Cannabinoid receptor ligand bias: implications in the central nervous system

Cannabidiol: metabolism and clinical efficacy in epileptic patients

INTRODUCTION

The landscape of epilepsy treatment has undergone a significant transformation with the emergence of cannabidiol as a potential therapeutic agent. Epidiolex, a pharmaceutical formulation of highly purified CBD, garnered significant attention not just for its therapeutic potential but also for being the first cannabis-derived medication to obtain approval from regulatory bodies.

AREA COVERED

In this narrative review the authors explore the intricate landscape of CBD as an antiseizure medication, deepening into its pharmacological mechanisms and clinical trials involving various epileptic encephalopathies. This exploration serves as a comprehensive guide, shedding light on a compound that holds promise for individuals contending with the significant challenges of drug-resistant epilepsy.

EXPERT OPINION

Rigorous studies highlight cannabidiol's efficacy, safety profile, and potential cognitive benefits, warranting further exploration for its approval in various drug-resistant epilepsy forms. As a promising therapeutic option, cannabidiol not only demonstrates efficacy in seizure control but also holds the potential for broader enhancements in the quality of life, especially for patients with epileptic encephalopathies.

The Sobering Sting: Oleoyl Serotonin Is a Novel Stephanoconus Snail Venom-Derived Antagonist of Cannabinoid Receptors That Counteracts Learning and Memory Deficits

Cannabinoid receptors (CB1 and CB2) are promising targets for a better understanding of neurological diseases. Nevertheless, only a few ligands of CB have reached clinical application so far. Venoms are considered as interesting sources of novel biologically active compounds. Here, we describe an endocannabinoid-like molecule, oleoyl serotonin (OS), present in the venom of Stephanoconus snails. Using electrophysiological assays, it was shown that OS inhibits CB1 and CB2. Structure-activity relationship studies using a chimeric CB1/2 revealed that the domain encompassing the transmembrane helix V (TMHV)- intracellular loop 3 (ICL3)-TMHVI of the CB2 is critical for the binding and function of OS. We concluded that OS binds to sites of the CB2 that are different from the binding sites of the non-selective CB agonist WIN55,212-2. Behavioral assays in mice showed that OS counteracted learning and memory deficits caused by WIN55,212-2. Furthermore, a selectivity screening of OS showed high selectivity for CB over various ion channels and receptors. Overall, OS may represent a new approach to the prevention and treatment of learning and memory cognition impairment in neurological diseases.

Research progress in the management of vascular disease with cannabidiol: a review

The morbidity and mortality rates associated with vascular disease (VD) have been gradually increasing. Currently, the most common treatment for VD is surgery, with the progress in drug therapy remaining slow. Cannabidiol (CBD) is a natural extract of Cannabis sativa L. with sedative, analgesic, and nonaddictive properties. CBD binds to 56 cardiovascular-related receptors and exerts extensive regulatory effects on the cardiovascular system, making it a potential pharmacological agent for the management of VD. However, most CBD studies have focused on neurological and cardiac diseases, and research on the management of VD with CBD is still rare. In this review, we summarize the currently available data on CBD in the management of VD, addressing four aspects: the major molecular targets of CBD in VD management, pharmacokinetic properties, therapeutic effects of CBD on common VDs, and side effects. The findings indicate that CBD has anti-anxiety, anti-oxidation, and anti-inflammatory properties and can inhibit abnormal proliferation and apoptosis of vascular smooth muscle and endothelial cells; these effects suggest CBD as a therapeutic agent for atherosclerosis, stress-induced hypertension, diabetes-related vasculopathy, ischemia-reperfusion injury, and vascular damage caused by smoking and alcohol abuse. This study provides a theoretical basis for further research on CBD in the management of VD.

Similarities and differences upon binding of naturally occurring Δ9-tetrahydrocannabinol-derivatives to cannabinoid CB1 and CB2 receptors

We have here assessed, using Δ⁹-tetrahydrocannabinol (Δ⁹-THC) for comparison, the effect of Δ⁹-tetrahydrocannabinolic acid (Δ⁹-THCA) and of Δ⁹-tetrahydrocannabivarin (Δ9-THCV) that is mediated by human versions of CB₁, CB₂, and CB₁-CB₂ receptor functional units, expressed in a heterologous system. Binding to the CB₁ and CB₂ receptors was addressed in living cells by means of a homogeneous assay. A biphasic competition curve for the binding to the CB₂ receptor, was obtained for Δ⁹-THCV in cells expressing the two receptors. Signaling studies included cAMP level determination, activation of the mitogen-activated protein kinase pathway and ß-arrestin recruitment were performed. The signaling triggered by Δ⁹-THCA and Δ⁹-THCV via individual receptors or receptor heteromers disclosed differential bias, i.e. the bias observed using a given phytocannabinoid depended on the receptor (CB₁, CB₂ or CB₁-CB₂) and on the compound used as reference to calculate the bias factor (Δ⁹-THC, a selective agonist or a non-selective agonist). These results are consistent with different binding modes leading to differential functional selectivity depending on the agonist structure, and the state (monomeric or heteromeric) of the cannabinoid receptor. In addition, on studying Gi-coupling we showed that Δ⁹-THCV and Δ⁹-THCA and Δ⁹-THCV were able to revert the effect of a selective CB₂ receptor agonist, but only Δ9-THCV, and not Δ9-THCA, reverted the effect of arachidonyl-2'-chloroethylamide (ACEA 100 nM) a selective agonist of the CB₁ receptor. Overall, these results indicate that cannabinoids may have a variety of binding modes that results in qualitatively different effects depending on the signaling pathway that is engaged upon cannabinoid receptor activation.

Axonal CB1 Receptors Mediate Inhibitory Bouton Formation via cAMP Increase and PKA

Experience-dependent formation and removal of inhibitory synapses are essential throughout life. For instance, GABAergic synapses are removed to facilitate learning, and strong excitatory activity is accompanied by the formation of inhibitory synapses to maintain coordination between excitation and inhibition. We recently discovered that active dendrites trigger the growth of inhibitory synapses via CB1 receptor-mediated endocannabinoid signaling, but the underlying mechanism remained unclear. Using two-photon microscopy to monitor the formation of individual inhibitory boutons in hippocampal organotypic slices from mice (both sexes), we found that CB1 receptor activation mediated the formation of inhibitory boutons and promoted their subsequent stabilization. Inhibitory bouton formation did not require neuronal activity and was independent of G_i/o-protein signaling, but was directly induced by elevating cAMP levels using forskolin and by activating G_s-proteins using DREADDs. Blocking PKA activity prevented CB1 receptor-mediated inhibitory bouton formation. Our findings reveal that axonal CB1 receptors signal via unconventional downstream pathways and that inhibitory bouton formation is triggered by an increase in axonal cAMP levels. Our results demonstrate an unexpected role for axonal CB1 receptors in axon-specific, and context-dependent, inhibitory synapse formation.SIGNIFICANCE STATEMENT Coordination between excitation and inhibition is required for proper brain function throughout life. It was previously shown that new inhibitory synapses can be formed in response to strong excitation to maintain this coordination, and this was mediated by endocannabinoid signaling via CB1 receptors. As activation of CB1 receptors generally results in the suppression of synaptic transmission, it remained unclear how CB1 receptors can mediate the formation of inhibitory synapses. Here we show that CB1 receptors on inhibitory axons signal via unconventional intracellular pathways and that inhibitory bouton formation is triggered by an increase in axonal cAMP levels and requires PKA activity. Our findings point to a central role for axonal cAMP signaling in activity-dependent inhibitory synapse formation.

