Cannabis use is increasingly pervasive among adolescents today, even more common than cigarette smoking. The evolving policy surrounding the legalization of cannabis reaffirms the need to understand the relationship between cannabis exposure early in life and psychiatric illnesses. cannabis contains psychoactive components, notably Δ9-tetrahydrocannabinol (THC), that interfere with the brain’s endogenous endocannabinoid system, which is critically involved in both pre- and post-natal neurodevelopment. Consequently, THC and related compounds could potentially usurp normal adolescent neurodevelopment, shifting the brain’s developmental trajectory toward a disease-vulnerable state, predisposing early cannabis users to motivational, affective, and psychotic disorders. Numerous human studies, including prospective longitudinal studies, demonstrate that early cannabis use is associated with major depressive disorder and drug addiction. A strong association between schizophrenia and cannabis use is also apparent, especially when considering genetic factors that interact with this environmental exposure. These human studies set a foundation for carefully controlled animal studies which demonstrate similar patterns following early cannabinoid exposure. Given the vulnerable nature of adolescent neurodevelopment and the persistent changes that follow early cannabis exposure, the experimental findings outlined should be carefully considered by policymakers. In order to fully address the growing issues of psychiatric illnesses and to ensure a healthy future, measures should be taken to reduce cannabis use among teens.
cannabis; drug addiction; negative affect; schizophrenia; adolescent
Endocannabinoids are small signaling lipids, with 2-arachidonoylglycerol (2-AG) implicated in modulating axonal growth and synaptic plasticity. The concept of short-range extracellular signaling by endocannabinoids is supported by the lack of trans-synaptic 2-AG signaling in mice lacking sn-1-diacylglycerol lipases (DAGLs), synthesizing 2-AG. Nevertheless, how far endocannabinoids can spread extracellularly to evoke physiological responses at CB1 cannabinoid receptors (CB1Rs) remains poorly understood. Here, we first show that cholinergic innervation of CA1 pyramidal cells of the hippocampus is sensitive to the genetic disruption of 2-AG signaling in DAGLα null mice. Next, we exploit a hybrid COS-7-cholinergic neuron co-culture system to demonstrate that heterologous DAGLα overexpression spherically excludes cholinergic growth cones from 2-AG-rich extracellular environments, and minimizes cell-cell contact in vitro. CB1R-mediated exclusion responses lasted 3 days, indicating sustained spherical 2-AG availability. Overall, these data suggest that extracellular 2-AG concentrations can be sufficient to activate CB1Rs along discrete spherical boundaries to modulate neuronal responsiveness.
Prenatal development is highly sensitive to maternal drug use due to the vulnerability for disruption of the fetal brain where the ongoing neurodevelopmental, resulting in lifelong consequences that can enhance risk for psychiatric disorders. Cannabis and cigarettes are the most commonly used illicit and licit substances, respectively, among pregnant women. While the behavioral consequences of prenatal cannabis and cigarette exposure have been well-documented in epidemiological and clinical studies, only recently have investigations into the molecular mechanisms associated with the developmental impact of early drug exposure been addressed. This article reviews the literature relevant to long-term gene expression disturbances in the human fetal brain in relation to maternal cannabis and cigarette use. To provide translational insights, we discuss animal models in which protracted molecular consequences of prenatal cannabis and cigarette exposure can be better explored and enable future evaluation of epigenetic pathways such as DNA methylation and histone modification that could potentially maintain abnormal gene regulation and related behavioral disturbances. Altogether, this information may help to address the current gaps of knowledge regarding the impact of early drug exposure that set in motion lifelong molecular disturbances that underlie vulnerability to psychiatric disorders.
Prenatal; fetal; marijuana; nicotine; epigenetics
Prenatal cannabis exposure has been linked to addiction vulnerability, but the neurobiology underlying this risk is unknown.
Striatal dopamine and opioid-related genes were studied in human fetal subjects exposed to cannabis (as well as cigarettes and alcohol). Cannabis-related gene disturbances observed in the human fetus were subsequently characterized using an animal model of prenatal delta-9-tetrahydrocannabinol (THC; 0.15 mg/kg) exposure.
