Lack of CB1 cannabinoid receptor impairs cocaine self-administration

The distribution of neurotransmitters in the brain circuitry: Mesolimbic pathway and addiction

Understanding the central nervous system (CNS) circuitry and its different neurotransmitters that underlie reward is essential to improve treatment for many common health issues, such as addiction. Here, we concentrate on understanding how the mesolimbic circuitry and neurotransmitters are organized and function, and how drug exposure affects synaptic and structural changes in this circuitry. While the role of some reward circuits, like the cerebral dopamine (DA)/glutamate (Glu)/gamma aminobutyric acid (GABA)ergic pathways, in drug reward, is well known, new research using molecular-based methods has shown functional alterations throughout the reward circuitry that contribute to various aspects of addiction, including craving and relapse. A new understanding of the fundamental connections between brain regions as well as the molecular alterations within these particular microcircuits, such as neurotrophic factor and molecular signaling or distinct receptor function, that underlie synaptic and structural plasticity evoked by drugs of abuse has been made possible by the ability to observe and manipulate neuronal activity within specific cell types and circuits. It is exciting that these discoveries from preclinical animal research are now being applied in the clinic, where therapies for human drug dependence, such as deep brain stimulation and transcranial magnetic stimulation, are being tested. Therefore, this chapter seeks to summarize the current understanding of the important brain regions (especially, mesolimbic circuitry) and neurotransmitters implicated in drug-related behaviors and the molecular mechanisms that contribute to altered connectivity between these areas, with the postulation that increased knowledge of the plasticity within the drug reward circuit will lead to new and improved treatments for addiction.

Lack of Bmal1 leads to changes in rhythmicity and impairs motivation towards natural stimuli

Maintaining proper circadian rhythms is essential for coordinating biological functions in mammals. This study investigates the effects of daily arrhythmicity using Bmal1-knockout (KO) mice as a model, aiming to understand behavioural and motivational implications. By employing a new mathematical analysis based on entropy divergence, we identified disrupted intricate activity patterns in mice derived by the complete absence of BMAL1 and quantified the difference regarding the activity oscillation's complexity. Changes in locomotor activity coincided with disturbances in circadian gene expression patterns. Additionally, we found a dysregulated gene expression profile particularly in brain nuclei like the ventral striatum, impacting genes related to reward and motivation. Further investigation revealed that arrhythmic mice exhibited heightened motivation for food and water rewards, indicating a link between circadian disruptions and the reward system. This research sheds light on how circadian clock alterations impact the gene expression regulating the reward system and how this, in turn, can lead to altered seeking behaviour and motivation for natural rewards. In summary, the present study contributes to our understanding of how reward processing is under the regulation of circadian clock machinery.

Roles of cannabinoid CB1 and CB2 receptors in the modulation of psychostimulant responses

Addiction to psychostimulant drugs, such as cocaine, D-amphetamine, and methamphetamine, is a public health issue that substantially contributes to the global burden of disease. Psychostimulant drugs promote an increase in dopamine levels within the mesocorticolimbic system, which is central to the rewarding properties of such drugs. Cannabinoid receptors (CB1R and CB2R) are expressed in the main areas of this system and implicated in the neuronal mechanisms underlying the rewarding effect of psychostimulant drugs. Here, we reviewed studies focusing on pharmacological intervention targeting cannabinoid CB1R and CB2R and their interaction in the modulation of psychostimulant responses.

CB1 Receptor Silencing Attenuates Ketamine-Induced Hyperlocomotion Without Compromising Its Antidepressant-Like Effects

Introduction: The antidepressant properties of ketamine have been extensively demonstrated in experimental and clinical settings. However, the psychotomimetic side effects still limit its wider use as an antidepressant. It was recently observed that endocannabinoids are inolved in ketamine induced reward properties. As an increase in endocannabinoid signaling induces antidepressant effects, this study aimed to investigate the involvement of cannabinoid type 1 receptors (CB1R) in the antidepressant and psychostimulant effects induced by ketamine. Methods: We tested the effects of genetic and pharmacological inhibition of CB1R in the hyperlocomotion and antidepressant-like properties of ketamine. The effects of ketamine (10-20 mg/kg) were assessed in the open-field and the forced swim tests (FSTs) in CB1R knockout (KO) and wild-type (WT) mice (male and female), and mice pre-treated with rimonabant (CB1R antagonist, 3-10 mg/kg). Results: We found that the motor hyperactivity elicited by ketamine was impaired in CB1R male and female KO mice. A similar effect was observed upon pharmacological blockade of CB1R in WT mice. However, genetic CB1R deletion did not modify the antidepressant effect of ketamine in male mice submitted to the FST. Surprisingly, pharmacological blockade of CB1R induced an antidepressant-like effect in both male and female mice, which was not further potentiated by ketamine. Conclusions: Our results support the hypothesis that CB1R mediate the psychostimulant side effects induced by ketamine, but not its antidepressant properties.

The FAAH inhibitor URB597 reduces cocaine intake during conditioned punishment and mitigates cocaine seeking during withdrawal

The endocannabinoid system is prominently implicated in the control of cocaine reinforcement due to its relevant role in synaptic plasticity and neurotransmitter modulation in the mesocorticolimbic system. The inhibition of fatty acid amide hydrolase (FAAH), and the resulting increase in anandamide and other N-acylethanolamines, represents a promising strategy for reducing drug seeking. In the present study, we aimed to assess the effects of the FAAH inhibitor URB597 (1 mg/kg) on crucial features of cocaine addictive-like behaviour in mice. Therefore, we tested the effects of URB597 on acquisition of cocaine (0.6 mg/kg/inf) self-administration, compulsive-like cocaine intake and cue-induced drug-seeking behaviour during withdrawal. URB597 reduced cocaine intake under conditioned punishment while having no impact on acquisition. This result was associated to increased cannabinoid receptor 1 gene expression in the ventral striatum and medium spiny neurons activation in the nucleus accumbens shell. Moreover, URB597 mitigated cue-induced drug-seeking behaviour during prolonged abstinence and prevented the withdrawal-induced increase in FAAH gene expression in the ventral striatum. In this case, URB597 decreased activation of medium spiny neurons in the nucleus accumbens core. Our findings evidence the prominent role of endocannabinoids in the development of cocaine addictive-like behaviours and support the potential of FAAH inhibition as a therapeutical target for the treatment of cocaine addiction.

Role of mesolimbic cannabinoid receptor 1 in stress-driven increases in cocaine self-administration in male rats

Stress is prevalent in the lives of those with substance use disorders (SUDs) and influences SUD outcomes. Understanding the neurobiological mechanisms through which stress promotes drug use is important for the development of effective SUD interventions. We have developed a model wherein exposure to a stressor, uncontrollable electric footshock, daily at the time of cocaine self-administration escalates intake in male rats. Here we test the hypothesis that stress-induced escalation of cocaine self-administration requires the CB1 cannabinoid receptor. Male Sprague-Dawley rats self-administered cocaine (0.5 mg/kg/inf, i.v.) during 2-h sessions comprised of four 30-min self-administration components separated by 5-min shock sequences or 5-min shock-free periods for 14 days. Footshock produced an escalation of cocaine self-administration that persisted following shock removal. Systemic administration of the cannabinoid receptor type 1 (CB1R) antagonist/inverse agonist, AM251, attenuated cocaine intake only in rats with a history of stress. This effect was localized to the mesolimbic system, as intra-nucleus accumbens (NAc) shell and intra-ventral tegmental area (VTA) micro-infusions of AM251 attenuated cocaine intake only in stress-escalated rats. Cocaine self-administration, regardless of stress history, increased CB1R binding site density in the VTA, but not NAc shell. Following extinction, cocaine-primed reinstatement (10 mg/kg, ip) was increased in rats with prior footshock during self-administration. AM251 attenuated reinstatement only in rats with a stress history. Altogether, these data demonstrate that mesolimbic CB1Rs are required to escalate intake and heighten relapse susceptibility and suggest that repeated stress at the time of cocaine use regulates mesolimbic CB1R activity through a currently unknown mechanism.

