Brain cannabinoid CB₂ receptors modulate cocaine's actions in mice

Hyperactivity induced by the dopamine D2/D3 receptor agonist quinpirole is attenuated by inhibitors of endocannabinoid degradation in mice

The present study was designed to investigate the effect of pharmacological inhibition of endocannabinoid degradation on behavioural actions of the dopamine D2/D3 receptor agonist quinpirole in male C57Bl/6J mice. In addition, we studied the effects of endocannabinoid degradation inhibition on both cocaine-induced psychomotor activation and behavioural sensitization. We analysed the effects of inhibition of the two main endocannabinoid degradation enzymes: fatty acid amide hydrolase (FAAH), using inhibitor URB597 (1 mg/kg); monoacylglycerol lipase (MAGL), using inhibitor URB602 (10 mg/kg). Administration of quinpirole (1 mg/kg) caused a temporal biphasic response characterized by a first phase of immobility (0-50 min), followed by enhanced locomotion (next 70 min) that was associated with the introduction of stereotyped behaviours (stereotyped jumping and rearing). Pretreatment with both endocannabinoid degradation inhibitors did not affect the hypoactivity actions of quinpirole. However, this pretreatment resulted in a marked decrease in quinpirole-induced locomotion and stereotyped behaviours. Administration of FAAH or MAGL inhibitors did not attenuate the acute effects of cocaine. Furthermore, these inhibitors did not impair the acquisition of cocaine-induced behavioural sensitization or the expression of cocaine-induced conditioned locomotion. Only MAGL inhibition attenuated the expression of an already acquired cocaine-induced behavioural sensitization. These results suggest that pharmacological inhibition of endocannabinoid degradation might exert a negative feedback on D2/D3 receptor-mediated hyperactivity. This finding might be relevant for therapeutic approaches for either psychomotor disorders (dyskinesia, corea) or disorganized behaviours associated with dopamine-mediated hyperactivity.

Human metabolites of synthetic cannabinoids JWH-018 and JWH-073 bind with high affinity and act as potent agonists at cannabinoid type-2 receptors

K2 or Spice is an emerging drug of abuse that contains synthetic cannabinoids, including JWH-018 and JWH-073. Recent reports indicate that monohydroxylated metabolites of JWH-018 and JWH-073 retain high affinity and activity at cannabinoid type-1 receptors (CB1Rs), potentially contributing to the enhanced toxicity of K2 compared to marijuana. Since the parent compounds also bind to cannabinoid type-2 receptors (CB2Rs), this study investigated the affinity and intrinsic activity of JWH-018, JWH-073 and several monohydroxylated metabolites at human CB2Rs (hCB2Rs). The affinity of cannabinoids for hCB2Rs was determined by competition binding studies employing CHO-hCB2 membranes. Intrinsic activity of compounds was assessed by G-protein activation and adenylyl cyclase (AC)-inhibition in CHO-hCB2 cells. JWH-073, JWH-018 and several of their human metabolites exhibit nanomolar affinity and act as potent agonists at hCB2Rs. Furthermore, a major omega hydroxyl metabolite of JWH-073 (JWH-073-M5) binds to CB2Rs with 10-fold less affinity than the parent molecule, but unexpectedly, is equipotent in regulating AC-activity when compared to the parent molecule. Finally, when compared to CP-55,940 and Δ(9)-tetrahydrocannabinol (Δ(9)-THC), JWH-018, JWH-018-M5 and JWH-073-M5 require significantly less CB2R occupancy to produce similar levels of AC-inhibition, indicating that these compounds may more efficiently couple CB2Rs to AC than the well characterized cannabinoid agonists examined. These results indicate that JWH-018, JWH-073 and several major human metabolites of these compounds exhibit high affinity and demonstrate distinctive signaling properties at CB2Rs. Therefore, future studies examining pharmacological and toxicological properties of synthetic cannabinoids present in K2 products should consider potential actions of these drugs at both CB1 and CB2Rs.

The role of fatty acid amide hydrolase inhibition in nicotine reward and dependence

The endogenous cannabinoid anandamide (AEA) exerts the majority of its effects at CB1 and CB2 receptors and is degraded by fatty acid amide hydrolase (FAAH). FAAH KO mice and animals treated with FAAH inhibitors are impaired in their ability to hydrolyze AEA and other non-cannabinoid lipid signaling molecules, such as oleoylethanolamide (OEA) and palmitoylethanolamide (PEA). AEA and these other substrates activate non-cannabinoid receptor systems, including TRPV1 and PPAR-α receptors. In this mini review, we describe the functional consequences of FAAH inhibition on nicotine reward and dependence as well as the underlying endocannabinoid and non-cannabinoid receptor systems mediating these effects. Interestingly, FAAH inhibition seems to mediate nicotine dependence differently in mice and rats. Indeed, pharmacological and genetic FAAH disruption in mice enhances nicotine reward and withdrawal. However, in rats, pharmacological blockade of FAAH significantly inhibits nicotine reward and has no effect in nicotine withdrawal. Studies suggest that non-cannabinoid mechanisms may play a role in these species differences.

Lipids and addiction: how sex steroids, prostaglandins, and cannabinoids interact with drugs of abuse

Lipidomics aims to identify and characterize all endogenous species of lipids and understand their roles in cellular signaling and, ultimately, the functioning of the organism. We are on the cusp of fully understanding the functions of many of the lipid signaling systems that have been identified for decades (e.g., steroids, prostaglandins), whereas our understanding of newer lipid signaling systems (e.g., endocannabinoids, N-acyl amides) still lags considerably behind. With an emphasis on their roles in the neurophysiology of addiction, we will examine three classes of lipids--sex steroids, prostaglandins, and cannabinoids--and how they work synergistically in the neurocircuitry of motivation. We will first give a brief overview of the biosynthesis for each class of lipid and its receptors, and then summarize what is known about the collective roles of the lipids in cocaine and alcohol abuse. This approach provides a novel view of lipid signaling as a class of molecules and their synergistic roles in addiction.

