Signals from the Fourth Dimension Regulate Drug Relapse

Transcriptional Dysregulation of Cholesterol Synthesis Underlies Hyposensitivity to GABA in the Ventral Tegmental Area During Acute Alcohol Withdrawal

BACKGROUND

The ventral tegmental area (VTA) is a dopaminergic brain area that is critical in the development and maintenance of addiction. During withdrawal from chronic ethanol exposure, the response of VTA neurons to GABA (gamma-aminobutyric acid) is reduced through an epigenetically regulated mechanism. In the current study, a whole-genome transcriptomic approach was used to investigate the underlying molecular mechanism of GABA hyposensitivity in the VTA during withdrawal after chronic ethanol exposure.

METHODS

We performed RNA sequencing of the VTA of Sprague Dawley male rats withdrawn for 24 hours from a chronic ethanol diet as well as sequencing of the VTA of control rats fed the Lieber-DeCarli diet. RNA sequencing data were analyzed using weighted gene coexpression network analysis to identify modules that contained coexpressed genes. Validation was performed with quantitative polymerase chain reaction, gas chromatography-mass spectrometry, and electrophysiological assays.

RESULTS

Pathway and network analysis of weighted gene coexpression network analysis module 1 revealed a significant downregulation of genes associated with the cholesterol synthesis pathway. Consistent with this association, VTA cholesterol levels were significantly decreased during withdrawal. Chromatin immunoprecipitation indicated a decrease in levels of acetylated H3K27 at the transcriptional control regions of these genes. Electrophysiological studies in VTA slices demonstrated that GABA hyposensitivity during withdrawal was normalized by addition of exogenous cholesterol. In addition, inhibition of cholesterol synthesis produced GABA hyposensitivity, which was reversed by adding exogenous cholesterol to VTA slices.

CONCLUSIONS

These results suggest that decreased expression of cholesterol synthesis genes may regulate GABA hyposensitivity of VTA neurons during alcohol withdrawal. Increasing cholesterol levels in the brain may be a novel avenue for therapeutic intervention to reverse detrimental effects of chronic alcohol exposure.

Cross-species epigenetic regulation of nucleus accumbens KCNN3 transcripts by excessive ethanol drinking

The underlying genetic and epigenetic mechanisms driving functional adaptations in neuronal excitability and excessive alcohol intake are poorly understood. Small-conductance Ca2+-activated K+ (KCa2 or SK) channels encoded by the KCNN family of genes have emerged from preclinical studies as a key contributor to alcohol-induced functional neuroadaptations in alcohol-drinking monkeys and alcohol-dependent mice. Here, this cross-species analysis focused on KCNN3 DNA methylation, gene expression, and single nucleotide polymorphisms, including alternative promoters in KCNN3, that could influence surface trafficking and function of KCa2 channels. Bisulfite sequencing analysis of the nucleus accumbens tissue from alcohol-drinking monkeys and alcohol-dependent mice revealed a differentially methylated region in exon 1A of KCNN3 that overlaps with a predicted promoter sequence. The hypermethylation of KCNN3 in the accumbens paralleled an increase in the expression of alternative transcripts that encode apamin-insensitive and dominant-negative KCa2 channel isoforms. A polymorphic repeat in macaque KCNN3 encoded by exon 1 did not correlate with alcohol drinking. At the protein level, KCa2.3 channel expression in the accumbens was significantly reduced in very heavy-drinking monkeys. Together, our cross-species findings on epigenetic dysregulation of KCNN3 represent a complex mechanism that utilizes alternative promoters to potentially impact the firing of accumbens neurons. Thus, these results provide support for hypermethylation of KCNN3 as a possible key molecular mechanism underlying harmful alcohol intake and alcohol use disorder.

Adaptor protein complex 2 in the orbitofrontal cortex predicts alcohol use disorder

Alcohol use disorder (AUD) is a life-threatening disease characterized by compulsive drinking, cognitive deficits, and social impairment that continue despite negative consequences. The inability of individuals with AUD to regulate drinking may involve functional deficits in cortical areas that normally balance actions that have aspects of both reward and risk. Among these, the orbitofrontal cortex (OFC) is critically involved in goal-directed behavior and is thought to maintain a representation of reward value that guides decision making. In the present study, we analyzed post-mortem OFC brain samples collected from age- and sex-matched control subjects and those with AUD using proteomics, bioinformatics, machine learning, and reverse genetics approaches. Of the 4,500+ total unique proteins identified in the proteomics screen, there were 47 proteins that differed significantly by sex that were enriched in processes regulating extracellular matrix and axonal structure. Gene ontology enrichment analysis revealed that proteins differentially expressed in AUD cases were involved in synaptic and mitochondrial function, as well as transmembrane transporter activity. Alcohol-sensitive OFC proteins also mapped to abnormal social behaviors and social interactions. Machine learning analysis of the post-mortem OFC proteome revealed dysregulation of presynaptic (e.g., AP2A1) and mitochondrial proteins that predicted the occurrence and severity of AUD. Using a reverse genetics approach to validate a target protein, we found that prefrontal Ap2a1 expression significantly correlated with voluntary alcohol drinking in male and female genetically diverse mouse strains. Moreover, recombinant inbred strains that inherited the C57BL/6J allele at the Ap2a1 interval consumed higher amounts of alcohol than those that inherited the DBA/2J allele. Together, these findings highlight the impact of excessive alcohol consumption on the human OFC proteome and identify important cross-species cortical mechanisms and proteins that control drinking in individuals with AUD.

Cross-species epigenetic regulation of nucleus accumbens KCNN3 transcripts by excessive ethanol drinking

The underlying genetic and epigenetic mechanisms driving functional adaptations in neuronal excitability and excessive alcohol intake are poorly understood. Small-conductance Ca2+-activated K+ (KCa2 or SK) channels encoded by the KCNN family of genes have emerged from preclinical studies as a key contributor to alcohol-induced functional neuroadaptations in alcohol-drinking monkeys and alcohol dependent mice. Here, this cross-species analysis focused on KCNN3 DNA methylation, gene expression, and single nucleotide polymorphisms including alternative promoters in KCNN3 that could influence surface trafficking and function of KCa2 channels. Bisulfite sequencing analysis of the nucleus accumbens tissue from alcohol-drinking monkeys and alcohol dependent mice revealed a differentially methylated region in exon 1A of KCNN3 that overlaps with a predicted promoter sequence. The hypermethylation of KCNN3 in the accumbens paralleled an increase in expression of alternative transcripts that encode apamin-insensitive and dominant-negative KCa2 channel isoforms. A polymorphic repeat in macaque KCNN3 encoded by exon 1 did not correlate with alcohol drinking. At the protein level, KCa2.3 channel expression in the accumbens was significantly reduced in very heavy drinking monkeys. Together, our cross-species findings on epigenetic dysregulation of KCNN3 represent a complex mechanism that utilizes alternative promoters to impact firing of accumbens neurons. Thus, these results provide support for hypermethylation of KCNN3 as a possible key molecular mechanism underlying harmful alcohol intake and alcohol use disorder.

Intrusive thinking: Circuit and synaptic mechanisms of a transdiagnostic psychiatric symptom

Spontaneous thought is an adaptive cognitive process that can produce novel and insightful thought sequences useful in guiding future behavior. In many psychiatric disorders, spontaneous thinking becomes intrusive and uncontrolled, and can trigger symptoms such as craving, repetitive negative thinking and trauma-related memories. We link studies using clinical imaging and rodent modeling towards understanding the neurocircuitry and neuroplasticity of intrusive thinking. We propose a framework in which drugs or stress change the homeostatic set point of brain reward circuitry, which then impacts subsequent plasticity induced by drug/stress conditioned cues (metaplastic allostasis). We further argue for the importance of examining not only the canonical pre- and postsynapse, but also the adjacent astroglial protrusions and extracellular matrix that together form the tetrapartite synapse and that plasticity throughout the tetrapartite synapse is necessary for cue-induced drug or stress behaviors. This analysis reveals that drug use or trauma cause long-lasting allostatic brain plasticity that sets the stage for subsequent drug/trauma-associated cues to induce transient plasticity that can lead to intrusive thinking.

