Synergistic interaction between the agonism of cebranopadol at nociceptin/orphanin FQ and classical opioid receptors in the rat spinal nerve ligation model.

Cebranopadol (trans-6'-fluoro-4',9'-dihydro-N,N-dimethyl-4-phenyl-spiro[cyclohexane-1,1'(3'H)-pyrano[3,4-b]indol]-4-amine) is a novel analgesic nociceptin/orphanin FQ opioid peptide (NOP) and classical opioid receptor (MOP, DOP, and KOP) agonist with highly efficacious and potent activity in a broad range of rodent models of nociceptive, inflammatory, and neuropathic pain as well as limited opioid-type side effects such as respiratory depression. This study was designed to explore contribution and interaction of NOP and classical opioid receptor agonist components to cebranopadol analgesia in the rat spinal nerve ligation (SNL) model. Assessing antihypersensitive activity in SNL rats intraperitoneal (IP) administration of cebranopadol resulted in ED 50 values of 3.3 and 3.58 µg/kg in two independent experiments. Pretreatment (IP) with J-113397 (4.64 mg/kg) a selective antagonist for the NOP receptor or naloxone (1 mg/kg), naltrindole (10 mg/kg), or nor-BNI (10 mg/kg), selective antagonists for MOP, DOP, and KOP receptors, yielded ED 50 values of 14.1, 16.9, 17.3, and 15 µg/kg, respectively. This 4-5 fold rightward shift of the dose-response curves suggested agonistic contribution of all four receptors to the analgesic activity of cebranopadol. Combined pretreatment with a mixture of the antagonists for the three classical opioid receptors resulted in an 18-fold potency shift with an ED 50 of 65.5 µg/kg. The concept of dose equivalence was used to calculate the expected additive effects of the parent compound for NOP and opioid receptor contribution and to compare them with the observed effects, respectively. This analysis revealed a statistically significant difference between the expected additive and the observed effects suggesting intrinsic synergistic analgesic interaction of the NOP and the classical opioid receptor components of cebranopadol. Together with the observation of limited respiratory depression in rats and humans the synergistic interaction of NOP and classical opioid receptor components in analgesia described in the current study may contribute to the favorable therapeutic index of cebranopadol observed in clinical trials.

Cebranopadol: a novel potent analgesic nociceptin/orphanin FQ peptide and opioid receptor agonist.

Cebranopadol (trans-6'-fluoro-4',9'-dihydro-N,N-dimethyl-4-phenyl-spiro[cyclohexane-1,1'(3'H)-pyrano[3,4-b]indol]-4-amine) is a novel analgesic nociceptin/orphanin FQ peptide (NOP) and opioid receptor agonist [Ki (nM)/EC50 (nM)/relative efficacy (%): human NOP receptor 0.9/13.0/89; human mu-opioid peptide (MOP) receptor 0.7/1.2/104; human kappa-opioid peptide receptor 2.6/17/67; human delta-opioid peptide receptor 18/110/105]. Cebranopadol exhibits highly potent and efficacious antinociceptive and antihypersensitive effects in several rat models of acute and chronic pain (tail-flick, rheumatoid arthritis, bone cancer, spinal nerve ligation, diabetic neuropathy) with ED50 values of 0.5-5.6 µg/kg after intravenous and 25.1 µg/kg after oral administration. In comparison with selective MOP receptor agonists, cebranopadol was more potent in models of chronic neuropathic than acute nociceptive pain. Cebranopadol's duration of action is long (up to 7 hours after intravenous 12 µg/kg; >9 hours after oral 55 µg/kg in the rat tail-flick test). The antihypersensitive activity of cebranopadol in the spinal nerve ligation model was partially reversed by pretreatment with the selective NOP receptor antagonist J-113397[1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one] or the opioid receptor antagonist naloxone, indicating that both NOP and opioid receptor agonism are involved in this activity. Development of analgesic tolerance in the chronic constriction injury model was clearly delayed compared with that from an equianalgesic dose of morphine (complete tolerance on day 26 versus day 11, respectively). Unlike morphine, cebranopadol did not disrupt motor coordination and respiration at doses within and exceeding the analgesic dose range. Cebranopadol, by its combination of agonism at NOP and opioid receptors, affords highly potent and efficacious analgesia in various pain models with a favorable side effect profile.

