The D2/3 dopamine receptor in pathological gambling: a positron emission tomography study with [11C]-(+)-propyl-hexahydro-naphtho-oxazin and [11C]raclopride

Dopamine mediates a directionally opposite correlation between empathy and the reinforcing effects of amphetamine and gambling in people with gambling disorder vs. healthy controls

Understanding the relationship between empathy, subjective effects of addictive reinforcers and dopamine function in people with gambling disorder (PGD) vs. healthy controls (HCs) may inform GD treatment. The current investigation addressed this issue via retrospective analysis of data from three studies using amphetamine and a slot machine (SLOTS) as reinforcers in PGD and HCs. The Empathy scale of Eysenck's Impulsiveness Questionnaire assessed trait Empathy. The Gamblers Beliefs Questionnaire assessed cognitive distortions. The Eysenck Lie scale assessed socially desirable responding. PET scans quantified dopamine receptor expression and amphetamine-induced dopamine release in Study 1. Pre-treatment with the D2-receptor (D2R)-preferring antagonist, haloperidol or D1R-D2R antagonist, fluphenazine before SLOTS tested the role of D2 autoreceptors and post-synaptic D2R in Study 2. Pre-treatment with the multi-system indirect dopamine agonist, modafinil before SLOTS assessed the reliability of correlations in PGD. Striatal D2R expression predicted greater Empathy and lower amphetamine 'Liking' in HCs, and predicted greater symptom severity in PGD. Empathy predicted lower 'Exciting' effects of SLOTS under placebo in HCs; no correlation emerged under either antagonist. Relative to placebo, haloperidol decreased, whereas fluphenazine increased, the positive correlation between Empathy and Exciting effects of SLOTS in PGD. Modafinil markedly reduced the positive correlation between Empathy and Exciting effects of SLOTS seen under placebo in PGD. Empathy predicted greater cognitive distortions in PGD in all studies. Lie scale variance influenced several primary effects. Prior research linking the insula with Empathy, reactivity to interoceptive signals for risky rewards (uncertainty), and cognitive distortions, provides a parsimonious account for these results.

Similarity and difference in large-scale functional network alternations between behavioral addictions and substance use disorder: a comparative meta-analysis

Behavioral addiction (BA) and substance use disorder (SUD) share similarities and differences in clinical symptoms, cognitive functions, and behavioral attributes. However, little is known about whether and how functional networks in the human brain manifest commonalities and differences between BA and SUD. Voxel-wise meta-analyses of resting-state functional connectivity (rs-FC) were conducted in BA and SUD separately, followed by quantitative conjunction analyses to identify the common and distinct alterations across both the BA and SUD groups. A total of 92 datasets with 2444 addicted patients and 2712 healthy controls (HCs) were eligible for the meta-analysis. Our findings demonstrated that BA and SUD exhibited common alterations in rs-FC between frontoparietal network (FPN) and other high-level neurocognitive networks (i.e. default mode network (DMN), affective network (AN), and salience network (SN)) as well as hyperconnectivity between SN seeds and the Rolandic operculum in SSN. In addition, compared with BA, SUD exhibited several distinct within- and between-network rs-FC alterations mainly involved in the DMN and FPN. Further, altered within- and between-network rs-FC showed significant association with clinical characteristics such as the severity of addiction in BA and duration of substance usage in SUD. The common rs-FC alterations in BA and SUD exhibited the relationship with consistent aberrant behaviors in both addiction groups, such as impaired inhibition control and salience attribution. By contrast, the distinct rs-FC alterations might suggest specific substance effects on the brain neural transmitter systems in SUD.

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Serotonergic and dopaminergic control of impulsivity in gambling disorder

Gambling disorder (GD) is major public health issue. The disorder is often characterized by elevated impulsivity with evidence from analogous substance use disorders underlining prominent roles of brain monoamines in addiction susceptibility and outcome. Critically, GD allows the study of addiction mechanisms without the confounder of the effects of chronic substances. Here, we assessed the roles of striatal dopamine transporter binding and extrastriatal serotonin transporter binding in GD as a function of impulsivity using [¹²³ I]FP-CIT SPECT imaging in 20 older adults with GD (DSM-5 criteria; mean age 64 years) and 40 non-GD age- and sex-matched controls. We focused on GD in older individuals because there are prominent age-related changes in neurotransmitter function and because there are no reported neuroimaging studies of GD in older adults. Volume-of-interest-based and voxelwise analyses were performed. GD patients scored clearly higher on impulsivity and had higher tracer binding in the ventromedial prefrontal cortex than controls (p < 0.001), likely reflecting serotonin transporter activity. The binding in the medial prefrontal cortex positively correlated with impulsivity over the whole sample (r = 0.62, p < 0.001) as well as separately in GD patients (r = 0.46, p = 0.04) and controls (r = 0.52, p < 0.001). Striatal tracer binding, reflecting dopamine transporter activity was also positively correlated with impulsivity but showed no group differences. These findings highlight the role of prefrontal serotonergic function in GD and impulsivity. They identify cerebral coordinates of a potential target for neuromodulation for both GD and high impulsivity, a core phenotypic dimensional cognitive marker in addictions.

Drug-Induced Gambling Disorder: Epidemiology, Neurobiology, and Management

Problematic gambling has been suggested to be a possible consequence of dopaminergic medications used mainly in neurological conditions, i.e. pramipexole and ropinirole, and possibly by one antipsychotic compound, aripiprazole. Patients with Parkinson's disease, restless legs syndrome and other conditions potentially treated with dopamine agonists, as well as patients treated for psychotic disorders, are vulnerable patient groups with theoretically increased risk of developing gambling disorder (GD), for example due to higher rates of mental ill-health in these groups. The aim of the present paper is to review the epidemiological, clinical, and neurobiological evidence of the association between dopaminergic medications and GD, and to describe risk groups and treatment options. The neurobiology of GD involves the reward and reinforcement system, based mainly on mesocorticolimbic dopamine projections, with the nucleus accumbens being a crucial area for developing addictions to substances and behaviors. The addictive properties of gambling can perhaps be explained by the reward uncertainty that activates dopamine signaling in a pathological manner. Since reward-related learning is mediated by dopamine, it can be altered by dopaminergic medications, possibly leading to increased gambling behavior and a decreased impulse control. A causal relationship between the medications and GD seems likely, but the molecular mechanisms behind this association have not been fully described yet. More research is needed in order to fully outline the clinical picture of GD developing in patient groups with dopaminergic medications, and data are needed on the differentiation of risk in different compounds. In addition, very few interventional studies are available on the management of GD induced by dopaminergic medications. While GD overall can be treated, there is need for treatment studies testing the effectiveness of tapering of the medication or other gambling-specific treatment modalities in these patient groups.

Life Habits and Mental Health: Behavioural Addiction, Health Benefits of Daily Habits, and the Reward System

In this review, the underlying mechanisms of health benefits and the risk of habitual behaviours such as internet use and media multitasking were explored, considering their associations with the reward/motivation system. The review highlights that several routines that are beneficial when undertaken normally may evolve into excessive behaviour and have a negative impact, as represented by "the inverted U-curve model". This is especially critical in the current era, where technology like the internet has become mainstream despite the enormous addictive risk. The understanding of underlying mechanisms of behavioural addiction and optimal level of habitual behaviours for mental health benefits are deepened by shedding light on some findings of neuroimaging studies to have hints to facilitate better management and prevention strategies of addictive problems. With the evolution of the world, and the inevitable use of some technologies that carry the risk of addiction, more effective strategies for preventing and managing addiction are in more demand than before, and the insights of this study are also valuable foundations for future research.

