Comparison of the µ-opioid receptor antagonists methocinnamox (MCAM) and naloxone to reverse and prevent the ventilatory depressant effects of fentanyl, carfentanil, 3-methylfentanyl, and heroin in male rats.

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.

The effects of eating a traditional high fat/high carbohydrate or a ketogenic diet on sensitivity of female rats to morphine.

Patients diagnosed with obesity are prescribed opioid medications at a higher rate than the general population; however, it is not known if eating a high fat diet might impact individual sensitivity to these medications. To explore the hypothesis that eating a high fat diet increases sensitivity of rats to the effects of morphine, 24 female Sprague-Dawley rats (n=8/diet) ate either a standard laboratory chow (17% kcal from fat), a high fat/low carbohydrate (ketogenic) chow (90.5% kcal from fat), or a traditional high fat/high carbohydrate chow (60% kcal from fat). Morphine-induced antinociception was assessed using a warm water tail withdrawal procedure, during which latency (in seconds) for rats to remove their tail from warm water baths was recorded following saline or morphine (0.32-56 mg/kg, IP) injections. Morphine was administered acutely and chronically, which involved 19 days of twice daily injections (increasing in 1/4 log dose increments every 3 days: 3.2-56 mg/kg, IP) to induce dependence and assess tolerance. The adverse effects of morphine (i.e., tolerance, withdrawal, changes in body temperature) were assessed throughout the study. Morphine induced comparable antinociception in rats eating different diets, and all rats developed tolerance following chronic morphine exposure. Additional adverse effects of morphine were also comparable among rats eating different diets; however, withdrawal-induced weight loss was less severe for rats eating ketogenic chow. These results suggest that dietary manipulation might modulate the severity of withdrawal-related weight loss, in ways that could be relevant for patients. Significance Statement The present study in female rats suggests that eating a high fat/low carbohydrate (ketogenic) or a traditional high fat/high carbohydrate diet does not impact the pain-relieving or adverse effects of opioids (i.e., tolerance or withdrawal). However, eating a ketogenic diet may have beneficial effects on opioid withdrawal-related weight loss. Individuals diagnosed with obesity taking opioids for pain-related conditions might therefore consider adopting a ketogenic diet when opioid administration is discontinued to potentially mitigate withdrawal-related weight loss.

Low-Efficacy Mu Opioid Agonists as Candidate Analgesics: Effects of Novel C-9 Substituted Phenylmorphans on Pain-Depressed Behavior in Mice.

Low efficacy mu opioid receptor (MOR) agonists may serve as novel candidate analgesics with improved safety relative to high-efficacy opioids. This study used a recently validated assay of pain-depressed behavior in mice to evaluate a novel series of MOR-selective C9-substituted phenylmorphan opioids with graded MOR efficacies. Intraperitoneal injection of dilute lactic acid (IP acid) served as a noxious stimulus to depress locomotor activity by mice in an activity chamber composed of two compartments connected by an obstructed door. Behavioral measures included (1) crosses between compartments (vertical activity over the obstruction) and (2) movement counts quantified as photobeam breaks summed across compartments (horizontal activity). Each drug was tested alone and as a pretreatment to IP acid. A charcoal-meal test and whole-body-plethysmography assessment of breathing in 5% CO2 were also used to assess gastrointestinal (GI) inhibition and respiratory depression, respectively. IP acid produced a concentration-dependent depression in crosses and movement that was optimally alleviated by intermediate- to low-efficacy phenylmorphans with sufficient efficacy to produce analgesia with minimal locomotor disruption. Follow-up studies with two low-efficacy phenylmorphans (JL-2-39 and DC-1-76.1) indicated that both drugs produced naltrexone-reversible antinociception with a rapid onset and a duration of ~1hr. Potency of both drugs increased when behavior was depressed by a lower IP-acid concentration, and neither drug alleviated behavioral depression by a non-pain stimulus (IP lithium chloride). Both drugs produced weaker GI inhibition and respiratory depression than fentanyl and attenuated fentanyl-induced GI inhibition and respiratory depression. Results support further consideration of selective, low-efficacy MOR agonists as candidate analgesics. Significance Statement This study used a novel set of mu opioid receptor (MOR)-selective opioids with graded MOR efficacies to examine the lower boundary of MOR efficacy sufficient to relieve pain-related behavioral depression in mice. Two novel low-efficacy opioids (JL-2-39, DC-1-76.1) produced effective antinociception with improved safety relative to higher- or lower-efficacy opioids, and results support further consideration of these and other low-efficacy opioids as candidate analgesics.

