Neuropathic Allodynia Research Articles

Growth Arrest and DNA-damage-inducible Protein 45β-mediated DNA Demethylation of Voltage-dependent T-type Calcium Channel 3.2 Subunit Enhances Neuropathic Allodynia after Nerve Injury in Rats.

Growth arrest and DNA-damage-inducible protein 45β reactivates methylation-silenced neural plasticity-associated genes through DNA demethylation. However, growth arrest and DNA-damage-inducible protein 45β-dependent demethylation contributes to neuropathic allodynia-associated spinal plasticity remains unclear. Adult male Sprague-Dawley rats (654 out of 659) received a spinal nerve ligation or a sham operation with or without intrathecal application of one of the following: growth arrest and DNA-damage-inducible protein 45β messenger RNA-targeted small interfering RNA, lentiviral vector expressing growth arrest and DNA-damage-inducible protein 45β, Ro 25-6981 (an NR2B-bearing N-methyl-D-aspartate receptor antagonist), or KN-93 (a calmodulin-dependent protein kinase II antagonist) were used for behavioral measurements, Western blotting, immunofluorescence, dot blots, detection of unmodified cytosine enrichment at cytosine-phosphate-guanine site, chromatin immunoprecipitation quantitative polymerase chain reaction analysis, and slice recordings. Nerve ligation-enhanced growth arrest and DNA-damage-inducible protein 45β expression (n = 6) in ipsilateral dorsal horn neurons accompanied with behavioral allodynia (n = 7). Focal knockdown of growth arrest and DNA-damage-inducible protein 45β expression attenuated ligation-induced allodynia (n = 7) by reducing the binding of growth arrest and DNA-damage-inducible protein 45β to the voltage-dependent T-type calcium channel 3.2 subunit promoter (n = 6) that decreased expression of and current mediated by the voltage-dependent T-type calcium channel 3.2 subunit (both n = 6). In addition, NR2B-bearing N-methyl-D-aspartate receptors and calmodulin-dependent protein kinase II act in an upstream cascade to increase growth arrest and DNA-damage-inducible protein 45β expression, hence enhancing demethylation at the voltage-dependent T-type calcium channel 3.2 subunit promoter and up-regulating voltage-dependent T-type calcium channel 3.2 subunit expression. Intrathecal administration of Ro 25-6981, KN-93, or a growth arrest and DNA-damage-inducible protein 45β-targeting small interfering RNA (n = 6) reversed the ligation-induced enrichment of unmodified cytosine at the voltage-dependent T-type calcium channel 3.2 subunit promoter by increasing the associated 5-formylcytosine and 5-carboxylcytosine levels. By converting 5-formylcytosine or 5-carboxylcytosine to unmodified cytosine, the NR2B-bearing N-methyl-D-aspartate receptor, calmodulin-dependent protein kinase II, or growth arrest and DNA-damage-inducible protein 45β pathway facilitates voltage-dependent T-type calcium channel 3.2 subunit gene demethylation to mediate neuropathic allodynia.

Tianeptine Reduces Mechanical Allodynia in Spinal Nerve-ligated and Chemotherapyinduced Neuropathic Mice

