Synthesis, characterization, and biological study of new synthetic opioid hemorphin-4 peptides containing sterically restricted nonnatural amino acids

Some new hemorphin-4 analogs with structures of Xxx-Pro-Trp-Thr-NH2 and Tyr-Yyy-Trp-Thr-NH2, where Xxx is 2-amino-3-(4-hydroxy-2,6-dimethylphenyl)propanoic acid or 2-amino-3-(4-dibenzylamino-2,6-dimethylphenyl)propanoic acid, and Yyy is (2S,4S)-4-amino-pyrrolidine-2-carboxylic acid, were synthesized and characterized by electrochemical and spectral analyses. In vivo anticonvulsant and antinociceptive activities of peptide derivatives were studied after intracerebroventricular injection in mice. The therapeutic effects of the modified peptides on seizures and pain in mice were evaluated to provide valuable insights into the potential applications of the novel compounds. Electrochemical characterization showed that the compounds behave as weak protolytes and that they are in a soluble, stable molecular form at physiological pH values. The antioxidant activity of the peptides was evaluated with voltammetric analyses, which were confirmed by applying the 2,2-Diphenyl-1-picrylhydrazyl method. The compounds showed satisfactory results regarding their structural stability, reaching the desired centers for the manifestation of biological activity without hydrolysis processes at 37°C and physiological pH. Dm-H4 and H4-P1 exhibited 100% and 83% potency to suppress the psychomotor seizures in the 6-Hz test compared to 67% activity of H4. Notably, only the H4-P1 had efficacy in blocking the tonic component in the maximal electroshock test with a potency comparable to H4. All investigated peptides containing unnatural conformationally restricted amino acids showed antinociceptive effects. The analogs Db-H4 and H4-P1 showed the most pronounced and long-lasting effect in both experimental models of pain induced by thermal and chemical stimuli. Dm-H4 produced a dose-dependent thermal antinociception and H4-P2 inhibited only formalin-induced pain behavior.

In Vitro and In Vivo Pharmacological Profiles of LENART01, a Dermorphin-Ranatensin Hybrid Peptide

Diverse chemical and pharmacological strategies are currently being explored to minimize the unwanted side effects of currently used opioid analgesics while achieving effective pain relief. The use of multitarget ligands with activity at more than one receptor represents a promising therapeutic approach. We recently reported a bifunctional peptide-based hybrid LENART01 combining dermorphin and ranatensin pharmacophores, which displays activity to the mu-opioid receptor (MOR) and dopamine D2 receptor (D2R) in rat brains and spinal cords. In this study, we investigated the in vitro binding and functional activities to the human MOR and the in vivo pharmacology of LENART01 in mice after subcutaneous administration. In vitro binding assays showed LENART01 to bind and be selective to the human MOR over the other opioid receptor subtypes and delta, kappa and nociceptin receptors. In the [35S]GTPγS binding assay, LENART01 acted as a potent and full agonist to the human MOR. In mice, LENART01 produced dose-dependent antinociceptive effects in formalin-induced inflammatory pain, with increased potency than morphine. Antinociceptive effects were reversed by naloxone, indicating MOR activation in vivo. Behavioral studies also demonstrated LENART01's properties to induce less adverse effects without locomotor dysfunction and withdrawal syndrome compared to conventional opioid analgesics, such as morphine. LENART01 is the first peptide-based MOR-D2R ligand known to date and the first dual MOR-dopamine D2R ligand for which in vivo pharmacology is reported with antinociceptive efficacy and reduced opioid-related side effects. Our current findings may pave the way to new pain therapeutics with limited side effects in acute and chronic use.

Antinociceptive Analysis of Natural Monoterpenes Eugenol, Menthol, Carvacrol and Thymol in a Zebrafish Larval Model

In the last decade, a considerable number of studies have broadened our knowledge of the nociceptive mechanisms of pain, a global health problem in both humans and animals. The use of herbal compounds such as eugenol, menthol, thymol, and carvacrol as analgesic agents has accompanied the growing interest in this area, offering a possible solution for this complex problem. Here, we aimed to explore how these natural substances-at three different concentrations (2, 5 and 10 mg/L)-affect the pain responses in zebrafish (Danio rerio) larvae exposed to 0.05% acetic acid (AA) for 1 min. By analysing the activity of acetylcholinesterase (AChE), 5'-ectonucleotidase and NTPDases, as well as aversion and exploratory behaviours, it was observed that that although all substances were effective in counteracting the pain stimulus, the concentration range within which they do so might be very limited. Eugenol, despite its acknowledged properties in fish anaesthesia, failed to alleviate the pain stimulus at low concentrations. Contrastingly, menthol exhibited the most promising results at the lowest concentrations tested. Overall, it is concluded that menthol might be a good analgesic for this species, qualifying it as a substance of interest for prospective studies.

Subcutaneous hydromorphone hydrochloride provides antinociception with transient adverse effects in four-toed hedgehogs (Atelerix albiventris)

OBJECTIVE

To evaluate the efficacy and safety of hydromorphone administered SC in four-toed hedgehogs (Atelerix albiventris).

ANIMALS

12 healthy adult hedgehogs.

METHODS

Hedgehogs underwent 2 randomized, blinded, placebo-controlled, complete crossover studies. Hind limb withdrawal latencies in response to an acute thermal noxious stimulus were measured to evaluate the antinociceptive efficacy of hydromorphone. Baseline latencies were obtained prior to injection and collected again at 0.5, 1, 2, 4, and 6 hours following injection. Based on pilot studies, single doses of SC hydromorphone at 0.15 and 0.3 mg/kg were evaluated for efficacy in crossover trials. Safety of single (0.15 and 0.3 mg/kg) and multiple doses of hydromorphone (0.3 mg/kg, SC, q 4 h, for 3 doses) was also assessed. In addition to monitoring behavior during latency measurements, animals were evaluated for overt sedation and daily changes in food intake, body weight, and running wheel activity for 6 days after injection to evaluate for adverse effects.

RESULTS

Hydromorphone at 0.15 mg/kg provided antinociception lasting < 4 hours, and 0.3 mg/kg provided antinociception lasting < 6 hours. Hydromorphone produced transient abnormal behaviors at both doses, including vocalization, chewing motions of the jaw, and paw raising. There were no statistically significant differences in body weight or running wheel activity between treatments for single or multiple doses of hydromorphone. Three doses of 0.3 mg/kg hydromorphone (q 4 h) produced a statistically significant decrease (median, -9.7%; range, -64% to 10%) in 6-day total food intake.

CLINICAL RELEVANCE

Subcutaneous hydromorphone (0.15 to 0.3 mg/kg) can be used for short-term antinociception with transient adverse effects in hedgehogs.

Novel endomorphin analogues CEMR-1 and CEMR-2 produce potent and long-lasting antinociception with a favourable side effect profile at the spinal level

BACKGROUND AND PURPOSE

Endomorphins have shown great promise as pharmaceutics for the treatment of pain. We have previously confirmed that novel endomorphin analogues CEMR-1 and CEMR-2 behaved as potent μ agonists and displayed potent antinociceptive activities at the supraspinal and peripheral levels. The present study was undertaken to evaluate the antinociceptive properties of CEMR-1 and CEMR-2 following intrathecal (i.t.) administration. Furthermore, their antinociceptive tolerance and opioid-like side effects were also determined.

EXPERIMENTAL APPROACH

The spinal antinociceptive effects of CEMR-1 and CEMR-2 were determined in a series of pain models, including acute radiant heat paw withdrawal test, spared nerve injury-induced neuropathic pain, complete Freund's adjuvant-induced inflammatory pain, visceral pain and formalin pain. Antinociceptive tolerance was evaluated in radiant heat paw withdrawal test.

KEY RESULTS

Spinal administration of CEMR-1 and CEMR-2 produced potent and prolonged antinociceptive effects in acute pain. CEMR-1 and CEMR-2 may produce their antinociception through distinct μ receptor subtypes. These two analogues also exhibited significant analgesic activities in neuropathic, inflammatory, visceral and formalin pain at the spinal level. It is noteworthy that CEMR-1 showed non-tolerance-forming analgesic properties, while CEMR-2 exhibited substantially reduced antinociceptive tolerance. Furthermore, both analogues displayed no or reduced side effects on conditioned place preference response, physical dependence, locomotor activity and gastrointestinal transit.

CONCLUSIONS AND IMPLICATIONS

The present investigation demonstrated that CEMR-1 and CEMR-2 displayed potent and long-lasting antinociception with a favourable side effect profile at the spinal level. Therefore, CEMR-1 and CEMR-2 might serve as promising analgesic compounds with minimal opioid-like side effects.