D3 Receptors and PET Imaging

This chapter encapsulates a short introduction to positron emission tomography (PET) imaging and the information gained by using this technology to detect changes of the dopamine 3 receptor (D3R) at the molecular level in vivo. We will discuss available D3R radiotracers, emphasizing [11C]PHNO. The focus, however, will be on PET findings in conditions including substance abuse, obesity, traumatic brain injury, schizophrenia, Parkinson's disease, and aging. Finally, there is a discussion about progress in producing next-generation selective D3R radiotracers.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

Unlucky punches: the vulnerability-stress model for the development of impulse control disorders in Parkinson's disease

Impulse-control disorders are commonly observed during dopamine-replacement therapy in Parkinson’s disease, but the majority of patients seems “immune” to this side effect. Epidemiological evidence suggests that a major risk factor may be a specific difference in the layout of the dopaminergic-reinforcement system, of which the ventral striatum is a central player. A series of imaging studies of the dopaminergic system point toward a presynaptic reduction of dopamine-reuptake transporter density and dopamine synthesis capacity. Here, we review current evidence for a vulnerability-stress model in which a relative reduction of dopaminergic projections to the ventral striatum and concomitant sensitization of postsynaptic neurons represent a predisposing (hypodopaminergic) vulnerability. Stress (hyperdopaminergic) is delivered when dopamine replacement therapy leads to a relative overdosing of the already-sensitized ventral striatum. These alterations are consistent with consecutive changes in reinforcement mechanisms, which stimulate learning from reward and impede learning from punishment, thereby fostering the development of impulse-control disorders. This vulnerability-stress model might also provide important insights into the development of addictions in the non-Parkinsonian population.

Does sodium oxybate inhibit brain dopamine release in humans? An exploratory neuroimaging study

OBJECTIVE

To establish in an exploratory neuroimaging study whether γ-hydroxybutyrate (sodium oxybate [SO]), a sedative, anti-narcoleptic drug with abuse potential, transiently inhibits striatal dopamine release in the human.

METHODS

Ten healthy participants (30 years; 6M, 4F) and one participant with narcolepsy received a baseline positron emission tomography scan of [C-11]raclopride, a D2/3 dopamine receptor radioligand sensitive to dopamine occupancy, followed approximately one week later by an oral sedative 3g dose of SO and two [C-11]raclopride scans (1 h, 7 h post SO). Plasma SO levels and drowsiness duration were assessed.

RESULTS

No significant changes were detected in [C-11]raclopride binding in striatum overall 1 or 7 h after SO, but a small non-significant increase in [C-11]raclopride binding, implying decreased dopamine occupancy, was noted in limbic striatal subdivision at one hour (+6.5%; p uncorrected = 0.045; +13.2%, narcolepsy participant), returning to baseline at 7 h. A positive correlation was observed between drowsiness duration and percent change in [C-11]raclopride binding in limbic striatum (r = 0.73; p = 0.017).

CONCLUSIONS

We did not find evidence in this sample of human subjects of a robust striatal dopamine change, as was reported in non-human primates. Our preliminary data, requiring extension, suggest that a 3g sedative SO dose might cause slight transient inhibition of dopamine release in limbic striatum.

Increased risk for developing gambling disorder under the treatment with pramipexole, ropinirole, and aripiprazole: A nationwide register study in Sweden

Gambling Disorder (GD) has recently been reclassified from an impulse-control disorder to a behavioural addiction and, as in other addictive disorders, the dopaminergic reward system is involved. According to neuroimaging studies, alterations within the striatal dopaminergic signalling can occur in GD. However, the findings to date are controversial and there has been no agreement yet on how the reward system is affected on a molecular basis. Within the last 20 years, there has been growing evidence for a higher risk to develop GD in response to certain dopaminergic medication. Especially the dopamine agonists pramipexole and ropinirole, and the dopamine modulator aripiprazole seem to increase the likelihood for GD. The goal of this study was to examine the association between a prescription for either of the three pharmaceuticals and a GD diagnosis in a large cross-sectional study of the Swedish population. Compared to patients with any other dopaminergic drug prescription (38.7% with GD), the diagnosis was more common in patients with a dopamine agonist prescription (69.8% with GD), resulting in an odds ratio of 3.2. A similar association was found between aripiprazole prescriptions and GD diagnoses, which were analysed within the subgroup of all patients with schizophrenia or a schizotypal, delusional, or another non-mood psychotic disorder. An aripiprazole prescription increased the likelihood of GD (88.8%) in comparison to patients without an aripiprazole prescription (71.2%) with an odds ratio of 3.4. This study contributes to the increasingly reliable evidence for an association between several dopaminergic drugs and a higher risk for developing GD. Therefore, one future research goal should be a better understanding of the neurobiology in GD to be able to design more selective dopaminergic medication with less severe side effects. Additionally, this knowledge could enable the development of pharmacotherapy in GD and other addictive disorders.

No evidence for decreased D2/3 receptor availability and frontal hypoperfusion in subjects with compulsive pornography use

Pornographic addiction refers to an addiction model associated with compulsive and repeated use of pornographic material. Whether the use of pornography may indeed become addictive remains a matter of debate. The current study investigated whether compulsive pornography use (CPU) is accompanied by reduced D2/3 receptor availability in the striatum and frontal hypofunctionality. Male subjects between 18 and 50 years of age with and without CPU were recruited using online and newspaper advertisements. Questionnaires were used to the assess the severity of compulsive pornography use (CIUS) and symptoms of depression, impulsivity and sensation seeking. Dopaminergic imaging was performed using [11C]-raclopride PET. Striatal binding potentials (BPND) and regional frontal cerebral influx values (R1) of [11C]-raclopride were calculated. Arterial Spin Labeling (ASL) MRI was performed to assess regional cerebral blood flow. No group differences between striatal BPND's of [11C]-raclopride in subjects with (n = 15) and without (n = 10) CPU were detected. In CPU subjects, no correlation was found between the CIUS score and striatal BPND's. Cerebral R1 values in frontal brain regions and cerebral blood flow measurements did not differ between groups. The current study fails to provide imaging support for sharing similar neurobiological alterations as previously has been reported in other addictive modalities.

Relationship between regional gray matter volumes and dopamine D2 receptor and transporter in living human brains

Although striatal dopamine neurotransmission is believed to be functionally linked to the formation of the corticostriatal network, there has been little evidence for this regulatory process in the human brain and its disruptions in neuropsychiatric disorders. Here, we aimed to investigate associations of striatal dopamine transporter (DAT) and D₂ receptor availabilities with gray matter (GM) volumes in healthy humans. Positron emission tomography images of D₂ receptor (n = 34) and DAT (n = 17) captured with the specific radioligands [¹¹C]raclopride and [¹⁸F]FE‐PE2I, respectively, were acquired along with T1‐weighted magnetic resonance imaging data in our previous studies, and were re‐analyzed in this work. We quantified the binding potentials (BP_ND) of these radioligands in the limbic, executive, and sensorimotor functional subregions of the striatum. Correlations between the radioligand BP_ND and regional GM volume were then examined by voxel‐based morphometry. In line with the functional and anatomical connectivity, [¹¹C]raclopride BP_ND in the limbic striatum was positively correlated with volumes of the uncal/parahippocampal gyrus and adjacent temporal areas. Similarly, we found positive correlations between the BP_ND of this radioligand in the executive striatum and volumes of the prefrontal cortices and their adjacent areas as well as between the BP_ND in the sensorimotor striatum and volumes of the somatosensory and supplementary motor areas. By contrast, no significant correlation was found between [¹⁸F]FE‐PE2I BP_ND and regional GM volumes. Our results suggest unique structural and functional corticostriatal associations involving D₂ receptor in healthy humans, which might be partially independent of the nigrostriatal pathway reflected by striatal DAT.

