CB1 cannabinoid receptor-mediated aggressive behavior

"Weeding out" violence? Translational perspectives on the neuropsychobiological links between cannabis and aggression

Recent shifts in societal attitudes towards cannabis have led to a dramatic increase in consumption rates in many Western countries, particularly among young people. This trend has shed light on a significant link between cannabis use disorder (CUD) and pathological reactive aggression, a condition involving disproportionate aggressive and violent reactions to minor provocations. The discourse on the connection between cannabis use and aggression is frequently enmeshed in political and legal discussions, leading to a polarized understanding of the causative relationship between cannabis use and aggression. However, integrative analyses from both human and animal research indicate a complex, bidirectional interplay between cannabis misuse and pathological aggression. On the one hand, emerging research reveals a shared genetic and environmental predisposition for both cannabis use and aggression, suggesting a common underlying biological mechanism. On the other hand, there is evidence that cannabis consumption can lead to violent behaviors while also being used as a self-medication strategy to mitigate the negative emotions associated with pathological reactive aggression. This suggests that the coexistence of pathological aggression and CUD may result from overlapping vulnerabilities, potentially creating a self-perpetuating cycle where each condition exacerbates the other, escalating into externalizing and violent behaviors. This article aims to synthesize existing research on the intricate connections between these issues and propose a theoretical model to explain the neurobiological mechanisms underpinning this complex relationship.

Putative Activation of Cannabinoid Receptor Type 1 Prevents Brain Oxidative Stress and Inhibits Aggressive-Like Behavior in Zebrafish

Background: Aggression is a set of complex behaviors commonly described in different neurological disorders, such as schizophrenia, autistim spectrum disorder, and anxiety. Previous studies have described that some changes in the redox status of the brain are closely associated with aggressive behavior in different species. In addition, the endocannabinoid system acts as a neuromodulator of the central nervous system, however, its participation in aggressive behavior needs to be elucidated. Danio rerio (zebrafish) is an important model in the study of aggression, in this context, the present study investigated whether the activation of type 1 cannabinoid receptors (CB1r) alters the cerebral redox state and aggressive behavior in zebrafish. Materials and Methods: We performed pharmacological manipulations with the CB1r agonist (ACEA) and antagonist (AM-251) to assess the role of this receptor in aggressive behavior. Individuals were isolated in pairs, without physical contact for 24 h, treated with the drugs of interest, and after 30 minutes of pharmacokinetics, the fights were filmed for 30 min, and the individuals were identified as dominant or subordinate. Results: A consistent decrease in the strike and bite aggressive behavior was observed in the group treated with the ACEA agonist compared with that in the control and AM-251 groups. When evaluating the cerebral redox state, we observed that treatment with the ACEA agonist reduced malondialdehyde (MDA) levels and increased the levels of sulfhydryl groups compared with those in the control group. These results indicate that the activation of CB1r by the ACEA agonist inhibited aggressiveness and attenuated the levels of oxidative stress in both subjects (dominant or subordinate) in the treated group. Conclusion: Thus, we suggest that zebrafish is an alternative model to study common aggressive behavior disorders among species and that CB1r represent a potential target for the development of treatments for aggressive disorders.

The endocannabinoid system as a putative target for the development of novel drugs for the treatment of psychiatric illnesses

Cannabis is well established to impact affective states, emotion and perceptual processing, primarily through its interactions with the endocannabinoid system. While cannabis use is quite prevalent in many individuals afflicted with psychiatric illnesses, there is considerable controversy as to whether cannabis may worsen these conditions or provide some form of therapeutic benefit. The development of pharmacological agents which interact with components of the endocannabinoid system in more localized and discrete ways then via phytocannabinoids found in cannabis, has allowed the investigation if direct targeting of the endocannabinoid system itself may represent a novel approach to treat psychiatric illness without the potential untoward side effects associated with cannabis. Herein we review the current body of literature regarding the various pharmacological tools that have been developed to target the endocannabinoid system, their impact in preclinical models of psychiatric illness and the recent data emerging of their utilization in clinical trials for psychiatric illnesses, with a specific focus on substance use disorders, trauma-related disorders, and autism. We highlight several candidate drugs which target endocannabinoid function, particularly inhibitors of endocannabinoid metabolism or modulators of cannabinoid receptor signaling, which have emerged as potential candidates for the treatment of psychiatric conditions, particularly substance use disorder, anxiety and trauma-related disorders and autism spectrum disorders. Although there needs to be ongoing clinical work to establish the potential utility of endocannabinoid-based drugs for the treatment of psychiatric illnesses, the current data available is quite promising and shows indications of several potential candidate diseases which may benefit from this approach.

Imaging and Genetic Tools for the Investigation of the Endocannabinoid System in the CNS

As central nervous system (CNS)-related disorders present an increasing cause of global morbidity, mortality, and high pressure on our healthcare system, there is an urgent need for new insights and treatment options. The endocannabinoid system (ECS) is a critical network of endogenous compounds, receptors, and enzymes that contribute to CNS development and regulation. Given its multifaceted involvement in neurobiology and its significance in various CNS disorders, the ECS as a whole is considered a promising therapeutic target. Despite significant advances in our understanding of the ECS's role in the CNS, its complex architecture and extensive crosstalk with other biological systems present challenges for research and clinical advancements. To bridge these knowledge gaps and unlock the full therapeutic potential of ECS interventions in CNS-related disorders, a plethora of molecular-genetic tools have been developed in recent years. Here, we review some of the most impactful tools for investigating the neurological aspects of the ECS. We first provide a brief introduction to the ECS components, including cannabinoid receptors, endocannabinoids, and metabolic enzymes, emphasizing their complexity. This is followed by an exploration of cutting-edge imaging tools and genetic models aimed at elucidating the roles of these principal ECS components. Special emphasis is placed on their relevance in the context of CNS and its associated disorders.

