Mitragynine and Morphine Produce Dose-dependent Bimodal Action on Food but not Water Intake in Rats.

Novel compounds such as mitragynine, the primary alkaloid in kratom (Mitragyna Speciosa), have emerged as alternative self-treatments for opioid use disorder. Mitragynine binds numerous receptor types, including opioid receptors, which are known to modulate food consumption. However, the ability of acute mitragynine to modulate food consumption remains unknown. The current study assessed the effects of acute mitragynine or morphine administration on unconditioned food and water intake in 16 Sprague Dawley rats. Food and water intake changes were monitored in response to morphine, mitragynine (1.78-56 mg/kg, intraperitoneal), saline, or vehicle controls for 12 hours starting at the onset of the dark cycle. Naltrexone pretreatment was utilized to examine pharmacological specificity. Both morphine and mitragynine demonstrated a biphasic food intake dose-effect, with low doses (5.6 mg/kg) increasing and high doses (56 mg/kg) decreasing food intake. All morphine doses reduced water intake; however, only the highest dose of mitragynine (56 mg/kg) reduced water intake. Naltrexone attenuated both stimulatory and inhibitory effects of morphine on food intake, but only the stimulatory effect of mitragynine. In conclusion, low doses of mitragynine stimulate food intake via opioid-related pathways, while high doses likely recruit other targets.

Pharmacokinetic and Pharmacodynamic Consequences of Cytochrome P450 3A Inhibition on Mitragynine Metabolism in Rats.

Mitragynine, an opioidergic alkaloid present in Mitragyna speciosa (kratom), is metabolized by cytochrome P450 3A (CYP3A) to 7-hydroxymitragynine, a more potent opioid receptor agonist. The extent to which conversion to 7-hydroxymitragynine mediates the in vivo effects of mitragynine is unclear. The current study examined how CYP3A inhibition (ketoconazole) modifies the pharmacokinetics of mitragynine in rat liver microsomes in vitro. The study further examined how ketoconazole modifies the discriminative stimulus and antinociceptive effects of mitragynine in rats. Ketoconazole [30 mg/kg, oral gavage (o.g.)] increased systemic exposure to mitragynine (13.3 mg/kg, o.g.) by 120% and 7-hydroxymitragynine exposure by 130%. The unexpected increase in exposure to 7-hydroxymitragynine suggested that ketoconazole inhibits metabolism of both mitragynine and 7-hydroxymitragynine, a finding confirmed in rat liver microsomes. In rats discriminating 3.2 mg/kg morphine from vehicle under a fixed-ratio schedule of food delivery, ketoconazole pretreatment increased the potency of both mitragynine (4.7-fold) and 7-hydroxymitragynine (9.7-fold). Ketoconazole did not affect morphine's potency. Ketoconazole increased the antinociceptive potency of 7-hydroxymitragynine by 4.1-fold. Mitragynine (up to 56 mg/kg, i.p.) lacked antinociceptive effects both in the presence and absence of ketoconazole. These results suggest that both mitragynine and 7-hydroxymitragynine are cleared via CYP3A and that 7-hydroxymitragynine is formed as a metabolite of mitragynine by other routes. These results have implications for kratom use in combination with numerous medications and citrus juices that inhibit CYP3A. SIGNIFICANCE STATEMENT: Mitragynine is an abundant kratom alkaloid that exhibits low efficacy at the µ-opioid receptor (MOR). Its metabolite, 7-hydroxymitragynine, is also an MOR agonist but with higher affinity and efficacy than mitragynine. Our results in rats demonstrate that cytochrome P450 3A (CYP3A) inhibition can increase the systematic exposure of both mitragynine and 7-hydroxymitragynine and their potency to produce MOR-mediated behavioral effects. These data highlight potential interactions between kratom and CYP3A inhibitors, which include numerous medications and citrus juices.

Interactive Effects of µ-Opioid and Adrenergic-α 2 Receptor Agonists in Rats: Pharmacological Investigation of the Primary Kratom Alkaloid Mitragynine and Its Metabolite 7-Hydroxymitragynine.

