Comparative actions of the opioid analgesics morphine, methadone and codeine in rat models of peripheral and central neuropathic pain

Rosuvastatin Synergistically Enhances the Antinociceptive Efficacy of Duloxetine in Paclitaxel-Induced Neuropathic Pain in Mice

Paclitaxel, a widely used cancer chemotherapeutic agent, has high incidence of neurotoxicity associated with the production of neuropathic pain, for which only duloxetine has shown significant but moderate analgesic effect. Since statins, classically used to reduce hypercholesterolemia, have shown antinociceptive effect in preclinical studies on neuropathic pain, we studied whether the antinociceptive efficacy of duloxetine could be synergistically potentiated by rosuvastatin in a model of paclitaxel-induced neuropathy in mice. The astrocytic and microglial responses in the spinal cord of paclitaxel-treated mice were also assessed by measuring GFAP and CD11b proteins, respectively. Paclitaxel treatment did not impair motor coordination and balance in rotarod testing. Rosuvastatin, duloxetine, and the rosuvastatin/duloxetine combination (combined at equieffective doses) dose-dependently decreased mechanical allodynia (ED₃₀, von Frey testing) and thermal hyperalgesia (ED₅₀, hot plate testing) in paclitaxel-treated mice. Isobolographic analysis showed a superadditive interaction for rosuvastatin and duloxetine, as both the ED₃₀ and ED₅₀ for the rosuvastatin/duloxetine combination contained only a quarter of each drug compared to the individual drugs. The rosuvastatin/duloxetine combination reversed paclitaxel-induced GFAP overexpression, indicating that such effects might depend in part on astrocyte inactivation. Results suggest that statins could be useful in synergistically enhancing the efficacy of duloxetine in some chemotherapy-induced neuropathic conditions.

Treatment of Pain in Rats, Mice, and Prairie Dogs

Recent myomorph and scuiromorph rodent analgesia studies are reviewed and evaluated for potential clinical application. Differences between laboratory animal studies and clinical use in diseased animals are discussed. Analgesia classes reviewed include local anesthetics, nonsteroidal anti-inflammatories, acetaminophen, opioids, and adjuvants such as anticonvulsants. Routes of administration including sustained-release mechanisms are discussed, as are reversal agents. Drug interactions are reviewed in the context of beneficial multimodal analgesia as well as potential adverse effects. Dosage recommendations for clinical patients are explored.

Combined effects of quetiapine and opioids: A study of autopsy cases, drug users and sedation in rats

Fatal opioid poisonings often involve methadone or morphine. This study aimed to elucidate if quetiapine, a widely used sedative antipsychotic medication, may increase the risk of fatal opioid poisoning by additive inhibitory effects on the central nervous system. We used data from 323 cases of fatal methadone or/and morphine poisonings autopsied from 2013 to 2020, a survey of 34 drug users, and performed blinded placebo‐controlled studies in 75 Flinders Resistant Line rats receiving three cumulative intraperitoneal doses of vehicle, methadone (2.5, 10 and 15 mg/kg), morphine (3.75, 15 and 22.5 mg/kg), quetiapine (3, 10 and 30 mg/kg) or quetiapine combined with methadone or morphine. Quetiapine was detected in 20.4% of fatal opioid poisonings with a significantly increased frequency over time, primarily in low or therapeutic concentrations, and was not associated with methadone or morphine concentrations. Use of quetiapine, most commonly in low‐to‐moderate doses to obtain a sleep‐inducing or tranquillizing effect, was reported by 67.6% of survey respondents. In the animal studies, a significant impairment of sedation score, performance on the rotarod and open field mobility was observed in all treatment groups compared with vehicle. However, the effect of quetiapine plus the opioid was not significantly different from that of the opioid alone. Thus, no additive sedative effects were observed in rats. Our results suggest that quetiapine is more often an innocent bystander than a contributor to fatal opioid poisoning. However, the combined effects on other parameters, including blood pressure, cardiac rhythm and respiratory rate, need investigation.

Anatomical Analysis of Transient Potential Vanilloid Receptor 1 (Trpv1+) and Mu-Opioid Receptor (Oprm1+) Co-expression in Rat Dorsal Root Ganglion Neurons

Primary afferent neurons of the dorsal root ganglia (DRG) transduce peripheral nociceptive signals and transmit them to the spinal cord. These neurons also mediate analgesic control of the nociceptive inputs, particularly through the μ-opioid receptor (encoded by Oprm1). While opioid receptors are found throughout the neuraxis and in the spinal cord tissue itself, intrathecal administration of μ-opioid agonists also acts directly on nociceptive nerve terminals in the dorsal spinal cord resulting in marked analgesia. Additionally, selective chemoaxotomy of cells expressing the TRPV1 channel, a nonselective calcium-permeable ion channel that transduces thermal and inflammatory pain, yields profound pain relief in rats, canines, and humans. However, the relationship between Oprm1 and Trpv1 expressing DRG neurons has not been precisely determined. The present study examines rat DRG neurons using high resolution multiplex fluorescent in situ hybridization to visualize molecular co-expression. Neurons positive for Trpv1 exhibited varying levels of expression for Trpv1 and co-expression of other excitatory and inhibitory ion channels or receptors. A subpopulation of densely labeled Trpv1+ neurons did not co-express Oprm1. In contrast, a population of less densely labeled Trpv1+ neurons did co-express Oprm1. This finding suggests that the medium/low Trpv1 expressing neurons represent a specific set of DRG neurons subserving the opponent processes of both transducing and inhibiting nociceptive inputs. Additionally, the medium/low Trpv1 expressing neurons co-expressed other markers implicated in pathological pain states, such as Trpa1 and Trpm8, which are involved in chemical nociception and cold allodynia, respectively, as well as Scn11a, whose mutations are implicated in familial episodic pain. Conversely, none of the Trpv1+ neurons co-expressed Spp1, which codes for osteopontin, a marker for large diameter proprioceptive neurons, validating that nociception and proprioception are governed by separate neuronal populations. Our findings support the hypothesis that the population of Trpv1 and Oprm1 coexpressing neurons may explain the remarkable efficacy of opioid drugs administered at the level of the DRG-spinal synapse, and that this subpopulation of Trpv1+ neurons is responsible for registering tissue damage.

