Role In Antinociception Research Articles

Peripheral 5-HT3 Receptors Are Involved in the Antinociceptive Effect of Bunodosine 391.

Bunodosine 391 (BDS 391), a low molecular weight compound isolated from the sea anemone Bunodosoma cangicum, increases the nociceptive threshold and inhibits inflammatory hyperalgesia. Serotonin receptors are involved in those effects. In this study, we have expanded the characterization of the antinociceptive effect of BDS 391 demonstrating that, in rats: (a) the compound inhibits (1.2–12 ng/paw) overt pain, in the formalin test, and mechanical hyperalgesia (0.6–6.0 ng/paw) detected in a model of neuropathic pain; (b) intraplantar administration of ondansetron, a selective 5-HT3 receptor antagonist, blocks the effect of BDS 391, whereas ketanserin, a 5-HT2 receptor antagonist, partially reversed this effect, indicating the involvement of peripheral 5-HT2 and 5-HT3 receptors in BDS 391 antinociception; and (c) in binding assay studies, BDS 391 was not able to displace the selective 5-HT receptor antagonists, suggesting that this compound does not directly bind to these receptors. The effect of biguanide, a selective 5-HT3 receptor agonist, was also evaluated. The agonist inhibited the formalin’s nociceptive response, supporting an antinociceptive role for 5-HT3 receptors. Our study is the first one to show that a non-peptidic low molecular weight compound obtained from a sea anemone is able to induce antinociception and that activation of peripheral 5-HT3 receptors contributes to this effect.

Repeated administration of mazindol reduces spontaneous pain-related behaviors without modifying bone density and microarchitecture in a mouse model of complete Freund's adjuvant-induced knee arthritis.

BackgroundThe role of dopaminergic system in the development of rheumatoid arthritis-related pain, a major symptom in this disease, has not been explored. Therefore, the anti-nociceptive effect of mazindol, a dopamine uptake inhibitor, was evaluated in a model of complete Freund’s adjuvant (CFA)-induced arthritis. Furthermore, as studies have shown that the dopaminergic system regulates bone metabolism, the effect of mazindol on bone mass and microarchitecture was determined.MethodsAdult ICR male mice received intra-articular injections of either CFA or saline into the right knee joint every week. Spontaneous pain-like behaviors (flinching and guarding) and locomotor activity were assessed at day 26 post-first CFA, following which, a single intraperitoneally (i.p.) administered dose of mazindol was given (1, 3 and 10 mg/kg). Then, the antinociceptive effect of a repeated administration of 3 mg/kg mazindol (daily, i.p.; day 15–day 26) was evaluated. Additionally, at day 26, the participation of D1-like, D2-like or opioid receptors in the antinociceptive effect of mazindol was evaluated. The effect of mazindol on bone density and microarchitecture was evaluated by micro-computed tomography.ResultsAcute administration of mazindol decreased the spontaneous pain-like behaviors in a dose-dependent manner without reducing the knee edema. However, mazindol at 10 mg/kg significantly increased the locomotor activity; therefore, 3 mg/kg mazindol was used for further studies. Repeated administration of 3 mg/kg mazindol significantly decreased the pain-like behaviors without modifying locomotor activity. The antinociceptive effect of mazindol was blocked by administration of a D2-like receptor antagonist (haloperidol), but not by administration of D1-like receptor antagonist (SCH 23390) or an opioid receptor antagonist (naloxone). Repeated administration of mazindol did not significantly modify the density and microarchitecture of periarticular bone of the arthritic and nonarthritic knee joints.ConclusionResults suggest that mazindol via D2-like receptors has an antinociceptive role in mice with CFA-induced knee arthritis without modifying the bone health negatively.

MiR-34c-5p functions as pronociceptive microRNA in cancer pain by targeting Cav2.3 containing calcium channels.