The primary kratom alkaloid mitragynine is proposed to act through multiple mechanisms, including actions at µ-opioid receptors (MORs) and adrenergic-α 2 receptors (Aα 2Rs), as well as conversion in vivo to a MOR agonist metabolite (i.e., 7-hydroxymitragynine). Aα 2R and MOR agonists can produce antinociceptive synergism. Here, contributions of both receptors to produce mitragynine-related effects were assessed by measuring receptor binding in cell membranes and, in rats, pharmacological behavioral effect antagonism studies. Mitragynine displayed binding affinity at both receptors, whereas 7-hydroxymitragynine only displayed MOR binding affinity. Compounds were tested for their capacity to decrease food-maintained responding and rectal temperature and to produce antinociception in a hotplate test. Prototypical MOR agonists and 7-hydroxymitragynine, but not mitragynine, produced antinociception. MOR agonist and 7-hydroxymitragynine rate-deceasing and antinociceptive effects were antagonized by the opioid antagonist naltrexone but not by the Aα 2R antagonist yohimbine. Hypothermia only resulted from reference Aα 2R agonists. The rate-deceasing and hypothermic effects of reference Aα 2R agonists were antagonized by yohimbine but not naltrexone. Neither naltrexone nor yohimbine antagonized the rate-decreasing effects of mitragynine. Mitragynine and 7-hydroxymitragynine increased the potency of the antinociceptive effects of Aα 2R but not MOR reference agonists. Only mitragynine produced hypothermic effects. Isobolographic analyses for the rate-decreasing effects of the reference Aα 2R and MOR agonists were also conducted. These results suggest mitragynine and 7-hydroxymitragynine may produce antinociceptive synergism with Aα 2R and MOR agonists. When combined with Aα 2R agonists, mitragynine could also produce hypothermic synergism. SIGNIFICANCE STATEMENT: Mitragynine is proposed to target the µ-opioid receptor (MOR) and adrenergic-α2 receptor (Aα2R) and to produce behavioral effects through conversion to its MOR agonist metabolite 7-hydroxymitragynine. Isobolographic analyses indicated supra-additivity in some dose ratio combinations. This study suggests mitragynine and 7-hydroxymitragynine may produce antinociceptive synergism with Aα2R and MOR agonists. When combined with Aα2R agonists, mitragynine could also produce hypothermic synergism.

The Lack of Contribution of 7-Hydroxymitragynine to the Antinociceptive Effects of Mitragynine in Mice: A Pharmacokinetic and Pharmacodynamic Study.

Kratom (Mitragyna speciosa), a Southeast Asian tree, has been used for centuries in pain relief and mitigation of opium withdrawal symptoms. Mitragynine (MTG), the major kratom alkaloid, is being investigated for its potential to provide analgesia without the deleterious effects associated with typical opioids. Concerns have been raised regarding the active metabolite of MTG, 7-hydroxymitragynine (7HMG), which has higher affinity and efficacy at µ-opioid receptors than MTG. Here we investigated the hotplate antinociception, pharmacokinetics, and tissue distribution of MTG and 7HMG at equianalgesic oral doses in male and female C57BL/6 mice to determine the extent to which 7HMG metabolized from MTG accounts for the antinociceptive effects of MTG and investigate any sex differences. The mechanism of action was examined by performing studies with the opioid receptor antagonist naltrexone. A population pharmacokinetic/pharmacodynamic model was developed to predict the behavioral effects after administration of various doses of MTG and 7HMG. When administered alone, 7HMG was 2.8-fold more potent than MTG to produce antinociception. At equivalent effective doses of MTG and 7HMG, there was a marked difference in the maximum brain concentration of 7HMG achieved, i.e., 11-fold lower as a metabolite of MTG. The brain concentration of 7HMG observed 4 hours post administration, producing an analgesic effect <10%, was still 1.5-fold higher than the maximum concentration of 7HMG as a metabolite of MTG. These results provide strong evidence that 7HMG has a negligible role in the antinociceptive effects of MTG in mice. SIGNIFICANCE STATEMENT: Mitragynine (MTG) is being investigated for its potential to aid in pain relief, opioid withdrawal syndrome, and opioid use disorder. The active metabolite of MTG, 7-hydroxymitragynine (7HMG), has been shown to have abuse potential and has been implicated in the opioid-like analgesic effect after MTG administration. The results of this study suggest a lack of involvement of 7HMG in the antinociceptive effects of MTG in mice.

The discriminative stimulus effects of baclofen and gamma hydroxybutyrate in C57BL/6J mice.

Baclofen and γ-hydroxybutyrate (GHB) exert γ-aminobutyric acid (GABA)B receptor agonism and have therapeutic utility but possess different pharmacological activities. We examined whether separate groups of mice could be trained to discriminate either baclofen or GHB, and the contribution of GABAB receptors to discriminative stimulus effects. Male C57BL/6J mice were trained to discriminate either baclofen (3.2 mg/kg, intraperitoneal) or GHB (178 mg/kg, intraperitoneal) from saline under a fixed-ratio 10 schedule. The GABAB antagonist 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348) was used to pharmacologically assess GABAB receptor involvement. The selectivity of the resulting discriminations was assessed with the opioid agonist morphine and the benzodiazepine midazolam. In baclofen-trained mice, both baclofen and GHB were readily discriminated. Baclofen produced a maximum of 86% baclofen-appropriate responding. CGP 35348 (320 mg/kg, i.p.) produced a 4.7-fold rightward shift in the dose-effect function. GHB produced a maximum of 85.8% baclofen-appropriate responding. In GHB-trained mice, both GHB and baclofen were readily discriminated. In GHB-trained mice, GHB produced a maximum of 85.3% drug-appropriate responding; CGP 35348 (320 mg/kg, i.p.) produced a 1.8-fold rightward shift in the GHB discrimination dose-effect function. Baclofen produced up to 70.0% GHB-appropriate responding. CGP 35348 (320 mg/kg, i.p.) significantly antagonized baclofen discrimination and baclofen produced up to 37% GHB-appropriate responding up to doses that disrupted operant responding. Morphine did not produce substitution for either baclofen or GHB. Midazolam produced partial substitution for both. GHB and baclofen discrimination assays in mice provide a useful approach for examining different receptor types mediating the effects of these two drugs.

