Potentiation Of Antinociception Research Articles

Potentiation of morphine antinociception and inhibition of diabetic neuropathic pain by the multi-chemokine receptor antagonist peptide RAP-103

AimsWe determined the ability of the multi-chemokine receptor (CCR2/CCR5/CCR8) antagonist RAP-103 to modulate pain behaviors in an acute model of surgical pain, with and without an added opioid (morphine), and by itself in a chronic model of Streptozotocin (STZ)-induced diabetic peripheral neuropathy (DPN). Materials and methodsPain behaviors were assessed by mechanical and thermal tests in rats. Cytokine and chemokine biomarkers in sciatic nerve and spinal cord were assessed by in situ qPCR. Key findingsIn the incisional pain assay, RAP-103 (0.01–1 mg/kg, i.p.) alone had no antiallodynic effect post-surgery. RAP-103 (0.5 mg/kg) when co-administered with morphine (0.5–5 mg/kg), reduced the ED50 of morphine from 3.19 mg/kg to 1.42 mg/kg. In a DPN model, rats exhibited persistent mechanical and cold allodynia. Oral administration of RAP-103 (0.5–0.02 mg/kg/day) resulted in a complete reversal of established hypersensitivity in DPN rats (P < .001), which gradually returned to pain hypersensitivity after the cessation of the treatment. The mRNA expression of cytokines, IL-1β, TNFα; chemokines CCL2, CCL3; and chemokine receptors CCR2 and CCR5 in DPN rat sciatic nerve, but not spinal cord, were significantly increased. RAP-103 resulted in significant reductions in sciatic nerve expression of IL-1β, TNFα and CCL3 in STZ-induced diabetic rats with trends toward lower levels for CCL2 and CCR5, while CCR2 was unchanged. SignificanceIn acute pain, co-administration of RAP-103 with morphine provided the same antinociceptive effect with a reduced dose of morphine, reducing opioid side-effects and risks. RAP-103 by itself is an effective non-opioid antinociceptive treatment for diabetic neuropathic pain.

Modulation of Morphine Analgesia, Antinociceptive Tolerance, and Mu-Opioid Receptor Binding by the Cannabinoid CB2 Receptor Agonist O-1966.

Acutely, non-selective cannabinoid (CB) agonists have been shown to increase morphine antinociceptive effects, and we and others have also demonstrated that non-selective CB agonists attenuate morphine antinociceptive tolerance. Activation of cannabinoid CB2 receptors reverses allodynia and hyperalgesia in models of chronic pain, and co-administration of morphine with CB2 receptor selective agonists has been shown to be synergistic. CB2 receptor activation has also been shown to reduce morphine-induced hyperalgesia in rodents, an effect attributed to CB2 receptor modulation of inflammation. In the present set of experiments, we tested both the acute and chronic interactions between morphine and the CB2 receptor selective agonist O-1966 treatments on antinociception and antinociceptive tolerance in C57Bl6 mice. Co-administration of morphine and O-1966 was tested under three dosing regimens: simultaneous administration, morphine pre-treated with O-1966, and O-1966 pre-treated with morphine. The effects of O-1966 on mu-opioid receptor binding were determined using [3H]DAMGO and [35S]GTPγS binding assays, and these interactions were further examined by FRET analysis linked to flow cytometry. Results yielded surprising evidence of interactions between the CB2 receptor selective agonist O-1966 and morphine that were dependent upon the order of administration. When O-1966 was administered prior to or simultaneous with morphine, morphine antinociception was attenuated and antinociceptive tolerance was exacerbated. When O-1966 was administered following morphine, morphine antinociception was not affected and antinociceptive tolerance was attenuated. The [35S]GTPγS results suggest that O-1966 interrupts functional activity of morphine at the mu-opioid receptor, leading to decreased potency of morphine to produce acute thermal antinociceptive effects and potentiation of morphine antinociceptive tolerance. However, O-1966 administered after morphine blocked morphine hyperalgesia and led to an attenuation of morphine tolerance, perhaps due to well-documented anti-inflammatory effects of CB2 receptor agonism.

Synergism between oral paracetamol and nefopam in a murine model of postoperative pain.

