Decrease In Mechanical Threshold Research Articles

111 Artificial Light-Triggered Sleep Deprivation May Lead to Allodynia in Rodent Model

IntroductionThe combination of artificial light and lack of exposure to natural light can delay the circadian clock, dysregulate the circadian cycle, and decrease alertness upon waking. This effect has been especially significant during the COVID-19 pandemic, where overexposure to artificial light at improper hours has contributed to increased rates of clinical insomnia. Artificial light may also contribute to concomitant neurological conditions such as primary headache, but the mechanisms by which light triggers sleep deprivation-induced headache are not well-understood.MethodsTo measure pain sensitivity, we habituated 13 wild-type male mice to von Frey filaments applied to the periorbital area until there was no response to 0.6g stimulus. We then applied 5 lux of continuous dim light to mice during their usual 12-hour dark cycle. The 12-hour light cycle remained unchanged with 200 lux continuous light. Three groups of mice experienced the dim light stimulus for one, three, or five consecutive days. Ambulation and rest activity were measured using SOF-812 Activity Monitor machines. After the experiment concluded, we waited 24 hours and measured mechanical threshold using von Frey filaments at 1, 3, 5, 8, and every 3 days subsequently until mice no longer responded to 0.6g stimulus.ResultsArtificial light triggered changes in circadian behavior including increased number of rest periods during 12-hour dark (dim light) cycle and shortened sleep duration during 12-hour light cycle. Following the artificial light stimulus, there was a significant decrease in mechanical threshold (P<0.05), representing allodynia. The one-day group displayed one day of significant allodynia. The three-day group displayed three days of significant allodynia. The five-day group displayed five days of significant allodynia.ConclusionArtificial light may trigger circadian dysregulation, and the duration of artificial light exposure seemed to be directly correlated to the duration of allodynia up to one week after the stimulus was removed. We will repeat these experiments and analyze CNS and PNS tissue samples to understand the underlying physiological and biochemical bases of how artificial light triggers sleep deprivation-induced headache. This knowledge could increase our understanding of the pathophysiology and comorbidity of sleep deprivation and headache.Support (if any)Funding was received from the National Institute of Health (NS104200).

Significance of medial preoptic area among the subcortical and cortical areas that are related to pain regulation in the rats with stress-induced hyperalgesia

Psychophysical stresses frequently increase sensitivity and response to pain, which is termed stress-induced hyperalgesia (SIH). However, the mechanism remains unknown. The subcortical areas such as medial preoptic area (MPO), dorsomedial nucleus of the hypothalamus (DMH), basolateral (BLA) and central nuclei of the amygdala (CeA), and the cortical areas such as insular (IC) and anterior cingulate cortices (ACC) play an important role in pain control via the descending pain modulatory system. In the present study we examined the expression of phosphorylated -cAMP-response element binding protein (pCREB) and the acetylation of histone H3 in these subcortical and cortical areas after repeated restraint stress to reveal changes in the subcortical and cortical areas that affect the function of descending pain modulatory system in the rats with SIH. The repeated restraint stress for 3 weeks induced a decrease in mechanical threshold in the rat hindpaw, an increase in the expression of pCREB in the MPO and an increase in the acetylation of histone H3 in the MPO, BLA and IC. The MPO was the only area that showed an increase in both the expression of pCREB and the acetylation of histone H3 among these examined areas after the repeated restraint stress. Furthermore, the number of pCREB-IR or acetylated histone H3-IR cells in the MPO was negatively correlated with the mechanical threshold. Together, our data represent the importance of the MPO among the subcortical and cortical areas that control descending pain modulatory system under the condition of SIH.