Spinal-supraspinal and intrinsic µ-opioid receptor agonist-norepinephrine reuptake inhibitor (MOR-NRI) synergy of tapentadol in diabetic heat hyperalgesia in mice.

Tapentadol is a µ-opioid receptor (MOR) agonist and norepinephrine reuptake inhibitor (NRI) with established efficacy in neuropathic pain in patients and intrinsic synergistic interaction of both mechanisms as demonstrated in rodents. In diabetic mice, we analyzed the central antihyperalgesic activity, the occurrence of site-site interaction, as well as the spinal contribution of opioid and noradrenergic mechanisms in a hotplate test. Tapentadol (0.1-3.16 µg/animal) showed full efficacy after intrathecal as well as after intracerebroventricular administration (ED50 0.42 µg/animal i.t., 0.18 µg/animal i.c.v.). Combined administration of equianalgesic doses revealed spinal-supraspinal synergy (ED50 0.053 µg/animal i.t. + i.c.v.). Morphine (0.001-10 µg/animal) also showed central efficacy and synergy (ED50 0.547 µg/animal i.t., 0.004 µg/animal i.c.v., 0.014 µg/animal i.t. + i.c.v.). Supraspinal potencies of tapentadol and morphine correlated with the 50-fold difference in their MOR affinities. In contrast, spinal potencies of both drugs were similar and correlated with their relative systemic potencies (ED50 0.27 mg/kg i.p. tapentadol, 1.1 mg/kg i.p. morphine). Spinal administration of the opioid antagonist naloxone or the α2-adrenoceptor antagonist yohimbine before systemic administration of equianalgesic doses of tapentadol (1 mg/kg i.p.) or morphine (3.16 mg/kg i.p.) revealed pronounced influence on opioidergic and noradrenergic pathways for both compounds. Tapentadol was more sensitive toward both antagonists than was morphine, with median effective dose values of 0.75 and 1.72 ng/animal i.t. naloxone and 1.56 and 2.04 ng/animal i.t. yohimbine, respectively. It is suggested that the antihyperalgesic action of systemically administered tapentadol is based on opioid spinal-supraspinal synergy, as well as intrinsic spinally mediated MOR-NRI synergy.

Pharmacological and behavioral profile of N-[(3R)-1-azabicyclo[2.2.2]oct-3-yl]-6-chinolincarboxamide (EVP-5141), a novel α7 nicotinic acetylcholine receptor agonist/serotonin 5-HT3 receptor antagonist.

RATIONALE AND OBJECTIVE: Agonists of α7 nicotinic acetylcholine receptors (nAChRs) may have therapeutic potential for the treatment of cognitive deficits. This study describes the in vitro pharmacology of the novel α7 nAChR agonist/serotonin 5-HT3 receptor (5-HT3R) antagonist N-[(3R)-1-azabicyclo[2.2.2]oct-3-yl]-6-chinolincarboxamide (EVP-5141) and its behavioral effects. RESULTS: EVP-5141 bound to α7 nAChRs in rat brain membranes (K i = 270 nM) and to recombinant human serotonin 5-HT3Rs (K i = 880 nM) but had low affinity for α4ß2 nAChRs (K i > 100 µM). EVP-5141 was a potent agonist at recombinant rat and human α7 nAChRs expressed in Xenopus oocytes. EVP-5141 acted as 5-HT3R antagonist but did not block α3ß4, α4ß2, and muscle nAChRs. Rats trained to discriminate nicotine from vehicle did not generalize to EVP-5141 (0.3-30 mg kg(-1), p.o.), suggesting that the nicotine cue is not mediated by the α7 nAChR and that EVP-5141 may not share the abuse liability of nicotine. EVP-5141 (0.3-3 mg kg(-1)) improved performance in the rat social recognition test. EVP-5141 (0.3 mg kg(-1), p.o.) ameliorated scopolamine-induced retention deficits in the passive avoidance task in rats. EVP-5141 (1 mg kg(-1), i.p.) improved spatial working memory of aged (26- to 32-month-old) rats in a water maze repeated acquisition task. In addition, EVP-5141 improved both object and social recognition memory in mice (0.3 mg kg(-1), p.o.). CONCLUSIONS: EVP-5141 improved performance in several learning and memory tests in both rats and mice, supporting the hypothesis that α7 nAChR agonists may provide a novel therapeutic strategy for the treatment of cognitive deficits in Alzheimer's disease or schizophrenia.

