Peripheral Opioid Receptors Research Articles

Phase I clinical trial evaluating the safety, tolerance, pharmacokinetics and pharmacodynamics of HSK21542 injection in healthy volunteers.

HSK21542 injection is a new peripheral kappa opioid receptor (KOR) agonist. To evaluate its safety, tolerability, pharmacokinetics and pharmacodynamics, this study was conducted in healthy volunteers, consisting of two parts: a single ascending dose (0.2-3.375 μg/kg, 15-min infusion) and different infusion durations (0.2 and 1μg/kg, 2- or 15-min infusion). The area under the plasma concentration-time curve (AUC) and peak concentration (Cmax) of HSK21542 were dose-linear among 0.2-3.375 μg/kg. After intravenous injection, HSK21542 was rapidly eliminated with a half-life (t1/2) of 1.5h, and the majority (48.02%) of the dose was excreted unchanged in urine. Pharmacodynamic results showed that HSK21542 increased prolactin release and reached a peak at 1-2h after administration but had no significant effect on vasopressin levels. There was a brief increase in urine volume within the initial 2h after administration. HSK21542 was well tolerated; most of the adverse effects (AEs) in the trial group were grade 1, and only 2 cases (4.0%) were grade 2. The main AE was paresthesia, which appeared in 42% (21/50) in the trial group. No serious adverse event (SAE) was observed. No subject withdrew early due to AEs. These results suggest that HSK21542 may be a potential treatment for pain and pruritic conditions.

Peripheral opioid receptor antagonism alleviates fentanyl-induced cardiorespiratory depression and is devoid of aversive effects.

In this study, we compare the central versus peripheral components underlying fentanyl-induced cardiorespiratory depression to prevent overdose deaths. Our data indicates that these effects are, at least partially, due to the activation of mu opioid receptors present in peripheral sites. These findings provide insight into peripheral contributions to fentanyl-induced overdoses and could potentially lead to the development of treatments selectively targeting the peripheral system, sparing individuals from the CNS-driven acute opioid withdrawal generally observed with the use of naloxone.

Advances in attenuating opioid-induced respiratory depression: A narrative review.

Opioids exert analgesic effects by agonizing opioid receptors and activating signaling pathways coupled to receptors such as G-protein and/or β-arrestin. Concomitant respiratory depression (RD) is a common clinical problem, and improvement of RD is usually achieved with specific antagonists such as naloxone; however, naloxone antagonizes opioid analgesia and may produce more unknown adverse effects. In recent years, researchers have used various methods to isolate opioid receptor-mediated analgesia and RD, with the aim of preserving opioid analgesia while attenuating RD. At present, the focus is mainly on the development of new opioids with weak respiratory inhibition or the use of non-opioid drugs to stimulate breathing. This review reports recent advances in novel opioid agents, such as mixed opioid receptor agonists, peripheral selective opioid receptor agonists, opioid receptor splice variant agonists, biased opioid receptor agonists, and allosteric modulators of opioid receptors, as well as in non-opioid agents, such as AMPA receptor modulators, 5-hydroxytryptamine receptor agonists, phosphodiesterase-4 inhibitors, and nicotinic acetylcholine receptor agonists.

Activation of Cardiac δ2-Opioid Receptors Increases Heart Tolerance to Reperfusion.

Coronary occlusion (45 min) and reperfusion (120 min) in male Wistar rats in vivo, as well as total ischemia (45 min) of an isolated rat heart followed by reperfusion (30 min) were reproduced. The selective δ2-opioid receptor agonist deltorphin II (0.12 mg/kg and 152 nmol/liter) was administered intravenously 5 min before reperfusion in vivo or added to the perfusion solution at the beginning of reperfusion of the isolated heart. The peripheral opioid receptor antagonist naloxone methiodide and δ2-opioid receptor antagonist naltriben were used in doses of 5 and 0.3 mg/kg, respectively. It was found that the infarct-limiting effect of deltorphin II is associated with the activation of δ2-opioid receptors. We have demonstrated that deltorphin II can improve the recovery of the contractility of the isolated heart after total ischemia.