A mechanistic overview of approaches for the treatment of psychostimulant dependence

Psychostimulant use disorder is a major health issue around the world with enormous individual, family-related and societal consequences, yet there are no effective pharmacological treatments available. In this review, a target-based overview of pharmacological treatments toward psychostimulant addiction will be presented. We will go through therapeutic approaches targeting different aspects of psychostimulant addiction with focus on three major areas; 1) drugs targeting signalling, and metabolism of the dopamine system, 2) drugs targeting either AMPA receptors or metabotropic glutamate receptors of the glutamate system and 3) drugs targeting the severe side-effects of quitting long-term psychostimulant use. For each of these major modes of intervention, findings from pre-clinical studies in rodents to clinical trials in humans will be listed, and future perspectives of the different treatment strategies as well as their potential side-effects will be discussed. Pharmaceuticals modulating the dopamine system, such as antipsychotics, DAT-inhibitors, and disulfiram, have shown some promising results. Cognitive enhancers have been found to increase aspects of behavioural control, and drugs targeting the glutamate system such as modulators of metabotropic glutamate receptors and AMPA receptors have provided interesting changes in relapse behaviour. Furthermore, CRF-antagonists directed toward alleviating the symptoms of the withdrawal stage have been examined with interesting resulting changes in behaviour. There are promising results investigating therapeutics for psychostimulant addiction, but further preclinical work and additional human studies with a more stratified patient selection are needed to prove sufficient evidence of efficacy and tolerability.

Cannabinoid use is enhanced by stress and changes conditioned stress responses

Individuals diagnosed with post-traumatic stress disorder (PTSD) are often comorbid for substance use disorders. Cannabis is widely used by PSTD patients, and the literature is mixed on whether cannabis use ameliorates or exacerbates patient responses to stress-associated conditioned stimuli (stress-CS). We determined if cannabis use affects responsivity to stress-CS in rats receiving 2 h stress in the presence of an odor stress-CS. Three weeks after acute stress, rats self-administered cannabinoids (delta9-tetrahydrocannabinol + cannabidiol; THC + CBD) for 15 days, and the stressed males consumed more THC + CBD than sham males. We then used the stress-CS or a novel odor (stress-NS) to reinstate THC + CBD seeking. Surprisingly, the stress-NS reinstated THC + CBD seeking, an effect blocked by N-acetylcysteine. Moreover, the stress-CS inhibited THC + CBD-CS induced reinstatement. To determine if the unexpected effects of stress-NS and -CS resulted from THC + CBD altering conditioned stress, the effect of THC + CBD use on stress-NS/CS-induced coping behaviors and spine morphology was quantified. In THC + CBD-treated rats, stress-NS increased active coping (burying). Conversely, stress-CS reduced active coping and increased passive coping (immobility) and other behavioral parameters associated with stress responses, including self-grooming and defecation. Transient spine head expansion in nucleus accumbens core is necessary for cue-induced drug seeking, and THC + CBD self-administration prevented the increase in head diameter by stress-CS in control rats. These data show THC + CBD self-administration altered the salience of environmental cues, causing neutral cues to promote active behavior (drug seeking and burying) and stress-CS to switch from active to passive behavior (inhibiting drug seeking and immobilization). We hypothesize that cannabis may exacerbate conditioned stress responses.

Extrasynaptic therapeutic targets in substance use and stress disorders

Treatments for substance use and stress disorders are based on ameliorating behavioral symptoms, not on reversing the synaptic pathology that has the potential to cure disorders. This failing arises in part from a research focus on how pre- and postsynaptic physiology is changed even though key neuropathology exists in the perisynaptic neuropil that homeostatically regulates synaptic transmission. We explore recent findings from the substance use and stress disorder literature pointing to a key role for perisynaptic astroglia and signaling in the extracellular matrix (ECM) in regulating synaptic pathology. We conclude that drugs and stress initiate long-lasting changes in brain synapses via enduring neuroadaptations in astroglia and the ECM, and that modulating extrasynaptic regulators may be therapeutically useful.

Chronic intermittent ethanol during adolescence and adulthood alters dendritic spines in the primary motor and visual cortex in rats

Prolonged adolescent binge drinking can disrupt sleep quality and increase the likelihood of alcohol-induced sleep disruptions in young adulthood in rodents and in humans. Striking changes in spine density and morphology have been seen in many cortical and subcortical regions after adolescent alcohol exposure in rats. However, there is little known about the impact of alcohol exposure on dendritic spines in the same motor and sensory cortices that EEG sleep is typically recorded from in rats. The aim of this study is to investigate whether an established model of chronic intermittent ethanol vapor in rats that has been demonstrated to disrupt sleep during adolescence or adulthood, also significantly alters cortical dendritic spine density and morphology. To this end, adolescent and adult Wistar rats were exposed to 5 weeks of ethanol vapor or control air exposure. After a 13-day withdrawal, primary motor cortex (M1) and primary/secondary visual cortex (V1/V2) layer V dendrites were analyzed for differences in spine density and morphology. Spines were classified into four categories (stubby, long, filopodia, and mushroom) based on the spine length and the width of the spine head and neck. The main results indicate an age-specific effect of adolescent intermittent ethanol exposure decreasing spine density in the M1 cortex compared to age-matched controls. Reductions in the density of M1 long-shaped spine subclassifications were seen in adolescent ethanol-exposed rats, but not adult-exposed rats, compared to their air-controls. Irrespective of age, there was an overall reduction produced by ethanol exposure on the density of filopodia and the length of long-shaped spines in V1/V2 cortex as compared to their air-exposed controls. Together, these data add to growing evidence that some cortical circuits are vulnerable to the effects of alcohol during adolescence and begin to elucidate potential mechanisms that may influence brain plasticity following early alcohol use.

Impaired glutamate homeostasis in the nucleus accumbens in human cocaine addiction

Cocaine addiction is characterized by overwhelming craving for the substance, which drives its escalating use despite adverse consequences. Animal models suggest a disrupted glutamate homeostasis in the nucleus accumbens to underlie addiction-like behavior. After chronic administration of cocaine, rodents show decreased levels of accumbal glutamate, whereas drug-seeking reinstatement is associated with enhanced glutamatergic transmission. However, due to technical obstacles, the role of disturbed glutamate homeostasis for cocaine addiction in humans remains only partially understood, and accordingly, no approved pharmacotherapy exists. Here, we applied a tailored proton magnetic resonance spectroscopy protocol that allows glutamate quantification within the human nucleus accumbens. We found significantly reduced basal glutamate concentrations in the nucleus accumbens in cocaine-addicted (N = 26) compared with healthy individuals (N = 30), and increased glutamate levels during cue-induced craving in cocaine-addicted individuals compared with baseline. These glutamatergic alterations, however, could not be significantly modulated by a short-term challenge of N-acetylcysteine (2400 mg/day on 2 days). Taken together, our findings reveal a disturbed accumbal glutamate homeostasis as a key neurometabolic feature of cocaine addiction also in humans. Therefore, we suggest the glutamatergic system as a promising target for the development of novel pharmacotherapies, and in addition, as a potential biomarker for a personalized medicine approach in addiction.

Drug addiction: from bench to bedside

Drug addiction is responsible for millions of deaths per year around the world. Still, its management as a chronic disease is shadowed by misconceptions from the general public. Indeed, drug consumers are often labelled as "weak", "immoral" or "depraved". Consequently, drug addiction is often perceived as an individual problem and not societal. In technical terms, drug addiction is defined as a chronic, relapsing disease resulting from sustained effects of drugs on the brain. Through a better characterisation of the cerebral circuits involved, and the long-term modifications of the brain induced by addictive drugs administrations, first, we might be able to change the way the general public see the patient who is suffering from drug addiction, and second, we might be able to find new treatments to normalise the altered brain homeostasis. In this review, we synthetise the contribution of fundamental research to the understanding drug addiction and its contribution to potential novel therapeutics. Mostly based on drug-induced modifications of synaptic plasticity and epigenetic mechanisms (and their behavioural correlates) and after demonstration of their reversibility, we tried to highlight promising therapeutics. We also underline the specific temporal dynamics and psychosocial aspects of this complex psychiatric disease adding parameters to be considered in clinical trials and paving the way to test new therapeutic venues.

Differential regulation of alcohol consumption and reward by the transcriptional cofactor LMO4

Repeated alcohol exposure leads to changes in gene expression that are thought to underlie the transition from moderate to excessive drinking. However, the mechanisms by which these changes are integrated into a maladaptive response that leads to alcohol dependence are not well understood. One mechanism could involve the recruitment of transcriptional co-regulators that bind and modulate the activity of transcription factors. Our results indicate that the transcriptional regulator LMO4 is one such candidate regulator. Lmo4-deficient mice (Lmo4gt/+) consumed significantly more and showed enhanced preference for alcohol in a 24 h intermittent access drinking procedure. shRNA-mediated knockdown of Lmo4 in the nucleus accumbens enhanced alcohol consumption, whereas knockdown in the basolateral amygdala (BLA) decreased alcohol consumption and reduced conditioned place preference for alcohol. To ascertain the molecular mechanisms that underlie these contrasting phenotypes, we carried out unbiased transcriptome profiling of these two brain regions in wild type and Lmo4gt/+ mice. Our results revealed that the transcriptional targets of LMO4 are vastly different between the two brain regions, which may explain the divergent phenotypes observed upon Lmo4 knockdown. Bioinformatic analyses revealed that Oprk1 and genes related to the extracellular matrix (ECM) are important transcriptional targets of LMO4 in the BLA. Chromatin immunoprecipitation revealed that LMO4 bound Oprk1 promoter elements. Consistent with these results, disruption of the ECM or infusion of norbinaltorphimine, a selective kappa opioid receptor antagonist, in the BLA reduced alcohol consumption. Hence our results indicate that an LMO4-regulated transcriptional network regulates alcohol consumption in the BLA.