Pathophysiological mechanisms underlying pain associated with cancer are poorly understood. microRNAs (miRNAs) are a class of noncoding RNAs with emerging functional importance in chronic pain. In a genome-wide screen for miRNAs regulated in dorsal root ganglia (DRG) neurons in a mouse model of bone metastatic pain, we identified miR-34c-5p as a functionally important pronociceptive miRNA. Despite these functional insights and therapeutic potential for miR-34c-5p, its molecular mechanism of action in peripheral sensory neurons remains unknown. Here, we report the identification and validation of key target transcripts of miRNA-34c-5p. In-depth bioinformatics analyses revealed Cav2.3, P2rx6, Oprd1, and Oprm1 as high confidence putative targets for miRNA-34c-5p. Of these, canonical and reciprocal regulation of miR-34c-5p and Cav2.3 was observed in cultured sensory neurons as well as in DRG in vivo in mice with cancer pain. Coexpression of miR-34c-5p and Cav2.3 was observed in peptidergic and nonpeptidergic nociceptors, and luciferase reporter assays confirmed functional binding of miR-34c-5p to the 3' UTR of Cav2.3 transcripts. Importantly, knocking down the expression of Cav2.3 specifically in DRG neurons led to hypersensitivity in mice. In summary, these results show that Cav2.3 is a novel mechanistic target for a key pronociceptive miRNA, miR-34c-5p, in the context of cancer pain and indicate an antinociceptive role for Cav2.3 in peripheral sensory neurons. The current study facilitates a deeper understanding of molecular mechanisms underlying cancer pain and suggests a potential for novel therapeutic strategies targeting miR-34c-5p and Cav2.3 in cancer pain.

GABAB receptors-mediated tonic inhibition of glutamate release from Aβ fibers in rat laminae III/IV of the spinal cord dorsal horn.

Presynaptic GABAB receptors (GABABRs) are highly expressed in dorsal root ganglion neurons and spinal cord dorsal horn. GABABRs located in superficial dorsal horn play an important antinociceptive role, by acting at both pre- and postsynaptic sites. GABABRs expressed in deep dorsal horn could be involved in the processing of touch sensation and possibly in the generation of tactile allodynia in chronic pain. The objective of this study was to characterize the morphological and functional properties of GABABRs expressed on Aβ fibers projecting to lamina III/IV and to understand their role in modulating excitatory synaptic transmission. We performed high-resolution electron microscopic analysis, showing that GABAB2 subunit is expressed on 71.9% of terminals in rat lamina III-IV. These terminals were engaged in axodendritic synapses and, for the 46%, also expressed glutamate immunoreactivity. Monosynaptic excitatory postsynaptic currents, evoked by Aβ fiber stimulation and recorded from lamina III/IV neurons in spinal cord slices, were strongly depressed by application of baclofen (0.1–2.5 µM), acting as a presynaptic modulator. Application of the GABABR antagonist CGP 55845 caused, in a subpopulation of neurons, the potentiation of the first of two excitatory postsynaptic currents recorded with the paired-pulse protocol, showing that GABABRs are endogenously activated. A decrease in the paired-pulse ratio accompanied the effect of CGP 55845, implying the involvement of presynaptic GABABRs. CGP 55845 facilitated only the first excitatory postsynaptic current also during a train of four consecutive stimuli applied to Aβ fibers. These results suggest that GABABRs tonically inhibit glutamate release from Aβ fibers at a subset of synapses in deep dorsal horn. This modulation specifically affects only the early phase of synaptic excitation in lamina III-IV neurons.

Antinociceptive effect of co-administered NMDA and histamine H4 receptor antagonists in a rat model of acute pain.

The histamine H4 receptor (H4R) has attracted a lot of attention in terms of its role in antinociception. The N-Methyl-d-aspartic acid (NMDA) receptor is a well-characterized participant in pain pathways. However, it's influence on H4R signaling is poorly understood. Thus, in the present study we investigated the effect of a selective H4R antagonist JNJ7777120 (25mg/kg) and a NMDA receptor antagonist MK-801 (0.1-10μg) on nociceptive thresholds in a rat acute pain model. The influence of intrathecally (it), intracerebroventricularly (icv) and intraplantarly (ipl) administered MK-801 co-injected with JNJ7777120 administered intraperitoneally (ip) was determined in the modified Randall-Selitto paw pressure, the tail flick and plantar tests. Both ip JNJ7777120 as well as MK-801 delivered it and ipl increased nociceptive pain thresholds. It and ipl pretreatment with MK-801 additively augmented JNJ7777120 mechanical antinociception. A weaker effect was also observed after it co-administration with MK-801 and JNJ7777120 in the tail flick test. Intracerebroventricular MK-801 failed to produce any effect nor did ipl MK-801 in the plantar test. The results show for the first time that the H4R and NMDA receptors play a significant role in antinociception in an acute non-inflammatory pain. Furthermore, the blockage of NMDA receptors at the peripheral and spinal, but not supraspinal sites produces a profound antinociceptive response upon simultaneous H4R antagonism. Thus, this study clearly demonstrates the relevance of NMDA and H4R receptors in the modulation of pain signals and that their role is not exclusively limited to their modulatory activity in inflammatory pain states.