Involvement of NMDA receptors in the development of mirror image pain

Damage on one side of the body evoked pain bilaterally on both the injured and uninjured sides. The abnormal phenomenon called mirror image pain, is generally observed in many clinical pain syndromes and in various animal pain models. However, its mechanism is not fully understood. In the present study, we investigated the mechanism of mirror image pain in the complete Freund's adjuvant (CFA)-induced inflammatory pain model measuring the revealed mechanical allodynia by von Frey filament test. After CFA injection, the paw withdrawal threshold to mechanical stimuli was significantly decreased not only in the ipsilateral paw (CFA injected paw) but also in the contralateral paw (CFA non-injected paw). The decrease in mechanical threshold of the contralateral paw, which is called mirror image pain, had delayed onset as compared to the ipsilateral mechanical allodynia. NMDA receptor antagonist treated after CFA injection did not attenuate bilaterally allodynia, however, pretreatment with NMDA receptor antagonist was significantly attenuated the development of mirror image pain, without affecting the ipsilateral mechanical allodynia. These results suggest that the mechanism of mirror image pain via NMDA receptor activation was different from ipsilateral mechanical allodynia.

Icariin, a flavonoid with anti-cancer effects, alleviated paclitaxel-induced neuropathic pain in a SIRT1-dependent manner.

BackgroundOne of the most common side effects of paclitaxel was dosage-dependently painful neuropathy. Various reports indicated that spinal neuroinflammation was involved in paclitaxel-induced neuropathic pain. This study investigated the effect of icariin on paclitaxel-induced neuroinflammation and peripheral neuropathy in rats.MethodsTwo parts were included in this study. In part one, the effect of icariin on paclitaxel-induced neuropathic pain was investigated. Mechanical thresholds were measured as primary outcomes. Production of proinflammatory factors (tumor necrosis factor-α, interleukin-1 β, and interleukin-6), activation of nuclear factor-κB (NF-κB(p65)) signal, and activation of astrocytes were detected as secondary outcomes. Spinal Sirtuin 1 (SIRT1) expression, H4 acetylation, and NAD+ content were measured to investigate the effect of icariin on spinal SIRT1 signal pathway. In part two, the role of SIRT1 signal on icariin-induced effect in rats was investigated, and EX527, a SIRT1 inhibitor, was employed.ResultsThe results showed paclitaxel treatment induced significant decrease in mechanical thresholds. Paclitaxel treatment also induced NF-κB(p65) activation and upregulation of proinflammatory factors (TNF-α, IL-1β, and IL-6). Paclitaxel also induced astrocyte activation in the spinal cord. However, 100 mg/kg icariin treatment significantly alleviated paclitaxel-induced mechanical allodynia and spinal neuroinflammation. Furthermore, icariin treatment dosage-dependently reversed paclitaxel-induced SIRT1 downregulation and H4 acetylation. EX527, a selective SIRT1 inhibitor, completely reversed icariin-induced anti-neuroinflammation and anti-allodynia effects in paclitaxel-induced neuropathic pain rats.ConclusionsThis meant that spinal SIRT1 activation was involved in icariin-induced effects in paclitaxel-induced neuropathic pain rats. Icariin could be a potential agent for the treatment of paclitaxel-induced neuropathic pain.

Acupuncture treatment for idiopathic trigeminal neuralgia: A longitudinal case-control double blinded study.

To evaluate the treatment effect of acupuncture on patients with idiopathic trigeminal neuralgia (ITN) by case-control longitudinal blinded study. Sixty ITN patients and 30 healthy subjects were included. The ITN patients were randomly assigned to acupuncture group (15 cases), sham-acupuncture group (15 cases) and carbamazepine group (30 cases), respectively. Clinical orofacial evaluation (including pain intensity and medication doses), research diagnostic criteria for temporomandibular disorders (RDC/TMD) and Helkimo indexes (for functional evaluation of the masticatory system), and quantitative sensory testing for sensory thresholds (gustative, olfactory, cold, warm, touch, vibration and superficial and deep pain) were evaluated before treatment, immediately after treatment, and 6 months after treatment. The mean pain intensity by the Visual Analogue Scale only decreased in the acupuncture group at the last evaluation (P=0.012). Patients in the sham-acupuncture group had an increase in carbamazepine doses according to the prescriptions (P<0.01). There was a reduction in secondary myofascial pain and mandibular limitations at the acupuncture and sham-acupuncture groups, however only the acupuncture group kept the changes after 6 months (P<0.01, P=0.023). There was a decrease in mechanical thresholds in the acupuncture group (tactile, P<0.01; vibration, P=0.027) and an increase in deep pain thresholds in both acupuncture and sham-acupuncture groups (P=0.013). Acupuncture can be an option in the treatment of ITN due to its analgesic effect in both ITN and secondary myofascial pain associated with it.