Tapentadol increases levels of noradrenaline in the rat spinal cord as measured by in vivo microdialysis.

Spinal noradrenaline is thought to play an important role in descending pain inhibitory pathways and the modulation of nociceptive information at the spinal level. Tapentadol is a µ-opioid receptor (MOR) agonist and noradrenaline reuptake inhibitor (NRI). We showed previously that tapentadol, in contrast to morphine, elevates levels of noradrenaline, but not serotonin, in the ventral hippocampus of rats. The aim of this study was to examine the effects of tapentadol, morphine and venlafaxine on spinal monoamine levels. Rats were implanted with spinal microdialysis probes. Drugs were administered intraperitoneally, and samples were collected for 3h in isoflurane-anesthetized animals and analysed for monoamine content using HPLC-MS/MS. In terms of area-under-curve (AUC, 0-180 min), tapentadol (4.64-21.5mg/kg) produced a dose-dependent, significant increase in extracellular spinal noradrenaline levels (9275±4346 min% at the highest dose versus -1047±889 min% for vehicle). A maximum increase of 182±32% of baseline was reached 60 min after administration of 10mg/kg tapentadol. Venlafaxine (10mg/kg) produced an effect of similar magnitude. In contrast, tapentadol decreased extracellular spinal serotonin levels (non-significantly compared to vehicle), while venlafaxine increased spinal serotonin to 267±74% of baseline. In contrast to tapentadol and venlafaxine, morphine slightly decreased levels of noradrenaline and serotonin. This study demonstrates that analgesic doses of tapentadol (and venlafaxine), but not morphine, increase spinal noradrenaline levels and that tapentadol is devoid of a relevant serotonergic effect. It supports the suggestion that the NRI component of tapentadol is functionally relevant and contributes to its mechanism of action.

Antinociceptive and antihyperalgesic effects of tapentadol in animal models of inflammatory pain.

The novel analgesic tapentadol HCl [(-)-(1R,2R)-3-(3-dimethylamino)-1-ethyl-2-methyl-propyl)-phenol hydrochloride] combines µ-opioid receptor (MOR) agonism and noradrenaline reuptake inhibition (NRI) in a single molecule and shows a broad efficacy profile in various preclinical pain models. This study analyzed the analgesic activity of tapentadol in experimental inflammatory pain. Analgesia was evaluated in the formalin test (pain behavior, rat and mouse), carrageenan-induced mechanical hyperalgesia (paw-pressure test, rat), complete Freund's adjuvant (CFA)-induced paw inflammation (tactile hyperalgesia, rat), and CFA knee-joint arthritis (weight bearing, rat). Tapentadol showed antinociceptive activity in the rat and mouse formalin test with an efficacy of 88 and 86% and ED(50) values of 9.7 and 11.3 mg/kg i.p., respectively. Tapentadol reduced mechanical hyperalgesia in carrageenan-induced acute inflammatory pain by 84% with an ED(50) of 1.9 mg/kg i.v. In CFA-induced tactile hyperalgesia, tapentadol showed 71% efficacy with an ED(50) of 9.8 mg/kg i.p. The decrease in weight bearing after CFA injection in one knee joint was reversed by tapentadol by 51% with an ED(25) of 0.9 mg/kg i.v. Antagonism studies were performed with the MOR antagonist naloxone and the α(2)-noradrenergic receptor antagonist yohimbine in the carrageenan- and CFA-induced hyperalgesia model. In the CFA model, the serotonergic receptor antagonist ritanserin was also tested. The effect of tapentadol was partially blocked by naloxone and yohimbine and completely blocked by the combination of both, but it was not affected by ritanserin. In summary, tapentadol showed antinococeptive/antihyperalgesic analgesic activity in each model of acute and chronic inflammatory pain, and the antagonism experiments suggest that both MOR activation and NRI contribute to its analgesic effects.

Synergistic antihypersensitive effects of pregabalin and tapentadol in a rat model of neuropathic pain.