The Endocannabinoid System and Cannabidiol: Past, Present, and Prospective for Cardiovascular Diseases

In the past, cannabis was commonly associated with mysticism and illegality. Fortunately, in recent years perspectives and discourses have changed. More prominence has been given to the rigorous scientific effort that led to the discovery of cannabis' many physiological actions and endogenous signalling mechanisms. The endocannabinoid system is a complex and heterogeneous pro-homeostatic network comprising different receptors with several endogenous ligands, numerous metabolic enzymes and regulatory proteins. Therefore, it is not surprising that alterations and dysfunctions of the endocannabinoid system are observed in almost every category of disease. Such high degree of pathophysiological involvement suggests the endocannabinoid system is a promising therapeutic target and prompted the translation of resurgent scientific findings into clinical therapies. Shifting attitudes toward cannabis also raised other matters such as increased patient awareness, prescription requests, self-medication, recreational use, recognition of new knowledge gaps, renewed scientific activity, and seemingly exponential growth of the cannabis industry. This review, following a general overview of cannabis and the endocannabinoid system, assiduously describes its role within the context of cardiovascular diseases, paying particular attention to the Janus influence that endocannabinoid system modulators can have on the cardiovascular system.

Cannabinoid receptor agonist WIN55212-2 reduces unpredictable mild stress-induced depressive behavior of rats

Background

Depression is a neurological disorder characterized by persistent low mood. A number of studies have suggested that the use of type 1 cannabinoid receptor (CB1R) agonists can reduce depressive behavior, but its effect on the depressive behavior and nerve damage of rats exposed to chronic unpredictable mild stress (CUMS) has not been reported.

Methods

Rats were exposed to CUMS for 4 weeks to induce depressive behavior. Male Sprague-Dawley (SD) rats aged 6–8 weeks were randomly divided into six groups: control group (control), depression group (CUMS), depression + fluoxetine group (Flu), depression + WIN55212-2 group (WIN), depression + NF-κB inhibitor group (PDTC), and depression + WIN + PDTC group (WIN + PDTC). We performed four behavioral experiments test to evaluate the depressive behaviors of rats. Hematoxylin and eosin (HE) and Nissl staining were performed to observe the neuron structures of the hippocampus. Enzyme-linked immunosorbent assay (ELISA) was used to measure the concentrations of interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and cyclooxygenase-2 (COX-2). Biochemical experiments were performed to evaluate the concentrations of nitric oxide (NO), malondialdehyde (MDA), reactive oxygen species (ROS), and superoxide dismutase (SOD). Fluorescence quantitative PCR was used to detect the mRNA expression of brain-derived neurotrophic factor (BDNF), tyrosine kinase receptor B (TrkB), and inducible nitric oxide synthase (iNOS) in the hippocampus, and western blot was performed to detect protein expression levels related to the NF-κB signaling pathway in the hippocampus.

Results

Compared with the normal control group, CUMS significantly induced abnormal behaviors in stressed rats. The concentrations of pro-inflammatory factors and oxidative stress injury factors in the hippocampus of the CUMS group increased significantly. The interventions of Flu, WIN, and PDTC significantly reduced neuroinflammation and oxidative stress injury. Compared with the WIN group, the WIN + PDTC intervention group had better results. In addition, WIN could significantly inhibit the activation of the NF-κB signaling pathway.

Conclusions

This study showed that cannabinoid receptor agonists can reduce the CUMS-induced depressive behaviors of rats by blocking the NF-κB signaling pathway to alleviate neuroinflammation and oxidative stress injury.

Systematic evaluation of a panel of 30 synthetic cannabinoid receptor agonists structurally related to MMB-4en-PICA, MDMB-4en-PINACA, ADB-4en-PINACA, and MMB-4CN-BUTINACA using a combination of binding and different CB1 receptor activation assays. Part III: The G protein pathway and critical comparison of different assays

The present work is the last of a three-part study investigating a panel of 30 systematically designed synthetic cannabinoid receptor agonists (SCRAs) including features such as the 4-pentenyl tail and varying head groups including amides and esters of l-valine (MMB, AB), l-tert-leucine (ADB), and l-phenylalanine (APP), as well as adamantyl (A) and cumyl moieties (CUMYL). Here, we evaluated these SCRAs for their capacity to activate the human cannabinoid receptor 1 (CB₁ ) via indirect measurement of G protein recruitment. Furthermore, we comparatively evaluated the results obtained from three in vitro assays, based on the recruitment of β-arrestin 2 (βarr2 assay) or Gα_i protein (mini-Gα_i assay), or binding of [³⁵ S]-GTPγS. The observed efficacies (E_max ) varied depending on the conducted assay. Statistical analysis suggests that the population means of the relative intrinsic activity (RA_i ) significantly differ for the [³⁵ S]-GTPγS assay and the other two assays, but the population means of the βarr2 and mini-Gα_i assays were not statistically different. Our data suggest that differences observed between the βarr2 and mini-Gα_i assays are the best predictor for 'biased agonism' towards βarr or G protein recruitment in our study. SCRAs carrying an ADB or MPP moiety as a head group tended to produce elevated E_max values in the βarr2 assay, which might result in a tendency of these compounds to cause pronounced tolerance in users-a hypothesis that should be evaluated further by future studies. In general, a comparison of efficacies derived from different assays is difficult and should only be conducted very cautiously.

Pharmacological selection of cannabinoid receptor effectors: Signalling, allosteric modulation and bias

The type-1 cannabinoid receptor (CB₁) is a promising drug target for a wide range of diseases. However, many existing and novel candidate ligands for CB₁ have shown only limited therapeutic potential. Indeed, no ligands are currently approved for the clinic except formulations of the phytocannabinoids Δ⁹-THC and CBD and a small number of analogues. A key limitation of many promising CB₁ ligands are their on-target adverse effects, notably including psychoactivity (agonists) and depression/suicidal ideation (inverse agonists). Recent drug development attempts have therefore focussed on altering CB₁ signalling profiles in two ways. Firstly, with compounds that enhance or reduce the signalling of endogenous (endo-) cannabinoids, namely allosteric modulators. Secondly, with compounds that probe the capability of selectively targeting specific cellular signalling pathways that may mediate therapeutic effects using biased ligands. This review will summarise the current paradigm of CB₁ signalling in terms of the intracellular transduction pathways acted on by the receptor. The development of compounds that selectively activate CB₁ signalling pathways, whether allosterically or via orthosteric agonist bias, will also be addressed.

Cannabinoid receptor type 2 ligands: an analysis of granted patents since 2010

The G-protein-coupled cannabinoid receptor type 2 (CB2R) is a key element of the endocannabinoid (EC) system. EC/CB2R signaling has significant therapeutic potential in major pathologies affecting humans such as allergies, neurodegenerative disorders, inflammation or ocular diseases. CB2R agonism exerts anti-inflammatory and tissue protective effects in preclinical animal models of cardiovascular, gastrointestinal, liver, kidney, lung and neurodegenerative disorders. Existing ligands can be subdivided into endocannabinoids, cannabinoid-like and synthetic CB2R ligands that possess various degrees of potency on and selectivity against the cannabinoid receptor type 1. This review is an account of granted CB2R ligand patents from 2010 up to the present, which were surveyed using Derwent Innovation®.