Effect of Bufalin-PLGA Microspheres in the Alleviation of Neuropathic Pain via the CCI Model

The treatment of neuropathic pain (NPP) is considered challenging, while the search for alternative medication is striving. NPP pathology is related with the expression of both the purinergic 2X7 (P2X7) receptor and the transient receptor potential vanilloid 1 receptor (TRPV1). Bufalin is a traditional Chinese medication derived from toad venom with pronounced antitumor, analgesic, and anti-inflammatory properties. However, poor solubility, rapid metabolism, and the knowledge gap on its pain alleviation mechanism have limited the clinical application of bufalin. Hence, the purpose of this study is to illustrate the NPP alleviation mechanism of bufalin via chronic constriction injury (CCI). To address the concern on fast metabolism, bufalin-PLGA microspheres (MS) were prepared via membrane emulsification to achieve prolonged pain-relieving effects. Western blot, real-time PCR, immunofluorescence, and molecular docking were employed to demonstrate the therapeutic action of bufalin on NPP. The results showed enhanced thermal withdrawal latency (TWL) and mechanical withdrawal threshold (MWT) after the administration of both bufalin and bufalin-PLGA MS in the CCI rats. Prolonged pain-relieving effects for up to 3 days with reduced dose frequency was achieved via bufalin-PLGA MS. In the CCI rats treated with bufalin-PLGA MS, the expression levels of protein and mRNA in TRPV1 and P2X7, both localized in the dorsal root ganglion (DRG), were reduced. Moreover, bufalin-PLGA MS effectively reduced the levels of IL-1β, IL-18, IL-6, and TNF-α in the CCI group. The results from molecular docking suggested a possible mechanism of NPP alleviation of bufalin through binding to P2X7 receptors directly. The administration of bufalin-PLGA MS prepared by membrane emulsification demonstrated promising applications for sustained effect on the alleviation of NPP.

Ventral tegmental area serotonin 5-HT1A receptors and corticolimbic cFos/BDNF/GFAP signaling pathways mediate dextromethorphan/morphine anti-allodynia

AIMS

The present study examined the role of ventral tegmental area (VTA) serotonergic 5HT_1A receptors in dextromethorphan/morphine-induced anti-allodynia and the possible changes of corticolimbic cFos, brain-derived neurotrophic factor (BDNF), and glial fibrillary acidic protein (GFAP) following the treatments.

MATERIALS AND METHODS

The VTA cannulation and the chronic constriction of the sciatic nerve were performed in male Wistar rats. Flexion withdrawal thresholds to mechanical stimulation in the hind-limb were determined using von Frey hairs. The expressions of cFos, BDNF, and GFAP were evaluated using the Western blotting technique.

KEY FINDINGS

BDNF (in the hippocampus), and GFAP (in the targeted sites) levels were increased following neuropathic pain. Morphine administration induced an anti-allodynic effect with a decrease in the amygdala BDNF level. Dextromethorphan/morphine-induced anti-allodynia was accompanied by the decrease of hippocampus/amygdala/PFC GFAP and amygdala cFos expressions. The PFC BDNF expression level was increased in dextromethorphan/morphine-treated rats. Intra-VTA microinjection of (S)-WAY100135 (1 µg/rat), a selective 5-HT_1A receptor antagonist, inhibited the anti-allodynic effect of dextromethorphan/morphine. This treatment increased the cFos level in the hippocampus and the amygdala while decreased the PFC level of cFos. The hippocampal BDNF expression was significantly increased, while the amygdala and the PFC expressions of BDNF were decreased under treatment. (S)-WAY100135 plus dextromethorphan/morphine increased the hippocampal/amygdala and PFC levels of GFAP.

SIGNIFICANCE

These findings indicate that dextromethorphan could potentiate the analgesic effect of morphine via the implication of the VTA serotonin 5-HT_1A receptors. It seems that the changes in the corticolimbic cFos/BDNF/GFAP signaling pathway may be involved in the observed anti-allodynic effect.

Spontaneous, Voluntary, and Affective Behaviours in Rat Models of Pathological Pain

In pain patients affective and motivational reactions as well as impairment of daily life activities dominate the clinical picture. In contrast, many rodent pain models have been established on the basis of mechanical hypersensitivity testing. Up to today most rodent studies on pain still rely on reflexive withdrawal responses only. This discrepancy has likely contributed to the low predictive power of preclinical pain models for novel therapies. Here, we used a behavioural test array for rats to behaviourally evaluate five aetiologically distinct pain models consisting of inflammatory-, postsurgical-, cephalic-, neuropathic- and chemotherapy-induced pain. We assessed paralleling clinical expressions and comorbidities of chronic pain with an array of behavioural tests to assess anxiety, social interaction, distress, depression, and voluntary/spontaneous behaviours. Pharmacological treatment of the distinct pain conditions was performed with pathology-specific and clinically efficacious analgesics as gabapentin, sumatriptan, naproxen, and codeine. We found that rats differed in their manifestation of symptoms depending on the pain model and that pathology-specific analgesics also reduced the associated behavioural parameters. Based on all behavioural test performed, we screened for tests that can discriminate experimental groups on the basis of reflexive as well as non-sensory, affective parameters. Together, we propose a set of non-evoked behaviours with a comparable predictive power to mechanical threshold testing for each pain model.

Effects of Three Consecutive Days of Morphine or Methadone Administration on Analgesia and Open-Field Activity in Mice with Ehrlich Carcinoma

This study assessed the exploratory behavioral responses in BALB/c mice inoculated with Ehrlich ascitic carcinoma after 3 consecutive days of treatment with morphine or methadone. Fifty-three female mice, 60 ± 10 d old, were used. Seven days after intraperitoneal tumor inoculation (2 × 10⁶ cells), the animals were randomized into 7 groups: morphine 5 mg/kg (MO₅), morphine 7.5 mg/kg (MO_7.5), morphine 10 mg/kg (MO₁₀), methadone 2.85 mg/kg (ME_2.85), methadone 4.3 mg/kg (ME_4.3), methadone 5.7 mg/kg (ME_5.7), and 0.9% NaCl (Saline) (n = 7). Drug treatments were administered subcutaneously every 6 h for 3 d. The animals were evaluated for analgesia using the mouse grimace scale (MGS) and for general activity using the open field test. The MGS was performed before tumor inoculation (day 0), on day 7 at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 at 40, 150, 240, and 360 min after drug injection. The open field test was performed before tumor inoculation (day 0), on day 7 after inoculation at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 after inoculation at 40, 150, and 360 min after drug injection. MGS results indicated that administration of morphine promoted analgesia for up to 240 min. Conversely, methadone reduced MGS scores only at 40 min. All tested doses promoted a significant dose-dependent increase in the total distance traveled and the average speed, and increase that was markedly pronounced on days 8 and 9 as compared with day 7. The frequencies of rearing and self-grooming decreased significantly after morphine or methadone administration. Despite the difference in analgesia, both drugs increased locomotion and reduced the frequency of rearing and self-grooming as compared with the untreated control animals.

