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.

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.

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.

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.

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.

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.

The serotonin/noradrenaline reuptake inhibitor venlafaxine attenuates acquisition, but not maintenance, of intravenous self-administration of heroin in rats.

Opioids and antidepressants are frequently used for the treatment of various pain conditions. A combination of both drug classes may be more effective than either treatment alone, and combined treatment with an antidepressant may result in an opiate-sparing effect. Although it has been shown that antidepressants can attenuate self-administration of psychomotor stimulant and depressant drugs, it is not known whether they also attenuate self-administration of opiates. To determine whether venlafaxine, a serotonin/noradrenaline reuptake inhibitor with antidepressive and analgesic properties, affects acquisition and maintenance of intravenous heroin self-administration in rats, male Long-Evans rats were trained to press a lever in order to receive heroin (0.05 mg/kg/infusion) under a fixed ratio or a progressive ratio schedule. A control group was trained in a fixed ratio food-reinforced operant procedure. The effect of venlafaxine on operant responding for heroin and food was assessed both during acquisition and, in separate groups of rats, during maintenance (i.e., after acquisition) of self-administration behaviour. Daily treatment with venlafaxine (10 mg/kg i.p.) before the operant session attenuated the acquisition of responding for heroin, but not for food. However, when tested during the maintenance phase in rats showing stable responding, acute treatment with venlafaxine only marginally affected operant responding for heroin under a fixed ratio:10 schedule of reinforcement, and neither acute nor subchronic (once daily during 4 weeks) venlafaxine treatment affected responding under a progressive ratio schedule. Thus, daily treatment with an antidepressant attenuates the acquisition of heroin self-administration in a behaviourally specific manner, while having only marginal effects on maintenance of heroin self-administration.

Broad analgesic profile of buprenorphine in rodent models of acute and chronic pain.

Buprenorphine is a potent opioid analgesic clinically used to treat moderate to severe pain. The present study assessed its analgesic efficacy in a broad range of rodent models of acute and chronic pain. In the phenylquinone writhing, hot plate, and tail flick mouse models of acute pain, full analgesic efficacy was obtained (ED50 values: 0.0084-0.16 mg/kg i.v.). Full analgesic efficacy was also obtained in yeast- and formalin-induced inflammatory pain (ED50 values: 0.0024-0.025 mg/kg i.v., rats and mice) and in mustard-oil-induced spontaneous pain, referred allodynia, and referred hyperalgesia in mice (ED50 values: 0.018-0.025 mg/kg i.v.). Buprenorphine strongly inhibited mechanical and cold allodynia in mononeuropathic rats, as well as mechanical hyperalgesia and cold allodynia in polyneuropathic rats (ED50 values: 0.055 and 0.036 mg/kg i.v. and 0.129 and 0.038 mg/kg i.p., respectively). It is concluded that buprenorphine shows a broad analgesic profile and offers the opportunity to treat different pain conditions, including neuropathic pain.

Effects of alpha 4/beta 2- and alpha 7-nicotine acetylcholine receptor agonists on prepulse inhibition of the acoustic startle response in rats and mice.

RATIONALE: Nicotine and agonists at subtypes of the nicotine acetylcholine receptor (nAChR) affect auditory gating, but the magnitude and direction of such effects appear highly variable. This variability may be due to differences in the tested dose range, selectivity of the test compound, species and strain, and suggests that nAChR subtypes are differentially involved in the control of auditory gating. OBJECTIVES AND METHODS: This study aimed to characterise the effects of nicotine and agonists with preferential activity at alpha4/beta2- and alpha7-nAChRs on auditory sensorimotor gating using a prepulse inhibition (PPI) paradigm. Similar experimental conditions were employed in rats and two strains of mice. The paradigm used startle stimuli of 120 dB and prepulse intensities of 3, 6 and 12 dB above a background of 70 dB. RESULTS: In Sprague-Dawley rats, nicotine disrupted PPI [minimal effective dose (MED): 1 mg/kg, SC] and this effect was mimicked by the potent nAChR agonist, epibatidine, (MED: < or = 0.001 mg/kg, IP) and the potent, and relatively selective, alpha4/beta2-nAChR agonist A-85380 (MED: < or = 0.1 mg/kg, IP). The effects of epibatidine, A-85380 and, to a lesser extent, nicotine were blocked by the non-selective nAChR antagonist mecamylamine. The relatively selective alpha7-nAChR agonists, GTS-21 and AR-R-17779, did not affect PPI in a consistent manner, both in rats and in DBA/2 mice, a strain expressing a disrupted gating phenotype, presumably due to altered activity of hippocampal alpha7-nAChRs. In BALB/c mice, a strain expressing a normal gating phenotype, nicotine (MED: 10 mg/kg, SC), epibatidine (MED: 0.03 mg/kg, IP) and A-85380 (MED: 0.3 mg/kg, IP) predominantly augmented PPI and mecamylamine attenuated these effects. CONCLUSIONS: The present results confirm that the effects of nAChR agonists on PPI are species dependent and suggest that stimulation of heteromeric nAChRs containing both alpha and beta subunits, and possibly of the alpha4/beta2 type, affect sensorimotor gating. Evidence supporting a role for alpha7-nAChRs in the control of PPI of the acoustic startle response was not obtained.