Evaluation of Habenaria aitchisonii Reichb. for antioxidant, anti-inflammatory, and antinociceptive effects with in vivo and in silico approaches

Habenaria aitchisonii Reichb was analyzed in this research, including its chemical composition and its in vitro antioxidant, anti-inflammatory, acute oral toxicity, and antinociceptive activity. The chloroform and ethyl acetate fractions were found to be the most powerful based on in vitro antioxidant, anti-inflammatory, and analgesic assays. The acute oral toxicity of the crude methanolic extract was determined before in vivo studies. The acetic acid and formalin tests were used to measure the antinociceptive effect, and the potential mechanisms involved in antinociception were explored. The carrageenan-induced paw edema test was used to examine the immediate anti-inflammatory effect, and many phlogistic agents were used to determine the specific mechanism. Furthermore, for ex vivo activities, the mice were sacrificed, the forebrain was isolated, and the antioxidant levels of glutathione (GSH), superoxide dismutase (SOD), thiobarbituric acid reactive substances (TBARS) and catalase (CAT) were estimated using a UV spectrophotometer. No toxicity was seen at oral dosages up to 3,000 mg/kg. The antinociceptive impact was much higher than the standard drug. Both the inflammatory and neurogenic phases of the formalin experiment revealed an analgesic effect in the chloroform and ethyl acetate fractions. In carrageenan anti-inflammatory assays, the chloroform fraction (Ha.Chf) was the most potent fraction. We further studied the GC-MS of crude plant extract and found a total of 18 compounds. In the anti-inflammatory mechanism, it was observed that the Ha.Chf inhibits the COX-2 as well as 5-LOX pathways. The results exhibited that this species is a good source of phytocomponents like germacrone, which can be employed as a sustainable and natural therapeutic agent, supporting its traditional use in folk medicine for inflammatory conditions and pain.

Effects of acute cannabidiol on behavior and the endocannabinoid system in HIV-1 Tat transgenic female and male mice

Background

Some evidence suggests that cannabidiol (CBD) has potential to help alleviate HIV symptoms due to its antioxidant and anti-inflammatory properties. Here we examined acute CBD effects on various behaviors and the endocannabinoid system in HIV Tat transgenic mice.

Methods

Tat transgenic mice (female/male) were injected with CBD (3, 10, 30 mg/kg) and assessed for antinociception, activity, coordination, anxiety-like behavior, and recognition memory. Brains were taken to quantify endocannabinoids, cannabinoid receptors, and cannabinoid catabolic enzymes. Additionally, CBD and metabolite 7-hydroxy-CBD were quantified in the plasma and cortex.

Results

Tat decreased supraspinal-related nociception and locomotion. CBD and sex had little to no effects on any of the behavioral measures. For the endocannabinoid system male sex was associated with elevated concentration of the proinflammatory metabolite arachidonic acid in various CNS regions, including the cerebellum that also showed higher FAAH expression levels for Tat(+) males. GPR55 expression levels in the striatum and cerebellum were higher for females compared to males. CBD metabolism was altered by sex and Tat expression.

Conclusion

Findings indicate that acute CBD effects are not altered by HIV Tat, and acute CBD has no to minimal effects on behavior and the endocannabinoid system.

Antinociceptive Effect of Dillenia indica (Linn.) Mediated by Opioid and Cannabinoid Systems: Pharmacological and Chemical Studies

Dillenia indica (Linn.) has been reported by several biological activities, including anti-inflammatory, antioxidant, antidiabetic, anti-hyperglycemic, antiproliferative, antimutagenic, anticholinesterase, and antimicrobial. In Brazilian traditional medicine, the fruits of D. indica have been used to treat general topical pain and inflammation, but with no scientific validation. Thus, aiming to study its chemical constitution and antinociceptive properties, the crude extract (CE) and fractions obtained from the fruits of D. indica were submitted to an in vivo pharmacological evaluation and a dereplication study by LC-MS/MS analysis, assisted by the Global Natural Product Social Molecular Networking (GNPS). The oral antinociceptive activity of the fruits of D. indica and the possible participation of the opioid and cannabinoid systems were demonstrated in the formalin-induced nociception model. The chemical dereplication study led us to identify several known chemical constituents, including flavonoids, such as caffeoylmalic acid, naringenin, quercetin, and kaempferol. According to literature data, our results are compatible with significant antinociceptive and anti-inflammatory activities. Therefore, the flavonoid constituents of the fruits of D. indica are probably responsible for its antioxidant, anti-inflammatory, and antinociceptive effects mediated by both opioid and cannabinoid systems, confirming its folk use in the treatment and relief of pain.

µ

The number of opioid overdose deaths has increased significantly over the past decade. The life-threatening effect of opioids is hypoventilation that can be reversed by the µ-opioid receptor (MOR) antagonist naloxone; however, because of the very short duration of action of naloxone, re-emergence of MOR agonist-induced hypoventilation can occur, requiring additional doses of naloxone. The MOR antagonist methocinnamox (MCAM) antagonizes hypoventilation by the non-morphinan fentanyl and the morphinan heroin in laboratory animals with an unusually long duration of action. Whole-body plethysmography was used to compare the potency and effectiveness of MCAM and naloxone for preventing and reversing hypoventilation by fentanyl, heroin, and the ultra-potent and longer-acting fentanyl analogs carfentanil and 3-methylfentanyl in male rats breathing normal air. Sessions comprised a 45-minute habituation period followed by intravenous (i.v.) administration of saline or an acute dose of MOR agonist. The rank order of potency to decrease ventilation was 3-methylfentanyl > carfentanil > fentanyl > heroin. MCAM (0.0001-0.1 mg/kg) and naloxone (0.0001-0.01 mg/kg) dose-dependently reversed hypoventilation by 3-methylfentanyl (0.01 mg/kg), carfentanil (0.01 mg/kg), fentanyl (0.1 mg/kg), or heroin (3.2 mg/kg). For prevention studies, MCAM, naloxone, or vehicle was administered i.v. 22, 46, or 70 hours prior to a MOR agonist. When administered 22 hours earlier, MCAM (0.1-1.0 mg/kg) but not naloxone (1.0 mg/kg) prevented hypoventilation by each MOR agonist. This study demonstrates the effectiveness of MCAM to reverse and prevent hypoventilation by MOR agonists including ultra-potent fentanyl analogs that have a long duration of action. Significance Statement The number of opioid overdose deaths increased over the past decade despite the availability of antagonists that can prevent and reverse the effects of opioids. This study demonstrates the effectiveness and long duration of action of the µ-opioid receptor (MOR) antagonist methocinnamox (MCAM) for reversing and preventing hypoventilation by MOR agonists including ultra-potent fentanyl analogs. These results provide support for the notion that MCAM has the potential to positively impact the ongoing opioid crisis by reversing and preventing opioid overdose.

Investigation of anti-nociceptive, anti-inflammatory potential and ADMET studies of pure compounds isolated from Isodon rugosus Wall. ex Benth

The strong ethnopharmacological utilization of Isodon rugosus Wall. Ex. Benth is evident in the treatment of several types of pain and inflammation, including toothache, earache, abdominal pain, gastric pain, and generalized body pain and inflammation. Based on this background, the antinociceptive effects of the crude extract, various fractions, and essential oil have been reported previously. In this research work, we isolate and characterize pure bioactive compounds from I. rugosus and evaluate possible mechanisms using various in vivo and in vitro models. The pure compounds were analyzed for analgesic and anti-inflammatory activities through various assays. The column chromatography of the chloroform fraction of I. rugosus led to the identification of two pure compounds, i.e., 1 and 2. Compound 1 demonstrated notable inhibition (62% writhing inhibition, 72.77% COX-2 inhibition, and 76.97% 5-LOX inhibition) and anti-inflammatory potential (>50% paw edema inhibition at various intervals). The possible mechanism involved in antinociception was considered primarily, a concept that has already been elucidated through the application of naloxone (an antagonist of opioid receptors). The involvement of adrenergic receptors was investigated using a hot plate model (an adrenergic receptor antagonist). The strong ethnomedicinal analgesic background of I. rugosus, supported by previous reports and current observations, leads to the conclusion that I. rugosus is a potential source of antinociceptive and anti-inflammatory bioactive compounds. It may be concluded from the results that the isolated analgesic compounds of I. rugosus may be a possible alternative remedy for pain and inflammation management with admirable efficacy and safety profiles.

A Novel Leu-Enkephalin Prodrug Produces Pain-Relieving and Antidepressant Effects

Persistent pain is a significant healthcare problem with limited treatment options. The high incidence of comorbid chronic pain and depression significantly reduces life quality and complicates the treatment of both conditions. Antidepressants are less effective for pain and depression than for depression alone and they induce severe side effects. Opioids are highly efficacious analgesics, but rapid development of tolerance, dependence, and debilitating side effects limit their efficacy and safe use. Leucine-enkephalin (Leu-ENK), the endogenous delta opioid receptor agonist, controls pain and mood and produces potent analgesia with reduced adverse effects compared to conventional opioids. High proteolytic instability, however, makes Leu-ENK ineffective after systemic administration and limits its clinical usefulness. KK-103, a Leu-ENK prodrug, was developed to overcome these limitations of Leu-ENK via markedly increased plasma stability in mice. We showed rapid and substantially increased systemic adsorption and blood plasma exposure of KK-103 compared to Leu-ENK. We also observed brain uptake of radiolabeled KK-103 after systemic administration, indicating a central effect of KK-103. We then established KK-103's prolonged antinociceptive efficacy in the ramped hot plate and formalin test. In both models, KK-103 produced a comparable dose to the maximum antinociceptive-effect relationship. The pain-alleviating effect of KK-103 primarily resulted from activating the delta opioid receptor after the likely conversion of KK-103 to Leu-ENK in vivo. Finally, KK-103 produced an antidepressant-like activity comparable to the antidepressant desipramine, but with minimal gastrointestinal inhibition and no incidence of sedation.