Abstract 986: Targeting nicotinic acetylcholine receptors for the prevention and reversal of chemotherapy-induced peripheral neuropathy

Abstract While chemotherapy has played a significant role in the survival of cancer patients, exposure to drugs such as the taxanes, platinum-based compounds, and bortezomib often results in chemotherapy-induced peripheral neuropathy (CIPN), a consequence of peripheral nerve fiber dysfunction or degeneration. CIPN is characterized by sensory symptoms such as numbness, burning pain, and allodynia, resulting in an overall decrease in quality of life. Paclitaxel (Taxol), a taxane that is commonly used to treat breast, lung, and ovarian cancers, has been found to cause CIPN in 59-78% of cancer patients. There is currently no effective preventative or therapeutic treatment for this side effect, which can be a dose-limiting factor or delay treatment. Our studies have established that the prototypical nicotinic acetylcholine receptor (nAChR) agonist, nicotine, is capable of preventing and reversing the development of paclitaxel-induced CIPN, and does not stimulate tumor growth or interfere with the cytotoxic properties of paclitaxel. Male C57BL/6J mice were treated with paclitaxel (8 mg/kg, i.p.) every other day for a total of four injections. Nicotine was either infused via a subcutaneous 7-day osmotic minipump (24 mg/kg/day) starting 48 hours prior to paclitaxel treatment, or acutely injected (0.9 mg/kg, i.p.) following paclitaxel treatment. Von Frey filament testing revealed that nicotine can prevent and reverse paclitaxel-induced mechanical allodynia; the latter effect of nicotine can be inhibited by the α7 nAChR antagonist methyllycaconitine (10 mg/kg, s.c.). Also, α7 nAChR knockout mice exhibit a greater sustained decrease in mechanical threshold after paclitaxel administration than wild-type mice, suggesting the involvement of this receptor in CIPN development and maintenance. This hypothesis was supported by the reversal of paclitaxel-induced mechanical allodynia by the α7 nAChR silent agonist R-47. While nicotine activates its receptors in the tumors cells, based on stimulation of the PI3K/Akt pathway, MTT/MTS colorimetric assays showed that concentrations of nicotine ranging from 0.1 to 10 µM fail to significantly increase the viability of A549 or H460 non-small cell lung cancer cells, murine Lewis lung carcinoma cells, or primary human lung cancer cells. Most importantly, the paclitaxel-induced decreases in H460 proliferation and LLC viability are not significantly attenuated by 1 µM nicotine. Moreover, 1 µM nicotine does not interfere with paclitaxel-induced apoptosis of A549 cells. Finally, R-47 also suppresses CIPN without increasing lung tumor cell viability and colony formation, or interfering with the cytotoxicity of paclitaxel. Our in vitro findings are supported by studies demonstrating that nicotine does not enhance tumor volume or cause premature death in tumor-bearing mice. These data suggest that nAChRs may be promising drug targets for the prevention and treatment of CIPN. Citation Format: S Lauren Kyte, Wisam Toma, Ganeshsingh Thakur, M Imad Damaj, David A. Gewirtz. Targeting nicotinic acetylcholine receptors for the prevention and reversal of chemotherapy-induced peripheral neuropathy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 986. doi:10.1158/1538-7445.AM2017-986

Electrical signal propagated across acupoints along Foot Taiyang Bladder Meridian in rats.