Discriminative stimulus effects of the structurally novel cannabinoid CB1/CB2 receptor partial agonist BAY 59-3074 in the rat.

BAY 59-3074 [3-[2-cyano-3-(trifluoromethyl)phenoxy]phenyl-4,4,4-trifluoro-1-butane-sulfonate] is a structurally novel cannabinoid CB1/CB2 receptor partial agonist with analgesic properties. The present study was performed to confirm its receptor binding profile in a highly sensitive in vivo assay. Rats (n=10) learned to discriminate BAY 59-3074 (0.5 mg/kg, p.o., t-1 h) from vehicle in a fixed-ratio: 10, food-reinforced two-lever procedure after a median number of 28 training sessions. BAY 59-3074 generalized dose-dependently (ED(50): 0.081 mg/kg, p.o.) and the cue was detectable between 0.25 and 4 h after administration. The selective cannabinoid CB1 receptor antagonist SR 141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] blocked the discriminative effects of BAY 59-3074 (ID50: 1.79 mg/kg, i.p.). Complete generalization was also obtained after i.p. administration of BAY 59-3074 (ED50 value: 0.41 mg/kg), and the reference cannabinoids BAY 38-7271 [(-)-(R)-3-(2-hydroxymethylindanyl-4-oxy)phenyl-4,4,4-trifluoro-1-butanesulfonate, 0.011 mg/kg], CP 55,940 [(-)-cis-3-[2-hydroxy-4(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol, 0.013 mg/kg], HU-210 [(-)-11-OH-Delta8-tetrahydrocannabinol dimethylheptyl, 0.022 mg/kg], WIN 55,212-2 [(R)-4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenylcarbonyl)-6H-pyrrolo [3,2,1-ij] quinolin-6-one, 0.41 mg/kg] and (-)-Delta9-tetrahydrocannabinol (0.41 mg/kg). Non-cannabinoids with analgesic properties, such as morphine, amitriptyline, carbamazepine, gabapentin and baclofen, did not generalize to the cue. It is concluded that the discriminative stimulus effects of BAY 59-3074 are specifically mediated by cannabinoid CB1 receptor activation.

Discriminative stimulus effects of BAY 38-7271, a novel cannabinoid receptor agonist.