WIN55,212-2, a Dual Modulator of Cannabinoid Receptors and G Protein-Coupled Inward Rectifier Potassium Channels

The coupling of cannabinoid receptors, CB1 and CB2, to G protein-coupled inward rectifier potassium channels, GIRK1 and GIRK2, modulates neuronal excitability in the human brain. The present study established and validated the functional expression in a Xenopus laevis oocyte expression system of CB1 and CB2 receptors, interacting with heteromeric GIRK1/2 channels and a regulator of G protein signaling, RGS4. This ex vivo system enables the discovery of a wide range of ligands interacting orthosterically or allosterically with CB1 and/or CB2 receptors. WIN55,212-2, a non-selective agonist of CB1 and CB2, was used to explore the CB1- or CB2-GIRK1/2-RGS4 signaling cascade. We show that WIN55,212-2 activates CB1 and CB2 at low concentrations whereas at higher concentrations it exerts a direct block of GIRK1/2. This illustrates a dual modulatory function, a feature not described before, which helps to explain the adverse effects induced by WIN55,212-2 in vivo. When comparing the effects with other typical cannabinoids such as Δ9-THC, CBD, CP55,940, and rimonabant, only WIN55,212-2 can significantly block GIRK1/2. Interestingly, the inward rectifier potassium channel, IRK1, a non-G protein-coupled potassium channel important for setting the resting membrane voltage and highly similar to GIRK1 and GIRK2, is not sensitive to WIN55,212-2, Δ9-THC, CBD, CP55,940, or rimonabant. From this, it is concluded that WIN55,212-2 selectively blocks GIRK1/2.

CB1 allosteric modulators and their therapeutic potential in CNS disorders

CB1 is the most abundant GPCR found in the mammalian brain. It has garnered considerable attention as a potential therapeutic drug target. CB1 is involved in a wide range of physiological and psychiatric processes and has the potential to be targeted in a wide range of disease states. However, most of the selective and non-selective synthetic CB1 agonists and antagonists/inverse agonists developed to date are primarily used as research tools. No novel synthetic cannabinoids are currently in the clinic for use in psychiatric illness; synthetic analogues of the phytocannabinoid THC are on the market to treat nausea and vomiting caused by cancer chemotherapy, along with off-label use for pain. Novel strategies are being explored to target CB1, but with emphasis on the elimination or mitigation of the potential psychiatric adverse effects that are observed by central agonism/antagonism of CB1. New pharmacological options are being pursued that may avoid these adverse effects while preserving the potential therapeutic benefits of CB1 modulation. Allosteric modulation of CB1 is one such approach. In this review, we will summarize and critically analyze both the in vitro characterization and in vivo validation of CB1 allosteric modulators developed to date, with a focus on CNS therapeutic effects.

Biased Coupling to β-Arrestin of Two Common Variants of the CB2 Cannabinoid Receptor

β-arrestins are partners of the G protein-coupled receptors (GPCRs), regulating their intracellular trafficking and signaling. Development of biased GPCR agonists, selectively targeting either G protein or β-arrestin pathways, are in the focus of interest due to their therapeutic potential in different pathological conditions. The CB₂ cannabinoid receptor (CB₂R) is a GPCR involved in various functions in the periphery and the central nervous system. Two common occurring variants of CB₂R, harboring Q63R or L133I missense mutations, have been implicated in the development of a diverse set of disorders. To evaluate the effect of these mutations, we characterized the binding profile of these mutant CB₂ receptors to G proteins and β-arrestin2. Although their ability to inhibit cAMP signaling was similar, the Q63R mutant had increased, whereas the L133I mutant receptor had decreased β-arrestin2 binding. In line with these observations, the variants also had altered intracellular trafficking. Our results show that two common variants of the CB₂ receptor have biased signaling properties, which may contribute to the pathogenesis of the associated disorders and may offer CB₂R as a target for further development of biased receptor activation strategies.

Psychiatric Disorders and Cannabinoid Receptors

With the increasing global use of medical and adult recreational use of cannabis and cannabinoids, this chapter provides overview of evidence from animal and human studies on psychiatric disorders and cannabinoid receptors. We review and present evaluation of the relationship between changes in the ECS and psychiatric disorders. Evidence suggests the existence of a relationship between changes in components of the ECS, and some of the symptoms present in psychiatric disorders. Both CB1Rs and CB2Rs are components of the endocannabinoid system with different cellular and tissue localization patterns that are differentially expressed in the CNS and PNS and are emerging targets for the treatment of number psychiatric disorders. As cannabis preparations are widely used for recreation globally, it is predictable that cannabis use disorders (CUDs) will increase and there is currently no available treatment for CUDs. Although major advances have been reported from cannabinoid and ECS research, there are gaps in scientific knowledge on long-term consequences of cannabis use. Adolescent and cannabis use during pregnancy presents further challenges, and more research will uncover the signaling pathways that couple the gut microbiota with the host ECS. Development of cannabis and cannabinoid nanomedicine for nanotherapy will certainly overcome some of the shortcomings and challenges in medicinal and recreational use of cannabis and cannabinoids. Thus, nanotechnology will allow targeted delivery of cannabinoid formulations with the potential to elevate their use to scientifically validated nanotherapeutic applications as the field of cannabis nanoscience matures.

Effects of Cannabinoid Agonists and Antagonists on Sleep in Laboratory Animals

The cannabinoids are a family of chemical compounds that can be either synthesized or naturally derived. These compounds have been shown to modulate a wide variety of biological processes. In this chapter, the studies detailing the effects of cannabinoids on sleep in laboratory animals are reviewed. Both exogenous and endogenous cannabinoids generally appear to decrease wakefulness and alter rapid eye movement (REM) and non-REM sleep in animal models. In addition, cannabinoids potentiate the effects of sedative-hypnotic drugs. However, the individual contributions of each cannabinoid on sleep processes is more nuanced and may depend on the site of action in the central nervous system. Many studies investigating the mechanism of cannabinoid effects on sleep suggest that the effects of cannabinoids on sleep are mediated via cannabinoid receptors; however, some evidence suggests that some sleep effects may be elicited via non-cannabinoid receptor-dependent mechanisms. More research is necessary to fully elucidate the role of each compound in modulating sleep processes.

Therapeutic potential and safety considerations for the clinical use of synthetic cannabinoids

The phytocannabinoid Δ9-tetrahydrocannabinol (THC) was isolated and synthesized in the 1960s. Since then, two synthetic cannabinoids (SCBs) targeting the cannabinoid 1 (CB1R) and 2 (CB2R) receptors were approved for medical use based on clinical safety and efficacy data: dronabinol (synthetic THC) and nabilone (synthetic THC analog). To probe the function of the endocannabinoid system further, hundreds of investigational compounds were developed; in particular, agonists with (1) greater CB1/2R affinity relative to THC and (2) full CB1/2R agonist activity. This pharmacological profile may pose greater risks for misuse and adverse effects relative to THC, and these SCBs proliferated in retail markets as legal alternatives to cannabis (e.g., novel psychoactive substances [NPS], "Spice," "K2"). These SCBs were largely outlawed in the U.S., but blanket policies that placed all SCB chemicals into restrictive control categories impeded research progress into novel mechanisms for SCB therapeutic development. There is a concerted effort to develop new, therapeutically useful SCBs that target novel pharmacological mechanisms. This review highlights the potential therapeutic efficacy and safety considerations for unique SCBs, including CB1R partial and full agonists, peripherally-restricted CB1R agonists, selective CB2R agonists, selective CB1R antagonists/inverse agonists, CB1R allosteric modulators, endocannabinoid-degrading enzyme inhibitors, and cannabidiol. We propose promising directions for SCB research that may optimize therapeutic efficacy and diminish potential for adverse events, for example, peripherally-restricted CB1R antagonists/inverse agonists and biased CB1/2R agonists. Together, these strategies could lead to the discovery of new, therapeutically useful SCBs with reduced negative public health impact.