Prenatal cannabis exposure decreased dopamine receptor D2 (DRD2) mRNA expression in the human ventral striatum (nucleus accumbens; NAc), a key brain reward region. No significant alterations were observed for the other genes in cannabis-exposed subjects. Maternal cigarette use was associated with reduced NAc prodynorphin mRNA expression and alcohol exposure induced broad alterations primarily in the dorsal striatum of most genes. To explore the mechanisms underlying the cannabis-associated disturbances, we exposed pregnant rats to THC and examined the epigenetic regulation of the NAc Drd2 gene in their offspring at postnatal day 2, comparable to the human fetal period studied, and in adulthood. Chromatin immunoprecipitation of the adult NAc revealed increased 2meH3K9 repressive mark and decreased 3meH3K4 and RNA polymerase II at the Drd2 gene locus in the THC-exposed offspring. Decreased Drd2 expression was accompanied by reduced D2R binding sites and increased sensitivity to opiate reward in adulthood.
These data suggest that maternal cannabis use alters developmental regulation of mesolimbic D2R in offspring through epigenetic mechanisms that regulate histone lysine methylation, and the ensuing reduction of D2R may contribute to addiction vulnerability later in life.
THC; addiction; development; enkephalin; dynorphin; D1 receptor
Glutamatergic transmission in the amygdala is hypothesized as an important mediator of stimulus-reward associations contributing to drug-seeking behavior and relapse. Insight is, however, lacking regarding the amygdala glutamatergic system in human drug abusers.
We examined glutamate receptors and scaffolding proteins associated with the post-synaptic density (PSD) of excitatory synapses in the human post-mortem amygdala. mRNA or protein levels were studied in a multi-drug (7 heroin, 8 cocaine, 7 heroin/cocaine and 7 control) or predominant heroin (29 heroin and 15 control) population of subjects.
The amygdala of drug abusers was characterized by a striking positive correlation (r > 0.8) between AMPA GluA1 and post-synaptic protein-95 (PSD-95) mRNA levels, which was not evident in controls. Structural equation multi-group analysis of protein correlations also identified the relationship between GluA1 and PSD-95 protein levels as the distinguishing feature of abusers. In line with the GluA1—PSD-95 implications of enhanced synaptic plasticity, Homer 1b/c protein expression was significantly increased in both heroin and cocaine users as was its binding partner dynamin-3, localized to the endocytic zone. Furthermore, there was a positive relationship between Homer 1b/c and dynamin-3 in drug abusers that reflected an increase in the direct physical coupling between the proteins. A noted age-related decline of Homer 1b/c—dynamin-3 interactions, as well as GluA1 levels, was blunted in abusers.
Impairment of key components of the amygdala PSD and coupling to the endocytic zone, critical for the regulation of glutamate receptor cycling, may underlie heightened synaptic plasticity in human drug abusers.
addiction; PSD-95; dynamin-3; post mortem; synaptic plasticity; glutamate
The Ca2+-binding proteins (CBPs) calbindin D28k, calretinin and parvalbumin are phenotypic markers of functionally diverse subclasses of neurons in the adult brain. The developmental dynamics of CBP expression are precisely timed: calbindin and calretinin are present in prospective cortical interneurons from mid-gestation, while parvalbumin only becomes expressed during the early postnatal period in rodents. Secretagogin (scgn) is a CBP cloned from pancreatic β and neuroendocrine cells. We hypothesized that scgn may be expressed by particular neuronal contingents during prenatal development of the mammalian telencephalon. We find that scgn is expressed in neurons transiting in the subpallial differentiation zone by embryonic day (E) 11 in mouse. From E12, scgn+ cells commute towards the extended amygdala and colonize the bed nucleus of stria terminalis, interstitial nucleus of the posterior limb of the anterior commissure, dorsal substantia innominata (SI), and the central and medial amygdaloid nuclei. Scgn+ neurons can acquire a cholinergic phenotype in the SI or differentiate into GABA cells in the central amygdala. We also uncover phylogenetic differences in scgn expression since this CBP defines not only neurons destined to the extended amygdala but also cholinergic projection cells and cortical pyramidal cells in the fetal non-human primate and human brains, respectively. Overall, our findings emphasize the developmentally shared origins of neurons populating the extended amygdala, and suggest that secretagogin can be relevant to the generation of functional modalities in specific neuronal circuitries.