Opioids and Immunosuppression: Clinical Evidence, Mechanisms of Action, and Potential Therapies.

In addition to the more well-known adverse effects of opioids, such as constipation, mounting evidence supports underlying immunosuppressive effects as well. In this study, we provide a narrative review of preclinical and clinical evidence of opioid suppression of the immune system as well as possible considerations for therapies. In vitro and animal studies have shown clear effects of opioids on inflammatory cytokine expression, immune cell activity, and pathogen susceptibility. Observational data in humans have so far supported preclinical findings, with multiple reports of increased rates of infections in various settings of opioid use. However, the extent to which this risk is due to the impact of opioids on the immune system compared with other risk factors associated with opioid use remains uncertain. Considering the data showing immunosuppression and increased risk of infection with opioid use, measures are needed to mitigate this risk in patients who require ongoing treatment with opioids. In preclinical studies, administration of opioid receptor antagonists blocked the immunomodulatory effects of opioids. As selective antagonists of peripheral opioid receptors, peripherally acting mu-opioid receptor (MOR) antagonists may be able to protect against immune impairment while still allowing for opioid analgesia. Future research is warranted to further investigate the relationship between opioids and infection risk as well as the potential application of peripherally acting MOR antagonists to counteract these risks.

TET1-Lipid Nanoparticle Encapsulating Morphine for Specific Targeting of Peripheral Nerve for Pain Alleviation.

Opioids are irreplaceable analgesics owing to the lack of alternative analgesics that offer opioid-like pain relief. However, opioids have many undesirable central side effects. Restricting opioids to peripheral opioid receptors could reduce those effects while maintaining analgesia. To achieve this goal, we developed Tet1-LNP (morphine), a neural-targeting lipid nanoparticle encapsulating morphine that could specifically activate the peripheral opioid receptor in the dorsal root ganglion (DRG) and significantly reduce the side effects caused by the activation of opioid receptors in the brain. Tet1-LNP (morphine) were successfully prepared using the thin-film hydration method. In vitro, Tet1-LNP (morphine) uptake was assessed in differentiated neuron-like PC-12 cells and dorsal root ganglion (DRG) primary cells. The uptake of Tet1-LNP (morphine) in the DRGs and the brain was assessed in vivo. Von Frey filament and Hargreaves tests were used to assess the antinociception of Tet1-LNP (morphine) in the chronic constriction injury (CCI) neuropathic pain model. Morphine concentration in blood and brain were evaluated using ELISA. Tet1-LNP (morphine) had an average size of 131 nm. Tet1-LNP (morphine) showed high cellular uptake and targeted DRG in vitro. CCI mice treated with Tet1-LNP (morphine) experienced prolonged analgesia for nearly 32 h compared with 3 h with free morphine (p < 0.0001). Notably, the brain morphine concentration in the Tet1-LNP (morphine) group was eight-fold lower than that in the morphine group (p < 0.0001). Our study presents a targeted lipid nanoparticle system for peripheral neural delivery of morphine. We anticipate Tet1-LNP (morphine) will offer a safe formulation for chronic neuropathic pain treatment, and promise further development for clinical applications.

Assessment of pain intensity and methods of its relief in patients with irritable bowel syndrome