Neuropathic pain is a clinical condition which remains poorly treated and combinations of pregabalin, an antagonist of the α2δ-subunit of Ca(2+) channels, with tapentadol, a µ-opioid receptor agonist/noradrenaline reuptake inhibitor, or with classical opioids such as oxycodone and morphine might offer increased therapeutic potential. In the rat spinal nerve ligation model, a dose dependent increase in ipsilateral paw withdrawal thresholds was obtained using an electronic von Frey filament after IV administration of pregabalin (1-10mg/kg), tapentadol (0.316-10mg/kg), morphine (1-4.64 mg/kg) and oxycodone (0.316-3.16 mg/kg), with ED(50) values (maximal efficacy) of 4.21 (67%), 1.65 (94%), 1.70 (96%) and 0.63 mg/kg (100%), respectively. Equianalgesic dose combinations of pregabalin and tapentadol (dose ratio 2.5:1), morphine (2.5:1) or oxycodone (6.5:1) resulted in ED(50) values (maximal efficacy) of 0.83 (89%), 2.33 (97%) and 1.14 mg/kg (100%), respectively. The concept of dose-equivalence suggested an additive interaction of pregabalin and either oxycodone or morphine, while a synergistic interaction was obtained with pregabalin and tapentadol (demonstrated by isobolographic analysis). There was no increase in contralateral paw withdrawal thresholds and no locomotor impairment, as measured in the open field, for the combination of pregabalin and tapentadol; while a significant increase and impairment was demonstrated for the combinations of pregabalin and either morphine or oxycodone. Because combination of pregabalin and tapentadol resulted in a synergistic antihypersensitive activity, it is suggested that, beside the use of either drug alone, this drug combination may offer a beneficial treatment option for neuropathic pain.

Critical evaluation of the use of extinction paradigms for the assessment of opioid-induced conditioned place preference in rats.

The rewarding effects of drugs of abuse are often studied by means of the conditioned place preference (CPP) paradigm. CPP is one of the most widely used models in behavioral pharmacology, yet its theoretical underpinnings are not well understood, and there are very few studies on the methodological and theoretical aspects of this model. An important drawback of the classical CPP paradigm is that it often does not show dose-dependent results. The persistence of the conditioned response, i.e. the time required until the CPP effect is extinct, may be related to the strength of conditioning, which in turn may be related to the rewarding efficacy of a drug. Resistance to extinction may therefore be a useful additional measure to quantify the rewarding effect of drugs. In the present study we examined the persistence of drug-environment associations after conditioning with morphine (1, 3 and 10 mg/kg i.p.), oxycodone (0.3, 1 and 3 mg/kg i.p.) and heroin (0.05, 0.25 and 0.5 mg/kg i.p.) by repeated retesting in the CPP apparatus (15-min sessions, 5 days/week) until the rats reached extinction (i.e. less than 55% preference over 3 consecutive sessions). Following an unbiased CPP protocol, morphine, oxycodone and heroin induced CPP with minimal effective doses of 3, 1 and 0.25 mg/kg, respectively, and with similar effect sizes for each CPP-inducing dose. The number of sessions required for extinction was positively correlated with the dose of the drug (experiment 1: 18 and 45 sessions for 3 and 10 mg/kg morphine, and 19 and 27 sessions for 1 and 3 mg/kg oxycodone; experiment 2: 12 and 24 sessions for 3 and 10 mg/kg morphine, and 10 and 14 sessions for 0.25 and 0.5 mg/kg heroin). These findings suggest that the use of an extinction paradigm can extend the quantitative assessment of the rewarding effect of drugs - however, within certain limits only. The present paradigm appears to be less suited for comparing the rewarding efficacy of different drugs due to great test-retest variability. Finally, the additional potential gain of information using this paradigm has to be weighed against the considerably large amount of additional time and effort.

The antinociceptive and antihyperalgesic effect of tapentadol is partially retained in OPRM1 (µ-opioid receptor) knockout mice.