The use of paracetamol or nefopam for postoperative pain control is limited by the need of high doses associated with unwanted effects. Previous works suggest positive interactions between both compounds that may be exploited to obtain potentiation of antinociception. Mechanical and heat antinociception induced by oral doses of paracetamol, nefopam or their combination was studied by isobolographic analysis in a murine model of postsurgical pain. The effective doses that produced 50% antinociception (ED50 ) were calculated from the log dose-response curves for each compound. Subsequently, the effects of ED8.7 s, ED12.5 s, ED17.5 s and ED35 s of nefopam and paracetamol combined were assessed. Oral paracetamol induced dose-dependent relief of postoperative sensitivity and showed higher efficacy reducing mechanical hypersensitivity (ED50 177.3±15.4mg/kg) than heat hyperalgesia (ED50 278.6±43mg/kg). Oral nefopam induced dose-dependent antinociception with similar efficacy for mechanical and heat hypersensitivity (ED50 s 5.42±0.81 vs. 5.83±0.72). Combinations of increasing isoeffective doses revealed that combined ED17.5 s (85.76mg/kg paracetamol and 1.9mg/kg nefopam) and ED35 s (132.67mg/kg and 3.73mg/kg) showed synergistic effects leading to 75% and 90% mechanical antinociception, respectively. These mixtures were defined by interaction indexes of 0.43 and 0.41 and ratios 45:1 and 35:1 paracetamol:nefopam, respectively. The same combinations showed additive effects for the inhibition of incisional thermal hyperalgesia. This work describes a synergistic antinociceptive interaction between low doses of nefopam and paracetamol for the treatment of postoperative hypersensitivity to peripheral stimuli. The promising results obtained on reflexive nociceptive responses of young male mice subjected to plantar surgery highlight the interest of further research evaluating the effects of this mixture on the affective-motivational component of pain and in females and additional age groups. Confirmation of pain-relieving efficacy and safety of this oral combination clinically available in European and Asian countries could provide a useful tool for postsurgical pain management. Early postoperative pain is currently undertreated and has been recognized as a relevant source of chronic postsurgical pain. Oral efficient treatments could facilitate fast-track surgeries and patient recovery at home. Here, we identify in a mouse model of postoperative pain a potent synergistic oral combination consisting of low paracetamol and nefopam doses that provides relief of postsurgical hypersensitivity to mechanical and thermal stimuli. Oral multimodal paracetamol-nefopam mixtures represent a potential clinically available pharmacological strategy for the relief of incisional sensitivity and the promotion of patient recovery.

Investigation of the Signaling Mechanisms of the Mu‐Delta Opioid Receptor Heterodimer

It has been hypothesized that the mu‐ and delta‐opioid receptors (MOR and DOR) can form a heterodimer (MDOR) and that the resulting heterodimer has a distinct pharmacology. Limited evidence from the literature suggests that MDOR‐selective antagonists could be a novel therapy to enhance opioid efficacy while decreasing side effects. However, due to the lack of selective tools, it is very difficult to study this complex system. We thus recently developed D24M, a novel, first‐in‐class selective MDOR antagonist. When D24M was combined with oxymorphone in vivo, we found potentiation of antinociception and reduction of withdrawal symptoms, validating the hypothesis of the MDOR as an anti‐opioid negative feedback loop and that MDOR antagonists could be novel therapeutics. These striking results prompted us to investigate the underlying signaling mechanisms of the MDOR. First, we treated male and female CD‐1 mice with oxymorphone and/or D24M, analyzed the brainstem using quantitative phospho‐proteomic analysis. Out of the thousands of hits, we found dozens of potential targets that became phosphorylated or dephosphorylated upon exposure to both oxymorphone and D24M. Based on this phospho‐proteomic database, we selected the signaling kinases CaMKII and Src to further investigate in vivo. We ran Western blot and tail‐flick anti‐nociception experiments in male and female CD‐1 mice in the presence or absence of specific kinase inhibitors (KN93 and Src I1) combined with oxymorphone and/or D24M. We found that both Src and CaMKII inhibitors blocked the enhanced anti‐nociception caused by D24M, while not impacting baseline opioid response. These results suggest that Src and CaMKII are both repressed by the MDOR to repress opioid anti‐nociception, which is reversed by D24M treatment. However, Western blot analysis did not show phosphorylation differences in the canonical activation loop sites of either kinase, suggesting that alternate phosphorylation sites identified in our proteomic analysis may be responsible for the signaling effects of these kinases downstream of the MDOR. Our results have thus identified 2 key nodes in the MDOR signal transduction cascade by which the MDOR blocks opioid anti‐nociception, and begun to explore their molecular signaling mechanisms. Future work will expand on the signaling cascades of the MDOR, as well as further establish MDOR antagonists as novel opioid therapeutics.Support or Funding InformationThese studies were funded by NIH R21DA044509 and UG3DA047717 to JMS. The authors have no relevant conflicts of interest to declare.

