgesia. Aspirin (100 mg/kg, i.v.) was without effect and morphine (0.5-4 mg/kg, i.v.) induced a dose-dependent antinociceptive effect but at doses twice as high as those used in normal rats. These results demonstrate the high pharmacological predictivity of this model of painful diabetes and suggest that in this pathological condition, among the drugs acting on monoaminergic transmission, noradrenergic drugs seem the most active.

Neuropathic pain is a common diabetic complication affecting 8-16% of diabetic patients. It is characterized by aberrant symptoms of spontaneous and stimulus-evoked pain including hyperalgesia and allodynia. Magnesium (Mg) deficiency has been proposed as a factor in the pathogenesis of diabetes-related complications, including neuropathy. In the central nervous system, Mg is also a voltage-dependant blocker of the N -methyl-d-aspartate receptor channels involved in abnormal processing of sensory information. We hypothesized that Mg deficiency might contribute to the development of neuropathic pain and the worsening of clinical and biological signs of diabetes and consequently, that Mg administration could prevent or improve its complications. We examined the effects of oral Mg supplementation (296 mg l −1 in drinking water for 3 weeks) on the development of neuropathic pain and on biological and clinical parameters of diabetes in streptozocin (STZ)-induced diabetic rats. STZ administration induced typical symptoms of type 1 diabetes. The diabetic rats also displayed mechanical hypersensitivity and tactile and thermal allodynia. The level of phosphorylated NMDA receptor NR1 subunit (pNR1) was higher in the spinal dorsal horn of diabetic hyperalgesic/allodynic rats. Magnesium supplementation failed to reduce hyperglycaemia, polyphagia and hypermagnesiuria, or to restore intracellular Mg levels and body growth, but increased insulinaemia and reduced polydipsia. Moreover, it abolished thermal and tactile allodynia, delayed the development of mechanical hypersensitivity, and prevented the increase in spinal cord dorsal horn pNR1. Thus, neuropathic pain symptoms can be attenuated by targeting the Mg-mediated blockade of NMDA receptors, offering new therapeutic opportunities for the management of chronic neuropathic pain.

Antidepressants are one of the first-line treatments for neuropathic pain. Despite the influence of serotonin (5-hydroxytryptamine, 5-HT) in pain modulation, selective serotonin reuptake inhibitors (SSRIs) are less effective than tricyclic antidepressants. Here, we show, in diabetic neuropathic rats, an alteration of the antihyperalgesic effect induced by stimulation of 5-HT(2A) receptors, which are known to mediate SSRI-induced analgesia. 5-HT(2A) receptor density was not changed in the spinal cord of diabetic rats, whereas postsynaptic density protein-95 (PSD-95), one of the PSD-95/disc large suppressor/zonula occludens-1 (PDZ) domain containing proteins interacting with these receptors, was upregulated. Intrathecal injection of a cell-penetrating peptidyl mimetic of the 5-HT(2A) receptor C-terminus, which disrupts 5-HT(2A) receptor-PDZ protein interactions, induced an antihyperalgesic effect in diabetic rats, which results from activation of 5-HT(2A) receptors by endogenous 5-HT. The peptide also enhanced antihyperalgesia induced by the SSRI fluoxetine. Its effects likely resulted from an increase in receptor responsiveness, because it revealed functional 5-HT(2A) receptor-operated Ca(2+) responses in neurons, an effect mimicked by knockdown of PSD-95. Hence, 5-HT(2A) receptor/PDZ protein interactions might contribute to the resistance to SSRI-induced analgesia in painful diabetic neuropathy. Disruption of these interactions might be a valuable strategy to design novel treatments for neuropathic pain and to increase the effectiveness of SSRIs.

Nociceptin/orphanin FQ (noci/OFQ), the endogenous ligand for the orphan ORL1 (opioid receptor-like1), has been shown to be anti- or pronociceptive and modify morphine analgesia in rats after central administration. We comparatively examined the effect of noci/OFQ on hyperalgesia and morphine analgesia in two experimental models of neuropathic pain: diabetic (D) and mononeuropathic (MN) rats. Noci/OFQ, when intrathecally (i.t.) injected (0.1, 0.3, or 1, to 10 microg/rat) was ineffective in normal rats, but reduced and suppressed mechanical hyperalgesia (paw-pressure test) in D and MN rats, respectively. This spinal inhibitory effect was suppressed by naloxone (10 microg/rat, i.t.) in both models. Combinations of systemic morphine with spinal noci/OFQ resulted in a strong potentiation of analgesia in D rats. In MN rats, an isobolographic analysis showed that the morphine+noci/OFQ association (i.t.) suppressed mechanical hyperalgesia in a superadditive manner. In summary, the present findings reveal that spinal noci/OFQ produces a differential antinociception in diabetic and traumatic neuropathic pain according to the etiology of neuropathy, an effect possibly mediated by opioid receptors. Moreover, noci/OFQ combined with morphine produces antinociceptive synergy in experimental neuropathy, opening new opportunities in the treatment of neuropathic pain.

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