Anti-nociceptive roles of the glia-specific metabolic inhibitor fluorocitrate in paclitaxel-evoked neuropathic pain.

Paclitaxel (Taxol) is a powerful chemotherapy drug used in breast cancers, but it often causes neuropathic pain, leading to the early cessation of therapy and poor treatment outcomes. Approaches for the management of paclitaxel-induced neuropathic pain are urgently needed. The involvement of spinal astrocytes in the pathogenesis of paclitaxel-induced neuropathy has been reported, but little is known about the role of fluorocitrate (FC), a selective inhibitor of astrocyte activation, during neuropathic pain related to paclitaxel treatment. In this study, we investigated the effects of FC on paclitaxel-induced neuropathic pain. Glial fibrillary acidic protein (GFAP) expression was determined to assess astrocyte activation. To explore the mechanisms involved, the expression of glial glutamate transporter 1 (GLT-1) and the activation of mitogen-activated protein kinases in the spinal dorsal horn were analyzed. The results showed that paclitaxel decreased the mechanical nociceptive thresholds and increased GFAP expression, leading to spinal astrocyte activation. After paclitaxel treatment, GLT-1 was significantly down-regulated, and the phosphorylation of ERK1/2 and JNK were obviously up-regulated. However, paclitaxel treatment did not increase p38 phosphorylation. Additional studies showed that paclitaxel-evoked mechanical hypersensitivity was reduced by FC treatment. Moreover, FC treatment inhibited the activation of astrocytes and reversed the changes in GLT-1 expression and MAPK phosphorylation. Further study indicated that FC did not influence the antitumor effect of paclitaxel, suggesting that FC blocked paclitaxel-induced neuropathic pain without antagonizing its antitumor effect. Together, these results suggested that paclitaxel induced astrocyte-specific activation, which may contribute to mechanical allodynia and hyperalgesia, and that FC could be a potential therapeutic agent for paclitaxel-induced neuropathic pain.

A path for two destinations.

We congratulate Dilli et al. (1) on their efforts to clarify the relevance of the greater occipital nerve (GON) block in the prevention of both episodic and chronic migraine. In 2001, our group carried out a similar study on episodic migraine patients (2). The methodology was also double-blind and included an active group (bupivacaine 0.5%) and a placebo group (saline 0.9%). After a 30-day baseline period, a first GON block and second GON block were performed 30 days apart. For both groups, there were no differences in the frequency and duration of migraine attacks, but in the active group, the migraine intensity worsened, possibly reflecting an antinociceptive role of the GON circuitry in migraine pathophysiology. The role of the GON in migraine pathophysiology is ambiguous. While anesthetic blockade of the GON is claimed to control migraine, stimulation of the GON is also reported to have the same effect (3,4). During a local anesthetic blockade of the GON, two opposite effects may occur: a) initially, there may be a nociceptive effect related to the puncture procedure and distension of the nerve endings (3); and b) an antinociceptive (anesthetic) effect may occur over the GON territory. The initial nociceptive input probably reaches the diffuse nociceptive inhibitory control (5) and may, in a manner akin to that observed with acupuncture and electrical stimulation of the GON, lead to migraine control (3,4). However, anesthetic block may lessen the GON tone over the nucleus trigeminalis caudalis, leading to an increase in the intensity of migraine attacks, as seen in our study (2).

