Behavioral and neural mechanisms of compulsive drug seeking

Abnormal Granger causal connectivity based on altered gray matter volume and associated neurotransmitters of adolescents with internet gaming disorder revealed by a multimodal neuroimaging study

Although prior studies have revealed alterations in gray matter volume (GMV) among individuals with internet gaming disorder (IGD). The brain's multifaceted functions hinge crucially on the intricate connections and communication among distinct regions. However, the intricate interaction of information between brain regions with altered GMV and other regions, and how they synchronize with various neurotransmitter systems, remains enigmatic. Therefore, we aimed to integrate structural, functional and molecular data to explore the GMV-based Granger causal connectivity abnormalities and their correlated neurotransmitter systems in IGD adolescents. Voxel-based morphometry (VBM) analysis was firstly performed to investigate GMV differences between 37 IGD adolescents and 35 matched controls. Brain regions with altered GMV were selected as seeds for further Granger causality analysis (GCA). Two-sample t tests were performed using the SPM12 toolkit to compare the GMV and Granger causal connectivity between IGD and control groups (GRF corrected, Pvoxel<0.005, Pcluster<0.05). Then, GMV-based Granger causal connectivity was spatially correlated with PET- and SPECT-derived maps covering multifarious neurotransmitter systems. Multiple comparison correction was performed using false discovery rate (FDR). Compared with controls, IGD adolescents showed higher GMV in the caudate nucleus and lingual gyrus. For the GCA, IGD adolescents showed higher Granger causal connectivity from insula, putamen, supplementary motor area (SMA) and middle cingulum cortex (MCC) to the caudate nucleus, and lower Granger causal connectivity from superior/inferior parietal gyrus (SPG/IPG) and middle occipital gyrus (MOG) to the lingual gyrus. Besides, GMV-based Granger causal connectivity of IGD adolescents were associated with the dopaminergic, serotonergic, GABAergic and noradrenaline systems. This study revealed that the caudate nucleus and lingual gyrus may be the key sites of neuroanatomical changes in IGD adolescents, and whole-brain Granger causal connectivity abnormalities based on altered GMV involved large brain networks including reward, cognitive control, and visual attention networks, and these abnormalities are associated with a variety of neurotransmitter systems, which may be associated with higher reward sensitivity, cognitive control, and attention control dysfunction.

The unbalanced behavioral activation and inhibition system sensitivity in internet gaming disorder: Evidence from resting-state Granger causal connectivity analysis

BACKGROUND

Patients with behavioral or substance addiction show an unbalanced behavioral activation system (BAS) and behavioral inhibition system (BIS) sensitivity. However, the relationship between internet gaming disorder (IGD) and BAS/BIS is obscure and the neurobiological mechanism underlying this relationship remains unclear.

METHODS

We recruited 154 IGDs and 229 recreational game users (RGUs) in the current study. First, we explored the relationship between BAS/BIS and IGD. Second, subjects were subdivided into subgroups by BAS/BIS sensitivity. Third, whole-brain Granger causal connectivity (GCC) of striatum and amygdala subdivisions was estimated for the subgroup. Fourth, mediation analysis was performed to explore the role of connectivity in the relationship between IGD and BAS/BIS sensitivity.

RESULTS

We found the IGD group scored higher than the RGU on BIS and BASf (fun-seeking) sensitivity. Then, we identified 4 (2*2) subgroups: low/high risk of IGD with low/high BAS/BIS sensitivity groups. Two-way ANCOVA main results of interaction effects showed that in the high BAS/BIS group, the RGU exhibited increased strength in the GCC from the left putamen to the right cuneus, and the IGD exhibited decreased strength in the GCC from the right medial frontal gyrus to the caudate, from the left superior frontal gyrus to the centromedial amygdala, and from the right superior parietal lobule to the left laterobasal amygdala. Moreover, the GCC from the centromedial amygdala to the middle frontal gyrus mediated the directional relationship between BIS and IAT (Young's internet addiction test) scores.

CONCLUSIONS

The IGD individuals exhibited higher BIS and BAS-fun seeking sensitivity. Moreover, IGD with unbalanced BAS/BIS sensitivity exhibited alternative connectivity patterns involving amygdala and striatum subdivisions. These findings suggest a neurobiological mechanism for an alternation between IGD and RGU with different BAS/BIS sensitivity.

New directions in modelling dysregulated reward seeking for food and drugs

Behavioral models are central to behavioral neuroscience. To study the neural mechanisms of maladaptive behaviors (including binge eating and drug addiction), it is essential to develop and utilize appropriate animal models that specifically focus on dysregulated reward seeking. Both food and cocaine are typically consumed in a regulated manner by rodents, motivated by reward and homeostatic mechanisms. However, both food and cocaine seeking can become dysregulated, resulting in binge-like consumption and compulsive patterns of intake. The speakers in this symposium for the 2021 International Behavioral Neuroscience Meeting utilize behavioral models of dysregulated reward-seeking to investigate the neural mechanisms of binge-like consumption, enhanced cue-driven reward seeking, excessive motivation, and continued use despite negative consequences. In this review, we outline examples of maladaptive patterns of intake and explore recent animal models that drive behavior to become dysregulated, including stress exposure and intermittent access to rewards. Lastly, we explore select behavioral and neural mechanisms underlying dysregulated reward-seeking for both food and drugs.

Neurochemical substrates linked to impulsive and compulsive phenotypes in addiction: A preclinical perspective

Drug compulsion manifests in some but not all individuals and implicates multifaceted processes including failures in top-down cognitive control as drivers for the hazardous pursuit of drug use in some individuals. As a closely related construct, impulsivity encompasses rash or risky behaviour without foresight and underlies most forms of drug taking behaviour, including drug use during adverse emotional states (i.e., negative urgency). While impulsive behavioural dimensions emerge from drug-induced brain plasticity, burgeoning evidence suggests that impulsivity also predates the emergence of compulsive drug use. Although the neural substrates underlying the apparently causal relationship between trait impulsivity and drug compulsion are poorly understood, significant advances have come from the interrogation of defined limbic cortico-striatal circuits involved in motivated behaviour and response inhibition, together with chemical neuromodulatory influences from the ascending neurotransmitter systems. We review what is presently known about the neurochemical mediation of impulsivity, in its various forms, and ask whether commonalities exist in the neurochemistry of compulsive drug-motivated behaviours that might explain individual risk for addiction.

Incentive motivation: 'wanting' roles of central amygdala circuitry

The central nucleus of amygdala (CeA) mediates positively-valenced reward motivation as well as negatively-valenced fear. Optogenetic or neurochemical stimulation of CeA circuitry can generate intense incentive motivation to pursue and consume a paired natural food, sex, or addictive drug reward, and even create maladaptive 'wanting what hurts' such as attraction to a shock rod. Evidence indicates CeA stimulations selectively amplify incentive motivation ('wanting') but not hedonic impact ('liking') of the same reward. Further, valence flips can occur for CeA contributions to motivational salience. That is, CeA stimulation can promote either incentive motivation or fearful motivation, even in the same individual, depending on situation. These findings may carry implications for understanding CeA roles in neuropsychiatric disorders involving aberrant motivational salience, ranging from addiction to paranoia and anxiety disorders.