Benzofuran Iboga-Analogs Modulate Nociception and Inflammation in an Acute Mouse Pain Model.

Pain management following acute injury or post-operative procedures is highly necessary for proper recovery and quality of life. Opioids and non-steroidal anti-inflammatory drugs (NSAIDS) have been used for this purpose, but opioids cause addiction and withdrawal symptoms whereas NSAIDS have several systemic toxicities. Derivatives of the naturally occurring iboga alkaloids have previously shown promising behavior in anti-addiction of morphine by virtue of their interaction with opioid receptors. On this frontier, four benzofuran analogs of the iboga family have been synthesized and their analgesic effects have been studied in formalin induced acute pain model in male Swiss albino mice at 30 mg/kg of body weight dose administered intraperitoneally. The antioxidant, anti-inflammatory and neuro-modulatory effects of the analogs were analyzed. Reversal of tail flick latency, restricted locomotion and anxiogenic behavior were observed in iboga alcohol, primary amide and secondary amide. Local neuroinflammatory mediators' substance P, calcitonin gene related peptide, cyclooxygenase-2 and p65 were significantly decreased whereas the depletion of brain derived neurotrophic factor and glia derived neurotrophic factor was overturned on iboga analog treatment. Behavioral patterns after oral administration of the best analog were also analyzed. Taken together, these results show that the iboga family of alkaloid has huge potential in pain management.

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.

Synthesis, molecular docking, electrochemical and fluorimetric analysis of new caffeic and cinnamic acid-conjugated hemorphin derivatives designed as potential anticonvulsant and antinociceptive agents.

Based on the pharmacophore model of opioid receptors, our team recently synthesized a series of short-chain hemorphin peptide analogs containing non-natural amino acids. They demonstrated anticonvulsant and antinociceptive activity with low neurotoxicity. In the present study, a series of novel bioconjugates of N-modified hemorphin analogs containing second pharmacophore cinnamic acids (CA) or caffeic (KA) were synthesized by a traditional solid-phase Fmoc chemistry method for peptide synthesis. Electrochemical and fluorimetric analysis, in vivo anticonvulsant and antinociceptive activity in mice were conducted on the compounds. The three CA acid- (H4-CA, H5-CA, and H7-CA) and three KA acid- (H4-KA, H5-KA, and H7-KA) conjugated hemorphin derivatives exhibited potency at the highest doses of 2 µg/5 µl, administered by intracerebroventricular (icv) mode, against seizure spread in the maximal electroshock test (MES) in mice. The KA-conjugated H5-KA derivate, at the lowest dose, was the only compound that suppressed clonic seizures in the subcutaneous pentylenetetrazol (scPTZ) test. Except for the H5-CA, all tested CA acid- and KA acid-conjugated peptide derivates had the potency to increase the latency for clonic seizures in a dose-dependent mode. The activity against the psychomotor seizures in the 6-Hz test was detected only for the H4-CA (0.5 µg) and H4-KA (0.5 µg and 1 µg), respectively. All investigated peptides showed a more pronounced antinociceptive effect in the "intraplantar formalin" test compared to the "hot plate" test. Shorter chain analogs showed a better antinociceptive profile against tonic pain. The data suggest a DOR and KOR-mediated mechanism of action. According to the docking analysis, H7-CA showed a different antinociceptive profile than other investigated peptides. The novel peptide derivates did not exhibit neurotoxicity in the rotarod test. Our findings suggest that conjugated CA and KA morphine peptides can be used to develop novel morphine-related analogs with anticonvulsant and antinociceptive activity.

Endogenous Cholinergic System Involved in Peripheral Analgesic Control in Mice Is Activated by TNF-α, CXCL-1, and IL-1ß.

