Nefopam potentiates morphine antinociception in allodynia and hyperalgesia in the rat

The synergistic anti-nociceptive effects of nefopam and gabapentinoids in inflammatory, osteoarthritis, and neuropathic pain mouse models

Pain is a common public health problem and remains as an unmet medical need. Currently available analgesics usually have limited efficacy or are accompanied by many adverse side effects. To achieve satisfactory pain relief by multimodal analgesia, new combinations of nefopam and gabapentinoids (pregabalin/gabapentin) were designed and assessed in inflammatory, osteoarthritis and neuropathic pain. Isobolographic analysis was performed to analyze the interactions between nefopam and gabapentinoids in carrageenan-induced inflammatory pain, mono-iodoacetate-induced osteoarthritis pain and paclitaxel-induced peripheral neuropathic pain in mice. The anti-inflammatory effect and motor performance of monotherapy or their combinations were evaluated in the carrageenan-induced inflammatory responses and rotarod test, respectively. Nefopam (1, 3, 5, 10, 30 mg/kg, p.o.), pregabalin (3, 6, 12, 24 mg/kg, p.o.) or gabapentin (25, 50, 75, 100 mg/kg, p.o.) dose-dependently reversed mechanical allodynia in three pain models. Isobolographic analysis indicated that the combinations of nefopam and gabapentinoids exerted synergistic anti-nociceptive effects in inflammatory, osteoarthritis, and neuropathic pain mouse models, as evidenced by the experimental ED50 (median effective dose) falling below the predicted additive line. Moreover, the combination of nefopam-pregabalin/gabapentin alleviated carrageenan-induced inflammation and edema, and also prevented gabapentinoids-related sedation or ataxia by lowering their effective doses. Collectively, the co-administration of nefopam and gabapentinoids showed synergistic analgesic effects and may result in improved therapeutic benefits for treating pain.

Antinociceptive properties of losmapimod in two acute pain models in rats: behavioural analysis, immunohistochemistry, dose response, and comparison with usual analgesic drugs

Background

The p38 protein is a ubiquitous mitogen-activated protein kinase involved in the proinflammatory signalling pathway and in the pain response after various noxious stimuli. Many p38 inhibitors have been developed and shown to provide effective analgesia in animal models. They are, however, mainly administered intrathecally or intravenously. Our study aimed to evaluate losmapimod, a novel oral p38 inhibitor, in two murine acute pain models.

Methods

Losmapimod (12 mg kg-1) was compared with paracetamol, ketamine, and morphine using thermal and mechanical stimulation after carrageenan injection. A dose-effect study was also performed with this model. Behavioural testing was also performed in a plantar incision model to confirm the analgesic effect of losmapimod. Expression of activated p38 in neurones, microglia, and astrocytes was also investigated at 2, 15, and 24 h after carrageenan injection.

Results

Losmapimod was both antiallodynic and antihyperalgesic in the carrageenan pain model and provided an antinociceptive effect similar to that of morphine. The dose of 12 mg kg-1 was shown to be the ED78 and ED64 after thermal and mechanical stimulation, respectively. After plantar incision, losmapimod provided a significant antinociceptive effect. No life-threatening side-effect was observed in the behavioural study. Losmapimod prevented neurone and microglial activation at 2 and 15 h after carrageenan injection, respectively, but no effect was found on astrocytic activation.

Conclusion

Losmapimod appears to be a promising drug in severe acute pain conditions. Losmapimod could also be helpful for postoperative pain control, as suggested by its effect after plantar incision.

Analgesic Effect of Low Dose Nefopam Hydrochloride after Arthroscopic Rotator Cuff Repair: A Randomized Controlled Trial