Heroin Seeking and Extinction From Seeking Activate Matrix Metalloproteinases at Synapses on Distinct Subpopulations of Accumbens Cells

BACKGROUND

Seeking addictive drugs is regulated by synaptic plasticity in the nucleus accumbens core and involves distinct plasticity in D1 and D2 receptor-expressing medium spiny neurons (D1/2-MSNs). However, it is unknown how differential plasticity between the two cell types is coordinated. Synaptic plasticity and seeking behavior induced by drug-paired cues depends not only on plasticity in the canonical pre- and postsynapse, but also on cue-induced changes in astrocytes and the extracellular matrix adjacent to the synapse. Drug cue-induced signaling in the extracellular matrix is regulated by catalytic activity of matrix metalloproteinases MMP-2,9. We hypothesized that the cell type-specific synaptic plasticity is associated with parallel cell-specific activity of MMP-2 and MMP-9.

METHODS

Transgenic rats were trained on a heroin self-administration protocol in which a light/tone cue was paired with heroin delivery, followed by 2 weeks of drug withdrawal, and then reinstated to heroin-conditioned cues. Confocal microscopy was used to make morphological measurements in membrane reporter-transduced D1- and D2-MSNs and astrocytes, and MMP-2,9 gelatinase activity adjacent to cell surfaces was quantified using in vivo zymography.

RESULTS

Presenting heroin-paired cues transiently increased MMP-9 activity around D1-MSN dendritic spines and synapse-proximal astroglial processes. Conversely, extinction training induced long-lasting increases in MMP-2 activity adjacent to D2-MSN synapses. Moreover, heroin-paired cues increased tissue inhibitor of metalloproteinases TIMP-1,2, which caused transient inhibition of MMP-2 activity around D2-MSNs during cue-induced heroin seeking.

CONCLUSIONS

The differential regulation of heroin seeking and extinguished seeking by different MMP subtypes on distinct cell populations poses MMP-2,9 activity as an important mediator and contributor in heroin-induced cell-specific synaptic plasticity.

Effects of ceftriaxone on ethanol drinking and GLT-1 expression in ethanol dependence and relapse drinking

Repeated cycles of chronic intermittent ethanol (CIE) exposure increase voluntary consumption of alcohol (ethanol) in mice. Previous reports from our laboratory show that CIE increases extracellular glutamate in the nucleus accumbens (NAc) and that manipulating accumbal glutamate concentrations will alter ethanol drinking, indicating that glutamate homeostasis plays a crucial role in ethanol drinking in this model. A number of studies have shown that ceftriaxone increases GLT-1 expression, the major glutamate transporter, and that treatment with this antibiotic reduces ethanol drinking. The present studies examined the effects of ceftriaxone on ethanol drinking and GLT-1 in a mouse model of ethanol dependence and relapse drinking. The results show that ceftriaxone did not influence drinking at any dose in either ethanol-dependent or non-dependent mice. Further, ceftriaxone did not increase GLT-1 expression in the accumbens core or shell, with the exception of the ethanol-dependent mice receiving the highest dose of ceftriaxone. Interestingly, ethanol-dependent mice treated with only vehicle displayed reduced expression of GLT-1 in the accumbens shell and of the presynaptic mGlu2 receptor in the accumbens core. The reduced expression of the major glutamate transporter (GLT-1), as well as a receptor that regulates glutamate release (mGlu2), may help explain, at least in part, increased glutamatergic transmission in this model of ethanol dependence and relapse drinking.

Oxytocin and Addiction: Potential Glutamatergic Mechanisms

Recently, oxytocin (OXT) has been investigated for its potential therapeutic role in addiction. OXT has been found to diminish various drug-seeking and drug-induced behaviors. Although its behavioral effects are well-established, there is not much consensus on how this neuropeptide exerts its effects. Previous research has given thought to how dopamine (DA) may be involved in oxytocinergic mechanisms, but there has not been as strong of a focus on the role that glutamate (Glu) has. The glutamatergic system is critical for the processing of rewards and the disruption of glutamatergic projections produces the behaviors seen in drug addicts. We introduce the idea that OXT has direct effects on Glu transmission within the reward processing pathway. Thus, OXT may reduce addictive behaviors by restoring abnormal drug-induced changes in the glutamatergic system and in its interactions with other neurotransmitters. This review offers insight into the mechanisms through which a potentially viable therapeutic target, OXT, could be used to reduce addiction-related behaviors.

Interaction of chronic intermittent ethanol and repeated stress on structural and functional plasticity in the mouse medial prefrontal cortex

Stress is a risk factor that plays a considerable role in the development and maintenance of alcohol (ethanol) abuse and relapse. Preclinical studies examining ethanol-stress interactions have demonstrated elevated ethanol drinking, cognitive deficits, and negative affective behaviors in mice. However, the neural adaptations in prefrontal cortical regions that drive these aberrant behaviors produced by ethanol-stress interactions are unknown. In this study, male C57BL/6J mice were exposed to chronic intermittent ethanol (CIE) and repeated forced swim stress (FSS). After two cycles of CIE x FSS, brain slices containing the prelimbic (PrL) and infralimbic (IfL) cortex were prepared for analysis of adaptations in dendritic spines and synaptic plasticity. In the PrL cortex, total spine density was increased in mice exposed to CIE. Immediately following induction of long-term potentiation (LTP), the fEPSP slope was increased in the PrL of CIE x FSS treated mice, indicative of a presynaptic adaptation on post-tetanic potentiation (PTP). In the IfL cortex, CIE exposure regardless of FSS experience resulted in an increase in spine density. FSS alone or when combined with CIE exposure increased PTP following LTP induction. Repeated FSS episodes increased IfL cortical paired-pulse facilitation, a second measure of presynaptic plasticity. In summary, CIE exposure resulted in structural adaptations while repeated stress exposure drove metaplastic changes in presynaptic function, demonstrating distinct morphological and functional changes in PrL and IfL cortical neurons. Thus, the structural and functional adaptations may be one mechanism underlying the development of excessive drinking and cognitive deficits associated with ethanol-stress interactions.

Psychostimulant-Induced Adaptations in Nucleus Accumbens Glutamatergic Transmission

Carrying different aspects of emotional and motivational signals, glutamatergic synaptic projections from multiple limbic and paralimbic brain regions converge to the nucleus accumbens (NAc), in which these arousing signals are processed and prioritized for behavioral output. In animal models of drug addiction, some key drug-induced alterations at NAc glutamatergic synapses underlie important cellular and circuit mechanisms that promote subsequent drug taking, seeking, and relapse. With the focus of cocaine, we review changes at NAc glutamatergic synapses that occur after different drug procedures and abstinence durations, and the behavioral impact of these changes.