Endocannabinoid system and drug addiction: new insights from mutant mice approaches

The involvement of the endocannabinoid system in drug addiction was initially studied by the use of compounds with different affinities for each cannabinoid receptor or for the proteins involved in endocannabinoids inactivation. The generation of genetically modified mice with selective mutations in these endocannabinoid system components has now provided important advances in establishing their specific contribution to drug addiction. These genetic tools have identified the particular interest of CB1 cannabinoid receptor and endogenous anandamide as potential targets for drug addiction treatment. Novel genetic tools will allow determining if the modulation of CB2 cannabinoid receptor activity and 2-arachidonoylglycerol tone can also have an important therapeutic relevance for drug addiction.

Cocaine-related behaviors in mice with deficient gliotransmission

RATIONALE

Astrocytes play an integral role in modulating synaptic transmission and plasticity, both key mechanisms underlying addiction. However, while astrocytes are capable of releasing chemical transmitters that can modulate neuronal function, the role of these gliotransmitters in mediating behaviors associated with drugs of abuse has been largely unexplored.

OBJECTIVES

The objective of the present study was to utilize mice with astrocytes that lack the ability to release chemical transmitters to evaluate the behavioral consequence of impaired gliotransmission on cocaine-related behaviors. These mice have previously been used to examine the role of gliotransmission in sleep homeostasis; however, no studies to date have utilized them in the study of addictive behaviors.

METHODS

Mice expressing a dominant-negative SNARE protein selectively in astrocytes (dnSNARE mice) were tested in a variety of behavioral paradigms examining cocaine-induced behavioral plasticity. These paradigms include locomotor sensitization, conditioned place preference followed by cocaine-induced reinstatement of CPP, and cocaine self-administration followed by cue-induced reinstatement of cocaine-seeking behavior.

RESULTS

Wild-type and dnSNARE mice demonstrated no significant differences in the development or maintenance of locomotor sensitization. While there were non-significant trends for reduced CPP following a low dose of cocaine, drug-induced reinstatement of CPP is completely blocked in dnSNARE mice. Similarly, while dnSNARE mice demonstrated a non-significant trend toward reduced cocaine self-administration compared with wild-type mice, dnSNARE mice do not demonstrate cue-induced reinstatement in this paradigm.

CONCLUSIONS

Gliotransmission is necessary for reinstatement of drug-seeking behaviors by cocaine or associated cues.

The cannabinoid CB1 receptor biphasically modulates motor activity and regulates dopamine and glutamate release region dependently

Cannabinoid administration modulates both dopaminergic and glutamatergic neurotransmission. The present study examines the effects of high and low dose WIN55,212-2, a CB1 receptor agonist, on extracellular dopamine and glutamate release in vivo via brain microdialysis in the nucleus accumbens (NAc), striatum and prefrontal cortex (PFC) in parallel to its effects on locomotor activity. WIN55,212-2 increased extracellular dopamine in the NAc (1 mg/kg i.p.), striatum (0.1 and 1 mg/kg i.p.) and PFC (1 mg/kg i.p.). Glutamate release was also elevated by WIN55,212-2 in the PFC (1 mg/kg i.p.) whereas in the NAc (0.1 and 1 mg/kg i.p.) and striatum, it was reduced (1 mg/kg i.p.). WIN55,212-2 administration produced hyperlocomotion at the lower dose (0.1 mg/kg i.p.) and hypolocomotion at the higher dose (1 mg/kg i.p.). Co-administration with the CB1 antagonist, SR-141716A (0.03 mg/kg i.p.), prevented the above effects. According to the present results, WIN55,212-2 affected locomotor activity biphasically while exerting converging effects on dopamine activity but diverging effects on glutamate release between cortical and subcortical regions, especially at the higher dose. These findings emphasize the involvement of the CB1 receptor in the simultaneous modulation of dopaminergic and glutamatergic neurotransmission in brain regions involved in reward and locomotion and suggest possible underlying mechanisms of acute cannabinoid exposure and its psychoactive and behavioural manifestations.

Cannabidiol inhibits the reward-facilitating effect of morphine: involvement of 5-HT1A receptors in the dorsal raphe nucleus

Cannabidiol is a non-psychotomimetic constituent of Cannabis sativa, which induces central effects in rodents. It has been shown that cannabidiol attenuates cue-induced reinstatement of heroin seeking. However, to the best of our knowledge, its effects on brain stimulation reward and the reward-facilitating effects of drugs of abuse have not yet been examined. Therefore, we investigated the effects of cannabidiol on brain reward function and on the reward-facilitating effect of morphine and cocaine using the intracranial self-stimulation (ICSS) paradigm. Rats were prepared with a stimulating electrode into the medial forebrain bundle (MFB), and a guide cannula into the dorsal raphe (microinjection experiments), and were trained to respond for electrical brain stimulation. A low dose of cannabidiol did not affect the reinforcing efficacy of brain stimulation, whereas higher doses significantly elevated the threshold frequency required for MFB ICSS. Both cocaine and morphine lowered ICSS thresholds. Cannabidiol inhibited the reward-facilitating effect of morphine, but not cocaine. This effect was reversed by pre-treatment with an intra-dorsal raphe injection of the selective 5-HT1A receptor antagonist WAY-100635. The present findings indicate that cannabidiol does not exhibit reinforcing properties in the ICSS paradigm at any of the doses tested, while it decreases the reward-facilitating effects of morphine. These effects were mediated by activation of 5-HT1A receptors in the dorsal raphe. Our results suggest that cannabidiol interferes with brain reward mechanisms responsible for the expression of the acute reinforcing properties of opioids, thus indicating that cannabidiol may be clinically useful in attenuating the rewarding effects of opioids.