Exploring Pathways to Gambling: Proposing the Integrated Risk and Protective Factors Model of Gambling Types

Three linked studies, testing key aspects of the Pathways towards Problem and Pathological Gambling Model (Blaszczynski and Nower in Addiction 87(5):487-499, 2002), are presented. Study one comprised 204 students and 490 gambling forum users. It predicted differences in gambling severity, mental health and substance use across different motives for gambling. Those with a primary social motive for gambling displayed less severe gambling and anxiety than those without, with the primary coping subgroup displaying the most anxiety and depression. Those who gambled primarily to enhance positive affect reported severe gambling. Study two comprised 404 gambling forum users and 265 students. Similar groups to the Pathways Model emerged, with a behaviourally conditioned and an emotionally vulnerable group. Unexpectedly, however, those in the emotionally vulnerable group reported more severe cognitive distortions than the behaviourally conditioned group. The final study, 378 gambling forum users and 201 students, found, as predicted, that three distinct gambling groups emerged; (1) those with lower levels of psychopathology and higher levels of protective factors; (2) those with heightened pre-existing anxiety and depression, and moderate levels of protective factors; and (3) those with heightened impulsivity, psychopathology, offending behaviour and the least protective factors. Three gambling groups are consequently presented (Social Gambler; Affect-Regulation Gambler; Antisocial Gambler) alongside the proposed Integrated Risk and Protective Factors Model of Gambling Types (IRPF-MGT). Directions for future research and implications for practice are outlined.

The neural basis of gambling disorder: An activation likelihood estimation meta-analysis

Previous imaging studies suggested that impairments of prefrontal-striatal and limbic circuits are correlated to excessive gambling. However, the neural underpinnings of gambling disorder (GD) continue to be the topic of debate. The present study aimed to identify structural changes in GD and differentiate the specific brain activity patterns associated with decision-making and reward-processing. We performed a systematic review complemented by Activation likelihood estimation (ALE) meta-analyses on morphometric and functional studies on neural correlates of GD. The ALE meta-analysis on structural studies revealed that patients with GD showed significant cortical grey-matter thinning in the right ventrolateral and ventromedial prefrontal cortex compared to healthy subjects. The ALE meta-analyses on functional studies revealed that patients with GD showed a significant hyperactivation in the medial prefrontal cortex and in the right ventral striatum during decision-making and gain processing compared to healthy subjects. These findings suggest that GD is related to an alteration of brain mechanisms underlying top-down control and appraisal of gambling-related stimuli and provided indications to develop new interventions in clinical practice.

Blunted endogenous opioid release following an oral dexamphetamine challenge in abstinent alcohol-dependent individuals

Addiction has been proposed as a 'reward deficient' state, which is compensated for with substance use. There is growing evidence of dysregulation in the opioid system, which plays a key role in reward, underpinning addiction. Low levels of endogenous opioids are implicated in vulnerability for developing alcohol dependence (AD) and high mu-opioid receptor (MOR) availability in early abstinence is associated with greater craving. This high MOR availability is proposed to be the target of opioid antagonist medication to prevent relapse. However, changes in endogenous opioid tone in AD are poorly characterised and are important to understand as opioid antagonists do not help everyone with AD. We used [11C]carfentanil, a selective MOR agonist positron emission tomography (PET) radioligand, to investigate endogenous opioid tone in AD for the first time. We recruited 13 abstinent male AD and 15 control participants who underwent two [11C]carfentanil PET scans, one before and one 3 h following a 0.5 mg/kg oral dose of dexamphetamine to measure baseline MOR availability and endogenous opioid release. We found significantly blunted dexamphetamine-induced opioid release in 5 out of 10 regions-of-interest including insula, frontal lobe and putamen in AD compared with controls, but no significantly higher MOR availability AD participants compared with HC in any region. This study is comparable to our previous results of blunted dexamphetamine-induced opioid release in gambling disorder, suggesting that this dysregulation in opioid tone is common to both behavioural and substance addictions.

Dopamine and Risky Decision-Making in Gambling Disorder

Gambling disorder is a behavioral addiction associated with impairments in value-based decision-making and cognitive control. These functions are thought to be regulated by dopamine within fronto-striatal circuits, but the role of altered dopamine neurotransmission in the etiology of gambling disorder remains controversial. Preliminary evidence suggests that increasing frontal dopamine tone might improve cognitive functioning in gambling disorder. We therefore examined whether increasing frontal dopamine tone via a single dose of the catechol-O-methyltransferase (COMT) inhibitor tolcapone would reduce risky choice in human gamblers (n = 14) in a randomized double-blind placebo-controlled crossover study. Data were analyzed using hierarchical Bayesian parameter estimation and a combined risky choice drift diffusion model (DDM). Model comparison revealed a nonlinear mapping from value differences to trial-wise drift rates, confirming recent findings. An increase in risk-taking under tolcapone versus placebo was about five times more likely, given the data, than a decrease [Bayes factor (BF) = 0.2]. Examination of drug effects on diffusion model parameters revealed that an increase in the value dependency of the drift rate under tolcapone was about thirteen times more likely than a decrease (BF = 0.073). In contrast, a reduction in the maximum drift rate under tolcapone was about seven times more likely than an increase (BF = 7.51). Results add to previous work on COMT inhibitors in behavioral addictions and to mounting evidence for the applicability of diffusion models in value-based decision-making. Future work should focus on individual genetic, clinical and cognitive factors that might account for heterogeneity in the effects of COMT inhibition.

High Recreational Gamblers Show Increased Stimulatory Effects of an Acute Laboratory Gambling Challenge

Gambling in moderation is a socially acceptable behavior and over 60% of the Swedish population gambles every year. It has been seen that slot machines are one of the most addictive and problematic forms of gambling and contribute highly to an addictive behavior. It is unclear why some individuals intensify their gambling behavior over time to extreme levels while others do not. Initial positive response of a drug or as in this case a gambling behavior, most likely influences the likelihood of continuing use in non-addicted individuals. Therefore, we wanted to investigate if recreational gamblers show an altered subjective response to an online gambling challenge, e.g. to casino gambling. The present study was designed to examine the subjective effects after an acute gambling challenge, in healthy recreational gamblers compared with non-gamblers. Eighty-two subjects participated in the study. They were challenged with an acute online slot machine gambling challenge and self-report questionnaires of mood and blood pressure were taken before and after gambling. The gamblers, and more specifically the high recreational gamblers, reported increased stimulative effects after the gambling challenge in comparison to the non-gamblers. Findings suggests that gamblers experience significantly higher arousal effects to an acute online slot machine challenge. This response may be a uniquely predictive behavior for increased risk of gambling addiction.