Basolateral Amygdala Cannabinoid CB1 Receptor Controls Formation and Elimination of Social Fear Memory

Patients with post-traumatic stress disorder (PTSD) usually manifest persistence of the traumatic memory for a long time after the event, also known as resistance to extinction learning. Numerous studies have shown that the endocannabinoid system, specifically the cannabinoid type-1 receptor (CB1R), plays an important role in traumatic memory. However, the effect of basolateral amygdala (BLA) CB1R in social fear memory formation and elimination is still unclear. Here, we built a mouse model of social avoidance induced by acute social defeat stress to investigate the role of BLA CB1R in social fear memory formation and anxiety- and depression-like behavior. Anterograde knockout of CB1R in BLA neurons facilitates social fear memory formation and manifests an anxiolytic effect but does not influence sociability and social novelty. Retrograde knockout of CB1R in BLA promotes social fear memory formation and shows an anxiogenic effect but does not affect sociability and social novelty. Moreover, intracerebral injection of the CB1R antagonist AM251 in BLA during the memory reconsolidation time window eliminates social fear memory. Our findings suggest the CB1R of BLA can be used as a novel molecular target in social fear memory formation and elimination and potential PTSD therapy with memory retrieval and AM251.

Endocannabinoid system and aggression across animal species

This narrative review article summarizes the current state of knowledge regarding the relationship between the endocannabinoid system (ECS) and aggression across multiple vertebrate species. Experimental evidence indicates that acute administration of phytocannabinoids, synthetic cannabinoids, and the pharmacological enhancement of endocannabinoid signaling decreases aggressive behavior in several animal models. However, research on the chronic effects of cannabinoids on animal aggression has yielded inconsistent findings, indicating a need for further investigation. Cannabinoid receptors, particularly cannabinoid receptor type 1, appear to be an important part of the endogenous mechanism involved in the dampening of aggressive behavior. Overall, this review underscores the importance of the ECS in regulating aggressive behavior and provides a foundation for future research in this area.

The 3Rs in Experimental Liver Disease

Patients with cirrhosis present multiple physiological and immunological alterations that play a very important role in the development of clinically relevant secondary complications to the disease. Experimentation in animal models is essential to understand the pathogenesis of human diseases and, considering the high prevalence of liver disease worldwide, to understand the pathophysiology of disease progression and the molecular pathways involved, due to the complexity of the liver as an organ and its relationship with the rest of the organism. However, today there is a growing awareness about the sensitivity and suffering of animals, causing opposition to animal research among a minority in society and some scientists, but also about the attention to the welfare of laboratory animals since this has been built into regulations in most nations that conduct animal research. In 1959, Russell and Burch published the book "The Principles of Humane Experimental Technique", proposing that in those experiments where animals were necessary, everything possible should be done to try to replace them with non-sentient alternatives, to reduce to a minimum their number, and to refine experiments that are essential so that they caused the least amount of pain and distress. In this review, a comprehensive summary of the most widely used techniques to replace, reduce, and refine in experimental liver research is offered, to assess the advantages and weaknesses of available experimental liver disease models for researchers who are planning to perform animal studies in the near future.

Aggression in Group-Housed Male Mice: A Systematic Review

Aggression among group-housed male mice is a major animal welfare concern often observed at animal facilities. Studies designed to understand the causes of male mice aggression have used different methodological approaches and have been heterogeneous, using different strains, environmental enrichments, housing conditions, group formations and durations. By conducting a systematic literature review based on 198 observed conclusions from 90 articles, we showed that the methodological approach used to study aggression was relevant for the outcome and suggested that home cage observations were better when studying home cage aggression than tests provoking aggression outside the home cage. The study further revealed that aggression is a complex problem; one solution will not be appropriate for all animal facilities and all research projects. Recommendations were provided on promising tools to minimize aggression, based on the results, which included what type of environmental enrichments could be appropriate and which strains of male mice were less likely to be aggressive.

Altered Expression of Genes Associated with Major Neurotransmitter Systems in the Reward-Related Brain Regions of Mice with Positive Fighting Experience

The main neurotransmitters in the brain-dopamine, γ-aminobutyric acid (GABA), glutamate, and opioids-are recognized to be the most important for the regulation of aggression and addiction. The aim of this work was to study differentially expressed genes (DEGs) in the main reward-related brain regions, including the ventral tegmental area (VTA), dorsal striatum (STR), ventral striatum (nucleus accumbens, NAcc), prefrontal cortex (PFC), and midbrain raphe nuclei (MRNs), in male mice with 20-day positive fighting experience in daily agonistic interactions. Expression of opioidergic, catecholaminergic, glutamatergic, and GABAergic genes was analyzed to confirm or refute the influence of repeated positive fighting experience on the development of "addiction-like" signs shown in our previous studies. High-throughput RNA sequencing was performed to identify differentially expressed genes in the brain regions of chronically aggressive mice. In the aggressive mice, upregulation of opioidergic genes was shown (Oprk1 in VTA, Pdyn in NAcc, Penk in PFC, and Oprd1 in MRNs and PFC), as was downregulation of genes Opcml and Oprk1 in STR and Pomc in VTA and NAcc. Upregulation of catecholaminergic genes in VTA (Ddc and Slc6a2) and in NAcc (Th and Drd2) and downregulation of some differentially expressed genes in MRNs (Th, Ddc, Dbh, Drd2, Slc18a2, and Sncg) and in VTA (Adra2c, Sncg, and Sncb) were also documented. The expression of GABAergic and glutamatergic genes that participate in drug addiction changed in all brain regions. According to literature data, the proteins encoded by genes Drd2, Oprk1, Oprd1, Pdyn, Penk, and Pomc are directly involved in drug addiction in humans. Thus, our results confirm our earlier claim about the formation of addiction-like signs following repeated positive fighting experience in mice, as shown previously in our biobehavioral studies.

Cannabidiol for the treatment of autism spectrum disorder: hope or hype?

RATIONALE

Autism spectrum disorder (ASD) is defined as a group of neurodevelopmental disorders whose symptoms include impaired communication and social interaction, restricted and repetitive patterns of behavior, and varying levels of intellectual disability. ASD is observed in early childhood and is one of the most severe chronic childhood disorders in prevalence, morbidity, and impact on society. It is usually accompanied by attention deficit hyperactivity disorder, anxiety, depression, sleep disorders, and epilepsy. The treatment of ASD has low efficacy, possibly because it has a heterogeneous nature, and its neurobiological basis is not clearly understood. Drugs such as risperidone and aripiprazole are the only two drugs available that are recognized by the Food and Drug Administration, primarily for treating the behavioral symptoms of this disorder. These drugs have limited efficacy and a high potential for inducing undesirable effects, compromising treatment adherence. Therefore, there is great interest in exploring the endocannabinoid system, which modulates the activity of other neurotransmitters, has actions in social behavior and seems to be altered in patients with ASD. Thus, cannabidiol (CBD) emerges as a possible strategy for treating ASD symptoms since it has relevant pharmacological actions on the endocannabinoid system and shows promising results in studies related to disorders in the central nervous system.