Potentiation of morphine-induced antinociception by harmaline: involvement of μ-opioid and ventral tegmental area NMDA receptors.

Morphine is one of the most well-known and potent analgesic agents; however, it can also induce various side effects. Thus, finding drugs and mechanisms which can potentiate the analgesic effects of low doses of morphine will be a good strategy for pain management. The involvement of μ-opioid receptors and ventral tegmental area (VTA) glutamatergic system in harmaline and morphine combination on the nociceptive response were investigated. Also, we examined reward efficacy and tolerance expression following the drugs. Animals were bilaterally cannulated in the VTA by stereotaxic instrument. A tail-flick (TF) apparatus and conditioned place preference (CPP) paradigm were used to measure nociceptive response and rewarding effects in male NMRI mice respectively. Morphine (2 mg/kg, i.p.) had no effect in TF test. Also, harmaline (1.25 and 5 mg/kg, i.p.) could not change pain threshold. Combination of a non-effective dose of harmaline (5 mg/kg) and morphine (2 mg/kg) produced antinociception and also prevented morphine tolerance but had no effect on the acquisition of CPP. Systemic administration of naloxone (0.5 and 1 mg/kg) and intra-VTA microinjection of NMDA (0.06 and 0.1 μg/mouse) before harmaline (5 mg/kg) plus morphine (2 mg/kg) prevented antinociception induced by the drugs. D-AP5 (0.5 and 1 μg/mouse, intra-VTA) potentiated the effect of low-dose harmaline (1.25 mg/kg) and morphine (2 mg/kg) and induced antinociception. Microinjection of the same doses of NMDA or D-AP5 into the VTA alone had no effect on pain threshold. The findings showed that harmaline potentiated the analgesic effect of morphine and reduced morphine tolerance. Glutamatergic and μ-opioidergic system interactions in the VTA seem to have a modulatory role in harmaline plus morphine-induced analgesia.

Supraspinal and Peripheral, but Not Intrathecal, σ1R Blockade by S1RA Enhances Morphine Antinociception.

Sigma-1 receptor (σ1R) antagonism increases the effects of morphine on acute nociceptive pain. S1RA (E-52862) is a selective σ1R antagonist widely used to study the role of σ1Rs. S1RA alone exerted antinociceptive effect in the formalin test in rats and increased noradrenaline levels in the spinal cord, thus accounting for its antinociceptive effect. Conversely, while systemic S1RA failed to elicit antinociceptive effect by itself in the tail-flick test in mice, it did potentiate the antinociceptive effect of opioids in this acute pain model. The present study aimed to investigate the site of action and the involvement of spinal noradrenaline on the potentiation of opioid antinociception by S1RA on acute thermal nociception using the tail-flick test in rats. Local administration was performed after intrathecal catheterization or intracerebroventricular and rostroventral medullar (RVM) cannulae implantation. Noradrenaline levels in the spinal cord were evaluated using the concentric microdialysis technique in awake, freely-moving rats. Systemic or supraspinal administration of S1RA alone, while having no effect on antinociception, enhanced the effect of morphine in rats. However, spinal S1RA administration did not potentiate the antinociceptive effect of morphine. Additionally, the peripherally restricted opioid agonist loperamide was devoid of antinociceptive effect but produced antinociception when combined with S1RA. Neurochemical studies revealed that noradrenaline levels in the dorsal horn of the spinal cord were not increased at doses exerting potentiation of the antinociceptive effect of the opioid. In conclusion, the site of action of σ1R for opioid modulation on acute thermal nociception is located at the peripheral and supraspinal levels, and the opioid-potentiating effect is independent of the spinal noradrenaline increase produced by S1RA.