Efficacy and safety of subcutaneous methadone hydrochloride in four-toed hedgehogs (Atelerix albiventris)

OBJECTIVE

To evaluate the efficacy and safety of SC methadone in four-toed hedgehogs.

ANIMALS

9 to 12 healthy adult four-toed hedgehogs (7 to 9 males and 3 females).

METHODS

Hedgehogs underwent 3 randomized, blinded, placebo-controlled, complete crossover studies. Hind limb withdrawal latencies in response to an acute thermal noxious stimulus were measured to evaluate the antinociceptive efficacy of methadone. Single doses of SC methadone were evaluated at 0.5 and 1 mg/kg for dose-dependent effects. Additionally, methadone (1.5 mg/kg) was administered at different concentrations to assess the effect of injection volume on antinociceptive efficacy. Finally, the safety of multiple doses of methadone (1.5 mg/kg, SC, q 2 h, for 3 doses) was also evaluated. In addition to monitoring behavior during latency measurements, animals were assessed for overt sedation. Food intake, body weight, and running wheel activity were assessed daily for 6 days following methadone administration to evaluate for adverse effects.

RESULTS

Methadone at 1 and 1.5 mg/kg provided antinociception lasting < 2 hours, and injection volume had no significant effect on efficacy. Methadone at 0.5 mg/kg did not induce antinociception. Methadone produced transient abnormal behaviors in all hedgehogs, with more animals affected at the 1.5-mg/kg dose. Behaviors included periods of standing motionless, vocalization, chewing motions, and paw raising. Single- or multiple-dose administration of methadone had no significant effect on total food intake, body weight, or running wheel activity.

CLINICAL RELEVANCE

The results of this study provide additional information on providing analgesia to hedgehogs. Subcutaneous methadone (1 to 1.5 mg/kg) can be used for short-term antinociception in hedgehogs.

Effect of the Location of Tetanic Stimulation on Autonomic Responses: A Randomized Cross-Over Pilot Study

Background

Tetanic stimuli are used as standardized noxious inputs to investigate nociception. Previous studies have applied tetanic stimuli to various anatomical locations without validating that the resulting physiological responses were independent of the location where tetanic stimuli were applied. Our aim was to investigate the effects of three anatomical tetanic stimulus application sites on physiological variables reflecting autonomic nervous system responses as measured by photoplethysmography (PPG).

Methods

Under general anesthesia, a five second, 100 hertz, 70 milliamp tetanic stimulus was applied to the ulnar nerve, medial side of the tibia, and thorax (T5 dermatome) (N=12). The effect of tetanic stimuli on PPG-derived variables (AC, DC, and ACDC) and pulse rate at each stimulus location was determined using repeated-measures analysis of variance (ANOVA) followed by Dunnett's post hoc test. Maximum tetanic stimulus-induced changes in PPG-derived variables and pulse rates were compared among the three stimulus locations using ANOVA.

Results

AC and ACDC values of PPG decreased, and the DC values of PPG increased in response to tetanic stimuli-induced vasoconstriction at each location (p<0.001 for all). The maximum changes in the AC, ACDC, and DC values did not differ between locations (p=NS). There were no significant changes in pulse rate (p=NS).

Conclusion

The results showed that tetanic stimulation at either of these three locations provides the same autonomic nervous system responses, as measured by PPG.

Clinical Trial Registration

ClinicalTrials.gov; NCT03648853.

Antinociceptive and antiinflammatory activities of crude leave extract and solvent fractions of Commelina latifolia Hochst. ex C.B.Clarke (Commelinaceae) leaves in murine model

Ethnopharmacological relevance: In the past, Ethiopian traditional medicine employed the leaves of the native Commelina latifolia Hochst. ex C.B. Clarke plant to treat wounds, pain, and malaria. Aim of the study: The crude extract and solvent fractions of C. latifolia Hochst. ex C.B. Clarke leaves were examined in the present investigation to determine their ability to have an antiinflammatory effect and provide an antinociceptive effect in animal models. Materials and methods: The leaves of C. latifolia were extracted with 80% methanol, and the CL crude extract was further fractionated with chloroform, pure methanol, and distilled water. The carrageenan-induced paw edema model was used to test the extracts' ability to reduce inflammation. The hotplate model and the acetic acid-induced writhing test on rodents were used to test the extracts' potential antinociceptive effect to reduce pain. Results: Inflammation was decreased by 64.59% with CL crude extract (400 mg/kg); 56.34% (400 mg/kg) of methanol fraction, 64.59% of aqueous fraction (400 mg/kg), and 38.27% of chloroform fraction in the carrageenan-induced inflammatory model. All extracts demonstrated a considerable lengthening of the nociception reaction time in the hot plate test, with a maximum antinociceptive effect of 78.98% (crude extract) and 71.65% (solvent fractions). At a dosage of 400 mg/kg, the natural C. latifolia crude extract and aqueous fraction demonstrated considerable antinociceptive effects against acetylsalicylic acid (ASA) during the writhing test (48.83% and 45.37than%, respectively). The current findings support Ethiopia's traditional user's assertions that the herb can alleviate inflammation and pain.

Experimental Study: Interleukin-31 Augments Morphine-Induced Antinociceptive Activity and Suppress Tolerance Development in Mice

Morphine-induced antinociception is partially reduced in interleukin-31 (IL-31) receptor A (IL-31RA)-deficient mice, indicating that IL-31RA is crucial for morphine-induced peripheral antinociception. Herein, we examined the combined effects of IL-31 and morphine on the antinociceptive activity and itch-associated scratching behavior (LLS) in mice and elucidated the regulatory mechanisms. A hot-plate test was used to assess antinociception. LLS was automatically detected and recorded via a computer. IL-31RA mRNA expression was assessed using real-time polymerase chain reaction. Repeated pre-treatment with IL-31 resulted in significant antinociceptive activity. Repeated administration of morphine decreased the morphine-induced antinociceptive activity, LLS counts, and regular dose and inhibited IL-31-induced LLS. These results suggested that the repeated administration of morphine depleted inter-neuronal IL-31RA levels, preventing morphine-induced antinociception. Therefore, IL-31 may be helpful as an adjunct analgesic to morphine. To explore the benefits of IL-31, its influence on morphine-induced antinociceptive tolerance in mice was examined. An IL-31 and morphine combination increased the analgesic action, which increased the expression of DRG neuronal IL-31RA, elucidating the site of peripheral antinociception of morphine. This site may induce exocytosis of IL-31RA in the sensory nervous system. Collectively, the suppressive effect of IL-31 on morphine-induced antinociceptive tolerance may result from IL-31RA supplementation in sensory nerves.

Identification and Pharmacological Characterization of a Low-Liability Antinociceptive Bifunctional MOR/DOR Cyclic Peptide

Peptide-based opioid ligands are important candidates for the development of novel, safer, and more effective analgesics to treat pain. To develop peptide-based safer analgesics, we synthesized a mixture-based cyclic pentapeptide library containing a total of 24,624 pentapeptides and screened the mixture-based library samples using a 55 °C warm water tail-withdrawal assay. Using this phenotypic screening approach, we deconvoluted the mixture-based samples to identify a novel cyclic peptide Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt), which produced dose- and time-dependent antinociception with an ED50 (and 95% confidence interval) of 0.70 (0.52-0.97) mg/kg i.p. mediated by the mu-opioid receptor (MOR). Additionally, higher doses (≥3 mg/kg, i.p.) of CycloAnt antagonized delta-opioid receptors (DOR) for at least 3 h. Pharmacological characterization of CycloAnt showed the cyclic peptide did not reduce breathing rate in mice at doses up to 15 times the analgesic ED50 value, and produced dramatically less hyperlocomotion than the MOR agonist, morphine. While chronic administration of CycloAnt resulted in antinociceptive tolerance, it was without opioid-induced hyperalgesia and with significantly reduced signs of naloxone-precipitated withdrawal, which suggested reduced physical dependence compared to morphine. Collectively, the results suggest this dual MOR/DOR multifunctional ligand is an excellent lead for the development of peptide-based safer analgesics.

Cleomin Exerts Acute Antinociceptive Effects in Mice via GABAB and Muscarinic Receptors

Cleomin, a 1,3-oxazolidine-2-thione, was recently isolated from Neocalyptrocalyx longifolium, a species traditionally used for treating painful conditions. Reports about the pharmacological activities of cleomin are lacking. Here, the antinociceptive effects of cleomin were investigated using mice models of pain, namely the formalin, the cold plate, and the tail flick tests. Motor integrity was assessed in the rota-rod test. Antagonism assays and in silico docking analyses were performed to investigate the putative mechanisms of action. Cleomin (12.5-25 mg/kg), at doses that did not induce motor impairment, induced dose-dependent antinociception in both early and late phases of the formalin test and reduced nociceptive behaviors in both the cold plate and tail flick tests. Pretreatments with phaclofen and atropine attenuated the antinociceptive effects of cleomin, implicating the involvement of GABAB and muscarinic receptors. In silico docking studies suggested satisfactory coupling between cleomin and GABAB and M2 receptors, hence corroborating their role in cleomin's activity. Pretreatments with naloxone, yohimbine, bicuculline, and methysergide did not affect the antinociception of cleomin. In silico pharmacokinetics prediction showed a good drug ability profile of cleomin. In conclusion, cleomin promoted antinociception mediated by GABAB and muscarinic receptors. These findings support further investigation of the analgesic potential of cleomin.