Relapse-Associated Transient Synaptic Potentiation Requires Integrin-Mediated Activation of Focal Adhesion Kinase and Cofilin in D1-Expressing Neurons

Relapse to drug use can be initiated by drug-associated cues. The intensity of cue-induced drug seeking in rodent models correlates with the induction of transient synaptic potentiation (t-SP) at glutamatergic synapses in the nucleus accumbens core (NAcore). Matrix metalloproteinases (MMPs) are inducible endopeptidases that degrade extracellular matrix (ECM) proteins, and reveal tripeptide Arginine-Glycine-Aspartate (RGD) domains that bind and signal through integrins. Integrins are heterodimeric receptors composed of αβ subunits, and a primary signaling kinase is focal adhesion kinase (FAK). We previously showed that MMP activation is necessary for and potentiates cued reinstatement of cocaine seeking, and MMP-induced catalysis stimulates β3-integrins to induce t-SP. Here, we determined whether β3-integrin signaling through FAK and cofilin (actin depolymerization factor) is necessary to promote synaptic growth during t-SP. Using a small molecule inhibitor to prevent FAK activation, we blocked cued-induced cocaine reinstatement and increased spine head diameter (dh). Immunohistochemistry on NAcore labeled spines with ChR2-EYFP virus, showed increased immunoreactivity of phosphorylation of FAK (p-FAK) and p-cofilin in dendrites of reinstated animals compared with extinguished and yoked saline, and the p-FAK and cofilin depended on β3-integrin signaling. Next, male and female transgenic rats were used to selectively label D1 or D2 neurons with ChR2-mCherry. We found that p-FAK was increased during drug seeking in both D1 and D2-medium spiny neurons (MSNs), but increased p-cofilin was observed only in D1-MSNs. These data indicate that β3-integrin, FAK and cofilin constitute a signaling pathway downstream of MMP activation that is involved in promoting the transient synaptic enlargement in D1-MSNs induced during reinstated cocaine by drug-paired cues.SIGNIFICANCE STATEMENT Drug-associated cues precipitate relapse, which is correlated with transient synaptic enlargement in the accumbens core. We showed that cocaine cue-induced synaptic enlargement depends on matrix metalloprotease signaling in the extracellular matrix (ECM) through β3-integrin to activate focal adhesion kinase (FAK) and phosphorylate the actin binding protein cofilin. The nucleus accumbens core (NAcore) contains two predominate neuronal subtypes selectively expressing either D1-dopamine or D2-dopamine receptors. We used transgenic rats to study each cell type and found that cue-induced signaling through cofilin phosphorylation occurred only in D1-expressing neurons. Thus, cocaine-paired cues initiate cocaine reinstatement and synaptic enlargement through a signaling cascade selectively in D1-expressing neurons requiring ECM stimulation of β3-integrin-mediated phosphorylation of FAK (p-FAK) and cofilin.

Direct administration of ifenprodil and citalopram into the nucleus accumbens inhibits cue-induced nicotine seeking and associated glutamatergic plasticity

Nicotine use disorder has been associated with glutamatergic alterations within the basal ganglia that might contribute to relapse. Specifically, initiation of cue-induced nicotine seeking produces rapid, transient synaptic potentiation (t-SP) in nucleus accumbens core (NAcore) medium spiny neurons (MSNs), defined as increases in spine head diameter and AMPA to NMDA current ratios (A/N). Ifenprodil, which inhibits nicotine reinstatement when administered systemically, antagonizes GluN2B-containing NMDA receptors, has affinity for serotonin receptors, and blocks serotonin transporters (SERT). The mechanisms underlying its therapeutic efficacy, however, remain unknown. Using pharmacological and genetic approaches, the current study examined the role of NAcore GluN2B receptors as well as SERT in mediating cue-induced nicotine seeking and associated MSN structure and physiology. Prior to reinstatement, rats received intra-NAcore injections of either ifenprodil, citalopram or artificial cerebral spinal fluid (15 min prior), or GluN2B or control siRNAs (3 consecutive days prior). Rats were sacrificed after a 15-min cue-induced reinstatement session for dendritic spine analysis, western blotting or whole-cell electrophysiology. Intra-NAcore ifenprodil blocked nicotine-seeking behavior and promoted a higher frequency of shorter spines on MSN dendrites. However, a decrease in membrane-bound GluN2B receptor expression did not prevent cue-induced nicotine seeking or associated MSN cell physiology. Interestingly, intra-NAcore citalopram, an SSRI, prevented cue-induced nicotine seeking. Together, these results indicate that the therapeutic effects of ifenprodil on cue-induced nicotine seeking may, in part, be due to its actions at SERT rather than GluN2B, which may be specific to nicotine-seeking as opposed to other drugs of abuse.

N-acetylcysteine for the treatment of comorbid alcohol use disorder and posttraumatic stress disorder: Design and methodology of a randomized clinical trial

Alcohol use disorder (AUD) and posttraumatic stress disorder (PTSD) are two prevalent psychiatric conditions in the U.S. The co-occurrence of AUD and PTSD is also common, and associated with a more severe clinical presentation and worse treatment outcomes across the biopsychosocial spectrum (e.g., social and vocational functioning, physical health) as compared to either disorder alone. Despite the high co-occurrence and negative outcomes, research on effective medications for AUD/PTSD is sparse and there is little empirical evidence to guide treatment decisions. The study described in this paper addresses this knowledge gap by testing the efficacy of N-acetylcysteine (NAC) in reducing alcohol use and PTSD symptoms. Animal studies and prior clinical research suggest a role for NAC in the treatment of substance use disorders and PTSD via glutamate modulation. NAC is a cysteine pro-drug that stimulates the cystine-glutamate exchanger, normalizes glial glutamate transporters, and restores glutamatergic tone on presynaptic receptors in reward regions of the brain. Moreover, NAC is available over-the-counter, has a long-established safety record, and does not require titration to achieve the target dose. This paper describes the rationale, study design, and methodology of a 12-week, randomized, double-blind, placebo-controlled trial of NAC (2400 mg/day) among adults with co-occurring AUD and PTSD. Functional magnetic resonance imaging (fMRI) and proton magnetic resonance spectroscopy (1H-MRS) are utilized to investigate the neural circuitry and neurochemistry underlying comorbid AUD/PTSD and identify predictors of treatment outcome. This study is designed to determine the efficacy of NAC in the treatment of co-occurring AUD/PTSD and provide new information regarding mechanisms of action implicated in co-occurring AUD/PTSD.

The Opioid-Addicted Tetrapartite Synapse

Opioid administration in preclinical models induces long-lasting adaptations in reward and habit circuitry. The latest research demonstrates that in the nucleus accumbens, opioid-induced excitatory synaptic plasticity involves presynaptic and postsynaptic elements as well as adjacent astroglial processes and the perisynaptic extracellular matrix. We outline opioid-induced modifications within each component of the tetrapartite synapse and provide a neurobiological perspective on how these adaptations converge to produce addiction-related behaviors in rodent models. By incorporating changes observed at each of the excitatory synaptic compartments into a unified framework of opioid-induced glutamate dysregulation, we highlight new avenues for restoring synaptic homeostasis that might limit opioid craving and relapse vulnerability.

Heroin Cue-Evoked Astrocytic Structural Plasticity at Nucleus Accumbens Synapses Inhibits Heroin Seeking

BACKGROUND

Opioid addiction is a critical medical and societal problem characterized by vulnerability to relapse. Glutamatergic synapses in the nucleus accumbens regulate the motivation to relapse to opioid use, and downregulation of glutamate transporters on astroglial processes adjacent to accumbens synapses contributes to heroin seeking induced by cues. However, it is not known how astroglial processes themselves respond to heroin cues or if changes in astroglial morphology are necessary for heroin seeking.

METHODS

Male Sprague Dawley rats (n = 62) were trained to self-administer heroin or sucrose and were reinstated by heroin-conditioned or sucrose-conditioned cues. Astroglial proximity to accumbens synapses was estimated using a confocal-based strategy, and the association between digitally isolated astroglia and the presynaptic marker synapsin I was quantified. To determine the functional consequence of astroglial morphological plasticity on cued heroin seeking, a morpholino antisense strategy was used to knock down expression of the actin binding protein ezrin, which is expressed almost exclusively in peripheral astroglial processes in the adult rat brain.

RESULTS

After heroin extinction, there was an enduring reduction in synaptic proximity by astroglia. Synaptic proximity was restored during 15 minutes of cued heroin seeking but returned to extinction levels by 120 minutes. Extinction from sucrose self-administration and reinstated sucrose seeking induced no changes in astroglial synaptic association. Ezrin knockdown reduced astroglial association with synapses and potentiated cued heroin seeking.

CONCLUSIONS

Cue-induced heroin seeking transiently increased synaptic proximity of accumbens astrocytes. Surprisingly, the reassociation of astroglia with synapses was compensatory, and preventing cue-induced morphological plasticity in astrocytes potentiated heroin seeking.

The Opioid Crisis and the Future of Addiction and Pain Therapeutics

Opioid misuse and addiction are a public health crisis resulting in debilitation, deaths, and significant social and economic impact. Curbing this crisis requires collaboration among academic, government, and industrial partners toward the development of effective nonaddictive pain medications, interventions for opioid overdose, and addiction treatments. A 2-day meeting, The Opioid Crisis and the Future of Addiction and Pain Therapeutics: Opportunities, Tools, and Technologies Symposium, was held at the National Institutes of Health (NIH) to address these concerns and to chart a collaborative path forward. The meeting was supported by the NIH Helping to End Addiction Long-Term^SM (HEAL) Initiative, an aggressive, trans-agency effort to speed scientific solutions to stem the national opioid crisis. The event was unique in bringing together two research disciplines, addiction and pain, in order to create a forum for crosscommunication and collaboration. The output from the symposium will be considered by the HEAL Initiative; this article summarizes the scientific presentations and key takeaways. Improved understanding of the etiology of acute and chronic pain will enable the discovery of novel targets and regulatable pain circuits for safe and effective therapeutics, as well as relevant biomarkers to ensure adequate testing in clinical trials. Applications of improved technologies including reagents, assays, model systems, and validated probe compounds will likely increase the delivery of testable hypotheses and therapeutics to enable better health outcomes for patients. The symposium goals were achieved by increasing interdisciplinary collaboration to accelerate solutions for this pressing public health challenge and provide a framework for focused efforts within the research community. SIGNIFICANCE STATEMENT: This article summarizes key messages and discussions resulting from a 2-day symposium focused on challenges and opportunities in developing addiction- and pain-related medications. Speakers and attendees came from 40 states in the United States and 15 countries, bringing perspectives from academia, industry, government, and healthcare by researchers, clinicians, regulatory experts, and patient advocates.