Background: Spinal nerve-ligated neuropathy and chemotherapy-induced neuropathy produce a persistent tactile allodynia in mice. Tianeptine is an antidepressant that exhibits structural similarities to tricyclic antidepressants but has distinct neurochemical properties. Objective: Here we examined the effects of intraperitoneal (i.p.) tianeptine on allodynia in spinal nerve-ligated and chemotherapy-induced neuropathic mice. Study Design: A randomized, experimental trial. Setting: Laboratory animal study. Methods: Spinal nerve-ligated neuropathy was induced in a Chung model made by ligating the L5 spinal nerve. Chemotherapy-induced neuropathy was induced by injecting vincristine (0.1 mg/kg/day; i.p.) on the following schedule: 5 days on, 2 days off, for14 days. Tianeptine (10, 30, and 50 mg/kg) and saline were administered, respectively, to both groups of neuropathic mice (n = 5 for each group). We evaluated mechanical allodynia using von Frey hairs prior to drug injections and at 30, 60, 90, 120, 180, and 240 minutes, and 24 hours after injections. We also measured the changes in activate transcription factor 3 (ATF3) level in the dorsal root ganglion (DRG) in each group in order to understand the analgesic mechanism of tianeptine. Results: Both spinal nerve-ligated and chemotherapy-induced neuropathic mice showed prominent allodynia. The control group showed no differences in mechanically induced allodynia compared to the experimental groups. For the tianeptine groups, paw-withdrawal thresholds in response to mechanical stimuli were significantly lower than the pre-administration values and values from the control group (P < 0.05). The increase in DRG ATF3 in neuropathic mice was reduced by tianeptine (P < 0.05). Limitations: Less is known about the transcription factors that affect inflammation signaling. Conclusions: Tianeptine administered i.p. reduces mechanical allodynia in spinal nerveligated and chemotherapy-induced neuropathic mice models. These effects were confirmed by attenuation of previously increased DRG ATF3. Key words: Tianeptine, spinal-nerve ligation, chemotherapy-induced neuropathic, activating transcription factor 3

Chronic Social Stress Time-Dependently Affects Neuropathic Pain-Related Cold Allodynia and Leads to Altered Expression of Spinal Biochemical Mediators.

Clinical data have shown that chronic exposure to stress may be accompanied by an enhancement of inflammation-related pain sensitivity. In this context, little is however known on the impact of stress on neuropathic pain. In the present study we addressed this issue by combining the chronic constriction injury (CCI) model with an ongoing social stress (OSS) paradigm. Cold plate and von Frey tests were performed in 48 rats divided into four groups: OSS exposed to OSS, CCI subjected to chronic nerve constriction, OSS+CCI with a combination of neuropathy and stress and CON, a control group lacking any manipulation. While we did not observe any stress-related differences in mechanical sensitivity throughout the observation period, CCI rats were more sensitive to cold stimulation than OSS+CCI in the initial phase of neuropathy. A switch was observed at a later stage, leading to a hypersensitivity of the OSS+CCI compared to the CCI rats. At this time point we investigated the spinal mRNA expression of neuron and glia related molecules potentially involved in neuropathic pain and stress. The combination of psychosocial stress and neuropathic pain seemed to enhance glial cell activation, pro-inflammatory cytokine and neurotrophic factor mRNA levels, rather than glutamatergic transmission. Our data show that long lasting social stress may lead to time-dependent alteration of neuropathy-related cold pain sensitivity while mechanically-induced pain remains unchanged.

RNA editing enzyme ADAR2 is a mediator of neuropathic pain after peripheral nerve injury.

Transcriptional and post-translational regulations are important in peripheral nerve injury-induced neuropathic pain, but little is known about the role of post-transcriptional modification. Our objective was to determine the possible effect of adenosine deaminase acting on RNA (ADAR) enzymes, which catalyze post-transcriptional RNA editing, in tactile allodynia, a hallmark of neuropathic pain. Seven days after L5 spinal nerve transection (SNT) in adult mice, we found an increase in ADAR2 expression and a decrease in ADAR3 expression in the injured, but not in the uninjured, dorsal root ganglions (DRGs). These changes were accompanied by elevated levels of editing at the D site of the serotonin (5-hydroxytryptamine) 2C receptor (5-HT2CR), at the I/V site of coatomer protein complex subunit α (COPA), and at the R/G site of AMPA receptor subunit GluA2 in the injured DRG. Compared to Adar2+/+/Gria2R/R littermate controls, Adar2-/-/Gria2R/R mice completely lacked the increased editing of 5-HT2CR, COPA, and GluA2 transcripts in the injured DRG and showed attenuated tactile allodynia after SNT. Furthermore, the antidepressant fluoxetine inhibited neuropathic allodynia after injury and reduced the COPA I/V site editing in the injured DRG. These findings suggest that ADAR2 is a mediator of injury-induced tactile allodynia and thus a potential therapeutic target for the treatment of neuropathic pain.-Uchida, H., Matsumura, S., Okada, S., Suzuki, T., Minami, T., Ito, S. RNA editing enzyme ADAR2 is a mediator of neuropathic pain after peripheral nerve injury.