Novel triaryl sulfonamide derivatives as selective cannabinoid receptor 2 inverse agonists and osteoclast inhibitors: discovery, optimization, and biological evaluation

Cannabinoid receptors have gained increasing attention as drug targets for developing potential therapeutic ligands. Here, we report the discovery and optimization of triaryl sulfonamides as a novel series possessing significant CB2 receptor affinity and selectivity. Four sets of triaryl ligands were designed and synthesized for further structural modifications and led to the identification of eight compounds as potent and selective CB2 inverse agonists with high binding affinity (CB2K(i) < 10 nM). Especially, compound 57 exhibited the strongest binding affinity on the CB2 receptor (CB2K(i) of 0.5 nM) and the best selectivity over the CB1 receptor (selectivity index of 2594). Importantly, 57 also showed potent inhibitory activity on osteoclast formation, and it was confirmed by a cell viability assay that the inhibition effects were not derived from the cytotoxicity. Finally, 3D QSAR studies confirmed our SAR findings that three bulky groups play an important role for CB2 receptor binding affinity.

Endogenous lipid activated G protein-coupled receptors: emerging structural features from crystallography and molecular dynamics simulations

Class A G-protein coupled receptors (GPCRs) are thought to have a common topology that includes seven transmembrane alpha helices (TMHs) that are arranged to form a closed bundle. This bundle forms the ligand binding pocket into which ligands are commonly thought to enter via the extracellular milieu. This ligand approach direction makes sense for GPCRs that have small positively charged ligands, such as the beta-2-adrenergic or the dopamine D2 receptor. However, there is a growing sub-group of Class A GPCRs that bind lipid-derived endogenous ligands, such as the cannabinoid CB1 and CB2 receptors (with endogenous ligands, N-arachidonoylethanolamine (anandamide) and sn-2-arachidonylglycerol (2-AG)) and the S1P1-5 receptors (with endogenous ligand, sphingosine-1-phosphate). Even the widely studied Class A GPCR, rhodopsin, binds a highly lipophillic chromophore (11-cis-retinal). For these receptors, ligand approach from the extracellular milieu has seemed unlikely given that the ligands of these receptors readily partition into lipid or are actually synthesized in the lipid bilayer. The recent X-ray-crystal structure of the sub-type 1 sphingosine-1-phosphate receptor (S1P1) provides important information on the key structural variations that may be the hallmarks for a Class A GPCR that binds lipid-derived ligands. These include an extracellular domain that is closed off to the extracellular milieu and the existence of an opening between transmembrane helices that may serve as a portal for ligand entry via the lipid bilayer. This review examines structural aspects that the cannabinoid receptors may share with the S1P1 receptor based upon sequence homology. This review also examines experimental and simulation results that suggest ligand entry via a lipid portal is quite likely for this emerging sub-group.

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.

Endocannabinoid signaling and synaptic function

Endocannabinoids are key modulators of synaptic function. By activating cannabinoid receptors expressed in the central nervous system, these lipid messengers can regulate several neural functions and behaviors. As experimental tools advance, the repertoire of known endocannabinoid-mediated effects at the synapse, and their underlying mechanism, continues to expand. Retrograde signaling is the principal mode by which endocannabinoids mediate short- and long-term forms of plasticity at both excitatory and inhibitory synapses. However, growing evidence suggests that endocannabinoids can also signal in a nonretrograde manner. In addition to mediating synaptic plasticity, the endocannabinoid system is itself subject to plastic changes. Multiple points of interaction with other neuromodulatory and signaling systems have now been identified. In this Review, we focus on new advances in synaptic endocannabinoid signaling in the mammalian brain. The emerging picture not only reinforces endocannabinoids as potent regulators of synaptic function but also reveals that endocannabinoid signaling is mechanistically more complex and diverse than originally thought.

Cannabinoid receptor agonists upregulate and enhance serotonin 2A (5-HT(2A)) receptor activity via ERK1/2 signaling

Recent behavioral studies suggest that nonselective agonists of cannabinoid receptors may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission. Two cannabinoids receptors are found in brain, CB1 and CB2 receptors, but the molecular mechanism by which cannabinoid receptors would regulate 5-HT(2A) receptor neurotransmission remains unknown. Interestingly, we have recently found that certain cannabinoid receptor agonists can specifically upregulate 5-HT(2A) receptors. Here, we present experimental evidence that rats treated with a nonselective cannabinoid receptor agonist (CP 55,940, 50 µg/kg, 7 days) showed increases in 5-HT(2A) receptor protein levels, 5-HT(2A) receptor mRNA levels, and 5-HT(2A) receptor-mediated phospholipase C beta (PLCβ) activity in prefrontal cortex (PFCx). Similar effects were found in neuronal cultured cells treated with CP 55,940 but these effects were prevented by selective CB2, but not selective CB1, receptor antagonists. CB2 receptors couple to the extracellular kinase (ERK) signaling pathway by Gα(i/o) class of G-proteins. Noteworthy, GP 1a (selective CB2 receptor agonist) produced a strong upregulation of 5-HT(2A) receptor mRNA and protein, an effect that was prevented by selective CB2 receptor antagonists and by an ERK1/2 inhibitor, PD 198306. In summary, our results identified a strong cannabinoid-induced upregulation of 5-HT(2A) receptor signaling in rat PFCx. Our cultured cell studies suggest that selective CB2 receptor agonists upregulate 5-HT(2A) receptor signaling by activation of the ERK1/2 signaling pathway. Activity of cortical 5-HT(2A) receptors has been associated with several physiological functions and neuropsychiatric disorders such as stress response, anxiety and depression, and schizophrenia. Therefore, these results may provide a molecular mechanism by which activation of cannabinoid receptors might be relevant to the pathophysiology of some cognitive and mood disorders in humans.