Magnesium Salt, a Simple Strategy to Improve Methadone Analgesia in Chronic Pain: An Isobolographic Preclinical Study in Neuropathic Mice

Analgesic efficacy of methadone in cancer and chronic non-cancer pains is greater than that of other opioids, probably because of its unique pharmacokinetics properties and also because it targets glutamatergic receptors in addition to µ-opioid receptors. However, methadone has drawbacks which are clearly related to dosing and treatment duration. The authors hypothesized that the antinociceptive efficacy of methadone could be synergistically potentiated by magnesium and copper salts in a preclinical mouse model of chronic pain, using the intraplantar formalin test as algesimetric tool. The spared nerve injury mice model was used to generate mononeuropathy. A low dose (0.25%) formalin was injected in the neuropathic limb in order to give rise only to Phase I response, resulting from direct activation by formalin of nociceptive primary afferents. Licking/biting of the formalin-injected limb was evaluated as nociceptive behavior during a 35-min observation period. Dose-response curves for intraperitoneal magnesium sulfate (10, 30, 100, and 300 mg/kg i.p.), copper sulfate (0.1, 0.3, 1, and 3 mg/kg i.p.) and methadone (0.1, 0.3, 1, and 3 mg/kg i.p.) allowed to combine them in equieffective doses and to determine their interaction by isobolographic analysis. Magnesium sulfate, copper sulfate and methadone dose-dependently decreased the nociceptive response evoked by formalin injection, the respective ED₅₀ being 76.38, 1.18, and 0.50 mg/kg i.p. Isobolographic analysis showed a superadditive interaction for magnesium and methadone. Indeed, despite that both ED₅₀ are obviously equieffective, the ED₅₀ for the MgSO4/methadone combination contained less than one third of the methadone having the ED₅₀ for methadone alone. For the CuSO₄/methadone combination, the interaction was only additive. Extrapolated to clinical settings, the results suggest that magnesium salts might be used to improve synergistically the efficacy of methadone in neuropathy, which would allow to reduce the dose of methadone and its associated side effects.

Neuropathic Pain Induced Alterations in the Opioidergic Modulation of a Descending Pain Facilitatory Area of the Brain

Opioids play a major role at descending pain modulation but the effects of neuropathic pain on the brain opioidergic system remain understudied. Since descending facilitation is enhanced during neuropathic pain, we studied the opioidergic modulation of the dorsal reticular nucleus (DRt), a medullary pain facilitatory area, in the spared nerve injury (SNI) model of neuropathic pain. We first performed a series of behavioral experiments in naïve-animals to establish the role of μ-opioid receptor (MOR) in the effects of endogenous and exogenous opioids at the DRt. Specifically, we showed that lentiviral-mediated MOR-knockdown at the DRt increased sensitivity to thermal and mechanical stimuli while the MOR agonist DAMGO induced the opposite effects. Additionally, we showed that MOR-knockdown and the pharmacological blockade of MOR by CTAP at the DRt decreased and inhibited, respectively, the analgesic effects of systemic morphine. Then, we performed in vivo microdialysis to measure enkephalin peptides in the DRt and evaluated MOR expression in the DRt at mRNA, protein and phosphorylated form levels by quantitative real-time PCR and immunohistochemistry, respectively. SNI-animals, compared to sham control, showed higher levels of enkephalin peptides, lower MOR-labeled cells without alterations in MOR mRNA levels, and higher phosphorylated MOR-labeled cells. Finally, we performed behavioral studies in SNI animals to determine the potency of systemic morphine and the effects of the pharmacologic and genetic manipulation of MOR at the DRt. We showed a reduced potency of the antiallodynic effects of systemic morphine in SNI-animals compared to the antinociceptive effects in sham animals. Increasing MOR-cells at the DRt of SNI-animals by lentiviral-mediated MOR-overexpression produced no effects on mechanical allodynia. DAMGO induced anti-allodynia only after MOR-overexpression. These results show that MOR inhibits DRt pain facilitatory actions and that this action contributes to the analgesic effects of systemic opioids. We further show that the inhibitory function of MOR is impaired during neuropathic pain. This is likely due to desensitization and degradation of MOR which are adaptations of the receptor that can be triggered by MOR phosphorylation. Skipping counter-regulatory pathways involved in MOR adaptations might restore the opioidergic inhibition at pain facilitatory areas.

Synergistic symptom-specific effects of ketorolac-tramadol and ketorolac-pregabalin in a rat model of peripheral neuropathy

BACKGROUND

Although current neuropathic pain treatment guidelines do not recommend the use of nonsteroidal anti-inflammatory drugs (NSAIDs), whether NSAIDs can serve as a useful adjuvant to conventional multimodal therapy remains unclear.

METHODS

The spared nerve injury (SNI) rats rapidly developed profound and long-lasting spontaneous and evoked pain behaviors, including mechanical and cold allodynia of the ipsilateral hind paw. At day 5, we first characterized the nociceptive responses to ketorolac, tramadol, pregabalin, and their combinations.

RESULTS

We found that tramadol and pregabalin exerted dose-dependent analgesic effects on both spontaneous and evoked behaviors. However, ketorolac alone did not suppress any behaviors regardless of the dose. Ketorolac-tramadol and ketorolac-pregabalin produced variable degrees of additive suppression of spontaneous and evoked behavioral responses. Cold allodynia was profoundly diminished after ketorolac was added to ineffective pregabalin or tramadol. Mechanical allodynia was markedly attenuated by ketorolac-pregabalin but less so by ketorolac-tramadol mixtures.

CONCLUSION

Our data demonstrated that an NSAID alone failed to relieve spontaneous or evoked pain behaviors in the rat SNI model, but when combined with a weak opioid and α-2-δ-ligand produced a profound synergistic analgesic effect on cold allodynia and discrepant efficacy for mechanical allodynia and spontaneous pain.

Exotic Mammal Orthopedics

Orthopedic disorders are a common clinical presentation for the exotic clinician. Before treating the fracture it is vital to stabilize the patient. Small exotic mammals are characterized by relatively thinner bones, adding to the difficulty the small size already represents. A combination of conservative and surgical treatment options are available. The principles of orthopedic surgery and ideas behind the treatment options remain the same as for small mammals, but not all techniques can be directly extrapolated. Historically, the tie-in fixator has been the preferred surgical choice whenever feasible, but further development in bone plates represents a promising advancement.

Is there a reasonable excuse for not providing post-operative analgesia when using animal models of peripheral neuropathic pain for research purposes?

Introduction

The induction of neuropathic pain-like behaviors in rodents often requires surgical intervention. This engages acute nociceptive signaling events that contribute to pain and stress post-operatively that from a welfare perspective demands peri-operative analgesic treatment. However, a large number of researchers avoid providing such care based largely on anecdotal opinions that it might interfere with model pathophysiology in the longer term.

Objectives

To investigate effects of various peri-operative analgesic regimens encapsulating different mechanisms and duration of action, on the development of post-operative stress/welfare and pain-like behaviors in the Spared Nerve Injury (SNI)-model of neuropathic pain.

