Cannabinoid agonists differentially substitute for the discriminative stimulus effects of Delta(9)-tetrahydrocannabinol in C57BL/6J mice

Δ9-tetrahydrocannabinol discrimination: Effects of route of administration in mice

Background

Route of administration is an important pharmacokinetic variable in development of translationally relevant preclinical models. Humans primarily administer cannabis through smoking, vaping, and edibles. In contrast, preclinical research has historically utilized injected Δ9-tetrahydrocannabinol (THC). The present study sought to examine how route of administration affected the potency and time course of THC's discriminative stimulus properties.

Methods

Adult female and male C57BL/6 mice were trained to discriminate intraperitoneal (i.p.) THC from vehicle in a drug discrimination procedure. After discrimination was acquired, a dose-effect curve was determined for i.p., oral (p.o.), subcutaneous (s.c.), and aerosolized THC. Subsequently, the time course of effects of each route of administration was determined.

Results

THC administered i.p., p.o., s.c., or via aerosolization fully substituted for i.p. THC. The potency of THC's psychoactive effects was similar for i.p., p.o., and s.c., except that THC was more potent when administered s.c. vs p.o. in females. All routes of administration had a similar potency in both sexes. The duration of THC's psychoactive effects was similar across i.p., s.c., and p.o. routes of administration, whereas aerosolized THC produced a faster onset and shorter duration of effects compared to the other routes.

Conclusion

THC administered via multiple routes of administration, including those commonly used in preclinical research (i.p. and s.c.) and more translationally relevant routes (aerosol and p.o.), produced THC-like discriminative stimulus effects in mice trained to discriminate i.p. THC. More precise predictions of THC's effects in humans may result from use of these translationally relevant routes of administration.

Acute cannabidiol treatment enhances social interaction in adult male mice

Cannabidiol (CBD) is a non-intoxicating phytochemical from Cannabis sativa that is increasingly used to manage pain. The potential for CBD to ameliorate dimensional behavior symptoms occurring in multiple psychiatric disorders was suggested, including social interaction impairments. To test this hypothesis, adult male BTBRT+Itpr3tf/J (BTBR) mice, a model of idiopathic autism exhibiting social preference deficits and restrictive repetitive behaviors, were acutely treated with vehicle or 0.1, 1, or 10 mg/kg CBD. Social interaction preference was assessed 50 min after treatment, followed by social novelty preference at 60 min, marble burying at 75 min and social dominance at 120 min. CBD (10 mg/kg) enhanced BTBR social interaction but not social novelty preference, marble burying or dominance, with serum levels = 29 ± 11 ng/mg at 3 h post-injection. Next, acute 10 mg/kg CBD was compared to vehicle treatment in male serotonin transporter (SERT) knock-out mice, since SERT deficiency is an autism risk factor, and in their wildtype background strain controls C57BL/6J mice. CBD treatment generally enhanced social interaction preference and attenuated social novelty preference, yet neither marble burying nor dominance was affected. These findings show acute treatment with as little as 10 mg/kg purified CBD can enhance social interaction preference in male mice that are otherwise socially deficient.

Age-related changes in CB1 receptor expression and function and the behavioral effects of cannabinoid receptor ligands

Cannabinoid use has increased among aging individuals. However, little information on age-related differences in the behavioral effects of these agents is available. To explore potential differences in the behavioral effects of cannabinoids, we determined effects of Δ⁹-tetrahydrocannabinol (THC, 1-10 mg/kg) or rimonabant (0.3-3.2 mg/kg) on operant fixed-ratio responding (FR10) for food in young adult (6 months) and aged (29 months) rats. THC dose-dependently decreased responding for food. Rimonabant alone had little or no effect on responding up to 1.0 mg/kg, but disrupted responding following a 3.2 mg/kg dose. Rimonabant (1.0 mg/kg) partially antagonized response disruption by THC. These effects were similar in young adult and aged rats. However, aging has been reported to change the neurobiology of cannabinoid CB1 receptors. To confirm our rats exhibited such differences, we assessed CB1 receptor binding sites and function in six subcortical (caudate, nucleus accumbens CA1, and CA2/CA3), and three cortical regions (medial prefrontal, temporal, entorhinal) in young adult (6 months) or aged (26 months) male Lewis rats using quantitative autoradiography. CB1 receptor binding sites were reduced in cortical, but not subcortical brain regions of aged rats. CB1 receptor function, at the level of receptor-G protein interaction, was not different in any region studied. Results indicate that down-regulation of CB1 receptor binding sites observed in cortical regions of aged rats was not accompanied by a commensurate decrease in CB1 receptor-stimulated [³⁵S]GTPγS binding, suggesting a compensatory increase in receptor function in cortical areas. Together, our results provide additional evidence of age-related changes in central CB1 receptor populations. However, the functional compensation for decreased CB1 receptor binding may mitigate changes in behavioral effects of cannabinoids. With the rising use of cannabinoid-based therapeutics among aging populations, further evaluation of age-related changes in the cannabinoid system and the impact of these changes on effects of this class of drugs is warranted.

Δ9-Tetrahydrocannabinol discrimination: Effects of route of administration in rats

Cannabis users typically smoke or vape cannabis or ingest it in edibles, whereas cannabinoids are typically administered via injection in rodent research. The present study examined the effects of route of administration (ROA) of Δ⁹-tetrahydrocannabinol (THC), the primary psychoactive constituent of cannabis. Adult female and male Long Evans rats were trained to discriminate intraperitoneal (i.p.) THC from vehicle in a drug discrimination procedure. Following acquisition, dose-effect curves were determined with THC using i.p., oral (p.o.), and subcutaneous (s.c.) injection in both sexes and aerosol exposure in males only, followed by a time course with one dose for each ROA. Both sexes acquired THC discrimination in a similar number of sessions, although baseline response rates were significantly lower in females than males. THC fully substituted for the 3 mg/kg i.p. training dose across all ROA. While potencies were similar for ROA involving first-pass metabolism (i.p. and p.o.), THC potency was lower with s.c. administration. During the time course analysis, aerosol administration had the shortest latency to onset of discriminative stimulus effects and the shortest duration of effect, whereas s.c. administration had the longest duration. The results of this examination of the effects of ROA on an abuse-related effect of THC provide an empirical foundation to facilitate choice of ROA for mechanistic investigation of THC's pharmacology. Further, animal models using translationally relevant ROA may facilitate more accurate predictions of their effects in humans.

Interactive effects of (±)-trans-U50488 and its stereoisomers with cannabinoids

The adverse effects of mu opioid agonists have spurred a renewed interest in using kappa opioid receptor (KOR) agonists as analgesics. KOR agonists also have potential for development as diuretics for the treatment of edema and hypertension. Here, we evaluated the discriminative stimulus, antinociceptive, and diuretic effects of the kappa agonist (±)-trans-U-50488 and its stereoisomers (-)-(1S,2S)-U-50488 or (+)-(1R,2R)-U-50488) alone and in combination with the cannabinoid agonist (-)-CP 55,940. To establish (±)-U-50488 as a discriminative stimulus, rats (n = 12) were trained to discriminate intraperitoneal (i.p.) administration of 5.6 mg/kg of (±)-trans-U-50488 from saline under a fixed-ratio 20 (FR-20) schedule of food reinforcement. Then, antinociception was assessed using two procedures: warm water tail withdrawal and von Frey paw withdrawal. Diuretic effects were assessed in separate rats (n = 6/group). Doses of (±)-U-50488 and (-)-U-50488 that served as discriminative stimuli produced significant increases in urine output, but at lower doses than those that produced antinociception. In contrast, (+)-U-50488 alone had no discriminative stimulus or diuretic effects at the doses tested, but did produce antinociception in the von Frey assay. When three cannabinoids and morphine were tested in the (±)-U-50488 discrimination procedure to determine the similarity of these drugs' discriminative stimulus effects to those for (±)-U-50488, the rank order similarity was (-)-CP 55,940 > (-)-trans-THC > (+)-WIN 55,212-2 ≥ morphine. (-)-CP 55,940 alone (0.056 mg/kg) partially substituted for the discriminative stimulus effects of (±)-U-50488 and produced significant diuretic and antinociceptive effects. (-)-CP 55,940 in combination with (±)-U-50488 also produced a two-fold leftward shift in the discriminative stimulus curve for (±)-U-50488, and near-additive antinociception with (±)-U-50488 and (+)-U-50488. Further, the diuretic effect of (-)-CP 55,940 was enhanced by a dose of (+)-U50488, which itself did not alter urine output. These data together indicate that a combination of cannabinoid and kappa opioid agonists can enhance diuresis, but may have limited potential for serving as opioid-sparing pharmacotherapeutics for treatment of pain.