Activation of the µ-opioid receptor (MOR) and noradrenaline reuptake inhibition (NRI) are well recognized as analgesic principles in acute and chronic pain indications. The novel analgesic tapentadol combines MOR agonism and NRI in a single molecule. The present study used OPRM1 (MOR) knockout (KO) mice to determine the relative contribution of MOR activation to tapentadol-induced analgesia in models of acute (nociceptive) and chronic (neuropathic) pain. Antinociceptive efficacy was inferred from paw withdrawal latencies on a 48 °C hot plate in naive animals. Antihyperalgesic efficacy was inferred from the number of nocifensive reactions in diabetic animals (streptozotocin-induced) and non-diabetic controls on a 50 °C hot plate. The effect of tapentadol (0.316-31.6 mg/kg IP) and the MOR agonist morphine (3-10 mg/kg IP) was determined in OPRM1 KO- and congenic wildtype mice. At baseline, diabetic OPRM1 KO mice showed reduced nocifensive reactions as compared to diabetic wildtype mice. In both pain models, morphine and tapentadol were effective in wildtype mice. In the KO mice, however, morphine failed to produce analgesia in either model. On the other hand, tapentadol still had clear effects, and when tested at a dose that was fully efficacious in wildtype mice, showed reduced but still significant antinociceptive efficacy in non-diabetic, and antihyperalgesic efficacy in diabetic OPRM1 KO mice. The remaining antinociceptive activity of tapentadol in OPRM1 KO mice was abolished by the α2-adrenoceptor antagonist yohimbine. In OPRM1 wildtype mice, the antihyperalgesic effect of tapentadol was 10 times more potent in diabetic animals (ED50=1.10 mg/kg) than its antinociceptive effect in naïve animals (ED50=10.8 mg/kg). This study supports the conclusion that the analgesic effect of tapentadol is only partly due to the activation of MOR, both under acute and chronic pain conditions, and that the efficacy of tapentadol against acute and chronic pain is based on its combined mechanism of action.

Pharmacological blockade or genetic knockout of the NOP receptor potentiates the rewarding effect of morphine in rats.

The Nociceptin/OrphaninFQ (NOP) system is believed to be involved in drug abuse and addiction. We have recently demonstrated that activation of the NOP receptor, by systemic administration of the NOP receptor agonist Ro65-6570, attenuated the rewarding effect of various opioids in conditioned place preference (CPP) in rats and this attenuating effect was reversed by the NOP receptor antagonist J-113397. The present study demonstrates that co-administration of J-113397 (4.64 mg/kg, i.p.) during conditioning, facilitates morphine-induced CPP. Moreover, we found that NOP receptor knockout rats (oprl1(-/-)) are more sensitive to the rewarding effect of morphine than wildtype control rats. Thus, pharmacological or genetic inactivation of the NOP system rendered rats more susceptible to the rewarding effect of morphine. These findings support the suggestion that the endogenous NOP system attenuates the rewarding effect of opioids and therefore offers a therapeutic target for the treatment of drug abuse and addiction.

The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) potentiates conditioned place preference induced by various addictive and non-addictive drugs in rats.

We have recently reported that the metabotropic glutamate receptor 5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) potentiates acquisition of conditioned place preference (CPP) induced by heroin and ketamine. The present study investigated to what extent this effect of MPEP can be generalized to other classes of drugs, such as the stimulants nicotine and cocaine, and to drugs that produce CPP in the rat despite a lack of abuse potential in humans, such as buspirone and clonidine. Adult male Sprague Dawley rats were subjected to a standard unbiased CPP protocol (six conditioning sessions lasting 20 minutes for nicotine and 40 minutes for the other compounds). Rats were conditioned with either nicotine (0.05-0.2 mg/kg, subcutaneously), cocaine [1-10 mg/kg, intraperitoneally (i.p.)], buspirone (0.3-3 mg/kg, i.p.) or clonidine (0.2-0.6 mg/kg, i.p.) in combination with MPEP (0 or 10 mg/kg, i.p.). For nicotine and cocaine, the minimal effective dose to induce CPP was lowered by pre-treatment with MPEP. While buspirone and clonidine did not induce CPP when given alone (i.e. combined with MPEP vehicle), both compounds induced CPP after pre-treatment with MPEP. It is concluded that MPEP consistently potentiates acquisition of drug-induced reward, independent of the mechanism of action of the co-administered drug. We suggest that the proposed anti-abuse effect of MPEP may be due to a substitution-like effect.

Dissociation of rewarding, anti-aversive and anti-nociceptive effects of different classes of anti-nociceptives in the rat.

