Long-term effects of juvenile nicotine exposure on abstinence-related social anxiety-like behavior and amygdalar cannabinoid receptor 1 (CB1R) mRNA expression in the novelty-seeking phenotype

Endocannabinoid-mediated synaptic plasticity and substance use disorders

Drugs impact brain reward circuits, causing dependence and addiction, in a condition currently described as substance use disorders. Mechanisms of synaptic plasticity in these circuits are crucial in the development of addictive behaviour, and endocannabinoids, particularly anandamide and 2-arachidonyl-glycerol, participate in normal neuroplasticity. Substance use disorders are known to be associated with disruption of endocannabinoid-mediated synaptic plasticity, among other phenomena. Endocannabinoids mediate neuroplasticity in the short and the long term. In the short term, we may stress «inhibitory» phenomena, such as depolarisation-induced suppression of inhibition and depolarisation-induced suppression of excitation, and such «disinhibitory» phenomena as long-lasting disinhibition of neuronal activity, particularly in the striatum, and suppression of hippocampal GABA release. Drugs of abuse can also disrupt normal endocannabinoid-mediated long-term potentiation and long-term depression. Endocannabinoids are also involved in the development of drug-induced hypofrontality and sensitisation. In summary, substance abuse causes a disruption in the synaptic plasticity of the brain circuits involved in addiction, with the alteration of normal endocannabinoid activity playing a prominent role. This facilitates abnormal changes in the brain and the development of the addictive behaviours that characterise substance use disorders.

Endocannabinoid-mediated synaptic plasticity and substance use disorders

Drugs impact brain reward circuits, causing dependence and addiction, in a condition currently described as substance use disorders. Mechanisms of synaptic plasticity in these circuits are crucial in the development of addictive behaviour, and endocannabinoids, particularly anandamide and 2-arachidonyl-glycerol, participate in normal neuroplasticity. Substance use disorders are known to be associated with disruption of endocannabinoid-mediated synaptic plasticity, among other phenomena. Endocannabinoids mediate neuroplasticity in the short and the long term. In the short term, we may stress "inhibitory" phenomena, such as depolarisation-induced suppression of inhibition and depolarisation-induced suppression of excitation, and such "disinhibitory" phenomena as long-lasting disinhibition of neuronal activity, particularly in the striatum, and suppression of hippocampal GABA release. Drugs of abuse can also disrupt normal endocannabinoid-mediated long-term potentiation and long-term depression. Endocannabinoids are also involved in the development of drug-induced hypofrontality and sensitisation. In summary, substance abuse causes a disruption in the synaptic plasticity of the brain circuits involved in addiction, with the alteration of normal endocannabinoid activity playing a prominent role. This facilitates abnormal changes in the brain and the development of the addictive behaviours that characterise substance use disorders.

Biomarkers of the Endocannabinoid System in Substance Use Disorders

Despite substance use disorders (SUD) being one of the leading causes of disability and mortality globally, available therapeutic approaches remain ineffective. The difficulty in accurately characterizing the neurobiological mechanisms involved with a purely qualitative diagnosis is an obstacle to improving the classification and treatment of SUD. In this regard, identifying central and peripheral biomarkers is essential to diagnosing the severity of drug dependence, monitoring therapeutic efficacy, predicting treatment response, and enhancing the development of safer and more effective pharmacological tools. In recent years, the crucial role that the endocannabinoid system (ECS) plays in regulating the reinforcing and motivational properties of drugs of abuse has been described. This has led to studies characterizing ECS alterations after exposure to various substances to identify biomarkers with potential diagnostic, prognostic, or therapeutic utility. This review aims to compile the primary evidence available from rodent and clinical studies on how the ECS components are modified in the context of different substance-related disorders, gathering data from genetic, molecular, functional, and neuroimaging experimental approaches. Finally, this report concludes that additional translational research is needed to further characterize the modifications of the ECS in the context of SUD, and their potential usefulness in the necessary search for biomarkers.

Rapid well-plate assays for motor and social behaviors in larval zebrafish

Behavior phenotypes are a powerful means of uncovering subtle xenobiotic chemical impacts on vertebrate nervous system development. Rodents manifest complex and informative behavior phenotypes but are generally not practical models in which to screen large numbers of chemicals. Zebrafish recapitulate much of the behavioral complexity of higher vertebrates, develop externally and are amenable to assay automation. Short duration automated assays can be leveraged to screen large numbers of chemicals or comprehensive dose-response for fewer chemicals. Here we describe a series of mostly automated assays including larval photomotor response, strobe light response, blue color avoidance, shoaling and mirror stimulus-response performed on the ZebraBox (ViewPoint Behavior Technologies) instrument platform. To explore the sensitivity and uniqueness of each assay endpoint, larval cohorts from 5 to 28 days post fertilization were acutely exposed to several chemicals broadly understood to impact different neuro-activities. We highlight the throughput advantages of using the same instrument platform for multiple assays and the ability of different assays to detect unique phenotypes among different chemicals.

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

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

Decreased Cannabinoid CB1 Receptors in Male Tobacco Smokers Examined With Positron Emission Tomography

BACKGROUND

Previous studies showed reduction of brain cannabinoid CB₁ receptors in adults with cannabis and alcohol use disorders. Preclinical data suggest that these receptors also contribute to nicotine reward and dependence. Tobacco smoking may confound clinical studies of psychiatric disorders because many patients with such disorders smoke tobacco. Whether human subjects who smoke tobacco but are otherwise healthy have altered CB₁ receptor binding in brain is unknown.