OBJECTIVES

Review the preclinical and clinical data supporting CBD's potential as a treatment for the symptoms and comorbidities associated with ASD, as well as discuss and provide information with the purpose of not trivializing the use of this drug.

Combinatorial approaches for treating neuropsychiatric social impairment

Social behaviour is an essential component of human life and deficits in social function are seen across multiple psychiatric conditions with high morbidity. However, there are currently no FDA-approved treatments for social dysfunction. Since social cognition and behaviour rely on multiple signalling processes acting in concert across various neural networks, treatments aimed at social function may inherently require a combinatorial approach. Here, we describe the social neurobiology of the oxytocin and endocannabinoid signalling systems as well as translational evidence for their use in treating symptoms in the social domain. We leverage this systems neurobiology to propose a network-based framework that involves pharmacology, psychotherapy, non-invasive brain stimulation and social skills training to combinatorially target trans-diagnostic social impairment. Lastly, we discuss the combined use of oxytocin and endocannabinoids within our proposed framework as an illustrative strategy to treat specific aspects of social function. Using this framework provides a roadmap for actionable treatment strategies for neuropsychiatric social impairment. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Sansoninto attenuates aggressive behavior and increases levels of homovanillic acid, a dopamine metabolite, in social isolation-reared mice

Background and aim

Early-life stress is thought to affect aggressive behavior in humans and rodents. Laboratory experiments have demonstrated that Sansoninto (SST; suān zǎo rén tāng), a traditional herbal medicine, attenuates stress-induced abnormal behavior in rodents. However, it is unknown whether SST attenuates stress-induced aggressive behavior. The current study examined the effects of SST on aggressive behavior of mice who suffered from social isolation (SI) stress in adolescence.

Experimental procedure

Five-week old mice were socially isolated for 6 weeks, and SST administration was started at 4 weeks after starting SI. Aggressive behavior and locomotor activity were examined in SST-treated mice. The content of dopamine and its metabolites in the hypothalamus were examined using high-performance liquid chromatography analysis. Gene expression analyses of monoamine oxidase-B (MAO-B), catechol-O-methyltransferase (COMT), and tyrosine hydroxylase in the hypothalamus were performed using quantitative reverse transcription polymerase chain reaction.

Results and conclusion

SST attenuated SI-induced aggressive behavior and increased levels of homovanillic acid, a metabolite of dopamine. However, SST did not affect dopamine levels. SI enhanced locomotion in a novel environment and increased COMT mRNA levels. In contrast, SST-treated mice showed no significant enhancement of locomotion. SST attenuated the increase in COMT mRNA levels. Given that the dopaminergic system has been implicated in aggressive behavior, these findings suggest that SST toned down dopaminergic signaling, resulting in amelioration of aggression. SST may be useful for treatment of aggressive behavior in patients with neurotic symptoms.

Astroglial cannabinoid signaling and behavior

In neuroscience, the explosion of innovative and advanced technical accomplishments is fundamental to understanding brain functioning. For example, the possibility to distinguish glial and neuronal activities at the synaptic level and/or the appearance of new genetic tools to specifically monitor and manipulate astroglial functions revealed that astrocytes are involved in several facets of behavioral control. In this sense, the discovery of functional presence of type-1 cannabinoid receptors in astrocytes has led to identify important behavioral responses mediated by this specific pool of cannabinoid receptors. Thus, astroglial type-1 cannabinoid receptors are in the perfect place to play a role in a complex scenario in which astrocytes sense neuronal activity, release gliotransmitters and modulate the activity of other neurons, ultimately controlling behavioral responses. In this review, we will describe the known behavioral implications of astroglial cannabinoid signaling and highlight exciting unexplored research avenues on how astroglial cannabinoid signaling could affect behavior.

Exogenous microbiota-derived metabolite trimethylamine N-oxide treatment alters social behaviors: Involvement of hippocampal metabolic adaptation

Increasing evidence indicates that gut microbiota and its metabolites can influence the brain function and the related behaviors. Trimethylamine N-oxide (TMAO), an indirect metabolite of gut microbiota, has been linked to aging, cognitive impairment, and many brain disorders. However, the potential effects of TMAO on social behaviors remain elusive. The present study investigated the effects of early life systemic TMAO exposure and intra-hippocampal TMAO infusion during adulthood on social behaviors in mice. We also analyzed the effects of intra-hippocampus infusion of TMAO during adulthood on levels of metabolites. The results showed that both systemic TMAO exposure in the post-weaning period and intra-hippocampal TMAO infusion during adulthood decreased social rank and reduced sexual preference in adult mice. Data from LC-MS metabolomics analysis showed that intra-hippocampal TMAO infusion induced a total 207 differential metabolites, which belongs to several metabolic or signaling pathways, especially FoxO signaling pathway and retrograde endocannabinoid signaling pathway. These data suggest that TMAO may affect social behaviors by regulating metabolites in the hippocampus, which may provide a new insight into the role of gut microbiota in regulating social behaviors.

Regulation of stress-provoked aggressive behavior using endocannabinoids

Reactive impulsive aggression is characterized by outbursts of rage and violence when subjects encounter threatening stressful events. Although impulsive aggression and violence create a high-cost burden on health and society, relatively little is known about treatment. Early adolescent social isolation (SI) alters brain development and functions. It induces hyper-excitatory in the ventral hippocampus (vHip) to promote acute stress-provoked outbursts of aggression, referred to as impulsive aggression, in mouse models. Cannabinoid type 1 receptors (CB1Rs) act on presynaptic sites and suppress neurotransmitter release into synapses. Given that CB1R activation inhibits neurotransmitter releases and modulates excitatory network activity, we tested the hypothesis that CB1R activation reduces impulsive aggression in SI mice through decreasing excitatory activity in the vHip. Here, we report that CB1R agonists, WIN-552122 (WIN) or arachidonylcyclopropylamide (ACPA), ameliorated acute stress-provoked attack behavior in the resident-intruder test without affecting general locomotion activity. Increasing endocannabinoids (eCBs) by inhibiting degradation enzymes in the vHip reduced impulsive aggression, and the effect was blunted by administration of AM251, a CB1R antagonist. Acute stress in SI mice induced c-Fos expression, a marker of neuronal activation, on vHip neurons projecting to the ventromedial hypothalamus (VMH), a well-known brain area that controls attack behavior. eCB augmentation inhibited c-Fos expression in VMH-projecting vHip neurons surrounded by CB1Rs. These results suggest that enhancing eCB signaling in order to activate CB1Rs suppresses impulsive aggression via suppressing vHip→VMH neural activity and point to a role of CB1R activation in ameliorating impulsive aggression in adults who have had adverse experiences during early adolescence.