Morphine in donkeys: Antinociceptive effect and preliminary pharmacokinetics

BACKGROUND

Morphine is the prototypical μ-opioid receptor agonist used to provide analgesia in veterinary species. Its effects are well-described in horses but not donkeys.

OBJECTIVES

To determine the antinociceptive effects of two doses of morphine in donkeys. To describe preliminary pharmacokinetic parameters of morphine in donkeys.

STUDY DESIGN

In vivo experiment.

METHODS

Eight adult castrated male donkeys were given intravenous (IV) 0.9% saline, morphine 0.1 mg/kg bwt (LDM), or morphine 0.5 mg/kg bwt (HDM) in a randomised order with a minimum 1-week washout period. Mechanical nociceptive thresholds (MNTs) were determined by a blinded investigator pre-injection and 15, 30, 45, 60, 90, 120, 150, 180, 210, 240, 300, and 360 min post-injection. Venous blood samples were collected pre-injection and 2, 5, 10, 15, 30, 45, 60, 90, and 120 min post-injection. Data were analysed using Friedman's test with Dunn's post hoc test for multiple comparisons. Pharmacokinetic parameters were calculated for the HDM treatment.

RESULTS

Baseline MNT was [median (interquartile range)] 8.9 (7.1-10.3) N and did not differ between treatments. Peak MNTs occurred at 60 min for both LDM (16.2 N) and HDM (25.0 N) treatments. MNTs after HDM treatment were higher than saline (p < 0.04) at 15, 60, 90, 120, 150, 180, 240, and 300 min post-injection. MNTs after LDM treatment were higher than baseline (p < 0.05) at 45 and 60 min post-injection. Terminal half-life for HDM was (mean ± SD) 51.0 ± 10.7 min, the volume of distribution at steady-state 2.07 ± 0.33 L/min and clearance 49.2 ± 4.16 ml * min/kg using noncompartmental analysis. The concentration of morphine-3-glucuronide (M3G) was higher than morphine-6-glucuronide (M6G) at all sampled time points.

MAIN LIMITATIONS

Short duration of plasma sampling for pharmacokinetic analysis; lack of objective measure of gastrointestinal function.

CONCLUSIONS

The HDM treatment provided mechanical antinociception in donkeys with no significant adverse effects.

Evaluation of Thermal Antinociceptive Effects of Intramuscular Hydromorphone Hydrochloride in Great Horned Owls (Bubo virginianus)

Across the Americas, great horned owls (Bubo virginianus) are often presented to veterinarians for conditions requiring pain management. Although recent studies have evaluated opioid drugs in raptor species, information in Strigiformes is lacking. The objective of this study was to evaluate the analgesic effect and duration of action of hydromorphone hydrochloride, a full µ-opioid receptor agonist, in great horned owls. In a randomized, blinded, balanced crossover study, 6 adult birds (5 females and 1 male) received hydromorphone (0.3 and 0.6 mg/kg) or saline (0.9% NaCl) solution (0.03 mL/kg; control) in the left pectoral muscle, with a 7-day washout interval between treatments. Each bird was assigned an agitation-sedation score, and the thermal foot withdrawal threshold (TFWT) was measured at predetermined times before (t = 0 hours) and after treatment administration (t = 0.5, 1.5, 3, and 6 hours). Measurements of the TFWT were obtained with a test box equipped with a thermal perch, which delivered a gradually increasing temperature 40-62°C (104-143.6°F) to the right plantar surface of the owl's foot. Compared with controls, hydromorphone at 0.3 mg/kg dose resulted in significantly higher mean TFWT at 0.5 hours (P < 0.001), 1.5 hours (P = 0.003), and 3 hours (P = 0.005), whereas the 0.6 mg/kg dose resulted in significantly higher mean TFWT from 0.5 hours (P = 0.035) to 1.5 hours (P = 0.001). Both hydromorphone doses were associated with a significant change in the agitation-sedation score (P = 0.001), consistent with mild to moderate sedation. Two owls were observed tremoring after administration of the 0.6 mg/kg dose, which was not noted after the 0.5-hour timepoint; no other adverse effects were identified. This study offers scientific evidence to support the use of a µ-opioid agonist in great horned owls for pain management. Pharmacokinetics and other pharmacodynamic studies of other pain models evaluating hydromorphone and other opioid drugs in this species are still needed.

Ketamine-Propofol Coadministration for Induction and Infusion Maintenance in Anesthetized Dogs: Effects on Electroencephalography and Antinociception

The effects of concurrent ketamine and propofol (ketofol) constant rate infusion (CRI) were examined in six dogs. The K:P ratio was 1:2, with an initial CRI of 0.25/0.5 mg/kg/min over ten minutes, followed by a 0.5 mg/kg ketamine bolus for induction. During induction, a comprehensive EEG frequency spectrum from delta to gamma was observed, accompanied by subanesthetic-dose ketofol-induced behavioral excitation, including nystagmus, tongue flicking, salivation and active muscle activity. The dogs were maintained on three 15 min decremental doses of ketofol CRI (0.8/1.6, 0.4/0.8 and 0.2/0.4 mg/kg/min). This phase featured a significant decrease in the Patient State Index, electromyographic activity and a shift to low beta waves (SEF95: 13-18 Hz). Additionally, profound antinociception to electric stimulation and a stable heart rate and blood pressure (MBP 81.5-110 mmHg) were observed, as well as a merging of ketamine and propofol EEG characteristics during maintenance. In the recovery phase, a return to beta and gamma EEG patterns and excitement behavior occurred, accompanied by a significant reduction in antinociception, highlighting features of low doses of ketofol. This study reveals biphasic EEG dynamic changes, associated behaviors and robust antinociception and cardiovascular function, suggesting the utility of ketofol as a total intravenous anesthetic combination in dogs.

Antinociceptive effect of Nephelium lappaceum L. fruit peel and the participation of nitric oxide, opioid receptors, and ATP-sensitive potassium channels

Introduction: Nephelium lappaceum L. (Sapindaceae) is a plant known as rambutan. It is used for various purposes in traditional medicine. Objective: We aimed to evaluate the antinociceptive effects of the ethanol extract of the fruit peel of N. lappaceum (EENL), the mechanisms involved in these effects, and the acute toxicity in zebrafish. Methods: We performed chromatography coupled to mass spectrometry, acute toxicity assay in zebrafish, and evaluation in mice submitted to models of nociception and locomotor activity. Results: We identified (epi)-catechin, procyanidin B, and ellagic acid and its derivatives in EENL. We did not find any toxicity in zebrafish embryos incubated with EENL. The locomotor activity of mice submitted to oral pretreatment with EENL was not changed, but it reduced the abdominal constrictions induced by acetic acid, the licking/biting time in both the first and second phase of formalin testing and capsaicin testing, and carrageenan-induced paw mechanical allodynia. Oral pretreatment with EENL increased latency time in the hot plate test. This antinociceptive effect was significantly reversed by naloxone, L-arginine, and glibenclamide respectively showing the participation of opioid receptors, nitric oxide, and KATP channels as mediators of EENL-induced antinociception. Conclusion: EENL causes antinociception with the participation of opioid receptors, nitric oxide, and KATP channels, and is not toxic to zebrafish.

Behavioral Effects and Analgesic Profile of Hemoglobin-Derived Valorphin and Its Synthetic Analog in Rodents

Valorphin (V1) is a naturally occurring peptide derived from hemoglobin that has been found to have an affinity for opioid receptors and exhibits antinociceptive and anticonvulsant activity. Some of its synthetic analogs containing an aminophosphonate moiety show structure-dependent potent antinociceptive effects. This study aimed to reveal a detailed picture of the antinociceptive mechanisms and behavioral effects of V1 and its recently synthesized phosphopeptide analog V2p in rodents using a range of methods. The studied peptides significantly reduced acute (mean V1-9.0, V2p-5.8 vs. controls-54.1 s) and inflammatory (mean V1-57.9 and V2p-53.3 vs. controls-107.6 s) nociceptive pain in the formalin test, as well as carrageenan-induced hyperalgesia (mean V1-184.7 and V2p-107.3 vs. controls-61.8 g) in the paw pressure test. These effects are mediated by activation of opioid receptors with a predominance of kappa in V1 antinociception and by delta, kappa, and mu receptors in V2p-induced antinociception. Both peptides did not change the levels of TNF-alpha and IL-1-beta in blood serum. V1 induces depression-like behavior, and V2p shows a tendency toward anxiolysis and short-term impairment of motor coordination without affecting exploratory behavior. The results characterize valorphin and its derivative as promising analgesics that exert their effects both centrally and peripherally, without causing severe behavioral changes in experimental animals. These encouraging data are a foundation for future studies focusing on the effects of hemorphins after long-term treatment.