CB1R chronic intermittent pharmacological activation facilitates amphetamine seeking and self-administration and changes in CB1R/CRFR1 expression in the amygdala and nucleus accumbens in rats

Patterns of drug ingestion may have a dissimilar impact on the brain, and therefore also the development of drug addiction. One pattern is binge intoxication that refers to the ingestion of a high amount of drug on a single occasion followed by an abstinence period of variable duration. In this study, our goal was to contrast the effect of continuous low amounts with intermittent higher amounts of Arachidonyl-chloro-ethylamide (ACEA), a CB1R agonist, on amphetamine seeking and ingestion, and describe the effects on the expression of CB1R and CRFR1 in the central nucleus of the amygdala (CeA) and in the nucleus accumbens shell (NAcS). Adult male Wistar rats were treated with a daily administration of vehicle or 20 μg of ACEA, or four days of vehicle followed by 100 μg of ACEA on the fifth day, for a total of 30 days. Upon completion of this treatment, the CB1R and CRFR1 expression in the CeA and NAcS was evaluated by immunofluorescence. Additional groups of rats were evaluated for their anxiety levels (elevated plus maze, EPM), amphetamine (AMPH) self-administration (ASA) and breakpoint (A-BP), as well as AMPH-induced conditioned place preference (A-CPP). Results indicated that ACEA induced changes in the CB1R and CRFR1 expression in both the NAcS and CeA. An increase in anxiety-like behavior, ASA, A-BP and A-CPP was also observed. Since the intermittent administration of 100 μg of ACEA induced the most evident changes in most of the parameters studied, we concluded that binge-like ingestion of drugs induces changes in the brain that may make the subject more vulnerable to developing drug addiction.

Crosstalk between the endocannabinoid and mid-brain dopaminergic systems: Implication in dopamine dysregulation

Endocannabinoids (eCBs) and the expanded endocannabinoid system (ECS)-"endocannabinoidome", consists of the endogenous ligands, eCBs, their canonical and non-canonical receptor subtypes, and their synthesizing and metabolizing enzymes. This system modulates a wide range of body functions and acts as a retrograde signaling system within the central nervous system (CNS) by inhibition of classical transmitters, and plays a vital modulatory function on dopamine, a major neurotransmitter in the CNS. Dopamine is involved in different behavioral processes and contributes to different brain disorders-including Parkinson's disease, schizophrenia, and drug addiction. After synthesis in the neuronal cytosol, dopamine is packaged into synaptic vesicles until released by extracellular signals. Calcium dependent neuronal activation results in the vesicular release of dopamine and interacts with different neurotransmitter systems. The ECS, among others, is involved in the regulation of dopamine release and the interaction occurs either through direct or indirect mechanisms. The cross-talk between the ECS and the dopaminergic system has important influence in various dopamine-related neurobiological and pathologic conditions and investigating this interaction might help identify therapeutic targets and options in disorders of the CNS associated with dopamine dysregulation.

Endocannabinoids at the synapse and beyond: implications for neuropsychiatric disease pathophysiology and treatment

Endocannabinoids (eCBs) are lipid neuromodulators that suppress neurotransmitter release, reduce postsynaptic excitability, activate astrocyte signaling, and control cellular respiration. Here, we describe canonical and emerging eCB signaling modes and aim to link adaptations in these signaling systems to pathological states. Adaptations in eCB signaling systems have been identified in a variety of biobehavioral and physiological process relevant to neuropsychiatric disease states including stress-related disorders, epilepsy, developmental disorders, obesity, and substance use disorders. These insights have enhanced our understanding of the pathophysiology of neurological and psychiatric disorders and are contributing to the ongoing development of eCB-targeting therapeutics. We suggest future studies aimed at illuminating how adaptations in canonical as well as emerging cellular and synaptic modes of eCB signaling contribute to disease pathophysiology or resilience could further advance these novel treatment approaches.

Therapeutic potential of PIMSR, a novel CB1 receptor neutral antagonist, for cocaine use disorder: evidence from preclinical research

Cannabinoid CB1 receptors (CB1Rs) have been major targets in medication development for the treatment of substance use disorders. However, clinical trials with rimonabant, a CB1R antagonist/inverse agonist, failed due to severe side effects. Here, we evaluated the therapeutic potential of PIMSR, a neutral CB1R antagonist lacking an inverse agonist profile, against cocaine's behavioral effects in experimental animals. We found that systemic administration of PIMSR dose-dependently inhibited cocaine self-administration under fixed-ratio (FR5), but not FR1, reinforcement, shifted the cocaine self-administration dose-response curve downward, decreased incentive motivation to seek cocaine under progressive-ratio reinforcement, and reduced cue-induced reinstatement of cocaine seeking. PIMSR also inhibited oral sucrose self-administration. Importantly, PIMSR alone is neither rewarding nor aversive as assessed by place conditioning. We then used intracranial self-stimulation (ICSS) to explore the possible involvement of the mesolimbic dopamine system in PIMSR's action. We found that PIMSR dose-dependently attenuated cocaine-enhanced ICSS maintained by electrical stimulation of the medial forebrain bundle in rats. PIMSR itself failed to alter electrical ICSS, but dose-dependently inhibited ICSS maintained by optical stimulation of midbrain dopamine neurons in transgenic DAT-Cre mice, suggesting the involvement of dopamine-dependent mechanisms. Lastly, we examined the CB1R mechanisms underlying PIMSR's action. We found that PIMSR pretreatment attenuated Δ⁹-tetrahydrocannabinol (Δ⁹-THC)- or ACEA (a selective CB1R agonist)-induced reduction in optical ICSS. Together, our findings suggest that the neutral CB1R antagonist PIMSR deserves further research as a promising pharmacotherapeutic for cocaine use disorder.

Molecular Alterations of the Endocannabinoid System in Psychiatric Disorders

The therapeutic benefits of the current medications for patients with psychiatric disorders contrast with a great variety of adverse effects. The endocannabinoid system (ECS) components have gained high interest as potential new targets for treating psychiatry diseases because of their neuromodulator role, which is essential to understanding the regulation of many brain functions. This article reviewed the molecular alterations in ECS occurring in different psychiatric conditions. The methods used to identify alterations in the ECS were also described. We used a translational approach. The animal models reproducing some behavioral and/or neurochemical aspects of psychiatric disorders and the molecular alterations in clinical studies in post-mortem brain tissue or peripheral tissues were analyzed. This article reviewed the most relevant ECS changes in prevalent psychiatric diseases such as mood disorders, schizophrenia, autism, attentional deficit, eating disorders (ED), and addiction. The review concludes that clinical research studies are urgently needed for two different purposes: (1) To identify alterations of the ECS components potentially useful as new biomarkers relating to a specific disease or condition, and (2) to design new therapeutic targets based on the specific alterations found to improve the pharmacological treatment in psychiatry.

Hippocampal Cannabinoid 1 Receptors Are Modulated Following Cocaine Self-administration in Male Rats

Cocaine addiction is a complex pathology inducing long-term neuroplastic changes that, in turn, contribute to maladaptive behaviors. This behavioral dysregulation is associated with transcriptional reprogramming in brain reward circuitry, although the mechanisms underlying this modulation remain poorly understood. The endogenous cannabinoid system may play a role in this process in that cannabinoid mechanisms modulate drug reward and contribute to cocaine-induced neural adaptations. In this study, we investigated whether cocaine self-administration induces long-term adaptations, including transcriptional modifications and associated epigenetic processes. We first examined endocannabinoid gene expression in reward-related brain regions of the rat following self-administered (0.33 mg/kg intravenous, FR1, 10 days) cocaine injections. Interestingly, we found increased Cnr1 expression in several structures, including prefrontal cortex, nucleus accumbens, dorsal striatum, hippocampus, habenula, amygdala, lateral hypothalamus, ventral tegmental area, and rostromedial tegmental nucleus, with most pronounced effects in the hippocampus. Endocannabinoid levels, measured by mass spectrometry, were also altered in this structure. Chromatin immunoprecipitation followed by qPCR in the hippocampus revealed that two activating histone marks, H3K4Me3 and H3K27Ac, were enriched at specific endocannabinoid genes following cocaine intake. Targeting CB1 receptors using chromosome conformation capture, we highlighted spatial chromatin re-organization in the hippocampus, as well as in the nucleus accumbens, suggesting that destabilization of the chromatin may contribute to neuronal responses to cocaine. Overall, our results highlight a key role for the hippocampus in cocaine-induced plasticity and broaden the understanding of neuronal alterations associated with endocannabinoid signaling. The latter suggests that epigenetic modifications contribute to maladaptive behaviors associated with chronic drug use.