Potentiation of Morphine-Induced Antinociception by Propranolol: The Involvement of Dopamine and GABA Systems.

Tolerance to the analgesic effect of morphine is a major clinical problem which can be managed by co-administration of another drug. This study investigated the ability of propranolol to potentiate the antinociceptive action of morphine and the possible mechanisms underlying this effect. Antinociception was assessed in three nociceptive tests (thermal, hot plate), (visceral, acetic acid), and (inflammatory, formalin test) in mice and quantified by measuring the percent maximum possible effect, the percent inhibition of acetic acid-evoked writhing response, and the area under the curve values of number of flinches for treated mice, respectively. The study revealed that propranolol (0.25–20 mg/Kg, IP) administration did not produce analgesia in mice. However, 10 mg/Kg propranolol, enhanced the antinociceptive effect of sub-analgesic doses of morphine (0.2, 1, and 2 mg/Kg, IP) in the three nociceptive tests. It also shifted the dose response curve of morphine to the left. The combined effect of propranolol and morphine was attenuated by haloperidol (D2 receptor antagonist, 1.5 mg/Kg, IP), and bicuculline (GABAA receptor antagonist, 2 mg/Kg, IP). Repeated daily administration of propranolol (10 mg/Kg, IP) did not alter the nociceptive responses in the three pain tests, but it significantly potentiated morphine-induced antinociception in the hot plate, acetic acid-evoked writhing, and in the second phase of formalin tests. Together, the data suggest that a cross-talk exists between the opioidergic and adrenergic systems and implicate dopamine and GABA systems in this synergistic effect of morphine-propranolol combination. Propranolol may serve as an adjuvant therapy to potentiate the effect of opioid analgesics.

Potentiation of morphine-induced antinociception and locomotion by citalopram is accompanied by anxiolytic-like effects

Morphine and related opioids are the mainstay of analgesic treatment, especially in patients suffering chronic pain. Besides their antinociceptive effects they may also exhibit anxiolytic-like properties that could contribute to pain relief. The pharmacological manipulation of the serotonergic system may not only modulate pain transmission and processing but also other behavioral effects of opioids. The present study aimed to analyze the effect of the concurrent treatment with citalopram, a selective serotonin reuptake inhibitor, on the antinociceptive, locomotor and anxiety-related effects induced by acute and subchronic administration of morphine in mice. Citalopram (15mg/kg) enhanced the acute antinociceptive effects of morphine when concurrently administered as evidenced by a two-fold increase in the ED50 for the antinociceptive effect of morphine in the hot-plate test. Chronic studies also revealed that concurrent citalopram treatment (15mg/kg) delayed the development of tolerance to the thermal antinociceptive effects of morphine. Additionally, morphine-induced hyperlocomotion was potentiated by citalopram as assessed in the open-field test and in the spontaneous activity recording in the home cage, a behavioral outcome to which tolerance or desensitization was not developed. Interestingly, chronic administration of both drugs promoted an anxiolytic effect as evidenced by the increased central activity in the open field test. Future investigations on this pharmacological interaction, such as the possible translational research in clinics, might have consequences in future strategies for the therapeutic management of pain.

St. John’s Wort Potentiates anti-Nociceptive Effects of Morphine in Mice Models of Neuropathic Pain

In this study, we compared the efficacy of a combination of PKC-blocker St. John's Wort (SJW) and morphine in mice with painful antiretroviral (2,3-dideoxycitidine [ddC]) and chemotherapic (oxaliplatin) neuropathy. Morphine (1 and 5 mg/Kg i.p.), SJW (1 and 5 mg/Kg o.s.), or their combination was administered by systemic injection, and antinociception was determined by using the hot and cold plate tests. Here we demonstrate the ability of SJW to relieve neuropathic pain in mice neuropathic models and a potentiation of morphine antinociception in thermal pain. The potentiating effect shown by SJW was not secondary to its antinociceptive activity as the increase of the morphine antinociceptive effect was produced at a dose (1mg/kg o.s.) devoid of any capability to modulate the pain threshold in neuropathic pain mice. Further examinations of the SJW main components revealed that hypericin was responsible for the potentiating properties whereas flavonoids were ineffective. These results show that SJW has notable antinociceptive activity for both neuropathic pain models and could be used in neuropathic pain relief alone or in combination with morphine. These data support the utility of combination SJW/opioid therapy in pain management for antinociceptive efficacy by enhancing opioid analgesia.