Untapped endocannabinoid pharmacological targets: Pipe dream or pipeline?

It has been established that the endogenous cannabinoid (endocannabinoid) system plays key modulatory roles in a wide variety of pathological conditions. The endocannabinoid system comprises both cannabinoid receptors, their endogenous ligands including 2-arachidonoylglycerol (2-AG), N-arachidonylethanolamine (anandamide, AEA), and enzymes that regulate the synthesis and degradation of endogenous ligands which include diacylglycerol lipase alpha (DAGL-α), diacylglycerol lipase beta (DAGL-β), fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL), α/β hydrolase domain 6 (ABHD6). As the endocannabinoid system exerts considerable involvement in the regulation of homeostasis and disease, much effort has been made towards understanding endocannabinoid-related mechanisms of action at cellular, physiological, and pathological levels as well as harnessing the various components of the endocannabinoid system to produce novel therapeutics. However, drug discovery efforts within the cannabinoid field have been slower than anticipated to reach satisfactory clinical endpoints and raises an important question into the validity of developing novel ligands that therapeutically target the endocannabinoid system. To answer this, we will first examine evidence that supports the existence of an endocannabinoid system role within inflammatory diseases, neurodegeneration, pain, substance use disorders, mood disorders, as well as metabolic diseases. Next, this review will discuss recent clinical studies, within the last 5 years, of cannabinoid compounds in context to these diseases. We will also address some of the challenges and considerations within the cannabinoid field that may be important in the advancement of therapeutics into the clinic.

Inhibition of monoacylglycerol lipase reduces nicotine reward in the conditioned place preference test in male mice

Nicotine, the primary psychoactive component in tobacco, plays a major role in the initiation and maintenance of tobacco dependence and addiction, a leading cause of preventable death worldwide. An essential need thus exists for more effective pharmacotherapies for nicotine-use cessation. Previous reports suggest that pharmacological and genetic blockade of CB1 receptors attenuate nicotine reinforcement and reward; while exogenous agonists enhanced these abuse-related behaviors. In this study, we utilized complementary genetic and pharmacologic approaches to test the hypothesis that increasing the levels of the endocannabinoid 2-arachindonoylglycerol (2-AG), will enhance nicotine reward by stimulating neuronal CB1 receptors. Contrary to our hypothesis, we found that inhibition of monoacylglycerol lipase (MAGL), the primary catabolic enzyme of 2-AG, attenuates nicotine conditioned place preference (CPP) in mice, through a non-CB1 receptor-mediated mechanism. MAGL inhibition did not alter palatable food reward or Lithium Chloride (LiCl) aversion. In support of our findings, repeated MAGL inhibition did not induce a reduction in CB1 brain receptor levels or hinder function. To explore the potential mechanism of action, we investigated if MAGL inhibition affected other fatty acid levels in our CPP paradigm. Indeed, MAGL inhibition caused a concomitant decrease in arachidonic acid (AA) levels in various brain regions of interest, suggesting an AA cascade-dependent mechanism. This idea is supported by dose-dependent attenuation of nicotine preference by the selective COX-2 inhibitors valdecoxib and LM-4131. Collectively, these findings, along with our reported studies on nicotine withdrawal, suggest that inhibition of MAGL represents a promising new target for the development of pharmacotherapies to treat nicotine dependence.

Finding order in chemical chaos - Continuing characterization of synthetic cannabinoid receptor agonists

Diversion of synthetic cannabinoids from the lab to drugs of abuse has become increasingly prevalent in recent years. Moreover, as earlier synthetic cannabinoids were banned, manufacturers introduced a new supply of novel compounds to serve as replacements. Hence, the chemical diversity of synthetic cannabinoid analogs has also rapidly increased. The present study examined 8 new synthetic cannabinoids: AM-1220, AM-2232, AM-2233, AM-679, EAM-2201, JWH-210, JHW-251, and MAM-2201. Each compound was assessed for binding affinity and functional activation of CB₁ and CB₂ receptors, and pharmacological equivalence with Δ⁹-tetrahydrocannabinol (THC) in THC drug discrimination. All compounds bound to and activated CB₁ and CB₂ receptors, although efficacy at the CB₂ receptor was reduced compared to that for the CB₁ receptor. Similarly, all compounds stimulated [³⁵S]GTPγS binding through the CB₁ receptor, and all compounds except AM-1220 and AM-2233 stimulated [³⁵S]GTPγS binding through the CB₂ receptor. Furthermore, these compounds, along with CP55,940, substituted for THC in THC drug discrimination. Rank order of potency in drug discrimination was correlated with CB₁ receptor binding affinity. Together, these results suggest that all test compounds share the THC-like subjective effects of marijuana. Interestingly, the most potent compounds in CB₁ binding in the present study were also the compounds that have been found recently in the U.S., MAM-2201, EAM-2201, JWH-210, AM-2233, and AM-1220. These results indicate that the evolution of the synthetic cannabinoid drug market may be focused toward compounds with increased potency. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Discriminative Stimulus Properties of Phytocannabinoids, Endocannabinoids, and Synthetic Cannabinoids

Psychoactive cannabinoids from the marijuana plant (phytocannabinoids), from the body (endocannabinoids), and from the research lab (synthetic cannabinoids) produce their discriminative stimulus effects by stimulation of CB1 receptors in the brain. Early discrimination work with phytocannabinoids confirmed that Δ9-tetrahydrocannabinol (Δ9-THC) is the primary psychoactive constituent of the marijuana plant, with more recent work focusing on characterization of the contribution of the major endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG), to Δ9-THC-like internal states. Collectively, these latter studies suggest that endogenous increases in both anandamide and 2-AG seem to be optimal for mimicking Δ9-THC's discriminative stimulus effects, although suprathreshold concentrations of anandamide also appear to be Δ9-THC-like in discrimination assays. Recently, increased abuse of synthetic cannabinoids (e.g., "fake marijuana") has spurred discrimination studies to inform regulatory authorities by predicting which of the many synthetic compounds on the illicit market are most likely to share Δ9-THC's abuse liability. In the absence of a reliable model of cannabinoid self-administration (specifically, Δ9-THC self-administration), cannabinoid discrimination represents the most validated and pharmacologically selective animal model of an abuse-related property of cannabinoids - i.e., marijuana's subjective effects. The influx of recent papers in which cannabinoid discrimination is highlighted attests to its continued relevance as a valuable method for scientific study of cannabinoid use and abuse.