Methods

Starting on the day of surgery, male Sprague-Dawley rats were administered either vehicle (s.c.), carprofen (5.0mg/kg, s.c.), buprenorphine (0.1mg/kg s.c. or 1.0mg/kg p.o. in Nutella^®), lidocaine/bupivacaine mixture (local irrigation) or a combination of all analgesics, with coverage from a single administration, and up to 72 hours. Post-operative stress and recovery were assessed using welfare parameters, bodyweight, food-consumption, and fecal corticosterone, and hindpaw mechanical allodynia was tested for assessing development of neuropathic pain for 28 days.

Results

None of the analgesic regimes compromised the development of mechanical allodynia. Unexpectedly, the combined treatment with 0.1mg/kg s.c. buprenorphine and carprofen for 72 hours and local irrigation with lidocaine/bupivacaine, caused severe adverse effects with peritonitis. This was not observed when the combination included a lower dose of buprenorphine (0.05mg/kg, s.c.), or when buprenorphine was administered alone (0.1mg/kg s.c. or 1.0mg/kg p.o.) for 72 hours. An elevated rate of wound dehiscence was observed especially in the combined treatment groups, underlining the need for balanced analgesia. Repeated buprenorphine injections had positive effects on body weight the first day after surgery, but depressive effects on food intake and body weight later during the first week.

Conclusion

Post-operative analgesia does not appear to affect established neuropathic hypersensitivity outcome in the SNI model.

Novel Endomorphin Analogs Are More Potent and Longer-Lasting Analgesics in Neuropathic, Inflammatory, Postoperative, and Visceral Pain Relative to Morphine

Activation of the mu-opioid receptor provides the gold standard for pain relief, but most opioids used clinically have adverse effects that have contributed to an epidemic of overdose deaths. We recently characterized mu-opioid receptor selective endomorphin (EM) analogs that provide potent antinociception with reduction or absence of a number of side effects of traditionally prescribed opioids including abuse liability, respiratory depression, motor impairment, tolerance, and inflammation. The current study explores the effectiveness of these EM analogs relative to morphine in four major pain models by intrathecal as well as intravenous administration in male Sprague Dawley rats and subcutaneous administration in male CD-1 mice. In the spared nerve injury model of neuropathic pain, mechanical allodynia and mechanical hyperalgesia were assessed with von Frey and Randall-Selitto tests, respectively. In the paw incision model of postoperative pain, von Frey testing was used to assess mechanical allodynia and thermal hyperalgesia was evaluated with Hargreaves testing. In the Complete Freund's Adjuvant model of inflammatory pain, thermal hyperalgesia was assessed using Hargreaves testing. In CD-1 mice, visceral pain was assessed with the acetic acid writhing test. In all cases, EM analogs had equal or greater potency and longer duration of action relative to morphine. The data suggest that EM analogs, particularly analog 4 (ZH853), could provide effective therapy for a diverse spectrum of pain conditions with low risk of adverse side effects compared with currently used opioids such as morphine.

PERSPECTIVE

Novel EM analogs show equal or greater potency and effectiveness relative to morphine in multiple pain models. Together with substantially reduced side effects, including abuse liability, the compounds show promise for addressing the critical need for effective pain relief as well as reducing the opioid overdose epidemic.

How to Improve Anesthesia and Analgesia in Small Mammals

The routine use of cardiovascular and respiratory monitor devices is essential for a good outcome in small mammal anesthesia. Physiologic differences between species and variation between individual animals should be considered when choosing an anesthetic protocol. The development of new pain assessment tools (eg, mouse grimace scale) can help recognize and alleviate pain.

Randall Selitto pressure algometry for assessment of bone-related pain in rats

BACKGROUND

Deep pain is neglected compared with cutaneous sources. Pressure algometry has been validated in the clinic for assessment of bone-related pain in humans. In animal models of bone-related pain, we have validated the Randall Selitto behavioural test for assessment of acute and pathological bone pain and compared the outcome with more traditional pain-related behaviour measures.

METHODS

Randall Selitto pressure algometry was performed over the anteromedial part of the tibia in naïve rats, sham-operated rats, and rats inoculated with MRMT-1 carcinoma cells in the left tibia, and the effect of morphine was investigated. Randall Selitto measures of cancer-induced bone pain were supplemented by von Frey testing, weight-bearing and limb use test. Contribution of cutaneous nociception to Randall Selitto measures were examined by local anaesthesia.

RESULTS

Randall Selitto pressure algometry over the tibia resulted in reproducible withdrawal thresholds, which were dose-dependently increased by morphine. Cutaneous nociception did not contribute to Randall Selitto measures. In cancer-bearing animals, compared with sham, significant differences in pain-related behaviours were demonstrated by the Randall Selitto test on day 17 and 21 post-surgery. A difference was also demonstrated by von Frey testing, weight-bearing and limb use tests.

CONCLUSION

Our results indicate that pressure applied by the Randall Selitto algometer on a region, where the bone is close to the skin, may offer a way to measure bone-related pain in animal models and could provide a supplement to the traditional behavioural tests and a means to study deep pain.

NS383 Selectively Inhibits Acid-Sensing Ion Channels Containing 1a and 3 Subunits to Reverse Inflammatory and Neuropathic Hyperalgesia in Rats

AIMS

Here, we investigate the pharmacology of NS383, a novel small molecule inhibitor of acid-sensing ion channels (ASICs).

METHODS

ASIC inhibition by NS383 was characterized in patch-clamp electrophysiological studies. Analgesic properties were evaluated in four rat behavioral models of pain.

RESULTS

NS383 inhibited H(+)-activated currents recorded from rat homomeric ASIC1a, ASIC3, and heteromeric ASIC1a+3 with IC50 values ranging from 0.61 to 2.2 μM. However, NS383 was completely inactive at homomeric ASIC2a. Heteromeric receptors containing AISC2a, such as ASIC1a+2a and ASIC2a+3, were only partially inhibited, presumably as a result of stoichiometry-dependent binding. NS383 (10-60 mg/kg, i.p.), amiloride (50-200 mg/kg, i.p.), acetaminophen (100-400 mg/kg, i.p.), and morphine (3-10 mg/kg, i.p.) all dose-dependently attenuated nocifensive behaviors in the rat formalin test, reversed pathological inflammatory hyperalgesia in complete Freund's adjuvant-inflamed rats, and reversed mechanical hypersensitivity in the chronic constriction injury model of neuropathic pain. However, in contrast to acetaminophen and morphine, motor function was unaffected by NS383 at doses at least 8-fold greater than those that were effective in pain models, whilst analgesic doses of amiloride were deemed to be toxic.

CONCLUSIONS

NS383 is a potent and uniquely selective inhibitor of rat ASICs containing 1a and/or 3 subunits. It is well tolerated and capable of reversing pathological painlike behaviors, presumably via peripheral actions, but possibly also via actions within central pain circuits.