Objective — to develop a questionnaire for assessing the condition of patients with irritable bowel syndrome (IBS); to evaluate clinical effectiveness of personalized diet, synthetic agonist of peripheral opioid receptors m‑, k‑, and d‑ trimebutine maleate, and complex pro‑prebiotic drug (Lactobacillus acidophilus 500 million, Lactobacillus rhamnosus 1 billion, Sacchromyces boulardii 30 million, Bifidobacterium lactis 275 million, Bifidobacterium longum 1 billion, Clostridium butyricum 2 million, Bacillus clausii spores 2 billion, Fructo oligosaccharides 100 mg) in patients with IBS.&#x0D; Materials and methods. To evaluate the clinical effectiveness of the treatment, 28 patients with IBS have been examined. The investigated group included 15 (53.6%) patients with IBS and diarrhea, 11 (39.3%) subjects with mixed IBS, and two (7.1%) patients with IBS and constipation. The age of the patients was 32.5±3.1 years; from them 17 (51.7%) were women and 11 (39.3%) men. Nine women (52.9%) and five (45.5%) men were married, and 11 (64.7%) women and 7 (63.3%) men had higher education. The average duration of the disease in women was 3.4±1.8 years, and 3.1±1.5 years in men. The diagnosis of IBS was established according to the Rome criteria IV revision. Defecation was evaluated according to the Bristol stool scale. All patients filled out the «Questionnaire for a patient with irritable bowel syndrome» before the treatment start and during the treatment. The examined patients were prescribed a personalized diet with account their nutritional status, and drug therapy that included trimebutine maleate (film‑coated tablets) in a dose of 200 to 600 mg depending on pain intensity, and synbiotic (Lactobacillus acidophilus, Lactobacillus rhamnosus, Sacchromyces boulardii, Bifidobacterium lactis, Bifidobacterium longum, Clostridium butyricum, Bacillus clausii spores, Fructo oligosaccharides) in a dose of one capsule twice a day, regardless of the severity of the pain syndrome.&#x0D; Results. Complex treatment with the use of personalized dietary recommendations, synthetic agonist of peripheral opioid receptors trimebutine maleate and synbiotic that included Lactobacillus acidophilus, Lactobacillus rhamnosus, Sacchromyces boulardii, Bifidobacterium lactis, Bifidobacterium longum, Clostridium butyricum, Bacillus clausii spores, Fructo oligosaccharides, promoted to the decrease of pain syndrome severity, improvement of the psychoemotional state and quality of life of patients.&#x0D; Conclusions. The developed questionnaire for a patient with IBS was comprehensible and easy to fill out by patients; it allowed a practitioner to quickly analyze the dynamics of the pain syndrome. Personalized dietary treatment, a differentiated approach in administration of the peripheral opioid receptor agonist trimebutine maleate and synbiotics (Lactobacillus acidophilus, Lactobacillus rhamnosus, Sacchromyces boulardii, Bifidobacterium lactis, Bifidobacterium longum, Clostridium butyricum, Bacillus clausii spores, IBS oligosaccharides) was clinically effective for patients with IBS, it promoted the significant pain relief and improvement of the quality of life of patients.

Pharmacokinetics, Mass Balance and Metabolism of [14C]HSK21542, a Novel Kappa Opioid Receptor Agonist, inHumans.

HSK21542, a synthetic short-chain polypeptide, is a selective peripheral kappa opioid receptor (KOR) agonist. In this single-centre, non-randomized, open-label study, the pharmacokinetics, mass balance, metabolism and excretion ofHSK21542 were investigated. A single intravenous dose of 2μg/0.212μCi/kg [14C]HSK21542 was administered to sixhealthy male subjects. Samples of blood, urine and faeces were collected for quantitative determination of total radioactivity and unchangedHSK21542, and identification of metabolites. The mean total recovery was 81.89% of the radiolabelled dose over 240h post-dose, with 35.60% and 46.30% excreted in faeces and urine, respectively. The mean maximum concentration (Cmax), thehalf-life (t1/2) and the area under the concentration-time curve (AUC0-t) of total radioactivity (TRA) in plasma were 20.4±4.16ngEq./g, 1.93±0.322h and 21.8±2.93h·ngEq./g, respectively, while the Cmax, t1/2 and the AUC0-t of unchangedHSK21542 were 18.3±3.36ng/mL, 1.66±0.185h and 18.4±2.24h·ng/mL, respectively. The blood-to-plasma ratios of TRA at several times ranged from 0.46 to 0.54. [14C]HSK21542 was detected as the main circulating substance in plasma, accounting for 92.17% of the AUC of TRA. The unchanged parent compound was the only major radioactive chemical in urine (100.00% of TRA) and faeces (93.53% of TRA). Metabolites were veryminor components. HSK21542 was barely metabolized in vivo and mainly excreted with unchangedHSK21542 as its main circulating component in plasma. It was speculated that renal excretion was the principal excretion pathway, and faecal excretion was the secondary pathway. NCT05835934.