Aquaporins in peripheral nociception

Aquaporins in peripheral nociception

The analgesic effect of orexin-A in a murine model of chemotherapy-induced neuropathic pain

Orexins are neuropeptides that are localized to neurons in the lateral and dorsal hypothalamus but its receptors are distributed to many different regions of the central nervous system. Orexins are implicated in a variety of physiological functions including sleep regulation, energy homeostats, and stress reactions. Furthermore, orexins administered exogenously have been shown to have analgesic effects in animal models. A type of intractable pain in patients is pain due to chemotherapy-induced peripheral neuropathy (CIPN). Several chemotherapeutic agents used for the treatment of malignant diseases induce dose-limiting neuropathic pain that compromises patients' quality of life. Here, we examined the analgesic effect of orexin-A in a murine model of CIPN, and compared it with the effect of duloxetine, the only drug recommended for the treatment of CIPN pain in patients. CIPN was induced in male BALB/c mice by repeated intraperitoneal injection of oxaliplatin, a platinum chemotherapeutic agent used for the treatment of advanced colorectal cancer. Neuropathic mechanical allodynia was assessed by the von Frey test, and the effect on acute thermal pain was assessed by the tail flick test. Intracerebroventricularly administered orexin-A dose-dependently attenuated oxaliplatin-induced mechanical allodynia and increased tail flick latencies. Oxaliplatin-induced mechanical allodynia was completely reversed by orexin-A at a low dose that did not increase tail flick latency. Duloxetine only partially reversed mechanical allodynia and had no effect on tail flick latency. The analgesic effect of orexin-A on oxaliplatin-induced mechanical allodynia was completely antagonized by prior intraperitoneal injection of SB-408124 (orexin type-1 receptor antagonist), but not by prior intraperitoneal injection of TCS-OX2-29 (orexin type-2 receptor antagonist). Our findings suggest that orexin-A is more potent than duloxetine in relieving pain CIPN pain and its analgesic effect is mediated by orexin type-1 receptors. Orexin type-1 receptor agonists may have potential therapeutic roles in the treatment of CIPN pain in patients.

Developmental Changes in Pain and Spinal Immune Gene Expression after Radicular Trauma in the Rat.

Neuropathic pain is chronic pain that develops after nerve injury and is less frequent in infants and children than in adults. Likewise, in animal models of neuropathic pain, allodynia and hyperalgesia are non-existent or attenuated in the infant, with a “switch” during development by which acute nerve injury transitions to chronic pain. Concomitant with the delay in neuropathic pain, there is a parallel delay in the ability of nerve injury to activate the immune system. Models of neuropathic pain in the infant have used various ligation methods and find that neuropathic pain does not occur under after postnatal days 21–28 (PN21–PN28), linked to activation of immune processes and developmental regulation of anti-inflammatory cytokines. We applied a model of neuropathic pain in the adult using a transient compression of the cervical nerve or nerve root in infant rats (injured at 10, 14, 21, or 28 days of age) to define transition periods during which injury results in no change in thermal and mechanical pain sensitivity or in short-term changes in pain. There was little to no hyperalgesia when the injury was imposed at PN10, but significant thermal hyperalgesia and mechanical allodynia 1 day after compression injury when performed at PN14, 21, or 28. Thermal withdrawal latencies returned to near baseline by 7 days postsurgery when the injuries were at PN14, and lasted up to 14 days when the injury was imposed at PN28. There was mechanical allodynia following injury at 1 day postinjury and at 14 days after injury at PN14. Measurements of mRNA from spinal cord at 1, 7, and 14 days postinjury at PN14, 21, and 28 showed that both the magnitude and duration of elevated immune markers and chemokines/cytokines were greater in the older animals, corresponding to the development of hyperalgesia. Thus, we confirm the late onset of neuropathic pain but found no evidence of emergent hyperalgesia if the injury was before PN21. This may be due to the use of a transient, and not sustained, compression ligation model.