To investigate the electrical signals propagated along Foot Taiyang Bladder Meridian (BL) in a rat model. The experiments were performed on Dark-Agouti (DA), DA.1U and Sprague Dawley (SD) rats. The antidromic electrical stimulation was applied on the nerve innervating "Pishu" (BL 20) to mimic the acupoint electro-acupuncture (EA). The activities recording from adjacent nerve innervating acupoint "Danshu" (BL 19) or "Weishu" (BL 21) were recorded as indics for acupoint, including the mechanical threshold and discharge rate. After mimic EA on BL 20, C and Aδ units from adjacent BL 19 or BL 21 were sensitized including the decrease in mechanical threshold and increase in discharge rates in DA, DA.1U and SD rats, especially in DA rats. The average discharge rate increased from 2.40±0.26 to 6.06±0.55 and from 1.92±0.42 to 6.17±1.10 impulse/min (P<0.01), and the mechanical threshold decreased from 0.52±0.12 to 0.24±0.05 and from 0.27±0.02 to 0.16±0.01 mmol/L (P<0.01) in C (n=15) and Aδ (n=18) units in DA rats. The net change in discharge rates from C units were 152.5%, 144.7% and 42.4% in DA, DA.1U and SD rats, respectively, among which DA rat's was the highest (P<0.05). In Aδ units, the net change in DA rats were also the highest (221.5%, 139.2% and 49.2% in DA, DA.1U and SD rats). These results showed that mimic acupoint EA activated adjacent acupoints along BL in three rat strains, which might be related to propagated sensation along meridians (PSM). In addition, DA rats were more sensitive and might be a good model animal for PSM research.

Blood-nerve barrier dysfunction contributes to the generation of neuropathic pain and allows targeting of injured nerves for pain relief

The blood-nerve barrier (BNB) is a selectively permeable barrier that creates an immunologically and biochemically privileged space for peripheral axons and supporting cells. The breakdown of the BNB allows access of blood-borne (hematogenous) cells and molecules to the endoneurium to engage in the local inflammatory cascade. This process was examined in a mouse model of trauma-associated neuropathic pain. The impact of nerve injury–triggered opening of the BNB in the development of chronic pain behavior was investigated. Partial ligation of the sciatic nerve led to a long-lasting disruption of the BNB distal to the site of injury. Vascular endothelial growth factor (VEGF) was expressed by resident macrophages after nerve injury. Intraneural injection of VEGF decreased mechanical thresholds while opening the BNB. Serum from nerve-injured or lipopolysaccharide-treated animals elicited mechanical allodynia in naive animals, when allowed to bypass the BNB by intraneural injection. Intraneural injection of fibrinogen, a clotting protein in plasma that was found to deposit in the nerve after nerve injury, also produced a decrease in mechanical thresholds when introduced into naive nerves. These results demonstrate that blood-borne molecules may play a role in the generation of neuropathic pain, suggesting that pain may be driven from infection or injury, at a distance from the nervous system. Furthermore, the breakdown of the BNB in neuropathic conditions was exploited to permit the entry of analgesic molecules that typically cannot pass the BNB, such as ProToxin-II, a BNB-impermeable Nav1.7 inhibitor. Therapeutics utilizing this mechanism could have selective access to injured nerves over healthy tissues.

Differential alternation of the antinociceptive effect of narcotic analgesics on the inflammatory pain state

Antinociceptive effect of narcotic analgesics, fentanyl, oxycodone and methadone in inflammatory pain state was described in the von Frey filament test using the complete Freund's adjuvant (CFA)-induced mouse inflammatory pain model. After the i.pl. injection of CFA, mechanical allodynia (decrease of mechanical threshold) was observed in ipsilateral paw. The antinociceptive effect of fentanyl and oxycodone injected s.c. against mechanical allodynia in inflammatory pain state was reduced bilaterally at 1 day after CFA pretreatment. However, the antinociceptive effect of methadone injected s.c. against mechanical allodynia in inflammatory pain state was reduced unilaterally at 1 day after CFA pretreatment. Moreover, the reduction of the antinociceptive effect of methadone in ipsilateral paw on inflammatory pain state was smaller than those of fentanyl or oxycodone. In conclusion, the alternation of the antinociceptive effect of narcotic analgesics in inflammatory pain state is variable among the narcotic analgesics used.

