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

Anxiety associated with palatable food withdrawal is reversed by the selective FAAH inhibitor PF-3845: A regional analysis of the contribution of endocannabinoid signaling machinery

OBJECTIVE

Consumption of energy-dense palatable "comfort" food can alleviate stress and negative emotions, while abrupt withdrawal from a palatable diet can worsen these symptoms, causing difficulties with adherence to weight-loss diets. Currently, no pharmacological treatment is effective for obesity-related anxiety, so we investigated the endocannabinoid system (ECS), and specifically the fatty acid amide hydrolase (FAAH), as an interesting emerging target in this context because of its key role in the regulation of both energy homeostasis and emotional behavior.

METHODS

Rats were subjected to exposure and subsequent abstinence from a palatable cafeteria diet. During abstinence period, rats were treated with the selective FAAH inhibitor PF-3845 (10 mg/kg; intraperitoneal administration every other day).

RESULTS

Abstinent rats displayed an anxiogenic-like behavior and changes in the proteins of ECS signaling machinery in brain areas involved both in anxiety and food intake regulation. In particular, withdrawal caused a reduction of the expression of cannabinoid receptors in the nucleus accumbens and of enzymes diacylglycerol lipase alpha and monoacylglycerol lipase (MAGL) in the amygdala. Pharmacological inhibition of FAAH exerted an anxiolytic-like effect in abstinent animals and increased both MAGL expression in amygdala and CB2 expression in prefrontal cortex.

DISCUSSION

Overall, our results suggest that emotional disturbances associated with dieting are coupled with region-specific alterations in the cerebral expression of the ECS and that the enhancement of the endocannabinoid signaling by FAAH inhibition might represent a novel pharmacological strategy for the treatment of anxiety related to abstinence from palatable food.

PUBLIC SIGNIFICANCE

The present study focused on evaluating the role of the endocannabinoid system in modulating withdrawal from naturally rewarding activities that have an impact on mood, such as feeding. The variations observed in the emotional behavior of abstinent rats was linked to neuroadaptations of the ECS in specific brain areas.

Synthesis and evaluation of dual fatty acid amide hydrolase-monoacylglycerol lipase inhibition and antinociceptive activities of 4-methylsulfonylaniline-derived semicarbazones

Fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) are promising targets for neuropathic pain and other CNS disorders. Based on our previous lead compound SIH 3, we designed and synthesized a series of 4-methylsulfonylphenyl semicarbazones and evaluated for FAAH and MAGL inhibition properties. Most of the compounds showed potency towards both enzymes with leading FAAH selectivity. Compound (Z)-2-(2,6-dichlorobenzylidene)-N-(4-(methylsulfonyl)phenyl)hydrazine-1-carboxamide emerged as the lead inhibitor against both FAAH (IC₅₀ = 11 nM) and MAGL (IC₅₀ = 36 nM). The lead inhibitor inhibited FAAH by non-competitive mode, but showed a mixed-type inhibition against MAGL. Molecular docking study unveiled that the docked ligands bind favorably to the active sites of FAAH and MAGL. The lead inhibitor interacted with FAAH and MAGL via π-π stacking via phenyl ring and hydrogen bonding through sulfonyl oxygen atoms or amide NH. Moreover, the stability of docked complexes was rationalized by molecular simulation studies. PAMPA assay revealed that the lead compound is suitable for blood-brain penetration. The lead compound showed better cell viability in lipopolysaccharide-induced neurotoxicity assay in SH-SY5Y cell lines. Further, in-vivo experiments unveiled that dual inhibitor was safe up to 2000 mg/kg with no hepatotoxicity. The dual FAAH-MAGL inhibitor produced significant anti-nociceptive effect in the CCI model of neuropathic pain without altering locomotion activity. Lastly, the lead compound exhibited promising ex-vivo FAAH/MAGL inhibition activity at the dose of 10 mg/kg and 20 mg/kg. Thus, these findings suggest that the semicarbazone-based lead compound can be a potential template for the development of agents for neuropathic pain.