Spice drugs are more than harmless herbal blends: a review of the pharmacology and toxicology of synthetic cannabinoids

"K2" and "Spice" drugs (collectively hereafter referred to as Spice) represent a relatively new class of designer drugs that have recently emerged as popular alternatives to marijuana, otherwise characterized as "legal highs". These drugs are readily available on the Internet and sold in many head shops and convenience stores under the disguise of innocuous products like herbal blends, incense, or air fresheners. Although package labels indicate "not for human consumption", the number of intoxicated people presenting to emergency departments is dramatically increasing. The lack of validated and standardized human testing procedures and an endless supply of potential drugs of abuse are primary reasons why researchers find it difficult to fully characterize clinical consequences associated with Spice. While the exact chemical composition and toxicology of Spice remains to be determined, there is mounting evidence identifying several synthetic cannabinoids as causative agents responsible for psychoactive and adverse physical effects. This review provides updates of the legal status of common synthetic cannabinoids detected in Spice and analytical procedures used to test Spice products and human specimens collected under a variety of clinical circumstances. The pharmacological and toxicological consequences of synthetic cannabinoid abuse are also reviewed to provide a future perspective on potential short- and long-term implications.

Lead discovery, chemistry optimization, and biological evaluation studies of novel biamide derivatives as CB2 receptor inverse agonists and osteoclast inhibitors

N,N'-((4-(Dimethylamino)phenyl)methylene)bis(2-phenylacetamide) was discovered by using 3D pharmacophore database searches and was biologically confirmed as a new class of CB(2) inverse agonists. Subsequently, 52 derivatives were designed and synthesized through lead chemistry optimization by modifying the rings A-C and the core structure in further SAR studies. Five compounds were developed and also confirmed as CB(2) inverse agonists with the highest CB(2) binding affinity (CB(2)K(i) of 22-85 nM, EC(50) of 4-28 nM) and best selectivity (CB(1)/CB(2) of 235- to 909-fold). Furthermore, osteoclastogenesis bioassay indicated that PAM compounds showed great inhibition of osteoclast formation. Especially, compound 26 showed 72% inhibition activity even at the low concentration of 0.1 μM. The cytotoxicity assay suggested that the inhibition of PAM compounds on osteoclastogenesis did not result from its cytotoxicity. Therefore, these PAM derivatives could be used as potential leads for the development of a new type of antiosteoporosis agent.

Increased vulnerability to cocaine in mice lacking dopamine D3 receptors

Neuroimaging studies using positron emission tomography suggest that reduced dopamine D(2) receptor availability in the neostriatum is associated with increased vulnerability to drug addiction in humans and experimental animals. The role of D(3) receptors (D(3)Rs) in the neurobiology of addiction remains unclear, however. Here we report that D(3)R KO (D(3)(-/-)) mice display enhanced cocaine self-administration and enhanced motivation for cocaine-taking and cocaine-seeking behavior. This increased vulnerability to cocaine is accompanied by decreased dopamine response to cocaine secondary to increased basal levels of extracellular dopamine in the nucleus accumbens, suggesting a compensatory response to decreased cocaine reward in D(3)(-/-) mice. In addition, D(3)(-/-) mice also display up-regulation of dopamine transporters in the striatum, suggesting a neuroadaptative attempt to normalize elevated basal extracellular dopamine. These findings suggest that D(3)R deletion increases vulnerability to cocaine, and that reduced D(3)R availability in the brain may constitute a risk factor for the development of cocaine addiction.

Cannabinoid exposure in adolescent female rats induces transgenerational effects on morphine conditioned place preference in male offspring

In the United States, marijuana is one of the drugs most abused by adolescents, with females representing a growing number of users. In previous studies, treatment of adolescent female rats with morphine significantly altered brain reward systems in future offspring. As both cannabinoid and opioid systems develop during adolescence, it was hypothesized that early exposure to cannabinoids would induce similar transgenerational effects. In the current study, female rats were treated with the cannabinoid receptor (CB1/CB2) agonist WIN 55,212-2 or its vehicle for three consecutive days during adolescent development (30 days of age), and were subsequently mated in adulthood (60 days of age). The adolescent and adult male offspring of these WIN 55,212-2 (WIN-F1)- or vehicle (VEH-F1)-treated females were tested for their response to morphine using the conditioned place preference (CPP) paradigm. Both adolescent and adult WIN-F1offspring exhibited greater sensitivity to morphine CPP than their VEH-F1 counterparts. Collectively, the findings provide additional evidence of transgenerational effects of adolescent drug use.

Interplay of cannabinoid 2 (CB2) receptors with nitric oxide synthases, oxidative and nitrative stress, and cell death during remote neurodegeneration

Remote neuronal degeneration and death/injury, which often occur in regions remote but functionally connected to the primary lesion site, may play a pivotal role in extending neuronal damage/dysfunction following traumatic brain injury, stroke, or peripheral nerve injury, as well as in chronic neurodegenerative diseases such as multiple sclerosis and amyotrophic lateral sclerosis. Even though the precise mechanisms of remote neuronal injury are poorly understood and no efficacious treatment options are available, it involves glial activation, inflammation, oxidative/nitrative stress, and apoptotic cell death. The newly discovered endocannabinoid signaling system consisting of endocannabinoids (endogenous bioactive lipid mediators), their synthetic and metabolizing enzymes, and their primary G protein-coupled cannabinoid 1 and 2 (CB(1) and CB(2)) receptors has been implicated in the regulation of numerous physiological and pathological processes/functions, including those associated with neurodegeneration. Using a well-characterized rodent model of remote neuronal degeneration, Oddi et al. (J Mol Med 2012, in press, DOI 10.1007/s00109-012-0884-1 ) have demonstrated that targeting CB(2) cannabinoid receptors may represent a promising novel approach to attenuate this pathological process. This editorial discusses the clinical significance of these interesting observations and the mechanisms of the possible interplay of CB(2) receptors with nitric oxide synthases, oxidative and nitrative stress, and cell death during remote neurodegeneration.