Accumbens brain-derived neurotrophic factor (BDNF) transmission inhibits cocaine seeking

Brain-derived neurotrophic factor (BDNF) regulates a variety of physiological processes, and several studies have explored the role of BDNF in addiction-related brain regions like the nucleus accumbens core (NAcore). We sought to understand the rapid effects of endogenous BDNF on cocaine seeking. Rats were trained to self-administer cocaine and extinguished. We then microinjected two inhibitors of BDNF stimulation of tropomyosin receptor kinase B (TrkB), the non-competitive receptor antagonist ANA-12 and TrkB/Fc, a fusion protein that binds BDNF and prevents TrkB stimulation. Blocking TrkB or inactivating BDNF in NAcore potentiated active lever pressing, showing that endogenous BDNF tone was present and supplying inhibitory tone on cue-induced reinstatement. To determine if exogenous BDNF also negatively regulated reinstatement, BDNF was microinjected into NAcore 15 minutes before cue-induced reinstatement. BDNF decreased cocaine seeking through TrkB receptor binding, but had no effect on inactive lever pressing, spontaneous or cocaine-induced locomotion, or on reinstated sucrose seeking. BDNF-infusion potentiated within trial extinction when microinjected in the NAcore during cue- and context + cue induced reinstatement, and the inhibition of lever pressing lasted at least 3 days post injection. Although decreased reinstatement endured for 3 days when BDNF was administered prior to a reinstatement session, when microinjected before an extinction session or in the home cage, BDNF did not alter subsequent cued-reinstatement. Together, these data show that endogenous BDNF acts on TrKB to provide inhibitory tone on reinstated cocaine seeking, and this effect was recapitulated by exogenous BDNF.

Extracellular Matrix Signaling Through β3 Integrin Mediates Cocaine Cue-Induced Transient Synaptic Plasticity and Relapse

BACKGROUND

Cue-induced relapse to drug use is a primary symptom of cocaine addiction. Cue-induced transient excitatory synaptic potentiation (t-SP) induced in the nucleus accumbens mediates cued cocaine seeking in rat models of relapse. Cue-induced t-SP depends on extracellular signaling by matrix metalloproteases (MMPs), but it is unknown how this catalytic activity communicates with nucleus accumbens neurons to induce t-SP and cocaine seeking.

METHODS

Male Sprague Dawley rats (N = 125) were trained to self-administer cocaine, after which self-administration was extinguished and then reinstated by cocaine-conditioned cues. We used a morpholino antisense strategy to knock down the β1 or β3 integrin subunits or inhibitors to prevent phosphorylation of the integrin signaling kinases focal adhesion kinase (FAK) or integrin-linked kinase. We quantified protein changes with immunoblotting and t-SP by measuring dendritic spine morphology and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate glutamate currents. Integrin signaling was stimulated by microinjecting an MMP activator or integrin peptide ligand into the accumbens.

RESULTS

Knockdown of β3 integrin or FAK inhibitor, but not β1 integrin or integrin-linked kinase inhibitor, prevented cue-induced cocaine seeking but not sucrose seeking. β3 integrin knockdown prevented t-SP as measured by preventing the cue-induced increases in both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate glutamate ratio and spine head diameter. Activating MMP gelatinases with tissue plasminogen activator potentiated cue-induced reinstatement, which was prevented by β3 integrin knockdown and FAK inhibition. Stimulating integrin receptors with the RGD ligand liberated by MMP gelatinase activity also potentiated cued cocaine seeking.

CONCLUSIONS

Activation of MMP gelatinase in the extracellular space is necessary for and potentiates cued cocaine seeking. This extracellular catalysis stimulates β3 integrins and activates FAK to induce t-SP and promote cue-induced cocaine seeking.

Leveraging calcium imaging to illuminate circuit dysfunction in addiction

Alcohol and drug use can dysregulate neural circuit function to produce a wide range of neuropsychiatric disorders, including addiction. To understand the neural circuit computations that mediate behavior, and how substances of abuse may transform them, we must first be able to observe the activity of circuits. While many techniques have been utilized to measure activity in specific brain regions, these regions are made up of heterogeneous sub-populations, and assessing activity from neuronal populations of interest has been an ongoing challenge. To fully understand how neural circuits mediate addiction-related behavior, we must be able to reveal the cellular granularity within brain regions and circuits by overlaying functional information with the genetic and anatomical identity of the cells involved. The development of genetically encoded calcium indicators, which can be targeted to populations of interest, allows for in vivo visualization of calcium dynamics, a proxy for neuronal activity, thus providing an avenue for real-time assessment of activity in genetically and anatomically defined populations during behavior. Here, we highlight recent advances in calcium imaging technology, compare the current technology with other state-of-the-art approaches for in vivo monitoring of neural activity, and discuss the strengths, limitations, and practical concerns for observing neural circuit activity in preclinical addiction models.

Chronic treatment with N-acetylcysteine decreases extinction responding and reduces cue-induced nicotine-seeking

N-acetylcysteine (NAC), a promising glutamatergic therapeutic agent, has shown some clinical efficacy in reducing nicotine use in humans and has been shown to reverse drug-induced changes in glutamatergic neurophysiology. In rats, nicotine-seeking behavior is associated with alterations in glutamatergic plasticity within the nucleus accumbens core (NAcore). Specifically, cue-induced nicotine-seeking is associated with rapid, transient synaptic plasticity (t-SP) in glutamatergic synapses on NAcore medium spiny neurons. The goal of the present study was to determine if NAC reduces nicotine-seeking behavior and reverses reinstatement-associated NAcore glutamatergic alterations. Rats were extinguished from nicotine self-administration, followed by subchronic NAC administration (0 or 100 mg/kg/d) for 4 days prior to cue-induced reinstatement. NAcore synaptic potentiation was measured via dendritic spine morphology and mRNA and protein of relevant glutamatergic genes were quantified. Nicotine-seeking behavior was not reduced by subchronic NAC treatment. Also, NAcore transcript and protein expression of multiple glutamatergic genes, as well as spine morphological measures, were unaffected by subchronic NAC. Finally, chronic NAC treatment (15 days total) during extinction and prior to reinstatement significantly decreased extinction responding and reduced reinstatement of nicotine-seeking compared to vehicle. Together, these results suggest that chronic NAC treatment is necessary for its therapeutic efficacy as a treatment strategy for nicotine addiction and relapse.

Enhancing the Utility of Preclinical Research in Neuropsychiatry Drug Development

Most large pharmaceutical companies have downscaled or closed their clinical neuroscience research programs in response to the low clinical success rate for drugs that showed tremendous promise in animal experiments intended to model psychiatric pathophysiology. These failures have raised serious concerns about the role of preclinical research in the identification and evaluation of new pharmacotherapies for psychiatry. In the absence of a comprehensive understanding of the neurobiology of psychiatric disorders, the task of developing "animal models" seems elusive. The purpose of this review is to highlight emerging strategies to enhance the utility of preclinical research in the drug development process. We address this issue by reviewing how advances in neuroscience, coupled with new conceptual approaches, have recently revolutionized the way we can diagnose and treat common psychiatric conditions. We discuss the implications of these new tools for modeling psychiatric conditions in animals and advocate for the use of systematic reviews of preclinical work as a prerequisite for conducting psychiatric clinical trials. We believe that work in animals is essential for elucidating human psychopathology and that improving the predictive validity of animal models is necessary for developing more effective interventions for mental illness.

A Model of Δ9-Tetrahydrocannabinol Self-administration and Reinstatement That Alters Synaptic Plasticity in Nucleus Accumbens

BACKGROUND

Cannabis is the most widely used illicit drug, but knowledge of the neurological consequences of cannabis use is deficient. Two primary components of cannabis are Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). We established a THC+CBD model of self-administration and reinstated drug seeking to determine if, similar to other addictive drugs, cannabis produces enduring synaptic changes in nucleus accumbens core (NAcore) thought to contribute vulnerability to drug reinstatement.