Finding order in chemical chaos - Continuing characterization of synthetic cannabinoid receptor agonists

Diversion of synthetic cannabinoids from the lab to drugs of abuse has become increasingly prevalent in recent years. Moreover, as earlier synthetic cannabinoids were banned, manufacturers introduced a new supply of novel compounds to serve as replacements. Hence, the chemical diversity of synthetic cannabinoid analogs has also rapidly increased. The present study examined 8 new synthetic cannabinoids: AM-1220, AM-2232, AM-2233, AM-679, EAM-2201, JWH-210, JHW-251, and MAM-2201. Each compound was assessed for binding affinity and functional activation of CB₁ and CB₂ receptors, and pharmacological equivalence with Δ⁹-tetrahydrocannabinol (THC) in THC drug discrimination. All compounds bound to and activated CB₁ and CB₂ receptors, although efficacy at the CB₂ receptor was reduced compared to that for the CB₁ receptor. Similarly, all compounds stimulated [³⁵S]GTPγS binding through the CB₁ receptor, and all compounds except AM-1220 and AM-2233 stimulated [³⁵S]GTPγS binding through the CB₂ receptor. Furthermore, these compounds, along with CP55,940, substituted for THC in THC drug discrimination. Rank order of potency in drug discrimination was correlated with CB₁ receptor binding affinity. Together, these results suggest that all test compounds share the THC-like subjective effects of marijuana. Interestingly, the most potent compounds in CB₁ binding in the present study were also the compounds that have been found recently in the U.S., MAM-2201, EAM-2201, JWH-210, AM-2233, and AM-1220. These results indicate that the evolution of the synthetic cannabinoid drug market may be focused toward compounds with increased potency. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Ketamine and MAG Lipase Inhibitor-Dependent Reversal of Evolving Depressive-Like Behavior During Forced Abstinence From Alcohol Drinking

Although alcoholism and depression are highly comorbid, treatment options that take this into account are lacking, and mouse models of alcohol (ethanol (EtOH)) intake-induced depressive-like behavior have not been well established. Recent studies utilizing contingent EtOH administration through prolonged two-bottle choice access have demonstrated depression-like behavior following EtOH abstinence in singly housed female C57BL/6J mice. In the present study, we found that depression-like behavior in the forced swim test (FST) is revealed only after a protracted (2 weeks), but not acute (24 h), abstinence period. No effect on anxiety-like behavior in the EPM was observed. Further, we found that, once established, the affective disturbance is long-lasting, as we observed significantly enhanced latencies to approach food even 35 days after ethanol withdrawal in the novelty-suppressed feeding test (NSFT). We were able to reverse affective disturbances measured in the NSFT following EtOH abstinence utilizing the N-methyl D-aspartate receptor (NMDAR) antagonist and antidepressant ketamine but not memantine, another NMDAR antagonist. Pretreatment with the monoacylglycerol (MAG) lipase inhibitor JZL-184 also reduced affective disturbances in the NSFT in ethanol withdrawn mice, and this effect was prevented by co-administration of the CB1 inverse agonist rimonabant. Endocannabinoid levels were decreased within the BLA during abstinence compared with during drinking. Finally, we demonstrate that the depressive behaviors observed do not require a sucrose fade and that this drinking paradigm may favor the development of habit-like EtOH consumption. These data could set the stage for developing novel treatment approaches for alcohol-withdrawal-induced mood and anxiety disorders.

Pharmacological characterization of repeated administration of the first generation abused synthetic cannabinoid CP47,497

A series of in vivo and in vitro assays were conducted to characterize the pharmacological effects of the first generation abused synthetic cannabinoid CP47,497, a racemic bicyclic cannabinoid that is similar in structure to the potent, high-efficacy synthetic cannabinoid CP55,940. CP47,497 was less efficacious than CP55,940 in activating G-proteins and dose-dependently produced common CB1 receptor-dependent pharmacological effects (i.e. catalepsy, hypothermia, antinociception, and hypolocomotion). CP47,497 also substituted for Δ9-tetrahydrocannabinol (THC) in the mouse drug discrimination, indicating that both drugs elicited a similar interceptive stimulus. The pharmacological effects of CP47,497 underwent tolerance following repeated administration and showed cross-tolerance following repeated THC administration, further suggesting a common cannabimimetic mechanism of action. Finally, the CB1 receptor antagonist rimonabant precipitated similar magnitudes of somatic withdrawal responses in mice treated repeatedly with THC or CP47,497. Taken together, these data verify the acute cannabimimetic effects of CP47,497, and indicate tolerance and dependence following repeated administration. The assays used here provide a straightforward approach to characterize the emerging next generation of abused synthetic cannabinoids.

Preclinical studies on the reinforcing effects of cannabinoids. A tribute to the scientific research of Dr. Steve Goldberg

RATIONALE

The reinforcing effects of most abused drugs have been consistently demonstrated and studied in animal models, although those of marijuana were not, until the demonstration 15 years ago that delta-9-tetrahydrocannabinol (THC) could serve as a reinforcer in self-administration (SA) procedures in squirrel monkeys. Until then, those effects were inferred using indirect assessments.

OBJECTIVES

The aim of this manuscript is to review the primary preclinical procedures used to indirectly and directly infer reinforcing effects of cannabinoid drugs.

METHODS

Results will be reviewed from studies of cannabinoid discrimination, intracranial self-stimulation (ICSS), conditioned place preference (CPP), as well as change in levels of dopamine assessed in brain areas related to reinforcement, and finally from self-administration procedures. For each procedure, an evaluation will be made of the predictive validity in detecting the potential abuse liability of cannabinoids based on seminal papers, with the addition of selected reports from more recent years especially those from Dr. Goldberg's research group.

RESULTS AND CONCLUSIONS

ICSS and CPP do not provide consistent results for the assessment of potential for abuse of cannabinoids. However, drug discrimination and neurochemistry procedures appear to detect potential for abuse of cannabinoids, as well as several novel "designer cannabinoid drugs." Though after 15 years transfer of the self-administration model of marijuana abuse from squirrel monkeys to other species remains somewhat problematic, studies with the former species have substantially advanced the field, and several reports have been published with consistent self-administration of cannabinoid agonists in rodents.

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.

Effects of acute and repeated dosing of the synthetic cannabinoid CP55,940 on intracranial self-stimulation in mice

BACKGROUND

Synthetic cannabinoids have emerged as a significant public health concern. To increase the knowledge of how these molecules interact on brain reward processes, we investigated the effects of CP55,940, a high efficacy synthetic CB1 receptor agonist, in a frequency-rate intracranial self-stimulation (ICSS) procedure.

METHODS

The impact of acute and repeated administration (seven days) of CP55,940 on operant responding for electrical brain stimulation of the medial forebrain bundle was investigated in C57BL/6J mice.

RESULTS

CP55,940 attenuated ICSS in a dose-related fashion (ED50 (95% C.L.)=0.15 (0.12-0.18)mg/kg). This effect was blocked by the CB1 receptor antagonist rimonabant. Tolerance developed quickly, though not completely, to the rate-decreasing effects of CP55,940 (0.3mg/kg). Abrupt discontinuation of drug did not alter baseline responding for up to seven days. Moreover, rimonabant (10mg/kg) challenge did not alter ICSS responding in mice treated repeatedly with CP55,940.

CONCLUSIONS

The finding that CP55,940 reduced ICSS in mice with no evidence of facilitation at any dose is consistent with synthetic cannabinoid effects on ICSS in rats. CP55,940-induced ICSS depression was mediated through a CB1 receptor mechanism. Additionally, tolerance and dependence following repeated CP55,940 administration were dissociable. Thus, CP55,940 does not produce reward-like effects in ICSS under these conditions.

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.