The interactions of alcohol and cocaine regulate the expression of genes involved in the GABAergic, glutamatergic and endocannabinoid systems of male and female rats

Although the pharmacological and behavioural interactions between cocaine and alcohol are well established, less is known about how polyconsumption of these drugs affects the neurotransmitter systems involved in their psychoactive effects and in particular, in the process of addiction. Here, rats of both sexes at two stages of development were studied under a chronic regime of intravenous cocaine and/or alcohol administration. Brain samples from the medial prefrontal cortex, nucleus accumbens, hippocampus and amygdala were extracted to analyse the mRNA expression of genes encoding subunits of the GABA, NMDA and AMPA receptors, as well as the expression of the CB1 receptor, and that of enzymes related to the biosynthesis and degradation of endocannabinoids. Moreover, two synaptic scaffold proteins related to GABA and NMDA receptors, gephyrin and PSD-95, were quantified in Western blots. Significant interactions between cocaine and alcohol were common, affecting the GABAergic and endocannabinoid systems in the medial prefrontal cortex and amygdala of young adults, whereas such interactions were evident in the glutamatergic and endocannabinoid systems in adults, as well as a more pronounced sex effect. Significant interactions between these drugs affecting the scaffold proteins were evident in the medial prefrontal cortex and nucleus accumbens of young adults, and in the nucleus accumbens and amygdala of adults, but not in the hippocampus. These results highlight the importance of considering the interactions between cocaine and alcohol on neurotransmitter systems in the context of polyconsumption, specifically when treating problems of abuse of these two substances.

Sex differences in cocaine-associated memory: The interplay between CB1, mGluR5, and estradiol

We know surprisingly little about the sex differences in the neurobiology of cocaine addiction, except females are more susceptible to the rewarding effects of cocaine than their male counterparts. Only a handful of recent studies have examined the neurobiology of cocaine-induced conditioned place preference (CPP) memory among female rodents. We contribute to this emerging line of research by documenting sex differences in cocaine-associated memory and illustrating the underlying signaling pathways in five experiments. Rimonabant (Rim), a cannabinoid CB₁ antagonist and inverse agonist, exerted a facilitating effect for low-dose cocaine and an impairing effect for high-dose cocaine CPP memory in male mice, as in our previous study, but not in female mice. Nor did we observe the effect exist among CB₁ knockout male mice, which indicated that the CB₁ receptors played a mediating role. We also found that the metabotropic glutamate receptor 5 (mGluR5) was located in the same signaling pathway as CB₁ in male mice. To clarify the mechanisms behind the sex differences, we used ovariectomized (OVX) female mice with estradiol benzoate (EB) replacement. In the OVX female mice, we showed that Rim-alone and EB-alone, but not Rim-and-EB-combined, facilitated the low-dose cocaine CPP memory. Moreover, 4-hydroxytamoxifen (4-OHT), an estrogen receptor (ER) antagonist, blocked Rim's and EB's facilitating effect. Finally, 2-methyl-6-(phenylethynyl)pyridine (MPEP), an mGluR5 antagonist, partially blocked EB's facilitating effect. In sum, we identified sex-specific effects of Rim on cocaine-induced CPP memory and the respective signaling pathways: mGluR5-CB₁ for male mice and ER-mGluR5-CB₁ for female mice. These findings may have merits for the development of sex-specific treatment for cocaine addiction.

Cannabidiol Modulates the Motivational and Anxiety-Like Effects of 3,4-Methylenedioxypyrovalerone (MDPV) in Mice

3,4-Methylenedioxypyrovalerone (MDPV) is a new psychoactive substance (NPS) and the most widespread and life-threatening synthetic cathinone of the "bath salts". Preclinical research has proven the cocaine-like psychostimulant effects of MDPV and its potential for abuse. Cannabidiol (CBD) is a non-psychotropic phytocannabinoid that has emerged as a new potential treatment for drug addiction. Here, we tested the effects of CBD (20 mg/kg) on MDPV (2 mg/kg)-induced conditioned place preference and MDPV (0.05 and 0.075 mg/kg/infusion) self-administration paradigms. In addition, we assessed the effects of the co-administration of CBD and MDPV (3 and 4 mg/kg) on anxiety-like behaviour using the elevated plus maze (EPM). CBD mitigated the MDPV-induced conditioned place preference. On the contrary, CBD administration throughout the MDPV (0.075 mg/kg/infusion) self-administration increased drug-seeking and taking behaviours, but only in the high-responders group of mice. Furthermore, CBD exerted anxiolytic-like effects, exclusively in MDPV-treated mice. Taken together, our results indicate that CBD modulation of MDPV-induced motivational responses in mice varies depending on the requirements of the learning task, resulting in a complex response. Therefore, further research attempting to decipher the behavioural and molecular interactions between CBD and MDPV is needed.

Cocaine-induced increases in motivation require 2-arachidonoylglycerol mobilization and CB1 receptor activation in the ventral tegmental area

A wide body of evidence supports an integral role for mesolimbic dopamine (DA) in motivated behavior. In brief, drugs that increase DA in mesolimbic terminal regions, like cocaine, enhance motivation, while drugs that decrease DA concentration reduce motivation. Data from our laboratory and others shows that phasic activation of mesolimbic DA requires signaling at cannabinoid type-1 (CB1) receptors in the ventral tegmental area (VTA), and systemic delivery of CB1 receptor antagonists reduces DA cell activity and attenuates motivated behaviors. Recent findings demonstrate that cocaine mobilizes the endocannabinoid 2-arachidonoylglycerol (2-AG) in the VTA to cause phasic activation of DA neurons and terminal DA release. It remains unclear, however, if cocaine-induced midbrain 2-AG signaling contributes to the motivation-enhancing effects of cocaine. To examine this, we trained male and female rats on a progressive ratio (PR) task for a food reinforcer. Each rat underwent a series of tests in which they were pretreated with cocaine alone or in combination with systemic or intra-VTA administration of the CB1 receptor antagonist rimonabant or the 2-AG synthesis inhibitor tetrahydrolipstatin (THL). Cocaine increased motivation, measured by augmented PR breakpoints, while rimonabant dose-dependently decreased motivation. Importantly, intra-VTA administration of rimonabant or THL, at doses that did not decrease breakpoints on their own, blocked systemic cocaine administration from increasing breakpoints in male and female rats. These data suggest that cocaine-induced increases in motivation require 2-AG signaling at CB1 receptors in the VTA and may provide critical insight into cannabinoid-based pharmacotherapeutic targets for the successful treatment of substance abuse.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Cannabinoid type 1 receptors in A2a neurons contribute to cocaine-environment association

RATIONALE

Cannabinoid type 1 receptors (CB1Rs) are widely expressed within the brain's reward circuits and are implicated in regulating drug induced behavioral adaptations. Understanding how CB1R signaling in discrete circuits and cell types contributes to drug-related behavior provides further insight into the pathology of substance use disorders.

OBJECTIVE AND METHODS

We sought to determine how cell type-specific expression of CB1Rs within striatal circuits contributes to cocaine-induced behavioral plasticity, hypothesizing that CB1R function in distinct striatal neuron populations would differentially impact behavioral outcomes. We crossed conditional Cnr1^fl/fl mice and striatal output pathway cre lines (Drd1a -cre; D1, Adora2a -cre; A2a) to generate cell type-specific CB1R knockout mice and assessed their performance in cocaine locomotor and associative behavioral assays.

RESULTS

Both knockout lines retained typical locomotor activity at baseline. D1-Cre x Cnr1^fl/fl mice did not display hyperlocomotion in response to acute cocaine dosing, and both knockout lines exhibited blunted locomotor activity across repeated cocaine doses. A2a-cre Cnr1^fl/fl, mice did not express a preference for cocaine paired environments in a two-choice place preference task.

CONCLUSIONS

This study aids in mapping CB1R-dependent cocaine-induced behavioral adaptations onto distinct striatal neuron subtypes. A reduction of cocaine-induced locomotor activation in the D1- and A2a-Cnr1 knockout mice supports a role for CB1R function in the motor circuit. Furthermore, a lack of preference for cocaine-associated context in A2a-Cnr1 mice suggests that CB1Rs on A2a-neuron inhibitory terminals are necessary for either reward perception, memory consolidation, or recall. These results direct future investigations into CB1R-dependent adaptations underlying the development and persistence of substance use disorders.