It was previously shown that morphine more potently reduces the affective as compared to the sensory component of nociception, and this effect is independent of morphine's rewarding properties. Here we investigated whether this finding can be generalized to other classes of anti-nociceptive drugs. The effect of oxycodone (0-10 mg/kg, i.p.), tramadol (0-10 mg/kg, i.p.), ibuprofen (0-300 mg/kg, i.p.) and pregabalin (0-31.6 mg/kg, i.p.) on negative affect and mechanical hypersensitivity accompanying carrageenan-induced (0.5% intraplantar) inflammatory nociception was assessed using conditioned place aversion (CPA) and Randall Selitto paw pressure test, respectively. The rewarding effect of these drugs was assessed using conditioned place preference (CPP). All four anti-nociceptive drugs dose-dependently reduced carrageenan-induced CPA and mechanical hypersensitivity. Furthermore all drugs induced CPP, except for ibuprofen. Similar to morphine, oxycodone and tramadol showed a large dissociation of anti-aversive versus anti-nociceptive potency, i.e. 10 times more potent against the affective versus the sensory component of nociception. Oxycodone and tramadol were 30 and 10 times more potent to produce CPP in animals under normal versus painful conditions. Ibuprofen and pregabalin also showed a dissociation of anti-aversive and anti-nociceptive potency, but less pronounced (i.e. three times more potent against the affective component). However, pregabalin showed no dissociation between rewarding potency under normal versus painful conditions. Taken together, these data suggest that the dissociation of rewarding potency in animals under normal versus painful conditions is limited to drugs with an opioid mechanism of action, while the dissociation of anti-aversive and anti-nociceptive potency applies to anti-nociceptive drugs with different mechanisms of action.

Effects of the NOP receptor agonist Ro65-6570 on the acquisition of opiate- and psychostimulant-induced conditioned place preference in rats.

Activation of the Nociceptin/Orphanin FQ (NOP) receptor may have anti-abuse effects. The present study examined the consequence of NOP receptor activation on the rewarding effect of opiates and psychostimulants in the conditioned place preference task in rats. First, the motivational effect of the NOP receptor agonists Ro64-6198 (0.316-3.16 mg/kg i.p.) and Ro65-6570 (1-10mg/kg i.p.) when administered alone, was assessed. Ro65-6570 was selected for further drug combination studies since, unlike Ro64-6198, it was devoid of an intrinsic motivational effect. Next, the minimal effective dose to induce reward for the opiates heroin (0.1-3.16 mg/kg i.p.), morphine (1-10mg/kg i.p.), hydrocodone (0.316-10mg/kg i.p.), tilidine (1-31.6 mg/kg i.p.), hydromorphone (0.1-10mg/kg i.p.), and oxycodone (0.0316-10mg/kg i.p.), as well as for the psychostimulants cocaine (3.16-31.6 mg/kg i.p.) and dexamphetamine (0.316-3.16 mg/kg i.p.) in combination with Ro 65-6570 (0 or 3.16 mg/kg i.p.) was determined. All drugs produced conditioned place preference, and for opiates and cocaine, but not for dexamphetamine, the minimal effective dose was higher when combined with Ro65-6570 (3.16 mg/kg i.p.). Attenuation of the rewarding effect of tilidine (3.16 mg/kg i.p.) and oxycodone (1mg/kg i.p.) by Ro65-6570 (3.16 mg/kg i.p.) could be reversed by pre-treatment with the NOP receptor antagonist J-113397 (4.64 mg/kg i.p.), suggesting that the attenuating effect of Ro65-6570 on opiates is due to activation of the NOP receptor. Taken together, the present study suggests that activation of NOP receptors effectively attenuates the rewarding effect of opiates, but may be less effective in reducing psychostimulant-induced reward.

Tapentadol, but not morphine, selectively inhibits disease-related thermal hyperalgesia in a mouse model of diabetic neuropathic pain.

Neuropathic pain in diabetic patients is a common distressing symptom and remains a challenge for analgesic treatment. Selective inhibition of pathological pain sensation without modification of normal sensory function is a primary aim of analgesic treatment in chronic neuropathic pain. Tapentadol is a novel analgesic with two modes of action, mu-opioid receptor (MOR) agonism and noradrenaline (NA) reuptake inhibition. Mice were rendered diabetic by means of streptozotocin, and neuropathic hyperalgesia was assessed in a 50 degrees C hot plate test. Normal nociception was determined in control mice. Tapentadol (0.1-1mg/kg i.v.) and morphine (0.1-3.16 mg/kg i.v.) dose-dependently attenuated heat-induced nociception in diabetic animals with full efficacy, reaching >80% at the highest doses tested. Tapentadol was more potent than morphine against heat hyperalgesia, with ED(50) (minimal effective dose) values of 0.32 (0.316) and 0.65 (1)mg/kg, respectively. Non-diabetic controls did not show significant anti-nociception with tapentadol up to the highest dose tested (1mg/kg). In contrast, 3.16 mg/kg morphine, the dose that resulted in full anti-hyperalgesic efficacy under diabetic conditions, produced significant anti-nociception in non-diabetic controls. Selective inhibition of disease-related hyperalgesia by tapentadol suggests a possible advantage in the treatment of chronic neuropathic pain when compared with classical opioids, such as morphine. It is hypothesized that this superior efficacy profile of tapentadol is due to simultaneous activation of MOR and inhibition of NA reuptake.