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Establishment and characterization of an optimized mouse model of multiple sclerosis-induced neuropathic pain using behavioral, pharmacologic, histologic and immunohistochemical methods

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) that causes debilitating central neuropathic pain in many patients. Although mouse models of experimental autoimmune encephalomyelitis (EAE) have provided insight on the pathobiology of MS-induced neuropathic pain, concurrent severe motor impairments confound quantitative assessment of pain behaviors over the disease course. To address this issue, we have established and characterized an optimized EAE-mouse model of MS-induced neuropathic pain. Briefly, C57BL/6 mice were immunized with MOG35-55 (200μg) and adjuvants comprising Quil A (45μg) and pertussis toxin (2×250ng). The traditionally used Freund's Complete Adjuvant (FCA) was replaced with Quil A, as FCA itself induces CNS neuroinflammation. Herein, EAE-mice exhibited a mild relapsing-remitting clinical disease course with temporal development of mechanical allodynia in the bilateral hindpaws. Mechanical allodynia was fully developed by 28-30days post-immunization (p.i.) and was maintained until study completion (52-60days p.i.), in the absence of confounding motor deficits. Single bolus doses of amitriptyline (1-7mg/kg), gabapentin (10-50mg/kg) and morphine (0.1-2mg/kg) evoked dose-dependent analgesia in the bilateral hindpaws of EAE-mice; the corresponding ED50s were 1.5, 20 and 1mg/kg respectively. At day 39 p.i. in EAE-mice exhibiting mechanical allodynia in the hindpaws, there was marked demyelination and gliosis in the brain and lumbar spinal cord, mirroring these pathobiologic hallmark features of MS in humans. Our optimized EAE-mouse model of MS-associated neuropathic pain will be invaluable for future investigation of the pathobiology of MS-induced neuropathic pain and for efficacy profiling of novel molecules as potential new analgesics for improved relief of this condition.

Effects of opioids on local anesthesia in the rat: a codeine and tramadol study

Opioids are central analgesics that act on the CNS (central nervous system) and PNS (peripheral nervous system). We investigated the effects of codeine (COD) and tramadol (TRAM) on local anesthesia of the sciatic nerve. Eighty Wistar male rats received the following SC injections in the popliteal fossa: local anesthetic with epinephrine (LA); local anesthetic without vasoconstrictor (LA WV); COD; TRAM; LA + COD; LA + TRAM; COD 20 minutes prior to LA (COD 20' + LA) or TRAM 20 minutes prior to LA (TRAM 20' + LA). As a nociceptive function, the blockade was considered the absence of a paw withdraw reflex. As a motor function, it was the absence of claudication. As a proprioceptive function, it was the absence of hopping and tactile responses. All data were compared using repeated-measures analysis of variance (ANOVA). Opioids showed a significant increase in the level of anesthesia, and the blockade duration of LA + COD was greater than that of the remaining groups (p < 0.05). The associated use of opioids improved anesthesia efficacy. This could lead to a new perspective in controlling dental pain.

Anti-hyperalgesic effect of a benzilidine-cyclohexanone analogue on a mouse model of chronic constriction injury-induced neuropathic pain: Participation of the κ-opioid receptor and KATP

The present study investigated the analgesic effect of a novel synthetic cyclohexanone derivative, 2,6-bis-4-(hydroxyl-3-methoxybenzilidine)-cyclohexanone or BHMC in a mouse model of chronic constriction injury-induced neuropathic pain. It was demonstrated that intraperitoneal administration of BHMC (0.03, 0.1, 0.3 and 1.0mg/kg) exhibited dose-dependent inhibition of chronic constriction injury-induced neuropathic pain in mice, when evaluated using Randall-Selitto mechanical analgesiometer. It was also demonstrated that pretreatment of naloxone (non-selective opioid receptor blocker), nor-binaltorphimine (nor-BNI, selective κ-opioid receptor blocker), but not β-funaltrexamine (β-FN, selective μ-opioid receptor blocker) and naltrindole hydrochloride (NTI, selective δ-opioid receptor blocker), reversed the anti-nociceptive effect of BHMC. In addition, the analgesic effect of BHMC was also reverted by pretreatment of 1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one (ODQ, soluble guanosyl cyclase blocker) and glibenclamide (ATP-sensitive potassium channel blocker) but not Nω-nitro-l-arginine (l-NAME, a nitric oxide synthase blocker). Taken together, the present study demonstrated that the systemic administration of BHMC attenuated chronic constriction, injury-induced neuropathic pain. We also suggested that the possible mechanisms include κ-opioid receptor activation and nitric oxide-independent cyclic guanosine monophosphate activation of ATP-sensitive potassium channel opening.

Tramadol and propentofylline coadministration exerted synergistic effects on rat spinal nerve ligation-induced neuropathic pain

Neuropathic pain is an intractable clinical problem. Drug treatments such as tramadol have been reported to effectively decrease neuropathic pain by inhibiting the activity of nociceptive neurons. It has also been reported that modulating glial activation could also prevent or reverse neuropathic pain via the administration of a glial modulator or inhibitor, such as propentofylline. Thus far, there has been no clinical strategy incorporating both neuronal and glial participation for treating neuropathic pain. Therefore, the present research study was designed to assess whether coadministration of tramadol and propentofylline, as neuronal and glial activation inhibitors, respectively, would exert a synergistic effect on the reduction of rat spinal nerve ligation (SNL)-induced neuropathic pain. Rats underwent SNL surgery to induce neuropathic pain. Pain behavioral tests were conducted to ascertain the effect of drugs on SNL-induced mechanical allodynia with von-Frey hairs. Proinflammatory factor interleukin-1β (IL-1β) expression was also detected by Real-time RT-PCR. Intrathecal tramadol and propentofylline administered alone relieved SNL-induced mechanical allodynia in a dose-dependent manner. Tramadol and propentofylline coadministration exerted a more potent effect in a synergistic and dose dependent manner than the intrathecal administration of either drug alone. Real-time RT-PCR demonstrated IL-1β up-expression in the ipsilateral spinal dorsal horn after the lesion, which was significantly decreased by tramadol and propentofylline coadministration. Inhibiting proinflammatory factor IL-1β contributed to the synergistic effects of tramadol and propentofylline coadministration on rat peripheral nerve injury-induced neuropathic pain. Thus, our study provided a rationale for utilizing a novel strategy for treating neuropathic pain by blocking the proinflammatory factor related pathways in the central nervous system.

Methadone in post-herpetic neuralgia: A pilot proof-of-concept study

OBJECTIVE

This research was designed as a pilot proof-of-concept study to evaluate the use of low-dose methadone in post-herpetic neuralgia patients who remained refractory after first and second line post-herpetic neuralgia treatments and had indications for adding an opioid agent to their current drug regimens.