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Early adolescent social isolation (SI) promotes impulsive aggression.

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Treatment of CB1R agonists reduces impulsive aggression in SI mice.

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Increasing endocannabinoids (eCBs) to activate CB1Rs reduces impulsive aggression.

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Augmenting eCBs suppresses stress-provoked neuronal activation in the hippocampus.

Cannabis Use in Older Adults: A Perspective

The prevalence of cannabis use among older adults (aged 65 and above) for both recreational and medicinal purposes has significantly increased in recent years. Information regarding the safety of cannabis in this population is important since aging is associated with metabolic changes, multiple morbidities, increases in prescription medication use, and an overall decline in functioning. In this Perspectives article, we review special considerations pertinent to older adults-specifically, the impact of cannabis on cognition and on falls and injuries, its drug interactions, and its potential medicinal applications for treating the behavioral and psychological symptoms of dementia. Knowledge about the role of cannabis in brain health, injury risk, and drug interactions remains limited since the available evidence stems primarily from adolescent and young adult cohorts, plus a limited number of small observational studies with older adults. In terms of utilizing certain cannabinoids to treat the behavioral and psychological symptoms of dementia, some studies have found promising results, but because of the lack of consistency in the literature, it is premature to draw conclusions. Controlled research trials specifically with geriatric participants are needed to understand the effects of cannabis use in this vulnerable population.

Lower amygdala fatty acid amide hydrolase in violent offenders with antisocial personality disorder: an [11C]CURB positron emission tomography study

Antisocial personality disorder (ASPD) imposes a high societal burden given the repetitive reactive aggression that affected individuals perpetrate. Since the brain endocannabinoid system (ECS) has been implicated in ASPD and aggressive behavior, we utilized [¹¹C]CURB positron emission tomography to investigate fatty acid amide hydrolase (FAAH), an enzyme of the ECS that degrades anandamide, in 16 individuals with ASPD and 16 control participants. We hypothesized that FAAH density would be lower in the amygdala for several reasons. First, decreased FAAH expression is associated with increased cannabinoid receptor 1 stimulation, which may be responsible for amygdala hyper-reactivity in reactive aggression. Second, the amygdala is the seat of the neural circuit mediating reactive aggression. Third, other PET studies of externalizing populations show reduced brain FAAH density. Conversely, we hypothesized that FAAH expression would be greater in the orbitofrontal cortex. Consistent with our hypothesis, we found that amygdala FAAH density was lower in the amygdala of ASPD (p = 0.013). Cerebellar and striatal FAAH expression were inversely related with impulsivity (cerebellum: r = -0.60, p = 0.017; dorsal caudate: r = -0.58, p = 0.023; dorsal putamen: r = -0.55, p = 0.034), while cerebellar FAAH density was also negatively associated with assaultive aggression (r = -0.54, p = 0.035). ASPD presents high levels of disruptive behavior with few, if any, efficacious treatment options. Novel therapeutics that increase FAAH brain levels in a region-specific manner could hold promise for attenuating certain symptom clusters of ASPD, although our results require replication.

Cannabinoid use and self-injurious behaviours: A systematic review and meta-analysis

BACKGROUND

The increasing availability of high-potency cannabis-derived compounds and the use of synthetic cannabinoids may be responsible for severe side effects like cognitive impairment, psychosis or self-injurious behaviours (SIB). In particular, SIB like non-suicidal self-injury (NSSI) and deliberate self-harm (DSH) raise growing concern as a possible consequence of cannabis use. However, the research to date has not addressed the relationship between cannabinoid use and SIB systematically.

METHODS

We conducted a systematic review on PubMed up to March 2020, using search terms related to cannabinoids and SIB.

RESULTS

The search yielded a total of 440 abstracts. Of those, 37 studies published between 1995 and 2020 were eligible for inclusion. Cannabinoid use was significantly associated with SIB at the cross-sectional (OR=1.569, 95%CI [1.167-2.108]) and longitudinal (OR=2.569, 95%CI [2.207-3.256]) level. Chronic use, presence of mental disorders, depressive symptoms, emotional dysregulation and impulsive traits might further increase the likelihood of self-harm in cannabis users. Synthetic cannabinoids may trigger highly destructive SIB mainly through the psychotomimetic properties of these compounds.

CONCLUSION

Cannabinoid use was associated with an increased prevalence of self-injury and may act as a causative factor with a duration-dependent manner. Emotional regulation and behavioural impulsivity functions might crucially moderate this association. Future studies should further investigate the mechanisms underlying this association, while exploring potential therapeutic applications of substances modulating the endocannabinoid system.

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.

Management of Dementia-Related Psychosis, Agitation and Aggression: A Review of the Pharmacology and Clinical Effects of Potential Drug Candidates

Along with cognitive decline, 90% of patients with dementia experience behavioral and psychological symptoms of dementia, such as psychosis, aggression, agitation, and depression. Atypical antipsychotics are commonly prescribed off-label to manage certain symptoms, despite warnings from the regulatory agencies regarding the increased risk of mortality associated with their use in elderly patients. Moreover, these compounds display a limited clinical efficacy, mostly owing to the fact that they were developed to treat schizophrenia, a disease characterized by neurobiological deficits. Thus, to improve clinical efficacy, it has been suggested that patients with dementia should be treated with exclusively designed and developed drugs that interact with pharmacologically relevant targets. Within this context, numerous studies have suggested druggable targets that might achieve therapeutically acceptable pharmacological profiles. Based on this, several different drug candidates have been proposed that are being investigated in clinical trials for behavioral and psychological symptoms of dementia. We highlight the recent advances toward the development of therapeutic agents for dementia-related psychosis and agitation/aggression and discuss the relationship between the relevant biological targets and their etiology. In addition, we review the compounds that are in the early stage of development (discovery or preclinical phase) and those that are currently being investigated in clinical trials for dementia-related psychosis and agitation/aggression. We also discuss the mechanism of action of these compounds and their pharmacological utility in patients with dementia.