Compulsivity in Alcohol Use Disorder and Obsessive Compulsive Disorder: Implications for Neuromodulation

Alcohol use Disorder (AUD) is one of the leading causes of morbidity and mortality worldwide. The progression of the disorder is associated with the development of compulsive alcohol use, which in turn contributes to the high relapse rate and poor longer term functioning reported in most patients, even with treatment. While the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) defines AUD by a cluster of symptoms, parsing its heterogeneous phenotype by domains of behavior such as compulsivity may be a critical step to improve outcomes of this condition. Still, neurobiological underpinnings of compulsivity need to be fully elucidated in AUD in order to better design targeted treatment strategies. In this manuscript, we review and discuss findings supporting common mechanisms between AUD and OCD, dissecting the construct of compulsivity and focusing specifically on characteristic disruptions in habit learning and cognitive control in the two disorders. Finally, neuromodulatory interventions are proposed as a probe to test compulsivity as key pathophysiologic feature of AUD, and as a potential therapy for the subgroup of individuals with compulsive alcohol use, i.e., the more resistant stage of the disorder. This transdiagnostic approach may help to destigmatize the disorder, and suggest potential treatment targets across different conditions.

The Molecular Basis of Drug Addiction: Linking Epigenetic to Synaptic and Circuit Mechanisms

Addiction is a disease in which, after a period of recreational use, a subset of individuals develops compulsive use that does not stop even in light of major negative consequences. Here, we review the evidence for underlying epigenetic remodeling in brain in two settings. First, excessive dopamine signaling during drug use may modulate gene expression, altering synaptic function and circuit activity and leading over time to maladaptive behaviors in vulnerable individuals. Second, on a longer timescale, life experience can shape the epigenetic landscape in brain and thereby may contribute to an individual's vulnerability by amplifying drug-induced changes in gene expression that drive the transition to addiction. We conclude by exploring how epigenetic mechanisms might serve as therapeutic targets for addiction treatments.

Stochastic synaptic plasticity underlying compulsion in a model of addiction

Activation of the mesolimbic dopamine system reinforces goal-directed behaviours. With repetitive stimulation-for example, by chronic drug abuse-the reinforcement may become compulsive and intake continues even in the face of major negative consequences. Here we gave mice the opportunity to optogenetically self-stimulate dopaminergic neurons and observed that only a fraction of mice persevered if they had to endure an electric shock. Compulsive lever pressing was associated with an activity peak in the projection terminals from the orbitofrontal cortex (OFC) to the dorsal striatum. Although brief inhibition of OFC neurons temporarily relieved compulsive reinforcement, we found that transmission from the OFC to the striatum was permanently potentiated in persevering mice. To establish causality, we potentiated these synapses in vivo in mice that stopped optogenetic self-stimulation of dopamine neurons because of punishment; this led to compulsive lever pressing, whereas depotentiation in persevering mice had the converse effect. In summary, synaptic potentiation of transmission from the OFC to the dorsal striatum drives compulsive reinforcement, a defining symptom of addiction.

Nicotine-induced CREB and DeltaFosB activity is modified by caffeine in the brain reward system of the rat

Coffee and nicotine consumption are frequently combined, indicating possible intensifying effect of caffeine on smoking behavior, although neurobiological background of this phenomenon remains unknown. We aimed at determining the effect of caffeine and nicotine, applied separately or simultaneously, on activation of six structures of the brain reward system: nucleus accumbens (NAc), ventral tegmental area (VTA), amygdala (Amg), hippocampus (Hip), medial prefrontal cortex (mPfr) and dorsal striatum (CdP) in the adult male Wistar rats. Activation of two transcription factors, the phosphorylated form of cyclic AMP-response element binding protein (pCREB) and DeltaFosB (ΔFosB) was assessed by immunohistochemistry after multiple-dose five-days psychostimulants administration followed by 20min and 24h survival, respectively. Nicotine evoked the highest increase of pCREB-immunoreactivity (-ir) in NAc, while caffeine exerted the weakest effect in mPfr and CdP. Nicotine/caffeine co-administration resulted in decrease of pCREB-ir in NAc and increase in Amg, compared with the effect of each psychostimulant used separately. Nicotine was the strongest psychostimulant activating ΔFosB-ir in Amg, whereas caffeine - in Hip. Nicotine/caffeine-exerted effect upon ΔFosB-ir in Amg was weaker, whereas in mPfr stronger, than nicotine-evoked effect in these structures. In summary, pCREB and ΔFosB activation is dependent on the type of stimulus, brain structure and functional context. Activation of both transcription factors is responsible for caffeine's modifying effect upon nicotine-related behaviors and must be taken into account while quitting cigarette smoking.

Free will in addictive behaviors: A matter of definition

Certain people are at risk for using alcohol or other drugs excessively and for developing problems with their use. Their susceptibility might arise from a variety of factors, including their genetic make-up, brain chemistry, family background, personality and other psychological variables, and environmental and sociocultural variables. Moreover, after substance use has become established, there are additional cognitive-motivational variables (e.g., substance-related attentional bias) that contribute to enacting behaviors consistent with the person's motivation to acquire and use the substance. People who are at such risk are likely to choose to use addictive substances even though doing so entails negative consequences. In the sense of complete freedom from being determined by causal factors, we believe that there is no such thing as free will, but defined as ability to make choices from among multiple options, even though the choices are ultimately governed by natural processes, addicted individuals are free to choose. Although they might appear unable to exercise this kind of free will in decisions about their substance use, addictive behaviors are ultimately always goal-directed and voluntary. Such goal pursuits manifest considerable flexibility. Even some severely addicted individuals can cease their use when the value of continuing the use abruptly declines or when the subjective cost of continuing the use is too great with respect to the incentives in other areas of their lives. Formal treatment strategies (e.g., contingency management, Systematic Motivational Counseling, cognitive training) can also be used to facilitate this reversal.

Gray matter deficits and altered resting-state connectivity in the superior temporal gyrus among individuals with problematic hypersexual behavior

Neuroimaging studies on the characteristics of hypersexual disorder have been accumulating, yet alternations in brain structures and functional connectivity in individuals with problematic hypersexual behavior (PHB) has only recently been studied. This study aimed to investigate gray matter deficits and resting-state abnormalities in individuals with PHB using voxel-based morphometry and resting-state connectivity analysis. Seventeen individuals with PHB and 19 age-matched healthy controls participated in this study. Gray matter volume of the brain and resting-state connectivity were measured using 3T magnetic resonance imaging. Compared to healthy subjects, individuals with PHB had significant reductions in gray matter volume in the left superior temporal gyrus (STG) and right middle temporal gyrus. Individuals with PHB also exhibited a decrease in resting-state functional connectivity between the left STG and left precuneus and between the left STG and right caudate. The gray matter volume of the left STG and its resting-state functional connectivity with the right caudate both showed significant negative correlations with the severity of PHB. The findings suggest that structural deficits and resting-state functional impairments in the left STG might be linked to PHB and provide new insights into the underlying neural mechanisms of PHB.

Altered Gray Matter Volume and Resting-State Connectivity in Individuals With Internet Gaming Disorder: A Voxel-Based Morphometry and Resting-State Functional Magnetic Resonance Imaging Study

Neuroimaging studies on the characteristics of individuals with Internet gaming disorder (IGD) have been accumulating due to growing concerns regarding the psychological and social problems associated with Internet use. However, relatively little is known about the brain characteristics underlying IGD, such as the associated functional connectivity and structure. The aim of this study was to investigate alterations in gray matter (GM) volume and functional connectivity during resting state in individuals with IGD using voxel-based morphometry and a resting-state connectivity analysis. The participants included 20 individuals with IGD and 20 age- and sex-matched healthy controls. Resting-state functional and structural images were acquired for all participants using 3 T magnetic resonance imaging. We also measured the severity of IGD and impulsivity using psychological scales. The results show that IGD severity was positively correlated with GM volume in the left caudate (p < 0.05, corrected for multiple comparisons), and negatively associated with functional connectivity between the left caudate and the right middle frontal gyrus (p < 0.05, corrected for multiple comparisons). This study demonstrates that IGD is associated with neuroanatomical changes in the right middle frontal cortex and the left caudate. These are important brain regions for reward and cognitive control processes, and structural and functional abnormalities in these regions have been reported for other addictions, such as substance abuse and pathological gambling. The findings suggest that structural deficits and resting-state functional impairments in the frontostriatal network may be associated with IGD and provide new insights into the underlying neural mechanisms of IGD.