Ca2+-binding protein; extended amygdala; forebrain; mouse; primate
Cocaine-induced alterations in gene expression cause changes in neuronal morphology and behavior that may underlie cocaine addiction. We identified an essential role for histone 3 lysine 9 (H3K9) dimethylation and the lysine dimethyltransferase G9a in cocaine-induced structural and behavioral plasticity. Repeated cocaine administration reduced global levels of H3K9 dimethylation in the nucleus accumbens. This reduction in histone methylation was mediated through the repression of G9a in this brain region, which was regulated by the cocaine-induced transcription factor ΔFosB. Using conditional mutagenesis and viral-mediated gene transfer, we found that G9a downregulation increased dendritic spine plasticity of nucleus accumbens neurons and enhanced preference for cocaine, thereby establishing a crucial role for histone methylation in the long-term actions of cocaine.
There remains debate regarding the impact of cannabis on neuropsychiatric disorders. Here, we examined the effects of cannabidiol (CBD), a nonpsychoactive constituent of cannabis on heroin self-administration and drug-seeking behavior using an experimental rat model. CBD (5-20 mg/kg) did not alter stable intake of heroin self-administration, extinction behavior, or drug seeking induced by a heroin prime injection. Instead, it specifically attenuated heroin-seeking behavior reinstated by exposure to a conditioned stimulus cue. CBD had a protracted effect with significance evident after 24h and even 2 weeks after administration. The behavioral effects were paralleled by neurobiological alterations in the glutamatergic and endocannabinoid systems. Discrete disturbances of AMPA GluR1 and cannabinoid type-1 receptor expression observed in the nucleus accumbens associated with stimulus cue-induced heroin seeking were normalized by CBD treatment. The findings highlight the unique contributions of distinct cannabis constituents to addiction vulnerability and suggest that CBD may be a potential treatment for heroin craving and relapse.
drug addiction; relapse; drug self-administration; endocannabinoid; glutamate receptor; rat
Serotonin 2C receptor (5-HT2CR) exerts a major inhibitory influence on dopamine (DA) neurotransmission within the mesocorticolimbic DA pathway that is implicated in drug reward and goal-directed behaviors. 5-HT2CR pre-mRNA undergoes adenosine-to-inosine editing generating numerous receptor isoforms in brain. Because editing influences 5-HT2CR efficacy, individual differences in editing might influence dopaminergic function and, thereby, contribute to inter-individual vulnerability to drug addiction.
Liability to drug-related behaviors in rats can be predicted by the level of motor activity in response to a novel environment. Rats with a high locomotor response (high responders; HRs) exhibit enhanced acquisition and maintenance of drug self-administration compared to rats with a low response (low responders; LRs). Here we examined 5-HT2CR mRNA editing and expression in HR and LR phenotypes in order to investigate the relationship between 5-HT2CR function and behavioral traits relevant to drug addiction vulnerability. Three regions of the mesocorticolimbic circuitry (ventral tegmental area (VTA), nucleus accumbens (NuAc) shell, and medial prefrontal cortex (PFC)) were examined.
5-HT2CR mRNA expression and editing was significantly higher in NuAc shell compared to both PFC and VTA, implying significant differences in function (including constitutive activity) among 5-HT2CR neuronal populations within the circuitry. The regional differences in editing could, at least in part, arise from the variations in expression levels of the editing enzyme, ADAR2, and/or from the variations in the ADAR2/ADAR1 ratio observed in the study. No differences in the 5-HT2CR expression were detected between the behavioral phenotypes. However, editing was higher in the PFC of HRs vs. LRs, implicating this region in the pathophysiology of drug abuse liability.
serotonin 2C receptor; mRNA editing; drug addiction; rat; high and low responders; prefrontal cortex
Alternative splicing of the mu opioid receptor genes to create multiple mu receptor subtypes has been demonstrated in animals and humans. Previously, we identified a number of C-terminal variants in mice, rats and human, followed by several N-terminal variants associated with a new upstream exon in mice (exon 11). Behavioral studies in exon 11 knockout mice suggest an important role for the exon 11 variants in the analgesic actions of heroin and morphine-6β-glucuronide, but not morphine or methadone. We now have identified a homologous human exon 11 and three similar human exon 11-associated variants, suggesting conservation of exon 11 and its associated variants across species. hMOR-1i has an additional 93 amino acids at the tip of the N-terminus but is otherwise identical to hMOR-1. When expressed in Chinese hamster ovary cells, the additional 93 amino acids in hMOR-1i had little effect on opioid binding, but significantly altered agonist-induced G-protein activation. hMOR-1G1 and hMOR-1G2 predicted six transmembrane domain variants, similar to those seen in mice. The regional expression of these exon 11-associated variants, as determined by RT-PCR, varied markedly, implying region-specific alternative splicing. The presence of exon 11-associated variants in humans raises questions regarding their potential role in heroin and morphine-6β-glucuronide actions in people as they do in mice.