Effects of Naloxone on Opioid-induced Hyperalgesia and Tolerance to Remifentanil under Sevoflurane Anesthesia in Rats

Opioid antagonists at ultra-low doses have been used with opioid agonists to prevent or limit opioid tolerance. The aim of this study was to evaluate whether an ultra-low dose of naloxone combined with remifentanil could block opioid-induced hyperalgesia and tolerance under sevoflurane anesthesia in rats. Male adult Wistar rats were allocated into one of four treatment groups (n = 7), receiving remifentanil (4 µg·kg·min) combined with naloxone (0.17 ng·kg·min), remifentanil alone, naloxone alone, or saline. Animals were evaluated for mechanical nociceptive thresholds (von Frey) and subsequently anesthetized with sevoflurane to determine the baseline minimum alveolar concentration (MAC). Next, treatments were administered, and the MAC was redetermined twice during the infusion. The experiment was performed three times on nonconsecutive days (0, 2, and 4). Hyperalgesia was considered to be a decrease in mechanical thresholds, whereas opioid tolerance was considered to be a decrease in sevoflurane MAC reduction by remifentanil. Remifentanil produced a significant decrease in mechanical thresholds compared with baseline values at days 2 and 4 (mean ± SD, 30.7 ± 5.5, 22.1 ± 6.4, and 20.7 ± 3.7g at days 0, 2, and 4, respectively) and an increase in MAC baseline values (2.5 ± 0.3, 3.0 ± 0.3, and 3.1 ± 0.3 vol% at days 0, 2, and 4, respectively). Both effects were blocked by naloxone coadministration. However, both remifentanil-treated groups (with or without naloxone) developed opioid tolerance determined by their decrease in MAC reduction. An ultra-low dose of naloxone blocked remifentanil-induced hyperalgesia but did not change opioid tolerance under inhalant anesthesia. Moreover, the MAC increase associated with hyperalgesia was also blocked by naloxone.

Back Pain and the Mineralocorticoid Receptor

Back Pain and the Mineralocorticoid Receptor

Involvement of monoamine oxidase B on models of postoperative and neuropathic pain in mice

In this study we assessed the involvement of monoamine oxidase B (MAO-B), a key enzyme implicated in monoamine metabolism, on postoperative (plantar incision) and neuropathic (partial sciatic nerve ligation) pain models in mice. Paw incision submitted mice showed a significant decrease in mechanical threshold compared with the sham-operated mice, characterizing the development of mechanical allodynia. The selective and irreversible MAO-B inhibitor selegiline, at a dose sufficient to selectively inhibit MAO-B activity (10mg/kg), showed an anti-allodynic effect from 0.5 to 6h after incision. Likewise, partial sciatic nerve ligation submitted mice also developed mechanical allodynia, which was reversed by selegiline (10mg/kg) from 2 to 6h after treatment. In addition, a significant increase on striatal MAO-B activity was observed in neuropathic mice compared with the sham-operated animals, which was reversed by selegiline treatment. Taken together, our results showed that MAO-B seems to exert a critical role in the development of postoperative and neuropathic pain.

Nitrous oxide (N2O) reduces postoperative opioid-induced hyperalgesia after remifentanil–propofol anaesthesia in humans

The aim of this study was to test if intraoperative administration of N(2)O during propofol-remifentanil anaesthesia prevented the onset of postoperative opioid-induced hyperalgesia (OIH). Fifty adult ASA I-II patients undergoing elective open septorhinoplasty under general anaesthesia were studied. Anaesthesia was with propofol, adjusted to bispectral index (40-50), and remifentanil (0.30 μg kg(-1) min(-1)). Patients were assigned to one of the two groups: with N(2)O (70%) and without N(2)O (100% oxygen). Mechanical pain thresholds were measured before surgery and 2 and 12-18 h after surgery. Pain measurements were performed on the arm using hand-held von Frey filaments. A non-parametric analysis of variance was used in the von Frey data analysis. P<0.05 was considered statistically significant. Baseline pain thresholds to mechanical stimuli were similar in both groups, with mean values of 69 [95% confidence interval (CI): 50.2, 95.1] g in the group without N(2)O and 71 (95% CI: 45.7, 112.1) g in the group with N(2)O. Postoperative pain scores and cumulative morphine consumption were similar between the groups. The analysis revealed a decrease in the threshold value in both groups. However, post hoc comparisons showed that at 12-18 h after surgery, the decrease in mechanical threshold was greater in the group without N(2)O than the group with N(2)O (post hoc analysis with Bonferroni's correction, P<0.05). Intraoperative 70% N(2)O administration significantly reduced postoperative OIH in patients receiving propofol-remifentanil anaesthesia.