Difference in the functional connectivity of the dorsolateral prefrontal cortex between smokers with nicotine dependence and individuals with internet gaming disorder

Background

It has been reported that internet gaming disorder (IGD) and smokers with nicotine dependence (SND) share clinical characteristics, such as over-engagement despite negative consequences and cravings. This study is to investigate the alterations in the resting-state functional connectivity (rsFC) of the dorsolateral prefrontal cortex (DLPFC) observed in SND and IGD. In this study, 27 IGD, 29 SND, and 33 healthy controls (HC) underwent a resting-state functional magnetic resonance imaging (rs-fMRI) scan. DLPFC connectivity was determined in all participates by investigating the synchronized low-frequency fMRI signal fluctuations using a temporal seed-based correlation method.

Results

Compared with the HC group, the IGD and SND groups showed decreased rsFC with DLPFC in the right insula and left inferior frontal gyrus with DLPFC. Compared with SND group, the IGD subjects exhibited increased rsFC in the left inferior temporal gyrus and right inferior orbital frontal gyrus and decreased rsFC in the right middle occipital gyrus, supramarginal gyrus, and cuneus with DLPFC.

Conclusion

Our results confirmed that SND and IGD share similar neural mechanisms related to craving and impulsive inhibitions. The significant difference in rsFC with DLPFC between the IGD and SND subjects may be attributed to the visual and auditory stimulation generated by long-term internet gaming.

How methamphetamine exposure during different neurodevelopmental stages affects social behavior of adult rats?

Social behavior involves complex of different forms of interactions between individuals that is essential for healthy mental and physical development throughout lifespan. Psychostimulants, including methamphetamine (MA), have neurotoxic effect, especially, if they are targeting CNS during its critical periods of development. The present study was aimed on evaluation of changes in social interactions (SI) following scheduled prenatal/neonatal MA treatment in combination with acute application in adulthood. Eight groups of male and eight groups of female rats were tested in adulthood: rats, whose mothers were exposed to MA (5mg/ml/kg) or saline (SA, 1ml/kg) during the first half of gestation (ED 1-11), the second half of gestation (ED 12-22) and neonatal period (PD 1-11). To do this, we compared indirect neonatal applications via the exposed dams with group of rat pups that received MA or SA directly through injections. In adulthood, half animals from each group were injected with MA (1mg/kg), second half with saline 45min prior to the Social Interaction Test. Females and males were observed for social and nonsocial activities of two unfamiliar individuals of the same sex and treatment in a familiar Open field arena. The present study demonstrated that prenatal/neonatal MA exposure leads to decrease the time spent in genital investigation, following and nonsocial activity. Acute dose of MA leads to a decrease in all SI patterns and to an increase in nonsocial activities relative to acute SA. Females were more active than males. Animals exposed to prenatal/neonatal treatment during the second half of gestation (ED 12-22) and throughout lactation period (PD 1-11 indirect/direct) had fewer SI and greater exploratory behavior than animals exposed during the first half of gestation (ED 1-11).

Granger causality reveals a dominant role of memory circuit in chronic opioid dependence

Resting-state magnetic resonance imaging has uncovered abnormal functional connectivity in heroin-dependent individuals (HDIs). However, it remains unclear how brain regions implicated in addictions are related in baseline state without conditioned cues in heroin dependent individuals during opioid maintenance treatment (HDIs-OMT). Previous connectivity analysis assessed the strength of correlated activity between brain regions but lacked the ability to infer directional neural interactions. In the current study, we employed Granger causality analysis to investigate directional causal influences among the brain circuits in HDIs-OMT and non-opioid users. The results revealed a weaker effective connectivity between the caudate nucleus implicated in mediating the reward circuit and other brain regions and also a weaker connectivity between the anterior cingulate cortex and medial prefrontal cortex implicated in mediating inhibitory control. Conversely, HDIs-OMT exhibited stronger effective connectivity between the hippocampus and amygdala implicated in mediating learning-memory, and the anterior cingulate cortex involved in mediating inhibitory control while the putamen mediated learned habits, suggesting that the hippocampus and amygdala may propel the memory circuit to override the control circuit and drive the learned habit in HDIs-OMT. Alterations in learning-memory and inhibitory control may contribute jointly and form a basis for relapse risk even after a period of heroin abstinence. Sustained neural effect of opioid dependence on methadone maintenance including hyperactivation in the memory circuit and impairment in the control circuit support the role of the memory circuitry in relapse and may help redefine targets for treatment.

Lewis and Fischer 344 rats as a model for genetic differences in spatial learning and memory: Cocaine effects

Lewis (LEW) and Fischer 344 (F344) rats are considered a model of genetic vulnerability to drug addiction. We previously showed important differences in spatial learning and memory between them, but in contrast with previous experiments demonstrating cocaine-induced enhanced learning in Morris water maze (MWM) highly demanding tasks, the eight-arm radial maze (RAM) performance was not modified either in LEW or F344 rats after chronic cocaine treatment. In the present work, chronically cocaine-treated LEW and F344 adult rats have been evaluated in learning and memory performance using the Y-maze, two RAM protocols that differ in difficulty, and a reversal protocol that tests cognitive flexibility. After one of the RAM protocols, we quantified dendritic spine density in hippocampal CA1 neurons and compared it to animals treated with cocaine but not submitted to RAM. LEW cocaine treated rats showed a better performance in the Y maze than their saline counterparts, an effect that was not evident in the F344 strain. F344 rats significantly took more time to learn the RAM task and made a greater number of errors than LEW animals in both protocols tested, whereas cocaine treatment induced deleterious effects in learning and memory in the highly difficult protocol. Moreover, hippocampal spine density was cocaine-modulated in LEW animals whereas no effects were found in F344 rats. We propose that differences in addictive-like behavior between LEW and F344 rats could be related to differences in hippocampal learning and memory processes that could be on the basis of individual vulnerability to cocaine addiction.

Cocaine cue-induced dopamine release in the human prefrontal cortex

BACKGROUND

Accumulating evidence indicates that drug-related cues can induce dopamine (DA) release in the striatum of substance abusers. Whether these same cues provoke DA release in the human prefrontal cortex remains unknown.

METHODS

We used high-resolution positron emission tomography with [18F]fallypride to measure cortical and striatal DA D2/3 receptor availability in the presence versus absence of drug-related cues in volunteers with current cocaine dependence.

RESULTS

Twelve individuals participated in our study. Among participants reporting a craving response (9 of 12), exposure to the cocaine cues significantly decreased [18F]fallypride binding potential (BPND) values in the medial orbitofrontal cortex and striatum. In all 12 participants, individual differences in the magnitude of craving correlated with BPND changes in the medial orbitofrontal cortex, dorsolateral prefrontal cortex, anterior cingulate, and striatum. Consistent with the presence of autoreceptors on mesostriatal but not mesocortical DA cell bodies, midbrain BPND values were significantly correlated with changes in BPND within the striatum but not the cortex. The lower the midbrain D2 receptor levels, the greater the striatal change in BPND and self-reported craving.

LIMITATIONS

Limitations of this study include its modest sample size, with only 2 female participants. Newer tracers might have greater sensitivity to cortical DA release.

CONCLUSION

In people with cocaine use disorders, the presentation of drug-related cues induces DA release within cortical and striatal regions. Both effects are associated with craving, but only the latter is regulated by midbrain autoreceptors. Together, the results suggest that cortical and subcortical DA responses might both influence drug-focused incentive motivational states, but with separate regulatory mechanisms.

