Mirogabalin Decreases Pain-like Behaviours and Improves Opioid and Ketamine Antinociception in a Mouse Model of Neuropathic Pain

Neuropathic pain remains a difficult clinical challenge due to its diverse aetiology and complex pathomechanisms, which are yet to be fully understood. Despite the variety of available therapies, many patients suffer from ineffective pain relief; hence, the search for more efficacious treatments continues. The new gabapentinoid, mirogabalin has recently been approved for clinical use. Although its main mechanism of action occurs at the α2σ-1 and α2σ-2 subunits of calcium channels and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level has not been clarified, which is crucial information from a clinical perspective. The findings of our study suggest that several indirect mechanisms may be responsible for the beneficial analgesic effect of mirogabalin. This is the first study to report that mirogabalin enhances the mRNA expression of spinal antinociceptive factors, such as IL-10 and IL-18BP, and reduces the concentration of the pronociceptive substance P. Importantly, mirogabalin improves the morphine-, buprenorphine-, oxycodone-, and ketamine-induced antinociceptive effects in a neuropathic pain model. Our findings support the hypothesis that enhancing opioid and ketamine analgesia by combining these drugs with mirogabalin may represent a new strategy for the effective pharmacotherapy of neuropathic pain.

Magnesium sulphate within multimodal analgesia, pre-emptive, or preventive analgesia

Background Magnesium (Mg) is a non-competitive N-methyl d-aspartate receptor antagonist with antinociceptive effects. Multimodal therapy is the optimal strategy for perioperative pain control to minimize the need for opioids. Inflammation caused by tissue trauma or direct nerve injury is responsible for the perioperative pain. The concept of “pre-emptive” analgesia, analgesic strategies administered prior to the stimulus, can modify the peripheral and central nervous system processing of noxious stimuli, thereby reducing central sensitization, hyperalgesia, and allodynia remains controversial. A more encompassing approach to the reduction of postoperative pain is the concept of “preventive” analgesia. The purpose of the study is to detect the proper use of MgSO4 as an analgesic being a non-competitive N-methyl d-aspartate (NMDA). Results There is no statistically significant difference in the haemodynamic parameters, intraoperative (33% vs 20%) and postoperative requirement for analgesics 6.6% vs 10% among groups I and II, respectively. There is no significant difference in the numerical analogue scale, where 16 vs 17 patients with no pain, 12 vs 10 with mild pain, and 2 vs 3 with moderate pain in groups I and II, respectively. Conclusion The use of MgSO4 in a bolus with or without infusion is comparable in the control of intraoperative and postoperative pain.

Antinociceptive Effects of Aza-Bicyclic Isoxazoline-Acylhydrazone Derivatives in Different Models of Nociception in Mice

Background: In a study recently published by our research group, the compounds isoxazoline-acylhydrazone derivatives R-99 and R-123 presented promising antinociceptive activity. However, the mechanism of action of this compound is still unknown. Objective: This study aimed to assess the mechanisms involved in the antinociceptive activity of these compounds in chemical models of pain. Methods: Animals were orally pretreated and evaluated in the acetic acid-, formalin-, capsaicin-, carrageenan- and Complete Freund's Adjuvant (CFA)-induced pain models in mice. The effects of the compounds after pretreatment with naloxone, prazosin, yohimbine, atropine, L-arginine, or glibenclamide were studied, using the acetic acid-induced writhing test to verify the possible involvement of opioid, α1-adrenergic, α2-adrenergic or cholinergic receptors, and nitric oxide or potassium channels pathways, respectively. Results: R-99 and R-123 compounds showed significant antinociceptive activity on pain models induced by acetic acid, formalin, and capsaicin. Both compounds decreased the mechanical hyperalgesia induced by carrageenan or CFA in mice. The antinociceptive effects of R-99 and R-123 on the acetic acid-induced writhing test were significantly attenuated by pretreatment with naloxone, yohimbine or atropine. R-99 also showed an attenuated response after pretreatment with atropine and glibenclamide. However, on the pretreatment with prazosin, there was no change in the animals' response to both compounds. Conclusion: R-99 and R-123 showed antinociceptive effects related to mechanisms that involve, at least in part, interaction with the opioid and adrenergic systems and TRPV1 pathways. The compound R-99 also interacts with the cholinergic pathways and potassium channels.