Overexpression of cannabinoid CB2 receptor in the brain induces hyperglycaemia and a lean phenotype in adult mice

It is well known that the endocannabinoid system, through cannabinoid CB1 receptor activation, has an important role in the main aspects of energy balance (i.e. food intake, energy expenditure and glucose and fat metabolism), orchestrating all the machinery involved in body weight control and energy homeostasis. A number of studies have revealed a crucial role of brain CB1 receptors in these processes. However, functional cannabinoid CB2 receptors have also been described in the brain, with no studies addressing their putative role in body weight control and glucose homeostasis. We have tested this hypothesis by analysing fasting-induced feeding, body weight, some hypothalamic neuropeptides, glucose tolerance and plasma hormones in an animal model specifically overexpressing CB2 receptors in the central nervous system. We found that specific overexpression of CB2 receptors in the brain promoted higher basal glucose levels, decreased fasting-induced feeding and, eventually, led to a lean phenotype and glucose intolerance. These findings could not be attributed to decreased locomotor activity, increased anxiety or depressive-like behaviours. The expression of relevant neuropeptides such as pro-opiomelanocortin and galanin in the arcuate nucleus of the hypothalamus was altered but not those of the CB1 receptor. Indeed, no changes in CB1 expression were found in the liver, skeletal muscle and adipose tissue. However, cannabinoid CB1 and CB2 receptor expression in the endocrine pancreas and glucagon plasma levels were decreased. No changes in plasma adiponectin, leptin, insulin and somatostatin were found. Taken together, these results suggest a role for central cannabinoid CB2 receptors in body weight control and glucose homeostasis.

The endocannabinoid system and the brain

The psychoactive constituent in cannabis, Δ(9)-tetrahydrocannabinol (THC), was isolated in the mid-1960s, but the cannabinoid receptors, CB1 and CB2, and the major endogenous cannabinoids (anandamide and 2-arachidonoyl glycerol) were identified only 20 to 25 years later. The cannabinoid system affects both central nervous system (CNS) and peripheral processes. In this review, we have tried to summarize research--with an emphasis on recent publications--on the actions of the endocannabinoid system on anxiety, depression, neurogenesis, reward, cognition, learning, and memory. The effects are at times biphasic--lower doses causing effects opposite to those seen at high doses. Recently, numerous endocannabinoid-like compounds have been identified in the brain. Only a few have been investigated for their CNS activity, and future investigations on their action may throw light on a wide spectrum of brain functions.

CB₂: therapeutic target-in-waiting

CB₂ cannabinoid receptor agonists hold promise as a new class of therapeutics for indications as diverse as pain, neuroinflammation, immune suppression and osteoporosis. These potential indications are supported by strong preliminary data from multiple investigators using diverse preclinical models. However, clinical trials for CB₂ agonists, when they have been reported have generally been disappointing. This review considers possible explanations for the mismatch between promising preclinical data and disappointing clinical data. We propose that a more careful consideration of CB₂ receptor pharmacology may help move CB₂ agonists from "promising" to "effective" therapeutics.

Sex differences in cannabinoid pharmacology: a reflection of differences in the endocannabinoid system?

Marijuana is the most widely used illicit drug in the U.S., and marijuana use by women is on the rise. Women have been found to be more susceptible to the development of cannabinoid abuse and dependence, have more severe withdrawal symptoms, and are more likely to relapse than men. The majority of research in humans suggests that women are more likely to be affected by cannabinoids than men, with reports of enhanced and decreased performance on various tasks. In rodents, females are more sensitive than males to effects of cannabinoids on tests of antinociception, motor activity, and reinforcing efficacy. Studies on effects of cannabinoid exposure during adolescence in both humans and rodents suggest that female adolescents are more likely than male adolescents to be deleteriously affected by cannabinoids. Sex differences in response to cannabinoids appear to be due to activational and perhaps organizational effects of gonadal hormones, with estradiol identified as the hormone that contributes most to the sexually dimorphic effects of cannabinoids in adults. Many, but not all sexually dimorphic effects of exogenous cannabinoids can be attributed to a sexually dimorphic endocannabinoid system in rodents, although the same has not yet been established firmly for humans. A greater understanding of the mechanisms underlying sexually dimorphic effects of cannabinoids will facilitate development of sex-specific approaches to treat marijuana dependence and to use cannabinoid-based medications therapeutically.

Decreased cocaine motor sensitization and self-administration in mice overexpressing cannabinoid CB₂ receptors