Developmental Approaches to Chronic Pain: A Narrative Review.

Chronic pain, which can potentially develop from acute pain, subacute pain, or breakthrough pain, is generally defined as pain persisting for greater than three months with minimal relief. Chronic pain can be associated with a myriad of medical conditions. It is also one of the most common causes of disability, physical suffering, depression, and reduced quality of life. Treatment can vary depending on the underlying pathophysiology and can involve physical therapy, non-pharmaceutical approaches, pharmaceutical drugs, and invasive procedures. Currently available pharmaceutical agents have been effective for short-term management of chronic pain conditions, but few options address chronic pain with long-term efficacy. First-line pharmaceutical agents can potentially include over-the-counter (OTC) or prescription-strength non-steroidal anti-inflammatory drugs (NSAIDs), which have been linked to numerous side effects. If chronic pain persists, steroids are frequently used to provide longer relief. For more progressive or resistant chronic pain and/or in conjunction with invasive procedures, opioids have been utilized for acute treatment and for long-term maintenance. While these agents have proven to be effective for both acute and long-term use due to their modulation at various peripheral and central opioid receptors, they can be associated with numerous side effects and tied to the risk of addiction. As such, an unmet need exists to identify treatment modalities that provide opioid-like pain relief without opioid-induced adverse effects and the potential for addiction. This narrative review will provide an overview of the currently available treatment modalities for chronic pain and their adverse event profiles, as well as a review of therapies that are currently in development and/or preclinical trials for the management and treatment of chronic pain.

Efficacy and safety of naldemedine treatment for opioid-induced constipation in gastrointestinal cancer: a retrospective analysis.

Gastrointestinal cancers are one of the most common cancer cases worldwide. Cancer treatment is multidisciplinary, which includes opioid pain management. Opioid analgesics cause opioid-induced constipation (OIC) with the onset of effect. Naldemedine, a peripheral opioid receptor antagonist, is an OIC-modifying agent, but no focused efficacy and safety analysis has been conducted for its use in gastrointestinal cancers. We retrospectively evaluated patients with gastrointestinal cancer treated with naldemedine at ten institutions in Japan from June 2017 to August 2019. Patients with gastrointestinal cancer who initiated treatment with opioids during hospitalization and were treated with naldemedine for the first time were included in the study. The gastrointestinal cancer types included were esophageal, gastric, small bowel, and colorectal cancers. We assessed the defecation frequency before and after the initiation of naldemedine use. Responders were defined as patients who defecated three or more times/week, with an increase from the baseline of one or more bowel movements/week over seven days after starting naldemedine. Thirty-three patients were observed for one week before and after starting naldemedine. Twentyone patients had an increase in defecation frequency of at least three times per week or at least once per week above the baseline. The response rate was 63.6% (95% CI: 46.6-77.9%). The median number of bowel movements for a week before and after the initiation of naldemedine treatment was 3 (range, 0-13) and 7 (range, 1-39), respectively, in the overall population (n=33), with a significant increase in defecation frequency following naldemedine administration (Wilcoxon signed rank test, P<0.005). Diarrhea was the predominant gastrointestinal symptom, with 13 (39.4%) patients experiencing grade 1 and none experiencing grade 3 or grade 4 adverse events. The frequency of other grade 1 adverse events was low abdominal pain in two patients, nausea in two patients, and anorexia in one patient, without any grade 2-4 adverse events. The results of the study suggest that naldemedine is effective and safe in clinical practice for gastrointestinal cancer treatment.

Peripheralization Strategies Applied to Morphinans and Implications for Improved Treatment of Pain.