Arthroscopic rotator cuff repair causes acute postoperative hyperalgesia. Multimodal analgesia is preferable to opioid-based intravenous patient-controlled analgesia (IV-PCA) due to postoperative nausea and vomiting (PONV). We evaluated the effect of nefopam as a postoperative non-opioid analgesic after shoulder surgeries. A total of 180 adult patients were enrolled for arthroscopic rotator cuff repair. They were randomly assigned to nefopam (N) or control (C) groups and each group was reclassified according to the interscalene block (B) into NB, CB and NX, CX. Nefopam was applied at a constant dose intravenously during recovery. Pain scores were measured with a Visual Analogue Scale (VAS) before (T0), immediately after (T1), 30 min (T2) and 12 h (T3), 24 h (T4) and 48 h (T5) after surgery. There was no significant difference in demographic data. The overall VAS scores did not differ with regard to nefopam use, except for the NB group at T4 in intention to treat (ITT) analysis (p < 0.05). PONV occurred more frequently in the N group than in the C group (p < 0.05). Neither individual nor all risk factors were associated with PONV occurrence (p > 0.10). In conclusion, nefopam alone did not show a definite decrease in postoperative pain. It instead increased PONV regardless of risk factors.

Assessment of thermal sensitivity in rats using the thermal place preference test: description and application in the study of oxaliplatin-induced acute thermal hypersensitivity and inflammatory pain models

Thermal sensitivity is an essential characteristic of some painful states, including oxaliplatin-induced neuropathy. The thermal place preference test (TPPT) was designed to finely assess thermal sensitivity in rodents. The TPPT monitors the time spent by unrestrained rodents on a test plate at fixed temperatures (5-50°C) compared with an adjacent reference plate at a neutral temperature (25°C). Here, we report the results of a study designed (i) to validate the optimal methodological parameters for measuring thermal sensitivity in rats, (ii) to assess the thermal sensitivity of healthy rats and animal models of pain and (iii) to explore the pharmacological effects of analgesic drugs. The most reproducible conditions occurred when the TPPT was performed in the morning and in the dark for 3 min with the reference plate set to 25°C. The temperature preferences of healthy rats were more than 17°C and less than 40°C. When compared with control animals, oxaliplatin-treated rats showed thermal hypersensitivity at 12, 20 and 35°C, and carrageenan-treated rats showed thermal hypersensitivity at 15 and 45°C. Duloxetine (2.5 mg/kg, intraperitoneal) reversed oxaliplatin-induced cold hypersensitivity (20°C) and morphine (1 mg/kg, intravenous) reversed carrageenan-induced heat hypersensitivity (45°C). We conclude that the TPPT enables a fine-grained assessment of thermal sensitivity that is relevant to the pathophysiological exploration of animal pain models and to the pharmacological assessment of analgesic drugs.

Spinal noradrenergic modulation and the role of the alpha-2 receptor in the antinociceptive effect of intrathecal nefopam in the formalin test

BACKGROUND

Nefopam has shown an analgesic effect on acute pain including postoperative pain. The reuptake of monoamines including serotonin and noradrenaline has been proposed as the mechanism of the analgesic action of nefopam, but it remains unclear. Although alpha-adrenergic agents are being widely used in the perioperative period, the role of noradrenergic modulation in the analgesic effect of nefopam has not been fully addressed.

METHODS

Changes in the antinociceptive effect of intrathecal (i.t.) nefopam against formalin-elicited flinching responses were explored in Sprague-Dawley rats pretreated with i.t. 6-hydroxydopamine (6-OHDA), which depletes spinal noradrenaline. In addition, antagonism to the effect of nefopam by prazosin and yohimbine was evaluated to further elucidate the antinociceptive mechanism of i.t. nefopam.

RESULTS

Pretreatment with i.t. 6-OHDA alone did not alter the flinching responses in either phase of the formalin test, while it attenuated the antinociceptive effect of i.t. nefopam significantly during phase 1, but not phase 2. The antagonist of the alpha-2 receptor, but not the alpha-1 receptor, reduced partially, but significantly, the antinociceptive effect of i.t. nefopam during phase 1, but not during phase 2.

CONCLUSIONS

This study demonstrates that spinal noradrenergic modulation plays an important role in the antinociceptive effect of i.t. nefopam against formalin-elicited acute initial pain, but not facilitated pain, and this action involves the spinal alpha-2 but not the alpha-1 receptor.

Antinociceptive effect of intrathecal nefopam and interaction with morphine in formalin-induced pain of rats

Background

Nefopam, a non-opiate analgesic, has been regarded as a substance that reduces the requirement for morphine, but conflicting results have also been reported. The inhibition of monoamine reuptake is a mechanism of action for the analgesia of nefopam. The spinal cord is an important site for the action of monoamines however, the antinociceptive effect of intrathecal nefopam was not clear. This study was performed to examine the antinociceptive effect of intrathecal (i.t.) nefopam and the pattern of pharmacologic interaction with i.t. morphine in the formalin test.