Phenotypic assessment of THC discriminative stimulus properties in fatty acid amide hydrolase knockout and wildtype mice

A number of studies have examined the ability of the endogenous cannabinoid anandamide to elicit Δ(9)-tetrahydrocannabinol (THC)-like subjective effects, as modeled through the THC discrimination paradigm. In the present study, we compared transgenic mice lacking fatty acid amide hydrolase (FAAH), the enzyme primarily responsible for anandamide catabolism, to wildtype counterparts in a THC discrimination procedure. THC (5.6 mg/kg) served as a discriminative stimulus in both genotypes, with similar THC dose-response curves between groups. Anandamide fully substituted for THC in FAAH knockout, but not wildtype, mice. Conversely, the metabolically stable anandamide analog O-1812 fully substituted in both groups, but was more potent in knockouts. The CB1 receptor antagonist rimonabant dose-dependently attenuated THC generalization in both groups and anandamide substitution in FAAH knockouts. Pharmacological inhibition of monoacylglycerol lipase (MAGL), the primary catabolic enzyme for the endocannabinoid 2-arachidonoylglycerol (2-AG), with JZL184 resulted in full substitution for THC in FAAH knockout mice and nearly full substitution in wildtypes. Quantification of brain endocannabinoid levels revealed expected elevations in anandamide in FAAH knockout mice compared to wildtypes and equipotent dose-dependent elevations in 2-AG following JZL184 administration. Dual inhibition of FAAH and MAGL with JZL195 resulted in roughly equipotent increases in THC-appropriate responding in both groups. While the notable similarity in THC's discriminative stimulus effects across genotype suggests that the increased baseline brain anandamide levels (as seen in FAAH knockout mice) do not alter THC's subjective effects, FAAH knockout mice are more sensitive to the THC-like effects of pharmacologically induced increases in anandamide and MAGL inhibition (e.g., JZL184).

Synthetic cannabinoids: epidemiology, pharmacodynamics, and clinical implications

BACKGROUND

Synthetic cannabinoids (SC) are a heterogeneous group of compounds developed to probe the endogenous cannabinoid system or as potential therapeutics. Clandestine laboratories subsequently utilized published data to develop SC variations marketed as abusable designer drugs. In the early 2000s, SC became popular as "legal highs" under brand names such as Spice and K2, in part due to their ability to escape detection by standard cannabinoid screening tests. The majority of SC detected in herbal products have greater binding affinity to the cannabinoid CB1 receptor than does Δ(9)-tetrahydrocannabinol (THC), the primary psychoactive compound in the cannabis plant, and greater affinity at the CB1 than the CB2 receptor. In vitro and animal in vivo studies show SC pharmacological effects 2-100 times more potent than THC, including analgesic, anti-seizure, weight-loss, anti-inflammatory, and anti-cancer growth effects. SC produce physiological and psychoactive effects similar to THC, but with greater intensity, resulting in medical and psychiatric emergencies. Human adverse effects include nausea and vomiting, shortness of breath or depressed breathing, hypertension, tachycardia, chest pain, muscle twitches, acute renal failure, anxiety, agitation, psychosis, suicidal ideation, and cognitive impairment. Long-term or residual effects are unknown. Due to these public health consequences, many SC are classified as controlled substances. However, frequent structural modification by clandestine laboratories results in a stream of novel SC that may not be legally controlled or detectable by routine laboratory tests.

METHODS

We present here a comprehensive review, based on a systematic electronic literature search, of SC epidemiology and pharmacology and their clinical implications.

Novel 3-substituted rimonabant analogues lack Δ(9) -tetrahydrocannabinol-like abuse-related behavioural effects in mice

BACKGROUND AND PURPOSE

Previous structure-activity relationship studies with analogues of the CB1 receptor antagonist rimonabant have demonstrated that a subset of these analogues with 3-substituent replacements of rimonabant's pyrazole core displayed cannabimimetic profiles seemingly independent of CB1 receptors. We sought to further evaluate these analogues in several behavioural models sensitive to detecting THC-like abuse liability.

EXPERIMENTAL APPROACH

Selected analogues were tested in a battery of tests in mice to replicate previous findings. Cross-generalization tests were conducted in mice trained to discriminate either THC or O-6629 from vehicle. Rimonabant and its analogues were also evaluated in substitution and challenge tests. Finally, development of cross-tolerance between THC and O-6211 in the mouse test battery was assessed.

KEY RESULTS

O-6629 and O-6658 produced dose-dependent acute cannabimimetic activity in mice, but neither substituted for nor antagonized THC's discriminative stimulus. Cross-substitution was observed with O-6658 in mice discriminating O-6629, whereas rimonabant neither substituted for nor attenuated the O-6629 discriminative stimulus. THC and morphine did not generate O-6629-like responding. Cross-tolerance did not develop in mice repeatedly treated with THC when tested with O-6211 in the mouse test battery.

CONCLUSIONS AND IMPLICATIONS

While some overlap exists between the pharmacological profiles of THC and these 3-substituent rimonabant analogues, the effects are mediated by distinct neural targets. Notably, these analogues are unlikely to possess marijuana-like abuse liability in humans, but general abuse liability has not yet been determined. Efforts to determine the mechanism(s) of action of this seemingly unique class of compounds are underway.

JWH-018 in rhesus monkeys: differential antagonism of discriminative stimulus, rate-decreasing, and hypothermic effects

Several effects of the abused synthetic cannabinoid JWH-018 were compared to those of Δ9-tetrahydrocannabinol (Δ9-THC) in rhesus monkeys. JWH-018 (0.1 mg/kg i.v.) was established as a discriminative stimulus and rimonabant was used to examine mechanisms responsible for discrimination as well as operant response rate-decreasing and hypothermic effects. JWH-018 dose-dependently increased drug-lever responding (ED50=0.01 mg/kg) and decreased response rate (ED50=0.064 mg/kg). Among various cannabinoids, the relative potency for producing discriminative stimulus and rate-decreasing effects was the same: CP-55940=JWH-018>Δ9-THC=WIN-55212-2=JWH-073. The benzodiazepine agonist midazolam and the NMDA antagonist ketamine did not exert JWH-018 like discriminative stimulus effects up to doses that disrupted responding. JWH-018 and Δ9-THC decreased rectal temperature by 2.2 and 2.8°C, respectively; the doses decreasing temperature by 2°C were 0.21 and 1.14 mg/kg, respectively. Antagonism did not differ between JWH-018 and Δ9-THC, but did differ among effects. The apparent affinities of rimonabant calculated in the presence of JWH-018 and Δ9-THC were not different from each other for antagonism of discriminative stimulus effects (6.58 and 6.59, respectively) or hypothermic effects (7.08 and 7.19, respectively). Apparent affinity estimates are consistent with the same receptors mediating the discriminative stimulus and hypothermic effects of both JWH-018 and Δ9-THC. However, there was more limited and less orderly antagonism of rate-decreasing effects, suggesting that an additional receptor mechanism is involved in mediating the effects of cannabinoids on response rate. Overall, these results strongly suggest that JWH-018 and Δ9-THC act at the same receptors to produce several of their shared psychopharmacological effects.

Endocannabinoid contribution to Δ9-tetrahydrocannabinol discrimination in rodents

The mechanism through which marijuana produces its psychoactive effects is Δ(9)-tetrahydrocannabinol (THC)-induced activation of cannabinoid CB1 receptors. These receptors are normally activated by endogenous lipids, including anandamide and 2-arachidonoyl glycerol (2-AG). A logical "first step" in determination of the role of these endocannabinoids in THC׳s psychoactive effects is to investigate the degree to which pharmacologically induced increases in anandamide and/or 2-AG concentrations through exogenous administration and/or systemic administration of inhibitors of their metabolism, fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL), respectively, share THC׳s discriminative stimulus effects. To this end, adult male mice and rats were trained to discriminate THC (5.6 and 3mg/kg, respectively). In Experiment 1, exogenous administration of anandamide or 2-AG did not substitute for THC in mice nor was substitution enhanced by co-administration of the FAAH or MAGL inhibitors, URB597 and N-arachidonyl maleimide (NAM), respectively. Significant decreases in responding may have prevented assessment of adequate endocannabinoid doses. In mice trained at higher baseline response rates (Experiment 2), the FAAH inhibitor PF3845 (10mg/kg) enhanced anandamide substitution for THC without producing effects of its own. The MAGL inhibitor JZL184 increased brain levels of 2-AG in vitro and in vivo, increased THC-like responding without co-administration of 2-AG. In rats, neither URB597 nor JZL184 engendered significant THC-appropriate responding, but co-administration of these two enzyme inhibitors approached full substitution. The present results highlight the complex interplay between anandamide and 2-AG and suggest that endogenous increases of both endocannabinoids are most effective in elicitation of THC-like discriminative stimulus effects.