Neuraxial administration of morphine combined with lidocaine induces regional antinociception in inland bearded dragons (Pogona vitticeps)

OBJECTIVE To assess the antinociceptive efficacy and safety of neuraxial morphine in inland bearded dragons (Pogona vitticeps). ANIMALS 10 healthy adult bearded dragons. PROCEDURES Animals were sedated with alfaxalone (15 mg/kg) SC prior to neuraxial injections. In a randomized, blinded, placebo-controlled, crossover design, animals received preservative-free morphine (0.5 mg/kg) combined with lidocaine (2 mg/kg) or lidocaine (2 mg/kg) only (control treatment). For both treatments, saline (0.9% NaCl) solution was used for dilution to a total volume of 0.3 mL/kg. If the initial injection did not result in motor block of the pelvic limbs or cloaca relaxation within 10 minutes, a second injection was performed. Measurements consisted of bilateral mechanical stimulation of the limbs and at 25%, 50%, and 75% of the trunk’s length as well as cloacal tone to assess spread and duration of motor block. Pelvic limb withdrawal latencies in response to a thermal noxious stimulus were measured over a 48-hour period to assess antinociception. RESULTS Success rate following the first injection was 90% (18/20 injections) and increased to 100% following a second injection. Motor block occurred within 5 minutes with both treatments. Pelvic limb withdrawal latencies were significantly prolonged following neuraxial morphine versus control treatment for at least 12 hours after injection. By 24 hours, no effect of morphine on pelvic limb latencies was detectable. CLINICAL RELEVANCE These results demonstrated that neuraxial administration of morphine results in regional antinociceptive effects for at least 12 hours and has no clinically relevant adverse effects in healthy bearded dragons. This technique has potential for providing regional analgesia in this species.

Investigating the antinociceptive effects of N-docosahexaenoyl ethanolamine and novel kappa opioid receptor agonists

<p>Chronic pain causes patients to endure prolonged suffering and discomfort, often having profound effects on quality of life. In New Zealand, one in five people currently suffer from chronic pain. To treat chronic pain, patients are typically prescribed drugs that activate the mu opioid receptor (MOPr), such as morphine, codeine and oxycodone. In recent years in the United States of America, there has been a rapid increase in the use of prescription and non-prescription opioid drugs, with opioid overdoses now the leading cause of accidental death. In New Zealand, daily doses of prescription opioids quadrupled in the ten year period from 2001-2011. Clearly, there is a need for the development of more effective and safe medications. This thesis evaluated two classes of non-addictive compounds: bioactive lipids and kappa opioid receptor (KOPr) agonists. N-docosahexaenoyl ethanolamine (DHEA) is an N-acyl ethanolamine class lipid that is structurally similar to the endocannabinoid anandamide. DHEA has previously been shown to have immune-modulatory effects in vitro, however, the in vivo effects have not previously been tested. Using the intraplantar 2% formaldehyde model in mice, DHEA reduced inflammatory and nociceptive pain via both intraperitoneal (i.p.) and local intraplantar (i.pl.) administration. DHEA significantly reduced formaldehyde-induced footpad oedema and reduced the infiltration of neutrophils into the inflamed tissue. The antinociceptive and anti-oedematous effects were not modulated by pre-treatment with either cannabinoid 1- or 2-type receptor antagonists. DHEA did not have any effect in a thermal nociceptive pain model and did not show any motor coordination impairment or changes in thermoregulation. In the search for non-addictive analgesics, KOPr agonists are a promising alternative. In contrast to MOPr agonists, KOPr agonists play a critical role in regulating the reward system. Salvinorin A (SalA) is a selective KOPr agonist that has antinociceptive and anti-inflammatory effects in vivo, with limited abuse potential. However, the short duration of action and aversive side effects limit the clinical usefulness. The present study aimed to investigate the antinociceptive effects of acute administration of novel analogues of SalA. In the dose-response tail withdrawal assay, SalA and the novel analogues 16-Ethynyl SalA and 16-Bromo SalA were more potent than the traditional KOPr agonist U50,488, and 16-Ethynyl SalA was more efficacious. 16-Ethynyl SalA and 16-Bromo SalA both had a longer duration of action in the warm water tail withdrawal assay and the hot plate test compared to SalA. In the intraplantar 2% formaldehyde test, SalA, 16-Ethynyl SalA and 16-Bromo SalA significantly reduced nociceptive pain and inflammatory pain, effects which were reversed by the KOPr antagonist nor-binaltorphimine. SalA, 16-Ethynyl SalA and 16-Bromo SalA reduced paw oedema and reduced the infiltration of neutrophils into the inflamed tissue. However, SalA, 16-Ethynyl SalA and 16-Bromo SalA produced motor incoordination effects. However, 16-Ethynyl SalA did not alter thermoregulation. The KOPr agonists were further assessed in a model of paclitaxel-induced neuropathic pain. In the acute dose-response experiment, 16-Ethynyl SalA was significantly more potent at reducing mechanical allodynia compared to morphine in both male and female mice. SalA and 16-Ethynyl SalA were more potent at reducing cold allodynia than morphine. In a chronic administration model over 22 days, for the treatment of cold and mechanical allodynia, all of the opioid treatments reduced pain, however, the traditional KOPr agonist U50,488, was the most potent, by reducing the male mechanical allodynia and cold allodynia in both sexes back to baseline levels. The ultrastructure of the sciatic nerves were studied, however, it was found that U50,488 did not reverse the effects of paclitaxel on myelin degeneration and mitochondrial damage. Overall, this study has identified DHEA as a modest treatment for inflammatory pain, with reduced side effects and a mechanism of action in contrast to other compounds with a similar structure. The novel KOPr agonists had significant effects in acute pain models with longer duration of action than the parent compound SalA. This is the first known study to investigate the effects of KOPr agonists in a paclitaxel-induced neuropathic pain model, showing that KOPr agonists are a potential therapeutic avenue for this debilitating condition.</p>