Discovery of Isatin-Based Carbohydrazones as Potential Dual Inhibitors of Fatty Acid Amide Hydrolase and Monoacylglycerol Lipase

Using ligand-based design strategy, a set of isatin-3-carbohydrazones was designed, synthesized and evaluated for dual fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) inhibition properties. Compound 5-chloro-N'-(5-chloro-2-oxoindolin-3-ylidene)-2-hydroxybenzohydrazide (13 b) emerged as a potent MAGL inhibitor with nanomolar activity (IC₅₀ =3.33 nM), while compound 5-chloro-N'-(1-(4-fluorobenzyl)-2-oxoindolin-3-ylidene)-2-hydroxybenzohydrazide (13 j) was the most potent selective FAAH inhibitor (IC₅₀ =37 nM). Compound 5-chloro-N'-(6-chloro-2-oxoindolin-3-ylidene)-2-hydroxybenzohydrazide (13 c) showed dual FAAH-MAGL inhibitory activity with an IC₅₀ of 31 and 29 nM respectively. Enzyme kinetics studies revealed that the isatin-based carbohydrazones are reversible inhibitors for both FAAH and MAGL. Further, blood-brain permeability assay confirmed that the lead compounds (13 b, 13 c, 13 g, 13 m and 13 q) are suitable as CNS candidates. Molecular dynamics simulation studies revealed the putative binding modes and key interactions of lead inhibitors within the enzyme active sites. The lead dual FAAH-MAGL inhibitor 13 c showed significant antioxidant activity and neuroprotection in the cell-based cytotoxicity assay. In summary, the study yielded three potent FAAH/MAGL inhibitor compounds (13 b, 13 c and 13 j) with acceptable pharmacokinetic profile and thus can be considered as promising candidates for treating neurological and mood disorders.

Behavioral Battery for Testing Candidate Analgesics in Mice. II. Effects of Endocannabinoid Catabolic Enzyme Inhibitors and ∆9-Tetrahydrocannabinol

Enhanced signaling of the endocannabinoid (eCB) system through inhibition of the catabolic enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) has received increasing interest for development of candidate analgesics. This study compared effects of MAGL and FAAH inhibitors with effects of ∆9-tetrahydrocannabinol (THC) using a battery of pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to stimulate two behaviors (stretching, facial grimace) and depress two behaviors (rearing, nesting). Nesting and locomotion were also assessed in the absence of IP acid as pain-independent behaviors. THC and a spectrum of six eCB catabolic enzyme inhibitors ranging from MAGL- to FAAH-selective were assessed for effectiveness to alleviate pain-related behaviors at doses that did not alter pain-independent behaviors. The MAGL-selective inhibitor MJN110 produced the most effective antinociceptive profile, with 1.0 mg/kg alleviating IP acid effects on stretching, grimace, and nesting without altering pain-independent behaviors. MJN110 effects on IP acid-depressed nesting had a slow onset and long duration (40 minutes to 6 hours), were blocked by rimonabant, and tended to be greater in females. As inhibitors increased in FAAH selectivity, antinociceptive effectiveness decreased. PF3845, the most FAAH-selective inhibitor, produced no antinociception up to doses that disrupted locomotion. THC decreased IP acid-stimulated stretching and grimace at doses that did not alter pain-independent behaviors; however, it did not alleviate IP acid-induced depression of rearing or nesting. These results support further consideration of MAGL-selective inhibitors as candidate analgesics for acute inflammatory pain. SIGNIFICANCE STATEMENT: This study characterized a spectrum of endocannabinoid catabolic enzyme inhibitors ranging in selectivity from monoacylglycerol lipase-selective to fatty acid amide hydrolase-selective in a battery of pain-stimulated, pain-depressed, and pain-independent behaviors previously pharmacologically characterized in a companion paper. This battery provides a method for prioritizing candidate analgesics by effectiveness to alleviate pain-related behaviors at doses that do not alter pain-independent behaviors, with inclusion of pain-depressed behaviors increasing translational validity and decreasing susceptibility to motor-depressant false positives.

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.