Effects of exposure to chronic uncertainty and a sensitizing regimen of amphetamine injections on locomotion, decision-making, and dopamine receptors in rats

Gambling disorder (GD) is a behavioral addiction that may be linked to alterations in dopamine (DA) systems. Gambling involves chronic exposure to uncertain reward, which can sensitize the activity of DA systems. Here we explored how combinations of Pavlovian and instrumental uncertainty impact DA sensitization and risky decision-making. Experiment 1: 40 rats underwent 66 uncertainty exposure (UE) sessions during which they responded for saccharin. Animal responding was reinforced according to a fixed or variable (FR/VR) ratio schedule that turned on a conditioned stimulus (CS; light), which predicted saccharin on 50% or 100% of trials. Animals responded under one of the four conditions: FR-CS100% (no uncertainty), VR-CS100%, FR-CS50%, and VR-CS50% (maximal uncertainty). DA sensitization was inferred from an enhanced locomotor response to d-amphetamine (d-AMPH; 0.5 mg/kg) challenge. The rat gambling task (rGT) was used to assess decision-making. Experiment 2: 24 rats received 5 weeks of sensitizing d-AMPH or saline doses, followed by locomotor activity and rGT testing. Experiment 3: Effects of UE and a sensitizing d-AMPH regimen on DA D1, D2, and D3 receptor binding were assessed in 44 rats using autoradiography. Compared to FR-CS100%, VR-CS100% and VR-CS50% rats displayed a greater locomotor response to d-AMPH, and VR-CS50% rats demonstrated riskier decision-making. Chronic d-AMPH-treated rats mirrored the effects of VR-CS50% groups on these two indices. Both VR-CS50% and d-AMPH-treated groups had increased striatal DA D2 receptor binding. These results suggest that chronic uncertainty exposure, similar to exposure to a sensitizing d-AMPH regimen, sensitized the function of DA systems and increased risky decision-making.

Unpacking common and distinct neuroanatomical alterations in cocaine dependent versus pathological gambling

Pathological gambling and cocaine dependence are highly pervasive disorders. Functional neuroimaging evidence implicates aberrant activity of prefrontal striatal pathways in both disorders. It is unclear if the neuroanatomy of these areas is also affected. Participants with pathological gambling (n = 18), cocaine dependence (n = 19) and controls (n = 21) underwent high-resolution structural MRI scan and cognitive assessments. In line with emerging functional neuroimaging findings, we hypothesised (i) lower volumes of corticostriatal areas ascribed to decision-making/inhibitory control, craving and reward processing (i.e., orbitofrontal cortex, inferior frontal gyrus, amygdala, striatum, insula) in both pathological gamblers and cocaine dependent participants versus controls; (ii) selected dopaminergic/glutamatergic pathways directly taxed by cocaine (i.e., superior, dorsolateral and anterior cingulate cortices) would be altered in cocaine dependent versus control participants only. Analyses were conducted with a bonferroni correction. Our results showed that both pathological gambling and cocaine dependent participants, compared to controls, had larger volumes of the right inferior frontal gyrus (ps <.01, ds = 0.66 and 0.62). Cocaine dependent participants had lower nucleus accumbens and medial orbitofrontal cortex volumes than pathological gamblers (ps <.05, ds = 0.51 and 0.72), with the latter being predicted by higher negative urgency scores. Inferior frontal gyrus volume may reflect common alterations of cocaine and gambling addictions, whereas cocaine dependence may be uniquely associated with reduced volume in dorsolateral and middle frontal regions. Cocaine's supra-physiological effects on mesolimbic neurons may explain reduced accumbens-orbitofrontal structure compared to gambling.

The anticipatory dopamine response in addiction: A common neurobiological underpinning of gambling disorder and substance use disorder?

The dopamine system is associated with reward processes in both gambling disorder and substance use disorder, and may constitute a common neurobiological underpinning in addiction. The present review examines differences and similarities of dopaminergic reward processes in gambling disorder and substance use disorder. First, it is suggested that baseline binding potentials of the dopamine system may not be a common pathway, since substance use disorder is associated with reduced binding potentials, whereas gambling disorder is not. Second, it is suggested that dopaminergic reward response may be not a common pathway, since substance use disorder is associated with a blunted dopamine response toward drugs, while conflicting findings of reward response has been reported in gambling disorder. Instead, it is suggested that the anticpatory dopamine response may constitute a common underpinning of gambling disorder and substance use disorder, which may be associated with increased dopamine activity in both types of disorder, and does not involve the intake of substances. The notion of the anticipatory dopamine response as a common underpinning of gambling disorder and substance use disorder is consistent with dopaminergic models of addictions such as the incentive-sensitization model, the ingrative neurodevelopmental model of vulnerability toward addiction and the reward prediction error model.

Dopamine and Gambling Disorder: Prospects for Personalized Treatment

Purpose of Review

To address variation in the severity of gambling disorder, this review evaluates the contribution of mesocorticolimbic dopamine neurons to potential behavioral endophenotypes, the influence of individual differences in the dopamine system on gambling and related behaviors, and the possible role for dopaminergic medications in the treatment of gambling disorder.

Recent Findings

Newer work has suggested that dopaminergic dysfunction can lead to increased reward anticipation and a greater sensitivity to uncertainty, which in turn may drive addictive gambling behaviors. In addition, increased impulsivity, a well-recognized risk factor for gambling disorder, has been linked to dopaminergic dysfunction. More recently, emerging evidence has suggested that dopaminergic medications can influence the discounting of delayed rewards.

Summary

Dopaminergic drugs that increase the salience of long-term over short-term goals may ameliorate symptoms of impulsive individuals with gambling disorder. More broadly, improved understanding of intermediate behavioral and other phenotypes with a defined neurobiological substrate may allow for personalized treatment of gambling disorder and other psychiatric conditions.

DNA-methylation of the dopamin receptor 2 gene is altered during alcohol withdrawal

The dopaminergic neurotransmission is known to be of crucial importance in addictive behavior. Epigenetic regulation like methylation of DNA influences the function of dopaminergic transmission. The present study investigated alterations of DNA methylation in the dopamine D2 receptor (DRD2)-gene in patients suffering from alcohol dependence. The study sample consists of 99 alcohol dependent males admitted for alcohol withdrawal treatment and a control group of 33 healthy participants. Blood samples underwent bisulfite sequencing to determine levels of DNA-methylation of the promoter region of the DRD2 gene. Mixed linear modeling was used to test differences between patients and controls, course of methylation during detoxification. While DRD2-gene methylation did not differ significantly between patients and controls, we found a significant increase of DRD2-gene methylation during alcohol withdrawal/early abstinence. Craving, measured with the Obsessive Compulsive Drinking Scale (OCDS), was significantly associated with DRD2-gene methylation. Furthermore, smoking significantly influenced DRD2-gene methylation in both, patients and controls. As in other types of addictive disorders, DRD2-gene methylation is altered during alcohol withdrawal/early abstinence. The findings regarding an association with alcohol craving and tobacco consumption point towards a crucial role of DRD2-gene methylation in the neurobiology of addictive behavior.

Striatal presynaptic dopaminergic dysfunction in gambling disorder: A 123 I-FP-CIT SPECT study

Although the involvement of dopamine in gambling disorder (GD) has long been hypothesized, its precise role remains unclear. The action of dopamine in the synapses is regulated by the dopamine transporter (DAT). We hereinafter present significant differences between a sample of 15 treatment-seeking GD subjects and 17 healthy controls in terms of striatal DAT availability, and we explore its association with reward-based decision making. We performed ¹²³ I-FP-CIT Single-photon emission computed tomography (SPECT) and correlated DAT binding ratios in the bilateral caudate and putamen with gambling symptoms (G-SAS, PG-YBOCS) and behaviors, as well as other psychometric variables (anhedonia and impulsivity). Gambling disorder (GD) subjects were also administered a computerized version of the Iowa gambling task (IGT) to assess reward-based decision making. We found reduced DAT availability in GD subjects compared with healthy controls (-13.30% in right caudate, -11.11% in right putamen, -11.44% in left caudate, and -11.46% in the left putamen). We also found that striatal DAT availability was inversely correlated with days spent gambling and IGT performance in GD subjects. These results provide evidence for a presynaptic dopaminergic dysfunction in striatal regions of GD subjects. Functional DAT down-regulation possibly sustains the transition towards compulsive gambling addiction, characterized both by hyperdopaminergic and hypodopaminergic states in the context of a sensitized dopaminergic system.