Methods

Male Sprague-Dawley rats were implanted with an i.t. catheter, and were randomly treated with a vehicle, nefopam, or morphine. Formalin was injected into the hind-paw 10 min. after an i.t. injection of the above experiment drugs. After obtaining antinociceptive ED₅₀ of nefopam and morphine, the mixture of nefopam and morphine was tested for the antinociceptive effect in the formalin test at a dose of 1/8, 1/4, 1/2 of ED₅₀, or ED₅₀ of each drug followed by an isobolographic analysis.

Results

Intrathecal nefopam significantly reduced the flinching responses in both phases of the formalin test in a dose-dependent manner. Its effect, however, peaked at a dose of 30 µg in phase 1 (39.8% of control) and 10 µg during phase 2 (37.6% of control). The isobolograhic analysis indicated an additive interaction of nefopam and morphine during phase 2, and a synergy effect in antinociception during phase 1.

Conclusions

This study demonstrated that i.t. nefopam produces an antinociceptive effect in formalin induced pain behavior during both phases of the formalin test, while interacting differently with i.t. morphine, synergistically during phase 1, and additively during phase 2.

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.

Carbamazepine potentiates morphine analgesia on postoperative pain in morphine-dependent rats

Postoperative pain and its control remain one of the most important issues in the field of surgery and health care systems. Morphine is a potent and effective analgesic, but substance abuse patients can manifest cross-tolerance to it, making it difficult to satisfy their analgesic/anesthetic requirements. As carbamazepine has shown antinociceptive properties in a variety of experimental and clinical settings, in the present study, we evaluated its potential antiallodynic effects on postoperative pain in naïve and morphine-dependent rats. Male rats were assigned to morphine-dependent and naïve groups and received intraperitoneally drug vehicles as control group, 3mg/kg morphine, 5, 10 or 15 mg/kg carbamazepine or 5mg/kg carbamazepine plus 3mg/kg morphine as a combination therapy 2 and 24h after surgery. Morphine-dependency was induced with multiple doses of morphine administered i.p. and plantar incision was made on the hind paw to simulate the postoperative pain. Paw withdrawal threshold (PWT) was obtained by von Frey filaments every 30 min after drug injection for up to 180 min. Morphine at 3mg/kg exerted antiallodynic effects in naïve rats and a decreased antinociception was observed in morphine-dependent rats. In contrast, 5mg/kg carbamazepine did not significantly alter PWT in naives but it was effective in dependent rats. 10 and 15 mg/kg carbamazepine attenuated allodynia following surgery in both groups. Co-administration of 5mg/kg carbamazepine with 3mg/kg morphine produced higher analgesia in morphine-dependent incised rats and prolonged antinociception as compared to morphine alone (P<0.05). Thus carbamazepine may potentiate the analgesic effect of chronically administered morphine on postoperative pain model in morphine-dependent rats.

A preliminary investigation into the practicality of use and duration of action of slow-release preparations of morphine and hydromorphone in laboratory rats

Significant advances have been made in our ability to assess pain and administer appropriate pain relief in laboratory animals. However, providing long-lasting analgesia using a route that does not involve animal restraint remains difficult. The objective of this study was to investigate whether oral administration of slow-release morphine or hydromorphone results in increased thermal nociception in laboratory rats. The results showed that 64 mg/kg morphine and 16 mg/kg hydromorphone induced comparable increases in foot withdrawal latencies for up to three hours postadministration; however, slow-release morphine increased response latencies for up to 11 hours. Whether these dose rates provide clinically effective pain relief has yet to be determined; however, these data suggest that using slow-release preparations could be an effective and highly practical method of elevating pain thresholds for a relatively prolonged period.