OC-069 Randomised Controlled Trial – Transcutaneous Auricular Electrical Vagal Nerve Stimulation Prevents the Development of Acid Induced Esophageal Hypersensitivity

Introduction We have previously demonstrated that physiologically increasing vagal tone, with deep breathing, prevents the development of acid induced oesophageal pain hypersensitivity.1 We aimed to determine whether electrical manipulation of parasympathetic tone influences the development of hypersensitivity in a validated model of acid induced oesophageal pain. Methods Prior to, and following, a 30 minute distal oesophageal infusion of 0.15 M hydrochloric acid, pain thresholds to electrical stimulation were determined in the proximal non-acid exposed oesophagus in 15 healthy subjects (11 male, mean age 30 years, range 21–42). Validated sympathetic (cardiac sympathetic index) and parasympathetic (cardiac vagal tone) parameters were measured at baseline and continuously thereafter. Subjects were randomised in a blinded crossover design to receive either external auricular electrical vagal nerve stimulation (VNS), or sham stimulation, during acid infusion. Results VNS increased cardiac vagal tone (31.6% ± 58.7 vs. -9.6 ± 20.6, p = 0.02) in comparison to sham stimulation. Mixed effects linear regression, controlling for age and gender, demonstrated that VNS prevented the development of acid-induced oesophageal hypersensitivity in comparison to sham stimulation (coefficient 15.4 mA /unit time (95% confidence interval 8.8 to 22.2), p = 0.001). Figure 1 demonstrates the percentage change in pain thresholds following a 30 distal oesophageal acid infusion. Conclusion The development of oesophageal hyperalgesia is prevented by electrically stimulating the vagus nerve by increasing parasympathetic tone. This study provides further evidence of the anti-nociceptive role of the parasympathetic nervous system within the oesophagus. Further work is warranted in patients groups such as those with recalcitrant non-erosive reflux disease. Reference 1 Botha C, et al . Gut 2014. Disclosure of Interest None Declared

Prohormone convertase 2 (PC2) null mice have increased mu opioid receptor levels accompanied by altered morphine-induced antinociception, tolerance and dependence

Chronic morphine treatment increases the levels of prohormone convertase 2 (PC2) in brain regions involved in nociception, tolerance and dependence. Thus, we tested if PC2 null mice exhibit altered morphine-induced antinociception, tolerance and dependence. PC2 null mice and their wild-type controls were tested for baseline hot plate latency, injected with morphine (1.25–10mg/kg) and tested for antinociception 30min later. For tolerance studies, mice were tested in the hot plate test before and 30min following morphine (5mg/kg) on day 1. Mice then received an additional dose so that the final dose of morphine was 10mg/kg on this day. On days 2–4, mice received additional doses of morphine (20, 40 and 80mg/kg on days 1, 2, 3, and 4, respectively). On day 5, mice were tested in the hot plate test before and 30min following morphine (5mg/kg). For withdrawal studies, mice were treated with the escalating doses of morphine (10, 20, 40 and 80mg/kg) for 4days, implanted with a morphine pellet on day 5 and 3 days later injected with naloxone (1mg/kg) and signs of withdrawal were recorded. Morphine dose-dependently induced antinociception and the magnitude of this response was greater in PC2 null mice. Tolerance to morphine was observed in wild-type mice and this phenomenon was blunted in PC2 null mice. Withdrawal signs were also reduced in PC2 null mice. Immunohistochemical studies showed up-regulation of the mu opioid receptor (MOP) protein expression in the periaqueductal gray area, ventral tegmental area, lateral hypothalamus, medial hypothalamus, nucleus accumbens, and somatosensory cortex in PC2 null mice. Likewise, naloxone specific binding was increased in the brains of these mice compared to their wild-type controls. The results suggest that the PC2-derived peptides may play a functional role in morphine-induced antinociception, tolerance and dependence. Alternatively, lack of opioid peptides led to up-regulation of the MOP and altered morphine-induced antinociception, tolerance and dependence.