Inhibition of microglia in the basolateral amygdala enhanced morphine-induced antinociception: Possible role of GABAA receptors

In clinical medicine, morphine is widely used to relieve many types of pain, but it has several side effects such as the development of tolerance and dependence. In order to decrease the side effects of morphine administration for the treatment of pain, the combination of minocycline as a glial inhibitor and morphine has been suggested in previous studies. It is important to understand which synaptic mechanisms are involved in the potentiative effect of minocycline on morphine antinociception. To this aim, male Wistar rats were bilaterally cannulated in the basolateral amygdala by srereotaxic instrument. A tail-flick apparatus was used to measure the pain threshold. The results revealed that intraperitoneal injection of morphine (2.5–7.5mg/kg) induced antinociception. Intra-basolateral amygdala microinjection of minocycline (5–10µg/rat) by itself had no effect on tail-flick latency, while the microinjection of the same doses of minocycline with an ineffective dose of morphine (5mg/kg) induced antinociception. Intra-basolateral amygdala microinjection of different doses of muscimol (0.001–0.005µg/rat) increased the minocycline-induced potentioation on morphine response in the tail-flick test. Intra-basolateral amygdala microinjection of muscimol by itself had no effect on tail-flick latency. On the other hand, intra-basolateral amygdala microinjection of bicuculline (0.01–0.1µg/rat) inhibited minocycline-induced potentiation of morphine antinociception. It should be noted that intra-basolateral amygdala bicucculine by itself had no effect on tail-flick latency. It can thus be concluded that intra-basolateral amygdala minocycline potentiates morphine response in the tail-flick test. Moreover, basolateral amygdala GABAergic system may be involved in the minocycline-induced potentiation of morphine response via GABAA receptors.

Transfer of Opiorphin Through a Blood-Brain Barrier Culture Model

Opioid peptides are potent analgesics with therapeutic potential in the treatment of acute and chronic pain. Their efficacy is limited by peptidases (enkephalinases). Opiorphin pentapeptide (QRFSR) is the first characterized human endogenous inhibitor of enkephalinases. The peptide is able to increase the binding and affinity of endogenous opiates to mu opioid receptors; thus, the mechanism of opiorphin may provide a new therapeutic approach in pain management. The analgesic effect of opiorphin was proven in several earlier published invitro and invivo studies. Our aim was to test the transfer of opiorphin through a blood-brain barrier model for the first time. The flux of opiorphin was tested on a blood-brain barrier culture model consisting of rat brain endothelial, glial and pericyte cells. Brain endothelial cells in this triple co-culture model form tight monolayers characterized by transendothelial electrical resistance measurement. Relative quantity of the peptide was estimated by mass spectrometry. The transfer of opiorphin through the blood-brain barrier model was estimated to be ∼3%, whereas the permeability coefficient was 0.53 ± 1.36 × 10(-6) cm/s (n= 4). We also observed rapid conversion of N-terminal glutamine into pyroglutamic acid during the transfer experiments. Our results indicate that opiorphin crosses cultured brain endothelial cells in the absence of serum factors in a significant amount. This is in agreement with previous invivo data showing potentiation of enkephalin-mediated antinociception. We suggest that opiorphin may have a potential as a centrally acting novel drug to treat pain.