Neural Substrates of Sexual Desire in Individuals with Problematic Hypersexual Behavior

Studies on the characteristics of individuals with hypersexual disorder have been accumulating due to increasing concerns about problematic hypersexual behavior (PHB). Currently, relatively little is known about the underlying behavioral and neural mechanisms of sexual desire. Our study aimed to investigate the neural correlates of sexual desire with event-related functional magnetic resonance imaging (fMRI). Twenty-three individuals with PHB and 22 age-matched healthy controls were scanned while they passively viewed sexual and nonsexual stimuli. The subjects' levels of sexual desire were assessed in response to each sexual stimulus. Relative to controls, individuals with PHB experienced more frequent and enhanced sexual desire during exposure to sexual stimuli. Greater activation was observed in the caudate nucleus, inferior parietal lobe, dorsal anterior cingulate gyrus, thalamus, and dorsolateral prefrontal cortex in the PHB group than in the control group. In addition, the hemodynamic patterns in the activated areas differed between the groups. Consistent with the findings of brain imaging studies of substance and behavior addiction, individuals with the behavioral characteristics of PHB and enhanced desire exhibited altered activation in the prefrontal cortex and subcortical regions. In conclusion, our results will help to characterize the behaviors and associated neural mechanisms of individuals with PHB.

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

Adaptation of the nervous system to different chemical and physiologic conditions is important for the homeostasis of brain processes and for learning and remembering appropriate responses to challenges. Although processes such as tolerance and dependence to various drugs of abuse have been known for a long time, it was recently discovered that even a single pharmacologically relevant dose of various drugs of abuse induces neuroplasticity in selected neuronal populations, such as the dopamine neurons of the ventral tegmental area, which persist long after the drug has been excreted. Prolonged (self-) administration of drugs induces gene expression, neurochemical, neurophysiological, and structural changes in many brain cell populations. These region-specific changes correlate with addiction, drug intake, and conditioned drugs effects, such as cue- or stress-induced reinstatement of drug seeking. In rodents, adolescent drug exposure often causes significantly more behavioral changes later in adulthood than a corresponding exposure in adults. Clinically the most impairing and devastating effects on the brain are produced by alcohol during fetal development. In adult recreational drug users or in medicated patients, it has been difficult to find persistent functional or behavioral changes, suggesting that heavy exposure to drugs of abuse is needed for neurotoxicity and for persistent emotional and cognitive alterations. This review describes recent advances in this important area of research, which harbors the aim of translating this knowledge to better treatments for addictions and related neuropsychiatric illnesses.

Context-induced reinstatement of methamphetamine seeking is associated with unique molecular alterations in Fos-expressing dorsolateral striatum neurons

Context-induced reinstatement of drug seeking is a well established animal model for assessing the neural mechanisms underlying context-induced drug relapse, a major factor in human drug addiction. Neural activity in striatum has previously been shown to contribute to context-induced reinstatement of heroin, cocaine, and alcohol seeking, but not yet for methamphetamine seeking. In this study, we found that context-induced reinstatement of methamphetamine seeking increased expression of the neural activity marker Fos in dorsal but not ventral striatum. Reversible inactivation of neural activity in dorsolateral but not dorsomedial striatum using the GABA agonists muscimol and baclofen decreased context-induced reinstatement. Based on our previous findings that Fos-expressing neurons play a critical role in conditioned drug effects, we assessed whether context-induced reinstatement was associated with molecular alterations selectively induced within context-activated Fos-expressing neurons. We used fluorescence-activated cell sorting to isolate reinstatement-activated Fos-positive neurons from Fos-negative neurons in dorsal striatum and used quantitative PCR to assess gene expression within these two populations of neurons. Context-induced reinstatement was associated with increased expression of the immediate early genes Fos and FosB and the NMDA receptor subunit gene Grin2a in only Fos-positive neurons. RNAscope in situ hybridization confirmed that Grin2a, as well as Grin2b, expression were increased in only Fos-positive neurons from dorsolateral, but not dorsomedial, striatum. Our results demonstrate an important role of dorsolateral striatum in context-induced reinstatement of methamphetamine seeking and that this reinstatement is associated with unique gene alterations in Fos-expressing neurons.

Different resting-state functional connectivity alterations in smokers and nonsmokers with Internet gaming addiction

This study investigated changes in resting-state functional connectivity (rsFC) of posterior cingulate cortex (PCC) in smokers and nonsmokers with Internet gaming addiction (IGA). Twenty-nine smokers with IGA, 22 nonsmokers with IGA, and 30 healthy controls (HC group) underwent a resting-state fMRI scan. PCC connectivity was determined in all subjects by investigating synchronized low-frequency fMRI signal fluctuations using a temporal correlation method. Compared with the nonsmokers with IGA, the smokers with IGA exhibited decreased rsFC with PCC in the right rectus gyrus. Left middle frontal gyrus exhibited increased rsFC. The PCC connectivity with the right rectus gyrus was found to be negatively correlated with the CIAS scores in the smokers with IGA before correction. Our results suggested that smokers with IGA had functional changes in brain areas related to motivation and executive function compared with the nonsmokers with IGA.

Habitual responding for alcohol depends upon both AMPA and D2 receptor signaling in the dorsolateral striatum

Chronic alcohol self-administration leads to alcohol-seeking behavior that is habitual and insensitive to changes in the value of the earned alcohol. Such behavior has been shown to rely on the dorsolateral region of the striatum in rats but the specific pharmacological control of output from this region is not yet understood. In the following experiments rats were trained to self-administer unsweetened 10% (v/v) ethanol in daily sessions for 8 weeks prior to testing for sensitivity to outcome devaluation. We examined the role of glutamatergic AMPA-receptor activation by testing the effects of the antagonist NBQX (0.3 and 1.0 μg/μl) infused specifically into the dorsolateral striatum (DLS) before devaluation testing. In a separate group of rats we examined the role of dopaminergic D2-receptor activation using the D2-receptor antagonist raclopride (0.2 and 1.0 μg/μl) infused into the DLS before devaluation testing. Following control (saline) infusions rats’ lever-press performance was insensitive to devaluation of ethanol thus showing evidence of habitual responding. NBQX and racolpride each restored goal-directed control of responding at doses that did not impair overall lever-press rates. These data demonstrate that expression of habitual alcohol seeking relies on glutamatergic inputs to the DLS and D2 receptors within the DLS.

Involvement of the brain renin-angiotensin system (RAS) in the neuroadaptive responses induced by amphetamine in a two-injection protocol

A single or repeated exposure to psychostimulants induces long-lasting neuroadaptative changes. Different neurotransmitter systems are involved in these responses including the neuropeptide angiotensin II. Our study tested the hypothesis that the neuroadaptative changes induced by amphetamine produce alterations in brain RAS components that are involved in the expression of the locomotor sensitization to the psychostimulant drug. Wistar male rats, pretreated with amphetamine were used 7 or 21 days later to study AT1 receptors by immunohistochemistry and western blot and also angiotensinogen mRNA and protein in caudate putamen and nucleus accumbens. A second group of animals was used to explore the possible role of Ang II AT1 receptors in the expression of behavioral sensitization. In these animals treated in the same way, bearing intra-cerebral cannula, the locomotor activity was tested 21 days later, after an amphetamine challenge injection and the animals received an AT1 blocker, losartan, or saline 5min before the amphetamine challenge. An increase of AT1 receptor density induced by amphetamine was found in both studied areas and a decrease in angiotensinogen mRNA and protein only in CPu at 21 days after treatment; meanwhile, no changes were established in NAcc. Finally, the increased locomotor activity induced by amphetamine challenge was blunted by losartan administration in CPu. No differences were detected in the behavioral sensitization when the AT1 blocker was injected in NAcc. Our results support the hypothesis of a key role of brain RAS in the neuroadaptative changes induced by amphetamine.