Violence and Cannabis Use: A Focused Review of a Forgotten Aspect in the Era of Liberalizing Cannabis

There has been a shift surrounding societal and legal perspectives on cannabis reflecting changing public attitudes towards the perceived safety and social acceptability of cannabis use. With cannabis liberalization internationally, the focus of most cannabis-related harms has been on effects with users themselves. Harm-to-others including injuries from violence have nevertheless been unfortunately largely overlooked. While studies remain heterogeneous, there is meta-analytical evidence pointing towards an association. The aims of this focused review are two-fold: (I) review the evidence from meta-analyses on the association between cannabis and violence; and (II) provide an overview of possible mechanisms relating cannabis use to violence. First, evidence from meta-analytical studies in youths, intimate partners, and individuals with severe mental disorders have shown that there is a global moderate association between cannabis use and violence, which is stronger in the latter more at-risk population. Preliminary data has even highlighted a potential dose-response relationship with larger effects in more frequent users. Although of importance, this subject has remained essentially forgotten as a public health concern. While literature remains inconclusive, data has suggested potential increases in cannabis use following liberalization policies. This may increase violent outcomes if the effect is directly related to the use of cannabis by means of its psychophysiological modifications. However, for the moment, the mechanisms associating cannabis use and violence remain to be clearly resolved. Considering the recency of policy changes on cannabis, further methodologically sound research using longitudinal designs should examine the effects that cannabis use may have on different forms of violence and the trends that emerge, while evaluating the effects of possible confounding factors (e.g. other substance use). In addition, as evidence-based research from meta-analyses have shown that cannabis use is associated with violence, measures must be taken to mitigate the risks.

Cannabidiol attenuates aggressive behavior induced by social isolation in mice: Involvement of 5-HT1A and CB1 receptors

Long-term single housing increases aggressive behavior in mice, a condition named isolation-induced aggression or territorial aggression, which can be attenuated by anxiolytic, antidepressant, and antipsychotic drugs. Preclinical and clinical findings indicate that cannabidiol (CBD), a non-psychotomimetic compound from Cannabis sativa, has anxiolytic, antidepressant, and antipsychotic properties. Few studies, however, have investigated the effects of CBD on aggressive behaviors. Here, we investigated whether CBD (5, 15, 30, and 60 mg/kg; i.p.) could attenuate social isolation-induced aggressive behavior in the resident-intruder test. Male Swiss mice (7-8 weeks) were single-housed for 10 days (resident mice) to induce aggressive behaviors, while conspecific mice of same sex and age (intruder mice) were group-housed. During the test, the intruder was placed into the resident's home-cage and aggressive behaviors initiated by the resident, including the latency for the first attack, number of attacks, and total duration of aggressive encounters, were recorded. The involvement of 5-HT1A and CB1 receptors (CB1R) in the effects of CBD was also investigated. All tested CBD doses induced anti-aggressive effects, indicated by a decrease in the number of attacks. CBD, at intermediary doses (15 and 30 mg/kg), also increased latency to attack the intruder and decreased the duration of aggressive encounters. No CBD dose interfered with locomotor behavior. CBD anti-aggressive effects were attenuated by the 5-HT1A receptor antagonist WAY100635 (0.3 mg/kg) and the CB1 antagonist AM251 (1 mg/kg), suggesting that CBD decreases social isolation-induced aggressive behaviors through a mechanism associated with the activation of 5-HT1A and CB1 receptors. Also, CBD decreased c-Fos protein expression, a neuronal activity marker, in the lateral periaqueductal gray (lPAG) in social-isolated mice exposed to the resident-intruder test, indicating a potential involvement of this brain region in the drug effects. Taken together, our findings suggest that CBD may be therapeutically useful to treat aggressive behaviors that are usually associated with psychiatric disorders.

Social isolation as a promising animal model of PTSD comorbid suicide: neurosteroids and cannabinoids as possible treatment options

Post-traumatic stress disorder (PTSD) is a psychiatric condition characterized by drastic alterations in mood, emotions, social abilities and cognition. Notably, one aspect of PTSD, particularly in veterans, is its comorbidity with suicide. Elevated aggressiveness predicts high-risk to suicide in humans and despite the difficulty in reproducing a complex human suicidal behavior in rodents, aggressive behavior is a well reproducible behavioral trait of suicide. PTSD animal models are based on a peculiar phenotype, including exaggerated fear memory and impaired fear extinction associated with neurochemical dysregulations in the brain circuitry regulating emotion. The endocannabinoid and the neurosteroid systems regulate emotions and stress responses, and recent evidence shows these two systems are interrelated and critically compromised in neuropsychiatric disorders. For instance, levels of the neurosteroid, allopregnanolone, as well as those of the endocannabinoids, anandamide and its congener, palmitoylethanolamide are decreased in PTSD. Similarly, the endocannabinoid system and neurosteroid biosynthesis are altered in suicidal individuals. Selective serotonin reuptake inhibitors (SSRIs), the only FDA-approved treatments for PTSD, fail to help half of the treatment-seeking patients. This highlights the need for developing biomarker-based efficient therapies. One promising alternative to SSRIs points to stimulation of allopregnanolone biosynthesis as a treatment and a valid end-point to predict treatment response in PTSD patients. This review highlights running findings on the role of the endocannabinoid and neurosteroid systems in PTSD and suicidal behavior both in a preclinical and clinical perspective. A specific focus is given to predictive PTSD/suicide animal models. Ultimately, we discuss the idea that disruption of neurosteroid and endocannabinoid biosynthesis may offer a novel promising biomarker axis to develop new treatments for PTSD and, perhaps, suicidal behavior.

Zebrafish models relevant to studying central opioid and endocannabinoid systems

The endocannabinoid and opioid systems are two interplaying neurotransmitter systems that modulate drug abuse, anxiety, pain, cognition, neurogenesis and immune activity. Although they are involved in such critical functions, our understanding of endocannabinoid and opioid physiology remains limited, necessitating further studies, novel models and new model organisms in this field. Zebrafish (Danio rerio) is rapidly emerging as one of the most effective translational models in neuroscience and biological psychiatry. Due to their high physiological and genetic homology to humans, zebrafish may be effectively used to study the endocannabinoid and opioid systems. Here, we discuss current models used to target the endocannabinoid and opioid systems in zebrafish, and their potential use in future translational research and high-throughput drug screening. Emphasizing the high degree of conservation of the endocannabinoid and opioid systems in zebrafish and mammals, we suggest zebrafish as an excellent model organism to study these systems and to search for the new drugs and therapies targeting their evolutionarily conserved mechanisms.