Functional brain mapping using specific sensory-circuit stimulation and a theoretical graph network analysis in mice with neuropathic allodynia.

Allodynia, a form of neuropathic pain, is defined as pain in response to a non-nociceptive stimulus. The brain regions responsible for pain, which are not normally activated, can be activated in allodynic mice by providing a suitable stimulus to Aβ-fibers, which transmit signals from tactile sensory fibers. Functional MRI (fMRI) can be used to objectively observe abnormal brain activation. In the present study, fMRI was conducted to investigate allodynia in mice; allodynia was generated by surgical injury at the L4 spinal nerve root, thus selectively stimulating sensory nerve fibers. In intact mice, only the primary somatosensory cortex (S1) was activated by stimulation of Aβ-fibers. Meanwhile, allodynic mice showed significantly higher BOLD signals in the anterior cingulate area (ACA) and thalamus. Using resting state fMRI, both degree and eigenvector centrality were significantly decreased in the contralateral S1, clustering coefficient and local efficiency were significantly increased in the ACA, and betweenness centrality was significantly higher in the ventral posterolateral nucleus of the thalamus. These results suggest that the observed abnormal BOLD activation is associated with defects in Aβ-fibers when Aβ-fibers in allodynic mice are selectively stimulated. The objective approach enabled by fMRI can improve our understanding of pathophysiological mechanisms and therapeutic efficacy.

Glucocorticoid regulation of ATP release from spinal astrocytes underlies diurnal exacerbation of neuropathic mechanical allodynia

Diurnal variations in pain hypersensitivity are common in chronic pain disorders, but the underlying mechanisms are enigmatic. Here, we report that mechanical pain hypersensitivity in sciatic nerve-injured mice shows pronounced diurnal alterations, which critically depend on diurnal variations in glucocorticoids from the adrenal glands. Diurnal enhancement of pain hypersensitivity is mediated by glucocorticoid-induced enhancement of the extracellular release of ATP in the spinal cord, which stimulates purinergic receptors on microglia in the dorsal horn. We identify serum- and glucocorticoid-inducible kinase-1 (SGK-1) as the key molecule responsible for the glucocorticoid-enhanced release of ATP from astrocytes. SGK-1 protein levels in spinal astrocytes are increased in response to glucocorticoid stimuli and enhanced ATP release by opening the pannexin-1 hemichannels. Our findings reveal an unappreciated circadian machinery affecting pain hypersensitivity caused by peripheral nerve injury, thus opening up novel approaches to the management of chronic pain.

The Suppressive Effects of Cinnamomi Cortex and Its Phytocompound Coumarin on Oxaliplatin-Induced Neuropathic Cold Allodynia in Rats.

Oxaliplatin, a chemotherapy drug, induces acute peripheral neuropathy characterized by cold allodynia, spinal glial activation and increased levels of pro-inflammatory cytokines. Herein, we determined whether Cinnamomi Cortex (C. Cortex), a widely used medicinal herb in East Asia for cold-related diseases, could attenuate oxaliplatin-induced cold allodynia in rats and the mechanisms involved. A single oxaliplatin injection (6 mg/kg, i.p.) induced significant cold allodynia signs based on tail immersion tests using cold water (4 °C). Daily oral administration of water extract of C. Cortex (WECC) (100, 200, and 400 mg/kg) for five consecutive days following an oxaliplatin injection dose-dependently alleviated cold allodynia with only a slight difference in efficacies between the middle dose at 200 mg/kg and the highest dose at 400 mg/kg. WECC at 200 mg/kg significantly suppressed the activation of astrocytes and microglia and decreased the expression levels of IL-1β and TNF in the spinal cord after injection with oxaliplatin. Furthermore, oral administration of coumarin (10 mg/kg), a major phytocompound of C. Cortex, markedly reduced cold allodynia. These results indicate that C. Cortex has a potent anti-allodynic effect in oxaliplatin-injected rats through inhibiting spinal glial cells and pro-inflammatory cytokines. We also suggest that coumarin might play a role in the anti-allodynic effect of C. Cortex.