Nefopam but not physostigmine affects the thermoregulatory response in mice via alpha(2)-adrenoceptors

Nefopam, a non-opioid, centrally acting benzoxazocine analgesic, proved to be as efficient in treatment of postanaesthetic thermoregulatory shivering as clonidine or meperidine. However, its exact mechanism of action is still unclear. Potent anti-shivering activity was also demonstrated for physostigmine primarily based on cholinergic but probably also different additional mechanisms of action. Hypothesizing an involvement of alpha(2)-adrenoceptors we studied their role in nefopam- and physostigmine-mediated thermoregulation in a mouse model of nonshivering thermogenesis. To differentiate possible alpha(2)-adrenoceptor subtype-specific interactions, we analysed wildtype mice and mice with deletion of the alpha(2A)-, alpha(2B)- or alpha(2C)-adrenoceptor (knock out). Ten mice of each genotype (n = 40) were administered saline, saline plus atipamezole, 1 mg/kg nefopam, 25 mg/kg nefopam, 25 mg/kg nefopam plus atipamezole, physostigmine and physostigmine plus atipamezole intraperitoneally. Each mouse was randomly subjected to each of the seven different treatments. Afterwards, the mice were positioned into a plexiglas chamber where rectal temperature and mixed expired carbon dioxide were measured during following whole body cooling. Thermoregulatory threshold temperature of nonshivering thermogenesis and maximum response intensity were analysed. Nefopam decreased the thermoregulatory threshold temperature in wildtype, alpha(2B)- and alpha(2C)-adrenoceptor mice. This effect was partially abolished by additional administration of the alpha(2)-adrenoceptor antagonist atipamezole. In alpha(2A)-adrenoceptor knock out mice, nefopam did not affect the thermoregulatory threshold. In contrast, physostigmine decreased the thermoregulatory threshold in wildtype and all alpha(2)-adrenoceptor knock out mice independently from additional atipamezole administration. Our results indicate an important role of the alpha(2A)-adrenoceptor in the thermoregulatory response induced by nefopam but not by physostigmine in mice.

Pharmacological characteristics of the hind paw weight bearing difference induced by chronic constriction injury of the sciatic nerve in rats

AIMS

We examined the possible involvement of spontaneous on-going pain in the rat chronic constriction injury (CCI) model of neuropathic pain.

MAIN METHODS

The development of weight bearing deficit, as an index of spontaneous on-going pain, was investigated in comparison to that of mechanical allodynia in CCI rats. We also examined the effects of morphine and a gabapentin analogue (1S, 3R)-3-methyl-gabapentin (3-M-gabapentin) on both the CCI-induced weight bearing deficit and mechanical allodynia.

KEY FINDINGS

Rats with CCI demonstrated a significant reduction in weight bearing of the injured limb with a peak at a week post-operation, which was followed by a gradual recovery for over 7 weeks. The time course of development and recovery of CCI-induced weight bearing deficit appeared to follow that of foot deformation of the affected hind limb. CCI also evoked mechanical allodynia that was fully developed on a week post-operation, but showed no recovery for at least 8 weeks. 3-M-gabapentin significantly inhibited CCI-induced mechanical allodynia, but not weight bearing deficit, at 100 mg/kg p.o. Likewise, morphine was without significant effect on CCI-induced weight bearing deficit at the dose (3 mg/kg, s.c.) that could almost completely inhibit mechanical allodynia, whereas it inhibited both mechanical allodynia and weight bearing deficit at 6 mg/kg, s.c.

SIGNIFICANCE

The present findings suggest that CCI-induced weight bearing deficit is not a consequence of mechanical allodynia, but is attributable to spontaneous on-going pain. The rat CCI model of neuropathic pain thus represents both spontaneous on-going pain and mechanical allodynia.