INTRODUCTION: Tissue injury results in the release of inflammatory mediators, including a cascade of algogenic substances, which contribute to the development of hyperalgesia. During this process, endogenous analgesic substances are peripherally released to counterbalance hyperalgesia. The present study aimed to investigate whether inflammatory mediators TNF-α, IL-1ß, CXCL1, norepinephrine (NE), and prostaglandin E2 (PGE2) may be involved in the deflagration of peripheral endogenous modulation of inflammatory pain by activation of the cholinergic system. METHODS: Male Swiss mice were subjected to paw withdrawal test. All the substances were injected via the intraplantar route. RESULTS: The main findings of this study were as follows: (1) carrageenan (Cg), TNF-α, CXCL-1, IL1-ß, NE, and PGE2 induced hyperalgesia; (2) the acetylcholinesterase enzyme inhibitor, neostigmine, reversed the hyperalgesia observed after Cg, TNF-α, CXCL-1, and IL1-ß injection; (3) the non-selective muscarinic receptor antagonist, atropine, and the selective muscarinic type 1 receptor (m1AChr) antagonist, telenzepine, potentiated the hyperalgesia induced by Cg and CXCL-1; (4) mecamylamine, a non-selective nicotinic receptor antagonist, potentiated the hyperalgesia induced by Cg, TNF-α, CXCL-1, and IL1-ß; (5) Cg, CXCL-1, and PGE2 increased the expression of the m1AChr and nicotinic receptor subunit α4protein. CONCLUSION: These results suggest that the cholinergic system may modulate the inflammatory pain induced by Cg, PGE2, TNF-α, CXCL-1, and IL1-ß.

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.

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.

Topical diosmetin attenuates nociception and inflammation in a ultraviolet B radiation-induced sunburn model in mice.

Burns are a global health problem and can be caused by several factors, including ultraviolet (UV) radiation. Exposure to UVB radiation can cause sunburn and a consequent inflammatory response characterised by pain, oedema, inflammatory cell infiltration, and erythema. Pharmacological treatments available to treat burns and the pain caused by them include nonsteroidal anti-inflammatory drugs (NSAIDs), opioids, antimicrobials and glucocorticoids, which are associated with adverse effects. Therefore, the search for new therapeutic alternatives is needed. Diosmetin, an aglycone of the flavonoid diosmin, has antinociceptive, antioxidant and anti-inflammatory properties. Thus, we evaluated the antinociceptive and anti-inflammatory effects of topical diosmetin (0.01, 0.1 and 1%) in a UVB radiation-induced sunburn model in mice. The right hind paw of the anaesthetised mice was exposed only once to UVB radiation (0.75 J/cm2) and immediately treated with diosmetin once a day for 5 days. The diosmetin antinociceptive effect was evaluated by mechanical allodynia and pain affective-motivational behaviour, while its anti-inflammatory activity was assessed by measuring paw oedema and polymorphonuclear cell infiltration. Mice exposed to UVB radiation presented mechanical allodynia, increased pain affective-motivational behaviour, paw oedema and polymorphonuclear cell infiltration into the paw tissue. Topical Pemulen® TR2 1% diosmetin reduced the mechanical allodynia, the pain affective-motivational behaviour, the paw oedema and the number of polymorphonuclear cells in the mice's paw tissue similar to that presented by Pemulen® TR2 0.1% dexamethasone. These findings indicate that diosmetin has therapeutic potential and may be a promising strategy for treating patients experiencing inflammatory pain, especially those associated with sunburn.

The Analgesic Potential of Litsea Species: A Systematic Review.

Various plant species from the Litsea genus have been claimed to be beneficial for pain relief. The PRISMA approach was adopted to identify studies that reported analgesic properties of plants from the Litsea genus. Out of 450 records returned, 19 primary studies revealed the analgesic potential of nine Litsea species including (1) Litsea cubeba, (2) Litsea elliptibacea, (3) Litsea japonica, (4) Litsea glutinosa, (5) Litsea glaucescens, (6) Litsea guatemalensis, (7) Litsea lancifolia, (8) Litsea liyuyingi and (9) Litsea monopetala. Six of the species, 1, 3, 4, 7, 8 and 9, demonstrated peripheral antinociceptive properties as they inhibited acetic-acid-induced writhing in animal models. Species 1, 3, 4, 8 and 9 further showed effects via the central analgesic route at the spinal level by increasing the latencies of heat stimulated-nocifensive responses in the tail flick assay. The hot plate assay also revealed the efficacies of 4 and 9 at the supraspinal level. Species 6 was reported to ameliorate hyperalgesia induced via partial sciatic nerve ligation (PSNL). The antinociceptive effects of 1 and 3 were attributed to the regulatory effects of their bioactive compounds on inflammatory mediators. As for 2 and 5, their analgesic effect may be a result of their activity with the 5-hydroxytryptamine 1A receptor (5-HT1AR) which disrupted the pain-stimulating actions of 5-HT. Antinociceptive activities were documented for various major compounds of the Litsea plants. Overall, the findings suggested Litsea species as good sources of antinociceptive compounds that can be further developed to complement or substitute prescription drugs for pain management.