Heteromer formation between cannabinoid type 1 and dopamine type 2 receptors is altered by combination cannabinoid and antipsychotic treatments

The cannabinoid type 1 (CB₁ ) receptor and the dopamine type 2 (D₂ ) receptor are co-localized on medium spiny neuron terminals in the globus pallidus where they modulate neural circuits involved in voluntary movement. Physical interactions between the two receptors have been shown to alter receptor signaling in cell culture. The objectives of the current study were to identify the presence of CB₁ /D₂ heteromers in the globus pallidus of C57BL/6J male mice, define how CB₁ /D₂ heteromer levels are altered following treatment with cannabinoids and/or antipsychotics, and determine if fluctuating levels of CB₁ /D₂ heteromers have functional consequences. Using in situ proximity ligation assays, we observed CB₁ /D₂ heteromers in the globus pallidus of C57BL/6J mice. The abundance of the heteromers increased following treatment with the nonselective cannabinoid receptor agonist, CP55,940. In contrast, treatment with the typical antipsychotic haloperidol reduced the number of CB₁ /D₂ heteromers, whereas the atypical antipsychotic olanzapine treatment had no effect. Co-treatment with CP55,940 and haloperidol had similar effects to haloperidol alone, whereas co-treatment with CP55,940 and olanzapine had similar effects to CP55,940. The observed changes were found to have functional consequences as the differential effects of haloperidol and olanzapine also applied to γ-aminobutyric acid release in STHdh^Q7/Q7 cells and motor function in C57BL/6J male mice. This work highlights the clinical relevance of co-exposure to cannabinoids and different antipsychotics over acute and prolonged time periods.

Neurobiology of cannabinoid receptor signaling

The endocannabinoid system (ECS) is a highly versatile signaling system within the nervous system. Despite its widespread localization, its functions within the context of distinct neural processes are very well discernable and specific. This is remarkable, and the question remains as to how such specificity is achieved. One key player in the ECS is the cannabinoid type 1 receptor (CB₁), a G protein-coupled receptor characterized by the complexity of its cell-specific expression, cellular and subcellular localization, and its adaptable regulation of intracellular signaling cascades. CB₁ receptors are involved in different synaptic and cellular plasticity processes and in the brain's bioenergetics in a context-specific manner. CB₂ receptors are also important in several processes in neurons, glial cells, and immune cells of the brain. As polymorphisms in ECS components, as well as external impacts such as stress and metabolic challenges, can both lead to dysregulated ECS activity and subsequently to possible neuropsychiatric disorders, pharmacological intervention targeting the ECS is a promising therapeutic approach. Understanding the neurobiology of cannabinoid receptor signaling in depth will aid optimal design of therapeutic interventions, minimizing unwanted side effects.
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Novel approaches and current challenges with targeting the endocannabinoid system

INTRODUCTION

The pathophysiological relevance of the endocannabinoid system has been widely demonstrated in a variety of diseases including cancer, neurological disorders, and metabolic issues. Therefore, targeting the receptors and the endogenous machinery involved in this system can provide a successful therapeutic outcome. Ligands targeting the canonical cannabinoid receptors, CB1 and CB2, along with inhibitors of the endocannabinoid enzymes have been thoroughly studied in diverse disease models. In fact, phytocannabinoids such as cannabidiol or Δ9-tetrahydrocannabinol are currently on the market for the management of neuropathic pain due to spasticity in multiple sclerosis or seizures in children epilepsy amongst others.

AREAS COVERED

Challenges in the pharmacology of cannabinoids arise from its pharmacokinetics, off-target effects, and psychoactive effects. In this context, the current review outlines the novel molecular approaches emerging in the field discussing their clinical potential.

EXPERT OPINION

Even if orthosteric CB1 and CB2 ligands are on the forefront in cannabinoid clinical research, emerging strategies such as allosteric or biased modulation of these receptors along with controlled off-targets effects may increase the therapeutic potential of cannabinoids.

Structural and Functional Insights into Cannabinoid Receptors

Cannabinoid receptors type 1 (CB1) and 2 (CB2) are widely expressed in the human body, and are attractive drug targets in the prevention and management of central nervous system (CNS) and immune system dysfunction, respectively. Recent breakthroughs in the structural elucidation of cannabinoid receptors and their signaling complexes with G proteins, provide the important molecular basis of ligand-receptor interactions, activation and signaling mechanism, which will facilitate the next-generation drug design and the precise modulation of the endocannabinoid system. Here, we provide an overview on the structural features of cannabinoid receptors in different functional states and the diverse ligand binding modes. The major challenges and new strategies for future therapeutic applications targeting the endocannabinoid system (ECS) are also discussed.

Activation of CB1R Promotes Lipopolysaccharide-Induced IL-10 Secretion by Monocytic Myeloid-Derived Suppressive Cells and Reduces Acute Inflammation and Organ Injury

Cannabis sativa and its principal components, Δ9-tetrahydrocannabinol (Δ9-THC) and cannabidiol, are increasingly being used to treat a variety of medical problems, including inflammatory conditions. Although studies suggest that the endocannabinoid system has immunomodulatory properties, there remains a paucity of information on the effects of cannabinoids on immunity and on outcomes of infection and injury. We investigated the effects and mechanism(s) of action of cannabinoid receptor agonists, including Δ9-THC, on inflammation and organ injury in endotoxemic mice. Administration of Δ9-THC caused a dramatic early upregulation of plasma IL-10 levels, reduced plasma IL-6 and CCL-2 levels, led to better clinical status, and attenuated organ injury in endotoxemic mice. The anti-inflammatory effects of Δ9-THC in endotoxemic mice were reversed by a cannabinoid receptor type 1 (CB₁R) inverse agonist (SR141716), and by clodronate-induced myeloid-cell depletion, but not by genetic invalidation or blockade of other putative Δ9-THC receptors, including cannabinoid receptor type 2, TRPV1, GPR18, GPR55, and GPR119. Although Δ9-THC administration reduced the activation of several spleen immune cell subsets, the anti-inflammatory effects of Δ9-THC were preserved in splenectomized endotoxemic mice. Finally, using IL-10-GFP reporter mice, we showed that blood monocytic myeloid-derived suppressive cells mediate the Δ9-THC-induced early rise in circulating IL-10. These results indicate that Δ9-THC potently induces IL-10, while reducing proinflammatory cytokines, chemokines, and related organ injury in endotoxemic mice via the activation of CB₁R. These data have implications for acute and chronic conditions that are driven by dysregulated inflammation, such as sepsis, and raise the possibility that CB₁R-signaling may constitute a novel target for inflammatory disorders.

The Endocannabinoid System and Synthetic Cannabinoids in Preclinical Models of Seizure and Epilepsy

Cannabinoids are compounds that are structurally and/or functionally related to the primary psychoactive constituent of Cannabis sativa, [INCREMENT]-tetrahydrocannabinol (THC). Cannabinoids can be divided into three broad categories: endogenous cannabinoids, plant-derived cannabinoids, and synthetic cannabinoids (SCs). Recently, there has been an unprecedented surge of interest into the pharmacological and medicinal properties of cannabinoids for the treatment of epilepsies. This surge has been stimulated by an ongoing shift in societal opinions about cannabinoid-based medicines and evidence that cannabidiol, a nonintoxicating plant cannabinoid, has demonstrable anticonvulsant activity in children with treatment-refractory epilepsy. The major receptors of the endogenous cannabinoid system (ECS)-the type 1 and 2 cannabinoid receptors (CB1R, CB2R)-have critical roles in the modulation of neurotransmitter release and inflammation, respectively; so, it is not surprising therefore that the ECS is being considered as a target for the treatment of epilepsy. SCs were developed as potential new drug candidates and tool compounds for studying the ECS. Beyond the plant cannabinoids, an extensive research effort is underway to determine whether SCs that directly target CB1R, CB2R, or the enzymes that breakdown endogenous cannabinoids have anticonvulsant effects in preclinical rodent models of epilepsy and seizure. This research demonstrates that many SCs do reduce seizure severity in rodent models and may have both positive and negative pharmacodynamic and pharmacokinetic interactions with clinically used antiepilepsy drugs. Here, we provide a comprehensive review of the preclinical evidence for and against SC modulation of seizure and discuss the important questions that need to be addressed in future studies.