Effect of tramadol on metamizol pharmacokinetics and pharmacodynamics after single and repeated administrations in arthritic rats

Combined administration of certain doses of opioid compounds with a non-steroidal anti-inflammatory drug can produce additive or supra-additive effects while reducing unwanted effects. We have recently reported that co-administration of metamizol with tramadol produces antinociceptive effect potentiation, after acute treatment. However, none information about the effect produced by the combination after chronic or repeated dose administration exists. The aims of this study were to investigate whether the antinociceptive synergism produced by the combination of metamizol and tramadol (177.8+17.8mg/kg, s.c. respectively) is maintained after repeated treatment and whether the effects observed are primarily due to pharmacodynamic interactions or may be related to pharmacokinetics changes. Administration of metamizol plus tramadol acute treatment significantly enhanced the antinociceptive effect of the drugs given alone (P<0.05). Nevertheless, this effect decreased about 53% after the chronic treatment (3 doses per day, for 4days). No pharmacokinetic interaction between metamizol and tramadol was found under acute treatment (P>0.05). The mechanism involved in the synergism of the antinociceptive effect observed with the combination of metamizol and tramadol in single dose cannot be attributed to a pharmacokinetic interaction, and other pharmacodynamic interactions have to be considered. On the other hand, when metamizol and tramadol were co-administered under repeated administrations, a pharmacokinetic interaction and tolerance development occurred. Differences found in metamizol active metabolites’ pharmacokinetics (P<0.05) were related to the development of tolerance produced by the combination after repeated doses. This work shows an additional preclinical support for the combination therapy. The clinical utility of this combination in a suitable dose range should be evaluated in future studies.

Antinociceptive effects of epidural magnesium sulphate alone and in combination with morphine in dogs

ObjectiveTo compare the antinociceptive effects of magnesium sulphate (MgSO4) when administered epidurally alone and in combination with morphine. Study designExperimental, randomized, ‘blinded’, crossover study. AnimalsSix healthy adult Beagle dogs. MethodsEvaluated treatments were MgSO4 (2.5 mg kg−1) alone (Mg), morphine (0.1 mg kg−1) alone (Mo), MgSO4 in combination with morphine (Mm), and sterile water (0.115 mL kg−1; Co) that were injected in the lumbosacral epidural space using an epidural catheter. Antinociception was measured using the von Frey mechanical threshold device applied to the carpal pads, both sides of the thorax and metatarsi. Measurements were obtained at time points: before treatment (baseline) and 0.5, 1, 2, 4, 6, 12, 18 and 24 hours after the epidural injection. Sedation, behaviour score and presence of motor deficits were assessed. Data were analyzed using a linear mixed model and Bonferroni adjustments, with significance set at p < 0.05. ResultsThere were significant effects of treatment and time in all regions. Overall threshold values in grammes force [median (interquartile range)] when stimulation regions were combined were significantly higher in Mg [164 (135–200)], Mo [156 (129–195)] and Mm [158 (131–192)] compared to Co [145 (120–179)]. Thresholds were significantly higher compared to Co in Mg, Mo and Mm at the thorax and metatarsi, but only in Mg and Mo at the carpal pads. No motor deficits were observed at any time point. Thresholds (combined regions) were increased from baseline at one or more time points with all treatments, including control. Conclusion and clinical relevanceEpidural MgSO4 produced an antinociceptive effect characterised by an increase in the mechanical thresholds of similar magnitude to that produced by epidural morphine, compared with the control group, without causing any motor deficits. No potentiation of morphine antinociception was observed. The onset and offset times of antinociception could not be clearly established. To what extent these results can be extrapolated to clinical cases requires further investigation.

Involvement of the vanilloid receptor 1 in the mechanisms of analgesic effect of amizonum