Investigating the antinociceptive effects of N-docosahexaenoyl ethanolamine and novel kappa opioid receptor agonists

<p>Chronic pain causes patients to endure prolonged suffering and discomfort, often having profound effects on quality of life. In New Zealand, one in five people currently suffer from chronic pain. To treat chronic pain, patients are typically prescribed drugs that activate the mu opioid receptor (MOPr), such as morphine, codeine and oxycodone. In recent years in the United States of America, there has been a rapid increase in the use of prescription and non-prescription opioid drugs, with opioid overdoses now the leading cause of accidental death. In New Zealand, daily doses of prescription opioids quadrupled in the ten year period from 2001-2011. Clearly, there is a need for the development of more effective and safe medications. This thesis evaluated two classes of non-addictive compounds: bioactive lipids and kappa opioid receptor (KOPr) agonists. N-docosahexaenoyl ethanolamine (DHEA) is an N-acyl ethanolamine class lipid that is structurally similar to the endocannabinoid anandamide. DHEA has previously been shown to have immune-modulatory effects in vitro, however, the in vivo effects have not previously been tested. Using the intraplantar 2% formaldehyde model in mice, DHEA reduced inflammatory and nociceptive pain via both intraperitoneal (i.p.) and local intraplantar (i.pl.) administration. DHEA significantly reduced formaldehyde-induced footpad oedema and reduced the infiltration of neutrophils into the inflamed tissue. The antinociceptive and anti-oedematous effects were not modulated by pre-treatment with either cannabinoid 1- or 2-type receptor antagonists. DHEA did not have any effect in a thermal nociceptive pain model and did not show any motor coordination impairment or changes in thermoregulation. In the search for non-addictive analgesics, KOPr agonists are a promising alternative. In contrast to MOPr agonists, KOPr agonists play a critical role in regulating the reward system. Salvinorin A (SalA) is a selective KOPr agonist that has antinociceptive and anti-inflammatory effects in vivo, with limited abuse potential. However, the short duration of action and aversive side effects limit the clinical usefulness. The present study aimed to investigate the antinociceptive effects of acute administration of novel analogues of SalA. In the dose-response tail withdrawal assay, SalA and the novel analogues 16-Ethynyl SalA and 16-Bromo SalA were more potent than the traditional KOPr agonist U50,488, and 16-Ethynyl SalA was more efficacious. 16-Ethynyl SalA and 16-Bromo SalA both had a longer duration of action in the warm water tail withdrawal assay and the hot plate test compared to SalA. In the intraplantar 2% formaldehyde test, SalA, 16-Ethynyl SalA and 16-Bromo SalA significantly reduced nociceptive pain and inflammatory pain, effects which were reversed by the KOPr antagonist nor-binaltorphimine. SalA, 16-Ethynyl SalA and 16-Bromo SalA reduced paw oedema and reduced the infiltration of neutrophils into the inflamed tissue. However, SalA, 16-Ethynyl SalA and 16-Bromo SalA produced motor incoordination effects. However, 16-Ethynyl SalA did not alter thermoregulation. The KOPr agonists were further assessed in a model of paclitaxel-induced neuropathic pain. In the acute dose-response experiment, 16-Ethynyl SalA was significantly more potent at reducing mechanical allodynia compared to morphine in both male and female mice. SalA and 16-Ethynyl SalA were more potent at reducing cold allodynia than morphine. In a chronic administration model over 22 days, for the treatment of cold and mechanical allodynia, all of the opioid treatments reduced pain, however, the traditional KOPr agonist U50,488, was the most potent, by reducing the male mechanical allodynia and cold allodynia in both sexes back to baseline levels. The ultrastructure of the sciatic nerves were studied, however, it was found that U50,488 did not reverse the effects of paclitaxel on myelin degeneration and mitochondrial damage. Overall, this study has identified DHEA as a modest treatment for inflammatory pain, with reduced side effects and a mechanism of action in contrast to other compounds with a similar structure. The novel KOPr agonists had significant effects in acute pain models with longer duration of action than the parent compound SalA. This is the first known study to investigate the effects of KOPr agonists in a paclitaxel-induced neuropathic pain model, showing that KOPr agonists are a potential therapeutic avenue for this debilitating condition.</p>