Altered Amygdala Excitation and CB1 Receptor Modulation of Aggressive Behavior in the Neuroligin-3R451C Mouse Model of Autism

Understanding neuronal mechanisms underlying aggression in patients with autism spectrum disorder (ASD) could lead to better treatments and prognosis. The Neuroligin-3 (NL3)^R451C mouse model of ASD has a heightened aggressive phenotype, however the biological mechanisms underlying this behavior are unknown. It is well established that NL3^R451C mice have imbalanced excitatory and inhibitory synaptic activity in the hippocampus and somatosensory cortex. The amygdala plays a role in modulating aggressive behavior, however potential changes in synaptic activity in this region have not previously been assessed in this model. We investigated whether aggressive behavior is robustly present in mice expressing the R451C mutation, following back-crossing onto a congenic background strain. Endocannabinoids influence social interaction and aggressive behavior, therefore we also studied the effects of cannabinoid receptor 1 (CB1) agonist on NL3^R451C mice. We report that NL3^R451C mice have increased amplitude of miniature excitatory postsynaptic currents (EPSCs) with a concomitant decrease in the amplitude of inhibitory postsynaptic currents (IPSCs) in the basolateral amygdala. Importantly, we demonstrated that NL3^R451C mice bred on a C57Bl/6 background strain exhibit an aggressive phenotype. Following non-sedating doses (0.3 and 1.0 mg/kg) of the CB1 receptor agonist WIN55,212-2 (WIN), we observed a significant reduction in aggressive behavior in NL3^R451C mice. These findings demonstrate altered synaptic activity in the basolateral amygdala and suggest that the NL3^R451C mouse model is a useful preclinical tool to understand the role of CB1 receptor function in aggressive behavior.

Neonatal DNA methylation and early-onset conduct problems: A genome-wide, prospective study

Early-onset conduct problems (CP) are a key predictor of adult criminality and poor mental health. While previous studies suggest that both genetic and environmental risks play an important role in the development of early-onset CP, little is known about potential biological processes underlying these associations. In this study, we examined prospective associations between DNA methylation (cord blood at birth) and trajectories of CP (4-13 years), using data drawn from the Avon Longitudinal Study of Parents and Children. Methylomic variation at seven loci across the genome (false discovery rate < 0.05) differentiated children who go on to develop early-onset (n = 174) versus low (n = 86) CP, including sites in the vicinity of the monoglyceride lipase (MGLL) gene (involved in endocannabinoid signaling and pain perception). Subthreshold associations in the vicinity of three candidate genes for CP (monoamine oxidase A [MAOA], brain-derived neurotrophic factor [BDNF], and FK506 binding protein 5 [FKBP5]) were also identified. Within the early-onset CP group, methylation levels of the identified sites did not distinguish children who will go on to persist versus desist in CP behavior over time. Overall, we found that several of the identified sites correlated with prenatal exposures, and none were linked to known genetic methylation quantitative trait loci. Findings contribute to a better understanding of epigenetic patterns associated with early-onset CP.

The Endocannabinoid System, Aggression, and the Violence of Synthetic Cannabinoid Use, Borderline Personality Disorder, Antisocial Personality Disorder, and Other Psychiatric Disorders

Endogenous and exogenous cannabinoids bind to central cannabinoid receptors to control a multitude of behavioral functions, including aggression. The first main objective of this review is to dissect components of the endocannabinoid system, including cannabinoid 1 and cannabinoid 2 receptors; the endogenous cannabinoids anandamide and 2-arachidonoylglycerol; and the indirect cannabinoid modulators fatty acid amide hydrolase and monoacylglycerol lipase; that have shown abnormalities in basic research studies investigating mechanisms of aggression. While most human research has concluded that the active ingredient of marijuana, Δ9-tetrahydrocannabinol, tends to dampen rather than provoke aggression in acute doses, recent evidence supports a relationship between the ingestion of synthetic cannabinoids and emergence of violent or aggressive behavior. Thus, another objective is to evaluate the emerging clinical data. This paper also discusses the relationship between prenatal and perinatal exposure to cannabis as well as use of cannabis in adolescence on aggressive outcomes. A final objective of the paper is to discuss endocannabinoid abnormalities in psychotic and affective disorders, as well as clinically aggressive populations, such as borderline personality disorder and antisocial personality disorder. With regard to the former condition, decreased anandamide metabolites have been reported in the cerebrospinal fluid, while some preliminary evidence suggests that fatty acid amide hydrolase genetic polymorphisms are linked to antisocial personality disorder and impulsive-antisocial psychopathic traits. To summarize, this paper will draw upon basic and clinical research to explain how the endocannabinoid system may contribute to the genesis of aggressive behavior.

Endocannabinoid Signaling in the Control of Social Behavior

Many mammalian species, including humans, exhibit social behavior and form complex social groups. Mechanistic studies in animal models have revealed important roles for the endocannabinoid signaling system, comprising G protein-coupled cannabinoid receptors and their endogenous lipid-derived agonists, in the control of neural processes that underpin social anxiety and social reward, two key aspects of social behavior. An emergent insight from these studies is that endocannabinoid signaling in specific circuits of the brain is context dependent and selectively recruited. These insights open new vistas on the neural basis of social behavior and social impairment.

Putative transmembrane transporter modulates higher-level aggression in Drosophila

By selection of winners of dyadic fights for 35 generations, we have generated a hyperaggressive Bully line of flies that almost always win fights against the parental wild-type Canton-S stock. Maintenance of the Bully phenotype is temperature dependent during development, with the phenotype lost when flies are reared at 19 °C. No similar effect is seen with the parent line. This difference allowed us to carry out RNA-seq experiments and identify a limited number of genes that are differentially expressed by twofold or greater in the Bullies; one of these was a putative transmembrane transporter, CG13646, which showed consistent and reproducible twofold down-regulation in Bullies. We examined the causal effect of this gene on the phenotype with a mutant line for CG13646, and with an RNAi approach. In all cases, reduction in expression of CG13646 by approximately half led to a hyperaggressive phenotype partially resembling that seen in the Bully flies. This gene is a member of a very interesting family of solute carrier proteins (SLCs), some of which have been suggested as being involved in glutamine/glutamate and GABA cycles of metabolism in excitatory and inhibitory nerve terminals in mammalian systems.