METHODS

This cross-over study was double blind and placebo controlled. Ten opioid naïve post-herpetic neuralgia patients received either methadone (5 mg bid) or placebo for three weeks, followed by a 15-day washout period and a second three-week treatment with either methadone or placebo, accordingly. Clinical evaluations were performed four times (before and after each three-week treatment period). The evaluations included the visual analogue scale, verbal category scale, daily activities scale, McGill pain questionnaire, adverse events profile, and evoked pain assessment. All patients provided written informed consent before being included in the study. ClinicalTrials.gov: NCT01752699 RESULTS: Methadone, when compared to placebo, did not significantly affect the intensity of spontaneous pain, as measured by the visual analogue scale. The intensity of spontaneous pain was significantly decreased after the methadone treatment compared to placebo on the category verbal scale (50% improved after the methadone treatment, none after the placebo, p=0.031). Evoked pain was reduced under methadone compared to placebo (50% improved after the methadone treatment, none after the placebo, p=0.031). Allodynia reduction correlated with sleep improvement (r=0.67, p=0.030) during the methadone treatment. The side effects profile was similar between both treatments.

CONCLUSIONS

Methadone seems to be safe and efficacious in post-herpetic neuralgia. It should be tried as an adjunctive treatment for post-herpetic neuralgia in larger prospective studies.

Repeated exposure to codeine alters morphine glucuronidation by affecting UGT gene expression in the rat

We have previously found that phenantrenic opioids, such as heroin or naltrexone, modulate morphine glucuronidation in the rat. Here we further investigated the effects of phenantrenic opioids on morphine glucuronidation comparing the effects of codeine and heroin. In particular, we measured the synthesis of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) from morphine: in the liver microsomal preparations obtained from rats repeatedly treated with two different doses of codeine (ex vivo study); in primary cultures of rat hepatocytes previously incubated for 72h with codeine, or heroin (in vitro study); in the latter conditions, the levels of expression of genes coding for uridine-5'-diphosphate-glucuronosyltransferases (UGTs) A1, A6, A7 and 2B1 were also determined; finally, the levels of glucuronic acid in rat hepatocytes previously incubated for 72h with codeine or heroin were assessed. The ex vivo study shows that codeine exposure in vivo stimulated liver microsomal M3G formation and de novo synthesis of M6G. Differently, in primary hepatocyte cultures both codeine and heroin inhibited M3G formation, whereas heroin only stimulated de novo synthesis of M6G; moreover, codeine significantly reduced UGT2B1 expression at 6h and caused a trend toward inhibition of UGT1A1 expression at 72h; heroin enhanced UGT2B1 expression and inhibited that of UGT1A1 at 72h; finally, both codeine and heroin depleted UDPGA content of hepatocytes. In conclusion, codeine affects liver glucuronidation of morphine enlightening the possible contribution of changes in the spectrum of UGT gene expression and co-factor synthesis in this phenomenon.

Progesterone produces antinociceptive and neuroprotective effects in rats with microinjected lysophosphatidic acid in the trigeminal nerve root

Background

In our present study, we studied the role of demyelination of the trigeminal nerve root in the development of prolonged nociceptive behavior in the trigeminal territory.

Results

Under anesthesia, the Sprague-Dawley rats were mounted onto a stereotaxic frame and 3 μL of lysophosphatidic acid (LPA, 1 nmol) was injected into the trigeminal nerve root to produce demyelination. This treatment decreased the air-puff thresholds, persisted until postoperative day 130, and then returned to the preoperative levels 160 days after LPA injection. The LPA-treated rats also showed a significant hyper-responsiveness to pin-prick stimulation. We further investigated the antinociceptive and neuroprotective effects of progesterone in rats undergoing demyelination of the trigeminal nerve root. Progesterone (8, 16 mg/kg/day) was administered subcutaneously, beginning on the operative day, for five consecutive days in the LPA-treated rats. Treatment with progesterone produced significant early anti-allodynic effects and delayed prolonged anti-allodynic effects. The expression of protein zero (P0) and peripheral myelin protein 22 (PMP22) were significantly down-regulated in the trigeminal nerve root on postoperative day 5 following LPA injection. This down-regulation of the P0 and PMP22 levels was blocked by progesterone treatment.

Conclusions

These results suggest that progesterone produces antinociceptive effects through neuroprotective action in animals with LPA-induced trigeminal neuropathic pain. Moreover, progesterone has potential utility as a novel therapy for trigeminal neuropathic pain relief at an appropriate managed dose and is therefore a possible future treatment strategy for improving the recovery from injury.

Blockade of microglial activation reduces mechanical allodynia in rats with compression of the trigeminal ganglion

The present study investigated the role of microglia and p38 MAPK in the development of mechanical allodynia in rats with compression of the trigeminal ganglion. Male Sprague-Dawley rats weighing 250-260 g were used. Under pentobarbital sodium anesthesia, the animals were mounted onto a stereotaxic frame and given injections of 4% agar solution (10 μL) to compress the trigeminal ganglion. The air-puff thresholds significantly decreased after compression of the trigeminal ganglion. On postoperative day 14, immunoreactivity to both OX-42 and p-p38 MAPK was up-regulated in the medullary dorsal horn as compared to the sham group. P-p38 MAPK was found to be co-localized with OX-42, but not with NeuN, a neuronal cell marker, or with GFAP, an astroglial cell marker. Intracisternal administration of 100 μg of minocycline significantly inhibited both mechanical allodynia and activation of microglia produced by compression of the trigeminal ganglion. Intracisternal administration of 0.1, 1, or 10 μg of SB203580, a p38 MAPK inhibitor, also significantly decreased mechanical allodynia and p38 MAPK activation in the trigeminal ganglion-compressed group. These results suggest that activation of p38 MAPK in the microglia is an important step in the development of mechanical allodynia in rats with compression of the trigeminal ganglion and that the targeted blockade of microglial p38 MAPK pathway is a potentially important new treatment strategy for trigeminal neuralgia-like nociception.

Enhancement of Antinociception by Co-administrations of Nefopam, Morphine, and Nimesulide in a Rat Model of Neuropathic Pain

Background

Neuropathic pain is a chronic pain due to disorder in the peripheral or central nervous system with different pathophysiological mechanisms. Current treatments are not effective. Analgesic drugs combined can reduce pain intensity and side effects. Here, we studied the analgesic effect of nimesulide, nefopam, and morphine with different mechanisms of action alone and in combination with other drugs in chronic constriction injury (CCI) model of neuropathic pain.