BAY 38-7271 [(-)-(R)-3-(2-hydroxymethylindanyl-4-oxy)phenyl-4,4,4-trifluoro-1-sulfonate] is a novel, highly potent and selective cannabinoid CB(1)/CB(2) receptor agonist with neuroprotective properties. It was the aim of the present study to further confirm its cannabinoid CB(1) receptor agonist properties in a highly sensitive in vivo assay. Male Wistar rats (n=24) were trained to discriminate BAY 38-7271 (0.05 mg/kg, i.p., t-30 min) from vehicle in a fixed-ratio:10, food-reinforced two-lever standard procedure. The animals acquired the discrimination after a median number of 52 training sessions. BAY 38-7271 generalized dose-dependently when tested after different routes of administration (ED(50): 0.018 mg/kg, i.p.; 0.001 microg/kg, i.v.; 0.18 mg/kg, p.o.). A time-dependency study indicated that the cue (0.05 mg/kg, i.p.) was detectable between 15 min and 4 h, with a maximum of generalization obtained at 30 min after administration. Pretreatment with the selective cannabinoid CB(1) receptor antagonist SR 141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] completely antagonized the effects of BAY 38-7271 (ID(50): 1.1 mg/kg, i.p.). Dose-dependent and complete generalization was also obtained after i.p. administration of the reference cannabinoid CB(1) receptor agonists HU-210 [(-)-11-OH-Delta(8)-tetrahydrocannabinol-dimethylheptyl, ED(50): 0.003 mg/kg], CP 55,940 [(-)-cis-3-[2-hydroxy-4(1,1-dimethyl-heptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol, 0.007 mg/kg], WIN 55,212-2 [(R)-4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphtalenylcarbonyl)-6H-pyrrolo [3,2,1-ij] quinolin-6-one, 0.28 mg/kg] and (-)-Delta(9)-tetrahydrocannabinol (0.34 mg/kg). The present study confirms that BAY 38-7271 is a highly potent cannabinoid CB(1) receptor agonist in vivo.

Pharmacological characterization of the chronic constriction injury model of neuropathic pain.

The chronic constriction injury model is a rat model of neuropathic pain based on a unilateral loose ligation of the sciatic nerve. The aim of the present study was to test its sensitivity to various clinically validated and experimental drugs. Mechanical allodynia and thermal hyperalgesia developed within one week post-surgery and were reliably present for at least 7 weeks. Mechanical allodynia was strongly attenuated by morphine (minimal effective dose in brackets: 8 mg/kg, p.o.) and the cannabinoids Delta9-tetrahydrocannabinol (3 mg/kg, p.o.) and (-)-cis-3-[2-hydroxy-4(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol (CP 55,940; 0.05 mg/kg, i.p.), and weakly/moderately attenuated by the anticonvulsants gabapentin (50 mg/kg, i.p.) and carbamazepine (32 mg/kg, i.p.), the muscle relaxant baclofen (3 mg/kg, i.p.), and the adenosine kinase inhibitor 4-amino-5-(3-bromophenyl)-7-(6-morpholino-pyridin-3-yl)pyrido[2,3-d]pyrimidine (ABT-702; 30 mg/kg, i.p.). Thermal hyperalgesia was strongly attenuated by morphine (16 mg/kg, p.o.), Delta9-tetrahydrocannabinol (6 mg/kg, p.o.), CP 55,940 (0.025 mg/kg, i.p.), carbamazepine (32 mg/kg, i.p.) and the antidepressant amitriptyline (32 mg/kg, p.o.), and weakly/moderately attenuated by gabapentin (50 mg/kg, i.p.), the anti-inflammatory cyclooxygenase-2 inhibitor rofecoxib (30 mg/kg, i.p.) and the adenosine A1 receptor positive allosteric modulator 2-amino-4,5,6,7-tetrahydrobenzo(b)thiophen-3-yl 4-chlorophenylmethanone (T62; 30 mg/kg, i.p.). Both symptoms were hardly or not affected by the nonselective N-methyl-d-aspartate receptor antagonists ketamine and dizocilpine, and the N-methyl-d-aspartate receptor NR2B-selective antagonists ifenprodil and R-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenyl-methyl)-1-piperidine propranol (Ro 25-6981). The finding that mechanical allodynia and/or thermal hyperalgesia are attenuated by various established compounds further supports the validity of the chronic constriction injury model for the study of neuropathic pain and its use for the identification of novel treatments.

Behavioral mechanisms underlying inhibition of food-maintained responding by the cannabinoid receptor antagonist/inverse agonist SR141716A.

This study investigated the possible behavioral mechanisms underlying the anorectic effect of the cannabinoid CB(1) receptor antagonist/inverse agonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A). Male or female rats were food-restricted and trained to emit stable responding in daily 10-min, fixed ratio 10 food-reinforced operant sessions. Under these conditions, as well as under free-feeding conditions, SR141716A inhibited food-maintained responding (ED(50) values ranging from 0.92 to 2.52 mg/kg, i.p.). In the same operant procedure, SR141716A suppressed intracranial self-stimulation with a potency which was slightly lower than the anorectic potency (ED(50): 4.50 mg/kg). As assessed during a 10-min test period SR141716A (1-10 mg/kg) did not affect activity counts; suggesting that the observed inhibition of operant behavior is not a direct consequence of impairment of locomotor activity. SR141716A, however, attenuated saccharin-preference in a conditioned taste aversion paradigm (ED(50): 6.45 mg/kg). Although the data support the suggestion that the anorectic effect of SR141716A results from an attenuating effect on the rewarding effect of food, the contribution of drug-induced aversion/malaise cannot be excluded.