Differentiation between low- and high-efficacy CB1 receptor agonists using a drug discrimination protocol for rats

RATIONALE

The "subjective high" from marijuana ingestion is likely due to Δ(9)-tetrahydrocannabinol (THC) activating the central cannabinoid receptor type 1 (CB1R) of the endocannabinoid signaling system. THC is a weak partial agonist according to in vitro assays, yet THC mimics the behavioral effects induced by more efficacious cannabinergics. This distinction may be important for understanding similarities and differences in the dose-effect spectra produced by marijuana/THC and designer cannabimimetics ("synthetic marijuana").

OBJECTIVE

We evaluated if drug discrimination is able to functionally detect/differentiate between a full, high-efficacy CB1R agonist [(±)AM5983] and the low-efficacy agonist THC in vivo.

MATERIALS AND METHODS

Rats were trained to discriminate between four different doses of AM5983 (0.10 to 0.56 mg/kg), and vehicle and dose generalization curves were determined for both ligands at all four training doses of AM5983. The high-efficacy WIN55,212-2 and the lower-efficacy (R)-(+)-methanandamide were examined at some AM5983 training conditions. Antagonism tests involved rimonabant and WIN55,212-2 and AM5983. The separate (S)- and (R)-isomers of (±)AM5983 were tested at one AM5983 training dose (0.30 mg/kg). The in vitro cyclic adenosine monophosphate (cAMP) assay examined AM5983 and the known CB1R agonist CP55,940.

RESULTS

Dose generalization ed50 values increased as a function of the training dose of AM5983, but more so for the partial agonists. The order of potency was (R)-isomer > (±)AM5983 > (S)-isomer and AM5983 > WIN55,212-2 ≥ THC > (R)-(+)-methanandamide. Surmountable antagonism of AM5983 and WIN55,212-2 occurred with rimonabant. The cAMP assay confirmed the cannabinergic nature of AM5983 and CP55,940.

CONCLUSIONS

Drug discrimination using different training doses of a high-efficacy, full CB1R agonist differentiated between low- and high-efficacy CB1R agonists.

Differential drug-drug interactions of the synthetic Cannabinoids JWH-018 and JWH-073: implications for drug abuse liability and pain therapy

Marijuana substitutes often contain blends of multiple psychoactive synthetic cannabinoids (SCBs), including the prevalent SCBs (1-pentyl-1H-indole-3-yl)-1-naphthalenyl-methanone (JWH-018) and (1-butyl-1H-indole-3-yl)-1-naphthalenyl-methanone (JWH-073). Because SCBs are frequently used in combinations, we hypothesized that coadministering multiple SCBs induces synergistic drug-drug interactions. Drug-drug interactions between JWH-018 and JWH-073 were investigated in vivo for Δ(9)-tetrahydrocannabinol (Δ(9)-THC)-like discriminative stimulus effects, analgesia, task disruption, and hypothermia. Combinations (JWH-018:JWH-073) of these drugs were administered to mice in assays of Δ(9)-THC discrimination, tail-immersion, and food-maintained responding, and rectal temperatures were measured. Synergism occurred in the Δ(9)-THC discrimination assay for two constant dose ratio combinations (1:3 and 1:1). A 1:1 and 2:3 dose ratio induced additivity and synergy, respectively, in the tail-immersion assay. Both 1:1 and 2:3 dose ratios were additive for hypothermia, whereas a 1:3 dose ratio induced subadditive suppression of food-maintained responding. In vitro drug-drug interactions were assessed using competition receptor-binding assays employing mouse brain homogenates and cannabinoid 1 receptor (CB1R)-mediated inhibition of adenylyl cyclase activity in Neuro2A wild-type cells. Interestingly, synergy occurred in the competition receptor-binding assay for two dose ratios (1:5 and 1:10), but not in the adenylyl cyclase activity assay (1:5). Altogether, these data indicate that drug-drug interactions between JWH-018 and JWH-073 are effect- and ratio-dependent and may increase the relative potency of marijuana substitutes for subjective Δ(9)-THC-like effects. Combinations may improve the therapeutic profile of cannabinoids, considering that analgesia but not hypothermia or task disruption was potentiated. Importantly, synergy in the competition receptor-binding assay suggests multiple CB1R-SCB binding sites.

Cannabinoids in disguise: Δ9-tetrahydrocannabinol-like effects of tetramethylcyclopropyl ketone indoles

Synthetic indole-derived cannabinoids have become commonly used recreational drugs and continue to be abused despite their adverse consequences. As compounds that were identified early in the epidemic (e.g., naphthoylindoles) have become legally banned, new compounds have appeared on the drug market. Two tetramethylcyclopropyl ketone indoles, UR-144 [(1-pentyl-1H-indol-3-yl)-(2,2,3,3-tetramethylcyclopropyl)methanone] and XLR-11 [(1-(5-fluoropentyl)-1H-indol-3-yl)-(2,2,3,3-tetramethylcyclopropyl)methanone], recently have been identified in confiscated products. These compounds are structurally related to a series of CB2-selective compounds explored by Abbott Labs. The purpose of the present study was to evaluate the extent to which UR-144 and XLR-11 shared cannabinoid effects with Δ9-tetrahydrocannabinol (Δ9-THC). Indices of in vitro and in vivo activity at cannabinoid receptors were assessed. Similar to other psychoactive cannabinoid agonists, XLR-11 and UR-144 showed low nanomolar (<30) affinity for CB1 and CB2 receptors, activated these receptors as full agonists, and produced dose-dependent effects that were blocked by rimonabant in mice, including antinociception, hypothermia, catalepsy and suppression of locomotor activity. The potency of both compounds was several-fold greater than Δ9-THC. XLR-11 and UR-144 also substituted for Δ9-THC in a Δ9-THC discrimination procedure in mice, effects that were attenuated by rimonabant. Analysis of urine from mice treated with the compounds revealed that both were extensively metabolized, with predominant urinary excretion as glucuronide conjugates. Together, these results demonstrate that UR-144 and XLR-11 share a pharmacological profile of in vitro and in vivo effects with Δ9-THC and other abused indole-derived cannabinoids and would be predicted to produce Δ9-THC-like subjective effects in humans.

Inhibition of FAAH and activation of PPAR: new approaches to the treatment of cognitive dysfunction and drug addiction

Enhancing the effects of endogenously-released cannabinoid ligands in the brain might provide therapeutic effects more safely and effectively than administering drugs that act directly at the cannabinoid receptor. Inhibitors of fatty acid amide hydrolase (FAAH) prevent the breakdown of endogenous ligands for cannabinoid receptors and peroxisome proliferator-activated receptors (PPAR), prolonging and enhancing the effects of these ligands when they are naturally released. This review considers recent research on the effects of FAAH inhibitors and PPAR activators in animal models of addiction and cognition (specifically learning and memory). These studies show that FAAH inhibitors can produce potentially therapeutic effects, some through cannabinoid receptors and some through PPAR. These effects include enhancing certain forms of learning, counteracting the rewarding effects of nicotine and alcohol, relieving symptoms of withdrawal from cannabis and other drugs, and protecting against relapse-like reinstatement of drug self-administration. Since FAAH inhibition might have a wide range of therapeutic actions but might also share some of the adverse effects of cannabis, it is noteworthy that at least one FAAH-inhibiting drug (URB597) has been found to have potentially beneficial effects but no indication of liability for abuse or dependence. Although these areas of research are new, the preliminary evidence indicates that they might lead to improved therapeutic interventions and a better understanding of the brain mechanisms underlying addiction and memory.