Cannabidiol Modulates Behavioural and Gene Expression Alterations Induced by Spontaneous Cocaine Withdrawal

The aim of this study was to evaluate the effects of cannabidiol (CBD) on the behavioural and gene expression changes in a new animal model of spontaneous cocaine withdrawal. For this purpose, male CD-1 mice were exposed to progressive increasing doses of cocaine for 12 days (15 to 60 mg/kg/day, i.p.), evaluating spontaneous cocaine withdrawal 6 h after the last cocaine administration. The effects of CBD (10, 20, and 40 mg/kg, i.p.) were evaluated on cocaine withdrawal-induced alterations in motor activity, somatic signs, and anxiety-like behaviour. Furthermore, gene expression changes in dopamine transporter (DAT) and tyrosine hydroxylase (TH) in the ventral tegmental area, and in cannabinoid receptors 1 (CNR1) and 2 (CNR2) in the nucleus accumbens, were analysed by real-time PCR. The results obtained in the study showed that mice exposed to the spontaneous cocaine withdrawal model presented increased motor activity, somatic withdrawal signs, and high anxiety-like behaviour. Interestingly, the administration of CBD normalized motor and somatic signs disturbances and induced an anxiolytic effect. Moreover, the administration of CBD blocked the increase of DAT and TH gene expression in mice exposed to the cocaine withdrawal, regulated the decrease of CNR1 and induced an additional upregulation of CNR2 gene expression. Thus, this model of spontaneous cocaine withdrawal induces clear behavioural and gene expression changes in mice. Interestingly, CBD alleviates these behavioural and gene expression alterations suggesting its potential for the management of cocaine withdrawal.

Protective role of neuronal and lymphoid cannabinoid CB2 receptors in neuropathic pain

Cannabinoid CB₂ receptor (CB₂) agonists are potential analgesics void of psychotropic effects. Peripheral immune cells, neurons and glia express CB₂; however, the involvement of CB₂ from these cells in neuropathic pain remains unresolved. We explored spontaneous neuropathic pain through on-demand self-administration of the selective CB₂ agonist JWH133 in wild-type and knockout mice lacking CB₂ in neurons, monocytes or constitutively. Operant self-administration reflected drug-taking to alleviate spontaneous pain, nociceptive and affective manifestations. While constitutive deletion of CB₂ disrupted JWH133-taking behavior, this behavior was not modified in monocyte-specific CB₂ knockouts and was increased in mice defective in neuronal CB₂ knockouts suggestive of increased spontaneous pain. Interestingly, CB₂-positive lymphocytes infiltrated the injured nerve and possible CB₂transfer from immune cells to neurons was found. Lymphocyte CB₂depletion also exacerbated JWH133 self-administration and inhibited antinociception. This work identifies a simultaneous activity of neuronal and lymphoid CB₂that protects against spontaneous and evoked neuropathic pain.

D3 dopamine receptors and a missense mutation of fatty acid amide hydrolase linked in mouse and men: implication for addiction

The endocannabinoid and dopaminergic systems have independently been implicated in substance use disorder and obesity. We investigated a potential interaction between genetically inherited variation in fatty acid amide hydrolase (FAAH, C385A), which metabolizes the cannabis-like endocannabinoid anandamide, and dopaminergic system, measured by dopamine receptor levels and mRNA. Binding of the dopamine D3 preferring probe [C-11]-(+)-PHNO was measured with positron emission tomography (PET) in 79 human subjects genotyped for the FAAH C385A polymorphism (36/79 AC + AA). Autoradiography with [H-3]-(+)-PHNO and in situ hybridization with a D3-specific S-35 riboprobe were carried out in 30 knock-in mice with the FAAH C385A polymorphism (20/30 AC + AA). We found that the FAAH genetic variant C385A was associated with significantly higher (+)-PHNO binding in both humans and in knock-in mice, and this effect was restricted to D3 selective brain regions (limbic striatum, globus pallidus, and ventral pallidum (9-14%; p < 0.04) in humans and Islands of Calleja (28%; p = 0.036) in mice). In situ hybridization with a D3-specific S-35 riboprobe in FAAH knock-in C385A mice confirmed significantly increased D3 receptor mRNA across examined regions (7-44%; p < 0.02). The association of reduced FAAH function with higher dopamine D3 receptors in human and mouse brain provide a mechanistic link between two brain systems that have been implicated in addiction-risk. This may explain the greater vulnerability for addiction and obesity in individuals with C385A genetic variant and by extension, suggest that a D3 antagonism strategy in substance use disorders should consider FAAH C385A polymorphism.

THC exposure during adolescence does not modify nicotine reinforcing effects and relapse in adult male mice

RATIONALE

Cannabis use is typically initiated during adolescence, and different studies suggest that adolescent cannabinoid exposure may increase the risk for drug addiction in adulthood.

OBJECTIVES

This study investigated the effects of adolescent exposure to the main psychoactive component of cannabis, ∆⁹-tetrahydrocannabinol (THC), in the reinforcing properties of nicotine in adult male mice. Possible alterations in relapse to nicotine-seeking behaviour in adult animals due to THC adolescent exposure were also evaluated.

METHODS

Adolescent mice were exposed to escalating doses of THC from PND35 to PND49. When mice reached adulthood (PND70), surgical procedures were applied for further behavioural evaluation. Nicotine self-administration sessions were conducted consecutively for 10 days. Following extinction, mice were tested for cue- and stress-induced reinstatement of nicotine-seeking behaviour.

RESULTS

Adolescent THC treatment did not modify acquisition and extinction of nicotine self-administration in adulthood. Moreover, THC exposure did not alter relapse to nicotine seeking induced by stress or nicotine-associated cues.

CONCLUSIONS

These results suggest that a history of exposure to THC during adolescence under these particular conditions does not modify the reinforcing effects and seeking behaviour of nicotine in the adult period.

Cannabinoid Modulation of Food-Cocaine Choice in Male Rhesus Monkeys

Marijuana and other cannabinoid compounds are widely used by cocaine users. Preclinical animal studies suggest that these compounds can increase the reinforcing effects of cocaine under some schedules of cocaine self-administration and reinstatement, but not in all cases. To date, no studies have used a food-cocaine concurrent choice procedure, which allows for assessment of drug effects on response allocation, not just changes in cocaine self-administration. The goal of the present study was to examine the effects of compounds differing in their efficacy at the cannabinoid receptor (CBR) on cocaine self-administration using a food-drug choice procedure in monkeys. Four adult male rhesus monkeys were trained to self-administer cocaine in the context of an alternative food (1.0-g banana-flavored pellets) reinforcer, such that complete cocaine dose-response curves (0, 0.003-0.1 mg/kg per injection) were determined each session. Monkeys were tested acutely with the CBR full agonist CP 55,940 (0.001-0.01 mg/kg); the CBR partial agonist Δ⁹-tetrahydrocannabinol (THC; 0.03-0.3 mg/kg), which is also the primary active ingredient in marijuana and the CBR antagonist rimonabant (0.3-3.0 mg/kg). Cocaine choice increased in a dose-dependent manner. Acute treatment with CP 55,940 decreased cocaine choice, whereas THC and rimonabant enhanced the reinforcing effects of cocaine. Chronic (7-day) treatment with CP 55,940 resulted in tolerance to the decreases in cocaine choice. These findings with Δ⁹-THC provide support for a potential mechanism for co-abuse of marijuana and cocaine. Additional research with chronic treatment with full CBR agonists on attenuating the reinforcing strength of cocaine is warranted. SIGNIFICANCE STATEMENT: Co-abuse of tetrahydrocannabinol and cocaine is a significant public health problem. The use of animal models allows for the determination of how cannabinoid receptor stimulation or blockade influences the reinforcing strength of cocaine.

Behavioral effects of psychostimulants in mutant mice with cell-type specific deletion of CB2 cannabinoid receptors in dopamine neurons

Activation of the endocannabinoid system modulate dopaminergic pathways that are involved in the effects of psychostimulants including amphetamine, cocaine, nicotine and other drugs of abuse. Genetic deletion or pharmacological activation of CB2 cannabinoid receptor is involved in the modulation of the effects of psychostimulants and their rewarding properties. Here we report on the behavioral effects of psychostimulants in DAT-Cnr2 conditional knockout (cKO) mice with selective deletion of type 2 cannabinoid receptors in dopamine neurons. There was enhanced psychostimulant induced hyperactivity in DAT-Cnr2 cKO mice, but the psychostimulant-induced sensitization was absent in DAT-Cnr2 cKO compared to the WT mice. Intriguingly, lower doses of amphetamine reduced locomotor activity of the DAT-Cnr2 cKO mice. While cocaine, amphetamine and methamphetamine produced robust conditioned place preference (CPP) in both DAT-Cnr2 cKO and WT mice, nicotine at the dose used induced CPP only in the WT but not in the DAT-Cn2 cKO mice. However, pre-treatment with the CB2R selective agonist JWH133, blocked cocaine and nicotine induced CPP in the WT mice. The deletion of CB2Rs in dopamine neurons modified the levels of tyrosine hydroxylase, and reduced the expression of dopamine transporter gene expression in DAT-Cnr2 cKO midbrain region. Taken together, our data suggest that CB2Rs play a role in the modulation of dopamine-related effects of psychostimulants and could be exploited as therapeutic target in psychostimulant addiction and other psychiatric disorders associated with dopamine dysregulation.