Pharmacological Comparison of Mitragynine and 7-Hydroxymitragynine: In Vitro Affinity and Efficacy for µ-Opioid Receptor and Opioid-Like Behavioral Effects in Rats.

Relationships between µ-opioid receptor (MOR) efficacy and effects of mitragynine and 7-hydroxymitragynine are not fully established. We assessed in vitro binding affinity and efficacy and discriminative stimulus effects together with antinociception in rats. The binding affinities of mitragynine and 7-hydroxymitragynine at MOR (Ki values 77.9 and 709 nM, respectively) were higher than their binding affinities at κ-opioid receptor (KOR) or δ-opioid receptor (DOR). [35S]guanosine 5'-O-[γ-thio]triphosphate stimulation at MOR demonstrated that mitragynine was an antagonist, whereas 7-hydroxymitragynine was a partial agonist (Emax = 41.3%). In separate groups of rats discriminating either morphine (3.2 mg/kg) or mitragynine (32 mg/kg), mitragynine produced a maximum of 72.3% morphine-lever responding, and morphine produced a maximum of 65.4% mitragynine-lever responding. Other MOR agonists produced high percentages of drug-lever responding in the morphine and mitragynine discrimination assays: 7-hydroxymitragynine (99.7% and 98.1%, respectively), fentanyl (99.7% and 80.1%, respectively), buprenorphine (99.8% and 79.4%, respectively), and nalbuphine (99.4% and 98.3%, respectively). In the morphine and mitragynine discrimination assays, the KOR agonist U69,593 produced maximums of 72.3% and 22.3%, respectively, and the DOR agonist SNC 80 produced maximums of 34.3% and 23.0%, respectively. 7-Hydroxymitragynine produced antinociception; mitragynine did not. Naltrexone antagonized all of the effects of morphine and 7-hydroxymitragynine; naltrexone antagonized the discriminative stimulus effects of mitragynine but not its rate-decreasing effects. Mitragynine increased the potency of the morphine discrimination yet decreased morphine antinociception. Here we illustrate striking differences in MOR efficacy, with mitragynine having less than 7-hydroxymitragynine. SIGNIFICANCE STATEMENT: At human µ-opioid receptor (MOR) in vitro, mitragynine has low affinity and is an antagonist, whereas 7-hydroxymitragynine has 9-fold higher affinity than mitragynine and is an MOR partial agonist. In rats, intraperitoneal mitragynine exhibits a complex pharmacology including MOR agonism; 7-hydroxymitragynine has higher MOR potency and efficacy than mitragynine. These results are consistent with 7-hydroxymitragynine being a highly selective MOR agonist and with mitragynine having a complex pharmacology that combines low efficacy MOR agonism with activity at nonopioid receptors.

Pharmacokinetics of Eleven Kratom Alkaloids Following an Oral Dose of Either Traditional or Commercial Kratom Products in Rats.

Kratom, Mitragyna speciosa Korth., is being widely consumed in the United States for pain management and the reduction of opioid withdrawal symptoms. The central nervous system (CNS) active alkaloids of kratom, including mitragynine, 7-hydroxymitragynine, and numerous additional compounds, are believed to derive their effects through opioid receptor activity. There is no literature describing the systemic exposure of many of these alkaloids after the consumption of kratom. Therefore, we have developed and validated a bioanalytical method for the simultaneous quantitation of 11 kratom alkaloids (mitragynine, 7-hydroxymitragynine, corynantheidine, speciogynine, speciociliatine, paynantheine, corynoxine, corynoxine-B, mitraphylline, ajmalicine, and isospeciofoline) in rat plasma. The validated method was used to analyze oral pharmacokinetic study samples of lyophilized kratom tea (LKT) and a marketed product, OPMS liquid shot, in rats. Among the 11 alkaloids, only mitragynine, 7-hydroxymitragynine, speciociliatine, and corynantheidine showed systemic exposure 8 h postdose, and the dose-normalized systemic exposure of these four alkaloids was higher (1.6-2.4-fold) following the administration of the commercial OPMS liquid. Paynantheine and speciogynine levels were quantifiable up to 1 h postdose, whereas none of the other alkaloids were detected. In summary, the method was successfully applied to quantify the exposure of individual kratom alkaloids after an oral dose of traditional or commercial products. This information will contribute to understanding the role of each alkaloid in the overall pharmacology of kratom and elucidating the pharmacokinetic differences between traditional and commercial kratom products.