Impulsivity moderates the effects of dopamine D2 and mixed D1-D2 antagonists in individuals with gambling disorder

BACKGROUND

The functional role of dopamine D1 and D2 receptors in gambling disorder (GD) remains unclear.

AIMS

This study aimed to investigate the role of D1 activation and the moderating effects of impulsivity, a trait linked with weaker D2-mediated inhibition of dopamine release, in GD subjects.

METHODS

Thirty (nine female) non-comorbid GD subjects with low (LI), moderate (MI), or high impulsivity (HI) received the preferential D2 antagonist haloperidol (HAL; 3 mg) or the mixed D1-D2 antagonist fluphenazine (FLU; 3 mg), on separate sessions before a 15-minute slot machine game or amphetamine (AMPH; 20 mg), in a placebo-controlled, double-blind, counterbalanced design.

RESULTS

On their own, HAL and FLU led to linear increases and decreases, respectively, in desire to gamble across increasing levels of impulsivity. The slot machine and AMPH each evoked an inverted-U pattern of desire to gamble across increasing impulsivity. HAL reversed this effect of the game, whereas FLU did not alter post-game desire. HAL and FLU decreased and increased psychostimulant-like effects of the game, respectively, in LI and MI subjects, but consistently reduced these effects in HI subjects. HAL also altered the salience of negative affective words on a reading task, such that greater salience of negative words coincided with lower post-game desire to gamble.

CONCLUSIONS

D1 receptors appear to gauge the incentive value of gambling in GD subjects. D1 activation has negative reinforcing effects in HI gamblers and positive reinforcing effects in LI gamblers. Medications that activate D1 could curtail chasing in HI gamblers. D1 blockade could benefit HI gamblers whose main concern is craving.

Gambling disorder

Gambling disorder is characterized by a persistent, recurrent pattern of gambling that is associated with substantial distress or impairment. The prevalence of gambling disorder has been estimated at 0.5% of the adult population in the United States, with comparable or slightly higher estimates in other countries. The aetiology of gambling disorder is complex, with implicated genetic and environmental factors. Neurobiological studies have implicated cortico-striato-limbic structures and circuits in the pathophysiology of this disorder. Individuals with gambling disorder often go unrecognized and untreated, including within clinical settings. Gambling disorder frequently co-occurs with other conditions, particularly other psychiatric disorders. Behavioural interventions, particularly cognitive-behavioural therapy but also motivational interviewing and Gamblers Anonymous, are supported in the treatment of gambling disorder. No pharmacological therapy has a formal indication for the treatment of gambling disorder, although placebo-controlled trials suggest that some medications, such as opioid-receptor antagonists, may be helpful. Given the associations with poor quality of life and suicide, improved identification, prevention, policy and treatment efforts are needed to help people with gambling disorder.

Neuroimaging of reward mechanisms in Gambling disorder: an integrative review

Gambling disorder (GD) was reclassified as a behavioral addiction in the DSM-5 and shares clinical and behavioral features with substance use disorders (SUDs). Neuroimaging studies of GD hold promise in isolating core features of the addiction syndrome, avoiding confounding effects of drug neurotoxicity. At the same time, a neurobiologically-grounded theory of how behaviors like gambling can become addictive remains lacking, posing a significant hurdle for ongoing decisions in addiction nosology. This article integrates research on reward-related brain activity (functional MRI) and neurotransmitter function (PET) in GD, alongside the consideration of structural MRI data as to whether these signals more likely reflect pre-existing vulnerability or neuroadaptive change. Where possible, we point to qualitative similarities and differences with established markers for SUDs. Structural MRI studies indicate modest changes in regional gray matter volume and diffuse reductions in white matter integrity in GD, contrasting with clear structural deterioration in SUDs. Functional MRI studies consistently identify dysregulation in reward-related circuitry (primarily ventral striatum and medial prefrontal cortex), but evidence is mixed as to the direction of these effects. The need for further parsing of reward sub-processes is emphasized, including anticipation vs outcome, gains vs. losses, and disorder-relevant cues vs natural rewards. Neurotransmitter PET studies indicate amplified dopamine (DA) release in GD, in the context of minimal differences in baseline DA D2 receptor binding, highlighting a distinct profile from SUDs. Preliminary work has investigated further contributions of opioids, GABA and serotonin. Neuroimaging data increasingly highlight divergent profiles in GD vs. SUDs. The ability of gambling to perpetually activate DA (via maximal uncertainty) may contribute to neuroimaging similarities between GD and SUDs, whereas the supra-physiological DA effects of drugs may partly explain differences in the neuroimaging profile of the two syndromes. Coupled with consistent observations of correlations with gambling severity and related clinical variables within GD samples, the overall pattern of effects is interpreted as a likely combination of shared vulnerability markers across GD and SUDs, but with further experience-dependent neuroadaptive processes in GD.

Efficacy of Pharmacological Interventions in Targeting Decision-Making Impairments across Substance and Behavioral Addictions

Decision-making impairments reflect tendencies towards risky or unwise choices as manifested by presence of psychiatric symptoms or cognitive impairment (e.g. representation of value, inhibitory control-response selection, learning). Such impairments are suggested by the hallmark symptoms of substance and behavioral addictions, which include escalation over time (of substance intake or a given behavior), lack of control, neglect of other domains of life, and cognitive distortions (such as ‘chasing losses’ in gambling disorder). Amongst the putative behavioral addictions, most epidemiological data exist for gambling disorder, which is now included in DSM-5 as a substance-related and addictive disorder. However, other disorders share parallels and may also constitute behavioral addictions, such as compulsive stealing (kleptomania), compulsive shopping, and compulsive sexual behavior. The current paper presents a narrative review of evidence for cognitive decision-making impairments in addictions, as well as pharmacological treatments of these disorders that may have relevance for improving decision-making. We find that objective decision-making deficits have been widely reported in patients with substance use disorders and gambling disorder, compared to controls. Decision-making in the other behavioral addictions is under-studied. Evidence-based pharmacological treatments for some of these addictive disorders, for example, opioid antagonists and glutamatergic agents, modulate neural systems playing key roles in decision-making. But clinical trials have seldom examined effects of such treatments on objective decision-making measures. Future research directions are discussed, including the need to include standardized outcome measures of decision-making (tasks and imaging) alongside traditional clinical measures, to better understand and enhance underlying treatment mechanisms.