Tapentadol hydrochloride: a next-generation, centrally acting analgesic with two mechanisms of action in a single molecule.

Tapentadol exerts its analgesic effects through micro opioid receptor agonism and noradrenaline reuptake inhibition in the central nervous system. Preclinical studies demonstrated that tapentadol is effective in a broad range of pain models, including nociceptive, inflammatory, visceral, mono- and polyneuropathic models. Moreover, clinical studies showed that tapentadol effectively relieves moderate to severe pain in various pain care settings. In addition, it was reported to be associated with significantly fewer treatment discontinuations due to a significantly lower incidence of gastrointestinal-related adverse events compared with equivalent doses of oxycodone. The combination of these reduced treatment discontinuation rates and tapentadol efficacy for the relief of moderate to severe nociceptive and neuropathic pain may offer an improvement in pain therapy by increasing patient compliance with their treatment regimen.

The mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) supports intravenous self-administration and induces conditioned place preference in the rat.

We recently reported that the mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) reduces intravenous self-administration of ketamine and, to a lesser extent, heroin in rats. We also found that MPEP potentiates conditioned place preference induced by these drugs, suggesting that the reduction of self-administration results from an MPEP-induced potentiation of the rewarding effect of the self-administered drug. The aim of the present study was to examine whether MPEP has intrinsic positive reinforcing and rewarding effects. In experiment 1, rats were trained to self-administer either ketamine [0.5 mg/kg/infusion, 2 h sessions, fixed-ratio (FR) 3] or heroin (0.05 mg/kg/infusion, 1 h sessions, FR 10), followed by a number of substitution sessions with MPEP (1 mg/kg/infusion) or saline. In experiment 2, drug-naïve rats were allowed to acquire intravenous self-administration of MPEP (1 mg/kg/infusion, 2 h sessions, FR 3) or saline. In experiment 3, rats were subjected to a single-trial unbiased conditioned place preference protocol with MPEP (0.3-10 mg/kg i.v., 20 min conditioning). It was found that (1) substitution with MPEP in rats which had learned to self-administer ketamine or heroin resulted in stable self-administration behavior, whereas substitution with saline resulted in a typical extinction profile, (2) drug-naïve rats learned to self-administer MPEP, but not saline, and self-administration remained stable for at least 7 sessions, and (3) MPEP induced dose-dependent place preference with a minimal effective dose of 3 mg/kg. These data clearly demonstrate that MPEP has (weak) positive reinforcing and rewarding effects when administered i.v.

2-Methyl-6-(phenylethynyl)-pyridine (MPEP) potentiates ketamine and heroin reward as assessed by acquisition, extinction, and reinstatement of conditioned place preference in the rat.

The mGlu(5) receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) has been shown to reduce intravenous self-administration of ketamine and, to a limited extent, heroin in rats. We investigated whether MPEP affects the rewarding effect of ketamine and heroin as assessed in a conditioned place preference (CPP) paradigm. Sprague Dawley rats were subjected to a standard unbiased CPP protocol. Rats were conditioned with either ketamine or heroin (0.316-31.6 and 0.0125-0.5 mg/kg i.p., respectively), in combination with MPEP (10 mg/kg, i.p.) or its vehicle. The effect of MPEP (10 mg/kg) on the duration of extinction and on reinstatement of ketamine- and heroin-induced CPP was also examined. Ketamine and heroin induced CPP with a minimal effective dose (MED) of 10 mg/kg and 0.25 mg/kg, respectively. MPEP (1-31.6 mg/kg) did not induce CPP by itself; however, co-treatment with MPEP resulted in a 10-fold and 5-fold leftward shift in the MED of ketamine and heroin for inducing CPP, respectively. MPEP slowed extinction of ketamine-induced CPP, but not of heroin-induced CPP, and once extinction was achieved, was able to reinstate CPP in both groups. These findings indicate that a moderate dose of MPEP (10 mg/kg i.p.) potentiates, rather than attenuates, the rewarding effect of ketamine and heroin. Moreover, these data suggest that the attenuating effect of MPEP on ketamine and heroin intravenous self-administration is due to an increase, rather than a decrease, of the rewarding/reinforcing effect of these compounds.