Assessment of Biased Agonism among Distinct Synthetic Cannabinoid Receptor Agonist Scaffolds

Cannabinoid receptor 1 (CB₁) is a key drug target for a number of diseases, including metabolic syndromes and neuropathic pain. Most of the typical cannabinoid ligands provoke psychotropic side effects that impair their therapeutic utility. As of today, it is not yet clearly known which structural features of cannabinoid ligands determine a preference toward specific signaling pathways. Distinct bioassays are typically used to elucidate signaling preferences. However, these are often based on different cell lines and use different principles and/or read-outs, which makes straightforward assessment of "ligand bias" difficult. Within this context, this study is the first to investigate ligand bias among synthetic cannabinoid receptor agonists (SCRAs) in as closely analogous conditions as possible, by applying a new functional complementation-based assay panel to assess the recruitment of Gα_i protein or β-arrestin2 to CB₁. In a panel of 21 SCRAs, chosen to cover a broad diversity in chemical structures, distinct, although often subtle, preferences toward specific signaling pathways were observed. Relative to CP55940, here considered as a "balanced" reference agonist, most of the selected SCRAs (e.g., 5F-APINACA, CUMYL-PEGACLONE, among others) displayed preferred signaling through the β-arrestin2 pathway, whereas MMB-CHMICA could serve as a potential "balanced" agonist. Interestingly, EG-018 was the only SCRA showing a significant (10-fold) preference toward G protein over β-arrestin2 recruitment. While it is currently unclear what this exactly means in terms of abuse potential and/or toxicity, the approach proposed here may allow construction of a knowledge base that in the end may allow better insight into the structure-"functional" activity relationship of these compounds. This may aid the development of new therapeutics with less unwanted psychoactive effects.

Medical Use of Cannabinoids

Cannabinoid receptors, endocannabinoids and the enzymes responsible for their biosynthesis and degradation constitute the endocannabinoid system. In recent decades, the endocannabinoid system has attracted considerable interest as a potential therapeutic target in numerous pathological conditions. Its involvement in several physiological processes is well known, such as in energy balance, appetite stimulation, blood pressure, pain modulation, embryogenesis, nausea and vomiting control, memory, learning and immune response, among others, as well as in pathological conditions where it exerts a protective role in the development of certain disorders. As a result, it has been reported that changes in endocannabinoid levels may be related to neurological diseases such as Parkinson's disease, Huntington's disease, Alzheimer's disease and multiple sclerosis, as well as anorexia and irritable bowel syndrome. Alterations in the endocannabinoid system have also been associated with cancer, affecting the growth, migration and invasion of some tumours. Cannabinoids have been tested in several cancer types, including brain, breast and prostate cancers. Cannabinoids have shown promise as analgesics for the treatment of both inflammatory and neuropathic pain. There is also evidence for a role of the endocannabinoid system in the control of emotional states, and cannabinoids could prove useful in decreasing and palliating post-traumatic stress disorder symptoms and anxiolytic disorders. The role of the endocannabinoid system in addictions has also been examined, and cannabinoids have been postulated as alternative and co-adjuvant treatments in some abuse syndromes, mainly in ethanol and opioid abuses. The expression of the endocannabinoid system in the eye suggests that it could be a potential therapeutic target for eye diseases. Considering the importance of the endocannabinoid system and the therapeutic potential of cannabinoids in this vast number of medical conditions, several clinical studies with cannabinoid-based medications are ongoing. In addition, some cannabinoid-based medications have already been approved in various countries, including nabilone and dronabinol capsules for the treatment of nausea and vomiting associated with chemotherapy, dronabinol capsules for anorexia, an oral solution of dronabinol for both vomiting associated with chemotherapy and anorexia, a Δ⁹-tetrahydrocannabinol/cannabidiol oromucosal spray for pain related to cancer and for spasticity and pain associated with multiple sclerosis, and an oral solution of cannabidiol for Dravet and Lennox-Gastaut syndromes. Here, we review the available efficacy, safety and tolerability data for cannabinoids in a range of medical conditions.

Cannabinoids in Gynecological Diseases

The endocannabinoid system (ECS) is a multifunctional homeostatic system involved in many physiological and pathological conditions. The ligands of the ECS are the endo-cannabinoids, whose actions are mimicked by exogenous cannabinoids, such as phytocannabinoids and synthetic cannabinoids. Responses to the ligands of the ECS are mediated by numerous receptors like the classical cannabinoid receptors (CB₁ and CB₂) as well as ECS-related receptors, e.g., G protein-coupled receptors 18 and 55 (GPR18 and GPR55), transient receptor potential ion channels, and nuclear peroxisome proliferator-activated receptors. The ECS regulates almost all levels of female reproduction, starting with oocyte production through to parturition. Dysregulation of the ECS is associated with the development of gynecological disorders from fertility disorders to cancer. Cannabinoids that act at the ECS as specific agonists or antagonists may potentially influence dysregulation and, therefore, represent new therapeutic options for the therapy of gynecological disorders.

The Endocannabinoid System as a Target in Cancer Diseases: Are We There Yet?

The endocannabinoid system (ECS) has been placed in the anti-cancer spotlight in the last decade. The immense data load published on its dual role in both tumorigenesis and inhibition of tumor growth and metastatic spread has transformed the cannabinoid receptors CB1 (CB1R) and CB2 (CB2R), and other members of the endocannabinoid-like system, into attractive new targets for the treatment of various cancer subtypes. Although the clinical use of cannabinoids has been extensively documented in the palliative setting, clinical trials on their application as anti-cancer drugs are still ongoing. As drug repurposing is significantly faster and more economical than de novo introduction of a new drug into the clinic, there is hope that the existing pharmacokinetic and safety data on the ECS ligands will contribute to their successful translation into oncological healthcare. CB1R and CB2R are members of a large family of membrane proteins called G protein-coupled receptors (GPCR). GPCRs can form homodimers, heterodimers and higher order oligomers with other GPCRs or non-GPCRs. Currently, several CB1R and CB2R-containing heteromers have been reported and, in cancer cells, CB2R form heteromers with the G protein-coupled chemokine receptor CXCR4, the G protein-coupled receptor 55 (GPR55) and the tyrosine kinase receptor (TKR) human V-Erb-B2 Avian Erythroblastic Leukemia Viral Oncogene Homolog 2 (HER2). These protein complexes possess unique pharmacological and signaling properties, and their modulation might affect the antitumoral activity of the ECS. This review will explore the potential of the endocannabinoid network in the anti-cancer setting as well as the clinical and ethical pitfalls behind it, and will develop on the value of cannabinoid receptor heteromers as potential new targets for anti-cancer therapies and as prognostic biomarkers.

Cannabinoid Signaling in the Skin: Therapeutic Potential of the "C(ut)annabinoid" System

The endocannabinoid system (ECS) has lately been proven to be an important, multifaceted homeostatic regulator, which influences a wide-variety of physiological processes all over the body. Its members, the endocannabinoids (eCBs; e.g., anandamide), the eCB-responsive receptors (e.g., CB₁, CB₂), as well as the complex enzyme and transporter apparatus involved in the metabolism of the ligands were shown to be expressed in several tissues, including the skin. Although the best studied functions over the ECS are related to the central nervous system and to immune processes, experimental efforts over the last two decades have unambiguously confirmed that cutaneous cannabinoid ("c[ut]annabinoid") signaling is deeply involved in the maintenance of skin homeostasis, barrier formation and regeneration, and its dysregulation was implicated to contribute to several highly prevalent diseases and disorders, e.g., atopic dermatitis, psoriasis, scleroderma, acne, hair growth and pigmentation disorders, keratin diseases, various tumors, and itch. The current review aims to give an overview of the available skin-relevant endo- and phytocannabinoid literature with a special emphasis on the putative translational potential, and to highlight promising future research directions as well as existing challenges.