The potential involvement of the cannabinoid CB₂ receptors (CB₂r) in the adaptive responses induced by cocaine was studied in transgenic mice overexpressing the CB₂r (CB₂xP) and in wild-type (WT) littermates. For this purpose, the acute and sensitized locomotor responses to cocaine, conditioned place preference, and cocaine intravenous self-administration were evaluated. In addition, we assessed whether CB₂r were localized in neurons and/or astrocytes, and whether they colocalized with dopamine D1 and D2 receptors (D1Dr and D2Dr). Dopamine (DA) extracellular levels in the nucleus accumbens (NAcc), and gene expression of tyrosine hydroxylase (TH) and DA transporter (DAT) in the ventral tegmental area (VTA), and μ-opioid and cannabinoid CB₁ receptors in the NAcc were also studied in both genotypes. CB₂xP mice showed decreased motor response to acute administration of cocaine (10-20 mg/kg) and cocaine-induced motor sensitization compared with WT mice. CB₂xP mice presented cocaine-induced conditioned place aversion and self-administered less cocaine than WT mice. CB₂r were found in neurons and astrocytes and colocalized with D2Dr in the VTA and NAcc. No significant differences in extracellular DA levels in the NAcc were observed between genotypes after cocaine administration. Under baseline conditions, TH and DAT gene expression was higher and μ-opioid receptor gene expression was lower in CB₂xP than in WT mice. However, both genotypes showed similar changes in TH and μ-opioid receptor gene expression after cocaine challenge independently of the pretreatment received. Importantly, the cocaine challenge decreased DAT gene expression to a lesser extent in cocaine-pretreated CB₂xP than in cocaine-pretreated WT mice. These results revealed that CB₂r are involved in cocaine motor responses and cocaine self-administration, suggesting that this receptor could represent a promising target to develop novel treatments for cocaine addiction.

Latest advances in novel cannabinoid CB(2) ligands for drug abuse and their therapeutic potential

The field of cannabinoid (CB) drug research is experiencing a challenge as the CB(1) antagonist Rimonabant, launched in 2006 as an anorectic/anti-obesity drug, was withdrawn from the European market due to the complications of suicide and depression as side effects. There is interest in developing CB(2) drugs without CB(1) psychotropic side effects for drug-abuse treatment and therapeutic medication. The CB(1) receptor was discovered predominantly in the brain, whereas the CB(2) is mainly expressed in peripheral cells and tissues, and is involved in immune signal transduction. Conversely, the CB(2) receptor was recently detected in the CNS, for example, in the microglial cells and the neurons. While the CB(2) neurons activity remains controversial, the CB(2) receptor is an attractive therapeutic target for neuropathic pain, immune system, cancer and osteoporosis without psychoactivity. This review addresses CB drug abuse and therapeutic potential with a focus on the most recent advances on new CB(2) ligands from the literature as well as patents.

CB₂ cannabinoid receptors inhibit synaptic transmission when expressed in cultured autaptic neurons

The role of CB₂ in the central nervous system, particularly in neurons, has generated much controversy. Fueling the controversy are imperfect tools, which have made conclusive identification of CB₂ expressing neurons problematic. Imprecise localization of CB₂ has made it difficult to determine its function in neurons. Here we avoid the localization controversy and directly address the question if CB₂ can modulate neurotransmission. CB₂ was expressed in excitatory hippocampal autaptic neurons obtained from CB₁ null mice. Whole-cell patch clamp recordings were made from these neurons to determine the effects of CB₂ on short-term synaptic plasticity. CB₂ expression restored depolarization induced suppression of excitation to these neurons, which was lost following genetic ablation of CB₁. The endocannabinoid 2-arachidonylglycerol (2-AG) mimicked the effects of depolarization in CB₂ expressing neurons. Interestingly, ongoing basal production of 2-AG resulted in constitutive activation of CB₂, causing a tonic inhibition of neurotransmission that was relieved by the CB₂ antagonist AM630 or the diacylglycerol lipase inhibitor RHC80267. Through immunocytochemistry and analysis of spontaneous EPSCs, paired pulse ratios and coefficients of variation we determined that CB₂ exerts its function at a presynaptic site of action, likely through inhibition of voltage gated calcium channels. Therefore CB₂ expressed in neurons effectively mimics the actions of CB₁. Thus neuronal CB₂ is well suited to integrate into conventional neuronal endocannabinoid signaling processes, with its specific role determined by its unique and highly inducible expression profile.

Cannabinoid receptors CB1 and CB2 form functional heteromers in brain

Exploring the role of cannabinoid CB(2) receptors in the brain, we present evidence of CB(2) receptor molecular and functional interaction with cannabinoid CB(1) receptors. Using biophysical and biochemical approaches, we discovered that CB(2) receptors can form heteromers with CB(1) receptors in transfected neuronal cells and in rat brain pineal gland, nucleus accumbens, and globus pallidus. Within CB(1)-CB(2) receptor heteromers expressed in a neuronal cell model, agonist co-activation of CB(1) and CB(2) receptors resulted in a negative cross-talk in Akt phosphorylation and neurite outgrowth. Moreover, one specific characteristic of CB(1)-CB(2) receptor heteromers consists of both the ability of CB(1) receptor antagonists to block the effect of CB(2) receptor agonists and, conversely, the ability of CB(2) receptor antagonists to block the effect of CB(1) receptor agonists, showing a bidirectional cross-antagonism phenomenon. Taken together, these data illuminate the mechanism by which CB(2) receptors can negatively modulate CB(1) receptor function.

Multiple functions of endocannabinoid signaling in the brain

Despite being regarded as a hippie science for decades, cannabinoid research has finally found its well-deserved position in mainstream neuroscience. A series of groundbreaking discoveries revealed that endocannabinoid molecules are as widespread and important as conventional neurotransmitters such as glutamate or GABA, yet they act in profoundly unconventional ways. We aim to illustrate how uncovering the molecular, anatomical, and physiological characteristics of endocannabinoid signaling has revealed new mechanistic insights into several fundamental phenomena in synaptic physiology. First, we summarize unexpected advances in the molecular complexity of biogenesis and inactivation of the two endocannabinoids, anandamide and 2-arachidonoylglycerol. Then, we show how these new metabolic routes are integrated into well-known intracellular signaling pathways. These endocannabinoid-producing signalosomes operate in phasic and tonic modes, thereby differentially governing homeostatic, short-term, and long-term synaptic plasticity throughout the brain. Finally, we discuss how cell type- and synapse-specific refinement of endocannabinoid signaling may explain the characteristic behavioral effects of cannabinoids.