PnPP-15, a Synthetic Peptide Derived from a Toxin from Phoneutria nigriventer Spider Venom, Alleviates Diabetic Neuropathic Pain and Acts Synergistically with Pregabalin in Mice

Diabetic neuropathic pain is one of the complications that affect a wide variety of the diabetic population and is often difficult to treat. Only a small number of patients experience pain relief, which usually comes with onerous side effects and low levels of satisfaction. The search for new analgesic drugs is necessary, given the limitations that current drugs present. Combining drugs to treat neuropathic pain has been attracting interest to improve their efficacy compared to single-drug monotherapies while also reducing dose sizes to minimize side effects. The aim of our study was to verify the antinociceptive effect of a synthetic peptide, PnPP-15, alone and combined with pregabalin, in male Swiss diabetic mice using the von Frey method. PnPP-15 is a synthetic peptide derived from PnPP19, a peptide representing a discontinuous epitope of the primary structure of the toxin PnTx2-6 from the venom of the spider Phoneutria nigriventer. The antinociceptive activity of both compounds was dose-dependent and showed synergism, which was verified by isobolographic analysis. Treatment with PnPP-15 did not cause spontaneous or forced motor changes and did not cause any damage or signs of toxicity in the analyzed organs (pancreas, lung, heart, kidney, brain, or liver). In conclusion, PnPP-15 is a great candidate for an analgesic drug against neuropathic pain caused by diabetes and exerts a synergistic effect when combined with pregabalin, allowing for even more efficient treatment.

N-palmitoylethanolamide synergizes the antinociception of morphine and gabapentin in the formalin test in mice

OBJECTIVE

The antinociceptive pharmacological interaction between N-palmitoylethanolamide (PEA) and morphine (MOR), as well as gabapentin (GBP), was investigated to obtain synergistic antinociception at doses where side effects were minimal. In addition, the possible antinociceptive mechanism of PEA + MOR or PEA + GBP combinations was explored.

METHODS

Individual dose-response curves (DRCs) of PEA, MOR and GBP were evaluated in female mice in which intraplantar nociception was induced with 2% formalin. Isobolographic method was used to detect the pharmacological interaction in the combination of PEA + MOR or PEA + GBP.

KEY FINDINGS

The ED50 was calculated from the DRC; the order of potency was MOR > PEA > GBP. The isobolographic analysis was obtained at a 1:1 ratio to determine the pharmacological interaction. The experimental values of flinching (PEA + MOR, Zexp = 2.72 ± 0.2 μg/paw and PEA + GBP Zexp = 2.77 ± 0.19 μg/paw) were significantly lower than those calculated theoretically (PEA + MOR Zadd = 7.78 ± 1.07 and PEA + GBP Zadd = 24.05 ± 1.91 μg/paw), resulting in synergistic antinociception. Pretreatment with GW6471 and naloxone demonstrated that peroxisome proliferator-activated receptor alpha (PPARα) and opioid receptors are involved in both interactions.

CONCLUSIONS

These results suggest that MOR and GBP synergistically enhance PEA-induced antinociception through PPARα and opioid receptor mechanisms. Furthermore, the results suggest that combinations containing PEA with MOR or GBP could be of interest in aiding the treatment of inflammatory pain.

Subcutaneous administration of hydromorphone (0.2 mg/kg) provides antinociception in ferrets (Mustela putorius furo)

OBJECTIVE

To evaluate antinociceptive efficacy of SC administration of hydromorphone hydrochloride and buprenorphine hydrochloride in ferrets (Mustela putorius furo).

ANIMALS

14 healthy adult ferrets (6 neutered males, 8 spayed females).

METHODS

In a randomized, blind, controlled, complete crossover design, all 14 ferrets received a single, SC injection of hydromorphone low dose (0.1 mg/kg), hydromorphone high dose (0.2 mg/kg), buprenorphine low dose (0.02 mg/kg), buprenorphine high dose (0.04 mg/kg), or saline solution (0.2 mL/kg). Sedation and forelimb withdrawal latency from a noxious thermal stimulation were evaluated, and behavior was recorded for a total of 8 hours postinjection.

RESULTS

Compared to saline, administration of hydromorphone at 0.2 mg/kg resulted in an estimated increase of withdrawal latencies of 7.4 seconds (95% CI, 3.2 to 11.6) at 60 minutes, of 6.6 seconds (2.4 to 10.8) at 90 minutes, of 6.0 seconds (1.8 to 10.2) at 120 minutes, of 7.0 seconds (2.9 to 11.1) at 180 minutes, and of 4.5 seconds (0.5 to 8.6) at 240 minutes. These differences were statistically significant. Hydromorphone administered at a lower dose and buprenorphine at either dose did not increase withdrawal latencies compared to saline. Based on the sedation score used in this study, signs of sedation increased over time in a similar fashion with all treatments, including saline. Erratic dysphoric-like behaviors occurred in all groups except for saline.

CLINICAL RELEVANCE

SC administration of hydromorphone at a dose of 0.2 mg/kg provided antinociception from 1 to 4 hours postinjection. Further validation of sedation scores in ferrets is warranted.

Ion channels in cancer-induced bone pain: from molecular mechanisms to clinical applications

Cancer-induced bone pain (CIBP) caused by bone metastasis is one of the most prevalent diseases, and current treatments rely primarily on opioids, which have significant side effects. However, recent developments in pharmaceutical science have identified several new mechanisms for CIBP, including the targeted modification of certain ion channels and receptors. Ion channels are transmembrane proteins, which are situated on biological cell membranes, which facilitate passive transport of inorganic ions across membranes. They are involved in various physiological processes, including transmission of pain signals in the nervous system. In recent years, there has been an increasing interest in the role of ion channels in chronic pain, including CIBP. Therefore, in this review, we summarize the current literature on ion channels, related receptors, and drugs and explore the mechanism of CIBP. Targeting ion channels and regulating their activity might be key to treating pain associated with bone cancer and offer new treatment avenues.

Oral morphine induces spinal 5-hydroxytryptamine (5-HT) release using an opioid receptor-independent mechanism

Morphine induces spinal 5-hydroxytryptamine (5-HT) release, but the role and mechanism of the spinal 5-HT release induced by morphine are not well understood. The purpose of this study was to define the role and mechanism of spinal 5-HT release induced by oral morphine. We also examined whether persistent pain affected the spinal 5-HT release induced by oral morphine. Spinal 5-HT release was measured using microdialysis of lumbar cerebrospinal fluid (CSF). Two opioids, morphine and oxycodone, were orally administered and 5-HT release was measured in awake rats. Naloxone and β-funaltrexamine (β-FNA) were used to determine whether the effect of morphine on 5-HT release was mediated by opioid receptor activation. To study persistent pain, a formalin test was used. At 45 min after oral morphine administration, the formalin test was started and spinal 5-HT release was measured. Oral morphine, but not oral oxycodone, increased 5-HT release at the spinal cord to approximately 4000% of the baseline value. This effect of morphine was not antagonized by either naloxone or β-FNA at a dose that antagonized the antinociceptive effect of morphine. Formalin-induced persistent pain itself had no effect on spinal 5-HT release but enhanced the oral morphine-induced spinal 5-HT release. Oral morphine-induced spinal 5-HT release was not mediated by opioid receptor activation. Spinal 5-HT induced by oral morphine did not play a major role in the antinociceptive effect of morphine in the hot plate test. Persistent pain increased oral morphine-induced spinal 5-HT release.

Analyzing the Antinociceptive Effect of Interleukin-31 in Mice

The theory that an itch inhibits pain has been refuted; however, previous research did not investigate this theory for an interleukin-31 (IL-31)-induced itch. Previously, we have found that morphine-induced antinociception was partially reduced in IL-31 receptor A (IL-31RA)-deficient (IL-31RAKI) mice, indicating that IL-31RA may play an important role in morphine-induced peripheral antinociception. In the present study, we evaluated the effect of IL-31-induced analgesia on a 2,4,6-trinitrochlorobenzene (TNCB)-sensitized mice using a hot-plate test. This test evaluated the antinociceptive activity of morphine and non-steroidal anti-inflammatory drugs (NSAIDs). Repeated pretreatment with IL-31 showed significant antinociceptive action. Furthermore, its combination with morphine, but not with NSAIDs, increased the analgesic action. In contrast, treatment with TNCB and capsaicin decreased antinociception. Moreover, TNCB increased IL-31RA expression in the dorsal root ganglia at 24 h, whereas capsaicin inhibited it. The comparative action of several analgesics on TNCB or capsaicin was evaluated using a hot-plate test, which revealed that the antinociceptive activity was decreased or disappeared in response to capsaicin-induced pain in IL-31RAKI mice. These results indicate that the analgesic action of IL-31 involves the peripheral nervous system, which affects sensory nerves. These results provide a basis for developing novel analgesics using this mechanism.

Treatment-Resistant Depression (TRD): Is the Opioid System Involved?