μ-Opioid receptors within subregions of the striatum mediate pair bond formation through parallel yet distinct reward mechanisms

The prairie vole is a socially monogamous rodent that is an excellent animal model for studies of the neurobiology of social attachment. Such studies have demonstrated that activation of reward circuitry during social interactions facilitates pair bond formation. Within this circuitry, μ-opioid receptors (MORs) modulate naturally rewarding behavior in an anatomically segregated manner; MORs located throughout the striatum (dorsal striatum, NAc core, and the entire NAc shell) are implicated in general motivational processes, whereas those located specifically within the dorsomedial NAc shell mediate positive hedonics (and are referred to as a "hedonic hotspot"). The purpose of the present study was to determine whether MORs within these distinct subregions differentially mediate pair bond formation. We first used receptor autoradiography to compare MOR binding densities between these regions. MOR binding was significantly higher in the NAc core and dorsomedial NAc shell compared with the ventral NAc shell. We next used partner preference testing to determine whether MORs within these subregions differentially mediate pair bonding. Blockade of MORs using 1 or 3 μg of H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 within the dorsal striatum decreased mating during the cohabitation period and inhibited partner preference formation. In contrast, blockade of MORs within dorsomedial NAc shell inhibited partner preference formation without effecting mating behavior, whereas other regions were not involved. Thus, MORs within the dorsal striatum mediate partner preference formation via impairment of mating, whereas those in the dorsomedial NAc shell appear to mediate pair bond formation through the positive hedonics associated with mating.

The neurobiological underpinnings of obesity and binge eating: a rationale for adopting the food addiction model

The food addiction model of overeating has been proposed to help explain the widespread advancement of obesity over the last 30 years. Parallels in neural substrates and neurochemistry, as well as corresponding motivational and behavioral traits, are increasingly coming to light; however, there are still key differences between the two disorders that must be acknowledged. We critically examine these common and divergent characteristics using the theoretical framework of prominent drug addiction models, investigating the neurobiological underpinnings of both behaviors in an attempt to justify whether classification of obesity and binge eating as an addictive disorder is merited.

Animal studies of addictive behavior

It is increasingly recognized that studying drug taking in laboratory animals does not equate to studying genuine addiction, characterized by loss of control over drug use. This has inspired recent work aimed at capturing genuine addiction-like behavior in animals. In this work, we summarize empirical evidence for the occurrence of several DSM-IV-like symptoms of addiction in animals after extended drug use. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, increased motivation for drugs, preference for drugs over nondrug rewards, and resistance to punishment. The fact that addiction-like behavior can occur and be studied in animals gives us the exciting opportunity to investigate the neural and genetic background of drug addiction, which we hope will ultimately lead to the development of more effective treatments for this devastating disorder.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

fMRI response in the medial prefrontal cortex predicts cocaine but not sucrose self-administration history

Repeated cocaine exposure induces long-lasting neuroadaptations that alter subsequent responsiveness to the drug. However, systems-level investigation of these neuroplastic consequences is limited. We employed a rodent model of drug addiction to investigate neuroadaptations associated with prolonged forced abstinence after long-term cocaine self-administration (SA). Since natural rewards also activate the mesolimbic reward system in a partially overlapping fashion as cocaine, our design also included a sucrose SA group. Rats were trained to self-administer cocaine or sucrose using a fixed-ratio one, long-access schedule (6 h/day for 20 days). A third group of naïve, sedentary rats served as a negative control. After 30 days of abstinence, the reactivity of the reward system was assessed with functional magnetic resonance imaging (fMRI) following an intravenous cocaine injection challenge. A strong positive fMRI response, as measured by fractional cerebral blood volume changes relative to baseline (CBV%), was seen in the sedentary control group in such cortico-limbic regions as medial prefrontal cortex and anterior cingulate cortex. In contrast, both the cocaine and sucrose SA groups demonstrated a very similar initial negative fMRI response followed by an attenuated positive response. The magnitude of the mPFC response was significantly correlated with the total amount of reinforcer intake during the training sessions for the cocaine SA but not for the sucrose SA group. Given that the two SA groups had identical histories of operant training and handling, this region-specific group difference revealed by regression analysis may reflect the development of neuroadaptive mechanisms specifically related to the emergence of addiction-like behavior that occurs only in cocaine SA animals.

Habitual alcohol seeking: time course and the contribution of subregions of the dorsal striatum

BACKGROUND

Addictions are defined by a loss of flexible control over behavior. The development of response habits might reflect early changes in behavioral control. The following experiments examined the flexibility of alcohol-seeking after different durations of self-administration training and tested the role of the dorsal striatum in the control of flexible and habitual alcohol self-administration.

METHODS

Rats were trained to lever-press to earn unsweetened ethanol (EtOH) (10%). The sensitivity of the lever-press response to devaluation was assessed by prefeeding the rats either EtOH or sucrose before an extinction test after different amounts of training (1, 2, 4, and 8 weeks). We subsequently tested the role of the dorsomedial striatum (DMS) and dorsolateral striatum (DLS) in controlling alcohol seeking with reversible inactivation techniques (baclofen/muscimol: 1.0/.1 mmol/L, .3 μL/side).

RESULTS

We find that operant responding for EtOH early in training is goal-directed and reduced by devaluation, but after 8 weeks of daily operant training, control has shifted to a habit-based system no longer sensitive to devaluation. Furthermore, after relatively limited training, when responding is sensitive to devaluation, inactivation of the DMS greatly attenuates the alcohol-seeking response, whereas inactivation of the DLS is without effect. In contrast, responding that is insensitive to devaluation after 8 weeks of training becomes sensitive to devaluation after inactivation of the DLS but is unaffected by inactivation of the DMS.

CONCLUSIONS

These experiments demonstrate that extended alcohol self-administration produces habit-like responding and that response control shifts from the DMS to the DLS across the course of training.

Amygdala 14-3-3ζ as a novel modulator of escalating alcohol intake in mice

Alcoholism is a devastating brain disorder that affects millions of people worldwide. The development of alcoholism is caused by alcohol-induced maladaptive changes in neural circuits involved in emotions, motivation, and decision-making. Because of its involvement in these processes, the amygdala is thought to be a key neural structure involved in alcohol addiction. However, the molecular mechanisms that govern the development of alcoholism are incompletely understood. We have previously shown that in a limited access choice paradigm, C57BL/6J mice progressively escalate their alcohol intake and display important behavioral characteristic of alcohol addiction, in that they become insensitive to quinine-induced adulteration of alcohol. This study used the limited access choice paradigm to study gene expression changes in the amygdala during the escalation to high alcohol consumption in C57BL/6J mice. Microarray analysis revealed that changes in gene expression occurred predominantly after one week, i.e. during the initial escalation of alcohol intake. One gene that stood out from our analysis was the adapter protein 14-3-3ζ, which was up-regulated during the transition from low to high alcohol intake. Independent qPCR analysis confirmed the up-regulation of amygdala 14-3-3ζ during the escalation of alcohol intake. Subsequently, we found that local knockdown of 14-3-3ζ in the amygdala, using RNA interference, dramatically augmented alcohol intake. In addition, knockdown of amygdala 14-3-3ζ promoted the development of inflexible alcohol drinking, as apparent from insensitivity to quinine adulteration of alcohol. This study identifies amygdala 14-3-3ζ as a novel key modulator that is engaged during escalation of alcohol use.