Increasing spinal 5-HT2A receptor responsiveness mediates anti-allodynic effect and potentiates fluoxetine efficacy in neuropathic rats. Evidence for GABA release

Antidepressants are one of the first line treatments for neuropathic pain but their use is limited by the incidence and severity of side effects of tricyclics and the weak effectiveness of selective serotonin reuptake inhibitors (SSRIs). Serotonin type 2A (5-HT2A) receptors interact with PDZ proteins that regulate their functionality and SSRI efficacy to alleviate pain. We investigated whether an interfering peptide (TAT-2ASCV) disrupting the interaction between 5-HT2A receptors and associated PDZ proteins would improve the treatment of traumatic neuropathic allodynia. Tactile allodynia was assessed in spinal nerve ligation-induced neuropathic pain in rats using von Frey filaments after acute treatment with TAT-2ASCV and/or 5-HT2A receptor agonist, alone or in combination with repeated treatment with fluoxetine. In vivo microdialysis was performed in order to examine the involvement of GABA in TAT-2ASCV/fluoxetine treatment-associated analgesia. TAT-2ASCV (100ng, single i.t. injection) improved SNL-induced tactile allodynia by increasing 5-HT2A receptor responsiveness to endogenous 5-HT. Fluoxetine alone (10mg/kg, five i.p. injections) slightly increased tactile thresholds and its co-administration with TAT-2ASCV (100ng, single i.t. injection) further enhanced the anti-allodynic effect. This effect depends on the integrity of descending serotonergic bulbospinal pathways and spinal release of GABA. The anti-allodynic effect of fluoxetine can be enhanced by disrupting 5-HT2A receptor-PDZ protein interactions. This enhancement depends on 5-HT2A receptor activation, spinal GABA release and GABAA receptor activation.

Effect of knockout of α2δ-1 on action potentials in mouse sensory neurons.

Gene deletion of the voltage-gated calcium channel auxiliary subunit α2δ-1 has been shown previously to have a cardiovascular phenotype, and a reduction in mechano- and cold sensitivity, coupled with delayed development of neuropathic allodynia. We have also previously shown that dorsal root ganglion (DRG) neuron calcium channel currents were significantly reduced in α2δ-1 knockout mice. To extend our findings in these sensory neurons, we have examined here the properties of action potentials (APs) in DRG neurons from α2δ-1 knockout mice in comparison to their wild-type (WT) littermates, in order to dissect how the calcium channels that are affected by α2δ-1 knockout are involved in setting the duration of individual APs and their firing frequency. Our main findings are that there is reduced Ca2+ entry on single AP stimulation, particularly in the axon proximal segment, reduced AP duration and reduced firing frequency to a 400 ms stimulation in α2δ-1 knockout neurons, consistent with the expected role of voltage-gated calcium channels in these events. Furthermore, lower intracellular Ca2+ buffering also resulted in reduced AP duration, and a lower frequency of AP firing in WT neurons, mimicking the effect of α2δ-1 knockout. By contrast, we did not obtain any consistent evidence for the involvement of Ca2+-activation of large conductance calcium-activated potassium (BK) and small conductance calcium-activated potassium (SK) channels in these events. In conclusion, the reduced Ca2+ elevation as a result of single AP stimulation is likely to result from the reduced duration of the AP in α2δ-1 knockout sensory neurons.This article is part of the themed issue ‘Evolution brings Ca2+ and ATP together to control life and death’.

The Red Nucleus Interleukin-6 Participates in the Maintenance of Neuropathic Pain Induced by Spared Nerve Injury.