2-Arachidonoylglycerol Modulation of Anxiety and Stress Adaptation: From Grass Roots to Novel Therapeutics

Over the past decade there has been a surge of interest in the development of endocannabinoid-based therapeutic approaches for the treatment of diverse neuropsychiatric conditions. Although initial preclinical and clinical development efforts focused on pharmacological inhibition of fatty acid amide hydrolase to elevate levels of the endocannabinoid anandamide, more recent efforts have focused on inhibition of monoacylglycerol lipase (MAGL) to enhance signaling of the most abundant and efficacious endocannabinoid ligand, 2-arachidonoylglycerol (2-AG). We review the biochemistry and physiology of 2-AG signaling and preclinical evidence supporting a role for this system in the regulation of anxiety-related outcomes and stress adaptation. We review preclinical evidence supporting MAGL inhibition for the treatment of affective, trauma-related, and stress-related disorders; describe the current state of MAGL inhibitor drug development; and discuss biological factors that could affect MAGL inhibitor efficacy. Issues related to the clinical advancement of MAGL inhibitors are also discussed. We are cautiously optimistic, as the field of MAGL inhibitor development transitions from preclinical to clinical and theoretical to practical, that pharmacological 2-AG augmentation could represent a mechanistically novel therapeutic approach for the treatment of affective and stress-related neuropsychiatric disorders.

Endogenous cannabinoid modulation of restraint stress-induced analgesia in thermal nociception

It is thought that endogenous cannabinoids have a role in the analgesia induced by specific forms of stress. We examined if the role of endogenous cannabinoids is also dependent upon the mode of nociception, and whether this could be altered by drugs which block their enzymatic degradation. In C57BL/6 mice, restraint stress produced analgesia in the hot-plate and plantar tests, two thermal pain assays that engage distinct supraspinal and spinal nociceptive pathways. Stress-induced analgesia in the hot-plate test was abolished by pre-treatment with the opioid receptor antagonist naltrexone but was unaffected by the cannabinoid receptor antagonist 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281). By contrast, stress-induced analgesia in the plantar test was abolished by pre-treatment with naltrexone plus AM281, but not by either antagonist individually. Remarkably, inhibiting the breakdown of endocannabinoids, with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195, rescued stress-induced analgesia in the hotplate test when endogenous opioid signalling was blocked by naltrexone. Furthermore, JZL195 recruited analgesia induced by sub-threshold restraint stress in both thermal pain assays. These findings indicate the role of endocannabinoids in stress-induced analgesia differs with the type of thermal pain behaviour. However, by inhibiting their breakdown, endocannabinoids can be recruited to substitute for endogenous opioid signalling when their activity is blocked, indicating a degree of redundancy between opioid and cannabinoid systems. Together these data suggest targeting endocannabinoid breakdown could provide an alternative, or adjuvant to mainstream analgesics such as opioids.

Endocannabinoid Signaling in the Central Amygdala and Bed Nucleus of the Stria Terminalis: Implications for the Pathophysiology and Treatment of Alcohol Use Disorder

High rates of relapse are a chronic and debilitating obstacle to effective treatment of alcohol use disorder (AUD); however, no effective treatments are available to treat symptoms induced by protracted abstinence. In the first part of this 2-part review series, we examine the literature supporting the effects of alcohol exposure within the extended amygdala (EA) neural circuitry. In Part 2, we focus on a potential way to combat negative affect associated with AUD, by exploring the therapeutic potential of the endogenous cannabinoid (eCB) system. The eCB system is a potent modulator of neural activity in the brain, and its ability to mitigate stress and negative affect has long been an area of interest for developing novel therapeutics. This review details the recent advances in our understanding of eCB signaling in 2 key regions of the EA, the central nucleus of the amygdala and the bed nucleus of the stria terminalis (BNST), and their role in regulating negative affect. Despite an established role for EA eCB signaling in reducing negative affect, few studies have examined the potential for eCB-based therapies to treat AUD-associated negative affect. In this review, we present an overview of studies focusing on eCB signaling in EA and cannabinoid modulation on EA synaptic activity. We further discuss studies suggesting dysregulation of eCB signaling in models of AUD and propose that pharmacological augmentation of eCB could be a novel approach to treat aspects of AUD. Lastly, future directions are proposed to advance our understanding of the relationship between AUD-associated negative affect and the EA eCB system that could yield new pharmacotherapies targeting negative affective symptoms associated with AUD.