Potential anti-nociceptive role of chemokine CXCL1 in peripheral nerve injury-induced neuropathic pain

Abstract Our previous studies have indicated that both microglial CD40 and spinal cord infiltrating CD4+ T cells play critical roles in the maintenance of peripheral nerve injury-induced neuropathic pain using a murine model of neuropathic pain, spinal nerve L5 transection (L5Tx). It is well accepted that chemokines play important role in the development of neuropathic pain. To further understand the role of microglial CD40 and CD4+ T cells in peripheral nerve injury-induced spinal cord chemokine response, we determined the levels of various chemokines in the lumbar spinal cord in CD40 knockout (KO), CD4 KO, and wild type (WT) BALB/c mice post-L5Tx. Using a multiplex assay, a total of eight chemokines CXCL1 (KC), CCL1 (TCA-3), CCL2 (MCP-1), CCL3 (MIP-1 alpha), CCL5 (RANTES), CCL11 (Eotaxin), CCL17 (TARC), and CCL22 (MDC) were examined. In WT mice, but not KO mice, L5Tx induced significant, gradual increases in CCL2, CCL3 and CCL5 up to day 21 post-L5Tx. At RNA level, L5Tx induced less pronounced increases of these chemokines in both types of KO mice compared to WT mice. These reduced chemokine response correlate with our previous observation that both CD40 KO and CD4 KO mice displayed significantly less mechanical hypersensitivity following L5Tx. Interestingly, L5Tx induced a significant increase in CXCL1 only in CD40 KO mice. Intrathecal injection of CXCL1 significantly reduced L5Tx-induced mechanical hypersensitivity in WT mice, indicating a potential protective effect of CXCL1 in neuropathic pain. Altogether, our data suggest chemokines’ roles in the development of neuropathic pain are chemokine type-specific.

Targeting Chronic Pain with Epigenetic Drugs: Focus on mGlu2 Receptors

Histone deacetylase (HDAC) enzymes regulate gene expression by affecting chromatin structure and/or the activity of transcription factors. We have recently demonstrated that histone deacetylase inhibitors (HDACIs) behave as epigenetic agents capable of inducing analgesia by up-regulating metabotropic glutamate type 2 (mGlu2) receptors. Specifically, the regulation of mGlu2 receptor expression appears to involve the acetylation of the NF- k B transcription factor. mGlu2 and mGlu3 receptors belong to class II metabotropic glutamate receptors. These receptors are coupled to G i/o proteins and play an important role in mediating antinociception in a variety of inflammatory and chronic pain models. We have shown that the HDACI-mediated mGlu2 receptor up-regulation occurs in the dorsal horn of the spinal cord and in the dorsal root ganglia, supporting a predominant role for mGlu2 receptors as mediators of analgesia in experimental animal models of chronic pain. We suggest that drugs that increase the expression of mGlu2 receptors, such as HDACIs or acetylating drugs (e.g. L-acetylcarnitine), may be effective in patients with chronic pain that are refractory to conventional analgesics.

Mu Opioid Receptor Modulation of Dopamine Neurons in the Periaqueductal Gray/Dorsal Raphe: A Role in Regulation of Pain.

The periaqueductal gray (PAG) is a brain region involved in nociception modulation, and an important relay center for the descending nociceptive pathway through the rostral ventral lateral medulla. Given the dense expression of mu opioid receptors and the role of dopamine in pain, the recently characterized dopamine neurons in the ventral PAG (vPAG)/dorsal raphe (DR) region are a potentially critical site for the antinociceptive actions of opioids. The objectives of this study were to (1) evaluate synaptic modulation of the vPAG/DR dopamine neurons by mu opioid receptors and to (2) dissect the anatomy and neurochemistry of these neurons, in order to assess the downstream loci and functions of their activation. Using a mouse line that expresses eGFP under control of the tyrosine hydroxylase (TH) promoter, we found that mu opioid receptor activation led to a decrease in inhibitory inputs onto the vPAG/DR dopamine neurons. Furthermore, combining immunohistochemistry, optogenetics, electrophysiology, and fast-scan cyclic voltammetry in a TH-cre mouse line, we demonstrated that these neurons also express the vesicular glutamate type 2 transporter and co-release dopamine and glutamate in a major downstream projection structure-the bed nucleus of the stria terminalis. Finally, activation of TH-positive neurons in the vPAG/DR using Gq designer receptors exclusively activated by designer drugs displayed a supraspinal, but not spinal, antinociceptive effect. These results indicate that vPAG/DR dopamine neurons likely play a key role in opiate antinociception, potentially via the activation of downstream structures through dopamine and glutamate release.