METHODS

We measured CB₁ receptors in brains of 18 healthy men who smoke tobacco (frequent chronic cigarette smokers), and 28 healthy men who do not smoke tobacco, using positron emission tomography and [¹⁸F]FMPEP-d₂, a radioligand for CB₁ receptors. We collected arterial blood samples during scanning to calculate the distribution volume (V_T), which is nearly proportional to CB₁ receptor density. Repeated-measures analysis of variance compared V_T between groups in various brain regions.

RESULTS

Brain CB₁ receptor V_T was about 20% lower in subjects who smoke tobacco than in subjects who do not. Decreased V_T was found in all brain regions, but reduction did not correlate with years of smoking, number of cigarettes smoked per day, or measures of nicotine dependence.

CONCLUSIONS

Tobacco-smoking healthy men have a widespread reduction of CB₁ receptor density in brain. Reduction of CB₁ receptors appears to be a common feature of substance use disorders. Future clinical studies on the CB₁ receptor should control for tobacco smoking.

Cognitive Dysfunction, Affective States, and Vulnerability to Nicotine Addiction: A Multifactorial Perspective

Although smoking prevalence has declined in recent years, certain subpopulations continue to smoke at disproportionately high rates and show resistance to cessation treatments. Individuals showing cognitive and affective impairments, including emotional distress and deficits in attention, memory, and inhibitory control, particularly in the context of psychiatric conditions, such as attention-deficit hyperactivity disorder, schizophrenia, and mood disorders, are at higher risk for tobacco addiction. Nicotine has been shown to improve cognitive and emotional processing in some conditions, including during tobacco abstinence. Self-medication of cognitive deficits or negative affect has been proposed to underlie high rates of tobacco smoking among people with psychiatric disorders. However, pre-existing cognitive and mood disorders may also influence the development and maintenance of nicotine dependence, by biasing nicotine-induced alterations in information processing and associative learning, decision-making, and inhibitory control. Here, we discuss the potential forms of contribution of cognitive and affective deficits to nicotine addiction-related processes, by reviewing major clinical and preclinical studies investigating either the procognitive and therapeutic action of nicotine or the putative primary role of cognitive and emotional impairments in addiction-like features.

Hippocampal Y2 receptor-mediated mossy fiber plasticity is implicated in nicotine abstinence-related social anxiety-like behavior in an outbred rat model of the novelty-seeking phenotype

Experimentally naïve outbred rats display varying rates of locomotor reactivity in response to the mild stress of a novel environment. Namely, some display high rates (HR) whereas some display low rates (LR) of locomotor reactivity. Previous reports from our laboratory show that HRs, but not LRs, develop locomotor sensitization to a low dose nicotine challenge and exhibit increased social anxiety-like behavior following chronic intermittent nicotine training. Moreover, the hippocampus, specifically hippocampal Y2 receptor (Y2R)-mediated neuropeptide Y signaling is implicated in these nicotine-induced behavioral effects observed in HRs. The present study examines the structural substrates of the expression of locomotor sensitization to a low dose nicotine challenge and associated social anxiety-like behavior following chronic intermittent nicotine exposure during adolescence in the LRHR hippocampi. Our data showed that the expression of locomotor sensitization to the low dose nicotine challenge and the increase in social anxiety-like behavior were accompanied by an increase in mossy fiber terminal field size, as well as an increase in spinophilin mRNA levels in the hippocampus in nicotine pre-trained HRs compared to saline pre-trained controls. Furthermore, a novel, selective Y2R antagonist administered systemically during 1 wk of abstinence reversed the behavioral, molecular and neuromorphological effects observed in nicotine-exposed HRs. These results suggest that nicotine-induced neuroplasticity within the hippocampus may regulate abstinence-related negative affect in HRs, and implicate hippocampal Y2R in vulnerability to the behavioral and neuroplastic effects of nicotine in the novelty-seeking phenotype.

Nicotine-induced anxiety-like behavior in a rat model of the novelty-seeking phenotype is associated with long-lasting neuropeptidergic and neuroplastic adaptations in the amygdala: effects of the cannabinoid receptor 1 antagonist AM251

A rat model of the novelty-seeking phenotype predicts vulnerability to the expression of behavioral sensitization to nicotine, where locomotor reactivity to novelty is used to screen experimentally-naïve rats for high (HR) versus low (LR) responders. The present study examines the long-term neuropeptidergic and neuroplastic adaptations associated with the expression of locomotor sensitization to a low dose nicotine challenge and social anxiety-like behavior following chronic intermittent nicotine exposure during adolescence in the LRHR phenotype. Our data show that the expression of behavioral sensitization to nicotine and abstinence-related anxiety are detected in nicotine pre-exposed HRs even across a long (3 wks) abstinence. Moreover, these behavioral effects of nicotine are accompanied by a persistent imbalance between neuropeptide Y and corticotrophin releasing factor systems, and a persistent increase in brain-derived neurotrophic factor (BDNF) and spinophilin mRNA levels in the amygdala. Furthermore, treatment with the cannabinoid receptor 1 antagonist, AM251 (5 mg/kg) during a short (1 wk) abstinence is ineffective in reversing nicotine-induced anxiety, fluctuations in BDNF and spinophilin mRNAs, and the neuropeptidergic dysregulations in the amygdala; although this treatment is effective in reversing the expression of locomotor sensitization to challenge nicotine even after a long abstinence. Interestingly, the identical AM251 treatment administered during the late phase of a long abstinence further augments anxiety and associated changes in BDNF and spinophilin mRNA in the basolateral nucleus of the amygdala in nicotine pre-exposed HRs. These findings implicate long-lasting neuropeptidergic and neuroplastic changes in the amygdala in vulnerability to the behavioral effects of nicotine in the novelty-seeking phenotype.