Previous studies have demonstrated that the red nucleus (RN) is involved in the regulation of neuropathic pain and plays both facilitated and inhibitory roles through different cytokines. Here, we aim to investigate the expression changes and roles of interleukin-6 (IL-6), a pleiotropic cytokine, as well as its receptor (IL-6R) in the RN of rats with neuropathic pain induced by spared nerve injury (SNI). Immunohistochemistry indicated that IL-6 and IL-6R were weakly expressed in the RN of normal rats, and were mainly co-localized with neurons and oligodendrocytes. Following SNI, the expression levels of IL-6 and IL-6R in the RN did not show obvious changes at 1week and 2weeks postinjury. However, both of them were significantly increased in the RN contralateral (but not ipsilateral) to the nerve ligation side at 3weeks postinjury, and co-localized not only with neurons and oligodendrocytes, but also with numerous astrocytes. Injection of different doses of anti-IL-6 antibody (100, 250, 500ng) into the RN contralateral to the nerve ligation side at 3weeks postinjury dose-dependently increased the paw withdrawal threshold (PWT) of rats and alleviated SNI-induced mechanical allodynia. Conversely, injection of different doses of recombinant rat IL-6 (5.0, 10, 20ng) into the unilateral RN of normal rats dose-dependently decreased the PWT of contralateral (but not ipsilateral) hind paw and evoked significant mechanical allodynia, which was similar to SNI-induced neuropathic allodynia. These results further support the conclusion that the RN is involved in the modulation of neuropathic pain, and suggest that IL-6 and IL-6R in the RN play a facilitated role in the later maintenance of SNI-induced neuropathic pain.

Botulinum toxin - neuropathic pain: Safety and efficacy of repeated injections of botulinum toxin A in peripheral neuropathic pain (BOTNEP): a randomised, double-blind, placebo-controlled trial.

When treated with botulinum toxin A, those patients with peripheral neuropathic pain and allodynia (triggering of pain from stimuli which do not normally provoke pain) at baseline, would appear to have a better outcome.

Cortical astrocytes rewire somatosensory cortical circuits for peripheral neuropathic pain.

Long-term treatments to ameliorate peripheral neuropathic pain that includes mechanical allodynia are limited. While glial activation and altered nociceptive transmission within the spinal cord are associated with the pathogenesis of mechanical allodynia, changes in cortical circuits also accompany peripheral nerve injury and may represent additional therapeutic targets. Dendritic spine plasticity in the S1 cortex appears within days following nerve injury; however, the underlying cellular mechanisms of this plasticity and whether it has a causal relationship to allodynia remain unsolved. Furthermore, it is not known whether glial activation occurs within the S1 cortex following injury or whether it contributes to this S1 synaptic plasticity. Using in vivo 2-photon imaging with genetic and pharmacological manipulations of murine models, we have shown that sciatic nerve ligation induces a re-emergence of immature metabotropic glutamate receptor 5 (mGluR5) signaling in S1 astroglia, which elicits spontaneous somatic Ca2+ transients, synaptogenic thrombospondin 1 (TSP-1) release, and synapse formation. This S1 astrocyte reactivation was evident only during the first week after injury and correlated with the temporal changes in S1 extracellular glutamate levels and dendritic spine turnover. Blocking the astrocytic mGluR5-signaling pathway suppressed mechanical allodynia, while activating this pathway in the absence of any peripheral injury induced long-lasting (>1 month) allodynia. We conclude that reawakened astrocytes are a key trigger for S1 circuit rewiring and that this contributes to neuropathic mechanical allodynia.

Inflammatory and neuropathic cold allodynia are selectively mediated by the neurotrophic factor receptor GFRα3

Tissue injury prompts the release of a number of proalgesic molecules that induce acute and chronic pain by sensitizing pain-sensing neurons (nociceptors) to heat and mechanical stimuli. In contrast, many proalgesics have no effect on cold sensitivity or can inhibit cold-sensitive neurons and diminish cooling-mediated pain relief (analgesia). Nonetheless, cold pain (allodynia) is prevalent in many inflammatory and neuropathic pain settings, with little known of the mechanisms promoting pain vs. those dampening analgesia. Here, we show that cold allodynia induced by inflammation, nerve injury, and chemotherapeutics is abolished in mice lacking the neurotrophic factor receptor glial cell line-derived neurotrophic factor family of receptors-α3 (GFRα3). Furthermore, established cold allodynia is blocked in animals treated with neutralizing antibodies against the GFRα3 ligand, artemin. In contrast, heat and mechanical pain are unchanged, and results show that, in striking contrast to the redundant mechanisms sensitizing other modalities after an insult, cold allodynia is mediated exclusively by a single molecular pathway, suggesting that artemin-GFRα3 signaling can be targeted to selectively treat cold pain.