Exploring the Chemistry of Alkaloids from Malaysian Mitragyna speciosa (Kratom) and the Role of Oxindoles on Human Opioid Receptors.

Ten indole and oxindole alkaloids (1-10) were isolated from the freshly collected leaves of Malaysian Mitragyna speciosa (Kratom). The chemical structures of these compounds were established on the basis of extensive 1D and 2D NMR and HRMS data analysis. The spectroscopic data of mitragynine oxindole B (4) are reported herein for the first time. The spatial configuration of mitragynine oxindole B (4) was confirmed by single-crystal X-ray diffraction. Simultaneous quantification of the isolated alkaloids in the M. speciosa leaf specimens collected from different locations in the northern region of Peninsular Malaysia was also performed using UPLC-MS/MS. The oxindole alkaloids (1-4) and the indole alkaloid (10) were assessed for binding affinity at opioid receptors. Corynoxine (1) showed high binding affinity to µ-opioid receptors with a Ki value of 16.4 nM. Further, corynoxine (1) was 1.8-fold more potent than morphine in rats subjected to a nociceptive hot plate assay. These findings have important implications for evaluating the combined effects of the minor oxindole alkaloids in the overall therapeutic activity of M. speciosa.

Nicotinic Acetylcholine Receptor Accessory Subunits Determine the Activity Profile of Epibatidine Derivatives.

Epibatidine is a potent analgetic agent with very high affinity for brain nicotinic acetylcholine receptors (nAChR). We determined the activity profiles of three epibatidine derivatives, RTI-36, RTI-76, and RTI-102, which have affinity for brain nAChR equivalent to that of epibatidine but reduced analgetic activity. RNAs coding for nAChR monomeric subunits and/or concatamers were injected into Xenopus oocytes to obtain receptors of defined subunit composition and stoichiometry. The epibatidine analogs produced protracted activation of high sensitivity (HS) α4- and α2-containing receptors with the stoichiometry of 2alpha:3beta subunits but not low sensitivity (LS) receptors with the reverse ratio of alpha and beta subunits. Although not strongly activated by the epibatidine analogs, LS α4- and α2-containing receptors were potently desensitized by the epibatidine analogs. In general, the responses of α4(2)ß2(2)α5 and ß3α4ß2α6ß2 receptors were similar to those of the HS α4ß2 receptors. RTI-36, the analog closest in structure to epibatidine, was the most efficacious of the three compounds, also effectively activating α7 and α3ß4 receptors, albeit with lower potency and less desensitizing effect. Although not the most efficacious agonist, RTI-76 was the most potent desensitizer of α4- and α2-containing receptors. RTI-102, a strong partial agonist for HS α4ß2 receptors, was effectively an antagonist for LS α4ß2 receptors. Our results highlight the importance of subunit stoichiometry and the presence or absence of specific accessory subunits for determining the activity of these drugs on brain nAChR, affecting the interpretation of in vivo studies since in most cases these structural details are not known. SIGNIFICANCE STATEMENT: Epibatidine and related compounds are potent ligands for the high-affinity nicotine receptors of the brain, which are therapeutic targets and mediators of nicotine addiction. Far from being a homogeneous population, these receptors are diverse in subunit composition and vary in subunit stoichiometry. We show the importance of these structural details for drug activity profiles, which present a challenge for the interpretation of in vivo experiments since conventional methods, such as in situ hybridization and immunohistochemistry, cannot illuminate these details.

The discriminative stimulus effects of epibatidine in C57BL/6J mice.

The α4ß2* nicotinic acetylcholine receptor (nAChR) subtypes are targeted for the development of smoking cessation aids, and the use of drug discrimination in mice provides a robust screening tool for the identification of drugs acting through nAChRs. Here, we established that the α4ß2* nAChR agonist epibatidine can function as a discriminative stimulus in mice. Male C57BL/6J mice discriminated epibatidine (0.0032 mg/kg, subcutaneously) and were tested with agonists varying in selectivity and efficacy for α4ß2* nAChRs. The discriminative stimulus effects of epibatidine were characterized with the nonselective, noncompetitive nicotinic antagonist mecamylamine, with the selective ß2-substype-containing nAChR antagonist dihydro-ß-erythroidine hydrobromide (DHßE), and the α7 antagonist methyllycaconitine (MLA). Nicotine (0.32-1.0 mg/kg, subcutaneously), the partial nAChR agonist cytisine (1.0-5.6 mg/kg, subcutaneously), and the α7 nAChR agonist N-[(3R)-1-azabicyclo[2.2.2]oct-3-yl]-4-chlorobenzamide (10-56 mg/kg, intraperitoneally) produced no more than 33% epibatidine-appropriate responding. The partial α4ß2* nAChR agonists varenicline and 2'-fluoro-3'-(4-nitro-phenyl)deschloroepibatidine produced 61 and 69% epibatidine-appropriate responding, respectively. DHßE and mecamylamine, but not MLA, significantly antagonized the discriminative stimulus effects of epibatidine. These results show that epibatidine may be trained as a discriminative stimulus in mice and has utility in elucidating the in-vivo pharmacology of α4ß2* nAChR ligands.