Methods

Male Wistar rats (n = 8) weighing 150-200 g were divided into 3 different groups: 1- Saline-treated CCI group, 2- Saline-treated sham group, and 3- Drug-treated CCI groups. Nimesulide (1.25, 2.5, and 5 mg/kg), nefopam (10, 20, and 30 mg/kg), and morphine (1, 3, and 5 mg/kg) were injected 30 minutes before surgery and continued daily to day 14 post-ligation. In the combination strategy, a nonanalgesic dose of drugs was used in combination such as nefopam + morphine, nefopam + nimesulide, and nimesulide + morphine. Von Frey filaments for mechanical allodynia and acetone test for cold allodynia were, respectively, used as pain behavioral tests. Experiments were performed on day 0 (before surgery) and days 1, 3, 5, 7, 10, and 14 post injury.

Results

Nefopam (30 mg/kg) and nimesulide (5 mg/kg) blocked mechanical and thermal allodynia; the analgesic effects of morphine (5 mg/kg) lasted for 7 days. Allodynia was completely inhibited in combination with nonanalgesic doses of nefopam (10 mg/kg), nimesulide (1.25 mg/kg), and morphine (3 mg/kg).

Conclusions

It seems that analgesic drugs used in combination, could effectively reduce pain behavior with reduced adverse effects.

Predictive validity of pharmacologic interventions in animal models of neuropathic pain

Introduction

The pathophysiologic and neurochemical characteristics of neuropathic pain must be considered in the search for new treatment targets. Breakthroughs in the understanding of the structural and biochemical changes in neuropathy have opened up possibilities to explore new treatment paradigms. However, long term sequels from the damage are still difficult to treat.

Aim of the study

To examine the validity of pharmacological treatments in humans and animals for neuropathic pain.

Method

An overview from the literature and own experiences of pharmacological treatments employed to interfere in pain behavior in different animal models was performed.

Results

The treatment principles tested in animal models of neuropathic pain may have predictive validity for treatment of human neuropathies. Opioids, neurotransmitter blockers, drugs interfering with the prostaglandin syntheses as well as voltage gated sodium channel blockers and calcium channel blockers are treatment principles having efficacy and similar potency in humans and in animals. Alternative targets have been identified and have shown promising results in the validated animal models. Modulators of the glutamate system with an increased expression of glutamate re-uptake transporters, inhibition of pain promoters as nitric oxide and prostaglandins need further exploration. Modulation of cytokines and neurotrophins in neuropathic pain implies new targets for study. Further, a combination of different analgesic treatments may as well improve management of neuropathic pain, changing the benefit/risk ratio.

Implications

Not surprisingly most pharmacologic principles that are tested in animal models of neuropathic pain are also found to be active in humans. Whereas many candidate drugs that were promising in animal models of neuropathic pain turned out not to be effective or too toxic in humans, animal models for neuropathic pain are still the best tools available to learn more about mechanisms of neuropathic pain. Better understanding of pathogenesis is the most hopeful approach to improve treatment of neuropathic pain.

GABAergic synaptic response and its opioidergic modulation in periaqueductal gray neurons of rats with neuropathic pain

Background

Neuropathic pain is a chronic and intractable symptom associated with nerve injury. The periaqueductal gray (PAG) is important in the endogenous pain control system and is the main site of the opioidergic analgesia. To investigate whether neuropathic pain affects the endogenous pain control system, we examined the effect of neuropathic pain induced by sacral nerve transection on presynaptic GABA release, the kinetics of postsynaptic GABA-activated Cl^- currents, and the modulatory effect of μ-opioid receptor (MOR) activation in mechanically isolated PAG neurons with functioning synaptic boutons.

Results

In normal rats, MOR activation inhibited the frequency of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) to 81.3% of the control without any alteration in their amplitude. In neuropathic rats, the inhibition of mIPSC frequency by MOR activation was 82.4%. The frequency of GABAergic mIPSCs in neuropathic rats was 151.8% of normal rats without any difference in the mIPSC amplitude. Analysis of mIPSC kinetics showed that the fast decay time constant and synaptic charge transfer of mIPSCs in neuropathic rats were 76.0% and 73.2% of normal rats, respectively.

Conclusions

These results indicate that although the inhibitory effect of MOR activation on presynaptic GABA release is similar in both neuropathic and normal rats, neuropathic pain may inhibit endogenous analgesia in the PAG through an increase in presynaptic GABA release.

A question of balance--positive versus negative allosteric modulation of GABA(A) receptor subtypes as a driver of analgesic efficacy in rat models of inflammatory and neuropathic pain

After injury GABA(A) receptor positive allosteric modulators (PAMs) mediate robust analgesia in animals via putative restoration of post-synaptic GABA(A)-α2 and -α3 receptor function within the spinal cord. GABA can also act at GABA(A) receptors localized on primary afferent neurones to inhibit presynaptic neurotransmitter release and produce analgesia via a process called primary afferent depolarization (PAD). Some forms of injury might sufficiently enhance PAD to shift it into a net excitatory process. Thus, negative allosteric modulators (NAMs) might also possess analgesic activity. We have compared compounds capable of either positively or negatively modulating GABA(A) receptors in rat models associated with injury-induced central sensitization. The subtype-selective PAMs NS11394 (1-10 mg/kg) and TPA023 (3-30 mg/kg) attenuated formalin-induced nocifensive behaviours. Similarly, both compounds reversed hindpaw mechanical hypersensitivity and weight bearing deficits in carrageenan-inflamed and nerve-injured rats. The non-selective PAM diazepam (1-5 mg/kg) was ineffective in all models. Surprisingly, both the non-selective NAM FG-7142 (3-30 mg/kg) and the α5-selective NAM α5IA-II (10-60 mg/kg) also attenuated formalin-induced nocifensive behaviours. In carrageenan-inflamed rats α5IA-II reversed mechanical hypersensitivity and weight bearing deficits whilst FG-7142 only attenuated weight bearing deficits. This picture was essentially reversed in nerve-injured rats for these two NAMs. With the exception of NS11394, all compounds attenuated exploratory motility behaviour in rats, either as a consequence of sedative or anxiogenic-like side-effects. These data indicate that the preferred selectivity and activity profiles for mediating analgesia upon activation of GABA(A) receptors might be more complex than previously anticipated, and is worthy of further exploration.

The animal model of spinal cord injury as an experimental pain model

Pain, which remains largely unsolved, is one of the most crucial problems for spinal cord injury patients. Due to sensory problems, as well as motor dysfunctions, spinal cord injury research has proven to be complex and difficult. Furthermore, many types of pain are associated with spinal cord injury, such as neuropathic, visceral, and musculoskeletal pain. Many animal models of spinal cord injury exist to emulate clinical situations, which could help to determine common mechanisms of pathology. However, results can be easily misunderstood and falsely interpreted. Therefore, it is important to fully understand the symptoms of human spinal cord injury, as well as the various spinal cord injury models and the possible pathologies. The present paper summarizes results from animal models of spinal cord injury, as well as the most effective use of these models.