Nefopam and ketoprofen synergy in rodent models of antinociception

Combinations of analgesics with different mechanisms of action offer the possibility of efficient analgesia with a decrease in side effects as a result of reduced dosages of one or both compounds. Based on a clinical observation of synergism between nefopam, a centrally acting non-opioid that inhibits monoamines reuptake, and ketoprofen, a non-steroidal anti-inflammatory drug, the objective of this study was to further explore this antinociceptive synergy in four distinct animal models of pain (both drugs were administered subcutaneously). Strong antinociceptive properties were observed in the mouse writhing abdominal test with ED50 values of 2.56+/-0.38 and 1.41+/-0.41 mg/kg for nefopam and ketoprofen, respectively. In the inflammatory phase of the mouse formalin test, both compounds significantly inhibited the licking time of the injected hind-paw with ED50 of 4.32+/-0.17 mg/kg for nefopam and 49.56+/-15.81 mg/kg for ketoprofen. Isobolographic analysis revealed that this drug combination is synergistic in the formalin test and additive in the writhing test. In rat carrageenan-induced tactile allodynia, single administration of nefopam or ketoprofen only partially reduced allodynia. Combination of low analgesic doses of nefopam (10 or 30 mg/kg) with low analgesic doses of ketoprofen (30 or 100 mg/kg) significantly reduced or reversed allodynia, with a more pronounced anti-allodynic effect and a longer duration efficacy. In a rat model of postoperative thermal hyperalgesia induced by incision, co-administration of nefopam at a low analgesic dose (10 mg/kg) with ketoprofen at non-analgesic doses (30 or 100 mg/kg) showed the appearance of a strong anti-hyperalgesic effect, maintained during at least 3 h. In conclusion, co-administration of nefopam with ketoprofen is synergistic, and should allow either to increase their analgesic efficacy and/or to reduce their side effects.

The Effect of Nefopam on Morphine Overconsumption Induced by Large-Dose Remifentanil During Propofol Anesthesia for Major Abdominal Surgery

Opioids may activate pain facilitatory systems opposing analgesia. We investigated whether large-dose remifentanil given during IV anesthesia caused postoperative morphine overconsumption and whether nefopam (a centrally acting analgesic) could reduce this. Sixty patients scheduled for abdominal surgery were included in this prospective, randomized study. The first 30 patients received either small-dose (Group S: 3 ng/mL) or large-dose (Group L: 8 ng/mL) remifentanil administrated by a target-controlled infusion during propofol anesthesia. Before skin closure, patients received morphine 0.15 mg/kg. Another 30 patients also received nefopam 20 mg intraoperatively. Postoperative pain was controlled by titration of morphine, followed by patient-controlled morphine analgesia (PCA). Morphine was requested earlier in Group L than in Group S (10 [1-63] min versus 37 [5-90] min, median [range]; P < 0.002). The dose of morphine by titration was larger in Group L than in Group S (0.28 [0.04-0.38] mg/kg versus 0.16 [0.03-0.41] mg/kg; P < 0.05). PCA morphine consumption and pain scores were similar. There were no differences between the nefopam groups in the time to first morphine request or in the dose of morphine by titration. Postoperative morphine overconsumption occurred after large-dose remifentanil and propofol anesthesia during the early postoperative period. Pretreatment with nefopam could be useful to prevent pain sensitization induced by opioids.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

A Comparison of the Antinociceptive and Adverse Effects of the mu-Opioid Agonist Morphine and the delta-Opioid Agonist SNC80

delta-Opioid receptor agonists have been postulated to induce analgesia without the adverse effects commonly associated with mu-opioids e.g. morphine. In the present study, we evaluated the occurrence of antinociceptive and opioid-like side effects in rats (n=5-7) treated with a single dose of subcutaneous morphine (0.01 to 40 mg/kg) or SNC80 (0.63 to 80 mg/kg). The antinociceptive effects of morphine and SNC80 were compared using a range of nociceptive tests including the tail withdrawal test, the acetic acid-induced abdominal constriction (writhing) assay, the automated formalin test and a model of inflammation-induced thermal hyperalgesia. The adverse effects of both drugs were examined in animal models for gastrointestinal (GI) inhibition (charcoal test; ricinus oil test), respiratory depression (blood-gas analysis), motor disturbances (automated rotarod model) and abuse liability (drug discrimination learning). Morphine displayed significant antinociceptive and adverse effects in all the animal models employed. SNC80 exhibited a significant effect in the writhing test and limited efficacy in a model of inflammation-induced thermal hypersensitivity. A delay in the occurrence of diarrhoea and some limited increases in PCO(2) were observed with the higher doses of SNC80 (> or =40 mg/kg). In conclusion, the delta-opioid agonist SNC80 lacks both the analgesic efficacy and adverse effects of mu-opioids. However, the activity of SNC80 in the inflammatory model suggests delta-opioid agonists may be useful analgesics in the treatment of some forms of inflammatory pain.