Modulation of vagal tone enhances gastroduodenal motility and reduces somatic pain sensitivity

The parasympathetic nervous system, whose main neural substrate is the vagus nerve, exerts a fundamental antinociceptive role and influences gastrointestinal sensori-motor function. Our research question was to whether combined electrical and physiological modulation of vagal tone, using transcutaneous electrical vagal nerve stimulation (t-VNS) and deep slow breathing (DSB) respectively, could increase musculoskeletal pain thresholds and enhance gastroduodenal motility in healthy subjects. Eighteen healthy subjects were randomized to a subject-blinded, sham-controlled, cross-over study with an active protocol including stimulation of auricular branch of the vagus nerve, and breathing at full inspiratory capacity and forced full expiration. Recording of cardiac derived parameters including cardiac vagal tone, moderate pain thresholds to muscle, and bone pressure algometry, conditioned pain modulation using a cold pressor test and a liquid meal ultrasonographic gastroduodenal motility test were performed. Cardiac vagal tone increased during active treatment with t-VNS and DSB compared to sham (p=0.009). In comparison to sham, thresholds to bone pain increased (p=0.001), frequency of antral contractions increased (p=0.004) and gastroduodenal motility index increased (p=0.016) with active treatment. However, no effect on muscle pain thresholds and conditioned pain modulation was seen. This experimental study suggests that this noninvasive approach with combined electrical and physiological modulation of vagal tone enhances gastroduodenal motility and reduces somatic pain sensitivity. These findings warrant further investigation in patients with disorders characterized with chronic pain and gastrointestinal dysmotility such as functional dyspepsia and irritable bowel syndrome.

Altered nociception and morphine tolerance in neuropeptide FF receptor type 2 over-expressing mice.

The neuropeptide FF system is thought to act as an anti-opioid modulator and plays a role in nociception, morphine antinociception and dependence. Two receptor subtypes, NPFFR1 and NPFFR2, have been identified, but their respective roles in these processes remain uncertain. In the present study, the role of NPFFR2 was investigated using transgenic mice over-expressing NPFFR2 in addition to a NPFFR2 agonist AC-263093. NPFFR2 Tg mice exhibited increased sensitivity to both mechanical and thermal noxious stimuli compared to the WT mice, while the antinociceptive effects of morphine at three different doses (6.25, 12.5 and 25mg/kg, s.c.) were similar in both strains. The development of tolerance to morphine antinociception after chronic morphine treatment (12.5mg/kg, s.c.; twice daily×5days) was attenuated in NPFFR2 Tg mice when compared to WT mice. Similarly, WT mice receiving AC-263093 pretreatment (2.5mg/kg, i.p.) showed attenuated morphine tolerance compared to vehicle controls. Most naloxone-precipitated morphine withdrawal symptoms were not attenuated in NPFFR2 Tg mice, with the exception of wet dog shake that was significantly reduced. Both NPFFR2 Tg and WT mice displayed similar degree of morphine rewarding. Our results suggest that neuropeptide FF R2 is mainly involved in the modulation of nociception and tolerance to morphine antinociception.

Epigallocatechin-3-gallate attenuates bone cancer pain involving decreasing spinal Tumor Necrosis Factor-α expression in a mouse model

Tumor metastasis to bone often elicits a wide array of symptoms, in which pain is a significant factor in catastrophic complications of bone cancer. The complete understanding of bone cancer-related pain is still unknown, while several pathophysiological components have been suggested, from tumor-stimulated osteolysis, nerve compression, stimulations of ion channels, and locally generated inflammatory cytokines. In particular, it has been shown that pro-inflammatory cytokine TNFα-mediated actions are necessary for the development of bone cancer pain. As a member of catechin family in green tea extracts, EGCG (Epigallocatechin-3-gallate) can reduce excess free radicals and attenuate overactive inflammatory signaling including TNFα. In addition, EGCG or its related molecules have been used to control neuropathic pain in various preclinical settings. However, its potential use in bone cancer-caused pain has not yet been reported. Here we show that treating a mouse model of bone cancer by EGCG, results in a dramatic reduction in pain behavior and a significant decrease of TNFα expression within the spinal cord of tumor-bearing mice. Thus, this study reveals an anti-nociceptive role for EGCG in the progression of pain caused by tumor bone metastasis, and highlights a potential scheme by using anti-TNFα as a therapeutic option for osteolytic pain.