Pharmacokinetics and Safety of Mitragynine in Beagle Dogs.

Mitragynine is the most abundant psychoactive alkaloid derived from the leaves of Mitragyna speciosa (kratom), a tropical plant indigenous to regions of Southeast Asia. Mitragynine displays a moderate affinity to opioid receptors, and kratom is often self-prescribed to treat pain and/or opioid addiction. The purpose of this study was to investigate the safety and pharmacokinetic properties of mitragynine in the dog. Single dose oral (5 mg/kg) and intravenous (0.1 mg/kg) pharmacokinetic studies of mitragynine were performed in female beagle dogs. The plasma concentrations of mitragynine were measured using ultra-performance liquid chromatography coupled with a tandem mass spectrometer, and the pharmacokinetic properties were analyzed using non-compartmental analysis. Following intravenous administration, mitragynine showed a large volume of distribution (Vd, 6.3 ± 0.6 L/kg) and high clearance (Cl, 1.8 ± 0.4 L/h/kg). Following oral mitragynine dosing, first peak plasma (Cmax, 278.0 ± 47.4 ng/mL) concentrations were observed within 0.5 h. A potent mu-opioid receptor agonist and active metabolite of mitragynine, 7-hydroxymitragynine, was also observed with a Cmax of 31.5 ± 3.3 ng/mL and a Tmax of 1.7 ± 0.6 h in orally dosed dogs while its plasma concentrations were below the lower limit of quantification (1 ng/mL) for the intravenous study. The absolute oral bioavailability of mitragynine was 69.6%. Administration of mitragynine was well tolerated, although mild sedation and anxiolytic effects were observed. These results provide the first detailed pharmacokinetic information for mitragynine in a non-rodent species (the dog) and therefore also provide significant information for allometric scaling and dose predictions when designing clinical studies.

The Effects of Morphine, Baclofen, and Buspirone Alone and in Combination on Schedule-Controlled Responding and Hot Plate Antinociception in Rats.

Better therapeutic options are needed for pain. Baclofen, buspirone, and morphine are characterized as having analgesic properties. However, little is known about potential interactions between analgesic effects of these drugs when combined. Furthermore, it is not known if the magnitude of these potential interactions will be similar for all drug effects. Thus, we tested the effects of these drugs alone and in combination for their capacity to produce thermal antinociception and to decrease food-maintained responding. Four male and four female Sprague-Dawley rats responded for food under a fixed-ratio 10 schedule; afterward they were immediately placed on a 52°C hot plate. Morphine, baclofen, and buspirone were examined alone and in 1:1 combinations, based upon ED50 values. Morphine and baclofen effects were evaluated with the opioid antagonist naltrexone and the GABAB antagonist (3-Aminopropyl)(diethoxymethyl)phosphinic acid (CGP35348), respectively. Morphine, baclofen, and buspirone dose dependently decreased operant responding, with the calculated ED50 values being 7.09, 3.42, and 0.57 mg/kg, respectively. The respective antinociception ED50 values were 16.15, 8.75, and 2.20 mg/kg. Analysis of 1:1 combinations showed the effects of morphine plus baclofen to decrease schedule-controlled responding and to produce thermal antinociception were synergistic. Effects of morphine plus buspirone and baclofen plus buspirone to decrease schedule-controlled responding were additive. Effects of the two combinations to produce thermal antinociception were synergistic. Naltrexone and CGP35348 antagonized the effects of morphine and baclofen, respectively. Synergistic antinociceptive effects, in conjunction with additive effects on food-maintained responding, highlight the therapeutic utility of opioid and non-opioid drug combinations.

Differential cross-tolerance to the effects of nicotinic acetylcholine receptor drugs in C57BL/6J mice following chronic varenicline.

Varenicline is a smoking cessation pharmacotherapy with a presumed mechanism of action of partial efficacy at the α4ß2 nicotinic acetylcholine receptor (nAChR); however, the extent to which daily varenicline use leads to changes in nAChR sensitivity is unclear. This study examined the consequences of daily varenicline treatment on disruptions in operant responding (i.e. rate-decreasing effects) and hypothermia induced by administration of nicotine, epibatidine, cytisine, and cocaine in C57BL/6J mice. Furthermore, mecamylamine was used to assess the involvement of nAChRs in the effects of varenicline. Mice were trained under a fixed ratio 20 of milk reinforcement, and rectal temperatures were measured after 30 min following drug-administration. Varenicline, nicotine, epibatidine, and cytisine produced dose-dependent decreases in response rate and rectal temperature. Chronic varenicline (30 mg/kg) engendered tolerance to varenicline, but more cross-tolerance to nicotine, for both disruptions in operant responding and hypothermia. Cross-tolerance only developed to the hypothermic effects of epibatidine, and no cross-tolerance developed to any effects of cytisine and cocaine. In varenicline-tolerant mice, mecamylamine did not antagonize the effects of varenicline. The varying magnitudes of tolerance and cross-tolerance among effects and drugs are indicative of a nonuniform nAChR pharmacology in vivo.