About 30% of major depression disorder patients fail to achieve remission, hence being diagnosed with treatment-resistant major depression (TRD). Opium had been largely used effectively to treat depression for centuries, but when other medications were introduced, its use was discounted due to addiction and other hazards. In a series of previous studies, we evaluated the antinociceptive effects of eight antidepressant medications and their interaction with the opioid system. Mice were tested with a hotplate or tail-flick after being injected with different doses of mianserin, mirtazapine, trazodone, venlafaxine, reboxetine, moclobemide, fluoxetine, or fluvoxamine to determine the effect of each drug in eliciting antinociception. When naloxone inhibited the antinociceptive effect, we further examined the effect of the specific opioid antagonists of each antidepressant drug. Mianserin and mirtazapine (separately) induced dose-dependent antinociception, each one yielding a biphasic dose-response curve, and they were antagonized by naloxone. Trazodone and venlafaxine (separately) induced a dose-dependent antinociceptive effect, antagonized by naloxone. Reboxetine induced a weak antinociceptive effect with no significant opioid involvement, while moclobemide, fluoxetine, and fluvoxamine had no opioid-involved antinociceptive effects. Controlled clinical studies are needed to establish the efficacy of the augmentation of opiate antidepressants in persons with treatment-resistant depression and the optimal dosage of drugs prescribed.

Kinins' Contribution to Postoperative Pain in an Experimental Animal Model and Its Implications

Postoperative pain causes discomfort and disability, besides high medical costs. The search for better treatments for this pain is essential to improve recovery and reduce morbidity and risk of chronic postoperative pain. Kinins and their receptors contribute to different painful conditions and are among the main painful inflammatory mediators. We investigated the kinin's role in a postoperative pain model in mice and reviewed data associating kinins with this painful condition. The postoperative pain model was induced by an incision in the mice's paw's skin and fascia with the underlying muscle's elevation. Kinin levels were evaluated by enzyme immunoassays in sham or operated animals. Kinin's role in surgical procedure-associated mechanical allodynia was investigated using systemic or local administration of antagonists of the kinin B₁ receptor (DALBk or SSR240612) or B₂ receptor (Icatibant or FR173657) and a kallikrein inhibitor (aprotinin). Kinin levels increased in mice's serum and plantar tissue after the surgical procedure. All kinin B₁ or B₂ receptor antagonists and aprotinin reduced incision-induced mechanical allodynia. Although controversial, kinins contribute mainly to the initial phase of postoperative pain. The kallikrein-kinin system can be targeted to relieve this pain, but more investigations are necessary, especially associations with other pharmacologic targets.

Antinociceptive, Sedative and Excitatory Effects of Intravenous Butorphanol Administered Alone or in Combination with Detomidine in Calves: A Prospective, Randomized, Blinded Cross-Over Study

(1) Background: The diagnostic and therapeutic procedures performed under sedation or general anesthesia in bovines are numerous. The analgesic drugs that can be legally used are few, making perioperative analgesia challenging. (2) Methods: Calves were administered butorphanol 0.1 mg kg^-1 alone (SB) or combined with 0.02 mg kg^-1 of a detomidine (DB) IV. The antinociceptive effect (trigeminocervical reflex threshold (TCRt)), as well as the behavioral (sedation and excitation) and physiological (heart and respiratory rate) changes were investigated. Five time windows were defined: BL (30 min pre-injection), T1 (0-30 min post-injection (PI)), T2 (31-60 min PI), T3 (61-90 min PI) and T4 (91-120 min PI). (3) Results: Both groups had a significative increase in TCRt at T1-T4 compared to the BL. The TCRt was significatively higher in DB than in SB at T1, T2 and T4. Heart rate decreased significatively in DB compared to that in BL. Calves were significantly more sedated in the DB group, and significantly more excited in the SB group compared to the BL. (4) Conclusions: Butorphanol alone has a statistically significant antinociceptive effect, but it elicits marked excitation, limiting its clinical applicability under this dosing regimen. The co-administration of detomidine eliminated the excitatory effect and induced consistent sedation and a significantly more pronounced antinociceptive effect.

Mitochondrial-Derived Peptide MOTS-c Ameliorates Spared Nerve Injury-Induced Neuropathic Pain in Mice by Inhibiting Microglia Activation and Neuronal Oxidative Damage in the Spinal Cord via the AMPK Pathway

MOTS-c, a recently discovered mitochondrial-derived peptide, plays an important role in many physiological and pathological functions via adenosine monophosphate-activated protein kinase (AMPK) activation. Numerous studies have demonstrated that AMPK is an emerging target for the modulation of neuropathic pain. Meanwhile, microglia-activation-evoked neuroinflammation is known to contribute to the development and progression of neuropathic pain. MOTS-c is also known to inhibit microglia activation, chemokine and cytokine expression, and innate immune responses. Accordingly, in this study, we evaluated the effects of MOTS-c on neuropathic pain and investigated the putative underlying mechanisms. We found that MOTS-c levels in plasma and spinal dorsal horn were significantly lower in mice with spared nerve injury (SNI)-induced neuropathic pain than in control animals. Accordingly, MOTS-c treatment produced pronounced dose-dependent antinociceptive effects in SNI mice; however, these effects were blocked by dorsomorphin, an AMPK inhibitor, but not naloxone, a nonselective opioid receptor antagonist. Moreover, intrathecal (i.t.) injection of MOTS-c significantly enhanced AMPKα_1/2 phosphorylation in the lumbar spinal cord of SNI mice. MOTS-c also significantly inhibited proinflammatory cytokine production and microglia activation in the spinal cord. The antinociceptive effects of MOTS-c were retained even when microglia activation in the spinal cord was inhibited by minocycline pretreatment, indicating that spinal cord microglia are dispensable for the antiallodynic effects of MOTS-c. In the spinal dorsal horn, MOTS-c treatment inhibited c-Fos expression and oxidative damage mainly in neurons rather than microglia. Finally, in contrast to morphine, i.t. administration of MOTS-c resulted in limited side effects relating to antinociceptive tolerance, gastrointestinal transit inhibition, locomotor function, and motor coordination. Collectively, the present study is the first to provide evidence that MOTS-c may be a promising therapeutic target for neuropathic pain.

Current Drug Development Overview: Targeting Voltage-Gated Calcium Channels for the Treatment of Pain

Voltage-gated calcium channels (VGCCs) are targeted to treat pain conditions. Since the discovery of their relation to pain processing control, they are investigated to find new strategies for better pain control. This review provides an overview of naturally based and synthetic VGCC blockers, highlighting new evidence on the development of drugs focusing on the VGCC subtypes as well as mixed targets with pre-clinical and clinical analgesic effects.

Melatonin promotes orofacial antinociception in adult zebrafish by modulating TRP channels

Melatonin is an endogenous hormone, known as the sleep hormone, which has already demonstrated its antinociceptive effect. This study aimed to evaluate the participation of TRP's channels in the orofacial antinociceptive effect of melatonin (MT) in adult zebrafish. Initially, the open field test was performed to evaluate the effect of MT on the locomotor activity of adult zebrafish. Then, the animals were pre-treated with MT (0.1, 0.3 or 1 mg/mL; gavage) and acute orofacial nociception was induced by the application of capsaicin (TRPV1 agonist), cinnamaldehyde (TRPA1 agonist) or menthol (TRPM8 agonist) applied into the animal's lip. Naive groups were included. MT, per se, did not alter the locomotor activity of the animals. MT reduced the nociceptive behaviour induced by the three agonists; however, the most significant effect was obtained with the lowest concentration tested (0.1 mg/mL) in the capsaicin test. The orofacial antinociceptive effect of melatonin was prevented by capsazepine (TRPV1 antagonist), but not by HC-030031 (TRPA1 antagonist). The molecular docking study indicated interaction between MT and the TRPV1, TRPA1 and TRPM8 channels and, in line with the in vivo results, there was greater affinity between MT and the TRPV1 channel. The results confirm the pharmacological relevance of melatonin as an inhibitor of orofacial nociception and this effect seems to be related to the modulation of TRP's channels.

Green light induces antinociception via visual-somatosensory circuits

Light has been shown to relieve pain, but the underlying neural mechanisms remain unknown. Here, we show that low-intensity (200 lux) green light treatment exerts antinociceptive effects through a neural circuit from the visual cortex projecting to the anterior cingulate cortex (ACC) in mice. Specifically, viral tracing, in vivo two-photon calcium imaging, and fiber photometry recordings show that green light activated glutamatergic projections from the medial part of the secondary visual cortex (V2MGlu) to GABAergic neurons in the ACC, which drives inhibition of local glutamatergic neurons (V2MGlu→ACCGABA→Glu). Optogenetic or chemogenetic activation of the V2MGlu→ACCGABA→Glu circuit mimics green-light-induced antinociception in both neuropathic and inflammatory pain model mice. Artificial inhibition of ACC-projecting V2MGlu neurons abolishes the antinociception induced by green light. Taken together, our study shows the V2M-ACC circuit as a potential candidate mediating green-light-induced antinociceptive effects.