METHODS

Sprague Dawley rats were trained to self-administer THC+CBD (n = 165) or were used as vehicle self-administering control animals (n = 24). Reinstatement was initiated by context, cues, drug priming, and stress (yohimbine injection). Enduring neuroadaptations produced by THC+CBD self-administration were assayed using four measures: dendritic spine morphology, long-term depression, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate ratios, and behavioral pharmacology.

RESULTS

We described a novel rodent model of cannabis relapse involving intravenous THC+CBD self-administration and drug seeking induced by conditioned context, cues, and stress. Cued reinstatement of THC+CBD seeking depended on a sequence of events implicated in relapse to other addictive drugs, as reinstatement was prevented by daily treatment with N-acetylcysteine or acute intra-NAcore pretreatment with a neuronal nitric oxide synthase or matrix metalloprotease-9 inhibitor, all of which normalize impaired glutamate homeostasis. The capacity to induce N-methyl-D-aspartate long-term depression in NAcore medium spiny neurons was abolished and dendritic spine density was reduced, but alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartate ratio was unaltered in THC+CBD-trained animals, akin to opioids, but not to psychostimulants.

CONCLUSIONS

We report enduring consequences of THC+CBD use on critical relapse circuitry and synaptic physiology in NAcore following rat self-administration and provide the first report of cue- and stress-induced reinstatement with this model.

Metaplasticity at the addicted tetrapartite synapse: A common denominator of drug induced adaptations and potential treatment target for addiction

In light of the current worldwide addiction epidemic, the need for successful therapies is more urgent than ever. Although we made substantial progress in our basic understanding of addiction, reliable therapies are lacking. Since 40-60% of patients treated for substance use disorder return to active substance use within a year following treatment discharge, alleviating the vulnerability to relapse is regarded as the most promising avenue for addiction therapy. Preclinical addiction research often focuses on maladaptive synaptic plasticity within the reward pathway. However, drug induced neuroadaptations do not only lead to a strengthening of distinct drug associated cues and drug conditioned behaviors, but also seem to increase plasticity thresholds for environmental stimuli that are not associated with the drug. This form of higher order plasticity, or synaptic metaplasticity, is not expressed as a change in the efficacy of synaptic transmission but as a change in the direction or degree of plasticity induced by a distinct stimulation pattern. Experimental addiction research has demonstrated metaplasticity after exposure to multiple classes of addictive drugs. In this review we will focus on the concept of synaptic metaplasticity in the context of preclinical addiction research. We will take a closer look at the tetrapartite glutamatergic synapse and outline forms of metaplasticity that have been described at the addicted synapse. Finally we will discuss the different potential avenues for pharmacotherapies that target glutamatergic synaptic plasticity and metaplasticity. Here we will argue that aberrant metaplasticity renders the reward seeking circuitry more rigid and hence less able to adapt to changing environmental contingencies. An understanding of the molecular mechanisms that underlie this metaplasticity is crucial for the development of new strategies for addiction therapy. The correction of drug-induced metaplasticity could be used to support behavioral and pharmacotherapies for the treatment of addiction.

Increased Alcohol Seeking in Mice Lacking Gpr88 Involves Dysfunctional Mesocorticolimbic Networks

BACKGOUND

Alcohol use disorder (AUD) is devastating and poorly treated, and innovative targets are actively sought for prevention and treatment. The orphan G protein-coupled receptor GPR88 is enriched in mesocorticolimbic pathways, and Gpr88 knockout mice show hyperactivity and risk-taking behavior, but a potential role for this receptor in drug abuse has not been examined.

METHODS

We tested Gpr88 knockout mice for alcohol-drinking and -seeking behaviors. To gain system-level understanding of their alcohol endophenotype, we also analyzed whole-brain functional connectivity in naïve mice using resting-state functional magnetic resonance imaging.

RESULTS

Gpr88 knockout mice showed increased voluntary alcohol drinking at both moderate and excessive levels, with intact alcohol sedation and metabolism. Mutant mice also showed increased operant responding and motivation for alcohol, while food and chocolate operant self-administration were unchanged. Alcohol place conditioning and alcohol-induced dopamine release in the nucleus accumbens were decreased, suggesting reduced alcohol reward in mutant mice that may partly explain enhanced alcohol drinking. Seed-based voxelwise functional connectivity analysis revealed significant remodeling of mesocorticolimbic centers, whose hallmark was predominant weakening of prefrontal cortex, ventral tegmental area, and amygdala connectional patterns. Also, effective connectivity from the ventral tegmental area to the nucleus accumbens and amygdala was reduced.

CONCLUSIONS

Gpr88 deletion disrupts executive, reward, and emotional networks in a configuration that reduces alcohol reward and promotes alcohol seeking and drinking. The functional connectivity signature is reminiscent of alterations observed in individuals at risk for AUD. The Gpr88 gene, therefore, may represent a vulnerability/resilience factor for AUD, and a potential drug target for AUD treatment.

Effect of N-acetylcysteine on motivation, seeking and relapse to ethanol self-administration

Alcohol use disorder is a chronic and highly relapsing disorder, characterized by a loss of control over alcohol consumption and craving. Several studies suggest a key role of glutamate in this disorder. In recent years, the modulation of cystine/glutamate exchange via the x_c^- system has emerged as a new therapeutic alternative for reducing the excitatory glutamatergic transmission observed after ethanol self-administration in both rats and humans. The objective of this study was to determine whether a treatment with N-acetylcysteine (NAC), a cystine prodrug, could reduce ethanol self-administration, ethanol-seeking behavior and reacquisition of ethanol self-administration. Male Long Evans rats were trained to self-administer 20 percent ethanol in operant cages for several weeks. Once the consumption surpassed 1 g of ethanol/kg body weight/15 minutes, the effect of an acute intraperitoneal injection of NAC (0, 25, 50 or 100 mg/kg) 1 hour before the beginning of each test was evaluated on different aspects of the operant self-administration behavior. We demonstrated antimotivational properties of NAC (100 mg/kg), as ethanol-reinforced responding was reduced in a fixed ratio (-35 percent) and in a progressive ratio schedule (-81 percent). NAC also reduced ethanol-seeking behavior (-77 percent) evaluated as extinction responding in a single extinction session. NAC was able to reduce reacquisition in rats that were abstinent for 17 days, while NAC had no effect on ethanol relapse in rats previously exposed to six extinction sessions. Overall, our results demonstrate that NAC limits motivation, seeking behavior and reacquisition in rats, making it a potential new treatment for the maintenance of abstinence.

Chronic Alcohol, Intrinsic Excitability, and Potassium Channels: Neuroadaptations and Drinking Behavior

Neural mechanisms underlying alcohol use disorder remain elusive, and this lack of understanding has slowed the development of efficacious treatment strategies for reducing relapse rates and prolonging abstinence. While synaptic adaptations produced by chronic alcohol exposure have been extensively characterized in a variety of brain regions, changes in intrinsic excitability of critical projection neurons are understudied. Accumulating evidence suggests that prolonged alcohol drinking and alcohol dependence produce plasticity of intrinsic excitability as measured by changes in evoked action potential firing and after-hyperpolarization amplitude. In this chapter, we describe functional changes in cell firing of projection neurons after long-term alcohol exposure that occur across species and in multiple brain regions. Adaptations in calcium-activated (K_Ca2), voltage-dependent (K_V7), and G protein-coupled inwardly rectifying (K_ir3 or GIRK) potassium channels that regulate the evoked firing and after-hyperpolarization parallel functional changes in intrinsic excitability induced by chronic alcohol. Moreover, there are strong genetic links between alcohol-related behaviors and genes encoding K_Ca2, K_V7, and GIRK channels, and pharmacologically targeting these channels reduces alcohol consumption and alcohol-related behaviors. Together, these studies demonstrate that chronic alcohol drinking produces adaptations in K_Ca2, K_V7, and GIRK channels leading to impaired regulation of the after-hyperpolarization and aberrant cell firing. Correcting the deficit in the after-hyperpolarization with positive modulators of K_Ca2 and K_V7 channels and altering the GIRK channel binding pocket to block the access of alcohol represent a potentially highly effective pharmacological approach that can restore changes in intrinsic excitability and reduce alcohol consumption in affected individuals.