SUMMARY The study of features of pharmacodynamics of a new analgesic is an important and urgent task of modern pharmacology. These data allow us to clarify the nosology for application of an analgesic and to create a theoretical background to optimize its use. An effect mediated by the transient receptor potential cation channel, subfamily V, member 1 (TRPV1) activation can also be an effective mechanism of the analgesic action. We evaluated the possibility of TRPV1 participation in implementation of the analgesic effect with the antiviral action of amizonum during the experiment. It is known that amino acids Tyr511 and Ser512 are the main components of the active site of TRPV1. In this connection, dipeptide Tyr-Ser has been completely synthesized as a model of the active site of TRPV1. In the experiment model this was shown, using the spectrophotometric method, with the formation of the “capsaicin - Tyr- Ser” intermolecular complex at the level of the stability constant Kkor=0.998 and Kr=0.3•10-4 L/mol and the “amizonum - Tyr-Ser” weak intermolecular complex Kr=0.05•104 L/mol; Kkor= 0.995, respectively. The data verification was carried out in experiments in vitro (isolated ratportal vein) and in vivo (Tail-flick model), with the TRPV1 agonist. It was shown that the amplitude of smooth muscle (SM) contraction of the portal vein at a capsaicin concentration 0.1 μmol/L, 0.5 μmol/L capsazepine, and 1.0 μmol/L amizonum was +30.3±5.3%, -3.2± 2.7% and +7.1±3.2% from initial level, respectivelly. In a combined application of amizonum with capsaicin or capsazepine, the amplitude of contraction of the SM portal vein was 20.1± 1.3% and -3.0±1.4%, respectively. This indicates the absence of action of amizonum under combined use of capsaicinoids. The Tail-flick model showed atypical potentiation of the amizonum antinociception with the use of capsaicin. The obtained data suggest the low probability of the participation of TRPV1 in the implementation of the antinociceptive action of amizonum.

Opioid Antagonists

Opioid Antagonists

Involvement of the vanilloid receptor 1 in the mechanism of analgetic effect of ketorolac

Abstract The existing ideas about the ketorolac mechanism of analgetic potency are associated with its effect on COX isoenzymes and its significant potency needs to be explained more completely. Its impact on vanilloid receptors 1 (TRPV1) can also be an effective mechanism of analgetic action. We have evaluated the analgetic potential of ketorolac on the basis of its influence on TRPV1. It is known that Tyr511 and Ser512 amino acids are active central components of TRPV1. Thereby the synthesis of the dipeptide Tyr-Ser has been conducted as a model of the TRPV1 active centre. In our model test, using spectrofluorometry, the formation of intermolecular complexes was shown: “ketorolac - Tyr-Ser” and “capsaicin - Tyr-Ser” at stability constants Kr=0.53 • 10-4 L/mol; Kkor= 0.998 and Kr =0.3 • 10-4 L/mol; Kkor=0.998, respectively. Verification of the data was proved in experiments in vitro (the preparation of portal vein) and in vivo (Tail-flick model), with the TRPV1 agonist and antagonist. On portal vein, it was established (from initial level) by Krebs solution that the range of relaxation of smooth muscles of the portal vein (SM) was as follows at the concentration of capsaicin 0.1 μmol/L +30.3%±5.3%, capsazepine 0.5 μmol/L -3.2±2.7% and ketorolac 1.0 μmol/L -60.0±7.0%. In the experiment, at the above mentioned doses of ketorolac on the background of the action of the capsaicin (capsazepine), the range of relaxation of smooth muscles (SM) of the portal vein was -59.3±5.0% and -63.0±6.0% respectively testifying to the fact that the effect of capsaicinoids was neutralised by ketorolac. On the Tail-flick model, an atypical potentiation of the ketorolac antinociception at simultaneous use with the capsaicin was shown. The obtained data allow suggesting that TRPV1 can activate the ketorolac antinociceptive effects.

Buprenorphine

Buprenorphine

Inhibition of Gβγ-subunit signaling potentiates morphine-induced antinociception but not respiratory depression, constipation, locomotion, and reward

Inhibition of Gβγ-subunit signaling to phospholipase C β3 has been shown to potentiate morphine-mediated antinociception while attenuating the development of tolerance and dependence in mice. The objective of this study was to determine the effect of Gβγ-subunit inhibition on antinociception and other pharmacological effects, such as respiratory depression, constipation, and hyperlocomotion, mediated by the μ-opioid receptor. The Gβγ-subunit inhibitor, gallein, was administered to C57BL/6J mice by intraperitoneal injection before morphine, and data were compared with mice treated with vehicle, morphine, or gallein alone. Morphine-induced antinociception was measured using the 55°C warm-water tail-withdrawal test. Pretreatment with gallein produced a dose-dependent potentiation of morphine-mediated antinociception, producing up to a 10-fold leftward shift in the morphine dose-response curve and extending the duration of antinociception induced by a single dose of morphine. Gallein pretreatment also prevented acute antinociceptive tolerance induced by morphine. In contrast, the dose-dependent respiratory depression and hyperlocomotion induced by morphine were not potentiated by gallein pretreatment. Similarly, gallein pretreatment did not potentiate morphine-conditioned place preference responses or morphine-induced constipation, as measured as a reduction in excreta. These results suggest that selectively inhibiting Gβγ-mediated signaling may selectively increase μ-opioid receptor-mediated antinociception without matching increases in adverse physiological effects.