Opioids are considered the most effective analgesics for the treatment of moderate to severe acute and chronic pain. However, the inadequate benefit/risk ratio of currently available opioids, together with the current 'opioid crisis', warrant consideration on new opioid analgesic discovery strategies. Targeting peripheral opioid receptors as effective means of treating pain and avoiding the centrally mediated side effects represents a research area of substantial and continuous attention. Among clinically used analgesics, opioids from the class of morphinans (i.e., morphine and structurally related analogues) are of utmost clinical importance as analgesic drugs activating the mu-opioid receptor. In this review, we focus on peripheralization strategies applied to N-methylmorphinans to limit their ability to cross the blood-brain barrier, thus minimizing central exposure and the associated undesired side effects. Chemical modifications to the morphinan scaffold to increase hydrophilicity of known and new opioids, and nanocarrier-based approaches to selectively deliver opioids, such as morphine, to the peripheral tissue are discussed. The preclinical and clinical research activities have allowed for the characterization of a variety of compounds that show low central nervous system penetration, and therefore an improved side effect profile, yet maintaining the desired opioid-related antinociceptive activity. Such peripheral opioid analgesics may represent alternatives to presently available drugs for an efficient and safer pain therapy.

Efficacy of dual enkephalinase inhibition in a preclinical migraine model is mediated by activation of peripheral delta opioid receptors.

The aim of this study was to evaluate whether elevating levels of enkephalin by inhibiting their degradation can attenuate stress-induced migraine-like behaviors in mice. Previous studies in animals have suggested the delta opioid receptor (DOR) as a novel migraine target. The primary endogenous ligands for DOR are enkephalins and their levels can be increased by pharmacological inhibition of enkephalinases; however, it is not clear whether enkephalinase inhibition can be efficacious in preclinical migraine models through activation of DOR or whether other opioid receptors might be involved. Further, it is not clear whether opioid receptors in the central nervous system are necessary for these effects. This study used a model of repetitive restraint stress in mice that induces periorbital hypersensitivity and priming to the nitric oxide donor sodium nitroprusside (SNP; 0.1 mg/kg). Von Frey filaments were used to measure periorbital mechanical thresholds and grimace scores were evaluated by observing mouse facial features. Animals were treated with the dual enkephalinase inhibitor (DENKI) PL37. On day two post-stress, PL37 given to mice via either intravenous injection (10 mg/kg) or oral gavage (20 mg/kg) significantly attenuated stress-induced periorbital hypersensitivity and facial grimace responses. Additionally, both intravenous (10 mg/kg) and oral gavage (20 mg/kg) of PL37 prior to SNP (0.1 mg/kg) administration on day 14 post-stress significantly reduced SNP-induced facial hypersensitivity. Injection of the DOR antagonist naltrindole (0.1 mg/kg) but not the mu-opioid receptor antagonist CTAP (1 mg/kg) prior to PL37 treatment blocked the effects. Finally, pretreatment of mice with the peripherally restricted opioid receptor antagonist naloxone methiodide (5 mg/kg) blocked the effects of PL37. These data demonstrate that inhibiting enkephalinases, and thus protecting enkephalins from degradation, attenuates stress-induced migraine-like behavior via activation of peripheral DOR. Peripheral targeting of endogenous opioid signaling may be an effective therapeutic strategy for migraine.

The Pharmacokinetics of Subcutaneous Methylnaltrexone Bromide in Rhesus Macaques (Macaca mulatta).