Accumbens Mechanisms for Cued Sucrose Seeking

Many studies support a perspective that addictive drugs usurp brain circuits used by natural rewards, especially for the dopamine-dependent reinforcing qualities of both drugs and natural rewards. Reinstated drug seeking in animal models of relapse relies on glutamate spillover from cortical terminals synapsing in the nucleus accumbens core (NAcore) to stimulate metabotropic glutamate receptor5 (mGluR5) on neuronal nitric oxide synthase (nNOS) interneurons. Contrasting the release of dopamine that is shared by sucrose and drugs of abuse, reinstated sucrose seeking does not induce glutamate spillover. We hypothesized that pharmacologically promoting glutamate spillover in the NAcore would mimic cocaine-induced adaptations and potentiate cued reinstatement of sucrose seeking. Inducing glutamate spillover by blocking astroglial glutamate transporters (GLT-1) had no effect on reinstated sucrose seeking. However, glutamate release probability is negatively regulated by presynaptic mGluR2/3, and sucrose reinstatement was potentiated following mGluR2/3 blockade. Potentiated sucrose reinstatement by mGluR2/3 blockade was reversed by antagonizing mGluR5, but reinstated sucrose seeking in the absence of mGluR2/3 blockade was not affected by blocking mGluR5. In cocaine-trained rodents mGluR5 stimulation reinstates drug seeking by activating nNOS, but activating mGluR5 did not promote reinstated sucrose seeking, nor was potentiated reinstatement after mGluR2/3 blockade reduced by blocking nNOS. However, chemogenetic activation of nNOS interneurons in the NAcore reinstated sucrose seeking. These data indicate that dysregulated presynaptic mGluR2/3 signaling is a possible site of shared signaling in drug seeking and potentiated reinstated sucrose seeking, but that downregulated glutamate transport and subsequent activation of nNOS by synaptic glutamate spillover is not shared.

Corticostriatal plasticity, neuronal ensembles, and regulation of drug-seeking behavior

The idea that interconnected neuronal ensembles code for specific behaviors has been around for decades; however, recent technical improvements allow studying these networks and their causal role in initiating and maintaining behavior. In particular, the role of ensembles in drug-seeking behaviors in the context of addiction is being actively investigated. Concurrent with breakthroughs in quantifying ensembles, research has identified a role for synaptic glutamate spillover during relapse. In particular, the transient relapse-associated changes in glutamatergic synapses on accumbens neurons, as well as in adjacent astroglia and extracellular matrix, are key elements of the synaptic plasticity encoded by drug use and the metaplasticity induced by drug-associated cues that precipitate drug-seeking behaviors. Here, we briefly review the recent discoveries related to ensembles in the addiction field and then endeavor to link these discoveries with drug-induced striatal plasticity and cue-induced metaplasticity toward deeper neurobiological understandings of drug seeking.

Persistent cognitive and morphological alterations induced by repeated exposure of adolescent rats to the abused inhalant toluene

While thepsychoactive inhalant toluene causes behavioral effects similarto those produced by other drugs of abuse, the persistent behavioral and anatomical abnormalities induced by toluene exposure are not well known. To mimic human "binge-like" inhalant intoxication, adolescent, male Sprague-Dawley rats were exposed to toluene vapor (5700ppm) twice daily for five consecutive days. These rats remained in their home cages until adulthood (P60), when they were trained in operant boxes to respond to a palatable food reward and then challenged with several different cognitive tasks. Rats that experienced chronic exposure to toluene plus abstinence ("CTA") showed enhanced performance in a strategy set-shifting task using a between-session, but not a within-session test design. CTA also blunted operant and classical conditioning without affecting responding during a progressive ratio task. While CTA rats displayed normal latent inhibition, previous exposure to a non-reinforced cue enhanced extinction of classically conditioned approach behavior of these animals compared to air controls. To determine whether CTA alters the structural plasticity of brain areas involved in set-shifting and appetitive behaviors, we quantified basal dendritic spine morphology in DiI-labeled pyramidal neurons in layer 5 of the medial prefrontal cortex (mPFC) and medium spiny neurons in the nucleus accumbens (NAc). There were no changes in dendritic spine density or subtype in the mPFC of CTA rats while NAc spine density was significantly increased due to an enhanced prevalence of long-thin spines. Together, these findings suggest that the persistent effects of CTA on cognition are related to learning and memory consolidation/recall, but not mPFC-dependent behavioral flexibility.

Glutaminase and MMP-9 Downregulation in Cortex and Hippocampus of LPA1 Receptor Null Mice Correlate with Altered Dendritic Spine Plasticity

Lysophosphatidic acid (LPA) is an extracellular lipid mediator that regulates nervous system development and functions acting through G protein-coupled receptors (GPCRs). Here we explore the crosstalk between LPA₁ receptor and glutamatergic transmission by examining expression of glutaminase (GA) isoforms in different brain areas isolated from wild-type (WT) and KOLPA₁ mice. Silencing of LPA₁ receptor induced a severe down-regulation of Gls-encoded long glutaminase protein variant (KGA) (glutaminase gene encoding the kidney-type isoforms, GLS) protein expression in several brain regions, particularly in brain cortex and hippocampus. Immunohistochemical assessment of protein levels for the second type of glutaminase (GA) isoform, glutaminase gene encoding the liver-type isoforms (GLS2), did not detect substantial differences with regard to WT animals. The regional mRNA levels of GLS were determined by real time RT-PCR and did not show significant variations, except for prefrontal and motor cortex values which clearly diminished in KO mice. Total GA activity was also significantly reduced in prefrontal and motor cortex, but remained essentially unchanged in the hippocampus and rest of brain regions examined, suggesting activation of genetic compensatory mechanisms and/or post-translational modifications to compensate for KGA protein deficit. Remarkably, Golgi staining of hippocampal regions showed an altered morphology of glutamatergic pyramidal cells dendritic spines towards a less mature filopodia-like phenotype, as compared with WT littermates. This structural change correlated with a strong decrease of active matrix-metalloproteinase (MMP) 9 in cerebral cortex and hippocampus of KOLPA₁ mice. Taken together, these results demonstrate that LPA signaling through LPA¹ influence expression of the main isoenzyme of glutamate biosynthesis with strong repercussions on dendritic spines maturation, which may partially explain the cognitive and learning defects previously reported for this colony of KOLPA¹ mice.

Chronic Ethanol During Adolescence Impacts Corticolimbic Dendritic Spines and Behavior

BACKGROUND

Risk for alcohol use disorders (AUDs) in adulthood is linked to alcohol drinking during adolescence, but understanding of the neural and behavioral consequences of alcohol exposure during adolescence remains incomplete. Here, we examined the neurobehavioral impact of adolescent chronic intermittent EtOH (CIE) vapor exposure in mice.

METHODS

C57BL/6J-background Thy1-EGFP mice were CIE-exposed during adolescence or adulthood and examined, as adults, for alterations in the density and morphology of dendritic spines in infralimbic (IL) cortex, prelimbic (PL) cortex, and basolateral amygdala (BLA). In parallel, adolescent- and adult-exposed C57BL/6J mice were tested as adults for 2-bottle EtOH drinking, sensitivity to EtOH intoxication (loss of righting reflex [LORR]), blood EtOH clearance, and measures of operant responding for food reward.

RESULTS

CIE during adolescence decreased IL neuronal spine density and increased the head width of relatively wide-head IL and BLA spines, whereas CIE decreased head width of relatively narrow-head BLA spines. Adolescents had higher EtOH consumption prior to CIE than adults, while CIE during adulthood, but not adolescence, increased EtOH consumption relative to pre-CIE baseline. CIE produced a tolerance-like decrease in LORR sensitivity to EtOH challenge, irrespective of the age at which mice received CIE exposure. Mice exposed to CIE during adolescence, but not adulthood, required more sessions than AIR controls to reliably respond for food reward on a fixed-ratio (FR) 1, but not subsequent FR3, reinforcement schedule. On a progressive ratio reinforcement schedule, break point responding was higher in the adolescent- than the adult-exposed mice, regardless of CIE. Finally, footshock punishment markedly suppressed responding for reward in all groups.

CONCLUSIONS

Exposure to CIE during adolescence altered dendritic spine density and morphology in IL and BLA neurons, in parallel with a limited set of behavioral alterations. Together, these data add to growing evidence that key corticolimbic circuits are vulnerable to the effects of alcohol during adolescence, with lasting, potentially detrimental, consequences for behavior.