Nicotine-like discriminative stimulus effects of acetylcholinesterase inhibitors and a muscarinic receptor agonist in Rhesus monkeys.

Acetylcholinesterase (AChE) inhibitors and positive allosteric nicotinic acetylcholine receptor (nAChR) modulators are potential pharmacotherapies for nicotine dependence. Because some smoking cessation aids (e.g. varenicline) appear to work by mimicking the effects of nicotine, we used drug discrimination to examine whether AChE inhibitors and nAChR allosteric modulators mimic the effects of nicotine. Rhesus monkeys discriminated 1.78 mg/kg of nicotine s.c. under an FR5 schedule of stimulus-shock termination. Nicotine and the AChE inhibitors donepezil and galantamine dose-dependently increased responding on the nicotine-appropriate lever with ED50 values of 0.35, 0.22, and 0.77 mg/kg, respectively. Donepezil (0.56 mg/kg) produced nicotine-like effects for at least 6 h, whereas the duration of action of galantamine (1.78 mg/kg) was less than 3 h. The positive allosteric nAChR modulator PNU-120596 (up to 10 mg/kg) and midazolam (up to 1.0 mg/kg) produced no more than 22% nicotine-lever responding. Oxotremorine, a muscarinic acetylcholine receptor agonist that was used to explore the extent to which muscarinic receptor agonism might contribute to the effects of AChE inhibitors, produced 94% nicotine-lever responding (ED50 value 0.013 mg/kg). The muscarinic antagonist atropine significantly antagonized the effects of both oxotremorine and nicotine; however, the dose of atropine antagonizing oxotremorine was smaller than the dose required to antagonize nicotine. Collectively, these results suggest that AChE inhibitors can mimic the effects of nicotine by indirectly stimulating both nicotinic and muscarinic receptors. Inasmuch as some smoking cessation aids work by exerting nicotine-like effects, the current results are consistent with the potential use of AChE inhibitors as novel smoking cessation aids.

Apparent Affinity Estimates and Reversal of the Effects of Synthetic Cannabinoids AM-2201, CP-47,497, JWH-122, and JWH-250 by Rimonabant in Rhesus Monkeys.

Synthetic cannabinoids have been prohibited due to abuse liability and toxicity. Four such synthetic cannabinoids, AM-2201 ([1-(5-fluoropentyl)indol-3-yl]-naphthalen-1-ylmethanone), CP-47,497 (2-[(1R,3S)-3-hydroxycyclohexyl]-5-(2-methyloctan-2-yl)phenol), JWH-122 [(4-methylnaphthalen-1-yl)-(1-pentylindol-3-yl)methanone], and JWH-250 [2-(2-methoxyphenyl)-1-(1-pentylindol-3-yl)ethanone], were tested for their capacity to produce CB1 receptor-mediated discriminative stimulus effects in two groups of rhesus monkeys. One group (n = 4) discriminated Δ9-tetrahydrocannabinol (∆9-THC; 0.1 mg/kg i.v.), and a second group (n = 4) discriminated the cannabinoid antagonist rimonabant (1 mg/kg i.v.) while receiving 1 mg/kg/12 hours of ∆9-THC. AM-2201, JWH-122, CP-47,497, JWH-250, and ∆9-THC increased ∆9-THC lever responding. Duration of action was 1-2 hours for AM-2201, JWH-122, and JWH-250 and 4-5 hours for CP-47,497 and ∆9-THC. Rimonabant (1 mg/kg) surmountably antagonized the discriminative stimulus effects of all cannabinoid agonists; the magnitude of rightward shift was 10.6-fold for AM-2201, 10.7-fold for JWH-122, 11.0-fold for CP-47,497, and 15.7-fold for JWH-250. The respective pKB values were not significantly different: 6.61, 6.65, 6.66, and 6.83. In ∆9-THC-treated monkeys discriminating rimonabant, AM-2201 (0.1 and 0.32 mg/kg), JWH-122 (0.32 and 1 mg/kg), JWH-250 (1 and 3.2 mg/kg), and CP-47,497 (0.32, 1, and 3.2 mg/kg) produced not only rate-decreasing effects that were reversed by rimonabant, but also dose-dependent, rightward shifts in the rimonabant discrimination dose-effect function. These results show striking similarity in the CB1 receptor mechanism mediating the subjective effects of AM-2201, JWH-122, JWH-250, and CP-47,497. For products containing AM-2201 and JWH-122, a short duration of action could lead to more frequent use; moreover, inattention to differences in potency among synthetic cannabinoids could underlie unexpected toxicity. Rapid reversal of effects by intravenous rimonabant has potential value in emergency situations.