Etanercept attenuates pain-related behavior following compression of the dorsal root ganglion in the rat

TNFα is an inflammatory mediator related to neuropathic pain including sciatica. Much basic research suggests that anti-TNFα therapy may be useful for the treatment of sciatica. The purpose of this study was to clarify the effects of etanercept in a dorsal root ganglion (DRG) compression model. Adult male Sprague-Dawley rats (200-250g, n=60) were used. An L-shaped stainless rod was used to compress the left L5 DRG in the saline and etanercept groups. No rod was used in the sham group. In the etanercept group, 1mg of etanercept was applied locally onto the DRG at the end of surgery. Saline was applied in the saline and sham groups. On day 3 and day 7 after surgery, the number of ED1-immunoreactive (IR) cells (macrophages) in the DRG was calculated by immunohistochemical methods (n=6). In addition, double-immunofluorescence labeling for ED1 and TNFα was performed. Behavioral testing with von Frey filaments and a heat stimulator was performed (n=12). ED1-IR cells in the DRG significantly increased in the control group compared with the sham group (p<0.05). Some ED1-IR cells were co-labeled for TNFα. In the etanercept group, decrease in mechanical threshold was significantly inhibited compared with the saline group (p<0.05). Thermal hyperalgesia was observed in the control group, but in neither the sham nor etanercept group (p<0.05). Etanercept attenuated the pain-related behavior induced by DRG compression. These findings suggest that mechanical effects on the DRG might be reduced by etanercept in addition to the effects on nucleus pulposus in lumbar disc herniation.

Neurobiological effects of pulsed magnetic field on diabetes‐induced neuropathy

In the clinic, although several pharmacological agents or surgical procedures are used to treat diabetes and diabetes-induced neuropathic pain, their success has been limited. Therefore, development of different alternatives in treatments is very important. The purpose of this study was to determine the efficacy of pulsed magnetic field (PMF) in improving signs and symptoms of diabetic neuropathy. In this study, the effects of PMF treatment were investigated in Streptozotocin (STZ)-induced acute and chronic diabetic rats by measuring the thermal latencies, mechanical thresholds, whole blood glucose levels and body weights. After STZ administration to rats, blood glucose level elevated and body weight decreased. Although PMF treatment did not affect changes in body weight, the blood glucose levels of PMF-treated diabetic rats exhibited a decrease during the treatments. Diabetic animals displayed marked decrease in mechanical thresholds and thermal latencies. While treatment of PMF partially restored the mechanical thresholds and thermal latency in acute diabetic rats, PMF caused a corrective effect on only mechanical threshold of chronic diabetic rats. These results suggested that treatment of PMF can potentially ameliorate the painful symptoms of diabetes, such as hyperalgesia and allodynia, by partially preventing the hyperglycemia.

Tonic inhibition of somatostatin on C and Aδ afferent fibers in rat dorsal skin in vivo

The purpose of this study was to investigate the effect of somatostatin (SST) in peripheral nerve terminals using local application of the SST receptor (SSTR) antagonist cyclo-somatostatin (c-SOM) injected into the receptive fields of cutaneous afferent fibers innervating the dorsal hairy skin in anesthetized rats. Single unit activity was recorded in teased filaments from the dorsal cutaneous nerve branch. Recordings were obtained from 206 primary afferent fibers. They were classified as C ( n = 70), Aδ ( n = 84) or Aβ ( n = 52) fibers based upon their conduction velocity. For C and Aδ fibers, mean discharge rate increased and mechanical threshold decreased significantly to 10 μL of 12.8 and 128 μM injected subcutaneously, but not to 0.128 and 1.28 μM c-SOM injection. For Aβ fibers, no dose of c-SOM changed their discharge rate or their mechanical sensitivity. In control experiments, injection of normal saline (NS) had no effect on any of the units tested. Octreotide (20 μM, 10 μL), an SSTR agonist, successfully reversed the increase in discharge rates and the decrease in mechanical thresholds in C and Aδ fibers when it was pre-administrated into the receptive field before c-SOM injection. These results provide evidence that somatostatin tonically inhibits the peripheral nerve terminals of small-diameter cutaneous afferent fibers.