Investigation of TRPV1 loss-of-function phenotypes in transgenic shRNA expressing and knockout mice.

The function of the transient receptor potential vanilloid 1 (TRPV1) cation channel was analyzed with RNA interference technologies and compared to TRPV1 knockout mice. Expression of shRNAs targeting TRPV1 in transgenic (tg) mice was proven by RNase protection assays, and TRPV1 downregulation was confirmed by reduced expression of TRPV1 mRNA and lack of receptor agonist binding in spinal cord membranes. Unexpectedly, TRPV3 mRNA expression was upregulated in shRNAtg but downregulated in knockout mice. Capsaicin-induced [Ca(2+)](i) changes in small diameter DRG neurons were significantly diminished in TRPV1 shRNAtg mice, and administration of capsaicin hardly induced hypothermia or nocifensive behaviour in vivo. Likewise, sensitivity towards noxious heat was reduced. Interestingly, spinal nerve injured TRPV1 knockout but not shRNAtg animals developed mechanical allodynia and hypersensitivity. The present study provides further evidence for the relevance of TRPV1 in neuropathic pain and characterizes RNA interference as valuable technique for drug target validation in pain research.

Differential effects of morphine on the affective and the sensory component of carrageenan-induced nociception in the rat.

Pain is generally considered to have a sensory and an affective component. Clinical research has suggested that morphine more potently attenuates the affective component as compared to the sensory component. Because preclinical nociception models typically focus on the sensory component of nociception, and do not assess the affective component, it is unclear whether this potency difference of morphine can also be found in preclinical models. We therefore adapted the place conditioning paradigm to investigate negative affect accompanying carrageenan-induced (0.5% intraplantar) inflammatory nociception in rats. We found that carrageenan produced clear conditioned place aversion (CPA). Morphine (0.01-10mg/kg i.p.) dose-dependently reduced carrageenan-induced CPA with a minimal effective dose (MED) of 0.03mg/kg. Since morphine has a rewarding effect by itself, morphine-induced conditioned place preference (CPP) was also investigated. Morphine induced CPP with a MED of 1mg/kg, suggesting that the rewarding effect of morphine was not responsible for reducing carrageenan-induced CPA. We also demonstrated that morphine reduced carrageenan-induced mechanical nociception as assessed in the Randall Selitto paradigm with a MED of 1mg/kg. It is concluded that the CPA model allows for an assessment of the negative affective component of carrageenan-induced nociception. Moreover, morphine was able to reduce the affective component of nociception at doses that did not affect the sensory component of nociception, and this effect was not due to its rewarding properties. The fact that this finding mirrors the clinical situation validates the use of the CPA model for assessing the affective component of nociception.

Effect of 2-methyl-6-(phenylethynyl) pyridine on intravenous self-administration of ketamine and heroin in the rat.

The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP) may be beneficial for drug abuse treatment, as it has been found to reduce self-administration of ethanol, nicotine and cocaine in preclinical models. This study investigated whether this finding can be extended to dissociative anaesthetics and opioids. Long Evans rats were trained to intravenously self-administer ketamine (0.5 mg/kg/infusion, 2 h sessions, fixed ratio 3) or heroin (0.05 mg/kg/infusion, 1 h sessions, fixed ratio 10). After reaching stable responding, the effect of MPEP pretreatment (1.25-20 mg/kg, intraperitoneal; t=-30 min) on intravenous self-administration (IVSA) of each compound was investigated. Behavioural specificity of MPEP on IVSA was assessed using a food-reinforcement procedure. IVSA of ketamine was dose-dependently reduced by MPEP pretreatment, with a minimal effective dose of 5 mg/kg and a 75% reduction at the highest dose tested. IVSA of heroin was only modestly reduced by the highest dose of MPEP (20% reduction). Food-reinforced behaviour was not altered by MPEP, either given alone or in combination with ketamine or heroin, indicating that the effect in the IVSA paradigm was behaviourally specific. It is suggested that the differential effect of MPEP on IVSA of ketamine and heroin is related to the particular class of the self-administered drug or its relative reinforcing efficacy.