Opioids are an integral component of pain management for nonhuman primates. These potent analgesics also adverse gastrointestinal (GI) effects that include constipation, bloating, and delayed gastric emptying. Methylnaltrexone bromide (MNTX) is a selective, peripherally acting μ- and κ-opioid receptor antagonist that can be used to mitigate the GI effects associated with opioid administration. Unlike naltrexone, a similar drug in this class, MNTX possesses an N-methyl-quaternary amine group that prevents it from crossing the blood brain barrier. This blockage allows inhibition of peripheral GI opioid receptors without affecting opioid-mediated analgesia in the central nervous system. We conducted a pharmacokinetic analysis of MNTX in serum and CSF of 6 healthy juvenile male rhesus macaques after subcutaneous administration of a 0.15-mg/kg dose. We hypothesized that the macaques would demonstrate a Tmax of 0.5 h, similar to that of humans, and that no MNTX would be detected in the CSF. This treatment resulted in a peak serum concentration of 114 ± 44 ng/mL at 0.25 ± 0.00 h; peak CSF at concentrations were 0.34 ± 0.07 ng/mL at the Tmax. These data show that subcutaneous administration of MNTX to rhesus macaques may block peripheral adverse effects of opioids without interfering with their central analgesic effects.

A Retrospective Study of the Efficacy and Safety of Naldemedine for Treatment of Opioid-Induced Constipation in Patients with Hepatobiliary Pancreatic Cancer.

Background and Objectives: Opioid analgesics, which are used for cancer-related pain management, cause opioid-induced constipation (OIC). Naldemedine, a peripheral opioid receptor antagonist, is an OIC-modifying agent, but no focused efficacy and safety analysis has been conducted for its use in hepatobiliary pancreatic cancers. We performed a multi-institutional study on the efficacy and safety of naldemedine in patients with hepatobiliary pancreatic cancer using opioids in clinical practice. Materials and Methods: We retrospectively evaluated patients with hepatobiliary pancreatic cancer (including liver, biliary tract, and pancreatic cancers) treated with opioids and naldemedine during hospitalization at ten institutions in Japan from June 2017 to August 2019. We assessed the frequency of bowel movements before and after the initiation of naldemedine therapy. Responders were defined as patients who defecated ≥3 times/week, with an increase from a baseline of ≥1 defecations/week over seven days after the initiation of naldemedine administration. Results: Thirty-four patients were observed for one week before and one week after starting naldemedine. The frequency of bowel movements increased by one over the baseline frequency or to at least thrice per week in 21 patients. The response rate was 61.7% (95% confidence interval: 45.4-78.0%). The median number of weekly bowel movements before and after naldemedine treatment was 2 (range: 0-9) and 6 (range: 1-17), respectively, in the overall population (n = 34); the increase in the number of bowel movements following naldemedine administration was statistically significant (Wilcoxon signed-rank test, p < 0.0001). Diarrhea was the predominant gastrointestinal symptom, and 10 (29.4%) patients experienced grade 1, grade 2, or grade 3 adverse events. The only other adverse event included fatigue in one patient; grade 2-4 adverse events were absent. Conclusions: Naldemedine is effective, and its use may be safe in clinical practice for patients with hepatobiliary pancreatic cancer receiving opioid analgesics.

Synergistic interaction between DAMGO-NH2 and NOP01 in peripherally acting antinociception in two mouse models of formalin pain

The most commonly used opioid analgesics are limited by their severe side-effects in the clinical treatment of pain. Preliminary reports indicate that the combination of classical opioids and N/OFQ receptor (NOP) ligands may be an effective strategy to reduce unwanted side-effects and improve antinociception. But the interaction of these two receptor ligands in pain regulation at the peripheral level remains unclear. In this study, the antinociception of a designed amide analogue of the mu opioid receptor (MOP) peptide agonist DAMGO, DAMGO-NH2, and its antinociceptive interaction with the peripherally limited NOP peptide agonist NOP01 was investigated in two mouse models of formalin pain. Our results showed that DAMGO-NH2 acted as a MOP agonist in in vitro functional assays. Moreover, local subcutaneous or intraplantar injection of DAMGO-NH2 exerted dose-related antinociception in both phases of the formalin orofacial and intraplantar pain, which could be mediated by the classical opioid receptor. Peripheral but not central pretreatment with the peripherally restricted opioid antagonist naloxone methiodide inhibited local DAMGO-NH2-induced antinociception, supporting the involvement of the peripheral opioid receptor in local DAMGO-NH2-induced antinociception. Furthermore, co-administration of the inactive doses of DAMGO-NH2 and NOP01 produced effective antinociception. More importantly, isobolographic analysis indicates that the combination of DAMGO-NH2 and NOP01 elicited supra-additive antinociception in these two models of formalin pain. In addition, the combination of DAMGO-NH2 and NOP01 did not change motor function of mice in rotarod test. In conclusion, these data suggest that peripheral DAMGO-NH2 and particularly its combination therapy with NOP01 may be effective for pain management.