The Endocannabinoid System and Cannabidiol's Promise for the Treatment of Substance Use Disorder

Substance use disorder is characterized by repeated use of a substance, leading to clinically significant distress, making it a serious public health concern. The endocannabinoid system plays an important role in common neurobiological processes underlying substance use disorder, in particular by mediating the rewarding and motivational effects of substances and substance-related cues. In turn, a number of cannabinoid drugs (e.g., rimonabant, nabiximols) have been suggested for potential pharmacological treatment for substance dependence. Recently, cannabidiol (CBD), a non-psychoactive phytocannabinoid found in the cannabis plant, has also been proposed as a potentially effective treatment for the management of substance use disorder. Animal and human studies suggest that these cannabinoids have the potential to reduce craving and relapse in abstinent substance users, by impairing reconsolidation of drug-reward memory, salience of drug cues, and inhibiting the reward-facilitating effect of drugs. Such functions likely arise through the targeting of the endocannabinoid and serotonergic systems, although the exact mechanism is yet to be elucidated. This article seeks to review the role of the endocannabinoid system in substance use disorder and the proposed pharmacological action supporting cannabinoid drugs' therapeutic potential in addictions, with a focus on CBD. Subsequently, this article will evaluate the underlying evidence for CBD as a potential treatment for substance use disorder, across a range of substances including nicotine, alcohol, psychostimulants, opioids, and cannabis. While early research supports CBD's promise, further investigation and validation of CBD's efficacy, across preclinical and clinical trials will be necessary.

Opposing roles of CB1 and CB2 cannabinoid receptors in the stimulant and rewarding effects of cocaine

BACKGROUND AND PURPOSE

The endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG) bind to CB₁ and CB₂ cannabinoid receptors in the brain and modulate the mesolimbic dopaminergic pathway. This neurocircuitry is engaged by psychostimulant drugs, including cocaine. Although CB₁ receptor antagonism and CB₂ receptor activation are known to inhibit certain effects of cocaine, they have been investigated separately. Here, we tested the hypothesis that there is a reciprocal interaction between CB₁ receptor blockade and CB₂ receptor activation in modulating behavioural responses to cocaine.

EXPERIMENTAL APPROACH

Male Swiss mice received i.p. injections of cannabinoid-related drugs followed by cocaine, and were then tested for cocaine-induced hyperlocomotion, c-Fos expression in the nucleus accumbens and conditioned place preference. Levels of endocannabinoids after cocaine injections were also analysed.

KEY RESULTS

The CB₁ receptor antagonist, rimonabant, and the CB₂ receptor agonist, JWH133, prevented cocaine-induced hyperlocomotion. The same results were obtained by combining sub-effective doses of both compounds. The CB₂ receptor antagonist, AM630, reversed the inhibitory effects of rimonabant in cocaine-induced hyperlocomotion and c-Fos expression in the nucleus accumbens. Selective inhibitors of anandamide and 2-AG hydrolysis (URB597 and JZL184, respectively) failed to modify this response. However, JZL184 prevented cocaine-induced hyperlocomotion when given after a sub-effective dose of rimonabant. Cocaine did not change brain endocannabinoid levels. Finally, CB₂ receptor blockade reversed the inhibitory effect of rimonabant in the acquisition of cocaine-induced conditioned place preference.

CONCLUSION AND IMPLICATIONS

The present data support the hypothesis that CB₁ and CB₂ receptors work in concert with opposing functions to modulate certain addiction-related effects of cocaine.

LINKED ARTICLES

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Role of the endocannabinoid system in drug addiction

Drug addiction is a chronic relapsing disorder that produces a dramaticglobal health burden worldwide. Not effective treatment of drug addiction is currently available probably due to the difficulties to find an appropriate target to manage this complex disease raising the needs for further identification of novel therapeutic approaches. The endocannabinoid system has been found to play a crucial role in the neurobiological substrate underlying drug addiction. Endocannabinoids and cannabinoid receptors are widely expressed in the main areas of the mesocorticolimbic system that participate in the initiation and maintenance of drug consumption and in the development of compulsion and loss of behavioral control occurring during drug addiction. The identification of the important role played by CB1 cannabinoid receptors in drug addiction encouraged the possible used of an early commercialized CB1 receptor antagonist for treating drug addiction. However, the incidence of serious psychiatric adverse events leaded to the sudden withdrawal from the market of this CB1 antagonist and all the research programs developed by pharmaceutical companies to obtain new CB1 antagonists were stopped. Currently, new research strategies are under development to target the endocannabinoid system for drug addiction avoiding these side effects, which include allosteric negative modulators of CB1 receptors and compounds targeting CB2 receptors. Recent studies showing the potential role of CB2 receptors in the addictive properties of different drugs of abuse have open a promising research opportunity to develop novel possible therapeutic approaches.

Deletion of Maged1 in mice abolishes locomotor and reinforcing effects of cocaine

Melanoma antigen genes (Mage) were first described as tumour markers. However, some of Mage are also expressed in healthy cells where their functions remain poorly understood. Here, we describe an unexpected role for one of these genes, Maged1, in the control of behaviours related to drug addiction. Mice lacking Maged1 are insensitive to the behavioural effects of cocaine as assessed by locomotor sensitization, conditioned place preference (CPP) and drug self-administration. Electrophysiological experiments in brain slices and conditional knockout mice demonstrate that Maged1 is critical for cortico-accumbal neurotransmission. Further, expression of Maged1 in the prefrontal cortex (PFC) and the amygdala, but not in dopaminergic or striatal and other GABAergic neurons, is necessary for cocaine-mediated behavioural sensitization, and its expression in the PFC is also required for cocaine-induced extracellular dopamine (DA) release in the nucleus accumbens (NAc). This work identifies Maged1 as a critical molecule involved in cellular processes and behaviours related to addiction.

Effects of Cocaine Self-Administration and Its Extinction on the Rat Brain Cannabinoid CB1 and CB2 Receptors

The aim of this study was to evaluate changes in the expression of cannabinoid type 1 (CB1) and 2 (CB2) receptor proteins in several brain regions in rats undergoing cocaine self-administration and extinction training. We used a triad-yoked procedure to distinguish between the motivational and pharmacological effects of cocaine. Using immunohistochemistry, we observed a significant decrease in CB1 receptor expression in the prefrontal cortex, dorsal striatum, and the basolateral and basomedial amygdala following cocaine (0.5 mg/kg/infusion) self-administration. Increased CB1 receptor expression in the ventral tegmental area in rats with previous cocaine exposure was also found. Following cocaine abstinence after 10 days of extinction training, we detected increases in the expression of CB1 receptors in the substantia nigra in both cocaine groups and in the subregions of the amygdala for only the yoked cocaine controls, while any method of cocaine exposure resulted in a decrease in CB2 receptor expression in the prefrontal cortex (p < 0.01), nucleus accumbens (p < 0.01), and medial globus pallidus (p < 0.01). Our findings further support the idea that the eCB system and CB1 receptors are involved in cocaine-reinforced behaviors. Moreover, we detected a cocaine-evoked adaptation in CB2 receptors in the amygdala, prefrontal cortex, and globus pallidus.

Epigenetic mechanisms associated with addiction-related behavioural effects of nicotine and/or cocaine: implication of the endocannabinoid system

The addictive use of nicotine (NC) and cocaine (COC) continues to be a major public health problem, and their combined use has been reported, particularly during adolescence. In neural plasticity, commonly induced by NC and COC, as well as behavioural plasticity related to the use of these two drugs, the involvement of epigenetic mechanisms, in which the reversible regulation of gene expression occurs independently of the DNA sequence, has recently been reported. Furthermore, on the basis of intense interactions with the target neurotransmitter systems, the endocannabinoid (ECB) system has been considered pivotal for eliciting the effects of NC or COC. The combined use of marijuana with NC and/or COC has also been reported. This article presents the addiction-related behavioural effects of NC and/or COC, based on the common behavioural/neural plasticity and combined use of NC/COC, and reviews the interacting role of the ECB system. The epigenetic processes inseparable from the effects of NC and/or COC (i.e. DNA methylation, histone modifications and alterations in microRNAs) and the putative therapeutic involvement of the ECB system at the epigenetic level are also discussed.