Cannabidiol reduces LPS-induced nociception via endocannabinoid system activation

Bacterial infections are often accompanied by fever and generalized muscle pain. However, the treatment of pain with an infectious etiology has been overlooked. Thus, we investigated the impact of cannabidiol (CBD) in bacterial lipopolysaccharide (LPS)-induced nociception. Male Swiss mice received intrathecal (i.t.) LPS injection, and the nociceptive threshold was measured by the von Frey filaments test. Spinal involvement of the cannabinoid CB₂ receptor, toll-like receptor 4 (TLR4), microglia and astrocytes were evaluated by i.t. administration of their respectively antagonists or inhibitors. Western blot, immunofluorescence, ELISA and liquid chromatography-mass spectrometry were used to assess Cannabinoid CB₂ receptors and TLR4 spinal expression, proinflammatory cytokines and endocannabinoid levels. CBD was administered intraperitoneally at 10 mg/kg. The pharmacological assay demonstrated TLR4 participation in LPS-induced nociception. In addition, spinal TLR4 expression and proinflammatory cytokine levels were increased in this process. CBD treatment prevented LPS-induced nociception and TLR4 expression. AM630 reversed antinociception and reduced CBD-induced endocannabinoids upregulation. Increased spinal expression of the cannabinoid CB₂ receptor was also found in animals receiving LPS, which was accompanied by reduced TLR4 expression in CBD-treated mice. Taken together, our findings indicated that CBD is a potential treatment strategy to control LPS-induced pain by attenuating TLR4 activation via the endocannabinoid system.

MMG22 Potently Blocks Hyperalgesia in Cisplatin-treated Mice

MMG22 is a bivalent ligand containing MOR agonist and mGluR5 antagonist pharmacophores connected by a 22-atom linker. Intrathecal (i.t.) administration of MMG22 to inflamed mice has been reported to produce fmol-range antinociception in the reversal of LPS-induced hyperalgesia. MMG22 reduced hyperalgesia in the spared nerve injury (SNI) model of neuropathic pain at 10 days after injury but not at 30 days after injury, perhaps related to the inflammation that occurs early after injury but subsequently subsides. The present study determined the efficacy of MMG22 in cisplatin-treated male mice in order to provide data relating to the efficacy of MMG22 in the treatment of neuropathic pain that is associated with inflammation. Groups of eight mice each received daily intraperitoneal (i.p.) injections of cisplatin for seven days to produce robust mechanical allodynia defined by the decrease in withdrawal threshold using an electronic von Frey applied to the plantar surface of the hind paw. Intrathecal administration of MMG22 potently reduced mechanical hyperalgesia (ED50 0.04 fmol/mouse) without tolerance, whereas MMG10 was essentially inactive. Morphine was less potent than MMG22 by >5-orders of magnitude and displayed tolerance. Subcutaneous MMG22 was effective (ED50 = 2.41 mg/kg) and devoid of chronic tolerance. We propose that MMG22 induces the formation of a MOR-mGluR5 heteromer through selective interaction with the upregulated NR2B subunit of activated NMDAR, in view of the 4600-fold reduction of i.t. MMG22 antinociception by the selective NR2B antagonist, Ro25-6981. A possible explanation for the substantially reduced potency for MMG22 in the SNI model is discussed.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Effects of neonatal fentanyl on late adolescent opioid-mediated behavior

Introduction

Because of the steady increase in the use of synthetic opioids in women of childbearing age, a large number of children are at risk of exposure to these drugs prenatally or postnatally through breast milk. While there is older literature looking at the effects of morphine and heroin, there are relatively few studies looking at the long-term effects of high-potency synthetic opioid compounds like fentanyl. Thus, in the present study, we assessed whether brief exposure to fentanyl in male and female rat pups during a period roughly equivalent to the third trimester of CNS development altered adolescent oral fentanyl self-administration and opioid-mediated thermal antinociception.

Methods

We treated the rats with fentanyl (0, 10, or 100 μg/kg sc) from postnatal day (PD) 4 to PD 9. The fentanyl was administered daily in two injections given 6 h apart. After the last injection on PD 9, the rat pups were left alone until either PD 40 where they began fentanyl self-administration training or PD 60 where they were tested for morphine- (0, 1.25, 2.5, 5, or 10 mg/kg) or U50,488- (0, 2.5, 5, 10, or 20 mg/kg) induced thermal antinociception.

Results

In the self-administration study, we found that female rats had more active nose pokes than male rats when receiving a fentanyl reward but not sucrose alone solution. Early neonatal fentanyl exposure did not significantly alter fentanyl intake or nose-poke response. In contrast, early fentanyl exposure did alter thermal antinociception in both male and female rats. Specifically, fentanyl (10 μg/kg) pre-treatment increased baseline paw-lick latencies, and the higher dose of fentanyl (100 μg/kg) reduced morphine-induced paw-lick latencies. Fentanyl pre-treatment did not alter U50,488-mediated thermal antinociception.

Conclusions

Although our exposure model is not reflective of typical human fentanyl use during pregnancy, our study does illustrate that even brief exposure to fentanyl during early development can have long-lasting effects on mu-opioid-mediated behavior. Moreover, our data suggest that females may be more susceptible to fentanyl abuse than males.

The Endogenous Cannabinoid and the Nitricoxidergic Systems in the Modulation of Stress Responses

The effects on stress-induced analgesia (SIA) from endogenous cannabinoid system (ECS) and nitric oxide (NO) interaction after 1 h of restraint stress were evaluated in male Wistar rats. The animals were subjected to 1 h of restraint and then injected with different combinations of cannabinoid receptor type 1 agonist anandamide (AEA) or antagonist AM251 along with an NO donor, NO precursor, or inhibitor of NO synthase. Nociception was evaluated using paw pressure (PP) or hot plate (HP) tests. AEA was administered immediately after the end of restraint-SIA (r-SIA). Administration of NO precursor reversed the pronociceptive effect of the CB1 agonist on r-SIA. Both the CB1 antagonist and the NOS inhibitor neutralized the pro-analgesic effect of L-arginine (L-arg). Administration of an NO donor, instead, increased r-SIA. Our experiments confirmed that the endogenous cannabinoid and the NO-ergic systems interact in the modulation of r-SIA. This interaction probably implies NO as a second messenger of the ECS.

Prostaglandin EP3 receptor activation is antinociceptive in sensory neurons via PI3Kγ, AMPK and GRK2

BACKGROUND AND PURPOSE

Prostaglandin E₂ is considered a major mediator of inflammatory pain, by acting on neuronal G_s protein-coupled EP2 and EP4 receptors. However, the neuronal EP3 receptor, colocalized with EP2 and EP4 receptor, is G_i protein-coupled and antagonizes the pronociceptive prostaglandin E₂ effect. Here, we investigated the cellular signalling mechanisms by which the EP3 receptor reduces EP2 and EP4 receptor-evoked pronociceptive effects in sensory neurons.

EXPERIMENTAL APPROACH

Experiments were performed on isolated and cultured dorsal root ganglion (DRG) neurons from wild type, phosphoinositide 3-kinase γ (PI3Kγ)^-/- , and PI3Kγ^{kinase dead (KD)/KD} mice. For subtype-specific stimulations, we used specific EP2, EP3, and EP4 receptor agonists from ONO Pharmaceuticals. As a functional readout, we recorded TTX-resistant sodium currents in patch-clamp experiments. Western blots were used to investigate the activation of intracellular signalling pathways. EP4 receptor internalization was measured using immunocytochemistry.

KEY RESULTS

Different pathways mediate the inhibition of EP2 and EP4 receptor-dependent pronociceptive effects by EP3 receptor stimulation. Inhibition of EP2 receptor-evoked pronociceptive effect critically depends on the kinase-independent function of the signalling protein PI3Kγ, and adenosine monophosphate activated protein kinase (AMPK) is involved. By contrast, inhibition of EP4 receptor-evoked pronociceptive effect is independent on PI3Kγ and mediated through activation of G protein-coupled receptor kinase 2 (GRK2), which enhances the internalization of the EP4 receptor after ligand binding.

CONCLUSION AND IMPLICATIONS

Activation of neuronal PI3Kγ, AMPK, and GRK2 by EP3 receptor activation limits cAMP-dependent pain generation by prostaglandin E₂ . These new insights hold the potential for a novel approach in pain therapy.

Antinociceptive effects of nortriptyline and desipramine hydrochloride in Speke's hinge-back tortoise (Kinixys Spekii)

Some of the most commonly used analgesic drugs in animals are of questionable efficacy or present adverse side effects among the various species of reptiles. Tricyclic antidepressants have been demonstrated to have antinociceptive effects in several animal models of pain and could be a good alternative for use in reptiles. The aim of the study was to investigate the antinociceptive effects of nortriptyline and desipramine hydrochloride in Speke's hinge-back tortoise. A total of 24 animals weighing 600-1000 g were used for nociceptive tests, i.e., formalin, capsaicin, and hot plate tests. Drugs were administered intracoelomically 30 min before starting the tests. The time spent in nocifensive behavior and the associated observable effects during the tests were recorded. Only the highest dose of 40 mg/kg of nortriptyline hydrochloride caused statistically significant decrease in nocifensive behavior in both the formalin and the capsaicin test. Desipramine hydrochloride at doses of 20 and 40 mg/kg caused statistically significant decrease in nocifensive behavior in the formalin test. Also, desipramine hydrochloride at doses of 15, 20, and 60 mg/kg caused statistically significant decrease in nocifensive behavior in the capsaicin test. None of the doses used for both drugs had any statistically significant effect on nocifensive behavior in the hot plate test. The results show that nortriptyline and desipramine hydrochloride have significant antinociceptive effects in the chemical but not thermal inflammatory pain-related behavior in the Speke's hinge-back tortoise. The most common associated side effect following administration of the higher doses of either of the drugs is excessive salivation.