Potential for endocannabinoid system modulation in ocular pain and inflammation: filling the gaps in current pharmacological options

Challenges in the management of ocular pain are an underappreciated topic. Currently available therapeutics lack both efficacy and clear guidelines for their use, with many also possessing unacceptable side effects. Promising novel agents would offer analgesic, anti-inflammatory, and possibly neuroprotective actions; have favorable ocular safety profiles; and show potential in managing neuropathic pain. Growing evidence supports a link between the endocannabinoid system (ECS) and a range of physiological and disease processes, notably those involving inflammation and pain. Both preclinical and clinical data suggest analgesic and anti-inflammatory actions of cannabinoids and ECS-modifying drugs in chronic pain conditions, including those of neuropathic origin. This review will examine existing evidence for the anatomical and physiological basis of ocular pain, specifically, ocular surface disease and the development of chronic ocular pain. The mechanism of action, efficacy, and limitations of currently available treatments will be discussed, and current knowledge related to ECS-modulation of ocular pain and inflammatory disease will be summarized. A perspective will be provided on the future directions of ECS research in terms of developing cannabinoid therapeutics for ocular pain.

Positive Allosteric Modulation of Cannabinoid Receptor Type 1 Suppresses Pathological Pain Without Producing Tolerance or Dependence

BACKGROUND

Activation of cannabinoid CB₁ receptors suppresses pathological pain but also produces unwanted central side effects. We hypothesized that a positive allosteric modulator of CB₁ signaling would suppress inflammatory and neuropathic pain without producing cannabimimetic effects or physical dependence. We also asked whether a CB₁ positive allosteric modulator would synergize with inhibitors of endocannabinoid deactivation and/or an orthosteric cannabinoid agonist.

METHODS

GAT211, a novel CB₁ positive allosteric modulator, was evaluated for antinociceptive efficacy and tolerance in models of neuropathic and/or inflammatory pain. Cardinal signs of direct CB₁-receptor activation were evaluated together with the propensity to induce reward or aversion and physical dependence. Comparisons were made with inhibitors of endocannabinoid deactivation (JZL184, URB597) or an orthosteric cannabinoid agonist (WIN55,212-2). All studies used 4 to 11 subjects per group.

RESULTS

GAT211 suppressed allodynia induced by complete Freund's adjuvant and the chemotherapeutic agent paclitaxel in wild-type but not CB₁ knockout mice. GAT211 did not impede paclitaxel-induced tumor cell line toxicity. GAT211 did not produce cardinal signs of direct CB₁-receptor activation in the presence or absence of pathological pain. GAT211 produced synergistic antiallodynic effects with fatty acid amide hydrolase and monoacylglycerol lipase inhibitors in paclitaxel-treated mice. Therapeutic efficacy was preserved over 19 days of chronic dosing with GAT211, but it was not preserved with the monoacylglycerol lipase inhibitor JZL184. The CB₁ antagonist rimonabant precipitated withdrawal in mice treated chronically with WIN55,212-2 but not in mice treated with GAT211. GAT211 did not induce conditioned place preference or aversion.

CONCLUSIONS

Positive allosteric modulation of CB₁-receptor signaling shows promise as a safe and effective analgesic strategy that lacks tolerance, dependence, and abuse liability.

The Endocannabinoid System and Oligodendrocytes in Health and Disease

Cannabinoid-based interventions are being explored for central nervous system (CNS) pathologies such as neurodegeneration, demyelination, epilepsy, stroke, and trauma. As these disease states involve dysregulation of myelin integrity and/or remyelination, it is important to consider effects of the endocannabinoid system on oligodendrocytes and their precursors. In this review, we examine research reports on the effects of the endocannabinoid system (ECS) components on oligodendrocytes and their precursors, with a focus on therapeutic implications. Cannabinoid ligands and modulators of the endocannabinoid system promote cell signaling in oligodendrocyte precursor survival, proliferation, migration and differentiation, and mature oligodendrocyte survival and myelination. Agonist stimulation of oligodendrocyte precursor cells (OPCs) at both CB₁ and CB₂ receptors counter apoptotic processes via Akt/PI3K, and promote proliferation via Akt/mTOR and ERK pathways. CB₁ receptors in radial glia promote proliferation and conversion to progenitors fated to become oligodendroglia, whereas CB₂ receptors promote OPC migration in neonatal development. OPCs produce 2-arachidonoylglycerol (2-AG), stimulating cannabinoid receptor-mediated ERK pathways responsible for differentiation to arborized, myelin basic protein (MBP)-producing oligodendrocytes. In cell culture models of excitotoxicity, increased reactive oxygen species, and depolarization-dependent calcium influx, CB₁ agonists improved viability of oligodendrocytes. In transient and permanent middle cerebral artery occlusion models of anoxic stroke, WIN55212-2 increased OPC proliferation and maturation to oligodendroglia, thereby reducing cerebral tissue damage. In several models of rodent encephalomyelitis, chronic treatment with cannabinoid agonists ameliorated the damage by promoting OPC survival and oligodendrocyte function. Pharmacotherapeutic strategies based upon ECS and oligodendrocyte production and survival should be considered.

Emerging strategies targeting CB2 cannabinoid receptor: Biased agonism and allosterism

During these last years, the CB₂ cannabinoid receptor has emerged as a potential anti-inflammatory target in diseases such as multiple sclerosis, amyotrophic lateral sclerosis, Huntington's disease, ischemic stroke, autoimmune diseases, osteoporosis, and cancer. However, the development of clinically useful CB₂ agonists reveals to be very challenging. Allosterism and biased-signaling mechanisms at CB₂ receptor may offer new avenues for the development of improved CB₂ receptor-targeted therapies. Although there has been some exploration of CB₁ receptor activation by new CB₁ allosteric or biased-signaling ligands, the CB₂ receptor is still at initial stages in this domain. In an effort to understand the molecular basis behind these pharmacological approaches, we have analyzed and summarized the structural data reported so far at CB₂ receptor.

Biased Ligands of G Protein-Coupled Receptors (GPCRs): Structure-Functional Selectivity Relationships (SFSRs) and Therapeutic Potential

G protein-coupled receptors (GPCRs) signal through both G-protein-dependent and G-protein-independent pathways, and β-arrestin recruitment is the most recognized one of the latter. Biased ligands selective for either pathway are expected to regulate biological functions of GPCRs in a more precise way, therefore providing new drug molecules with superior efficacy and/or reduced side effects. During the past decade, biased ligands have been discovered and developed for many GPCRs, such as the μ opioid receptor, the angiotensin II receptor type 1, the dopamine D₂ receptor, and many others. In this Perspective, recent advances in this field are reviewed by discussing the structure-functional selectivity relationships (SFSRs) of GPCR biased ligands and the therapeutic potential of these molecules. Further understanding of the biological functions associated with each signaling pathway and structural basis for biased signaling will facilitate future drug design in this field.

Insights into the effects of the endocannabinoid system in cancer: a review

In the last few decades, the endocannabinoid system has attracted a great deal of interest in terms of its applications to clinical medicine. In particular, its applications in cancer probably represent one of the therapeutic areas with most promise. On the one hand, expression of the endocannabinoid system is altered in numerous types of tumours, compared to healthy tissue, and this aberrant expression has been related to cancer prognosis and disease outcome, suggesting a role of this system in tumour growth and progression that depends on cancer type. On the other hand, cannabinoids exert an anticancer activity by inhibiting the proliferation, migration and/or invasion of cancer cells and also tumour angiogenesis. However, some cannabinoids, at lower concentrations, may increase tumour proliferation, inducing cancer growth. Enough data has been provided to consider the endocannabinoid system as a new therapeutic target in cancer, although further studies to fully establish the effect of cannabinoids on tumour progression are still needed.