Pharmacological sensitivity and gene expression analysis of the tibial nerve injury model of neuropathic pain.

The tibial nerve injury model is a novel, surgically uncomplicated, rat model of neuropathic pain based on a unilateral transection (neurotomy) of the tibial branch of the sciatic nerve. The aim of the present study was to describe some behavioral and molecular features of the model, and to test its sensitivity to a number of drugs which are currently used for the treatment of neuropathic pain. The model was characterized by a pronounced mechanical allodynia which was present in all subjects and a less robust thermal hyperalgesia. Mechanical allodynia developed within 2 weeks post-surgery and was reliably present for at least 9 weeks. Neurotomized rats showed no autotomy and their body weight developed normally. Gene expression in ipsilateral L5 dorsal root ganglia, analyzed by quantitative polymerase chain reaction (PCR), showed a pronounced up-regulation of galanin and vasointestinal peptide (VIP). This up-regulation developed rapidly (within 1 to 2 days following neurotomy) and remained present for at least 12 days. On the other hand, expression of calcitonin gene-related peptide (CGRP) and substance P mRNA was down-regulated 12 days following neurotomy. Mechanical allodynia was completely reversed by morphine [minimal effective dose (MED): 8 mg/kg, i.p.] and partially reversed by carbamazepine (MED: 64 mg/kg, i.p.), baclofen (MED: 3 mg/kg, i.p.) and amitriptyline (trend for efficacy at 32 mg/kg, i.p.), but not by gabapentin (50-100 mg/kg, i.p.). The finding that the tibial nerve injury model shows a robust and persistent mechanical allodynia which is sensitive to a number of established analgesics, as well as a gene expression profile which is compatible with that obtained in other models of neuropathic pain, further supports its validity as a reliable and surgically uncomplicated model for the study of neuropathic pain.

Role of 5-hT2C receptors in the hypophagic effect of m-CPP, ORG 37684 and CP-94,253 in the rat.

Compounds that stimulate 5-HT2C and/or 5-HT1B receptors induce hypophagia, but the relative role of these receptors in the control of feeding behaviour remains to be unequivocally demonstrated. The objectives of the present study were: (a) comparison of the hypophagic effect of the mixed 5-HT2C/1B receptor agonist, m-CPP, with that of ORG 37684 and CP-94,253, a relatively selective 5-HT2C and 5-HT1B receptor agonist, respectively; (b) verification of the contribution of 5-HT2C receptors to the hypophagic effect of these compounds by antagonism experiments; and (c) to test whether cotreatment with ORG 37684 and CP-94,253 leads to a more pronounced reduction of food intake as compared with treatment with either compound alone. Food intake was measured in a free feeding experimental protocol employing female Wistar rats. m-CPP was more potent in suppressing food intake than ORG 37684 and CP-94,253 (ED50 values for the first hour of access: 0.45, 1.84 and 3.48 mg/kg ip, respectively). The 5-HT2C receptor antagonists, metergoline and SB 242.084, completely reversed the hypophagic effect of ORG 37684, but not that of CP-94,253 and m-CPP. The hypophagic effect of ORG 37684 was potentiated by a low (inactive) dose of CP-94,253 (ED50: 4.95 and 2.44 mg/kg ip after vehicle and CP-94,253 pretreatment, respectively) and vice versa (ED50 values: 4.02 and 0.62 mg/kg ip). It is concluded that the hypophagic effect of ORG 37684-but not that of m-CPP and CP-94,253--is exclusively mediated by activation of 5-HT2C receptors. The results further indicate that simultaneous activation of 5-HT2C and 5-HT1B receptors underlies the higher potency of m-CPP in reducing food intake, as compared with other, more selective, compounds.

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.

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.

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.