Differential antagonism and tolerance/cross-tolerance among nicotinic acetylcholine receptor agonists: scheduled-controlled responding and hypothermia in C57BL/6J mice.

The tobacco-dependence pharmacotherapies varenicline and cytisine act as partial α4ß2 nAChR agonists. However, the extent to which α4ß2 nicotinic acetylcholine receptors (nAChRs) mediate their in-vivo effects remains unclear. Nicotine, varenicline, cytisine, and epibatidine were studied in male C57BL/6J mice for their effects on rates of fixed ratio responding and rectal temperature alone and in combination with the nonselective nAChR antagonist mecamylamine and the α4ß2 nAChR antagonist dihydro-ß-erythroidine. The effects of nicotine, varenicline, cytisine, epibatidine, and cocaine were assessed before and during chronic nicotine treatment. The rate-decreasing and hypothermic effects of nicotine, varenicline, cytisine, and epibatidine were antagonized by mecamylamine (1 mg/kg), but only the effects of nicotine and epibatidine were antagonized by dihydro-ß-erythroidine (3.2 mg/kg). Chronic nicotine produced 4.7 and 5.1-fold rightward shifts in the nicotine dose-effect functions to decrease response rate and rectal temperature, respectively. Nicotine treatment decreased the potency of epibatidine to decrease response rate and rectal temperature 2.2 and 2.9-fold, respectively, and shifted the varenicline dose-effect functions 2.0 and 1.7-fold rightward, respectively. Cross-tolerance did not develop from nicotine to cytisine. These results suggest that the in-vivo pharmacology of tobacco cessation aids cannot be attributed to a single nAChR subtype; instead, multiple receptor subtypes differentially mediate their effects.

Simultaneous inhibition of fatty acid amide hydrolase and monoacylglycerol lipase shares discriminative stimulus effects with Δ9-tetrahydrocannabinol in mice.

Monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) inhibitors exert preclinical effects indicative of therapeutic potential (i.e., analgesia). However, the extent to which MAGL and FAAH inhibitors produce unwanted effects remains unclear. Here, FAAH and MAGL inhibition was examined separately and together in a Δ(9)-tetrahydrocannabinol (Δ(9)-THC; 5.6 mg/kg i.p.) discrimination assay predictive of subjective effects associated with cannabis use, and the relative contribution of N-arachidonoyl ethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) in the prefrontal cortex, hippocampus, and caudate putamen to those effects was examined. Δ(9)-THC dose-dependently increased Δ(9)-THC appropriate responses (ED50 value = 2.8 mg/kg), whereas the FAAH inhibitors PF-3845 [N-3-pyridinyl-4-[[3-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenyl]methyl]-1-piperidinecarboxamide] and URB597 [(3'-​(aminocarbonyl)[1,​1'-​biphenyl]-​3-​yl)-​cyclohexylcarbamate] or a MAGL inhibitor JZL184 [4-​nitrophenyl-​4-​(dibenzo[d][1,​3]dioxol-​5-​yl(hydroxy)methyl)piperidine-​1-​carboxylate] alone did not substitute for the Δ(9)-THC discriminative stimulus. The nonselective FAAH/MAGL inhibitors SA-57 [4-[2-(4-chlorophenyl)ethyl]-1-piperidinecarboxylic acid 2-(methylamino)-2-oxoethyl ester] and JZL195 [4-​nitrophenyl 4-​(3-​phenoxybenzyl)piperazine-​1-​carboxylate] fully substituted for Δ(9)-THC with ED50 values equal to 2.4 and 17 mg/kg, respectively. Full substitution for Δ(9)-THC was also produced by a combination of JZL184 and PF-3845, but not by a combination of JZL184 and URB597 (i.e., 52% maximum). Cannabinoid receptor type 1 antagonist rimonabant attenuated the discriminative stimulus effects of Δ(9)-THC, SA-57, JZL195, and the combined effects of JZL184 and PF-3845. Full substitution for the Δ(9)-THC discriminative stimulus occurred only when both 2-AG and AEA were significantly elevated, and the patterns of increased endocannabinoid content were similar among brain regions. Overall, these results suggest that increasing both endogenous 2-AG and AEA produces qualitatively unique effects (i.e., the subjective effects of cannabis) that are not obtained from increasing either 2-AG or AEA separately.

Full Fatty Acid Amide Hydrolase Inhibition Combined with Partial Monoacylglycerol Lipase Inhibition: Augmented and Sustained Antinociceptive Effects with Reduced Cannabimimetic Side Effects in Mice.