N-Acyl pyrazoles: Effective and tunable inhibitors of serine hydrolases

A series of N-acyl pyrazoles was examined as candidate serine hydrolase inhibitors in which the active site acylating reactivity and the leaving group ability of the pyrazole could be tuned not only through the nature of the acyl group (reactivity: amide > carbamate > urea), but also through pyrazole C4 substitution with electron-withdrawing or electron-donating substituents. Their impact on enzyme inhibitory activity displayed pronounced effects with the activity improving substantially as one alters both the nature of the reacting carbonyl group (urea > carbamate > amide) and the pyrazole C4 substituent (CN > H > Me). It was further demonstrated that the acyl chain of the N-acyl pyrazole ureas can be used to tailor the potency and selectivity of the inhibitor class to a targeted serine hydrolase. Thus, elaboration of the acyl chain of pyrazole-based ureas provided remarkably potent, irreversible inhibitors of fatty acid amide hydrolase (FAAH, apparent K_i = 100-200 pM), dual inhibitors of FAAH and monoacylglycerol hydrolase (MGLL), or selective inhibitors of MGLL (IC₅₀ = 10-20 nM) while simultaneously minimizing off-target activity (e.g., ABHD6 and KIAA1363).

Loren Parsons' contribution to addiction neurobiology

Loren (Larry) H. Parsons passed away at the age of 51. In spite of his premature departure, Larry much contributed to the drug abuse field. Since his graduate studies for the Ph.D. in Chemistry in J.B. Justice lab, microdialysis is the tread that links Larry's research topics, namely, the role of dopamine (DA), serotonin (5-HT), gamma-aminobutyric acid (GABA), glutamate and endocannabinoids (eCBs) in drug reinforcement and dependence. Larry was the first to show that abstinence from chronic cocaine reduces extracellular DA in the NAc, consistent with the so called 'dopamine depletion hypothesis' of cocaine addiction. Another Larry's major contributions are the studies on 5-HT and 5-HT receptors' role in cocaine stimulant actions, which resulted in the identification of 5-HT1B receptors as a critical substrate of cocaine reinforcement. By applying mass spectrometry to eCBs analysis in brain dialysates, Larry's lab showed that ethanol, heroin, nicotine and cocaine differentially affect anandamide and 2-arachidonoylglicerol overflow in the NAc shell, a critical site of drugs of abuse DA stimulant actions. Larry also applied microdialysis to study GABA and glutamate's role in ethanol dependence and heroin reinforcement, providing in vivo evidence for a sensitization of corticotropin-releasing factor-dependent release of GABA in the central amygdala in withdrawal from chronic ethanol and for a reduction of GABA transmission in the ventral pallidum in heroin but not cocaine intravenous self-administration. Larry showed the wide possibilities of microdialysis as a general purpose methodology for monitoring neurotransmitters and neuromodulators in the brain extracellular compartment. From this viewpoint, he stands as the best advocate for microdialysis.

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.

Just add water: cannabinoid discrimination in a water T-maze with FAAH(-/-) and FAAH(+/+) mice

Incomplete overlap in the discriminative stimulus effects of Δ-tetrahydrocannabinol (THC) and the endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol has been reported in food-reinforced tasks. The aim of this study was to examine cannabinoid discriminative stimulus effects in a nonappetitive procedure. Adult male mice lacking the gene for AEA's major metabolic enzyme, fatty acid amide hydrolase (FAAH), and FAAH mice were trained to discriminate THC or AEA in a water T-maze, in which the response was swimming to an escape platform on the injection-appropriate side. JZL184, a monoacylglycerol lipase inhibitor, was also tested. FAAH mice showed faster acquisition than FAAH mice. THC and AEA fully substituted, with only minor cross-procedure potency variations. Incomplete substitution of JZL184 was observed in THC-trained FAAH mice in the water-maze task, as contrasted with full substitution in a food-reinforced nose-poke procedure. Stress-induced changes in AEA and/or 2-arachidonoylglycerol concentrations in the brain may have mediated this attenuation. JZL184 also partially substituted in AEA-trained FAAH mice in the water maze, suggesting incomplete overlap in the stimulus effects of AEA and JZL184. Through the use of a novel water-maze procedure, the present study supports the work of previous behavioral pharmacologists in showing the robustness of the discrimination paradigm.

