Discriminative stimulus effects of the cannabinoid CB1antagonist SR 141716A in rhesus monkeys pretreatedwith 9-tetrahydrocannabinol
Lance R. McMahon
Received: 22 February 2006 /Accepted: 29 June 2006 / Published online: 5 September 2006# Springer-Verlag 2006
AbstractRationale Drug discrimination can be used to examinetolerance and dependence in agonist-treated animals byestablishing an appropriate antagonist as a discriminativestimulus.Objective Establish intravenous SR 141716A as a discrim-inative stimulus in four rhesus monkeys pretreated with arelatively small dose of9-tetrahydrocannabinol (9-THC).Methods Rhesus monkeys received i.v. 9-THC(0.32 mg/kg) and discriminated i.v. SR 141716A (1 mg/kg) from vehicle while responding under a fixed ratio (FR) 5schedule of stimulus-shock termination.Results The discriminative stimulus effects of SR 141716Awere dose-dependent (ED50=0.33 mg/kg) and were mim-icked by the CB1 antagonist AM 251 (ED50=0.98 mg/kg),but not by a benzodiazepine (midazolam) or an N-methyl-D-aspartate antagonist (ketamine). An additional dose(0.32 mg/kg in addition to 0.32 mg/kg administered beforethe session) of 9-THC shifted the SR 141716A doseeffect curve 3-fold rightward. Omitting 9-THC before testsessions resulted in responding on the SR 141716A leverthat was attenuated by subsequent administration of 9-THC (ED50=0.13 mg/kg), CP 55940 (ED50=0.013 mg/kg),and WIN 55212-2 (ED50=0.35 mg/kg); midazolam andketamine did not attenuate responding on the SR 141716Alever. SR 141716A (1 mg/kg) shifted the 9-THC and CP55940 doseeffect curves 3.4-fold rightward; the WIN55212-2 doseeffect curve was not significantly modifiedby a dose of 1 mg/kg of SR 141716A.
Conclusions SR 141716A can be established as a discrim-inative stimulus in animals pretreated with 9-THC, andthis assay is selective for cannabinoid activity. Differentialantagonism of cannabinoids by SR 141716A might indicatethat the mechanism of action of WIN 55212-2 is notidentical to other cannabinoids. This study demonstratesthat, under the appropriate conditions, drug discriminationhas utility for examining cannabinoid dependence andwithdrawal.
Keywords Antagonist . Cannabinoid . Cannabis .
9-tetrahydrocannabinol . Drug discrimination .
Rhesus monkey . Rimonabant . SR 141716A
The CB1 antagonist SR 141716A (rimonabant) is a usefulpharmacologic probe for examining CB1 receptors thatmediate the effects of naturally occurring cannabinoids[9-tetrahydrocannabinol (9-THC) and other cannabi-noids in Cannabis sativa] and synthetic cannabinoidagonists (e.g., CP 55940 and WIN 55212-2). Cannabinoidagonists have a variety of effects in animals, e.g., theydecrease body temperature, induce catalepsy, increase heartrate, and disrupt short-term memory, and these effects areattenuated by SR 141716A (Compton et al. 1996; Huestiset al. 2001; Lichtman and Martin 1996). Moreover,cannabinoid agonists have discriminative stimulus effectsthat provide a pharmacologically selective measure ofcannabinoid activity in vivo inasmuch as cannabinoidagonists share discriminative stimulus effects with eachother and not with noncannabinoids (Balster and Prescott1992), and these effects are attenuated by SR 141716A
Psychopharmacology (2006) 188:306314DOI 10.1007/s00213-006-0500-6
L. R. McMahon (*)Department of Pharmacology,The University of Texas Health Science Center at San Antonio,7703 Floyd Curl Drive,San Antonio, TX 78229-3900, USAe-mail: email@example.com
(Jrbe et al. 2001; McMahon et al. 2005; Wiley et al.1995b). That SR 141716A attenuates the discriminativestimulus effects of cannabinoid agonists is consistent withclinical studies reporting that SR 141716A attenuates thesubjective effects of smoked marijuana (Huestis et al.2001). Collectively, most studies with SR 141716A showit to be an appropriate antagonist of the behavioral effectsof 9-THC and provide ample pharmacologic evidence tosupport the hypothesis that CB1 receptors mediate thebehavioral effects of cannabinoid agonists.