Peripherally injected canabidiol reduces neuropathic pain in mice: Role of the 5-HT1A and TRPV1 receptors.

Cannabidiol (CBD) is the most abundant non-psychoactive component found in plants of the genus Cannabis. Its analgesic effect for the treatment of neuropathy has been widely studied. However, little is known about its effects in the acute treatment when Cannabidiol is administered peripherally. Because of that, this research was aimed to evaluate the antinociceptive effects of the CBD when administered peripherally for the treatment of acute neuropathic pain and check the involvement of the 5-HT1A and the TRPV1 receptors in this event. Neuropathic pain was induced with the constriction of the sciatic nerve while the nociceptive threshold was measured using the pressure test of the mouse paw. The technique used proved to be efficient to induce neuropathy, and the CBD (5, 10 and 30 µg/paw) induced the antinociception in a dosage-dependent manner. The dosage used that induced a more potent effect (30 µg/paw), did not induce a systemic response, as demonstrated by both the motor coordination assessment test (RotaRod) and the antinociceptive effect restricted to the paw treated with CBD. The administration of NAN-190 (10 µg/paw), a selective 5-HT1A receptor antagonist, and SB-366791 (16 µg/paw), a selective TRPV1 antagonist, partially reversed the CBD-induced antinociception. The results of the research suggest that the CBD produces the peripheral antinociception during the acute treatment of the neuropathic pain and it partially involved the participation of the 5-HT1A and TRPV1 receptors.

Disgusted snails, oxytocin, and the avoidance of infection threat.

Disgust is considered to be a fundamental affective state associated with triggering the behavioral avoidance of infection and parasite/pathogen threat. In humans, and other vertebrates, disgust affects how individuals interact with, and respond to, parasites, pathogens and potentially infected conspecifics and their sensory cues. Here we show that the land snail, Cepaea nemoralis, displays a similar "disgust-like" state eliciting behavioral avoidance responses to the mucus associated cues of infected and potentially infected snails. Brief exposure to the mucus of snails treated with the Gram-negative bacterial endotoxin, lipopolysaccharide (LPS), elicited dose-related behavioral avoidance, including acute antinociceptive responses, similar to those expressed by mammals. In addition, exposure to the mucus cues of LPS treated snails led to a subsequent avoidance of unfamiliar individuals, paralleling the recognition of and avoidance responses exhibited by vertebrates exposed to potential pathogen risk. Further, the avoidance of, and antinociceptive responses to, the mucus of LPS treated snails were attenuated in a dose-related manner by the oxytocin (OT) receptor antagonist, L-368,899. This supports the involvement of OT and OT receptor homologs in the expression of infection avoidance, and consistent with the roles of OT in the modulation of responses to salient social and infection threats by rodents and other vertebrates. These findings with land snails are indicative of evolutionarily conserved disgust-like states associated with OT/OT receptor homolog modulated behavioral avoidance responses to infection and pathogen threat.

Adenosine signaling mediate pain transmission in the central nervous system.

Pain is a common clinical symptom that seriously affects the quality of life in a variety of patient populations. In recent years, research on the role of adenosine signaling in pain modulation has made great progress. Adenosine is a purine nucleoside and a neuromodulator, and regulates multiple physiological and pathophysiological functions through the activation of four G protein-coupled receptors, which are classified as A1, A2A, A2B, and A3 adenosine receptors (ARs). Adenosine and its receptors that are widespread in the central nervous system (CNS) play an important role in the processing of nociceptive sensory signals in different pain models. A1Rs have the highest affinity to adenosine, and the role in analgesia has been well investigated. The roles of A2ARs and A2BRs in the modulation of pain are controversial because they have both analgesic and pronociceptive effects. The analgesic effects of A3Rs are primarily manifested in neuropathic pain. In this article, we have reviewed the recent studies on ARs in the modulation of neuropathic pain, inflammatory pain, postoperative pain, and visceral pain in the CNS. Furthermore, we have outlined the pathways through which ARs contribute to pain regulation, thereby shedding light on how this mechanism can be targeted to provide effective pain relief.