Contrasting the Role of xCT and GLT-1 Upregulation in the Ability of Ceftriaxone to Attenuate the Cue-Induced Reinstatement of Cocaine Seeking and Normalize AMPA Receptor Subunit Expression

Long-term treatment with ceftriaxone attenuates the reinstatement of cocaine seeking while increasing the function of the glutamate transporter 1 (GLT-1) and system xC- (Sxc) in the nucleus accumbens core (NAc). Sxc contributes the majority of nonsynaptic extracellular glutamate in the NAc, while GLT-1 is responsible for the majority of glutamate uptake. Here we used antisense to decrease the expression of GLT-1 and xCT (a catalytic subunit of Sxc) to determine the relative importance of both proteins in mediating the ability of ceftriaxone to prevent cue-induced reinstatement of cocaine seeking and normalize glutamatergic proteins in the NAc of rats. Intra-NAc xCT knockdown prevented ceftriaxone from attenuating reinstatement and from upregulating GLT-1 and resulted in increased surface expression of AMPA receptor subunits GluA1 and GluA2. Intra-NAc GLT-1 knockdown also prevented ceftriaxone from attenuating reinstatement and from upregulating xCT expression, without affecting GluA1 and GluA2 expression. In the absence of cocaine or ceftriaxone treatment, xCT knockdown in the NAc increased the expression of both GluA1 and GluA2 without affecting GLT-1 expression while GLT-1 knockdown had no effect. PCR and immunoprecipitation of GLT-1 revealed that ceftriaxone does not upregulate GLT-1 and xCT through a transcriptional mechanism, and their coregulation by ceftriaxone is not mediated by physical interaction. These data support important and distinct roles for xCT and GLT-1 in the actions of ceftriaxone and add to a body of literature finding evidence for coregulation of these transporters. Our results also point to xCT expression and subsequent basal glutamate levels as being a key mediator of AMPA receptor expression in the NAc.SIGNIFICANCE STATEMENT Ceftriaxone attenuates the reinstatement of cocaine, alcohol, and heroin seeking. The mechanism of action of this behavioral effect has been attributed to glutamate transporter 1 (GLT-1) and xCT (a catalytic subunit of Sxc)/Sxc upregulation in the nucleus accumbens core. Here we used an antisense strategy to knock down GLT-1 or xCT in the nucleus accumbens core and examined the behavioral and molecular consequences. While upregulation of both xCT and GLT-1 are essential to the ability of ceftriaxone to attenuate cue-induced reinstatement of cocaine seeking, each protein uniquely affects the expression of other glutamate receptor and transporter proteins. We also report that reducing basal glutamate levels through the manipulation of xCT expression increases the surface expression of AMPA receptor subunits, providing insight to the mechanism by which cocaine alters AMPA surface expression.

Promising pharmacogenetic targets for treating alcohol use disorder: evidence from preclinical models

Inherited genetic variants contribute to risk factors for developing an alcohol use disorder, and polymorphisms may inform precision medicine strategies for treating alcohol addiction. Targeting genetic mutations linked to alcohol phenotypes has provided promising initial evidence for reducing relapse rates in alcoholics. Although successful in some studies, there are conflicting findings and the reports of adverse effects may ultimately limit their clinical utility, suggesting that novel pharmacogenetic targets are necessary to advance precision medicine approaches. Here, we describe promising novel genetic variants derived from preclinical models of alcohol consumption and dependence that may uncover disease mechanisms that drive uncontrolled drinking and identify novel pharmacogenetic targets that facilitate therapeutic intervention for the treatment of alcohol use disorder.

Orbitofrontal Neuroadaptations and Cross-Species Synaptic Biomarkers in Heavy-Drinking Macaques

Cognitive impairments, uncontrolled drinking, and neuropathological cortical changes characterize alcohol use disorder. Dysfunction of the orbitofrontal cortex (OFC), a critical cortical subregion that controls learning, decision-making, and prediction of reward outcomes, contributes to executive cognitive function deficits in alcoholic individuals. Electrophysiological and quantitative synaptomics techniques were used to test the hypothesis that heavy drinking produces neuroadaptations in the macaque OFC. Integrative bioinformatics and reverse genetic approaches were used to identify and validate synaptic proteins with novel links to heavy drinking in BXD mice. In drinking monkeys, evoked firing of OFC pyramidal neurons was reduced, whereas the amplitude and frequency of postsynaptic currents were enhanced compared with controls. Bath application of alcohol reduced evoked firing in neurons from control monkeys, but not drinking monkeys. Profiling of the OFC synaptome identified alcohol-sensitive proteins that control glutamate release (e.g., SV2A, synaptogyrin-1) and postsynaptic signaling (e.g., GluA1, PRRT2) with no changes in synaptic GABAergic proteins. Western blot analysis confirmed the increase in GluA1 expression in drinking monkeys. An exploratory analysis of the OFC synaptome found cross-species genetic links to alcohol intake in discrete proteins (e.g., C2CD2L, DIRAS2) that discriminated between low- and heavy-drinking monkeys. Validation studies revealed that BXD mouse strains with the D allele at the C2cd2l interval drank less alcohol than B allele strains. Thus, by profiling of the OFC synaptome, we identified changes in proteins controlling glutamate release and postsynaptic signaling and discovered several proteins related to heavy drinking that have potential as novel targets for treating alcohol use disorder.SIGNIFICANCE STATEMENT Clinical research identified cognitive deficits in alcoholic individuals as a risk factor for relapse, and alcoholic individuals display deficits on cognitive tasks that are dependent upon the orbitofrontal cortex (OFC). To identify neurobiological mechanisms that underpin OFC dysfunction, this study used electrophysiology and integrative synaptomics in a translational nonhuman primate model of heavy alcohol consumption. We found adaptations in synaptic proteins that control glutamatergic signaling in chronically drinking monkeys. Our functional genomic exploratory analyses identified proteins with genetic links to alcohol and cocaine intake across mice, monkeys, and humans. Future work is necessary to determine whether targeting these novel targets reduces excessive and harmful levels of alcohol drinking.

The good and bad news about glutamate in drug addiction

In 1998 we published a perspective review describing how drug-induced neuroadaptations might serve towards understanding drug craving. We proposed experimental perspectives to help discern data relevant to long-lasting brain changes, and to distinguish dopamine-related changes that were largely pharmacological from glutamatergic changes that were based on drug-environment associations. These perspectives are embedded in drug abuse research, and the last 18 years has witnessed marked development in understanding addiction-associated corticostriatal glutamate plasticity. Here we propose three new perspectives on how the field might approach integrating and using the emerging data on glutamatergic adaptations. (1) Consider adaptations produced in kind across drug classes as most useful towards understanding shared characteristics of addiction, such as relapse. (2) Consider how drug-induced changes in glia and the extracellular matrix may contribute to synaptic alterations. (3) Make measurements not only at late withdrawal, but also during drug seeking events to capture transient changes that mediate active drug seeking that are shared across drug classes.

Structural and functional plasticity of dendritic spines - root or result of behavior?

Dendritic spines are multifunctional integrative units of the nervous system and are highly diverse and dynamic in nature. Both internal and external stimuli influence dendritic spine density and morphology on the order of minutes. It is clear that the structural plasticity of dendritic spines is related to changes in synaptic efficacy, learning and memory and other cognitive processes. However, it is currently unclear whether structural changes in dendritic spines are primary instigators of changes in specific behaviors, a consequence of behavioral changes, or both. In this review, we first examine the basic structure and function of dendritic spines in the brain, as well as laboratory methods to characterize and quantify morphological changes in dendritic spines. We then discuss the existing literature on the temporal and functional relationship between changes in dendritic spines in specific brain regions and changes in specific behaviors mediated by those regions. Although technological advancements have allowed us to better understand the functional relevance of structural changes in dendritic spines that are influenced by environmental stimuli, the role of spine dynamics as an underlying driver or consequence of behavior still remains elusive. We conclude that while it is likely that structural changes in dendritic spines are both instigators and results of behavioral changes, improved research tools and methods are needed to experimentally and directly manipulate spine dynamics in order to more empirically delineate the relationship between spine structure and behavior.

Effects of Ethanol on Brain Extracellular Matrix: Implications for Alcohol Use Disorder

The brain extracellular matrix (ECM) occupies the space between cells and is involved in cell-matrix and cell-cell adhesion. However, in addition to providing structural support to brain tissue, the ECM activates cell signaling and controls synaptic transmission. The expression and activity of brain ECM components are regulated by alcohol exposure. This review will discuss what is currently known about the effects of alcohol on the activity and expression of brain ECM components. An interpretation of how these changes might promote alcohol use disorder (AUD) will be also provided. Ethanol (EtOH) exposure decreases levels of structural proteins involved in the interstitial matrix and basement membrane, with a concomitant increase in proteolytic enzymes that degrade these components. In contrast, EtOH exposure generally increases perineuronal net components. Because the ECM has been shown to regulate both synaptic plasticity and behavioral responses to drugs of abuse, regulation of the brain ECM by alcohol may be relevant to the development of alcoholism. Although investigation of the function of brain ECM in alcohol abuse is still in early stages, a greater understanding of the interplay between ECM and alcohol might lead to novel therapeutic strategies for treating AUD.