TRPC1 and TRPC6 Channels Cooperate with TRPV4 to Mediate Mechanical Hyperalgesia and Nociceptor Sensitization

The transient receptor potential vanilloid 4 (TRPV4) contributes to mechanical hyperalgesia of diverse etiologies, presumably as part of a mechanoreceptor signaling complex (Alessandri-Haber et al., 2008). To investigate the hypothesis that a functional interaction between TRPV4 and stretch-activated ion channels (SACs) is involved in this mechanical transduction mechanism, we used a selective SACs inhibitor, GsMTx-4. Intradermal injection of GsMTx-4 in the rat hindpaw reversed the mechanical hyperalgesia induced by intradermal injection of inflammatory mediators. In vivo single fiber recordings showed that GsMTx-4 reversed inflammatory mediator-induced decrease in mechanical threshold in half of sensitized C-fibers. Furthermore, GsMTx-4 reduced hyperalgesia to both mechanical and hypotonic stimuli in different models of inflammatory and neuropathic pain, although it had no effect on baseline mechanical nociceptive thresholds. TRPC1 and TRPC6, two GsMTx-4-sensitive SACs, are expressed in dorsal root ganglion (DRG) neurons. Single-cell reverse transcription-PCR showed that messenger RNAs for TRPV4, TRPC1, and TRPC6 are frequently coexpressed in DRG neurons. Spinal intrathecal administration of oligodeoxynucleotides antisense to TRPC1 and TRPC6, like that to TRPV4, reversed the hyperalgesia to mechanical and hypotonic stimuli induced by inflammatory mediators without affecting baseline mechanical nociceptive threshold. However, antisense to TRPC6, but not to TRPC1, reversed the mechanical hyperalgesia induced by a thermal injury or the TRPV4-selective agonist 4alpha-PDD (4 alpha-phorbol 12,13-didecanoate). We conclude that TRPC1 and TRPC6 channels cooperate with TRPV4 channels to mediate mechanical hyperalgesia and primary afferent nociceptor sensitization, although they may have distinctive roles.

Kainate-induced excitation and sensitization of nociceptors in normal and inflamed rat glabrous skin

This study investigates contributions of peripheral kainate receptors to acute nociception and persistent inflammatory pain in rat. Immunohistochemical analysis of kainate receptor expression using antibodies recognizing glutamate receptor subunits 5, 6, and 7 demonstrates that 28% of unmyelinated axons in normal digital nerve are positively labeled. Following intraplantar injection of complete Freund’s adjuvant, a significant increase in glutamate receptor subunits 5, 6, and 7-labeled axons occurs at 2 days (40%), but not 7 (31%) or 14 days (28%) post-complete Freund’s adjuvant. In behavioral studies, we confirm an increased mechanical sensitivity in complete Freund’s adjuvant-injected hind paws. Furthermore, activation of kainate receptors following intraplantar injection of 1.0mM kainate in normal animals results in a mechanical sensitivity similar to that observed in inflamed animals. A 1.0mM kainate injection into inflamed hind paws further enhances the mechanical sensitivity. Injection of the non– N-methyl- d-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (0.1mM) reverses complete Freund’s adjuvant-induced mechanical sensitivity through a local effect. In single unit recordings from nociceptors in a glabrous skin-nerve preparation, mechanical sensitization is present in inflamed skin evidenced by a decrease in mechanical threshold and an increase in discharge rate during a suprathreshold, constant force stimulus. Thermal sensitization is also present evidenced by a decrease in heat threshold. There is a dose-dependent increase in kainate-induced nociceptor activity in both normal and inflamed skin but the kainate required to induce activation is reduced in inflamed skin. Although proportions of kainate-activated nociceptors are the same in normal and inflamed skin, the kainate-induced mean discharge rate is significantly enhanced in inflamed skin. Exposure of normal and inflamed nociceptors to 0.3mM kainate sensitizes fibers to re-application of kainate and heat. This sensitization is blocked in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione or the glutamate receptor subunit 5 selective antagonist 3 S,4a R,6 S,8a R-6-[4-carboxy-phenyl] methyl-1,2,3,4,4a,5,6,7,8,8a-deca-hydroisoquinoline-3-carboxylic acid. The data indicate that peripheral kainate receptors not only play an important role in normal nociception but also contribute to mechanical sensitivity and heat sensitization accompanying inflammatory pain.