AM2389, a high-affinity, in vivo potent CB1-receptor-selective cannabinergic ligand as evidenced by drug discrimination in rats and hypothermia testing in mice

RATIONALE

The endocannabinoid signaling system (ECS) has been targeted for developing novel therapeutics since ECS dysfunction has been implicated in various pathologies. Current focus is on chemical modifications of the hexahydrocannabinol (HHC) nabilone (Cesamet(®)).

OBJECTIVE

To characterize the novel, high-affinity cannabinoid receptor 1 (CB(1)R) HHC-ligand AM2389 [9β-hydroxy-3-(1-hexyl-cyclobut-1-yl)-hexahydrocannabinol in two rodent pre-clinical assays.

MATERIALS AND METHODS

CB(1)R mediation of AM2389-induced hypothermia in mice was evaluated with AM251, a CB(1)R-selective antagonist/inverse agonist. Additionally, two groups of rats discriminated the full cannabinergic aminoalkylindole AM5983 (0.18 and 0.56 mg/kg) from vehicle 20 min post-injection in a two-choice operant conditioning task motivated by 0.1% saccharin/water. Generalization/substitution tests were conducted with AM2389, AM5983, and Δ(9)-tetrahydrocannabinol (Δ(9)-THC).

RESULTS

Δ(9)-THC (30 mg/kg)-induced hypothermia exhibited a faster onset and shorter duration of action compared with AM2389 (0.1 and 0.3 mg/kg). AM251 (3 and 10 mg/kg) attenuated/blocked hypothermia induced by 0.3 mg/kg AM2389. In drug discrimination, the order of potency was AM2389 > AM5983 > Δ(9)-THC with ED(50) values of 0.0025, 0.0571, and 0.2635 mg/kg, respectively, in the low-dose condition. The corresponding ED(50) values in the high-dose condition were 0.0069, 0.1246, and 0.8438 mg/kg, respectively. Onset of the effects of AM2389 was slow with a protracted time-course; the functional, perceptual in vivo half-life was approximately 17 h.

CONCLUSIONS

This potent cannabinergic HHC exhibited a slow onset of action with a protracted time-course. The AM2389 chemotype appears well suited for further drug development, and AM2389 currently is used to probe behavioral consequences of sustained ECS activation.

Excitability of prefrontal cortical pyramidal neurons is modulated by activation of intracellular type-2 cannabinoid receptors

The endocannabinoid (eCB) system is widely expressed throughout the central nervous system (CNS) and the functionality of type-1 cannabinoid receptors in neurons is well documented. In contrast, there is little knowledge about type-2 cannabinoid receptors (CB(2)Rs) in the CNS. Here, we show that CB(2)Rs are located intracellularly in layer II/III pyramidal cells of the rodent medial prefrontal cortex (mPFC) and that their activation results in IP(3)R-dependent opening of Ca(2+)-activated Cl(-) channels. To investigate the functional role of CB(2)R activation, we induced neuronal firing and observed a CB(2)R-mediated reduction in firing frequency. The description of this unique CB(2)R-mediated signaling pathway, controlling neuronal excitability, broadens our knowledge of the influence of the eCB system on brain function.

Effects of a selective cannabinoid CB2 agonist and antagonist on intravenous nicotine self administration and reinstatement of nicotine seeking

Over the last decade there have been significant advances in the discovery and understanding of the cannabinoid system along with the development of pharmacologic tools that modulate its function. Characterization of the crosstalk between nicotine addiction and the cannabinoid system may have significant implications on our understanding of the neurobiological mechanisms underlying nicotine dependence. Two types of cannabinoid receptors (CB1 and CB2) have been identified. CB1 receptors are expressed in the brain and modulate drug taking and drug seeking for various drugs of abuse, including nicotine. CB2 receptors have been recently identified in the brain and have been proposed to play a functional role in mental disorders and drug addiction. Our objective was to explore the role of CB2 receptors on intravenous nicotine self administration under two schedules of reinforcement (fixed and progressive ratio) and on nicotine seeking induced by nicotine priming or by nicotine associated cues. For this, we evaluated the effects of various doses of the selective CB2 antagonist AM630 (1.25 to 5 mg/kg) and CB2 agonist AM1241 (1 to 10 mg/kg) on these behavioral responses in rats. Different groups of male Long Evans rats were trained to lever press for nicotine at a unit dose of 30 µg/kg/infusion. Subsequently, animals were randomized using a Latin-square design and injected with either AM1241 or AM630 using a counterbalanced within subject design. Administration of the CB2 ligands did not affect either nicotine-taking nicotine-seeking behavior. Our results do not support the involvement of CB2 receptors in nicotine-taking or nicotine-seeking behavior.

Cocaine effects are modulated by central CB2 receptors in mice: new hope in a cannabinoid target for addiction and other CNS disorders

Evaluation of: Xi ZX, Peng XQ, Li X et al. Brain cannabinoid CB2 receptors modulate cocaine’s actions in mice. Nat. Neurosci. 14(9), 1160–1166 (2011). This study aimed to investigate the possible involvement of brain CB2 receptors in cocaine’s behavioral and neurochemical effects by using highly selective CB2 receptor agonists and antagonists, combined with CB1 and CB2 receptor knockout mice. They found that systemic or intracerebral administration of the CB2 selective agonist, JWH133, dose-dependently inhibited intravenous cocaine self-administration, cocaine-induced locomotion and cocaine-induced nucleus accumbens extracellular dopamine in wild-type and CB1 receptor knockout mice, but not in CB2 receptor knockout mice. These findings indicate that brain CB2 receptors modulate cocaine’s reinforcing and locomotor-stimulating effects, likely through a dopamine-dependent mechanism, and further suggest that selective ligands targeting brain CB2 receptors may represent a novel target for cocaine and other drug addictions as well as other CNS disorders.