In addition to providing a highly quantitative andpharmacologically selective measure of acute drug actionin vivo, drug discrimination can be used to examine theconsequences of daily drug administration (e.g., toleranceand dependence). For example, in animals that discriminatean agonist from vehicle, the discriminative stimulus effectsof drugs can be compared before and after a period inwhich discrimination training is suspended and the trainingdrug, or a pharmacologic equivalent, is administeredchronically. This approach has been used to examinetolerance to opioids (Sannerud and Young 1987), benzo-diazepines (Pugh et al. 1992), and cannabinoids (Wiley etal. 1993). Alternatively, animals can be treated chronicallywith an agonist and trained to discriminate an antagonist;the consequences of agonist treatment are assessed bycomparing the effects of drugs in agonist-treated animals(that discriminate an antagonist) to the effects of drugs inuntreated animals that discriminate the same agonist or apharmacologic equivalent. For example, the discriminativestimulus effects of a benzodiazepine antagonist in benzo-diazepine-treated animals have been used to show thatbenzodiazepine treatment produces differential cross-toler-ance among drugs with benzodiazepine-like discriminativestimulus effects (McMahon et al. 2001). Moreover, underthe appropriate conditions, the discriminative stimuluseffects of an antagonist in agonist-treated animals can bepredictive of dependence and withdrawal.
In a previous study, SR 141716A (1 mg/kg) wasestablished as a discriminative stimulus in rhesus monkeysthat were treated daily with 9-THC, and the discrimina-tion appeared to be related to antagonism of 9-THC(McMahon and France 2003). However, there were markedindividual differences in the effects of cannabinoids,perhaps due to individual differences in the absorptionand metabolism of cannabinoids administered intramuscu-larly or subcutaneously, and limited information wascollected regarding the pharmacologic profile of the SR141716A discriminative stimulus in that study. Relative tothe intramuscular route, the time required for i.v. 9-THCto be absorbed and metabolized is more consistent acrossindividual rhesus monkeys (Perlin et al. 1985). To increasehomogeneity in the behavioral effects of cannabinoidsacross individuals, monkeys in this study received i.v. 9-
THC (0.32 mg/kg) immediately before sessions in whichthey discriminated i.v. SR 141716A (1 mg/kg) fromvehicle. A relatively small dose of 9-THC was chosenfor study so that the effects of 9-THC would beminimized on days when 9-THC was not administeredbefore test sessions. Relative to the previous study(McMahon and France 2003), discriminative stimuluseffects were more consistent among monkeys in the presentstudy, and a more comprehensive pharmacologic profilewas established by studying the CB1-selective antagonistAM 251 and the cannabinoid agonists CP 55940 and WIN55212-2, alone and in combination with SR 141716A.
Materials and methods
Subjects Four adult (two female and two male) rhesusmonkeys (Macaca mulatta) were housed individually on a14-h light/10-h dark schedule; were maintained at 95% free-feeding weight (range 5.59.4 kg) with a diet consisting ofprimate chow (High Protein Monkey Diet; Harlan Teklad,Madison, WI), fresh fruit, and peanuts; and were providedwater in the home cage. All monkeys had been previouslytrained to discriminate i.m. SR 141716A (1 mg/kg) whilereceiving s.c. 9-THC (1.12 mg/kg/day; McMahon andFrance 2003). Monkeys were maintained in accordance withthe Institutional Animal Care and Use Committee, TheUniversity of Texas Health Science Center at San Antonio,and the Guide for the Care and Use of Laboratory Animals(National Research Council 1996).
Surgery Using aseptic surgical procedures, monkeys re-ceived chronic indwelling catheters (heparin coated poly-urethane, OD=1.68 mm, ID=1.02 mm; Instech Solomon,Plymouth Meeting, PA). Upon anesthesia with ketamine(10 mg/kg i.m.) and isoflurane (1.53.0%, inhaled via facemask), a catheter was inserted into a subclavian or femoralvein and advanced 5 cm. Suture silk (coated vicryl, EthiconInc., Somerville, New Jersey) was used to anchor thecatheter to the vessel and to ligate the section of the vesselproximal to the catheter insertion. The other end of thecatheter, which passed subcutaneously and exited at themidscapular region of the back, was protected by a custom-made nylon-mesh jacket (Lomir Biomedical, Toronto, ON).
Apparatus During experimental sessions, monkeys wereseated in chairs (Model R001; Primate Products, Miami,FL) that provided restraint and were placed in ventilated,sound-attenuating chambers equipped with two responselevers and stimulus lights. Feet were placed in shoescontaining brass electrodes through which a brief electricstimulus (3 mA, 250 ms) could be delivered from an A/Cgenerator. The operant conditioning equipment was
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