Review: brain neurobiology of gambling disorder based on rodent models

Different literature reviews of gambling disorder (GD) neurobiology have been focused on human studies, others have focused on rodents, and others combined human and rodent studies. The main question of this review was: which are the main neurotransmitters systems and brain structures relevant for GD based on recent rodent studies? This work aims to review the experimental findings regarding the rodent´s neurobiology of GD. A search in the Pub Med database was set (October 2012-October 2017) and 162 references were obtained. After screening, 121 references were excluded, and only 41 references remained from the initial output. More, other 25 references were added to complement (introduction section, neuroanatomical descriptions) the principal part of the work. At the end, a total of 66 references remained for the review. The main conclusions are: 1) according to studies that used noninvasive methods for drug administration, some of the neurotransmitters and receptors involved in behaviors related to GD are: muscarinic, N-methyl-D-aspartate (NMDA), cannabinoid receptor 1 (CB₁), cannabinoid receptor 2 (CB₂), dopamine 2 receptor (D₂), dopamine 3 receptor (D₃), and dopamine 4 receptor (D₄); 2) moreover, there are other neurotransmitters and receptors involved in GD based on studies that use invasive methods of drug administration (eg, brain microinjection); example of these are: serotonin 1A receptor (5-HT_1A), noradrenaline receptors, gamma-aminobutyric acid receptor A (GABA_A), and gamma-aminobutyric acid receptor B (GABA_B); 3) different brain structures are relevant to behaviors linked to GD, like: amygdala (including basolateral amygdala (BLA)), anterior cingulate cortex (ACC), hippocampus, infralimbic area, insular cortex (anterior and rostral agranular), nucleus accumbens (NAc), olfactory tubercle (island of Calleja), orbitofrontal cortex (OFC), medial prefrontal cortex (mPFC), prefrontal cortex (PFC) - subcortical network, striatum (ventral) and the subthalamic nucleus (STN); and 4) the search for GD treatments should consider this diversity of receptor/neurotransmitter systems and brain areas.

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high‐affinity state postulates that a certain subset of G‐protein‐coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high‐affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high‐affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high‐affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high‐affinity state in vivo are discussed.

Pharmacological insights into impulsive-compulsive spectrum disorders associated with dopaminergic therapy

Impulsive-compulsive spectrum disorders are associated with dopamine agonist therapy in some patients. These untoward outcomes occur with direct-acting, full and partial agonists at D2 dopamine family receptors. The disorders typically emerge during chronic treatment, and exhibit common features that are independent of the neurological or psychiatric pathology for which the initial therapy was indicated. It is well-documented that the brain is 'plastic', changing in response to alterations to internal factors (e.g., disease processes), as well as external factors (e.g., therapies). The complexities of these clinical scenarios have eluded a clear depiction of the neurobiology for impulsive-compulsive spectrum disorders and engendered considerable debate regarding the mechanistic underpinnings of the disorders. In this opinion, we use pharmacological concepts related to homeostatic compensation subsequent to chronic receptor activation to provide a unifying construct. This construct helps explain the occurrence of impulsive-compulsive spectrum disorders across disease states, and during therapy with full and partial agonists.

Clinical neuropsychiatric considerations regarding nonsubstance or behavioral addictions

Over the past several decades, non-substance-use behaviors like gambling, gaming, and sex have received greater consideration as possible foci of addictions. In this article, I will review the recent history and current status of non-substance or behavioral addictions. A main focus will involve gambling and gambling disorder, given that the latter is currently the sole non-substance addictive disorder described in the main text of the current (fifth) edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Internet gaming disorder, currently in the DSM-5 section addressing conditions that may need additional research, will also be considered, as will the concept of Internet addiction. Compulsive sexual behaviors (including problematic pornography use) will be considered, particularly with respect to how behavioral addictions may be considered in the forthcoming 11th edition of the International Classification of Diseases (ICD-11).

A potential link between gambling addiction severity and central dopamine levels: Evidence from spontaneous eye blink rates

Accumulating evidence points at similarities between substance use disorders (SUD) and gambling disorder on the behavioral and neural level. In SUD, attenuation of striatal D2/3-receptor availability is a consistent finding, at least for stimulating substances. For gambling disorder, no clear association with striatal D2/3-receptor availability has been unveiled so far. With its presumably negligible dopaminergic toxicity, possible differences in receptor availability in gambling disorder might constitute a vulnerability marker. Spontaneous eye blink rate (sEBR) is discussed as a potential proxy measure for striatal dopamine D2/3-receptor availability. Here we examined sEBR in 21 male problem gamblers and 20 healthy control participants. In addition, participants completed a screening questionnaire for overall psychopathology and self-reported measures of alcohol and nicotine consumption. We found no significant difference in sEBR between gamblers and controls. However, in gamblers, sEBR was negatively associated with gambling severity and positively associated with psychopathology. A final exploratory analysis revealed that healthy controls with low sEBR displayed higher alcohol and nicotine consumption than healthy participants with high sEBR. Although the exact association between dopamine transmission and sEBR is still debated, our findings reveal that sEBR is sensitive to inter-individual differences in gambling disorder severity in problem gamblers.

Is Internet Addiction a Clinical Symptom or a Psychiatric Disorder? A Comparison With Bipolar Disorder

The general purpose of this review is to present an updated literature overview of neurobiological/clinical aspects of Internet addiction (IA), particularly of overlaps and differences with bipolar affective disorder (BPAD). Articles with clinical/neurobiological aspects of IA or similarities/differences with BPAD as main topics, from 1990 to present and written in English language, were included. Comorbidity between IA and other psychiatric disorders, including BPAD, is common. Dysfunctions in dopaminergic pathways have been found both in IA and in mood disorders. Most of investigations in IA support a chronic hypodopaminergic dysfunctional state in brain reward circuit and an excessive reward experience during mood elevation. Neuroimaging studies show prefrontal cortex abnormalities shared between addictive and bipolar patients. BPAD and IA present numerous overlaps, such as polymorphisms in nicotinic receptors genes, anterior cingulate/prefrontal cortex abnormalities, serotonin/dopamine dysfunctions, and good response to mood stabilizers. The future is to clarify diagnostic criteria to better define the IA/BPAD relationship.

Increased Striatal Dopamine Synthesis Capacity in Gambling Addiction

BACKGROUND

The hypothesis that dopamine plays an important role in the pathophysiology of pathological gambling is pervasive. However, there is little to no direct evidence for a categorical difference between pathological gamblers and healthy control subjects in terms of dopamine transmission in a drug-free state. Here we provide evidence for this hypothesis by comparing dopamine synthesis capacity in the dorsal and ventral parts of the striatum in 13 pathological gamblers and 15 healthy control subjects.

METHODS

This was achieved using [¹⁸F]fluoro-levo-dihydroxyphenylalanine dynamic positron emission tomography scans and striatal regions of interest that were hand-drawn based on visual inspection of individual structural magnetic resonance imaging scans.

RESULTS

Our results show that dopamine synthesis capacity was increased in pathological gamblers compared with healthy control subjects. Dopamine synthesis was 16% higher in the caudate body, 17% higher in the dorsal putamen, and 17% higher in the ventral striatum in pathological gamblers compared with control subjects. Moreover, dopamine synthesis capacity in the dorsal putamen and caudate head was positively correlated with gambling distortions in pathological gamblers.

CONCLUSIONS

Taken together, these results provide empirical evidence for increased striatal dopamine synthesis in pathological gambling.