Inhibition of the endocannabinoid-regulating enzyme monoacylglycerol lipase elicits a CB1 receptor-mediated discriminative stimulus in mice

Substantial challenges exist for investigating the cannabinoid receptor type 1 (CB₁)-mediated discriminative stimulus effects of the endocannabinoids, 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anandamide; AEA), compared with exogenous CB₁ receptor agonists, such as Δ⁹-tetrahydrocannabinol (THC) and the synthetic cannabinoid CP55,940. Specifically, each endocannabinoid is rapidly degraded by the respective hydrolytic enzymes, monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH). Whereas MAGL inhibitors partially substitute for THC and fully substitute for CP55,940, FAAH inhibitors do not substitute for either drug. Interestingly, combined FAAH-MAGL inhibition results in full THC substitution, and the dual FAAH-MAGL inhibitor SA-57 serves as its own discriminative training stimulus. Because MAGL inhibitors fully substitute for SA-57, we tested whether the selective MAGL inhibitor MJN110 would serve as a training stimulus. Twelve of 13 C57BL/6J mice learned to discriminate MJN110 from vehicle, and the CB₁ receptor antagonist rimonabant dose-dependently blocked its discriminative stimulus. CP55,940, SA-57, and another MAGL inhibitor JZL184, fully substituted for MJN110. In contrast, the FAAH inhibitor PF-3845 failed to substitute for the MJN110 discriminative stimulus, but produced a 1.6 (1.1-2.2; 95% confidence interval) leftward shift in the MJN110 dose-response curve. Inhibitors of other relevant enzymes (i.e., ABHD6, COX-2) and nicotine did not engender substitution. Diazepam partially substituted for MJN110, but rimonabant failed to block this partial effect. These findings suggest that MAGL normally throttles 2-AG stimulation of CB₁ receptors to a magnitude insufficient to produce cannabimimetic subjective effects. Accordingly, inhibitors of this enzyme may release this endogenous brake producing effects akin to those produced by exogenously administered cannabinoids.

Discriminative Stimulus Properties of the Endocannabinoid Catabolic Enzyme Inhibitor SA-57 in Mice

Whereas the inhibition of fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL), the respective major hydrolytic enzymes of N-arachidonoyl ethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), elicits no or partial substitution for Δ(9)-tetrahydrocannabinol (THC) in drug-discrimination procedures, combined inhibition of both enzymes fully substitutes for THC, as well as produces a constellation of cannabimimetic effects. The present study tested whether C57BL/6J mice would learn to discriminate the dual FAAH-MAGL inhibitor SA-57 (4-[2-(4-chlorophenyl)ethyl]-1-piperidinecarboxylic acid 2-(methylamino)-2-oxoethyl ester) from vehicle in the drug-discrimination paradigm. In initial experiments, 10 mg/kg SA-57 fully substituted for CP55,940 ((-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol), a high-efficacy CB1 receptor agonist in C57BL/6J mice and for AEA in FAAH (-/-) mice. Most (i.e., 23 of 24) subjects achieved criteria for discriminating SA-57 (10 mg/kg) from vehicle within 40 sessions, with full generalization occurring 1 to 2 hours postinjection. CP55,940, the dual FAAH-MAGL inhibitor JZL195 (4-​nitrophenyl 4-​(3-​phenoxybenzyl)piperazine-​1-​carboxylate), and the MAGL inhibitors MJN110 (2,5-dioxopyrrolidin-1-yl 4-(bis(4-chlorophenyl)methyl)piperazine-1-carboxylate) and JZL184 (4-[Bis(1,3-benzodioxol-5-yl)hydroxymethyl]-1-piperidinecarboxylic acid 4-nitrophenyl ester) fully substituted for SA-57. Although the FAAH inhibitors PF-3845 ((N-3-pyridinyl-4-[[3-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenyl]methyl]-1-piperidinecarboxamide) and URB597 (cyclohexylcarbamic acid 3'-(aminocarbonyl)-[1,1'-biphenyl]-3-yl ester) did not substitute for SA-57, PF-3845 produced a 2-fold leftward shift in the MJN110 substitution dose-response curve. In addition, the CB1 receptor antagonist rimonabant blocked the generalization of SA-57, as well as substitution of CP55,940, JZL195, MJN110, and JZL184. These findings suggest that MAGL inhibition plays a major role in the CB1 receptor-mediated SA-57 training dose, which is further augmented by FAAH inhibition.

Effects of fatty acid amide hydrolase (FAAH) inhibitors on working memory in rats

RATIONALE

Manipulations of the endocannabinoid system could potentially produce therapeutic effects with minimal risk of adverse cannabis-like side effects. Inhibitors of fatty acid amide hydrolase (FAAH) increase endogenous levels of the cannabinoid-receptor agonist, anandamide, and show promise for treating a wide range of disorders. However, their effects on learning and memory have not been fully characterized.