Effect of nitric oxide-cyclic GMP-K+ channel pathway blockers, naloxone and metformin, on the antinociception induced by the diuretic pamabrom

Pamabrom is a diuretic that is effective in treating premenstrual syndrome and primary dysmenorrhea. The aim of this study was to examine the effect of metformin and modulators of the opioid receptor-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP)-K⁺ channel pathway on the local antinociception induced by pamabrom. The rat paw 1% formalin test was used to assess the effects. Rats were treated with local administration of pamabrom (200-800 µg/paw) or indomethacin (200-800 µg/paw). The antinociception of pamabrom or indomethacin was evaluated with and without the local pretreatment of the blockers. Local administration of pamabrom and indomethacin produced dose-dependent antinociception during the second phase of the test. Local pretreatment of the paws with naloxone (50 µg/paw), l-nitro-arginine methyl ester (10-100 µg/paw), or 1H-(1,2,4)-oxadiazolo[4,2-a]quinoxalin-1-one (10-100 µg/paw) reverted the antinociception induced by local pamabrom, but not of indomethacin. Similarly, the K⁺ channel blockers glibenclamide, glipizide, 4-aminopyridine, tetraethylammonium, charybdotoxin, or apamin reverted the pamabrom-induced antinociception, but not of indomethacin. Metformin significantly blocked the antinociception of pamabrom and indomethacin. Our data suggest that pamabrom could activate the opioid receptor-NO-cGMP-K⁺ channel pathway to produce its peripheral antinociception in the formalin test. Likewise, a biguanide-dependent mechanism could be activated by pamabrom and indomethacin to generate antinociception.

Design of κ-Opioid Receptor Agonists for the Development of Potential Treatments of Pain with Reduced Side Effects

The κ-opioid receptor (KOR) has recently emerged as an alternative therapeutic target for the development of pain medications, without deleterious side effects associated with the μ-opioid receptor (MOR). However, modulation of KOR is currently under investigation for the treatment of depression, mood disorders, psychiatric comorbidity, and specific drug addictions. However, KOR agonists also trigger adverse effects including sedation, dysphoria, and hallucinations. In this respect, there is currently much debate on alternative paradigms. Recent effort has been devoted in search of biased ligands capable of selectively activating favorable signaling over signaling associated with unwanted side effects. On the other hand, the use of partial agonists is expected to allow the analgesia to be produced at dosages lower than those required to produce the adverse effects. More empirically, the unwanted central effects can be also avoided by using peripherally restricted agonists. In this review, we discuss the more recent trends in the design of KOR-selective, biased or partial, and finally, peripherally acting agonists. Special emphasis is given on the discussion of the most recent approaches for controlling functional selectivity of KOR-specific ligands.

The Antinociceptive Effect of a Hydroalcoholic Extract of Polygala altomontana and Its Chemical Profile Using UPLC-ESI-QTOF-HR-MS

The hydroalcoholic extract of Polygala altomontana (30, 100, and 300 mg/kg, i.g.) showed a dose-dependent antinociceptive action during the inflammatory phase of the formalin test. In addition, the preparation (30 and 300 mg/kg, i.g.) showed anti-hyperalgesic action when tested on a mechanical nociception model. UPLC-ESI-QTOF-MS data indicated the active extract contained phenylpropanoid sucrose esters, glycosylated quercetin derivatives, styrylpyrones, and coumarins. Some identified compounds, including styrylpyrones and coumarins, have previously demonstrated antinociceptive action. The results also show that P. altomontana shows potential for developing pain-relieving herbal remedies and drugs.

OFP011 Cyclic Peptide as a Multifunctional Agonist for Opioid/Neuropeptide FF Receptors with Improved Blood-Brain Barrier Penetration

Mounting evidence indicates that the neuropeptide FF (NPFF) system is involved in the side effects of opioid usage, including antinociceptive tolerance, hyperalgesia, abuse, constipation, and respiratory depression. Our group recently discovered that the multitarget opioid/NPFF receptor agonist DN-9 exhibits peripheral antinociceptive activity. To improve its metabolic stability, antinociceptive potency, and duration, in this study, we designed and synthesized a novel cyclic disulfide analogue of DN-9, OFP011, and examined its bioactivity through in vitro cyclic adenosine monophosphate (cAMP) functional assays and in vivo behavioral experiments. OFP011 exhibited multifunctional agonistic effects at the μ-opioid and the NPFF₁ and NPFF₂ receptors and partial agonistic effects at the δ- and κ-opioid in vitro, as determined via the cAMP functional assays. Pharmacokinetic and pharmacological experiments revealed improvement in its blood-brain barrier permeability after systemic administration. In addition, subcutaneous OFP011 exhibited potent and long-lasting antinociceptive activity via the central μ- and κ-opioid receptors, as observed in different physiological and pathological pain models. At the highest antinociceptive doses, subcutaneous OFP011 exhibited limited tolerance, gastrointestinal transit, motor coordination, addiction, reward, and respiration depression. Notably, OFP011 exhibited potent oral antinociceptive activities in mouse models of acute, inflammatory, and neuropathic pain. These results suggest that the multifunctional opioid/NPFF receptor agonists with improved blood-brain barrier penetration are a promising strategy for long-term treatment of moderate to severe nociceptive and pathological pain with fewer side effects.

(2R,6R)-hydroxynorketamine (HNK) reverses mechanical hypersensitivity in a model of localized inflammatory pain

The ketamine metabolite (2R,6R)-hydroxynorketamine, or (2R,6R)-HNK, was recently reported to evoke antinociception in response to a noxious thermal stimulus in healthy mice and reverse mechanical hypersensitivity in a murine model of neuropathic pain. This study reports the behavioral effects of (2R,6R)-HNK in male and female C57BL/6J mice exposed to a localized inflammatory pain condition and the broad pharmacological mechanism underlying this effect. Hind paw intraplantar injection of λ-carrageenan (CARR) caused inflammation and mechanical hypersensitivity in mice within 2 hours, lasting at least 48 hours. Intraperitoneal administration of (2R,6R)-HNK (10-30 mg/kg i.p.) 2 hours following CARR injection significantly reversed mechanical hypersensitivity within 1 hour in male and female mice, and the effect persisted for 24 hours following a single dose. The magnitude and timing of the analgesic effect of (2R,6R)-HNK were comparable to the non-steroidal anti-inflammatory drug carprofen. The reversal of hypersensitivity by (2R,6R)-HNK was blocked at 4 and 24 hours after administration by pretreatment with the AMPA receptor antagonist NBQX and was not accompanied by changes in locomotor activity. These findings reinforce the growing evidence supporting (2R,6R)-HNK as a novel analgesic in multiple preclinical pain models and further support an AMPAR-dependent mechanism of action. SIGNIFICANCE: The ketamine metabolite (2R,6R)-HNK reversed mechanical hypersensitivity associated with localized inflammation with onset less than one hour and duration greater than 24 hours in an effect comparable to the NSAID carprofen. Reversal of mechanical hypersensitivity by (2R,6R)-HNK is AMPAR-dependent.

Decursinol-mediated antinociception and anti-allodynia in acute and neuropathic pain models in male mice: Tolerance and receptor profiling

Korean scientists have shown that oral administration of Angelica gigas Nakai (AGN) root alcoholic extract and the metabolite of its pyranocoumarins, decursinol, have antinociceptive properties across various thermal and acute inflammatory pain models. The objectives of this study were 1) to assess whether tolerance develops to the antinociceptive effects of once-daily intraperitoneally administered decursinol (50 mg/kg) in acute thermal pain models, 2) to establish its anti-allodynic efficacy and potential tolerance development in a model of chemotherapy-evoked neuropathic pain (CENP) and 3) to probe the involvement of select receptors in mediating the pain-relieving effects with antagonists. The results show that decursinol induced antinociception in both the hot plate and tail-flick assays and reversed mechanical allodynia in mice with cisplatin-evoked neuropathic pain. Tolerance was detected to the antinociceptive effects of decursinol in the hot plate and tail-flick assays and to the anti-allodynic effects of decursinol in neuropathic mice. Pretreatment with either the 5-HT2 antagonist methysergide, the 5-HT2A antagonist volinanserin, or the 5-HT2C antagonist SB-242084 failed to attenuate decursinol-induced antinociception in the tail-flick assay. While pretreatment with the cannabinoid inverse agonists rimonabant and SR144528 failed to modify decursinol-induced anti-allodynia, pretreatment with the opioid antagonist naloxone partially attenuated the anti-allodynic effects of decursinol. In conclusion, our data support decursinol as an active phytochemical of AGN having both antinociceptive and anti-allodynic properties. Future work warrants a more critical investigation of potential receptor mechanisms as they are likely more complicated than initially reported.