Passiflora incarnata attenuation of neuropathic allodynia and vulvodynia apropos GABA-ergic and opioidergic antinociceptive and behavioural mechanisms.

BackgroundPassiflora incarnata is widely used as an anxiolytic and sedative due to its putative GABAergic properties. Passiflora incarnata L. methanolic extract (PI-ME) was evaluated in an animal model of streptozotocin-induced diabetic neuropathic allodynia and vulvodynia in rats along with antinociceptive, anxiolytic and sedative activities in mice in order to examine possible underlying mechanisms.MethodsPI-ME was tested preliminary for qualitative phytochemical analysis and then quantitatively by proximate and GC-MS analysis. The antinociceptive property was evaluated using the abdominal constriction assay and hot plate test. The anxiolytic activity was performed in a stair case model and sedative activity in an open field test. The antagonistic activities were evaluated using naloxone and/or pentylenetetrazole (PTZ). PI-ME was evaluated for prospective anti-allodynic and anti-vulvodynic properties in a rat model of streptozotocin induced neuropathic pain using the static and dynamic testing paradigms of mechanical allodynia and vulvodynia.ResultsGC-MS analysis revealed that PI-ME contained predominant quantities of oleamide (9-octadecenamide), palmitic acid (hexadecanoic acid) and 3-hydroxy-dodecanoic acid, among other active constituents. In the abdominal constriction assay and hot plate test, PI-ME produced dose dependant, naloxone and pentylenetetrazole reversible antinociception suggesting an involvement of opioidergic and GABAergic mechanisms. In the stair case test, PI-ME at 200 mg/kg increased the number of steps climbed while at 600 mg/kg a significant decrease was observed. The rearing incidence was diminished by PI-ME at all tested doses and in the open field test, PI-ME decreased locomotor activity to an extent that was analagous to diazepam. The effects of PI-ME were antagonized by PTZ in both the staircase and open field tests implicating GABAergic mechanisms in its anxiolytic and sedative activities. In the streptozotocin-induced neuropathic nociceptive model, PI-ME (200 and 300 mg/kg) exhibited static and dynamic anti-allodynic effects exemplified by an increase in paw withdrawal threshold and paw withdrawal latency. PI-ME relieved only the dynamic component of vulvodynia by increasing flinching response latency.ConclusionsThese findings suggest that Passiflora incarnata might be useful for treating neuropathic pain. The antinociceptive and behavioural findings inferring that its activity may stem from underlying opioidergic and GABAergic mechanisms though a potential oleamide-sourced cannabimimetic involvement is also discussed.Electronic supplementary materialThe online version of this article (doi:10.1186/s12906-016-1048-6) contains supplementary material, which is available to authorized users.

Melatonin relieves neuropathic allodynia through spinal MT2-enhanced PP2Ac and downstream HDAC4 shuttling-dependent epigenetic modification of hmgb1 transcription.