Zebrafish: a model for the study of addiction genetics

Drug abuse and dependence are multifaceted disorders with complex genetic underpinnings. Identifying specific genetic correlates is challenging and may be more readily accomplished by defining endophenotypes specific for addictive disorders. Symptoms and syndromes, including acute drug response, consumption, preference, and withdrawal, are potential endophenotypes characterizing addiction that have been investigated using model organisms. We present a review of major genes involved in serotonergic, dopaminergic, GABAergic, and adrenoreceptor signaling that are considered to be directly involved in nicotine, opioid, cannabinoid, and ethanol use and dependence. The zebrafish genome encodes likely homologs of the vast majority of these loci. We also review the known expression patterns of these genes in zebrafish. The information presented in this review provides support for the use of zebrafish as a viable model for studying genetic factors related to drug addiction. Expansion of investigations into drug response using model organisms holds the potential to advance our understanding of drug response and addiction in humans.

Endocannabinoid signaling, glucocorticoid-mediated negative feedback, and regulation of the hypothalamic-pituitary-adrenal axis

The hypothalamic-pituitary-adrenal (HPA) axis regulates the outflow of glucocorticoid hormones under basal conditions and in response to stress. Within the last decade, a large body of evidence has mounted indicating that the endocannabinoid system is involved in the central regulation of the stress response; however, the specific role endocannabinoid signaling plays in phases of HPA axis regulation, and the neural sites of action mediating this regulation, were not mapped out until recently. This review aims to collapse the current state of knowledge regarding the role of the endocannabinoid system in the regulation of the HPA axis to put together a working model of how and where endocannabinoids act within the brain to regulate outflow of the HPA axis. Specifically, we discuss the role of the endocannabinoid system in the regulation of the HPA axis under basal conditions, activation in response to acute stress, and glucocorticoid-mediated negative feedback. Interestingly, there appears to be some anatomical specificity to the role of the endocannabinoid system in each phase of HPA axis regulation, as well as distinct roles of both anandamide and 2-arachidonoylglycerol in these phases. Overall, the current level of information indicates that endocannabinoid signaling acts to suppress HPA axis activity through concerted actions within the prefrontal cortex, amygdala, and hypothalamus.

Marijuana, endocannabinoids, and epilepsy: potential and challenges for improved therapeutic intervention

Phytocannabinoids isolated from the cannabis plant have broad potential in medicine that has been well recognized for many centuries. It is presumed that these lipid soluble signaling molecules exert their effects in both the central and peripheral nervous system in large part through direct interaction with metabotropic cannabinoid receptors. These same receptors are also targeted by a variety of endogenous cannabinoids including 2-arachidonoyl glycerol and anandamide. Significant effort over the last decade has produced an enormous advance in our understanding of both the cellular and the synaptic physiology of endogenous lipid signaling systems. This increase in knowledge has left us better prepared to carefully evaluate the potential for both natural and synthetic cannabinoids in the treatment of a variety of neurological disorders. In the case of epilepsy, long standing interest in therapeutic approaches that target endogenous cannabinoid signaling systems are, for the most part, not well justified by available clinical data from human epileptics. Nevertheless, basic science experiments have clearly indicated a key role for endogenous cannabinoid signaling systems in moment to moment regulation of neuronal excitability. Further it has become clear that these systems can both alter and be altered by epileptiform activity in a wide range of in vitro and in vivo models of epilepsy. Collectively these observations suggest clear potential for effective therapeutic modulation of endogenous cannabinoid signaling systems in the treatment of human epilepsy, and in fact, further highlight key obstacles that would need to be addressed to reach that goal.

Functional selectivity in CB(2) cannabinoid receptor signaling and regulation: implications for the therapeutic potential of CB(2) ligands

Receptor internalization increases the flexibility and scope of G protein-coupled receptor (GPCR) signaling. CB(1) and CB(2) cannabinoid receptors undergo internalization after sustained exposure to agonists. However, it is not known whether different agonists internalize CB(2) to different extents. Because CB(2) is a promising therapeutic target, understanding its trafficking in response to different agonists is necessary for a complete understanding of its biology. Here we profile a number of cannabinoid receptor ligands and provide evidence for marked functional selectivity of cannabinoid receptor internalization. Classic, aminoalkylindole, bicyclic, cannabilactone, iminothiazole cannabinoid, and endocannabinoid ligands varied greatly in their effects on CB(1) and CB(2) trafficking. Our most striking finding was that (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo-[1,2,3-d,e]-1,4-benzoxazin-6-yl]-1-naphthalenyl-methanone (WIN55,212-2) (and other aminoalkylindoles) failed to promote CB(2) receptor internalization, whereas 5-(1,1-dimethylheptyl)-2-(5-hydroxy-2-(3-hydroxypropyl)cyclohexyl)phenol (CP55,940) robustly internalized CB(2) receptors. Furthermore, WIN55,212-2 competitively antagonized CP55,940-induced CB(2) internalization. Despite these differences in internalization, both compounds activated CB(2) receptors as measured by extracellular signal-regulated kinase 1/2 phosphorylation and recruitment of β-arrestin(2) to the membrane. In contrast, whereas CP55,940 inhibited voltage-gated calcium channels via CB(2) receptor activation, WIN55,212-2 was ineffective on its own and antagonized the effects of CP55,940. On the basis of the differences we found between these two ligands, we also tested the effects of other cannabinoids on these signaling pathways and found additional evidence for functional selectivity of CB(2) ligands. These novel data highlight that WIN55,212-2 and other cannabinoids show strong functional selectivity at CB(2) receptors and suggest that different classes of CB(2) ligands may produce diverse physiological effects, emphasizing that each class needs to be separately evaluated for therapeutic efficacy.