Feed-forward mechanisms: addiction-like behavioral and molecular adaptations in overeating

Food reward, not hunger, is the main driving force behind eating in the modern obesogenic environment. Palatable foods, generally calorie-dense and rich in sugar/fat, are thus readily overconsumed despite the resulting health consequences. Important advances have been made to explain mechanisms underlying excessive consumption as an immediate response to presentation of rewarding tastants. However, our understanding of long-term neural adaptations to food reward that oftentimes persist during even a prolonged absence of palatable food and contribute to the reinstatement of compulsive overeating of high-fat high-sugar diets, is much more limited. Here we discuss the evidence from animal and human studies for neural and molecular adaptations in both homeostatic and non-homeostatic appetite regulation that may underlie the formation of a "feed-forward" system, sensitive to palatable food and propelling the individual from a basic preference for palatable diets to food craving and compulsive, addiction-like eating behavior.

Conditioned response evoked by nicotine conditioned stimulus preferentially induces c-Fos expression in medial regions of caudate-putamen

Nicotine has both unconditioned and conditioned stimulus properties. Conditioned stimulus properties of nicotine may contribute to the tenacity of nicotine addiction. The purpose of this experiment was to use neurohistochemical analysis of rapidly developing c-Fos protein to elucidate neurobiological loci involved in the processing of nicotine as an interoceptive conditioned stimulus (CS). Rats were injected (SC) in an intermixed fashion with saline or nicotine (16 sessions of each) and placed in conditioning chambers where they were given one of the three conditions depending on group assignment: (a) nicotine paired 100% of the time with intermittent access to sucrose (nicotine-CS condition), (b) nicotine and saline each paired 50% of the time with sucrose (chamber-CS condition), or (c) no sucrose US control (CS-alone condition). Rats in the nicotine-CS condition acquired the discrimination as evidenced by goal-tracking (ie, increased dipper entries before initial sucrose delivery) only on nicotine sessions. The chamber-CS condition showed goal-tracking on all sessions; no goal-tracking was seen in the CS-alone condition. On the test day, rats in each condition were challenged with saline or nicotine and later assessed for c-Fos immunoreactivity. In concordance with previous reports, nicotine induced c-Fos expression in the majority of areas tested; however, learning-dependent expression was specific to dorsomedial and ventromedial regions of caudate-putamen (dmCPu, vmCPu). Only rats in the nicotine-CS condition, when challenged with nicotine, had higher c-Fos expression in the dmCPu and vmCPu. These results suggest that medial areas of CPu involved in excitatory conditioning with an appetitive nicotine CS.

The glial cell modulators, ibudilast and its amino analog, AV1013, attenuate methamphetamine locomotor activity and its sensitization in mice

Over 800,000 Americans abuse the psychomotor stimulant, methamphetamine, yet its abuse is without an approved medication. Methamphetamine induces hypermotor activity, and sensitization to this effect is suggested to represent aspects of the addiction process. Methamphetamine's regulation of 3'-5'-cyclic adenosine monophosphate (cAMP) levels may be partially responsible for its behavioral effects, and compounds that inhibit phosphodiesterase (PDE), the enzyme that degrades cAMP, can alter methamphetamine-induced behaviors. Methamphetamine also activates glial cells and causes a subsequent increase in pro-inflammatory cytokine levels. Modulation of glial cell activation is associated with changes in behavioral responses, and substances that oppose inflammatory activity can attenuate drug-induced behaviors. Ibudilast (aka AV411; 3-isobutyryl-2-isopropylpyrazolo-[1,5-a]pyridine), inhibits both PDE and glial pro-inflammatory activity. Ibudilast's amino analog, AV1013, modulates similar glial targets but negligibly inhibits PDE. The present study determined whether ibudilast and AV1013 would attenuate methamphetamine-induced locomotor activity and its sensitization in C57BL/6J mice. Mice were treated b.i.d. with ibudilast (1.8-13 mg/kg), AV1013 (10-56 mg/kg) or their vehicles intraperitoneally for 7 days, beginning 48 h before 5 days of daily 1-h locomotor activity tests. Each test was initiated by either a methamphetamine (3 mg/kg) or a saline injection. Ibudilast significantly (P<0.05) reduced the acute, chronic, and sensitization effects of methamphetamine's locomotor activity without significantly affecting activity by itself. AV1013 had similar anti-methamphetamine effects, suggesting that glial cell activity, by itself, can modulate methamphetamine's effects and perhaps serve as a medication target for its abuse.

The persistence of maladaptive memory: addiction, drug memories and anti-relapse treatments

Addiction is a chronic, relapsing disorder, characterised by the long-term propensity of addicted individuals to relapse. A major factor that obstructs the attainment of abstinence is the persistence of maladaptive drug-associated memories, which can maintain drug-seeking and taking behaviour and promote unconscious relapse of these habits. Thus, addiction can be conceptualised as a disorder of aberrant learning of the formation of strong instrumental memories linking actions to drug-seeking and taking outcomes that ultimately are expressed as persistent stimulus-response habits; of previously neutral environmental stimuli that become associated with drug highs (and/or withdrawal states) through pavlovian conditioning, and of the subsequent interactions between pavlovian and instrumental memories to influence relapse behaviour. Understanding the psychological, neurobiological and molecular basis of these drug memories may produce new methods of pro-abstinence, anti-relapse treatments for addiction.

Loss in risk-taking: absence of optimal gain or reduction in one's own resources?

Determining how living beings react to tasks that reflect realistic situations of risk has given rise to a vast literature. However, I argue that the methodologies traditionally used to test humans and nonhumans relative to risk often fail to achieve their goal. When risk is modelled in laboratory, potential decision cost (or potential loss) typically denotes an absence of optimal gain. In contrast, when risk occurs in real-life situations, potential loss denotes the reduction in an individual's limited resources - whether energetic, social, financial, etc. This conceptual difference about the nature of risk may have important implications for the understanding of the parameters that control risk-taking behaviour.

Distinct contributions of dopamine in the dorsolateral striatum and nucleus accumbens shell to the reinforcing properties of cocaine

Dopaminergic neurotransmission in the dorsal and ventral striatum is thought to be involved in distinct aspects of cocaine addiction. Ventral striatal dopamine mediates the acute reinforcing properties of cocaine, whereas dopamine in the dorsolateral striatum (DLS) is thought to become involved in later stages of the addiction process to mediate well-established cue-controlled drug seeking. However, it is unclear whether the DLS also has a role in the reinforcing properties of cocaine itself. Therefore, we systematically investigated the involvement of dopamine in dorsal and ventral striatal regions in cocaine self-administration, using various schedules of reinforcement in animals with limited drug taking experience. Intra-DLS infusion of the dopamine receptor antagonist α-flupenthixol did not affect the acquisition of cocaine self-administration, increased cocaine self-administration under a fixed ratio-1 (FR-1) schedule of reinforcement, caused a rightward and downward shift of the dose-response curve of cocaine under an FR-1 schedule of reinforcement and decreased responding for cocaine under a progressive ratio (PR) schedule of reinforcement. Infusion of α-flupenthixol into the ventral nucleus accumbens (NAcc) shell inhibited the acquisition of cocaine self-administration, reduced responding for the drug under FR-1 and PR schedules of reinforcement, and caused a downward shift of the dose-response curve of cocaine self-administration under an FR-1 schedule of reinforcement. These data show that dopamine in both the DLS and NAcc shell is involved in cocaine reinforcement. We suggest that the DLS and the NAcc shell mediate somewhat distinct facets of the reinforcing properties of cocaine, related to its rewarding and motivational aspects, respectively.

Seeking-taking chain schedules of cocaine and sucrose self-administration: effects of reward size, reward omission, and α-flupenthixol

RATIONALE

In heterogeneous seeking-taking (ST) chain schedules of self-administration, seeking rewards and taking rewards are distinct actions, giving animals explicit control over their intake of the reward. However, the neurobehavioral characteristics of ST chain schedules are relatively unexplored.