The neurobiological basis of human aggression: A review on genetic and epigenetic mechanisms

Aggression is an evolutionary conserved behavior present in most species including humans. Inadequate aggression can lead to long-term detrimental personal and societal effects. Here, we differentiate between proactive and reactive forms of aggression and review the genetic determinants of it. Heritability estimates of aggression in general vary between studies due to differing assessment instruments for aggressive behavior (AB) as well as age and gender of study participants. In addition, especially non-shared environmental factors shape AB. Current hypotheses suggest that environmental effects such as early life stress or chronic psychosocial risk factors (e.g., maltreatment) and variation in genes related to neuroendocrine, dopaminergic as well as serotonergic systems increase the risk to develop AB. In this review, we summarize the current knowledge of the genetics of human aggression based on twin studies, genetic association studies, animal models, and epigenetic analyses with the aim to differentiate between mechanisms associated with proactive or reactive aggression. We hypothesize that from a genetic perspective, the aminergic systems are likely to regulate both reactive and proactive aggression, whereas the endocrine pathways seem to be more involved in regulation of reactive aggression through modulation of impulsivity. Epigenetic studies on aggression have associated non-genetic risk factors with modifications of the stress response and the immune system. Finally, we point to the urgent need for further genome-wide analyses and the integration of genetic and epigenetic information to understand individual differences in reactive and proactive AB. © 2015 Wiley Periodicals, Inc.

Aggression in non-human vertebrates: Genetic mechanisms and molecular pathways

Aggression is an adaptive behavioral trait that is important for the establishment of social hierarchies and competition for mating partners, food, and territories. While a certain level of aggression can be beneficial for the survival of an individual or species, abnormal aggression levels can be detrimental. Abnormal aggression is commonly found in human patients with psychiatric disorders. The predisposition to aggression is influenced by a combination of environmental and genetic factors and a large number of genes have been associated with aggression in both human and animal studies. In this review, we compare and contrast aggression studies in zebrafish and mouse. We present gene ontology and pathway analyses of genes linked to aggression and discuss the molecular pathways that underpin agonistic behavior in these species. © 2015 Wiley Periodicals, Inc.

Cannabinoid Receptors Are Overexpressed in CLL but of Limited Potential for Therapeutic Exploitation

The cannabinoid receptors 1 and 2 (CNR1&2) are overexpressed in a variety of malignant diseases and cannabinoids can have noteworthy impact on tumor cell viability and tumor growth. Patients diagnosed with chronic lymphocytic leukemia (CLL) present with very heterogeneous disease characteristics translating into highly differential risk properties. To meet the urgent need for refinement in risk stratification at diagnosis and the search for novel therapies we studied CNR expression and response to cannabinoid treatment in CLL. Expression levels of CNR1&2 were determined in 107 CLL patients by real-time PCR and analyzed with regard to prognostic markers and survival. Cell viability of primary CLL cells was determined in suspension and co-culture after incubation in increasing cannabinoid concentrations under normal and reduced serum conditions and in combination with fludarabine. Impact of cannabinoids on migration of CLL cells towards CXCL12 was determined in transwell plates. We found CNR1&2 to be overexpressed in CLL compared to healthy B-cells. Discriminating between high and low expressing subgroups, only high CNR1 expression was associated with two established high risk markers and conferred significantly shorter overall and treatment free survival. Viability of CLL primary cells was reduced in a dose dependent fashion upon incubation with cannabinoids, however, healthy cells were similarly affected. Under serum reduced conditions, no significant differences were observed within suspension and co-culture, respectively, however, the feeder layer contributed significantly to the survival of CLL cells compared to suspension culture conditions. No significant differences were observed when treating CLL cells with cannabinoids in combination with fludarabine. Interestingly, biologic activity of cannabinoids was independent of both CNR1&2 expression. Finally, we did not observe an inhibition of CXCL12-induced migration by cannabinoids. In contrast to other tumor entities, our data suggest a limited usability of cannabinoids for CLL therapy. Nonetheless, we could define CNR1 mRNA expression as novel prognostic marker.

Neural endocannabinoid CB1 receptor expression, social status, and behavior in male European starlings

Many species modify behavior in response to changes in resource availability or social status; however, the neural mechanisms underlying these modifications are not well understood. Prior work in male starlings demonstrates that status-appropriate changes in behavior involve brain regions that regulate social behavior and vocal production. Endocannabinoids are ubiquitously distributed neuromodulators that are proposed to play a role in adjusting behavior to match social status. As an initial step to provide insight into this hypothesis we observed flocks of male starlings in outdoor aviaries during the breeding season. We used quantitative real-time PCR to measure expression of endocannabinoid CB1 receptors in brain regions involved in social behavior and motivation (lateral septum [LS], ventral tegmental area [VTA], medial preoptic nucleus [POM]) and vocal behavior (Area X and robust nucleus of the arcopallium; RA). Males with nesting sites sang to females and displaced other males more than males without nesting sites. They also had higher levels of CB1 receptor expression in LS and RA. CB1 expression in LS correlated positively with agonistic behaviors. CB1 expression in RA correlated positively with singing behavior. CB1 in VTA also correlated positively with singing when only singing birds were considered. These correlations nicely map onto the well-established role of LS in agonistic behavior and the known role of RA in song production and VTA in motivation and song production. Studies are now needed to precisely characterize the role of CB1 receptors in these regions in the production of status-appropriate social behaviors.