The Infarct-Reducing Effect of the δ2 Opioid Receptor Agonist Deltorphin II: The Molecular Mechanism.

The search for novel drugs for the treatment of acute myocardial infarction and reperfusion injury of the heart is an urgent aim of modern pharmacology. Opioid peptides could be such potential drugs in this area. However, the molecular mechanism of the infarct-limiting effect of opioids in reperfusion remains unexplored. The objective of this research was to study the signaling mechanisms of the cardioprotective effect of deltorphin II in reperfusion. Rats were subjected to coronary artery occlusion (45 min) and reperfusion (2 h). The ratio of infarct size/area at risk was determined. This study indicated that the cardioprotective effect of deltorphin II in reperfusion is mediated via the activation of peripheral δ2 opioid receptor (OR), which is most likely localized in cardiomyocytes. We studied the role of guanylyl cyclase, protein kinase Cδ (PKCδ), phosphatidylinositol-3-kinase (PI3-kinase), extracellular signal-regulated kinase-1/2 (ERK1/2-kinase), ATP-sensitive K+-channels (KATP channels), mitochondrial permeability transition pore (MPTP), NO synthase (NOS), protein kinase A (PKA), Janus 2 kinase, AMP-activated protein kinase (AMPK), the large conductance calcium-activated potassium channel (BKCa-channel), reactive oxygen species (ROS) in the cardioprotective effect of deltorphin II. The infarct-reducing effect of deltorphin II appeared to be mediated via the activation of PKCδ, PI3-kinase, ERK1/2-kinase, sarcolemmal KATP channel opening, and MPTP closing.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors.

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca2+ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown. We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors. Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist. Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Effect of deuteration on the single dose pharmacokinetic properties and postoperative analgesic activity of methadone

Although methadone is effective in the management of acute pain, the complexity of its absorption-distribution-metabolism-excretion profile limits its use as an opioid of choice for perioperative analgesia. Because deuteration is known to improve the pharmacokinetic, pharmacodynamic and toxicological properties of some drugs, here we characterized the single dose pharmacokinetic properties and post-operative analgesic efficacy of d9-methadone.The pharmacokinetic profiles of d9-methadone and methadone administered intravenously to CD-1 male mice revealed that deuteration leads to a 5.7- and 4.4-fold increase in the area under the time-concentration curve and maximum concentration in plasma, respectively, as well as reduction in clearance (0.9 ± 0.3 L/h/kg vs 4.7 ± 0.8 L/h/kg). The lower brain-to-plasma ratio of d9-methadone compared to that of methadone (0.35 ± 0.12 vs 2.05 ± 0.62) suggested that deuteration decreases the transfer of the drug across the blood-brain barrier. The estimated LD50 value for a single intravenous dose of d9-methadone was 2.1-fold higher than that for methadone. Moreover, d9-methadone outperformed methadone in the efficacy against postoperative pain by primarily activating peripheral opioid receptors. Collectively, these data suggest that the replacement of three hydrogen atoms in three methyl groups of methadone altered its pharmacokinetic properties, improved safety, and enhanced its analgesic efficacy.