Selective elimination of isolectin B4-binding trigeminal neurons enhanced formalin-induced nocifensive behavior in the upper lip of rats and c-Fos expression in the trigeminal subnucleus caudalis

The functional significance of non-peptidergic C-fibers in orofacial pain processing is largely unknown. The present study examined the effects of the selective elimination of isolectin B4 (IB4)-binding (IB4+) neurons on formalin-induced face rubbing behavior (FRB) in the upper lip of rats and c-Fos-immunoreactive (c-Fos-IR) cells in the trigeminal subnucleus caudalis (Vc). IB4 conjugated to neurotoxin, saporin (IB4-Sap), blank-saporin (Bl-Sap), or saline (Sal) was injected into the cisterna magna. IB4-Sap treatments significantly decreased IB4+ terminals in lamina II of Vc and IB4+ trigeminal ganglia neurons, whereas Sal- and BI-Sap treatments did not. The number of formalin-induced FRB 15–30min after the formalin injection was significantly higher in IB4-Sap-treated rats than in Sal- or Bl-Sap-treated rats, and was associated with an increase in c-Fos-IR cells. The systemic preadministration of the GABAA antagonist, bicuculline, and agonist, muscimol, had stronger decreasing effects on FRB and c-Fos-IR cells in IB4-Sap-treated rats than the preadministration of Sal, whereas the opposite effects were observed in Sal- and Bl-Sap-treated rats. These results indicate that IB4+ neurons in the trigeminal nerve play antinociceptive regulatory roles in formalin-induced orofacial pain processing and that GABAA receptor functions at segmental and supratrigeminal sites have complex modulatory influences on antinociceptive roles.

Antinociceptive roles of galanin receptor 1 in nucleus accumbens of rats in a model of neuropathic pain.

It has been reported that galanin and its receptors might be involved in the modulation and transmission of nociception in the central nervous system. Our previous research has also demonstrated that galanin induces antinociception in the nucleus accumbens (NAc) of intact rats. However, the interaction between galanin and its receptors in the NAc and the underlying mechanism of suppressing pain transmission remain unclear. The present study seeks to determine the antinociception induced by galanin receptor (GalR)-1 stimulation in the NAc of rats with neuropathic pain. The left sciatic nerve of rats was ligated to mimic a neuropathic pain model. Western blots showed that the expression of GalR1 was significantly upregulated in the NAc of rats with neuropathic pain. Intra-NAc injection of GalR1 agonist M617 induced a dose-dependent increase in hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulations in rats with neuropathic pain. Also, the effect of M617 was attenuated by M35, a GalR1/2 antagonist; at the same time, M35 reduced the galanin-induced antinociception, suggesting that GalR1 mediates antinociception induced by galanin in the NAc of rats with neuropathic pain. Furthermore, we found that M617-induced antinociception in rats with neuropathic pain was stronger than the antinociception in intact rats. We also found that injections of M617 and galanin each induced significant increases in HWL, but the galanin-induced antinociception was stronger than that of M617. All these results suggest that GalR1 plays an important role in antinociception and that other GalRs also are involved in pain modulation induced by galanin in the NAc of rats with neuropathic pain.

Is Acid Painful or not?