Nigrostriatal and Mesolimbic D2/3 Receptor Expression in Parkinson's Disease Patients with Compulsive Reward-Driven Behaviors

The nigrostriatal and mesocorticolimbic dopamine networks regulate reward-driven behavior. Regional alterations to mesolimbic dopamine D_2/3 receptor expression are described in drug-seeking and addiction disorders. Parkinson's disease (PD) patients are frequently prescribed D₂-like dopamine agonist (DAgonist) therapy for motor symptoms, yet a proportion develop clinically significant behavioral addictions characterized by impulsive and compulsive behaviors (ICBs). Until now, changes in D_2/3 receptor binding in both striatal and extrastriatal regions have not been concurrently quantified in this population. We identified 35 human PD patients (both male and female) receiving DAgonist therapy, with (n = 17) and without (n = 18) ICBs, matched for age, disease duration, disease severity, and dose of dopamine therapy. In the off-dopamine state, all completed PET imaging with [¹⁸F]fallypride, a high affinity D₂-like receptor ligand that can measure striatal and extrastriatal D_2/3 nondisplaceable binding potential (BP_ND). Striatal differences between ICB+/ICB- patients localized to the ventral striatum and putamen, where ICB+ subjects had reduced BP_ND In this group, self-reported severity of ICB symptoms positively correlated with midbrain D_2/3 receptor BP_ND Group differences in regional D_2/3 BP_ND relationships were also notable: ICB+ (but not ICB-) patients expressed positive correlations between midbrain and caudate, putamen, globus pallidus, and amygdala BP_NDs. These findings support the hypothesis that compulsive behaviors in PD are associated with reduced ventral and dorsal striatal D_2/3 expression, similar to changes in comparable behavioral disorders. The data also suggest that relatively preserved ventral midbrain dopaminergic projections throughout nigrostriatal and mesolimbic networks are characteristic of ICB+ patients, and may account for differential DAgonist therapeutic response.SIGNIFICANCE STATEMENT The biologic determinants of compulsive reward-based behaviors have broad clinical relevance, from addiction to neurodegenerative disorders. Here, we address biomolecular distinctions in Parkinson's disease patients with impulsive compulsive behaviors (ICBs). This is the first study to image a large cohort of ICB+ patients using positron emission tomography with [18F]fallypride, allowing quantification of D_2/3 receptors throughout the mesocorticolimbic network. We demonstrate widespread differences in dopaminergic networks, including (1) D2-like receptor distinctions in the ventral striatum and putamen, and (2) a preservation of widespread dopaminergic projections emerging from the midbrain, which is associated with the severity of compulsive behaviors. This clearly illustrates the roles of D_2/3 receptors and medication effects in maladaptive behaviors, and localizes them specifically to nigrostriatal and extrastriatal regions.

The neurobiology of impulse control disorders in Parkinson's disease: from neurotransmitters to neural networks

Impulse control disorders (ICD) are common neuropsychiatric disorders that can arise in Parkinson’s disease (PD) patients after commencing dopamine replacement therapy. Approximately 15% of all patients develop these disorders and many more exhibit subclinical symptoms of impulsivity. ICD is thought to develop due to an interaction between the use of dopaminergic medication and an as yet unknown neurobiological vulnerability that either pre-existed before PD onset (possibly genetic) or is associated with neural alterations due to the PD pathology. This review discusses genes, neurotransmitters and neural networks that have been implicated in the pathophysiology of ICD in PD. Although dopamine and the related reward system have been the main focus of research, recently, studies have started to look beyond those systems to find new clues to the neurobiological underpinnings of ICD and come up with possible new targets for treatment. Studies on the whole-brain connectome to investigate the global alterations due to ICD development are currently lacking. In addition, there is a dire need for longitudinal studies that are able to disentangle the contributions of individual (genetic) traits and secondary effects of the PD pathology and chronic dopamine replacement therapy to the development of ICD in PD.

Decision-Making Under Risk, but Not Under Ambiguity, Predicts Pathological Gambling in Discrete Types of Abstinent Substance Users

This study explored how different forms of reward-based decision-making are associated with pathological gambling (PG) among abstinent individuals with prior dependence on different classes of drugs. Participants had lifetime histories of either "pure" heroin dependence (n = 64), "pure" amphetamine dependence (n = 51), or polysubstance dependence (n = 89), or had no history of substance dependence (n = 133). Decision-making was assessed via two neurocognitive tasks: (1) the Iowa Gambling Task (IGT), a measure of decision-making under ambiguity (i.e., uncertain risk contingencies); and (2) the Cambridge Gambling task (CGT), a measure of decision-making under risk (i.e., explicit risk contingencies). The main effects of neurocognitive performance and drug class on PG (defined as ≥3 DSM-IV PG symptoms) as well as their interactional effects were assessed via multiple linear regression. Two CGT indices of decision-making under risk demonstrated positive main effects on PG. Interaction effects indicated that the effects of decision-making under risk on PG were largely consistent across participant groups. Notably, a linear relationship between greater CGT Risk-Taking and PG symptoms was not observed among amphetamine users, whereas IGT performance was selectively and positively associated with PG in polysubstance users. Overall, results indicate that reward-based decision-making under risk may represent a risk factor for PG across substance users, with some variations in these relationships influenced by specific class of substance of abuse.

Effects of disulfiram on choice behavior in a rodent gambling task: association with catecholamine levels

RATIONALE

Gambling disorder is a growing societal concern, as recognized by its recent classification as an addictive disorder in the DSM-5. Case reports have shown that disulfiram reduces gambling-related behavior in humans.

OBJECTIVES

The purpose of the present study was to determine whether disulfiram affects performance on a rat gambling task, a rodent version of the Iowa gambling task in humans, and whether any changes were associated with alterations in dopamine and/or norepinephrine levels.

METHODS

Rats were administered disulfiram prior to testing on the rat gambling task or prior to analysis of dopamine or norepinephrine levels in brain homogenates. Rats in the behavioral task were divided into two subgroups (optimal vs suboptimal) based on their baseline levels of performance in the rat gambling task. Rats in the optimal group chose the advantageous strategy more, and rats in the suboptimal group (a parallel to problem gambling) chose the disadvantageous strategy more. Rats were not divided into optimal or suboptimal groups prior to neurochemical analysis.

RESULTS

Disulfiram administered 2 h, but not 30 min, before the task dose-dependently improved choice behavior in the rats with an initial disadvantageous "gambling-like" strategy, while having no effect on the rats employing an advantageous strategy. The behavioral effects of disulfiram were associated with increased striatal dopamine and decreased striatal norepinephrine.

CONCLUSIONS

These findings suggest that combined actions on dopamine and norepinephrine may be a useful treatment for gambling disorders.

The effects of buspirone on occupancy of dopamine receptors and the rat gambling task

BACKGROUND

The dopamine D3 receptor (DRD₃) has been proposed as a target for drug development for the treatment of addictive disorders. Recently, the anxiolytic buspirone has been shown to have affinity for DRD₃ and DRD₄, and interest in repurposing it for addictive disorders has grown.

METHODS

Binding of [³H]-(+)-PHNO in the rat cerebellum and striatum was used to measure occupancy by buspirone of DRD₃ or DRD₂, respectively. Effects of buspirone in the rat gambling task (rGT) and the five-choice serial reaction time task (5-CSRTT) were examined.

RESULTS

Buspirone occupied both the DRD₂ and DRD₃ at high doses and the DRD₃, but not the DRD₂, in the narrow dose range of 3 mg/kg. At 10 mg/kg, a disruption of performance on rGT was observed. All measures of performance on the rGT, except for perseverations, were affected at 3 mg/kg. On the 5-CSRTT, omissions were increased. Impairments in the rGT were not mimicked by the effects induced by satiation. Further, buspirone did not impair food-maintained responding under a progressive ratio schedule of reinforcement at any dose, suggesting that the effects of buspirone on the rGT cannot be explained by non-selective actions.

CONCLUSIONS

Although buspirone had effects on the rGT at the dose that selectively occupied the DRD₃, the effects found do not parallel those found in previous studies of the effects of selective DRD₃ antagonists on the rGT. Thus, buspirone may impair performance on the rGT through actions at multiple receptor sites.