OBJECTIVES

We determined the effects of five structurally different FAAH inhibitors in an animal model of working memory known to be sensitive to impairment by delta-9 tetrahydrocannabinol (THC).

METHODS

A delayed nonmatching-to-position procedure was used in rats. Illuminated nosepoke holes were used to provide sample cues (left versus right) and record responses (correct versus incorrect) after delays ranging from 0 to 28 s. Various test drugs were given acutely up to two times per week before daily sessions.

RESULTS

One FAAH inhibitor, AM3506 (3 mg/kg), decreased accuracy in the memory task. Four other FAAH inhibitors (URB597, URB694, PF-04457845, and ARN14633) and a monoacylglycerol lipase inhibitor (JZL184, which blocks the degradation of the endocannabinoid 2-arachidonoylglycerol) had no effect. Testing of AM3506 in combination with antagonists for receptors known to be affected by anandamide and other fatty acid amides indicated that the impairment induced by AM3506 was mediated by cannabinoid CB1 receptors, and not by alpha-type peroxisome proliferator-activated receptors (PPAR-alpha) or vanilloid transient receptor potential cation channels (TRPV1).

CONCLUSIONS

FAAH inhibitors differ with respect to their potential for memory impairment, abuse liability, and probably other cannabis-like effects, and they should be evaluated individually for specific therapeutic and adverse effects.

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.

Systemic and spinal administration of FAAH, MAGL inhibitors and dual FAAH/MAGL inhibitors produce antipruritic effect in mice

The increase of endocannabinoid tonus by inhibiting fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL) represents a promising therapeutic approach in a variety of disease to overcome serious central side effects of exocannabinoids. Recent studies reported that systemic administration of FAAH and MAGL inhibitors produce antipruritic action. Dual FAAH/MAGL inhibitors have also been described to get enhanced endocannabinoid therapeutic effect. In this study, we examined and compared dose-related antipruritic effects of systemic (intraperitoneal; ip) or intrathecal (it) administration of selective FAAH inhibitor PF-3845 (5, 10, and 20 mg/kg, i.p.; 1, 5, and 10 µg, i.t.), MAGL inhibitor JZL184 (4, 20, and 40 mg/kg, i.p.; 1, 5, and 10 µg, i.t.) and dual FAAH/MAGL inhibitor JZL195 (2, 5, and 20 mg/kg, i.p.; 1, 5, and 10 µg, i.t.) on serotonin (5-HT)-induced scratching model. Serotonin (25 μg) was injected intradermally in a volume of 50 μl into the rostral part of skin on the back of male Balb-C mice. Both systemic or intrathecal administration of PF-3845, JZL184 or JZL195 produced similar dose-dependent antipruritic effects. Our results suggest that endocannabinoid-degrading enzymes FAAH and MAGL are involved in pruritic process at spinal level. FAAH, MAGL or dual FAAH/MAGL inhibitors have promising antipruritic effects, at least, in part through spinal site of action.

Monoacylglycerol Lipase Regulates Fever Response

Cyclooxygenase inhibitors such as ibuprofen have been used for decades to control fever through reducing the levels of the pyrogenic lipid transmitter prostaglandin E2 (PGE₂). Historically, phospholipases have been considered to be the primary generator of the arachidonic acid (AA) precursor pool for generating PGE₂ and other eicosanoids. However, recent studies have demonstrated that monoacyglycerol lipase (MAGL), through hydrolysis of the endocannabinoid 2-arachidonoylglycerol, provides a major source of AA for PGE₂ synthesis in the mammalian brain under basal and neuroinflammatory states. We show here that either genetic or pharmacological ablation of MAGL leads to significantly reduced fever responses in both centrally or peripherally-administered lipopolysaccharide or interleukin-1β-induced fever models in mice. We also show that a cannabinoid CB1 receptor antagonist does not attenuate these anti-pyrogenic effects of MAGL inhibitors. Thus, much like traditional nonsteroidal anti-inflammatory drugs, MAGL inhibitors can control fever, but appear to do so through restricted control over prostaglandin production in the nervous system.

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.