Potentiation of morphine-induced mechanical antinociception by σ1 receptor inhibition: Role of peripheral σ1 receptors

We studied the modulation of morphine-induced mechanical antinociception and side effects by σ1 receptor inhibition. Both wild-type (WT) and σ1 receptor knockout (σ1-KO) mice showed similar responses to paw pressure (100–600 g). The systemic (subcutaneous) or local (intraplantar) administration of σ1 antagonists (BD-1063, BD-1047, NE-100 and S1RA) was devoid of antinociceptive effects in WT mice. However, σ1-KO mice exhibited an enhanced mechanical antinociception in response to systemic morphine (1–16 mg/kg). Similarly, systemic treatment of WT mice with σ1 antagonists markedly potentiated morphine-induced antinociception, and its effects were reversed by the selective σ1 agonist PRE-084. Although the local administration of morphine (50–200 μg) was devoid of antinociceptive effects in WT mice, it induced dose-dependent antinociception in σ1-KO mice. This effect was limited to the injected paw. Enhancement of peripheral morphine antinociception was replicated in WT mice locally co-administered with σ1 antagonists and the opioid. None of the σ1 antagonists tested enhanced morphine-antinociception in σ1-KO mice, confirming a σ1-mediated action. Morphine-induced side-effects (hyperlocomotion and inhibition of gastrointestinal transit) were unaltered in σ1-KO mice. These results cannot be explained by a direct interaction of σ1 ligands with μ-opioid receptors or adaptive changes of μ-receptors in σ1-KO mice, given that [3H]DAMGO binding in forebrain, spinal cord, and hind-paw skin membranes was unaltered in mutant mice, and none of the σ1 drugs tested bound to μ-opioid receptors. These results show that σ1 receptor inhibition potentiates morphine-induced mechanical analgesia but not its acute side effects, and that this enhanced analgesia can be induced at peripheral level.

Potentiation of oxycodone antinociception in mice by agmatine and BMS182874 via an imidazoline I2 receptor-mediated mechanism

The potentiation of oxycodone antinociception by BMS182874 (endothelin-A (ETA) receptor antagonist) and agmatine (imidazoline receptor/α2-adrenoceptor agonist) is well-documented. It is also known that imidazoline receptors but not α2-adrenoceptors are involved in potentiation of oxycodone antinociception by agmatine and BMS182874 in mice. However, the involvement of specific imidazoline receptor subtypes (I1, I2, or both) in this interaction is not clearly understood. The present study was conducted to determine the involvement of imidazoline I1 and I2 receptors in agmatine- and BMS182874-induced potentiation of oxycodone antinociception in mice. Antinociceptive (tail flick and hot-plate) latencies were determined in male Swiss Webster mice treated with oxycodone, agmatine, BMS182874, and combined administration of oxycodone with agmatine or BMS182874. Efaroxan (imidazoline I1 receptor antagonist) and BU224 (imidazoline I2 receptor antagonist) were used to determine the involvement of I1 and I2 imidazoline receptors, respectively. Oxycodone produced significant antinociceptive response in mice which was not affected by efaroxan but was blocked by BU224. Agmatine-induced potentiation of oxycodone antinociception was blocked by BU224 but not by efaroxan. Similarly, BMS182874-induced potentiation of oxycodone antinociception was blocked by BU224 but not by efaroxan. This is the first report demonstrating that BMS182874- or agmatine-induced enhancement of oxycodone antinociception is blocked by BU224 but not by efaroxan. We conclude that imidazoline I2 receptors but not imidazoline I1 receptors are involved in BMS182874- and agmatine-induced potentiation of oxycodone antinociception in mice.