A real time screening assay for cannabinoid CB1 receptor-mediated signaling

The cannabinoid CB1 receptor is expressed throughout the central nervous system where it functions to regulate neurotransmitter release and synaptic plasticity. While the CB1 receptor has been identified as a target for both natural and synthetic cannabinoids, the specific downstream signaling pathways activated by these various ligands have not been fully described. In this study, we developed a real-time membrane potential fluorescent assay for cannabinoids using pituitary AtT20 cells that endogenously express G protein-gated inward rectifier K⁺ (GIRK) channels and were stably transfected with the CB1 receptor using a recombinant lentivirus. In whole-cell patch clamp experiments application of the cannabinoid agonist WIN 55,212-2 to AtT20 cells expressing the CB1 receptor (AtT20/CB1) activated GIRK currents that were blocked by BaCl₂. WIN 55,212-2 activation of the GIRK channels was associated with a time- and concentration-dependent (EC₅₀ = 309 nM) hyperpolarization of the membrane potential in the AtT20/CB1 cells when monitored using a fluorescent membrane potential-sensitive dye. The WIN 55,212-2-induced fluorescent signal was inhibited by pretreatment of the cells with either the GIRK channel blocker tertiapin-Q or the CB1 receptor antagonist SR141716. The cannabinoids displayed a response of WIN 55,212-2 ≈ anandamide (AEA) > CP 55,940 > Δ⁹-tetrahydrocannabinol (THC) when maximal concentrations of the four ligands were tested in the assay. Thus, the AtT20/CB1 cell fluorescent assay will provide a straightforward and efficient methodology for examining cannabinoid-stimulated G_i signaling.

The future of type 1 cannabinoid receptor allosteric ligands

Allosteric modulation of the type 1 cannabinoid receptor (CB1R) holds great therapeutic potential. This is because allosteric modulators do not possess intrinsic efficacy, but instead augment (positive allosteric modulation) or diminish (negative allosteric modulation) the receptor's response to endogenous ligand. Consequently, CB1R allosteric modulators have an effect ceiling which allows for the tempering of CB1R signaling without the desensitization, tolerance, dependence, and psychoactivity associated with orthosteric compounds. Pain, movement disorders, epilepsy, obesity are all potential therapeutic targets for CB1R allosteric modulation. Several challenges exist for the development of CB1R allosteric modulators, such as receptor subtype specificity, translation to in vivo systems, and mixed allosteric/agonist/inverse agonist activity. Despite these challenges, elucidation of crystal structures of CB1R and compound design based on structure-activity relationships will advance the field. In this review, we will cover recent progress for CB1R allosteric modulators and discuss the future promise of this research.

Bioactive lipids in gintonin-enriched fraction from ginseng

Background

Ginseng is a traditional herbal medicine for human health. Ginseng contains a bioactive ligand named gintonin. The active ingredient of gintonin is lysophosphatidic acid C18:2 (LPA C18:2). We previously developed a method for gintonin-enriched fraction (GEF) preparation to mass-produce gintonin from ginseng. However, previous studies did not show the presence of other bioactive lipids besides LPAs. The aim of this study was to quantify the fatty acids, lysophospholipids (LPLs), and phospholipids (PLs) besides LPAs in GEF.

Methods

We prepared GEF from white ginseng. We used gas chromatography-mass spectrometry for fatty acid analysis and liquid chromatography-tandem mass spectrometry for PL analysis, and quantified the fatty acids, LPLs, and PLs in GEF using respective standards. We examined the effect of GEF on insulin secretion in INS-1 cells.

Results

GEF contains about 7.5% linoleic (C18:2), 2.8% palmitic (C16:0), and 1.5% oleic acids (C18:1). GEF contains about 0.2% LPA C18:2, 0.06% LPA C16:0, and 0.02% LPA C18:1. GEF contains 0.08% lysophosphatidylcholine, 0.03% lysophosphatidylethanolamine, and 0.13% lysophosphatidylinositols. GEF also contains about 1% phosphatidic acid (PA) 16:0-18:2, 0.5% PA 18:2-18:2, and 0.2% PA 16:0-18:1. GEF-mediated insulin secretion was not blocked by LPA receptor antagonist.

Conclusion

We determined four characteristics of GEF through lipid analysis and insulin secretion. First, GEF contains a large amount of linoleic acid (C18:2), PA 16:0-18:2, and LPA C18:2 compared with other lipids. Second, the main fatty acid component of LPLs and PLs is linoleic acid (C18:2). Third, GEF stimulates insulin secretion not through LPA receptors. Finally, GEF contains bioactive lipids besides LPAs.

Targeting Cannabinoid Signaling in the Immune System: "High"-ly Exciting Questions, Possibilities, and Challenges

It is well known that certain active ingredients of the plants of Cannabis genus, i.e., the "phytocannabinoids" [pCBs; e.g., (-)-trans-Δ⁹-tetrahydrocannabinol (THC), (-)-cannabidiol, etc.] can influence a wide array of biological processes, and the human body is able to produce endogenous analogs of these substances ["endocannabinoids" (eCB), e.g., arachidonoylethanolamine (anandamide, AEA), 2-arachidonoylglycerol (2-AG), etc.]. These ligands, together with multiple receptors (e.g., CB₁ and CB₂ cannabinoid receptors, etc.), and a complex enzyme and transporter apparatus involved in the synthesis and degradation of the ligands constitute the endocannabinoid system (ECS), a recently emerging regulator of several physiological processes. The ECS is widely expressed in the human body, including several members of the innate and adaptive immune system, where eCBs, as well as several pCBs were shown to deeply influence immune functions thereby regulating inflammation, autoimmunity, antitumor, as well as antipathogen immune responses, etc. Based on this knowledge, many in vitro and in vivo studies aimed at exploiting the putative therapeutic potential of cannabinoid signaling in inflammation-accompanied diseases (e.g., multiple sclerosis) or in organ transplantation, and to dissect the complex immunological effects of medical and "recreational" marijuana consumption. Thus, the objective of the current article is (i) to summarize the most recent findings of the field; (ii) to highlight the putative therapeutic potential of targeting cannabinoid signaling; (iii) to identify open questions and key challenges; and (iv) to suggest promising future directions for cannabinoid-based drug development.

Synergistic interaction of the cannabinoid and death receptor systems - a potential target for future cancer therapies?

Cannabinoid receptors have been shown to interact with other receptors, including tumor necrosis factor receptor superfamily (TNFRS) members, to induce cancer cell death. When cannabinoids and death-inducing ligands (including TNF-related apoptosis-inducing ligand) are administered together, they have been shown to synergize and demonstrate enhanced antitumor activity in vitro. Certain cannabinoid ligands have been shown to sensitize cancer cells and synergistically interact with members of the TNFRS, thus suggesting that the combination of cannabinoids with death receptor (DR) ligands induces additive or synergistic tumor cell death. This review summarizes recent findings on the interaction of the cannabinoid and DR systems and suggests possible clinical co-application of cannabinoids and DR ligands in the treatment of various malignancies.