Inherited pain hypersensitivity and increased anxiety-like behaviors are associated with genetic epilepsy in Wistar Audiogenic Rats: Short- and long-term effects of acute and chronic seizures on nociception and anxiety.

Anxiety and pain hypersensitivity are neurobehavioral comorbidities commonly reported by patients with epilepsies, and preclinical models are suitable to investigate the neurobiology of behavioral and neuropathological alterations associated with these epilepsy-related comorbidities. This work aimed to characterize endogenous alterations in nociceptive threshold and anxiety-like behaviors in the Wistar Audiogenic Rat (WAR) model of genetic epilepsy. We also assessed the effects of acute and chronic seizures on anxiety and nociception. WARs from acute and chronic seizure protocols were divided into two groups to assess short- and long-term changes in anxiety (1 day or 15 days after seizures, respectively). To assess anxiety-like behaviors, the laboratory animals were submitted to the open field, light-dark box, and elevated plus maze tests. The von Frey, acetone, and hot plate tests were used to measure the endogenous nociception in seizure-free WARs, and postictal antinociception was recorded at 10, 30, 60, 120, 180 min, and 24 h after seizures. Seizure-free WARs presented increased anxiety-like behaviors and pain hypersensitivity, displaying mechanical and thermal allodynia (to heat and cold stimuli) in comparison to nonepileptic Wistar rats. Potent postictal antinociception that persisted for 120 to 180 min was detected after acute and chronic seizures. Additionally, acute and chronic seizures have magnified the expression of anxiety-like behaviors when assessed at 1 day and 15 days after seizures. Behavioral analysis indicated more severe and persistent anxiogenic-like alterations in WARs submitted to acute seizures. Therefore, WARs presented pain hypersensitivity and increased anxiety-like behaviors endogenously associated with genetic epilepsy. Acute and chronic seizures induced postictal antinociception in response to mechanical and thermal stimuli and increased anxiety-like behaviors when assessed 1 day and 15 days later. These findings support the presence of neurobehavioral alterations in subjects with epilepsy and shed light on the use of genetic models to characterize neuropathological and behavioral alterations associated with epilepsy.

Epidural lidocaine, butorphanol, and butorphanol - lidocaine combination in dromedary camels.

BACKGROUND: The use of general anesthesia in dromedary camels is constrained by risks related to decubitus. Caudal epidural analgesia is an alternative convenient technique providing loco-regional analgesia for numerous invasive and noninvasive painful conditions. Lidocaine is probably the most commonly used local anesthetic in clinical practice, but has a relatively short duration and may not provide significant long term analgesic benefits. Epidural administration of an opioid-local anesthetic mixture would improve the quality and length of analgesia and minimizes the adverse motor effects provoked by local anesthetics. Butorphanol (potent agonist-antagonist opioid) has been used to improve the duration of epidural analgesia in some animal species, but not in camels. Therefore, our purpose was to investigate the onset and duration of analgesia as well as the clinical and hemato-biochemical effects produced by the epidural administration of butorphanol (0.04 mg/kg), lidocaine (0.22 mg/ kg), and butorphanol-lidocaine (0.04 mg/kg-0.22 mg/ kg) mixture in nine adult dromedary camels in a crossover experimental study. RESULTS: The onset of analgesia was not statistically different between lidocaine (6.5 ± 2.3 min) and butorphanol-lidocaine (7.3 ± 1.5 min) combination. Delayed onset of analgesia was reported after butorphanol administration (14.7 ± 3.5 min). Butorphanol-lidocaine combination produced marked longer duration (175 ± 8.7 min) than lidocaine (55 ± 6.8 min) and butorphanol (158 ± 5.3 min). Mild ataxia was observed in the butorphanol-lidocaine and lidocaine treated animals and slight sedation was reported after butorphanol and butorphanol-lidocaine administration. A transient significant increase in the glucose levels was recorded after all treatments. CONCLUSIONS: Epidural administration of butorphanol augments the analgesic effects and duration of lidocaine with minimal adverse effects.

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.

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.

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.