OBJECTIVES

This study was made to evaluate two variants of ST chain schedules of self-administration to measure seeking and taking of sucrose and cocaine in rats.

METHODS

Rats had to respond on one lever (seeking lever) under a random interval (RI) or under a progressive ratio (PR) schedule, to gain access to a second lever (taking lever), responding on which under a fixed-ratio 1 (FR-1) schedule of reinforcement delivered the reward. We assessed the effects of reward size, reward omission, and administration of the dopamine receptor antagonist α-flupenthixol. The effects of α-flupenthixol on responding for cocaine or sucrose under an FR-1 schedule of reinforcement were also assessed.

RESULTS

Cocaine seeking under both schedules was reduced by decreasing reward size, reward omission, and α-flupenthixol treatment. Cocaine taking was decreased by α-flupenthixol treatment and reward omission, but not by altering reward size. Sucrose seeking was not affected by reward size, but was reduced by α-flupenthixol and reward omission. Sucrose taking was diminished by reward omission only. α-Flupenthixol increased cocaine but not sucrose intake under an FR-1 schedule of reinforcement.

CONCLUSIONS

Both ST(PR) and ST(RI) schedules can be used to assess seeking and taking of sucrose and cocaine. Dopaminergic neurotransmission mediates the positive subjective properties of cocaine but not sucrose and the motivational properties of both sucrose and cocaine.

Increased expression of 5-HT₆ receptors in dorsolateral striatum decreases habitual lever pressing, but does not affect learning acquisition of simple operant tasks in rats

Serotonin-6 (5-HT(6)) receptors are densely expressed in the dorsolateral striatum (DLS), a brain region linked to habits. Medications acting on the serotonergic system, including 5-HT(6) receptors, can diminish habitual and repetitive behaviors associated with clinical syndromes such as obsessive-compulsive disorder, and may have implications for addiction as well. To examine the role of 5-HT(6) receptors in the acquisition and persistence of habitual behavior, we manipulated 5-HT(6) receptor expression in the DLS with herpes simplex virus vectors in combination with different behavioral procedures; control rats received a vector expressing enhanced green fluorescent protein. In one set of experiments, rats were tested under conditions that favor the acquisition of either discrete action-outcome responding or repetitive responding; increased 5-HT(6) receptor expression in DLS did not alter learning in either paradigm. In the next experiment, rats were over-trained on fixed- then variable-interval schedules, resulting in an escalation of lever pressing over sessions far in excess of that necessary to receive sucrose pellets. After training, rats received viral vector infusion into the DLS. Subsequently, half of each group underwent an omission contingency training session in which they received reinforcement for refraining from pressing the lever, while the other half served as yoked controls. A probe session under extinction conditions was performed the following day. Only rats that received both the 5-HT(6) vector and omission contingency training showed reduced lever pressing during the probe session. These results suggest that increasing 5-HT(6) receptor signaling in the DLS facilitates behavioral flexibility in the face of changing contingencies.

Neurobiology of overeating and obesity: the role of melanocortins and beyond

The alarming increase in the incidence of obesity and obesity-associated disorders makes the etiology of obesity a widely studied topic today. As opposed to 'homeostatic feeding', where food intake is restricted to satisfy one's biological needs, the term 'non-homeostatic' feeding refers to eating for pleasure or the trend to over-consume (palatable) food. Overconsumption is considered a crucial factor in the development of obesity. Exaggerated consumption of (palatable) food, coupled to a loss of control over food intake despite awareness of its negative consequences, suggests that overeating may be a form of addiction. At a molecular level, insulin and leptin resistance are hallmarks of obesity. In this review, we specifically address the question how leptin resistance contributes to enhanced craving for (palatable) food. Since dopamine is a key player in the motivation for food, the interconnection between dopamine, leptin and neuropeptides related to feeding will be discussed. Understanding the mechanisms by which these neuropeptidergic systems hijack the homeostatic feeding mechanisms, thus leading to overeating and obesity is the primary aim of this review. The melanocortin system, one of the crucial neuropeptidergic systems modulating feeding behavior will be extensively discussed. The inter-relationship between neuronal populations in the arcuate nucleus and other areas regulating energy homeostasis (lateral hypothalamus, paraventricular nucleus, ventromedial hypothalamus etc.) and reward circuitry (the ventral tegmental area and nucleus accumbens) will be evaluated and scrutinized.

Glycine transporter-1 blockade leads to persistently reduced relapse-like alcohol drinking in rats

BACKGROUND

Residual dysfunction of multiple neurotransmitter systems due to chronic alcohol use is likely responsible for the occurrence of compulsive alcohol seeking during abstinence and relapse behavior. There is increasing evidence that glycine, which activates both glycine and N-methyl-D-aspartate receptors, contributes to excessive alcohol consumption. We therefore hypothesized that the blockade of glycine transporter 1 might interfere with compulsive alcohol consumption and relapse behavior.

METHODS

We used our animal model of alcoholism--long-term alcohol consumption with repeated deprivation phases in rats--to study the effects of a selective blocker of glycine transporter 1 Org25935. The abstinence-promoting drug acamprosate was used as a reference compound. Subsequently, we examined alterations in dorsal striatal gene expression caused by chronic ethanol (EtOH) consumption, focusing on glycinergic and glutamatergic signaling-related genes. Gene expression profiles of Org25935-treated EtOH-drinking rats were compared with vehicle-treated EtOH-drinking versus age-matched EtOH-naive rats.

RESULTS

We found that repeated treatment with Org25935 reduced compulsive relapse-like drinking without the development of tolerance. Importantly, these antirelapse properties were maintained for at least 6 weeks in a treatment-free period. This persistent effect was paralleled by a reversal of altered expression levels of a set of glycinergic and glutamatergic signaling-related genes to the levels found in EtOH-naive control rats.

CONCLUSIONS

This study shows that treatment of rats with Org25935 leads to a reduction of compulsive alcohol consumption and relapse-like drinking behavior--an effect that persists into treatment-free periods. This long-term antirelapse effect might result from a restoration of normal glycinergic and glutamatergic signaling function.

Motivation for alcohol becomes resistant to quinine adulteration after 3 to 4 months of intermittent alcohol self-administration

BACKGROUND

Continued consumption of alcohol despite deleterious consequences is a hallmark of alcoholism and represents a critical challenge to therapeutic intervention. Previous rat studies showed that enduring alcohol self-administration despite pairing alcohol with normally aversive stimuli was only observed after very long-term intake (>8 months). Aversion-resistant alcohol intake has been previously interpreted to indicate pathological or compulsive motivation to consume alcohol. However, given the time required to model compulsive alcohol seeking in previous studies, there is considerable interest in developing more efficient and quantitative rodent models of aversion-resistant alcohol self-administration.

METHODS

Outbred Wistar rats underwent 3 to 4 months or approximately 1.5 months of intermittent, home-cage, two-bottle access (IAA) to 20% alcohol (v/v) or water. Then, after brief operant training, the effect of the bitter-tasting quinine (0.1 g/l) on the motivation to seek alcohol was quantified via progressive ratio (PR). Motivation for quinine-adulterated 2% sucrose under PR was assayed in a separate cohort of 3 to 4 months IAA rats. The effects of quinine on home-cage alcohol consumption in IAA rats and rats with continuous access to alcohol were also examined. Finally, a dose-response for quinine taste preference in IAA and continuous-access animals was determined.