Interactive effects of morphine and nicotine on memory function depend on the central amygdala cannabinoid CB1 receptor function in rats

The present study investigated the possible involvement of the central amygdala (CeA) cannabinoid receptors type-1 (CB1Rs) in the interactive effects of morphine and nicotine on memory formation in a passive avoidance learning task. Our results showed that systemic administration of morphine (3 and 6mg/kg, s.c.) immediately after training phase impaired memory consolidation and induced amnesia. Administration of nicotine (0.3 and 0.6mg/kg, s.c.) before testing phase significantly restored morphine-induced amnesia, suggesting a cross state-dependent learning between morphine and nicotine. The results showed that while the administration of the lower dose of nicotine (0.1mg/kg, s.c.) per se did not induce a significant effect on morphine-induced amnesia, intra-CeA injection of arachidonylcyclopropylamide (ACPA), a cannabinoid CB1 receptor agonist (3 and 4ng/rat), significantly potentiated the nicotine response. Furthermore, the blockade of the CeA cannabinoid CB1 receptors by the injection of AM251 (0.75 and 1ng/rat) reversed the potentiative effect of nicotine (0.6mg/kg, s.c.) on morphine-induced amnesia. It should be considered that bilateral injection of the same doses of ACPA or AM251 (0.5-1ng/rat) into the CeA by itself had no effect on morphine response in a passive avoidance learning task. Confirmed by the cubic interpolation planes, the dose-response data revealed a cross-state-dependent learning between morphine and nicotine which may be mediated by the CeA endocannabinoid system via CB1 receptors.

Serotonin 2B Receptors in Mesoaccumbens Dopamine Pathway Regulate Cocaine Responses

Addiction is a maladaptive pattern of behavior following repeated use of reinforcing drugs in predisposed individuals, leading to lifelong changes. Common among these changes are alterations of neurons releasing dopamine in the ventral and dorsal territories of the striatum. The serotonin 5-HT_2B receptor has been involved in various behaviors, including impulsivity, response to antidepressants, and response to psychostimulants, pointing toward putative interactions with the dopamine system. Despite these findings, it remains unknown whether 5-HT_2B receptors directly modulate dopaminergic activity and the possible mechanisms involved. To answer these questions, we investigated the contribution of 5-HT_2B receptors to cocaine-dependent behavioral responses. Male mice permanently lacking 5-HT_2B receptors, even restricted to dopamine neurons, developed heightened cocaine-induced locomotor responses. Retrograde tracing combined with single-cell mRNA amplification indicated that 5-HT_2B receptors are expressed by mesolimbic dopamine neurons. In vivo and ex vivo electrophysiological recordings showed that 5-HT_2B-receptor inactivation in dopamine neurons affects their neuronal activity and increases AMPA-mediated over NMDA-mediated excitatory synaptic currents. These changes are associated with lower ventral striatum dopamine activity and blunted cocaine self-administration. These data identify the 5-HT_2B receptor as a pharmacological intermediate and provide mechanistic insight into attenuated dopamine tone following exposure to drugs of abuse.SIGNIFICANCE STATEMENT Here we report that mice lacking 5-HT_2B receptors totally or exclusively in dopamine neurons exhibit heightened cocaine-induced locomotor responses. Despite the sensitized state of these mice, we found that associated changes include lower ventral striatum dopamine activity and lower cocaine operant self-administration. We described the selective expression of 5-HT_2B receptors in a subpopulation of dopamine neurons sending axons to the ventral striatum. Increased bursting in vivo properties of these dopamine neurons and a concomitant increase in AMPA synaptic transmission to ex vivo dopamine neurons were found in mice lacking 5-HT_2B receptors. These data support the idea that the chronic 5-HT_2B-receptor inhibition makes mice behave like animals already exposed to cocaine with higher cocaine-induced locomotion associated with changes in dopamine neuron reactivity.

Interactions between the Kynurenine and the Endocannabinoid System with Special Emphasis on Migraine

Both the kynurenine and the endocannabinoid systems are involved in several neurological disorders, such as migraine and there are increasing number of reports demonstrating that there are interactions of two systems. Although their cooperation has not yet been implicated in migraine, there are reports suggesting this possibility. Additionally, the individual role of the endocannabinoid and kynurenine system in migraine is reviewed here first, focusing on endocannabinoids, kynurenine metabolites, in particular kynurenic acid. Finally, the function of NMDA and cannabinoid receptors in the trigeminal system-which has a crucial role in the pathomechanisms of migraine-will also be discussed. The interaction of the endocannabinoid and kynurenine system has been demonstrated to be therapeutically relevant in a number of pathological conditions, such as cannabis addiction, psychosis, schizophrenia and epilepsy. Accordingly, the cross-talk of these two systems may imply potential mechanisms related to migraine, and may offer new approaches to manage the treatment of this neurological disorder.

Binge ethanol drinking during adolescence modifies cocaine responses in mice

Binge ethanol drinking is an emerging pattern of excessive consumption among adolescents and young adults. Repeated ethanol intoxication has negative consequences during critical periods of brain development. Therefore, binge ethanol intake represents a vulnerability factor that promotes subsequent manifestations of neuropsychiatric disorders. In this study, we investigated the effects of oral binge ethanol intake during adolescence on the subsequent effects of cocaine in C57BL/6 mice. Firstly, we evaluated the oral ethanol intake of two binge ethanol procedures with different ethanol concentrations (20% v/v versus 30%, v/v). The highest ethanol intake was found in mice exposed to the lower ethanol concentration (20% v/v). In a second experiment, mice exposed to binge ethanol procedure were evaluated to study the effects of cocaine on locomotor activity, behavioural sensitization, and the reinforcing effects of cocaine in the self-administration paradigm. Mice exposed to ethanol binging showed discrete detrimental effects in responses to cocaine in the different experiments evaluated. Our findings revealed that the pattern of binge ethanol consumption in adolescent mice here evaluated produced a weak facilitation of cocaine responses. The present study highlights the importance of interventions to limit the deleterious effects of binge ethanol drinking during adolescence.

Effects of repeated social defeat on adolescent mice on cocaine-induced CPP and self-administration in adulthood: integrity of the blood-brain barrier

Social stress in adulthood enhances cocaine self-administration, an effect that has been related with an increase in extracellular signal-regulated kinase and p38α mitogen-activated protein kinase phosphorylation. A detrimental effect of cocaine on blood-brain barrier (BBB) integrity has also been reported. This study evaluates the effects of repeated social defeat (RSD) during adolescence on the reinforcing and motivational effects of cocaine in adult mice and the changes induced by RSD on BBB permeability. Cocaine self-administration, conditioned place preference and quantitative analysis of claudin-5, laminin, collagen-IV and IgG immunoreactivity took place 3 weeks after RSD. Mice socially defeated during adolescence developed conditioned place preference and exhibited reinstated preference with a non-effective dose of cocaine (1 mg/kg). RSD mice needed significantly more sessions than control animals for the preference induced by 25 mg/kg of cocaine to be extinguished. However, acquisition of cocaine self-administration (0.5 mg/kg per injection) was delayed in the RSD group. Mice exposed to RSD displayed significant changes in BBB structure in adulthood, with a marked reduction in expression of the tight junction protein claudin-5 and an increase in basal laminin degradation (reflected by a decrease in laminin and collagen-IV expression) in the nucleus accumbens and hippocampus. The detrimental effect induced by cocaine (25 mg/kg) on collagen-IV expression in the hippocampus was more pronounced in RSD mice. In summary, our findings suggest that stress and cocaine can increase the long-term vulnerability of the brain to subsequent environmental insults as a consequence of a sustained disruption of the BBB.

The CB1 receptor is required for the establishment of the hyperlocomotor phenotype in developmentally-induced hypothyroidism in mice

Alterations in motor functions are well-characterized features observed in humans and experimental animals with thyroid hormone dysfunctions during development. We have previously suggested the implication of the endocannabinoid system in the hyperlocomotor phenotype observed in developmentally induced hypothyroidism in rats. In this work we have further analyzed the implication of endocannabinoids in the effect of hypothyroidism on locomotor activity. To this end, we evaluated the locomotor activity in adult mice lacking the cannabinoid receptor type 1 (CB1R^-/-) and in their wild type littermates (CB1R^+/+), whose hypothyroidism was induced in day 12 of gestation and maintained during the experimental period. Our results show that hypothyroidism induced a hyperlocomotor phenotype only in CB1R^+/+, but not in CB1R^-/- mice. In contrast with our previous results in rats, the expression of CB1R in striatum and the motor response to the cannabinoid agonist HU210 was unaltered in hypothyroid CB1R^+/+ mice suggesting that the cannabinoid system is not altered by hypothyroidism. Also, no effect of HU210 was observed in locomotion of CB1R^-/- mice. Finally, since the dopaminergic system plays a major role in the control of locomotor activity we studied its function in hypothyroid wild type and knockout animals. Our results show no alteration in the behavioral response induced by the dopamine D1 receptor agonist SKF38393. However we observed a decreased response to the dopamine D2 receptor antagonist haloperidol only in hypothyroid CB1R^+/+ mice, which might indicate potential alterations in D2R signaling in these animals. In conclusion, our data suggest that the cannabinoid system is necessary for the induction of hyperlocomotor phenotype in mice with developmentally induced hypothyroidism.