Acute and chronic ethanol exacerbates formalin pain in neonatal rats

We have previously reported that withdrawal from acute ethanol (EtOH) exposure lowers mechanical thresholds in post-natal day 7 (P7) and post-natal day 21 (P21) rats. The present study tested the hypothesis that daily administration of 4 g/kg 15% EtOH for 5 days in rats during the human developmental equivalent of the third trimester, but not at a later time in development, would alter mechanical thresholds and formalin-induced pain behaviors. A transient decrease in mechanical thresholds (allodynia) was observed in P7 rats upon withdrawal from repeated EtOH between P3 and P7. When challenged with intraplantar formalin on P11, rats exposed to acute or chronic EtOH had enhanced phase II pain behaviors. In contrast to chronic EtOH administration to rats between P3 and P7, prolonged mechanical allodynia was observed in P21 rats upon withdrawal from chronic EtOH between P17 and P21. Formalin responses were unchanged in P25 rats exposed to acute or chronic EtOH. The affects of EtOH on somatosensory processing are dependent upon the age at which exposure occurs.

Spinal δ-opioid receptors mediate suppression of systemic SNC80 on excitability of the flexor reflex in normal and inflamed rat

Due to low central nervous system (CNS) bioavailability of δ-opioid peptides, little is known about the effect of systemic administration of δ-opioid receptor ligands. The present study examined the effect of non-peptidergic δ-opioid receptor agonists, (+)-4-[(α R)-α-((2 R,5 R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]- N, N-diethylbenzamide (SNC80) and (−)dibenzoyl- l-tartaric acid salt (SNC86), on the activity of α−motoneurons in decerebrate-spinal rats. The flexor reflex was facilitated by C-afferent conditioning inputs, shown by a decrease in mechanical threshold and increase in touch- and pinch-evoked responses. Systemic administration of SNC80 (10 μmol/kg) prevented and reversed the neuronal hyperactivity. We further examined the effect of this agonist on the hypersensitivity of the flexor reflex induced by intraplantar injection of Freund's adjuvant. SNC80 dose-dependently (1, 3, 5 and 10 μmol/kg) increased the mechanical threshold and decreased touch-, pinch- and Aβ-afferent inputs-evoked responses. Similar effects were seen with SNC86 (5 μmol/kg). Pretreatment with either naloxone (20 μmol/kg, i.p.) or (Cyclopropylmethyl)-6,7-dehydro-4,5α-epoxy-14β-ethoxy-5β-methylindolo [2′,3′:6′,7′]morphinan-3-ol hydrochloride (SH378; 5 μmol/kg, intraarterially (i.a.)), a novel selective δ-opioid receptor antagonist, completely abolished the anti-hypersensitivity effect of SNC80. The effect of SNC80 remained following intrathecal administration of μ-opioid receptor antagonist d-Phe-Cys-Tyr- d-Trp-Orn-Thr-Pen-Thr-NH 2 (CTOP; 1.5 nmol). These results indicate that systemic injection of SNC80 exerted antihypersensitivity in models of both acute and tonic nociception and these effects are mediated mainly through a spinal δ-opioid mechanism.