Topical Calendula officinalis L. inhibits inflammatory pain through antioxidant, anti-inflammatory and peripheral opioid mechanisms

ObjectiveThe present study evaluated the antinociceptive activity of Calendula officinalis L. (Ca) cream on inflammatory hypernociception. MethodsCreams with different Ca concentrations were tested for their ability to ameliorate pain-related behavior and edema in rats using formalin test, carrageenan (Cg)-induced acute inflammation model, bradykinin (BK)-induced acute inflammation model, and complete Freund’s adjuvant (CFA)-induced chronic inflammation model. Using the formalin test, we also examined the implication of peripheral opioid receptors in the antinociceptive mechanisms of Ca cream, by means of Q-naloxone, a peripherally acting nonselective opioid antagonist. Furthermore, the effects of Ca cream compared with diclofenac on BK-induced edema were examined when the kininase II in tissue was preserved or inhibited by captopril. The local production of redox biomarkers in formalin model, tumor necrosis factor-α (TNF-α) in Cg model and histopathological changes in CFA model were also evaluated. ResultsA single application of Ca cream at a dose of 10% or 30% (w/w) decreased the formalin-induced licking/biting behavior during both phases of this test in a Q-naloxone-sensitive manner. This effect was associated with the reduction of oxidative stress in the injured paw and the edema associated with the later phase of formalin-induced pain. A single application of Ca cream (10% or 30%) reduced paw edema and thermal hypernociception in Cg-induced acute inflammation, corresponding with a local decrease in TNF-α. Ca cream also inhibited BK-induced spontaneous nociceptive behavior and paw inflammation in both paradigms studied. Repeated applications of Ca cream also decreased CFA-induced chronic inflammation, mechanical hypernociception and histopathological changes in the paw. ConclusionThese results reveal the topical antinociceptive and antiedematogenic effects of Ca cream. A modulatory action on peripheral opioid receptors associated with its antioxidant mechanism may be involved, at least in part, in its analgesic effects. These findings may have an impact on the clinical management of painful inflammatory diseases.

NEUROBEHAVIOURAL AND ANALGESIC EFFECT OF Ocimum gratissimum LINN. LEAVES ESSENTIAL OIL IN Wistar albino MICE

Studies have shown that pain relieving medications may be neuroprotective. Ocimum gratissimum Linn. that is widely used in traditional medicine for debility and many other illnesses neuropharmacologically related has not been fully explored. Aim. This study was designed to investigate the safety of intake, neurobehavioral and analgesic effects of the Essential Oil of Ocimum gratissimum Linn leaves (EOOG) in mice. Methods. Acute toxicity of EOOG was determined following standard method while the neurobehavioural properties were assessed using the open field for Novelty-Induced Rearing (NIR), Novelty-Induced Grooming (NIG) and locomotor activity in mice. The hole board apparatus was used for the frequency of head dips. The Y-maze was used for short- working memory. Mechanistic studies were conducted with Atropine (muscarinic blocker, 0.5 mg/kg), Propanolol (non-selective ß-adrenoceptor blocker, 0.2 mg/kg), Haloperidol (dopamine receptor blocker, 0.2 mg/kg), Cyproheptadine (Serotoninergic antagonist, 0.5 mg/kg) and Yohimbine (ά-2 adrenergic blocker, 1 mg/kg). The analgesic activity of Ocimum gratissimum was investigated using acetic acid writhing test and thermally-induced pain. Results. The median lethal dose (LD50) of Ocimum gratissimum was 2449 mg/kg. The EOOG significantly reduced novelty-induced behaviour in a dose-dependent manner. The exploratory activity of animals treated with the EOOG was observed to decrease non-dose dependently with the highest dose (40 mg/kg) showing no activity on the hole board apparatus. The EOOG produced a significant reduction in locomotor activity in all the doses in a non-dose dependent manner but at the lowest dose. In the Y-maze, EOOG did not produce any significant effect on working memory as the percentage alternation produced was not significantly different from the control. The EOOG in hot plate analgesic assay showed increased reaction time suggesting central nervous system analgesic property. Conclusions. The results of the investigation showed that EOOG might possess sedative properties due to its ability to inhibit NIR and NIG, head dips, and locomotor activity. Furthermore, the inhibition of nociception marked in this research advocates antinociceptive activity which might be through the peripheral or central opioid receptor..