RESULTS

Motivation for alcohol after 3 to 4 months IAA, measured using an operant PR procedure, was not altered by adulteration of alcohol with 0.1 g/l quinine. In contrast, after 3 to 4 months of IAA, motivation for sucrose under PR was significantly reduced by adulteration of sucrose with 0.1 g/l quinine. In addition, motivation for alcohol after only approximately 1.5 months IAA was significantly reduced by adulteration of alcohol with 0.1 g/l quinine. Furthermore, home-cage alcohol intake by IAA rats was insensitive to quinine at concentrations (0.01, 0.03 g/l) that significantly reduced alcohol drinking in animals with continuous access to alcohol. Finally, no changes in quinine taste preference after 3 to 4 months IAA or continuous access to alcohol were observed.

CONCLUSIONS

We have developed a novel and technically simple hybrid operant/IAA model in which quinine-resistant motivation for alcohol is evident after an experimentally tractable period of time (3 to 4 months vs. 8 months). Quinine dramatically reduced sucrose and water intake by IAA rats, indicating that continued responding for alcohol in IAA rats despite adulteration with the normally aversive quinine might reflect maladaptive or compulsive motivation for alcohol. This model could facilitate identification of novel therapeutic interventions for pathological alcohol seeking in humans.

Neuronal nicotinic acetylcholine receptors as pharmacotherapeutic targets for the treatment of alcohol use disorders

Alcohol use disorders (AUDs) are complex, and developing effective treatments will require the combination of novel medications and cognitive behavioral therapy approaches. Epidemiological studies have shown there is a high correlation between alcohol consumption and tobacco use, and the prevalence of smoking in alcoholics is as high as 80% compared to about 30% for the general population. Both preclinical and clinical data provide evidence that nicotine administration increases alcohol intake and non-specific nicotinic receptor antagonists reduce alcohol-mediated behaviors. As nicotine interacts specifically with the neuronal nicotinic acetylcholine receptor (nAChR) system, this suggests that nAChRs play an important role in the behavioral effects of alcohol. In this review, we discuss the importance of nAChRs for the treatment of AUDs and argue that the use of FDA approved nAChR ligands, such as varenicline and mecamylamine, approved as smoking cessation aids may prove to be valuable treatments for AUDs. We also address the importance of combining effective medications with behavioral therapy for the treatment of alcohol dependent individuals.

Inflexible and indifferent alcohol drinking in male mice

BACKGROUND

Alcoholism is characterized by compulsive alcohol intake, but this critical feature of alcoholism is seldom captured in preclinical studies. Here, we evaluated whether alcohol-preferring C57BL/6J mice develop compulsive alcohol drinking patterns, using adulteration of the alcohol solution with quinine, in a limited access choice paradigm. We assessed 2 independent aspects of compulsive drinking: (i) inflexible alcohol intake by testing whether mice would drink bitter alcohol solutions if this was their only source of alcohol and (ii) indifferent drinking by comparing intake of aversive and nonaversive alcohol solutions.

METHODS

Male C57BL/6J mice consumed alcohol for 2 or 8 consecutive weeks. The alcohol solution was then adulterated with graded quinine concentrations, and the effect on alcohol intake was determined.

RESULTS

C57BL/6J mice rapidly developed compulsive alcohol drinking patterns. Adulteration of the alcohol solution with an aversive quinine concentration failed to reduce intake, indicative of inflexible drinking behavior, after only 2 weeks of alcohol experience, although quinine adulteration did suppress the acquisition of alcohol drinking in naïve mice. After 8 weeks of alcohol consumption, the mice also became indifferent to quinine. They consumed an aversive, quinine-containing alcohol solution, despite the simultaneous availability of an unadulterated alcohol solution. Prolonged alcohol ingestion did not alter the sensitivity to the bitter taste of quinine itself.

CONCLUSION

These findings demonstrate the staged occurrence in mice of 2 distinct behavioral characteristics of alcoholism, i.e., inflexible and indifferent alcohol drinking.

Repeated amphetamine administration in rats revealed consistency across days and a complete dissociation between locomotor and hypothalamic-pituitary-adrenal axis effects of the drug

RATIONALE

Most drugs of abuse stimulate both locomotor activity and the hypothalamic-pituitary-adrenal (HPA) axis, but the relationship between the two responses within the same subjects and their reliabilities has been scarcely studied. Our objectives were to study: (1) the consistency and stability across time of locomotor and HPA activation induced by repeated d-amphetamine (AMPH); (2) the relationship between locomotor and hormonal responses to AMPH; and (3) the relationship between novelty-induced activity and both types of responses to the drug.

METHODS

Male adult rats were exposed to a novel environment to study the locomotor response. Later, they were injected with AMPH (2 mg/kg, sc) for 5 days. In Experiment 1, Plasma adrenocorticotropin (ACTH) and corticosterone levels in response to AMPH were studied on days 1, 3, and 5, and locomotor response on days 2 and 4. In Experiment 2, ACTH and corticosterone responses were studied on days 2 and 4, and locomotor response on days 1, 3, and 5.

RESULTS

Across days, both locomotor and HPA responses to the drug were consistent, but independent measures, unrelated to the reactivity to novelty. As measured by the area under the curve, the HPA response to AMPH desensitized with the repeated injection, whereas the initial locomotor response to the drug increased.

CONCLUSIONS

Dissociation exists between HPA and locomotor activation induced by AMPH, which seemed to be both reliable individual traits. Locomotor reactivity to novelty was related neither to HPA nor to locomotor responses to AMPH.

Frontal-striatal abnormalities underlying behaviours in the compulsive-impulsive spectrum

In this paper, we tentatively bring together the psychiatric, neurological and addiction perspectives on the impulsive-compulsive spectrum of neuropsychiatric disorders, in order to understand the pathophysiology of impulse control disorders (ICDs) in Parkinson's disease. In an attempt to try to pool the various levels of information we will therefore focus on three disorders within the impulse-compulsive spectrum, i.e., obsessive-compulsive disorder (OCD), ICDs in Parkinson's disease, and cocaine seeking behaviour. Whereas there are large differences between these three domains, each with their own nomenclature, hypotheses and study results, they share the focus on an imbalance within and between the frontal-striatal circuits as underlying substrate for the behaviours. For each disorder, we summarize the results from recent studies in order to describe in which way alterations in the frontal-striatal circuits contribute to the phenotype. The phenomenological overlap between ICDs in Parkinson's disease, addiction and OCD needs further investigation, since better understanding of the overlapping and differentiating characteristics will contribute to our understanding of the pathophysiology of the disturbances and treatment alternatives.

Compulsive alcohol drinking in rodents

Upon prolonged alcohol exposure, the behaviour of an individual can gradually switch from controlled to compulsive. Our review is focused on the neurobiological mechanisms that might underlie this transition as well as the factors that are influencing it. Animal studies suggest that temporally increased alcohol consumption during post-abstinence drinking is accompanied by a loss of flexibility of the behaviour and therefore, could serve as a model for compulsive alcohol drinking. However, studies using different alcohol-preferring rat lines in the post-abstinence drinking model suggest that high alcohol consumption does not necessarily lead to the development of compulsive drinking. This indicates the significance of genetic predisposition to compulsive behaviour. Neuroimaging data show that chronic alcohol consumption affects the activity of several brain regions such as the extrapyramidal motor system and several areas of the prefrontal cortex including the orbitofrontal and anterior cingulate cortex. Similar changes in brain activity is seen in patients suffering from obsessive-compulsive disorder at baseline conditions and during provocation of obsessive thoughts and urge to perform compulsive-like rituals. This indicates that dysfunction of these regions may be responsible for the expression of compulsive components of alcohol drinking behaviour. Several brain neurotransmitter systems seem to be responsible for the switch from controlled to compulsive behaviour. In particular, hypofunctioning of monoaminergic systems and hyperfunctioning of glutamatergic systems may play a role in compulsive alcohol drinking.