JWH-018 and JWH-073: Δ⁹-tetrahydrocannabinol-like discriminative stimulus effects in monkeys

Products containing naphthalen-1-yl-(1-pentylindol-3-yl) methanone (JWH-018) and naphthalen-1-yl-(1-butylindol-3-yl) methanone (JWH-073) are emerging drugs of abuse. Here, the behavioral effects of JWH-018 and JWH-073 were examined in one behavioral assay selective for cannabinoid agonism, rhesus monkeys (n = 4) discriminating Δ⁹-tetrahydrocannabinol (Δ⁹-THC; 0.1 mg/kg i.v.), and another assay sensitive to cannabinoid withdrawal, i.e., monkeys (n = 3) discriminating the cannabinoid antagonist rimonabant (1 mg/kg i.v.) during chronic Δ⁹-THC (1 mg/kg s.c. 12 h) treatment. Δ⁹-THC, JWH-018, and JWH-073 increased drug-lever responding in monkeys discriminating Δ⁹-THC; the ED₅₀ values were 0.044, 0.013, and 0.058 mg/kg, respectively and the duration of action was 4, 2, and 1 h, respectively. Rimonabant (0.32-3.2 mg/kg) produced surmountable antagonism of Δ⁹-THC, JWH-018, and JWH-073. Schild analyses and single-dose apparent affinity estimates yielded apparent pA₂/pK(B) values of 6.65, 6.68, and 6.79 in the presence of Δ⁹-THC, JWH-018, and JWH-073, respectively. In Δ⁹-THC-treated monkeys discriminating rimonabant, the training drug increased responding on the rimonabant lever; the ED₅₀ value of rimonabant was 0.20 mg/kg. Δ⁹-THC (1-10 mg/kg), JWH-018 (0.32-3.2 mg/kg), and JWH-073 (3.2-32 mg/kg) dose-dependently attenuated the rimonabant-discriminative stimulus (i.e., withdrawal). These results suggest that Δ⁹-THC, JWH-018, and JWH-073 act through the same receptors to produce Δ⁹-THC-like subjective effects and attenuate Δ⁹-THC withdrawal. The relatively short duration of action of JWH-018 and JWH-073 might lead to more frequent use, which could strengthen habitual use by increasing the frequency of stimulus-outcome pairings. This coupled with the possible greater efficacy of JWH-018 at cannabinoid 1 receptors could be associated with greater dependence liability than Δ⁹-THC.

Cannabinergic aminoalkylindoles, including AM678=JWH018 found in 'Spice', examined using drug (Δ(9)-tetrahydrocannabinol) discrimination for rats

We examined four different cannabinergic aminoalkylindole ligands, including one drug (AM678=JWH018) found in herbal 'Spice' concoctions, for their ability to substitute for Δ(9)-tetrahydrocannabinol (THC), and the ability of the cannabinoid receptor 1-selective antagonist/inverse agonist rimonabant to block the substitution, 30 and 90 min after intraperitoneal injection. Rats trained to discriminate the effects of vehicle from those produced by 3 mg/kg of THC were used. The order of potency was: AM5983≥AM678>AM2233>WIN55212-2 at both test intervals. AM5983 and AM678 appeared eight times more potent than THC, followed by AM2233 (about twice as potent as THC), and WIN55212-2 approximately THC at the 30-min test interval. The aminoalkylindoles showed reduced potency (i.e. an increased ED50 value) at the longer injection-to-test interval of 90 min compared with testing at 30 min. The rightward shifts by coadministration of rimonabant were approximately 8-fold to 12-fold for AM5983 and AM678, compared with an approximately 3-fold rightward shift for the WIN55212-2 curve. AM2233 (1.8 mg/kg) substitution was also blocked by 1 mg/kg of rimonabant. In conclusion, AM5983 and AM678=JWH018 are potent cannabimimetics derived from an aminoalkylindole template. WIN55212-2 seemed to interact differently with rimonabant, compared with either AM5983 or AM678, indicating potential differences in the mechanism(s) of action among cannabinergic aminoalkylindoles.

Dissimilar cannabinoid substitution patterns in mice trained to discriminate Δ(9)-tetrahydrocannabinol or methanandamide from vehicle

Δ(9)-Tetrahydrocannabinol (THC) discrimination in rodents is a behavioral assay that has been used to probe differences among classes of cannabinoids in rats. The purpose of this study was to determine whether traditional and anandamide-like cannabinoids were distinguishable in cannabinoid discrimination procedures in mice. Male mice were trained to discriminate 30 mg/kg THC or 70 mg/kg methanandamide from vehicle in a two-lever milk-reinforced drug discrimination procedure. After acquisition, agonist tests with THC, methanandamide, CP 55940, and anandamide were conducted, as were antagonism tests with rimonabant. Substitution (agonism) and antagonism tests were also carried out in female mice trained to discriminate THC. THC and CP 55940 fully substituted in THC-trained mice of both sexes. Further, THC substitution was rimonabant reversible. In contrast, mice injected with methanandamide or anandamide failed to respond substantially on the THC lever, even up to doses that decreased overall responding. In methanandamide-trained mice, methanandamide fully generalized to the methanandamide training dose. Rimonabant did not reverse this generalization. Although THC, CP 55940, and anandamide also increased responding on the methanandamide lever, the magnitude of substitution was less than for methanandamide. These results suggest incomplete overlap in the underlying mechanisms mediating endocannabinoid pharmacology and marijuana intoxication. Further, they suggest that methanandamide discrimination may involve a non-CB(1) receptor mechanism that is particularly prominent at higher doses.

Enhancement of the behavioral effects of endogenous and exogenous cannabinoid agonists by phenylmethyl sulfonyl fluoride

Marijuana's effects in humans are most often reported as intoxicating or therapeutic; yet, some humans report dysphoria or other negative affect. To evaluate whether differences in endocannabinoid levels might account for this variability, the present study examined whether sensitivity to cannabinoids changed when anandamide (AEA) metabolism was inhibited through administration of phenylmethyl sulfonyl fluoride (PMSF) a non-specific irreversible amidase inhibitor. Male Long Evans rats were trained to discriminate 3 mg/kg Δ(9)-tetrahydrocannabinol (THC) versus vehicle in 2-lever drug discrimination procedure. ED(50)s for THC and CP 55,940 were lower when administered with PMSF than alone. PMSF administration also potentiated characteristic cannabimimetic effects of THC in ICR mice. Potentiation of AEA's in vivo effects by PMSF were also observed, primarily as a consequence of PMSF inhibition of the enzyme fatty acid amide hydrolase. Enhancement of the effects of THC and CP 55,940 through this mechanism is unlikely, as these cannabinoids are predominantly metabolized through the P450 system. Mass spectrometry revealed that, in the presence of THC, endogenous AEA levels in the brain decreased and that this decrease was prevented by PMSF, suggesting that increased AEA levels may have acted additively with exogenously administered cannabinoids to increase cannabimimetic effects. These findings may account for the varying affect in response to marijuana in humans or cannabinoids in animals while also suggesting that metabolic inhibitors of AEA may potentiate marijuana's intoxicating effects in humans. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Central mediation and differential blockade by cannabinergics of the discriminative stimulus effects of the cannabinoid CB1 receptor antagonist rimonabant in rats

RATIONALE

Discovery of an endocannabinoid signaling system launched the development of the blocker rimonabant, a cannabinoid CB1 receptor (CB(1)R) antagonist/inverse agonist. Due to untoward effects, this medication was withdrawn and efforts have been directed towards discovering chemicals with more benign profiles.

OBJECTIVE

This study aims to comparatively evaluate new ligands using a rimonabant discriminated drinking aversion procedure.