Antinociceptive activities of a novel diarylpentanoid analogue, 2-benzoyl-6-(3-bromo-4-hydroxybenzylidene)cyclohexen-1-ol, and its possible mechanisms of action in mice

A novel synthetic compound from the 2-benzoyl-6-benzylidenecyclohexanone analogue, namely 2-benzoyl-6-(3-bromo-4-hydroxybenzylidene)cyclohexen-1-ol (BBHC), showed pronounced nitric oxide inhibition in IFN-γ/LPS-induced RAW 264.7 cells. Based on this previous finding, our present study aimed to investigate the antinociceptive effects of BBHC via chemical and thermal stimuli in vivo. The investigation of the antinociceptive activity of BBHC (0.1, 0.3, 1.0 and 3.0 mg/kg, i.p.) was initiated with 3 preliminary screening tests, then BBHC was subjected to investigate its possible involvement with excitatory neurotransmitters and opioid receptors. The potential acute toxicity of BBHC administration was also studied. Administration of BBHC significantly inhibited acetic acid-induced abdominal constrictions, formalin-induced paw licking activity and developed notable increment in the latency time. BBHC’s ability to suppress capsaicin- and glutamate-induced paw licking activities, as well as to antagonise the effect of naloxone, had indicated the possible involvement of its antinociception with TRPV1, glutamate and opioid receptors, respectively. The antinociceptive activities of BBHC was not related to any sedative action and no evidence of acute toxic effect was detected. The present study showed that BBHC possessed significant peripheral and central antinociceptive activities via chemical- and thermal-induced nociceptive murine models without any locomotor alteration and acute toxicity.

Investigating the analgesic properties of Kurkinorin, a novel mu-opioid receptor analogue of Salvinorin A

<p>Background: The mu-opioid receptor (MOPr) activating drugs such as morphine, fentanyl, etorphine and methadone are used to treat moderate to severe pain. However, their long-term use produces serious adverse effects such as respiratory depression, sedation, tolerance, nausea, dependence, and constipation and this signifies the search for an alternate pain therapeutic agent. Here we report the investigation of antinociceptive and side effect profiles of a structurally unique MOPr-activating drug, kurkinorin from Salvinorin A (Sal A) that was compared with morphine and herkinorin. Methods: Adult male B6-SJL mice (22-29 g) were used to investigate the antinociceptive effects of kurkinorin, herkinorin and morphine utilising the 50° C warm-water tail-withdrawal assay. The 2% intra-dermal formalin assay was used to evaluate acute nociceptive and inflammatory pain and paw oedema. The side effect profiles were evaluated by measuring core-body temperature and utilising behavioural tests of motor co-ordination (accelerating rotarod test). Kurkinorin’s rewarding properties were assessed using the conditioned place preference (CPP) assay in male Sprague-Dawley rats (240-350 g). Results: Kurkinorin produced significant antinociceptive effects in the tail-withdrawal assay at both 5 (p<0.01, 10 min, p<0.001, 15-60 min) and 10 mg/kg (p<0.001, 5-90 min, p<0.01, 120 min) and attenuated both nociceptive and inflammatory pain in the 2% intra-dermal formalin model in mice. The analgesic effects of kurkinorin at 10 mg/kg were similar to the analgesic effects of morphine at the same dose. The decrease in pain score in the intra-dermal formalin assay with kurkinorin and morphine produced a corresponding reduction of paw oedema. In comparison, herkinorin had reduced analgesic effects in the tail-withdrawal assay (10 mg/kg, p<0.05, 30 min) and attenuated inflammatory pain in the intra-dermal formalin assay (10 mg/kg, p<0.001) with reduced paw oedema (10 mg/kg, p<0.05). Morphine produced significant motor incoordination effects from 15-60 min post injection whereas kurkinorin produced no significant motor impairment. Kurkinorin and herkinorin (5 mg/kg, i.p) did not produce rewarding effects, whereas morphine produced a significant, rewarding effect in the CPP assay. Kurkinorin produced no change in the core body temperature while morphine significantly reduced the body temperature. Conclusions: Kurkinorin is central acting and is as potent as morphine in attenuating acute nociceptive and inflammatory pain. It produced no significant sedative and rewarding effects. Therefore, kurkinorin has been identified as a structurally new class of mu-opioid analgesic, displaying improvements compared to morphine.</p>