Genetics of aggressive behavior: An overview

The Research Domain Criteria (RDoC) address three types of aggression: frustrative non-reward, defensive aggression and offensive/proactive aggression. This review sought to present the evidence for genetic underpinnings of aggression and to determine to what degree prior studies have examined phenotypes that fit into the RDoC framework. Although the constructs of defensive and offensive aggression have been widely used in the animal genetics literature, the human literature is mostly agnostic with regard to all the RDoC constructs. We know from twin studies that about half the variance in behavior may be explained by genetic risk factors. This is true for both dimensional, trait-like, measures of aggression and categorical definitions of psychopathology. The non-shared environment seems to have a moderate influence with the effects of shared environment being unclear. Human molecular genetic studies of aggression are in an early stage. The most promising candidates are in the dopaminergic and serotonergic systems along with hormonal regulators. Genome-wide association studies have not yet achieved genome-wide significance, but current samples are too small to detect variants having the small effects one would expect for a complex disorder. The strongest molecular evidence for a genetic basis for aggression comes from animal models comparing aggressive and non-aggressive strains or documenting the effects of gene knockouts. Although we have learned much from these prior studies, future studies should improve the measurement of aggression by using a systematic method of measurement such as that proposed by the RDoC initiative.

Role of CB2 receptors in social and aggressive behavior in male mice

RATIONALE

Male CB1KO mice exhibit stronger aggressive responses than wild-type mice.

OBJECTIVE

This study was designed to examine the role of cannabinoid CB2r in social and aggressive behavior.

METHODS

The social interaction test and resident-intruder paradigm were performed in mice lacking CB2r (CB2KO) and in wild-type (WT) littermates. The effects of the CB2r selective agonist JWH133 (1 and 2 mg/kg) on aggression were also evaluated in Oncins France 1 (OF1) mice. Gene expression analyses of monoamine oxidase-A (MAO-A), catechol-o-methyltransferase (COMT), 5-hydroxytryptamine transporter (5-HTT), and 5-HT1B receptor (5HT1Br) in the dorsal raphe nuclei (DR) and the amygdala (AMY) were carried out using real-time PCR.

RESULTS

Group-housed CB2KO mice exhibited higher levels of aggression in the social interaction test and displayed more aggression than resident WT mice. Isolation increased aggressive behavior in WT mice but did not affect CB2KO animals; however, the latter mice exhibited higher levels of social interaction with their WT counterparts. MAO-A and 5-HTT gene expression was significantly higher in grouped CB2KO mice. The expression of 5HT1Br, COMT, and MAO-A in the AMY was more pronounced in CB2KO mice than in WT counterparts. Acute administration of the CB2 agonist JWH133 significantly reduced the level of aggression in aggressive isolated OF1 mice, an effect that decreased after pretreatment with the CB2 receptor antagonist AM630.

CONCLUSION

Our results suggest that CB2r is implicated in social interaction and aggressive behavior and deserves further consideration as a potential new target for the management of aggression.

Cannabinoids for the Treatment of Agitation and Aggression in Alzheimer's Disease

Alzheimer's disease (AD) is frequently associated with neuropsychiatric symptoms (NPS) such as agitation and aggression, especially in the moderate to severe stages of the illness. The limited efficacy and high-risk profiles of current pharmacotherapies for the management of agitation and aggression in AD have driven the search for safer pharmacological alternatives. Over the past few years, there has been a growing interest in the therapeutic potential of medications that target the endocannabinoid system (ECS). The behavioural effects of ECS medications, as well as their ability to modulate neuroinflammation and oxidative stress, make targeting this system potentially relevant in AD. This article summarizes the literature to date supporting this rationale and evaluates clinical studies investigating cannabinoids for agitation and aggression in AD. Letters, case studies, and controlled trials from four electronic databases were included. While findings from six studies showed significant benefits from synthetic cannabinoids—dronabinol or nabilone—on agitation and aggression, definitive conclusions were limited by small sample sizes, short trial duration, and lack of placebo control in some of these studies. Given the relevance and findings to date, methodologically rigorous prospective clinical trials are recommended to determine the safety and efficacy of cannabinoids for the treatment of agitation and aggression in dementia and AD.

Involvement of 2-arachidonoylglycerol signaling in social challenge responding of male CD1 mice

RATIONALE

Endocannabinoids are strong modulators of emotionality and present a novel target for psychotropic drug development. Increasing evidence suggests that endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG) affect behavior differentially. While the roles of anandamide have been investigated extensively, studies regarding the specific roles of 2-AG became possible only recently, and its involvement in social behaviors has not yet been studied.

OBJECTIVE

We studied the impact of 2-AG signaling on aggression as a first attempt to characterize the role of this endocannabinoid in social behaviors.

METHODS

2-AG signaling was enhanced by the monoacylglycerol lipase inhibitor JZL184 (8, and 16 mg/kg) in mice later submitted to the resident/intruder paradigm.

RESULTS

JZL184 near completely abolished aggressiveness in residents and increased victimization (i.e., attacks by the opponent). Interestingly, the level of defensiveness remained unaltered, despite the large increase in bites received. The CB1 receptor blocker AM251 (0.5 mg/kg) did not influence the effects of JZL184. In intruders, JZL184 near completely suppressed bites and offensive behavior in a fashion similar to residents, but it also increased agitation and defensiveness during, and the corticosterone response to, aggressive encounters. Experiments involving the corticosterone synthesis inhibitor metyrapone (30 mg/kg) suggest that the suppression of biting and offensive behavior is directly influenced by JZL184, whereas increased agitation and defensiveness (seen in intruders only) are a secondary development of the stress-endocrine effects of JZL184.

CONCLUSIONS

2-AG signaling emerges as a surprisingly strong negative modulator of aggressiveness, which warrants further studies into its general role in social behavior and the target receptors involved.

Genetic Manipulation of the Endocannabinoid System

The physiological and pathophysiological functions of the endocannabinoid system have been studied extensively using transgenic and targeted knockout mouse models. The first gene deletions of the cannabinoid CB(1) receptor were described in the late 1990s, soon followed by CB(2) and FAAH mutations in early 2000. These mouse models helped to elucidate the fundamental role of endocannabinoids as retrograde transmitters in the CNS and in the discovery of many unexpected endocannabinoid functions, for example, in the skin, bone and liver. We now have knockout mouse models for almost every receptor and enzyme of the endocannabinoid system. Conditional mutant mice were mostly developed for the CB(1) receptor, which is widely expressed on many different neurons, astrocytes and microglia, as well as on many cells outside the CNS. These mouse strains include "floxed" CB(1) alleles and mice with a conditional re-expression of CB(1). The availability of these mice made it possible to decipher the function of CB(1) in specific neuronal circuits and cell populations or to discriminate between central and peripheral effects. Many of the genetic mouse models were also used in combination with viral expression systems. The purpose of this review is to provide a comprehensive overview of the existing genetic models and to summarize some of the most important discoveries that were made with these animals.