METHODS

Rats discriminated between rimonabant (5.6 mg/kg) and vehicle. The 30 min saccharin (0.1%) drinking after rimonabant pretreatment was followed by injection of lithium chloride (120 mg/kg) in the experimental animals. After vehicle pretreatment, experimental animals were given i.p. NaCl (10 ml/kg). Postdrinking treatment for controls was NaCl, irrespective of pretreatment condition (rimonabant or vehicle).

RESULTS

The centrally acting neutral CB(1)R antagonist AM4113, but not the limited brain penetrating CB(1)R neutral antagonist AM6545, substituted for rimonabant. The CB(1)R agonists THC (1-10 mg/kg), AM1346 (1-10 mg/kg) did not substitute. The rimonabant-induced conditioned suppression of saccharin drinking was attenuated when CB(1)R agonists AM5983 (0.01-1 mg/kg) and THC (10 mg/kg), but not the CB(1)R agonist AM1346 (0.1-18 mg/kg), were combined with rimonabant (5.6 mg/kg). By varying the injection-to-test interval, we gauged the relative duration of the cueing effects of rimonabant, and the in vivo functional half-life was estimated to be approximately 1.5 h.

CONCLUSION

A neutral CB(1)R antagonist (AM4113) produced cueing effects similar to those of rimonabant and generalization likely was centrally mediated. The functional cueing effects of rimonabant are relatively short-acting, pharmacologically selective, and differentially blocked by cannabinergics.

Endocannabinoid influence in drug reinforcement, dependence and addiction-related behaviors

The endogenous cannabinoid system is an important regulatory system involved in physiological homeostasis. Endocannabinoid signaling is known to modulate neural development, immune function, metabolism, synaptic plasticity and emotional state. Accumulating evidence also implicates brain endocannabinoid signaling in the etiology of drug addiction which is characterized by compulsive drug seeking, loss of control in limiting drug intake, emergence of a negative emotional state in the absence of drug use and a persistent vulnerability toward relapse to drug use during protracted abstinence. In this review we discuss the effects of drug intake on brain endocannabinoid signaling, evidence implicating the endocannabinoid system in the motivation for drug consumption, and drug-induced alterations in endocannabinoid function that may contribute to various aspects of addiction including dysregulated synaptic plasticity, increased stress responsivity, negative affective states, drug craving and relapse to drug taking. Current knowledge of genetic variants in endocannabinoid signaling associated with addiction is also discussed.

The endogenous cannabinoid anandamide shares discriminative stimulus effects with ∆(9)-tetrahydrocannabinol in fatty acid amide hydrolase knockout mice

The endogenous cannabinoid system has been noted for its therapeutic potential, as well as the psychoactivity of cannabinoids such as Δ9-tetrahydrocannabinol (THC). However, less is known about the psychoactivity of anandamide (AEA), an endocannabinoid ligand. Thus, the goals of this study were to establish AEA as a discriminative stimulus in transgenic mice lacking fatty acid amide hydrolase (i.e., FAAH -/- mice unable to rapidly metabolize AEA), evaluate whether THC or oleamide, a fatty acid amide, produced AEA-like responding, and assess for CB(1) mediation of AEA's discriminative stimulus. Mice readily discriminated between 6mg/kg AEA and vehicle in a two-lever drug discrimination task. AEA dose-dependently generalized to itself. THC elicited full AEA-like responding, whereas oleamide failed to substitute. The CB(1) antagonist rimonabant attenuated AEA- and THC-induced AEA-appropriate responding, demonstrating CB(1) mediation of AEA's discriminative stimulus. These findings suggest that, in the absence of FAAH, AEA produces intoxication comparable to THC, and consequently to marijuana.

Structural and pharmacological analysis of O-2050, a putative neutral cannabinoid CB(1) receptor antagonist

Rimonabant, the prototypic antagonist of cannabinoid CB(1) receptors, has been reported to have inverse agonist properties at higher concentrations, which may complicate its use as a tool for mechanistic evaluation of cannabinoid pharmacology. Consequently, recent synthesis efforts have concentrated on discovery of a neutral antagonist using a variety of structural templates. The purpose of this study was to evaluate the pharmacological properties of the putative neutral cannabinoid CB(1) receptor antagonist O-2050, a sulfonamide side chain analog of Δ(8)-tetrahydrocannabinol. O-2050 and related sulfonamide cannabinoids exhibited good affinity for both cannabinoid CB(1) and CB(2) receptors. While the other sulfonamide analogs produced cannabinoid agonist effects in vivo (e.g., activity suppression, antinociception, and hypothermia), O-2050 stimulated activity and was inactive in the other two tests. O-2050 also decreased food intake in mice, an effect that was reminiscent of that produced by rimonabant. Unlike rimonabant, however, O-2050 did not block the effects of cannabinoid agonists in vivo, even when administered i.c.v. In contrast, O-2050 antagonized the in vitro effects of cannabinoid agonists in [(35)S]GTPγS and mouse vas deferens assays without having activity on its own in either assay. Further evaluation revealed that O-2050 fully and dose-dependently substituted for Δ(9)-tetrahydrocannabinol in a mouse drug discrimination procedure (a cannabinoid agonist effect) and that it inhibited forskolin-stimulated cyclic AMP signaling with a maximum efficacy of approximately half that of the full agonist CP55,940 [(-)-cis-3-[2-hydroxy-4(1,1-dimethyl-heptyl)phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol]. Together, these results suggest that O-2050 is not a viable candidate for classification as a neutral cannabinoid CB(1) receptor antagonist.

Antagonism of ∆⁹-THC induced behavioral effects by rimonabant: time course studies in rats

The objective was to examine the time course of the cannabinoid 1 receptor antagonist/inverse agonist rimonabant's ability to antagonize in vivo cannabinergic agonist effects. We used two behavioral procedures sensitive to the effects of ∆⁹-tetrahydrocannabinol (∆⁹-THC): rat drug discrimination (EXP-1) and suppression of fixed-ratio responding (FR) for food reinforcement (EXP-2). Two training doses of ∆⁹-THC (1.8 and 3 mg/kg) served as discriminative cues in two groups discriminating ∆⁹-THC from vehicle; injections were i.p. 20 min before session onset. Tests assessed the dose-response functions of ∆⁹-THC and the time course for rimonabant in its ability to block the discriminative stimulus effects of ∆⁹-THC. For antagonism testing, the training doses of ∆⁹-THC were used and the rimonabant dose was 1mg/kg. Tests were 20, 60, 120, and 240 min post rimonabant administration; ∆⁹-THC was always administered 20 min prior to testing. For EXP-2, only one response lever was activated and every 10th (FR-10) press on that lever resulted in food delivery. Once the response rate stabilized, tests occurred with ∆⁹-THC, rimonabant and combinations of the drugs. The ED(50) estimates for the dose-response functions were 0.38 (±0.28-0.51) and 0.50 (±0.40-0.63) mg/kg for the training doses of 1.8 and 3 mg/kg ∆⁹-THC, respectively. The time course studies suggested functional half-life estimates of 128.4 (±95.7-172.2) and 98.4 (±64.2-150.7) min by rimonabant for the two groups in EXP-1, respectively. Similarly, the functional half-life of rimonabant was 118.9 (±66.1-213.9) min in EXP-2. Thus, antagonism of ∆⁹-THC by rimonabant is relatively short lasting.

Discriminative stimulus functions of methanandamide and delta(9)-THC in rats: tests with aminoalkylindoles (WIN55,212-2 and AM678) and ethanol

OBJECTIVE

The aim of the study was to characterize in vivo the aminoalkylindoles WIN55,212-2 (WIN) and AM678 (naphthalen-1-yl(1-pentyl-1H-indol-3-yl)methanone) as cannabinoid receptor (CB(1)R) ligands using drug discrimination. Tests also involved delta(9)-tetrahydrocannabinol (THC) and R-(+)-methanandamide (mAEA), a metabolically stable analog of the endogenous ligand anandamide, as well as the CB(1)R selective antagonist/inverse agonist rimonabant; tests with ethanol assessed pharmacological specificity. We used two different drug discriminations (mAEA and THC) allowing us to explore potential differences in CB(1)R activation which could be attributed to variations in their respective CB(1)R signaling mechanisms.