Inhibition of fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL), the primary hydrolytic enzymes for the respective endocannabinoids N-arachidonoylethanolamine (AEA) and 2-arachidonylglycerol (2-AG), produces antinociception but with minimal cannabimimetic side effects. Although selective inhibitors of either enzyme often show partial efficacy in various nociceptive models, their combined blockade elicits augmented antinociceptive effects, but side effects emerge. Moreover, complete and prolonged MAGL blockade leads to cannabinoid receptor type 1 (CB1) receptor functional tolerance, which represents another challenge in this potential therapeutic strategy. Therefore, the present study tested whether full FAAH inhibition combined with partial MAGL inhibition would produce sustained antinociceptive effects with minimal cannabimimetic side effects. Accordingly, we tested a high dose of the FAAH inhibitor PF-3845 (N-​3-​pyridinyl-​4-​[[3-​[[5-​(trifluoromethyl)-​2-​pyridinyl]oxy]phenyl]methyl]-​1-​piperidinecarboxamide; 10 mg/kg) given in combination with a low dose of the MAGL inhibitor JZL184 [4-nitrophenyl 4-(dibenzo[d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxylate] (4 mg/kg) in mouse models of inflammatory and neuropathic pain. This combination of inhibitors elicited profound increases in brain AEA levels (>10-fold) but only 2- to 3-fold increases in brain 2-AG levels. This combination produced significantly greater antinociceptive effects than single enzyme inhibition and did not elicit common cannabimimetic effects (e.g., catalepsy, hypomotility, hypothermia, and substitution for Δ(9)-tetrahydrocannabinol in the drug-discrimination assay), although these side effects emerged with high-dose JZL184 (i.e., 100 mg/kg). Finally, repeated administration of this combination did not lead to tolerance to its antiallodynic actions in the carrageenan assay or CB1 receptor functional tolerance. Thus, full FAAH inhibition combined with partial MAGL inhibition reduces neuropathic and inflammatory pain states with minimal cannabimimetic effects.

The discriminative stimulus effects of mecamylamine in nicotine-treated and untreated rhesus monkeys.

The extent to which chronic nicotine treatment can alter the effects of the nicotinic acetylcholine receptor antagonist mecamylamine, and whether those effects can be attenuated by nicotine have not been clearly established in the literature. Here, the discriminative stimulus effects of mecamylamine were compared between one group of rhesus monkeys receiving a continuous infusion of nicotine base (5.6 mg/kg/day subcutaneously) and another group of monkeys not receiving nicotine treatment. Both groups responded under a fixed ratio 5 schedule of stimulus-shock termination. Stimulus control was obtained at doses of 1.78 mg/kg mecamylamine in monkeys receiving continuous nicotine and 5.6 mg/kg mecamylamine in monkeys not receiving continuous nicotine treatment. Nicotine did not attenuate the discriminative stimulus effects of mecamylamine in either group. Discontinuation of continuous nicotine produced responding on the mecamylamine lever within 24 h in some but not all monkeys. This may indicate a qualitative difference in the discriminative stimulus effects of mecamylamine between groups, perhaps reflecting antagonism of nicotine and nicotine withdrawal in monkeys receiving continuous nicotine. The failure of nicotine to reverse the effects of mecamylamine is consistent with a noncompetitive interaction at nicotinic acetylcholine receptors and indicates that mecamylamine-induced withdrawal cannot be readily modified by nicotine.

Characterization of commercially available murine fibrosarcoma NCTC-2472 cells both in vitro and as a model of bone cancer pain in vivo.

For many cancer patients tumor burden negatively impacts quality of life due to associated pain onset. Neuropathic pain is commonly associated with late cancer stages, and is resultant of tumor metastasis to bone, herein referred to as cancer-induced bone pain. Given the severe impact on quality of life and clinical treatment strategies focusing on symptom management, novel therapeutics are needed to alleviate cancer-induced bone pain and/or reduce cancer burden. In the current study we characterized a commercially available murine fibrosarcoma cell line, NCTC-2472 in vitro, which can be used to assess the capacity of novel compounds to impact cellular viability. We found that dimethyl sulfoxide, a known cytotoxic agent and common drug preparation compound, significantly decreased cell viability in a dose-related manner. We then characterized the in vivo tumor development and associated pain behavior characteristics following implantation of NCTC-2472 fibrosarcoma into male and female C3H/HeJ mice. The C3H/HeJ strain was utilized as these mice are syngeneic with NCTC-2472 fibrosarcoma and their use reduces potential implantation failure. We found that tumor development in mice resulted in the development of mechanical allodynia but not thermal hyperalgesia. Gabapentin, a clinically relevant analgesic, produced dose-related mechanical allodynia reversal. These studies provide further characterization of a cancer-induced bone pain model that can be used to examine novel compounds as anti-cancer and analgesic therapeutics.