Characterization of structurally novel G protein biased CB1 agonists: Implications for drug development

The human cannabinoid subtype 1 receptor (hCB₁R) is highly expressed in the CNS and serves as a therapeutic target for endogenous ligands as well as plant-derived and synthetic cannabinoids. Unfortunately, acute use of hCB₁R agonists produces unwanted psychotropic effects and chronic administration results in development of tolerance and dependence, limiting the potential clinical use of these ligands. Studies in β-arrestin knockout mice suggest that interaction of certain GPCRs, including μ-, δ-, κ-opioid and hCB₁Rs, with β-arrestins might be responsible for several adverse effects produced by agonists acting at these receptors. Indeed, agonists that bias opioid receptor activation toward G-protein, relative to β-arrestin signaling, produce less severe adverse effects. These observations indicate that therapeutic utility of agonists acting at hCB₁Rs might be improved by development of G-protein biased hCB₁R agonists. Our laboratory recently reported a novel class of indole quinulidinone (IQD) compounds that bind cannabinoid receptors with relatively high affinity and act with varying efficacy. The purpose of this study was to determine whether agonists in this novel cannabinoid class exhibit ligand bias at hCB₁ receptors. Our studies found that a novel IQD-derived hCB₁ receptor agonist PNR-4-20 elicits robust G protein-dependent signaling, with transduction ratios similar to the non-biased hCB₁R agonist CP-55,940. In marked contrast to CP-55,940, PNR-4-20 produces little to no β-arrestin 2 recruitment. Quantitative calculation of bias factors indicates that PNR-4-20 exhibits from 5.4-fold to 29.5-fold bias for G protein, relative to β-arrestin 2 signaling (when compared to G protein activation or inhibition of forskolin-stimulated cAMP accumulation, respectively). Importantly, as expected due to reduced β-arrestin 2 recruitment, chronic exposure of cells to PNR-4-20 results in significantly less desensitization and down-regulation of hCB₁Rs compared to similar treatment with CP-55,940. PNR-4-20 (i.p.) is active in the cannabinoid tetrad in mice and chronic treatment results in development of less persistent tolerance and no significant withdrawal signs when compared to animals repeatedly exposed to the non-biased full agoinst JWH-018 or Δ⁹-THC. Finally, studies of a structurally similar analog PNR- 4-02 show that it is also a G protein biased hCB₁R agonist. It is predicted that cannabinoid agonists that bias hCB₁R activation toward G protein, relative to β-arrestin 2 signaling, will produce fewer and less severe adverse effects both acutely and chronically.

The In Vivo Effects of the CB1-Positive Allosteric Modulator GAT229 on Intraocular Pressure in Ocular Normotensive and Hypertensive Mice

PURPOSE

Orthosteric cannabinoid receptor 1 (CB₁) activation leads to decreases in intraocular pressure (IOP). However, use of orthosteric CB₁ agonists chronically has several disadvantages, limiting their usefulness as clinically relevant drugs. Allosteric modulators interact with topographically distinct sites to orthosteric ligands and may be useful to circumvent some of these disadvantages. The purpose of this study was to investigate the effects of the novel CB₁-positive allosteric modulator (PAM) GAT229 on IOP.

METHODS

IOP was measured using rebound tonometry in anesthetized normotensive C57Bl/6 mice and in a genetic model of ocular hypertension [nose, eyes, ears (nee) mice] before drug administration, and at 1, 6, and 12 h thereafter.

RESULTS

In normotensive mice, topical administration of 5 μL GAT229 alone at either 0.2% or 2% did not reduce IOP. However, a subthreshold dose (0.25%) of the nonselective orthosteric CB₁ agonist WIN 55,212-2, when combined with 0.2% GAT229, significantly reduced IOP compared with vehicle at 6 and 12 h. Similarly, combination of subthreshold Δ⁹-tetrahydrocannabinol (a nonselective orthosteric CB₁ agonist; 1 mg/kg) with topical 0.2% GAT229 produced IOP lowering at 6 h. In nee mice, administration of topical 0.2% GAT229 or 10 mg/kg GAT229 alone was sufficient to lower IOP at 6 and 12 h, and 12 h, respectively.

CONCLUSIONS

The CB₁ PAM GAT229 reduces IOP in ocular hypertensive mice and enhanced CB₁-mediated IOP reduction when combined with subthreshold CB₁ orthosteric ligands in normotensive mice. Administration of CB₁ PAMs may provide a novel approach to reduce IOP with fewer of the disadvantages associated with orthosteric CB₁ activation.

Assessing Allosteric Modulation of CB1 at the Receptor and Cellular Levels

The cannabinoid CB1 receptor is abundant in the central nervous system and regulates neuronal transmission and other key physiological processes including those leading to pain, inflammation, memory, and feeding behavior. CB1 is activated by the endogenous ligands, arachidonoyl ethanolamine and 2-arachidonoyl glycerol, by various synthetic ligands (e.g., CP55940), and by Δ9-tetrahydrocannabinol, the psychoactive component of Cannabis sativa. These CB1 ligands are orthosteric and transduce downstream signals by binding CB1 and primarily inducing Gi coupling, but Gs and β-arrestin coupling are also possible. Recently, allosteric modulators for CB1 were discovered that bind to topographically distinct sites and can noncompetitively impact the potency and efficacy of orthosteric compounds. These offer the exciting potential for mechanistic analyses and for developing therapeutics. Yet, it is critical to elucidate whether a compound is a positive allosteric modulator or a negative allosteric modulator of orthosteric ligand-induced CB1 profiles to understand pathway specificity and ameliorate diseases. In this chapter, we present equilibrium and kinetic binding analysis to reveal the impact of allosteric modulators on CB1. Also described are activities consistent with CB1 activation (or inactivation) and include cellular internalization of CB1 and downstream signaling patterns. Since many CB1 allosteric modulators do not enhance G protein coupling, it is critical to distinguish CB1 activation and biased signaling patterns via β-arrestin from CB1 inactivation. These strategies can illuminate pathway specificity and are valuable for the fine-tuning of CB1 function.

CB1 and CB2 Receptor Pharmacology

The CB1 and CB2 cannabinoid receptors (CB1R, CB2R) are members of the G protein-coupled receptor (GPCR) family that were identified over 20 years ago. CB1Rs and CB2Rs mediate the effects of Δ9-tetrahydrocannabinol (Δ9-THC), the principal psychoactive constituent of marijuana, and subsequently identified endogenous cannabinoids (endocannabinoids) anandamide and 2-arachidonoyl glycerol. CB1Rs and CB2Rs have both similarities and differences in their pharmacology. Both receptors recognize multiple classes of agonist and antagonist compounds and produce an array of distinct downstream effects. Natural polymorphisms and alternative splice variants may also contribute to their pharmacological diversity. As our knowledge of the distinct differences grows, we may be able to target select receptor conformations and their corresponding pharmacological responses. This chapter will discuss their pharmacological characterization, distribution, phylogeny, and signaling pathways. In addition, the effects of extended agonist exposure and how that affects signaling and expression patterns of the receptors are considered.

Functional selectivity at G-protein coupled receptors: Advancing cannabinoid receptors as drug targets

The phenomenon of functional selectivity, whereby a ligand preferentially directs the information output of a G-protein coupled receptor (GPCR) along (a) particular effector pathway(s) and away from others, has redefined traditional GPCR signaling paradigms to provide a new approach to structure-based drug design. The two principal cannabinoid receptors (CBRs) 1 and 2 belong to the class-A GPCR subfamily and are considered tenable therapeutic targets for several indications. Yet conventional orthosteric ligands (agonists, antagonists/inverse agonists) for these receptors have had very limited clinical utility due to their propensity to incite on-target adverse events. Chemically distinct classes of cannabinergic ligands exhibit signaling bias at CBRs towards individual subsets of signal transduction pathways. In this review, we discuss the known signaling pathways regulated by CBRs and examine the current evidence for functional selectivity at CBRs in response to endogenous and exogenous cannabinergic ligands as biased agonists. We further discuss the receptor and ligand structural features allowing for selective activation of CBR-dependent functional responses. The design and development of biased ligands may offer a pathway to therapeutic success for novel CBR-targeted drugs.