Antinociceptive Effect of Memantine and Morphine on Vincristine-induced Peripheral Neuropathy in Rats

Background

Vincristine-induced peripheral neuropathy is a major dose limiting side effect and thus effective therapeutic strategy is required. In this study, we investigated the antinociceptive effect of memantine and morphine on a vincristine-induced peripheral neuropathy model in rats.

Methods

Male Sprague-Dawley rats weighing 220-240 g were used in all experiments. Rats subsequently received daily intraperitoneal injections of either vincristine sulfate (0.1 ml/kg/day) or saline (0.1 ml/kg/day) over 12 days, immediately following behavioral testing. For assessment of mechanical allodynia, mechanical stimuli using von Frey filament was applied to the paw to measure withdrawal threshold. The effects of N-methyl-D-aspartate receptors antagonist (memantine; 2.5, 5, 10 mg/kg intraperitoneal), opioid agonist (morphine; 2.5, 5, 10 mg/kg intraperitoneal) and vehicle (saline) on vicristine-induced neuropathy were evaluated.

Results

Mechanical allodynia developed over the course of ten daily injections of vincristine relative to groups receiving saline at the same time. Morphine abolished the reduction in paw withdrawal threshold compared to vehicle and produced dose-responsiveness. Only the highest dose of memantine (10 mg/kg) was able to increase paw withdrawal threshold compared to vehicle.

Conclusions

Systemic morphine and memantine have an antinociceptive effect on the vincristine-induced peripheral neuropathy model in rats. These results suggest morphine and memantine may be an alternative approach for the treatment of vincristine-induced peripheral neuropathic pain.

Thermal nociception is decreased by hypocretin-1 and an adenosine A1 receptor agonist microinjected into the pontine reticular formation of Sprague Dawley rat

Clinical and preclinical data concur that sleep disruption causes hyperalgesia, but the brain mechanisms through which sleep and pain interact remain poorly understood. Evidence that pontine components of the ascending reticular activating system modulate sleep and nociception encouraged the present study testing the hypothesis that hypocretin-1 (orexin-A) and an adenosine receptor agonist administered into the pontine reticular nucleus, oral part (PnO) each alter thermal nociception. Adult male rats (n = 23) were implanted with microinjection guide tubes aimed for the PnO. The PnO was microinjected with saline (control), hypocretin-1, the adenosine A(1) receptor agonist N(6)-p-sulfophenyladenosine (SPA), the hypocretin receptor-1 antagonist N-(2-Methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl-urea (SB-334867), and hypocretin-1 plus SB-334867. As an index of antinociceptive behavior, the latency (in seconds) to paw withdrawal away from a thermal stimulus was measured following each microinjection. Compared to control, antinociception was significantly increased by hypocretin-1 and by SPA. SB-334867 increased nociceptive responsiveness, and administration of hypocretin-1 plus SB-334867 blocked the antinociception caused by hypocretin-1. These results suggest for the first time that hypocretin receptors in rat PnO modulate nociception.

PERSPECTIVE

Widely distributed and overlapping neural networks regulate states of sleep and pain. Specifying the brain regions and neurotransmitters through which pain and sleep interact is an essential step for developing adjunctive therapies that diminish pain without disrupting states of sleep and wakefulness.

[Opioid effectiveness for neuropathic pain in a patient with glossopharyngeal neuralgia]

A 55-year-old man complained of intermittent piercing pain on the right side of his throat. In a visit to our hospital, he was diagnosed with glossopharyngeal neuralgia and was treated with carbamazepine. But he obtained no pain relief and had drug-induced hypersensitivity syndrome due to carbamazepine on admission. We discontinued the carbamazepine, so the severity of the throat pain increased. We then administered codeine, which alleviated the pain; an increase in the dosage led to complete pain remission. After his general condition improved, he underwent an operation for microvascular decompression. In general, opioids are not efficacious against neuropathic pain, including glossopharyngeal neuralgia. However, recent studies do show their effectiveness against such pain via various mechanisms. They may control neuropathic pain through their effects on cortical brain regions and the thalamus, they may affect the descending antinociceptive pathway via actions on the periaqueductal gray, and they may modulate pain transmission in the spinal dorsal horn. We believe this to be the first case report to mention the effectiveness of opioids for glossopharyngeal neuralgia. If the control of pain is difficult in cases of glossopharyngeal neuralgia, opioids may be a useful therapeutic option.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Comparative antiallodynic activity of morphine, pregabalin and lidocaine in a rat model of neuropathic pain produced by one oxaliplatin injection

A single infusion of oxaliplatin, a drug active against colorectal cancer, induces specific painful syndrome characterized by neurosensitive symptoms triggered or aggravated in cold conditions. In an animal model that reproduces such hypersensitivity to cold for five days after a single oxaliplatin administration (6mg/kg, i.p.), we assessed the antinociceptive efficacy of intravenously administered drugs such as morphine, lidocaine and pregabalin using the rat tail immersion test in cold water (10 degrees C). The antinociceptive efficacy was first ranked by ratio of the pharmacological effect (versus time) to dose: pregabalin (2mg/kg)>lidocaine (3mg/kg)>morphine (4mg/kg). Our results show that pregabalin may be a good choice to treat cold hypersensitivity after one oxaliplatin injection.

Axotomy-induced up-regulation of tumor necrosis factor-alpha in the dorsal root ganglia

OBJECTIVES

Neuropathic pain is a chronic pain syndrome associated with drug, injury or disease-induced damage or destruction of sensory afferent fibers of the dorsal root ganglia (DRG). Although the exact underlying pathologic mechanisms are not known, pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) are recognized as potential modulators of peripheral and central nervous system inflammatory responses. They play a crucial role in injury and the pathologic development of chronic pain syndromes such as neuropathic pain.

METHODS

Twenty-four rats were divided into a naive control (n=6), sham (surgery exposing sciatic nerve, n=6), and peripheral nerve lesion group (unilateral axotomy of sciatic nerve, n=12).

RESULTS

The results of this study demonstrate a transient up-regulation of TNF-alpha expression within ipsi- and contralateral DRG following complete unilateral sciatic nerve axotomy as confirmed by immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time PCR. Elevated expression of TNF-alpha was noted to occur within the first 7 days post-axotomy, which subsequently normalized to baseline levels by day 14. This transient up-regulation was also associated with a switch in cellular source from predominant satellite cell expression at baseline to that involving satellite cells and abundant numbers of sensory neurons.

DISCUSSION

These results support the role of TNF-alpha in the upstream cascade of cellular events involved in the underlying pathogenesis of neuropathic pain. Strategies targeting the early attenuation of TNF-alpha within the DRG during the first week post-injury may have significant clinical impact in preventing the downstream cascade of events involved in the underlying cellular pathology of neuropathic pain.