Acid (or proton) is an effective algogen to excite primary afferent nociceptors and evoke pain. Pain associated with tissue acidosis occurs where there is/are tissue injury, inflammation, ischemia, fatiguing exercise, or tumors. However, a recent study in mice has virtually changed our view regarding the significance of acid signaling in nociception. In the study, an acid-induced muscle pain model was used to probe the endogenous antinociceptive signaling pathway. Surprisingly, instead of acid-induced pain, the authors discovered that acid could also elicit an antinociceptive effect on mice, when specific acid sensors of muscle nociceptors, such as ASIC3 and TRPV1, were blocked. The acid-mediated antinociceptive pathway might act via a subset of acid-sensitive muscle afferent neurons to release substance P (SP), which would act on neurokinin 1 (NK1) receptors to reduce acid-induced depolarization via activation of an M-type potassium channel. The newly uncovered antinociceptive role for acid has largely changed our concept of pain biology and brought new insight for the analgesic drug development.

Involvement of Descending Serotonergic and Noradrenergic Systems and their Spinal Receptor Subtypes in the Antinociceptive Effect of Dipyrone.

The antinociceptive effect of dipyrone is partly due to its action upon pain-related central nervous system structures. Despite intensive research, the precise mechanisms mediating its analgesic effects remain unclear. Here, we aimed to determine whether neurotoxic destruction of descending inhibitory pathways affect dipyrone-induced antinociception and whether various spinal serotonergic and adrenergic receptors are involved in this antinociception. The nociceptive response was assessed by the tail-flick test. Mice injected with dipyrone (150, 300, 600 mg/kg, i.p.) elicited dose-related antinociception. The neurotoxins 5,7-dihydroxytryptamine (50 μg/mouse) and 6-hydroxydopamine (20 μg/mouse) are applied intrathecally to deplete serotonin and noradrenaline in the spinal cord. 3 days after neurotoxin injections, a significant reduction in the antinociceptive effect of dipyrone was observed. Intrathecal administration of monoaminergic antagonists (10 μg/mouse), the 5-HT2a antagonist ketanserin, the 5-HT3 antagonist ondansetron, the 5-HT7 antagonist SB-258719, α1-adrenoceptor antagonist prazosin, α2-adrenoceptor antagonist yohimbine, and the β-adrenoceptor antagonist propranolol also attenuated dipyrone antinociception. We propose that descending serotonergic and noradrenergic pathways play pivotal role in dipyrone-induced antinociception and spinal 5-HT2a, 5-HT3, and 5-HT7-serotonergic and α1, α2, and β-adrenergic receptors mediate this effect.

Oxytocin-induced membrane hyperpolarization in pain-sensitive dorsal root ganglia neurons mediated by Ca2+/nNOS/NO/KATP pathway

Oxytocin (OT) plays an important role in pain modulation and antinociception in the central nervous system. However, little is known about its peripheral effects. This study was conducted to investigate the effect of OT on the electrical properties of neurons in the dorsal root ganglia (DRG) and the underlying mechanisms. DRG neurons from adult rats were acutely dissociated and cultured. Intracellular Ca2+ was determined by fluorescent microscopy using an indicator dye. The electrical properties of DRG neurons were tested by patch-clamp recording. The oxytocin receptor (OTR) and neuronal nitric oxide synthase (nNOS) on DRG neurons were assessed with immunofluorescence assays. OTR co-localized with nNOS in most of Isolectin B4 (IB4)-binding cultured DRG neurons in rats. OT decreased the excitability, increased the outward current, and evoked the membrane hyperpolarization in cultured DRG neurons. Sodium nitroprusside (SNP), the donor of nitric oxide (NO), exerted similar effects as OT on the membrane potential of cultured DRG neurons. OT increased the production of NO in DRGs and cultured DRG neurons. Pre-treatment of the OTR antagonist atosiban or the selective nNOS inhibitor N-Propyl-l-arginine (NPLA) significantly attenuated the hyperpolarization effect evoked by OT. OT produced a concentration-dependent increase in intracellular Ca2+ in DRG neurons that responds to capsaicin, which can be attenuated by atosiban, but not by NPLA. OT-evoked membrane hyperpolarization and increase of outward current were distinctly attenuated by glibenclamide, a blocker of ATP-sensitive K+ (KATP) channel. OT might be an endogenous antinociceptive agent and the peripheral antinociceptive effects of OT are mediated by activation of the Ca2+/nNOS/NO/KATP pathway in DRG neurons.