Maternal Separation Impairs Cocaine-Induced Behavioural Sensitization in Adolescent Mice

Adverse early-life conditions induce persistent disturbances that give rise to negative emotional states. Therefore, early life stress confers increased vulnerability to substance use disorders, mainly during adolescence as the brain is still developing. In this study, we investigated the consequences of maternal separation, a model of maternal neglect, on the psychotropic effects of cocaine and the neuroplasticity of the dopaminergic system. Our results show that mice exposed to maternal separation displayed attenuated behavioural sensitization, while no changes were found in the rewarding effects of cocaine in the conditioned place preference paradigm and in the reinforcing effects of cocaine in the self-administration paradigm. The evaluation of neuroplasticity in the striatal dopaminergic pathways revealed that mice exposed to maternal separation exhibited decreased protein expression levels of D2 receptors and increased levels of the transcriptional factor Nurr1. Furthermore, animals exposed to maternal separation and treated with cocaine exhibited increased DA turnover and protein expression levels of DAT and D2R, while decreased Nurr1 and Pitx3 protein expression levels were observed when compared with saline-treated mice. Taken together, our data demonstrate that maternal separation caused an impairment of cocaine-induced behavioural sensitization possibly due to a dysfunction of the dopaminergic system, a dysfunction that has been proposed as a factor of vulnerability for developing substance use disorders.

Attenuation of Cocaine-Induced Conditioned Place Preference and Motor Activity via Cannabinoid CB2 Receptor Agonism and CB1 Receptor Antagonism in Rats

BACKGROUND

Studies have shown the involvement of cannabinoid (CB) receptors in the behavioral and neurobiological effects of psychostimulants. Most of these studies have focused on the role of CB1 receptors in the psychostimulant effects of cocaine, while very few have investigated the respective role of CB2 receptors. Further studies are warranted to elucidate the extent of CB receptor involvement in the expression of cocaine-induced effects.

METHODS

The role of CB1 and CB2 receptors in the rewarding and motor properties of cocaine was assessed in conditioned place preference, conditioned motor activity, and open field activity in rats.

RESULTS

The CB1 receptor antagonist rimonabant (3 mg/kg) decreased the acquisition and the expression of conditioned place preference induced by cocaine (20 mg/kg). Rimonabant inhibited cocaine-elicited conditioned motor activity when administered during the expression of cocaine-induced conditioned place preference. Rimonabant decreased ambulatory and vertical activity induced by cocaine. The CB2 receptor agonist JWH-133 (10 mg/kg) decreased the acquisition and the expression of cocaine-induced conditioned place preference. JWH-133 inhibited cocaine-elicited conditioned motor activity when administered during the acquisition and the expression of cocaine-induced conditioned place preference. JWH-133 decreased ambulatory activity and abolished vertical activity induced by cocaine. The effects of JWH-133 on cocaine conditioned and stimulated responses were abolished when the CB2 receptor antagonist/inverse agonist AM630 (5 mg/kg) was preadministered.

CONCLUSIONS

Cannabinoid CB1 and CB2 receptors modulate cocaine-induced rewarding behavior and appear to have opposite roles in the regulation of cocaine's reinforcing and psychomotor effects.

A Highly Polymorphic Copy Number Variant in the NSF Gene is Associated with Cocaine Dependence

Cocaine dependence is a complex psychiatric disorder involving both genetic and environmental factors. Several neurotransmitter systems mediate cocaine’s effects, dependence and relapse, being the components of the neurotransmitter release machinery good candidates for the disorder. Previously, we identified a risk haplotype for cocaine dependence in the NSF gene, encoding the protein N-Ethylmaleimide-Sensitive Factor essential for synaptic vesicle turnover. Here we examined the possible contribution to cocaine dependence of a large copy number variant (CNV) that encompasses part of the NSF gene. We performed a case-control association study in a discovery sample (359 cases and 356 controls) and identified an association between cocaine dependence and the CNV (P = 0.013), that was confirmed in the replication sample (508 cases and 569 controls, P = 7.1e-03) and in a pooled analysis (P = 1.8e-04), with an over-representation of low number of copies in cases. Subsequently, we studied the functional impact of the CNV on gene expression and found that the levels of two NSF transcripts were significantly increased in peripheral blood mononuclear cells (PBMC) along with the number of copies of the CNV. These results, together with a previous study from our group, support the role of NSF in the susceptibility to cocaine dependence.

Differential Control of Cocaine Self-Administration by GABAergic and Glutamatergic CB1 Cannabinoid Receptors

The type 1 cannabinoid receptor (CB1) modulates numerous neurobehavioral processes and is therefore explored as a target for the treatment of several mental and neurological diseases. However, previous studies have investigated CB1 by targeting it globally, regardless of its two main neuronal localizations on glutamatergic and GABAergic neurons. In the context of cocaine addiction this lack of selectivity is critical since glutamatergic and GABAergic neuronal transmission is involved in different aspects of the disease. To determine whether CB1 exerts different control on cocaine seeking according to its two main neuronal localizations, we used mutant mice with deleted CB1 in cortical glutamatergic neurons (Glu-CB1) or in forebrain GABAergic neurons (GABA-CB1). In Glu-CB1, gene deletion concerns the dorsal telencephalon, including neocortex, paleocortex, archicortex, hippocampal formation and the cortical portions of the amygdala. In GABA-CB1, it concerns several cortical and non-cortical areas including the dorsal striatum, nucleus accumbens, thalamic, and hypothalamic nuclei. We tested complementary components of cocaine self-administration, separating the influence of primary and conditioned effects. Mechanisms underlying each phenotype were explored using in vivo microdialysis and ex vivo electrophysiology. We show that CB1 expression in forebrain GABAergic neurons controls mouse sensitivity to cocaine, while CB1 expression in cortical glutamatergic neurons controls associative learning processes. In accordance, in the nucleus accumbens, GABA-CB1 receptors control cocaine-induced dopamine release and Glu-CB1 receptors control AMPAR/NMDAR ratio; a marker of synaptic plasticity. Our findings demonstrate a critical distinction of the altered balance of Glu-CB1 and GABA-CB1 activity that could participate in the vulnerability to cocaine abuse and addiction. Moreover, these novel insights advance our understanding of CB1 neuropathophysiology.

Khat (Catha edulis F.) and cannabinoids: Parallel and contrasting behavioral effects in preclinical and clinical studies

After a brief outline of Catha edulis F. (khat) and the cannabinoid systems, the interactions between the pharmacological effects of khat and cannabinoids will be reviewed. Khat chewing is a widespread habit that has a deep-rooted sociocultural tradition in Africa and the Middle East. Experimental studies conducted to investigate khat's central and peripheral effects have revealed an amphetamine-like mechanism of action mediated through the dopaminergic system. The endocannabinoid system comprises the receptors, the endogenous agonists and the related biochemical machinery responsible for synthesizing these substances and terminating their actions. Endocannabinoids are synthesized "on demand" from membrane phospholipids and then rapidly cleared by cellular uptake and enzymatic degradation. Khat and cannabinoids produce a body of parallel and contrasting behavioral effects. Concurrent consumption of khat and cannabinoids may increase the risk of getting or precipitating psychosis, has rewarding and motivational effect, increases the threshold of pain perception and impairs learning and memory. On the other hand, the action of cannabis to enhance food intake is likely to reduce khat's appetite suppressant effects.