Evidence for GABA-A receptor dysregulation in gambling disorder: correlation with impulsivity

As a behavioural addiction, gambling disorder (GD) provides an opportunity to characterize addictive processes without the potentially confounding effects of chronic excessive drug and alcohol exposure. Impulsivity is an established precursor to such addictive behaviours, and GD is associated with greater impulsivity. There is also evidence of GABAergic dysregulation in substance addiction and in impulsivity. This study therefore investigated GABA_A receptor availability in 15 individuals with GD and 19 healthy volunteers (HV) using [¹¹C]Ro15‐4513, a relatively selective α5 benzodiazepine receptor PET tracer and its relationship with impulsivity. We found significantly higher [¹¹C]Ro15‐4513 total distribution volume (V_T) in the right hippocampus in the GD group compared with HV. We found higher levels of the ‘Negative Urgency’ construct of impulsivity in GD, and these were positively associated with higher [¹¹C]Ro15‐4513 V_T in the amygdala in the GD group; no such significant correlations were evident in the HV group. These results contrast with reduced binding of GABAergic PET ligands described previously in alcohol and opiate addiction and add to growing evidence for distinctions in the neuropharmacology between substance and behavioural addictions. These results provide the first characterization of GABA_A receptors in GD with [¹¹C]Ro15‐4513 PET and show greater α5 receptor availability and positive correlations with trait impulsivity. This GABAergic dysregulation is potential target for treatment.

BAS-drive trait modulates dorsomedial striatum activity during reward response-outcome associations

According to the Reinforcement Sensitivity Theory, behavioral studies have found that individuals with stronger reward sensitivity easily detect cues of reward and establish faster associations between instrumental responses and reward. Neuroimaging studies have shown that processing anticipatory cues of reward is accompanied by stronger ventral striatum activity in individuals with stronger reward sensitivity. Even though establishing response-outcome contingencies has been consistently associated with dorsal striatum, individual differences in this process are poorly understood. Here, we aimed to study the relation between reward sensitivity and brain activity while processing response-reward contingencies. Forty-five participants completed the BIS/BAS questionnaire and performed a gambling task paradigm in which they received monetary rewards or punishments. Overall, our task replicated previous results that have related processing high reward outcomes with activation of striatum and medial frontal areas, whereas processing high punishment outcomes was associated with stronger activity in insula and middle cingulate. As expected, the individual differences in the activity of dorsomedial striatum correlated positively with BAS-Drive. Our results agree with previous studies that have related the dorsomedial striatum with instrumental performance, and suggest that the individual differences in this area may form part of the neural substrate responsible for modulating instrumental conditioning by reward sensitivity.

[18F]AV-1451 binding to neuromelanin in the substantia nigra in PD and PSP

This study investigated binding of [¹⁸F]AV-1451 to neuromelanin in the substantia nigra of patients with Parkinson's disease (PD) and progressive supranuclear palsy (PSP). [¹⁸F]AV-1451 is a positron emission tomography radiotracer designed to bind pathological tau. A post-mortem study using [¹⁸F]AV-1451 discovered off-target binding properties to neuromelanin in the substantia nigra. A subsequent clinical study reported a 30% decrease in [¹⁸F]AV-1451 binding in the midbrain of PD patients. A total of 12 patients and 10 healthy age-matched controls were recruited. An anatomical MRI and a 90-min PET scan, using [¹⁸F]AV-1451, were acquired from all participants. The standardized uptake value ratio (SUVR) from 60 to 90 min post-injection was calculated for the substantia nigra, using the cerebellar cortex as the reference region. The substantia nigra was delineated using automated region of interest software. An independent samples ANOVA and LSD post hoc testing were used to test for differences in [¹⁸F]AV-1451 SUVR between groups. Substantia nigra SUVR from 60 to 90 min was significantly greater in HC compared to both PSP and PD groups. Although the PD group had the lowest SUVR, there was no significant difference in substantia nigra uptake between PD and PSP. [¹⁸F]AV-1451 may be the first PET radiotracer capable of imaging neurodegeneration of the substantia nigra in parkinsonisms. Further testing must be done in PD and atypical parkinsonian disorders to support this off-target use of [¹⁸F]AV-1451.

Dopamine and Opioid Neurotransmission in Behavioral Addictions: A Comparative PET Study in Pathological Gambling and Binge Eating

Although behavioral addictions share many clinical features with drug addictions, they show strikingly large variation in their behavioral phenotypes (such as in uncontrollable gambling or eating). Neurotransmitter function in behavioral addictions is poorly understood, but has important implications in understanding its relationship with substance use disorders and underlying mechanisms of therapeutic efficacy. Here, we compare opioid and dopamine function between two behavioral addiction phenotypes: pathological gambling (PG) and binge eating disorder (BED). Thirty-nine participants (15 PG, 7 BED, and 17 controls) were scanned with [¹¹C]carfentanil and [¹⁸F]fluorodopa positron emission tomography using a high-resolution scanner. Binding potentials relative to non-displaceable binding (BP_ND) for [¹¹C]carfentanil and influx rate constant (K_i) values for [¹⁸F]fluorodopa were analyzed with region-of-interest and whole-brain voxel-by-voxel analyses. BED subjects showed widespread reductions in [¹¹C]carfentanil BP_ND in multiple subcortical and cortical brain regions and in striatal [¹⁸F]fluorodopa K_i compared with controls. In PG patients, [¹¹C]carfentanil BP_ND was reduced in the anterior cingulate with no differences in [¹⁸F]fluorodopa K_i compared with controls. In the nucleus accumbens, a key region involved in reward processing, [¹¹C]Carfentanil BP_ND was 30-34% lower and [¹⁸F]fluorodopa K_i was 20% lower in BED compared with PG and controls (p<0.002). BED and PG are thus dissociable as a function of dopaminergic and opioidergic neurotransmission. Compared with PG, BED patients show widespread losses of mu-opioid receptor availability together with presynaptic dopaminergic defects. These findings highlight the heterogeneity underlying the subtypes of addiction and indicate differential mechanisms in the expression of pathological behaviors and responses to treatment.

Is there a relation between novelty seeking, striatal dopamine release and frontal cortical thickness?

BACKGROUND

Novelty-seeking (NS) and impulsive personality traits have been proposed to reflect an interplay between fronto-cortical and limbic systems, including the limbic striatum (LS). Although neuroimaging studies have provided some evidence for this, most are comprised of small samples and many report surprisingly large effects given the challenges of trying to relate a snapshot of brain function or structure to an entity as complex as personality. The current work tested a priori hypotheses about associations between striatal dopamine (DA) release, cortical thickness (CT), and NS in a large sample of healthy adults.

METHODS

Fifty-two healthy adults (45M/7F; age: 23.8±4.93) underwent two positron emission tomography scans with [11C]raclopride (specific for striatal DA D2/3 receptors) with or without amphetamine (0.3 mg/kg, p.o.). Structural magnetic resonance image scans were acquired, as were Tridimensional Personality Questionnaire data. Amphetamine-induced changes in [11C]raclopride binding potential values (ΔBPND) were examined in the limbic, sensorimotor (SMS) and associative (AST) striatum. CT measures, adjusted for whole brain volume, were extracted from the dorsolateral sensorimotor and ventromedial/limbic cortices.

RESULTS

BPND values were lower in the amphetamine vs. no-drug sessions, with the largest effect in the LS. When comparing low vs. high LS ΔBPND groups (median split), higher NS2 (impulsiveness) scores were found in the high ΔBPND group. Partial correlations (age and gender as covariates) yielded a negative relation between ASTS ΔBPND and sensorimotor CT; trends for inverse associations existed between ΔBPND values in other striatal regions and frontal CT. In other words, the greater the amphetamine-induced striatal DA response, the thinner the frontal cortex.

CONCLUSIONS

These data expand upon previously reported associations between striatal DA release in the LS and both NS related impulsiveness and CT in the largest sample reported to date. The findings add to the plausibility of these associations while suggesting that the effects are likely weaker than has been previously proposed.