Melatonin (MLT; N-acetyl-5-methoxytryptamine) exhibits analgesic properties in chronic pain conditions. While researches linking MLT to epigenetic mechanisms have grown exponentially over recent years, very few studies have investigated the contribution of MLT-associated epigenetic modification to pain states. Here, we report that together with behavioral allodynia, spinal nerve ligation (SNL) induced a decrease in the expression of catalytic subunit of phosphatase 2A (PP2Ac) and enhanced histone deacetylase 4 (HDAC4) phosphorylation and cytoplasmic accumulation, which epigenetically alleviated HDAC4-suppressed hmgb1 gene transcription, resulting in increased high-mobility group protein B1 (HMGB1) expression selectively in the ipsilateral dorsal horn of rats. Focal knock-down of spinal PP2Ac expression also resulted in behavioral allodynia in association with similar protein expression as observed with SNL. Notably, intrathecal administration with MLT increased PP2Ac expression, HDAC4 dephosphorylation and nuclear accumulation, restored HDAC4-mediated hmgb1 suppression and relieved SNL-sensitized behavioral pain; these effects were all inhibited by spinal injection of 4P-PDOT (a MT2 receptor antagonist, 30 minutes before MLT) and okadaic acid (OA, a PP2A inhibitor, 3 hr after MLT). Our findings demonstrate a novel mechanism by which MLT ameliorates neuropathic allodynia via epigenetic modification. This MLT-exhibited anti-allodynia is mediated by MT2-enhanced PP2Ac expression that couples PP2Ac with HDAC4 to induce HDAC4 dephosphorylation and nuclear import, herein increases HDAC4 binding to the promoter of hmgb1 gene and upregulates HMGB1 expression in dorsal horn neurons.

Involvement of spinal muscarinic and serotonergic receptors in the anti-allodynic effect of electroacupuncture in rats with oxaliplatin-induced neuropathic pain.

This study was performed to investigate whether the spinal cholinergic and serotonergic analgesic systems mediate the relieving effect of electroacupuncture (EA) on oxaliplatin-induced neuropathic cold allodynia in rats. The cold allodynia induced by an oxaliplatin injection (6 mg/kg, i.p.) was evaluated by immersing the rat's tail into cold water (4℃) and measuring the withdrawal latency. EA stimulation (2 Hz, 0.3-ms pulse duration, 0.2~0.3 mA) at the acupoint ST36, GV3, or LI11 all showed a significant anti-allodynic effect, which was stronger at ST36. The analgesic effect of EA at ST36 was blocked by intraperitoneal injection of muscarinic acetylcholine receptor antagonist (atropine, 1 mg/kg), but not by nicotinic (mecamylamine, 2 mg/kg) receptor antagonist. Furthermore, intrathecal administration of M2 (methoctramine, 10 µg) and M3 (4-DAMP, 10 µg) receptor antagonist, but not M1 (pirenzepine, 10 µg) receptor antagonist, blocked the effect. Also, spinal administration of 5-HT3 (MDL-72222, 12 µg) receptor antagonist, but not 5-HT1A (NAN-190, 15 µg) or 5-HT2A (ketanserin, 30 µg) receptor antagonist, prevented the anti-allodynic effect of EA. These results suggest that EA may have a signifi cant analgesic action against oxaliplatin-induced neuropathic pain, which is mediated by spinal cholinergic (M2, M3) and serotonergic (5-HT3) receptors.

Dorsal Horn Parvalbumin Neurons Are Gate-Keepers of Touch-Evoked Pain after Nerve Injury.

SUMMARYNeuropathic pain is a chronic debilitating disease that results from nerve damage, persists long after the injury has subsided, and is characterized by spontaneous pain and mechanical hypersensitivity. Although loss of inhibitory tone in the dorsal horn of the spinal cord is a major contributor to neuropathic pain, the molecular and cellular mechanisms underlying this disinhibition are unclear. Here, we combined pharmacogenetic activation and selective ablation approaches in mice to define the contribution of spinal cord parvalbumin (PV)-expressing inhibitory interneurons in naive and neuropathic pain conditions. Ablating PV neurons in naive mice produce neuropathic pain-like mechanical allodynia via disinhibition of PKCγ excitatory interneurons. Conversely, activating PV neurons in nerve-injured mice alleviates mechanical hypersensitivity. These findings indicate that PV interneurons are modality-specific filters that gate mechanical but not thermal inputs to the dorsal horn and that increasing PV inter-neuron activity can ameliorate the mechanical hypersensitivity that develops following nerve injury.