METHODS

There were two concurrently trained groups of rats. One group discriminated between i.p. injected vehicle and 10 mg/kg mAEA. The other group was trained to discriminate between vehicle and 1.8 mg/kg THC.

RESULTS

Dose generalization curves for AM678, WIN55,212-2, THC, and mAEA suggested the following rank order of potency: AM678 > WIN55,212-2 > or = THC > mAEA in both drug discrimination groups. Challenge by 1 mg/kg rimonabant resulted in shifts to the right of the generalization curves for the two aminoalkylindoles (4.4-fold for AM678 and 11.3-fold for WIN in the mAEA group, whereas for the THC group, the corresponding values were 13 and 2.6, respectively), suggesting surmountable antagonism. Ethanol did not generalize in either of the two groups, suggesting pharmacological specificity.

CONCLUSION

Data are congruent with the general observation that there is substantial overlap in the discriminative stimulus effects of CB(1)R ligands across different chemical classes. However, the quantitative differences in the interactions between the two aminoalkylindoles and rimonabant in the two discrimination groups suggest subtle variations in the ligand-receptor activation(s).

In vivo pharmacology of endocannabinoids and their metabolic inhibitors: therapeutic implications in Parkinson's disease and abuse liability

This review focuses on the behavioral pharmacology of endogenous cannabinoids (endocannabinoids) and indirect-acting cannabinoid agonists that elevate endocannabinoid tone by inhibiting the activity of metabolic enzymes. Similarities and differences between prototype cannabinoid agonists, endocannabinoids and inhibitors of endocannabinoid metabolism are discussed in the context of endocannabinoid pharmacokinetics in vivo. The distribution and function of cannabinoid and non-CB(1)/CB(2) receptors are also covered, with emphasis on their role in disorders characterized by dopamine dysfunction, such as drug abuse and Parkinson's disease. Finally, evidence is presented to suggest that FAAH inhibitors lack the abuse liability associated with CB(1) agonists, although they may modify the addictive properties of other drugs, such as alcohol.

Discriminative stimulus properties of delta9-tetrahydrocannabinol (THC) in C57Bl/6J mice

Primarily, rats have served as subjects in Delta(9)-tetrahydrocannabinol's (THC) discrimination studies although other species such as monkeys and pigeons have been used. While the introduction of the knockout and transgenic mice has vastly stimulated the study of the discriminative stimulus effects of drugs there is only a single published report of mice trained to discriminate THC. Thus, this study extended those results by providing a systematic replication that THC serves as an effective discriminative stimulus in mice and by further investigating the mechanisms of action involved in the THC discrimination model in the mouse. Male C57BL/6J mice were trained to discriminate 10 mg/kg THC from vehicle in 2-lever drug discrimination. THC fully and dose dependently substituted for itself. Cannabinoid indoles, except one with low cannabinoid CB(1) receptor affinity, substituted for THC. Anandamide failed to substitute for THC when administered alone but completely substituted when administered with the non-specific fatty acid amide hydrolase inhibitor, phenylmethylsulphonyl fluoride. As expected, nicotine failed to substitute for THC. Lastly, the cannabinoid CB(1) receptor antagonist rimonabant blocked THC's discriminative stimulus effects. Taken together these studies demonstrate THC's ability to produce discriminative stimulus effects as well as demonstrate its pharmacological specificity and mechanism of action in a two-lever drug discrimination mouse model.

Discriminative stimulus functions in rats of AM1346, a high-affinity CB1R selective anandamide analog

OBJECTIVE

To characterize in vivo the high-affinity CB(1) cannabinoid receptor (CB(1)R) selective anandamide analog AM1346 [alkoxyacid amide of N-eicosa-tetraenylamine] using drug discrimination. Substitution tests involved Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and R-(+)-methanandamide (mAEA), a metabolically stable analog of anandamide (AEA), as well as the CB(1)R antagonist/inverse agonist rimonabant; D: -amphetamine and morphine were also examined to assess pharmacological specificity.

MATERIALS AND METHODS

Rats were initially trained to discriminate between i.p.-injected vehicle and 3 mg/kg AM1346 (group 3 mg/kg; t' = 20 min); subsequently, the rats were retrained with 5.6 mg/kg AM1346 (group 5.6 mg/kg; t' = 20 min).

RESULTS

Dose-generalization curves of AM1346, Delta(9)-THC, and mAEA suggested the following order of potency: Delta(9)-THC > AM1346 > mAEA both for rats discriminating between 3 and 5.6 mg/kg AM1346 from vehicle. In group 3 mg/kg, challenge by 1 mg/kg rimonabant resulted in parallel shifts to the right of the dose-generalization curves for Delta(9)-THC and AM1346, suggesting surmountable antagonism. Surmountable antagonism was not demonstrated with rimonabant-mAEA combinations. A long duration of effect was indicated when 3 mg/kg AM1346 was examined after different time intervals following i.p. administration (group 3 mg/kg). The in vivo half-life was close to 5 h. Neither D: -amphetamine nor morphine generalized in either of groups 3 mg/kg and 5.6 mg/kg, suggesting pharmacological specificity.

CONCLUSION

Unlike mAEA, the surmountable antagonism between rimonabant and AM1346 showed that the structural features of AEA can be modified to produce novel ligands that reduce the dissociation between the discriminative stimulus and rate decreasing effects of CB(1)R agonists derived from an AEA template.

Apparent affinity estimates of rimonabant in combination with anandamide and chemical analogs of anandamide in rhesus monkeys discriminating Delta9-tetrahydrocannabinol

RATIONALE

Anandamide and Delta(9)-tetrahydrocannabinol (Delta(9)-THC) sometimes produce different discriminative stimulus effects and, therefore, appear to differ in their mechanism of action. In order to understand the widespread use of cannabis and the therapeutic potential of cannabinoids, mechanisms responsible for behavioral effects need to be identified.

OBJECTIVE

Drug discrimination was used to compare the mechanism of action of Delta(9)-THC, anandamide, and two structural analogs of anandamide in rhesus monkeys.

MATERIALS AND METHODS

Monkeys discriminated Delta(9)-THC (0.1 mg/kg i.v.) from vehicle. Delta(9)-THC, anandamide, methanandamide, and arachidonylcyclopropylamide (ACPA) were administered i.v. alone and in combination with at least one dose of rimonabant. Schild analysis and single-dose apparent affinity estimates were used to estimate the potency of rimonabant as an antagonist of each cannabinoid; these values were compared to examine whether the same receptors mediated discriminative stimulus effects.

RESULTS

Delta(9)-THC, ACPA, methanandamide, and anandamide produced greater than 96% of responses on the Delta(9)-THC lever. The ED(50) values were 0.024 mg/kg for Delta(9)-THC, 0.14 mg/kg for ACPA, 0.28 mg/kg for methanandamide, and 1.7 mg/kg for anandamide. The duration of action of Delta(9)-THC was 4-6 h and longer than the duration of action ACPA, methanandamide, and anandamide (i.e., each less than 50 min). Rimonabant surmountably antagonized the discriminative stimulus effects of each agonist, and the apparent affinity estimates (pA (2) and pK (B) values) were 6.24-6.83.

CONCLUSIONS

Rimonabant can produce surmountable antagonism of the behavioral effects of not only Delta(9)-THC but also anandamide, methanandamide, and ACPA, and the interactions appear simple, competitive, and reversible. These cannabinoid agonists act at the same receptors to produce discriminative stimulus effects.