Cocaine-induced behavioral sensitization decreases the expression of endocannabinoid signaling-related proteins in the mouse hippocampus

In the reward mesocorticolimbic circuits, the glutamatergic and endocannabinoid systems are implicated in neurobiological mechanisms underlying cocaine addiction. However, the involvement of both systems in the hippocampus, a critical region to process relational information relevant for encoding drug-associated memories, in cocaine-related behaviors remains unknown. In the present work, we studied whether the hippocampal gene/protein expression of relevant glutamate signaling components, including glutamate-synthesizing enzymes and metabotropic and ionotropic receptors, and the hippocampal gene/protein expression of cannabinoid type 1 (CB1) receptor and endocannabinoid metabolic enzymes were altered following acute and/or repeated cocaine administration resulting in conditioned locomotion and locomotor sensitization. Results showed that acute cocaine administration induced an overall down-regulation of glutamate-related gene expression and, specifically, a low phosphorylation level of GluA1. In contrast, locomotor sensitization to cocaine produced an up-regulation of several glutamate receptor-related genes and, specifically, an increased protein expression of the GluN1 receptor subunit. Regarding the endocannabinoid system, acute and repeated cocaine administration were associated with an increased gene/protein expression of CB1 receptors and a decreased gene/protein expression of the endocannabinoid-synthesis enzymes N-acyl phosphatidylethanolamine D (NAPE-PLD) and diacylglycerol lipase alpha (DAGLα). These changes resulted in an overall decrease in endocannabinoid synthesis/degradation ratios, especially NAPE-PLD/fatty acid amide hydrolase and DAGLα/monoacylglycerol lipase, suggesting a reduced endocannabinoid production associated with a compensatory up-regulation of CB1 receptor. Overall, these findings suggest that repeated cocaine administration resulting in locomotor sensitization induces a down-regulation of the endocannabinoid signaling that could contribute to the specifically increased GluN1 expression observed in the hippocampus of cocaine-sensitized mice.

Effects of Cannabinoid Exposure during Adolescence on the Conditioned Rewarding Effects of WIN 55212-2 and Cocaine in Mice: Influence of the Novelty-Seeking Trait

Adolescent exposure to cannabinoids enhances the behavioural effects of cocaine, and high novelty-seeking trait predicts greater sensitivity to the conditioned place preference (CPP) induced by this drug. Our aim was to evaluate the influence of novelty-seeking on the effects of adolescent cannabinoid exposure. Adolescent male mice were classified as high or low novelty seekers (HNS and LNS) in the hole-board test. First, we evaluated the CPP induced by the cannabinoid agonist WIN 55212-2 (0.05 and 0.075 mg/kg, i.p.) in HNS and LNS mice. Then, HNS and LNS mice were pretreated i.p. with vehicle, WIN 55212-2 (0.1 mg/kg), or cannabinoid antagonist rimonabant (1 mg/kg) and were subsequently conditioned with WIN 55212-2 (0.05 mg/kg, i.p.) or cocaine (1 or 6 mg/kg, i.p.). Only HNS mice conditioned with the 0.075 mg/kg dose acquired CPP with WIN 55212-2. Adolescent exposure to this cannabinoid agonist increased the rewarding effects of 1 mg/kg of cocaine in both HNS and LNS mice, and in HNS mice it also increased the reinstating effect of a low dose of cocaine. Our results endorse a role for individual differences such as a higher propensity for sensation-seeking in the development of addiction.

The absence of VGLUT3 predisposes to cocaine abuse by increasing dopamine and glutamate signaling in the nucleus accumbens

Tonically active cholinergic interneurons (TANs) from the nucleus accumbens (NAc) are centrally involved in reward behavior. TANs express a vesicular glutamate transporter referred to as VGLUT3 and thus use both acetylcholine and glutamate as neurotransmitters. The respective roles of each transmitter in the regulation of reward and addiction are still unknown. In this study, we showed that disruption of the gene that encodes VGLUT3 (Slc17a8) markedly increased cocaine self-administration in mice. Concomitantly, the amount of dopamine (DA) release was strongly augmented in the NAc of VGLUT3(-/-) mice because of a lack of signaling by metabotropic glutamate receptors. Furthermore, dendritic spines and glutamatergic synaptic transmission on medium spiny neurons were increased in the NAc of VGLUT3(-/-) mice. Increased DA and glutamate signaling in the NAc are hallmarks of addiction. Our study shows that TANs use glutamate to reduce DA release and decrease reinforcing properties of cocaine in mice. Interestingly, we also observed an increased frequency of rare variations in SLC17A8 in a cohort of severe drug abusers compared with controls. Our findings identify VGLUT3 as an unexpected regulator of drug abuse.

Cocaine-Induced Endocannabinoid Mobilization in the Ventral Tegmental Area

Cocaine is a highly addictive drug that acts upon the brain’s reward circuitry via the inhibition of mono-amine uptake. Endogenous cannabinoids (eCB) are lipid molecules released from midbrain dopamine (DA) neurons that modulate cocaine’s effects through poorly understood mechanisms. We find that cocaine stimulates release of the eCB, 2-arach-idonoylglycerol (2-AG), in the rat ventral midbrain to suppress GABAergic inhibition of DA neurons, through activation of presynaptic cannabinoid CB1 receptors. Cocaine mobilizes 2-AG via inhibition of norepinephrine uptake and promotion of a cooperative interaction between G_q/11-coupled type-1 metabotropic glutamate and α₁-adrenergic receptors to stimulate internal calcium stores and activate phospholipase C. The disinhibition of DA neurons by cocaine-mobilized 2-AG is also functionally relevant because it augments DA release in the nucleus accumbens in vivo. Our results identify a mechanism through which the eCB system can regulate the rewarding and addictive properties of cocaine.

Sex differences in the long-lasting consequences of adolescent ethanol exposure for the rewarding effects of cocaine in mice

RATIONALE

The practice of binge drinking is very common among adolescents of both sexes. It can have long-term consequences with respect to drug consumption during adulthood, but knowledge on these effects in females is limited.

OBJECTIVES

The long-lasting effects of intermittent exposure to ethanol (EtOH) during adolescence on different cocaine-elicited behaviours, including locomotor reactivity, conditioned place preference (CPP) and intravenous self-administration, were evaluated in male and female adult mice. It was hypothesized that an EtOH binge during adolescence would increase sensitivity to the effects of a sub-threshold dose of cocaine and has a differential impact on the drug's effects in males and females.

METHODS

Adolescent OF1 mice (postnatal day (PND) 26) underwent a 2-week pre-treatment schedule consisting of 16 doses of EtOH (2.5 g/kg) or saline (twice daily administrations separated by a 4-h interval i.p.) administered on two consecutive days separated by an interval of 2 days. Three weeks later (PND > 60), we assessed locomotor activity responses induced by an acute injection of different doses of cocaine in experiment 1 and the rewarding effects of cocaine on the CPP (1 mg/kg) and intravenous self-administration (1 mg/kg/infusion) paradigms in experiment 2.

RESULTS

Pre-exposure to EtOH during adolescence altered motor reactivity to cocaine in a dose- and sex-dependent manner, increased sensitivity to cocaine in CPP and enhanced self-administration in adult mice.

CONCLUSIONS

The effects of intermittent exposure to ethanol during adolescence are evident in adulthood, during which greater sensitivity and intake of cocaine is observed and differ in each sex.

Cognitive Impairment Induced by Delta9-tetrahydrocannabinol Occurs through Heteromers between Cannabinoid CB1 and Serotonin 5-HT2A Receptors

Activation of cannabinoid CB1 receptors (CB1R) by delta9-tetrahydrocannabinol (THC) produces a variety of negative effects with major consequences in cannabis users that constitute important drawbacks for the use of cannabinoids as therapeutic agents. For this reason, there is a tremendous medical interest in harnessing the beneficial effects of THC. Behavioral studies carried out in mice lacking 5-HT2A receptors (5-HT2AR) revealed a remarkable 5-HT2AR-dependent dissociation in the beneficial antinociceptive effects of THC and its detrimental amnesic properties. We found that specific effects of THC such as memory deficits, anxiolytic-like effects, and social interaction are under the control of 5-HT2AR, but its acute hypolocomotor, hypothermic, anxiogenic, and antinociceptive effects are not. In biochemical studies, we show that CB1R and 5-HT2AR form heteromers that are expressed and functionally active in specific brain regions involved in memory impairment. Remarkably, our functional data shows that costimulation of both receptors by agonists reduces cell signaling, antagonist binding to one receptor blocks signaling of the interacting receptor, and heteromer formation leads to a switch in G-protein coupling for 5-HT2AR from Gq to Gi proteins. Synthetic peptides with the sequence of transmembrane helices 5 and 6 of CB1R, fused to a cell-penetrating peptide, were able to disrupt receptor heteromerization in vivo, leading to a selective abrogation of memory impairments caused by exposure to THC. These data reveal a novel molecular mechanism for the functional interaction between CB1R and 5-HT2AR mediating cognitive impairment. CB1R-5-HT2AR heteromers are thus good targets to dissociate the cognitive deficits induced by THC from its beneficial antinociceptive properties.