G protein; mu opioid receptor (MOR); morphine; mu opioid receptor-1; opioid receptor; splicing
Alcohol dependence and associated cognitive impairments apparently result from neuroadaptations to chronic alcohol consumption involving changes in expression of multiple genes. Here we investigated whether transcription factors of Nuclear Factor-kappaB (NF-κB) family, controlling neuronal plasticity and neurodegeneration, are involved in these adaptations in human chronic alcoholics.
Methods and Findings
Analysis of DNA-binding of NF-κB (p65/p50 heterodimer) and the p50 homodimer as well as NF-κB proteins and mRNAs was performed in postmortem human brain samples from 15 chronic alcoholics and 15 control subjects. The prefrontal cortex involved in alcohol dependence and cognition was analyzed and the motor cortex was studied for comparison. The p50 homodimer was identified as dominant κB binding factor in analyzed tissues. NF-κB and p50 homodimer DNA-binding was downregulated, levels of p65 (RELA) mRNA were attenuated, and the stoichiometry of p65/p50 proteins and respective mRNAs was altered in the prefrontal cortex of alcoholics. Comparison of a number of p50 homodimer/NF-κB target DNA sites, κB elements in 479 genes, down- or upregulated in alcoholics demonstrated that genes with κB elements were generally upregulated in alcoholics. No significant differences between alcoholics and controls were observed in the motor cortex.
We suggest that cycles of alcohol intoxication/withdrawal, which may initially activate NF-κB, when repeated over years downregulate RELA expression and NF-κB and p50 homodimer DNA-binding. Downregulation of the dominant p50 homodimer, a potent inhibitor of gene transcription apparently resulted in derepression of κB regulated genes. Alterations in expression of p50 homodimer/NF-κB regulated genes may contribute to neuroplastic adaptation underlying alcoholism.
Many young people experiment with cannabis, yet only a subgroup progress to dependence suggesting individual differences that could relate to factors such as genetics and behavioral traits. Dopamine receptor D2 (DRD2) and proenkephalin (PENK) genes have been implicated in animal studies with cannabis exposure. Whether polymorphisms of these genes are associated with cannabis dependence and related behavioral traits is unknown.
Healthy young adults (18–27 years) with cannabis dependence and without a dependence diagnosis were studied (N = 50/group) in relation to a priori-determined single nucleotide polymorphisms (SNPs) of the DRD2 and PENK genes. Negative affect, Impulsive Risk Taking and Neuroticism-Anxiety temperamental traits, positive and negative reward-learning performance and stop-signal reaction times were examined. The findings replicated the known association between the rs6277 DRD2 SNP and decisions associated with negative reinforcement outcomes. Moreover, PENK variants (rs2576573 and rs2609997) significantly related to Neuroticism and cannabis dependence. Cigarette smoking is common in cannabis users, but it was not associated to PENK SNPs as also validated in another cohort (N = 247 smokers, N = 312 non-smokers). Neuroticism mediated (15.3%–19.5%) the genetic risk to cannabis dependence and interacted with risk SNPs, resulting in a 9-fold increase risk for cannabis dependence. Molecular characterization of the postmortem human brain in a different population revealed an association between PENK SNPs and PENK mRNA expression in the central amygdala nucleus emphasizing the functional relevance of the SNPs in a brain region strongly linked to negative affect.
Overall, the findings suggest an important role for Neuroticism as an endophenotype linking PENK polymorphisms to cannabis-dependence vulnerability synergistically amplifying the apparent genetic risk.
Despite abundant expression of DNA methyltransferases (Dnmt’s) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We find that Dnmt3a expression is regulated in mouse nucleus accumbens (NAc) by chronic cocaine and chronic social defeat stress. Moreover, NAc specific manipulations that block DNA methylation potentiate cocaine reward and exert antidepressant-like effects, whereas NAc specific Dnmt3a overexpression attenuates cocaine reward and is pro-depressant. On a cellular level, we show that chronic cocaine selectively increases thin dendritic spines on NAc neurons and that DNA methylation is both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli.