Potential vulnerabilities of neuronal reward, risk, and decision mechanisms to addictive drugs

Focused Ultrasound Combined With Microbubbles Attenuate Symptoms in Heroin-Addicted Mice

OBJECTIVE

To explore the efficacy and mechanisms of stimulating the nucleus accumbens (NAc) in heroin-addicted mice using focused ultrasound and microbubbles (MBs).

METHODS

The conditioned place preference (CPP) method was employed to establish a heroin-addicted mice model. Mice were randomized into control (C), heroin (H), heroin + ultrasound (H + U) and H + U + MBs. Ultrasound (2 MHz fundamental frequency, 1.34 MPa peak-negative pressure, 1 MHz pulse repetition frequency, 5% duty cycle, 15 min/d, over 2 d) was applied to stimulate the NAc in the latter 2 groups. Whereas H + U + MBs received an injection of sulfur hexafluoride MBs during the stimulation. Subsequently, CPP scores, open-field test (OFT), and elevated plus-maze test (EPMT) were conducted to assess behavioral changes in addiction memory, anxiety and exercise status. HE staining was performed to detect pathological structures. Neurotransmitters such as dopamine (DA), serotonin (5-HT) and glutamate (Glu) were detected using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Transmission electron microscopy (TEM) was used to observe ultrastructural changes of synapses in NAc. Immunohistochemistry (IHC) was utilized to detect Cleaved Caspase-3 in the NAc region. Western blotting (WB) was used to detect the protein expression of Cleaved Caspase-3, Bax and Bcl-2 in NAc.

RESULTS

HE staining showed small patches of erythrocyte exudation were observed in the NAc and adjacent areas in H + U + MBs. The CPP scores of H + U + MBs were lower (p < 0.05) than H. After ultrasound treatment, all indices of the OFT and EPMT in H + U + MBs were significantly higher than H (p < 0.05). UPLC-MS/MS revealed that the levels of DA, 5-HT and Glu in H + U + MBs were lower than H (p < 0.01). TEM showed decrease the number of synapses (p < 0.05), and noticeable swelling of mitochondria, membrane damage, as well as damage to the cristae. Further detection by IHC and WB showed that the pro-apoptotic proteins Cleaved Caspase-3 and Bax increased and Bcl-2 decreased as anti-apoptotic proteins after ultrasound combined with MBs (p < 0.05).

CONCLUSION

Focused ultrasound combined with MBs stimulate the NAc can weaken the addictive memory and improve anxiety of heroin-related mice. The mechanical effect of ultrasound combined with the cavitation effect may be a potential treatment for addiction.

Blunted anterior midcingulate response to reward in opioid users is normalized by prefrontal transcranial magnetic stimulation

Introduction

Abnormalities in goal-directed behavior, mediated by mesocorticolimbic reward function and structure, contribute to worse clinical outcomes including higher risk of treatment dropout and drug relapse in opioid users (OU).

Material and Method

In a sham-controlled randomized study design, we measured whether robot-assisted 10Hz transcranial magnetic stimulation (TMS) applied to the prefrontal cortex was able to modulate anterior midcingulate cortex (MCC) electrophysiological response to rewards, in OU and matched healthy controls.

Results

We show that OU exhibit a blunted anterior MCC reward response, compared to healthy controls (t(39) = 2.62, p = 0.01, d = 0.84), and that this is normalized following 10-Hz excitatory TMS (t (36) = .82, p = 0.42, d = 0.17).

Conclusions

Excitatory TMS modulated the putative reward function of the MCC in OU. Further work with increased sample sizes and TMS sessions is required to determine whether restoring MCC reward function increases reward-directed behaviors, which may enhance treatment success through the maintenance of treatment goals.

Learning and memory processes in behavioural addiction: A systematic review

Similar to addictive substances, addictive behaviours such as gambling and gaming are associated with maladaptive modulation of key brain areas and functional networks implicated in learning and memory. Therefore, this review sought to understand how different learning and memory processes relate to behavioural addictions and to unravel their underlying neural mechanisms. Adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, we systematically searched four databases - PsycINFO, PubMed, Scopus, and Web of Science using the agreed-upon search string. Findings suggest altered executive function-dependent learning processes and enhanced habit learning in behavioural addiction. Whereas the relationship between working memory and behavioural addiction is influenced by addiction type, working memory aspect, and task nature. Additionally, long-term memory is incoherent in individuals with addictive behaviours. Consistently, neurophysiological evidence indicates alterations in brain areas and networks implicated in learning and memory processes in behavioural addictions. Overall, the present review argues that, like substance use disorders, alteration in learning and memory processes may underlie the development and maintenance of behavioural addictions.

Recording Neural Reward Signals in a Naturalistic Operant Task Using Mobile-EEG and Augmented Reality

The electrophysiological response to rewards recorded during laboratory tasks has been well documented, yet little is known about the neural response patterns in a more naturalistic setting. Here, we combined a mobile-EEG system with an augmented reality headset to record event-related brain potentials (ERPs) while participants engaged in a naturalistic operant task to find rewards. Twenty-five participants were asked to navigate toward a west or east goal location marked by floating orbs, and once participants reached the goal location, the orb would then signify a reward (5 cents) or no-reward (0 cents) outcome. Following the outcome, participants returned to a start location marked by floating purple rings, and once standing in the middle, a 3 s counter signaled the next trial, for a total of 200 trials. Consistent with previous research, reward feedback evoked the reward positivity, an ERP component believed to index the sensitivity of the anterior cingulate cortex to reward prediction error signals. The reward positivity peaked ∼230 ms with a maximal at channel FCz (M = -0.695 μV, ±0.23) and was significantly different than zero (p < 0.01). Participants took ∼3.38 s to reach the goal location and exhibited a general lose-shift (68.3% ±3.5) response strategy and posterror slowing. Overall, these novel findings provide support for the idea that combining mobile-EEG with augmented reality technology is a feasible solution to enhance the ecological validity of human electrophysiological studies of goal-directed behavior and a step toward a new era of human cognitive neuroscience research that blurs the line between laboratory and reality.

D1 and D2 medium spiny neurons in the nucleus accumbens core have distinct and valence-independent roles in learning

At the core of value-based learning is the nucleus accumbens (NAc). D1- and D2-receptor-containing medium spiny neurons (MSNs) in the NAc core are hypothesized to have opposing valence-based roles in behavior. Using optical imaging and manipulation approaches in mice, we show that neither D1 nor D2 MSNs signal valence. D1 MSN responses were evoked by stimuli regardless of valence or contingency. D2 MSNs were evoked by both cues and outcomes, were dynamically changed with learning, and tracked valence-free prediction error at the population and individual neuron level. Finally, D2 MSN responses to cues were necessary for associative learning. Thus, D1 and D2 MSNs work in tandem, rather than in opposition, by signaling specific properties of stimuli to control learning.

Dopamine determines how reward overrides risk

Why do animals pursue reward in the face of punishment? Dopamine-releasing neurons that promote reward-seeking behaviour indirectly impair those that encode punishment avoidance, affecting decisions on risk.

Recording neural reward signals in the real-world using mobile-EEG and augmented reality

The electrophysiological response to rewards recorded during laboratory-based tasks has been well documented over the past two decades, yet little is known about the neural response patterns in 'real-world' settings. To address this issue, we combined a mobile-EEG system with an augmented reality headset (which blends high definition "holograms" within the real-world) to record event-related brain potentials (ERP) while participants navigated an operant chamber to find rewards. 25 participants (age = 18-43, Male=6, Female=19) were asked to choose between two floating holograms marking a west or east goal-location in a large room, and once participants reached the goal location, the hologram would turn into a reward (5 cents) or no-reward (0 cents) cue. Following the feedback cue, participants were required to return to a hologram marking the start location, and once standing in it, a 3 second counter hologram would initiate the next trial. This sequence was repeated until participants completed 200 trials. Consistent with previous research, reward feedback evoked the reward positivity, an ERP component believed to index the sensitivity of the anterior cingulate cortex to reward prediction error signals. The reward positivity peaked around 235ms post-feedback with a maximal at channel FCz (M=-2.60μV, SD=1.73μV) and was significantly different than zero (p < 0.01). At a behavioral level, participants took approximately 3.38 seconds to reach the goal-location and exhibited a general lose-shift (68.3% ± 3.5) response strategy and were slightly slower to return to the start location following negative feedback (2.43 sec) compared to positive feedback (2.38 sec), evidence of post-error slowing. Overall, these findings provide the first evidence that combining mobile-EEG with augmented reality technology is a feasible solution to enhance the ecological validity of human electrophysiological studies of goal-directed behavior and a step towards a new era of human cognitive neuroscience research that blurs the line between laboratory and reality.

Neurocognitive predictors of addiction-related outcomes: A systematic review of longitudinal studies

It is well-established that addiction is typically associated with a distinct pattern of neurocognitive functioning with a consensus that it is typified by impaired top-down executive control and aberrant risk-reward processing. Despite a consensus that neurocognition plays an important role in characterizing and maintaining addictive disorders, there is a lack of systematic, bottom-up synthesis of quantitative evidence showing that neurocognition predicts addictive behaviors, and which neurocognitive constructs have the best predictive validity. This systematic review aimed to assess whether cognitive control and risk-reward processes as defined by the Research Domain Criteria (RDoC) predict the development and maintenance of addictive behaviors specifically, consumption, severity, and relapse. The findings from this review expose the substantial lack of evidence for neurocognition predicting addiction outcomes. However, there is evidence that suggests reward-related neurocognitive processes may be important for the detection of early risk for addiction, as well as a potentially viable target for designing novel, more effective interventions.

The scratch card gambler: a hidden reality

Scratch cards-otherwise known as scratch tickets or instant lotteries-are a highly accessible type of lottery, due to its relative accessibility and affordability. In Portugal, the popularity of scratch cards has experienced substantial growth, with almost no regulatory reaction whatsoever. This study aims to describe the sociodemographic characteristics of scratch card gamblers, prevalent gambling habits, and their perceptions regarding scratch card gambling. This study also determines the constancy of pathological scratch card gambling, and the possible impact of regulatory measures. We found that about half of the participants studied were at risk of pathological gambling and scratch cards seem to have a clear potential for enticing higher spending in vulnerable consumers, compared to other forms of gambling games. Perception biases regarding gambling are frequent and almost no one afflicted seeks help. Regulatory measures are crucial to regulate potential problematic behaviors, specifically in high-risk persons. There is an urgent need to act, made clear by both present findings and numerous precedent warnings regarding scratch card gambling hazards.

Differential transcriptome profile underlying risky choice in a rat gambling task

BACKGROUND AND AIMS

Proper measurement of expected risk is important for making rational decisions, and maladaptive decision making may underlie various psychiatric disorders. However, differentially expressed genetic profiling involved in this process is still largely unknown. A rodent version of the gambling task (rGT) has been developed to measure decision-making by adopting the same principle of Iowa Gambling Task in humans. In the present study, we examined using next-generation sequencing (NGS) technique whether there are differences in gene expression profiles in the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAc) when rats make different choices toward risk in rGT.

METHODS

Rats were trained in a touch screen chamber to learn the relationships between 4 different light signals on the window of the screen and accompanied reward outcomes or punishments set up with different magnitudes and probabilities. Once they showed a stabilized pattern of preference upon free choice, rats were classified into risk-averse or risk-seeking groups. After performing the rGT, rats were decapitated, the mPFC and the NAc was dissected out, and NGS was performed with the total RNA extracted.

RESULTS

We found that 477 and 36 genes were differentially expressed (approximately 75 and 83% out of them were downregulated) in the mPFC and the NAc, respectively, in risk-seeking compared to risk-averse rats. Among those, we suggested a few top ranked genes that may contribute to promoting risky choices.

DISCUSSION AND CONCLUSIONS

Our findings provide insights into transcriptional components underlying risky choices in rats.

A Systematic Review on Common and Distinct Neural Correlates of Risk-taking in Substance-related and Non-substance Related Addictions

Both substance-related as well as non-substance-related addictions may include recurrent engagement in risky actions despite adverse outcomes. We here apply a unified approach and review task-based neuroimaging studies on substance-related (SRAs) and non-substance related addictions (NSRAs) to examine commonalities and differences in neural correlates of risk-taking in these two addiction types. To this end, we conducted a systematic review adhering to the PRISMA guidelines. Two databases were searched with predefined search terms to identify neuroimaging studies on risk-taking tasks in individuals with addiction disorders. In total, 19 studies on SRAs (comprising a total of 648 individuals with SRAs) and 10 studies on NSRAs (comprising a total of 187 individuals with NSRAs) were included. Risk-related brain activation in SRAs and NSRAs was summarized individually and subsequently compared to each other. Results suggest convergent altered risk-related neural processes, including hyperactivity in the OFC and the striatum. As characteristic for both addiction types, these brain regions may represent an underlying mechanism of suboptimal decision-making. In contrast, decreased DLPFC activity may be specific to SRAs and decreased IFG activity could only be identified for NSRAs. The precuneus and posterior cingulate show elevated activity in SRAs, while findings regarding these areas were mixed in NSRAs. Additional scarce evidence suggests decreased ventral ACC activity and increased dorsal ACC activity in both addiction types. Associations between identified activation patterns with drug use severity underpin the clinical relevance of these findings. However, this exploratory evidence should be interpreted with caution and should be regarded as preliminary. Future research is needed to evaluate the findings gathered by this review.

[Positive cognitive neuroscience : Positive valence systems of the Research Domain Criteria initiative]

In diesem Artikel werden die Domäne „positive Valenzsysteme“ (PVS) der Research-Domain-Criteria(RDoC)-Matrix sowie ihre Subkonstrukte dargestellt und erläutert. Unter PVS fallen im Wesentlichen verschiedene Formen und Prozesse der Belohnungsverarbeitung. Diese werden in der Psychiatrie schon seit Jahrzehnten im Bereich von Sucht, Schizophrenie und Depression untersucht und letztere sind daher nicht Gegenstand dieses Artikels. Hier soll vielmehr die heuristische Fruchtbarkeit der RDoC-Systematik für das Verständnis anderer Erkrankungen und Konstrukte dargestellt werden und zwar für das transdiagnostische Konstrukt der Anhedonie sowie für die Autismusspektrumstörung und die Gruppe der Essstörungen. Weiterhin wird gezeigt, wie die PVS-Domäne auch klinisch den Blick über die traditionelle Psychopathologie erweitert und wie sie die Entwicklung neuer behavioraler Messinstrumente angeregt hat. Abschließend wird auf Limitationen und mögliche zukünftige Erweiterungen des Ansatzes eingegangen.

Linking RDoC and HiTOP: A new interface for advancing psychiatric nosology and neuroscience

The Research Domain Criteria (RDoC) and the Hierarchical Taxonomy of Psychopathology (HiTOP) represent major dimensional frameworks proposing two alternative approaches to accelerate progress in the way psychopathology is studied, classified, and treated. RDoC is a research framework rooted in neuroscience aiming to further the understanding of transdiagnostic biobehavioral systems underlying psychopathology and ultimately inform future classifications. HiTOP is a dimensional classification system, derived from the observed covariation among symptoms of psychopathology and maladaptive traits, which seeks to provide more informative research and treatment targets (i.e., dimensional constructs and clinical assessments) than traditional diagnostic categories. This article argues that the complementary strengths of RDoC and HiTOP can be leveraged in order to achieve their respective goals. RDoC's biobehavioral framework may help elucidate the underpinnings of the clinical dimensions included in HiTOP, whereas HiTOP may provide psychometrically robust clinical targets for RDoC-informed research. We present a comprehensive mapping between dimensions included in RDoC (constructs and subconstructs) and HiTOP (spectra and subfactors) based on narrative review of the empirical literature. The resulting RDoC-HiTOP interface sheds light on the biobehavioral correlates of clinical dimensions and provides a broad set of dimensional clinical targets for etiological and neuroscientific research. We conclude with future directions and practical recommendations for using this interface to advance clinical neuroscience and psychiatric nosology. Ultimately, we envision that this RDoC-HiTOP interface has the potential to inform the development of a unified, dimensional, and biobehaviorally-grounded psychiatric nosology.

Drug overdose deaths, addiction neuroscience and the challenges of translation

In this article, we argue that the rapid rise in drug overdose deaths in America is a tragedy that draws attention to fundamental conceptual and experimental problems in addiction science that have significant human consequences. Despite enormous economic investment, political support and claims to have revolutionised addiction medicine, neurobiological models are yet to produce a treatment for substance addiction. This is partly, we claim, because neurobiology is unable to explain essential features of addiction and relapse that neurobehavioral models of addiction are better placed to investigate. We show how addiction neuroscience turned to long-term memory to explain the chronicity of addiction and persistent relapses long after neurochemical traces have left the body. The turn to memory may in time help to close the translational gap facing addiction medicine, but it is our view in this article that the primary value of memory theory lays in its potential to create new critical friendships between biological and social sciences that are attuned to the lived experience and suffering of stigmatised people. The value of the memory turn may rest upon the capacity of these critical friendships to wean addiction science off its long-term dependence on disease concepts of human distress.

Crisis, what crisis? Addiction neuroscience and the challenges of translation

In this article we interrogate the claim that there is an opioid crisis: a dramatic rise in drug overdose fatalities in the United States over the past two decades that is also spreading to other countries.  The usual argument is that this crisis is largely explained by errant prescription practices leading to an oversupply of opioids, leading to addiction, premature mortality and drug overdose deaths, both among those prescribed opioids for pain relief, and those obtaining them on the illegal market.  We argue, that this view is highly problematic and that it is likely to entrench deeper problems with how substance addiction has been perceived and known. In this article, we develop an alternative picture of the addiction crisis based on four years of research and collaboration with addiction neuroscientists. Drug overdose deaths, we claim, are symptoms of what we term the ‘structural distribution of social despair.’ We argue that this is compounded by a translation crisis at the heart of addiction neuroscience. For all its dominance, the ‘dopamine hypothesis’ of addiction that shaped understandings for some three decades, has still not produced a single effective treatment. However, this translation crisis also represents an opportunity for ‘the memory turn’ in addiction neuroscience as it seeks to translate its emerging conception of addiction as a problem of memory into effective forms of treatment. We conclude by arguing that, for the ‘memory turn’ to underpin effective interventions into ‘the opioid crisis’, a new relation between neuroscientists and social scientists of addiction is needed, one that proceeds from the lived experience of human beings.

The opioid crisis: a contextual, social-ecological framework

The prevalence of opioid use and misuse has provoked a staggering number of deaths over the past two and a half decades. Much attention has focused on individual risks according to various characteristics and experiences. However, broader social and contextual domains are also essential contributors to the opioid crisis such as interpersonal relationships and the conditions of the community and society that people live in. Despite efforts to tackle the issue, the rates of opioid misuse and non-fatal and fatal overdose remain high. Many call for a broad public health approach, but articulation of what such a strategy could entail has not been fully realised. In order to improve the awareness surrounding opioid misuse, we developed a social-ecological framework that helps conceptualise the multivariable risk factors of opioid misuse and facilitates reviewing them in individual, interpersonal, communal and societal levels. Our framework illustrates the multi-layer complexity of the opioid crisis that more completely captures the crisis as a multidimensional issue requiring a broader and integrated approach to prevention and treatment.

Deep brain stimulation of the orbitofrontal cortex prevents the development and reinstatement of morphine place preference

The orbitofrontal cortex (OFC) is involved in compulsive drug seeking and drug relapse. Its involvement in cue-, context-, and stress-induced reinstatement of drug seeking has also been confirmed in animal models. Deep brain stimulation (DBS) was proposed to be an effective intervention for patients with treatment-refractory addiction. Therefore, in the present study, we investigated the potential efficacy of DBS in the OFC for controlling addictive-like behaviors in rats. Rats were bilaterally implanted with electrodes in the OFC and trained to the morphine conditioned place preference (CPP; 3, 5, and 7 mg/kg). High-frequency (HF; 130 Hz) or low-frequency (LF; 13 Hz) DBS-like stimulation was applied during the conditioning (40 minutes, once daily, 3 days) or extinction (20 minutes, once daily, 6-10 days) trials. Following the extinction, morphine preference was reinstated by a priming dose of morphine (2 mg/kg). When applied during the conditioning phase, HF-DBS significantly decreased preference for the morphine-associated context. HF-DBS during the extinction phase of morphine CPP reduced the number of days to full extinction of morphine preference and prevented morphine priming-induced recurrence of morphine preference. LF-DBS did not change any of these addictive behaviors. HF-DBS had no significant effect on novel object recognition memory. In conclusion, HF-DBS of the OFC prevented morphine preference, facilitated extinction of morphine preference, and blocked drug priming-induced reinstatement of morphine seeking. These findings may indicate a potential applicability of DBS in the treatment of relapse to drug use. Further studies will be necessary to assess the translatability of these findings to the clinic.

Reward, Control & Decision-Making in Cannabis Use Disorder: Insights from Functional MRI

The recreational consumption of cannabis has increased significantly across the world with an estimated 180 million people currently using. In the United States, 4.1 million are currently diagnosed with cannabis use disorder. Cannabis dependence and abuse was combined into a single entity as a behavioral disorder with a problematic pattern of cannabis use and termed cannabis use disorder by the Diagnostic and Statistical Manual of Mental Disorders. Chronic use of cannabis has been linked with region-specific effects across the brain mediating reward processing, cognitive control and decision-making that are central to understanding addictive behaviors. This review presents a snapshot of the current literature assessing the effects of chronic cannabis use on human brain function via functional MRI. Studies employing various paradigms and contrasting cognitive activation amongst cannabis users and non-users were incorporated. The effects of trans-del-ta-9-tetrahydrocannabinol (Δ9-THC) in marijuana and other preparations of cannabis are mediated by the endocannabinoid system, which is also briefly introduced.Much variation exists in the current literature regarding the functional changes associated with chronic cannabis use. One possible explanation for this variation is the heterogeneity in study designs, with little implementation of standardized diagnostic criteria when selecting chronic users, distinct time points of participant assessment, differing cognitive paradigms and imaging protocols. As such, there is an urgent requirement for future investigations that further characterize functional changes associated with chronic cannabis use.

Cell-type and projection-specific dopaminergic encoding of aversive stimuli in addiction

Drug addiction is a major public health concern across the world for which there are limited treatment options. In order to develop new therapies to correct the behavioral deficits that result from repeated drug use, we need to understand the neural circuit dysfunction that underlies the pathophysiology of the disorder. Because the initial reinforcing effects of drugs are dependent on increases in dopamine in reward-related brain regions such as the mesolimbic dopamine pathway, a large focus of addiction research has centered on the dysregulation of this system and its control of positive reinforcement and motivation. However, in addition to the processing of positive, rewarding stimuli, there are clear deficits in the encoding and valuation of information about potential negative outcomes and how they control decision making and motivation. Further, aversive stimuli can motivate or suppress behavior depending on the context in which they are encountered. We propose a model where rewarding and aversive information guides the execution of specific motivated actions through mesocortical and mesolimbic dopamine acting on D1- and D2- receptor containing neuronal populations. Volitional drug exposure alters the processing of rewarding and aversive stimuli through remodeling of these dopaminergic circuits, causing maladaptive drug seeking, self-administration in the face of negative consequences, and drug craving. Together, this review discusses the dysfunction of the circuits controlling different types of aversive learning as well as how these guide specific discrete behaviors, and provides a conceptual framework for how they should be considered in preclinical addiction models.

Revealing Dissociable Attention Biases in Chronic Smokers Through an Individual-Differences Approach

Addiction is accompanied by attentional biases (AB), wherein drug-related cues grab attention independently of their perceptual salience. AB have emerged in different flavours depending on the experimental approach, and their clinical relevance is still debated. In chronic smokers we sought evidence for dissociable attention abnormalities that may play distinct roles in the clinical manifestations of the disorder. Fifty smokers performed a modified visual probe-task measuring two forms of AB and their temporal dynamics, and data on their personality traits and smoking history/status were collected. Two fully dissociable AB effects were found: A Global effect, reflecting the overall impact of smoke cues on attention, and a Location-specific effect, indexing the impact of smoke cues on visuospatial orienting. Importantly, the two effects could be neatly separated from one another as they: (i) unfolded with dissimilar temporal dynamics, (ii) were accounted for by different sets of predictors associated with personality traits and smoking history and (iii) were not correlated with one another. Importantly, the relevance of each of these two components in the single individual depends on a complex blend of personality traits and smoking habits, a result that future efforts addressing the clinical relevance of addiction-related AB should take into careful consideration.

In Cocaine Dependence, Neural Prediction Errors During Loss Avoidance Are Increased With Cocaine Deprivation and Predict Drug Use

BACKGROUND

In substance-dependent individuals, drug deprivation and drug use trigger divergent behavioral responses to environmental cues. These divergent responses are consonant with data showing that short- and long-term adaptations in dopamine signaling are similarly sensitive to state of drug use. The literature suggests a drug state-dependent role of learning in maintaining substance use; evidence linking dopamine to both reinforcement learning and addiction provides a framework to test this possibility.

METHODS

In a randomized crossover design, 22 participants with current cocaine use disorder completed a probabilistic loss-learning task during functional magnetic resonance imaging while on and off cocaine (44 sessions). Another 54 participants without Axis I psychopathology served as a secondary reference group. Within-drug state and paired-subjects' learning effects were assessed with computational model-derived individual learning parameters. Model-based neuroimaging analyses evaluated effects of drug use state on neural learning signals. Relationships among model-derived behavioral learning rates (α+, α-), neural prediction error signals (δ+, δ-), cocaine use, and desire to use were assessed.

RESULTS

During cocaine deprivation, cocaine-dependent individuals exhibited heightened positive learning rates (α+), heightened neural positive prediction error (δ+) responses, and heightened association of α+ with neural δ+ responses. The deprivation-enhanced neural learning signals were specific to successful loss avoidance, comparable to participants without psychiatric conditions, and mediated a relationship between chronicity of drug use and desire to use cocaine.

CONCLUSIONS

Neurocomputational learning signals are sensitive to drug use status and suggest that heightened reinforcement by successful avoidance of negative outcomes may contribute to drug seeking during deprivation. More generally, attention to drug use state is important for delineating substrates of addiction.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Reward Learning over Weeks Versus Minutes Increases the Neural Representation of Value in the Human Brain

Over the past few decades, neuroscience research has illuminated the neural mechanisms supporting learning from reward feedback. Learning paradigms are increasingly being extended to study mood and psychiatric disorders as well as addiction. However, one potentially critical characteristic that this research ignores is the effect of time on learning: human feedback learning paradigms are usually conducted in a single rapidly paced session, whereas learning experiences in ecologically relevant circumstances and in animal research are almost always separated by longer periods of time. In our experiments, we examined reward learning in short condensed sessions distributed across weeks versus learning completed in a single "massed" session in male and female participants. As expected, we found that after equal amounts of training, accuracy was matched between the spaced and massed conditions. However, in a 3-week follow-up, we found that participants exhibited significantly greater memory for the value of spaced-trained stimuli. Supporting a role for short-term memory in massed learning, we found a significant positive correlation between initial learning and working memory capacity. Neurally, we found that patterns of activity in the medial temporal lobe and prefrontal cortex showed stronger discrimination of spaced- versus massed-trained reward values. Further, patterns in the striatum discriminated between spaced- and massed-trained stimuli overall. Our results indicate that single-session learning tasks engage partially distinct learning mechanisms from distributed training. Our studies begin to address a large gap in our knowledge of human learning from reinforcement, with potential implications for our understanding of mood disorders and addiction.SIGNIFICANCE STATEMENT Humans and animals learn to associate predictive value with stimuli and actions, and these values then guide future behavior. Such reinforcement-based learning often happens over long time periods, in contrast to most studies of reward-based learning in humans. In experiments that tested the effect of spacing on learning, we found that associations learned in a single massed session were correlated with short-term memory and significantly decayed over time, whereas associations learned in short massed sessions over weeks were well maintained. Additionally, patterns of activity in the medial temporal lobe and prefrontal cortex discriminated the values of stimuli learned over weeks but not minutes. These results highlight the importance of studying learning over time, with potential applications to drug addiction and psychiatry.

Neural encoding of choice during a delayed response task in primate striatum and orbitofrontal cortex

Reward outcomes are available in many diverse situations and all involve choice. If there are multiple outcomes each rewarding, then decisions regarding relative value lead to choosing one over another. Important factors related to choice context should be encoded and utilized for this form of adaptive choosing. These factors can include the number of alternatives, the pacing of choice behavior and the possibility to reverse one's choice. An essential step in understanding if the context of choice is encoded is to directly compare choice with a context in which choice is absent. Neural activity in orbitofrontal cortex and striatum encodes potential value parameters related to reward quality and quantity as well as relative preference. We examined how neural activations in these brain regions are sensitive to choice situations and potentially involved in a prediction for the upcoming outcome selection. Neural activity was recorded and compared between a two-choice spatial delayed response task and an imperative 'one-option' task. Neural activity was obtained that extended from the instruction cue to the movement similar to previous work utilizing the identical imperative task. Orbitofrontal and striatal neural responses depended upon the decision about the choice of which reward to collect. Moreover, signals to predictive instruction cues that precede choice were selective for the choice situation. These neural responses could reflect chosen value with greater information on relative value of individual options as well as encode choice context itself embedded in the task as a part of the post-decision variable.

Restoration of Kv7 Channel-Mediated Inhibition Reduces Cued-Reinstatement of Cocaine Seeking

Cocaine addicts display increased sensitivity to drug-associated cues, due in part to changes in the prelimbic prefrontal cortex (PL-PFC). The cellular mechanisms underlying cue-induced reinstatement of cocaine seeking remain unknown. Reinforcement learning for addictive drugs may produce persistent maladaptations in intrinsic excitability within sparse subsets of PFC pyramidal neurons. Using a model of relapse in male rats, we sampled >600 neurons to examine spike frequency adaptation (SFA) and afterhyperpolarizations (AHPs), two systems that attenuate low-frequency inputs to regulate neuronal synchronization. We observed that training to self-administer cocaine or nondrug (sucrose) reinforcers decreased SFA and AHPs in a subpopulation of PL-PFC neurons. Only with cocaine did the resulting hyperexcitability persist through extinction training and increase during reinstatement. In neurons with intact SFA, dopamine enhanced excitability by inhibiting Kv7 potassium channels that mediate SFA. However, dopamine effects were occluded in neurons from cocaine-experienced rats, where SFA and AHPs were reduced. Pharmacological stabilization of Kv7 channels with retigabine restored SFA and Kv7 channel function in neuroadapted cells. When microinjected bilaterally into the PL-PFC 10 min before reinstatement testing, retigabine reduced cue-induced reinstatement of cocaine seeking. Last, using cFos-GFP transgenic rats, we found that the loss of SFA correlated with the expression of cFos-GFP following both extinction and re-exposure to drug-associated cues. Together, these data suggest that cocaine self-administration desensitizes inhibitory Kv7 channels in a subpopulation of PL-PFC neurons. This subpopulation of neurons may represent a persistent neural ensemble responsible for driving drug seeking in response to cues.SIGNIFICANCE STATEMENT Long after the cessation of drug use, cues associated with cocaine still elicit drug-seeking behavior, in part by activation of the prelimbic prefrontal cortex (PL-PFC). The underlying cellular mechanisms governing these activated neurons remain unclear. Using a rat model of relapse to cocaine seeking, we identified a population of PL-PFC neurons that become hyperexcitable following chronic cocaine self-administration. These neurons show persistent loss of spike frequency adaptation, reduced afterhyperpolarizations, decreased sensitivity to dopamine, and reduced Kv7 channel-mediated inhibition. Stabilization of Kv7 channel function with retigabine normalized neuronal excitability, restored Kv7 channel currents, and reduced drug-seeking behavior when administered into the PL-PFC before reinstatement. These data highlight a persistent adaptation in a subset of PL-PFC neurons that may contribute to relapse vulnerability.

Presynaptic α2-adrenoceptor modulates glutamatergic synaptic transmission in rat nucleus accumbens in vitro

The nucleus accumbens (NAc), integrating information from the prefrontal cortex and limbic structures, plays a critical role in reward and emotion regulation. Previous studies have reported that the NAc shell receives direct noradrenergic projections, and activation of α₂-adrenoceptor (α₂-AR) in the NAc shell decreases the fear or anxiety level of rats. However, the underlying mechanism is still little known. Intriguingly, glutamatergic neurotransmission in the NAc shell is closely related to reward and emotion. Here, using brain slice preparations and whole-cell patch clamp recordings, we examined the effect of activation of α₂-AR on glutamatergic neurotransmission in the NAc shell. Perfusing slice with α₂-AR selective agonist clonidine (CLON) reduced the evoked excitatory postsynaptic currents (EPSCs) on the NAc shell neurons. This inhibitory effect on AMPA-mediated glutamatergic EPSCs was blocked by the α₂-AR selective antagonist yohimbine (YOH). Notably, CLON reduced the frequency but not the amplitude of miniature EPSCs. Furthermore, CLON decreased the first EPSC amplitude but increased the paired-pulse facilitation on the NAc shell neurons, and it did not affect postsynaptic AMPA/NMDA ratio, revealing a presynaptic mechanism of α₂-AR-mediated inhibition on glutamatergic transmission. In addition, the modulation on glutamatergic transmission by α₂-AR was independent of presynaptic NMDA receptor. These results suggest that noradrenergic afferent inputs may suppress glutamatergic synaptic transmission via presynaptic α₂-AR in the NAc shell, and actively participate in rewarding and emotional processes via the NAc.

Dopamine terminals from the ventral tegmental area gate intrinsic inhibition in the prefrontal cortex

Spike frequency adaptation (SFA or accommodation) and calcium‐activated potassium channels that underlie after‐hyperpolarization potentials (AHP) regulate repetitive firing of neurons. Precisely how neuromodulators such as dopamine from the ventral tegmental area (VTA) regulate SFA and AHP (together referred to as intrinsic inhibition) in the prefrontal cortex (PFC) remains unclear. Using whole cell electrophysiology, we measured intrinsic inhibition in prelimbic (PL) layer 5 pyramidal cells of male adult rats. Results demonstrate that bath application of dopamine reduced intrinsic inhibition (EC₅₀: 25.0 μmol/L). This dopamine action was facilitated by coapplication of cocaine (1 μmol/L), a blocker of dopamine reuptake. To evaluate VTA dopamine terminals in PFC slices, we transfected VTA dopamine cells of TH::Cre rats in vivo with Cre‐dependent AAVs to express channelrhodopsin‐2 (ChR2) or designer receptors exclusively activated by designer drugs (DREADDS). In PFC slices from these animals, stimulation of VTA terminals with either blue light to activate ChR2 or bath application of clozapine‐N‐oxide (CNO) to activate Gq‐DREADDs produced a similar reduction in intrinsic inhibition in PL neurons. Electrophysiological recordings from cells expressing retrograde fluorescent tracers showed that this plasticity occurs in PL neurons projecting to the accumbens core. Collectively, these data highlight an ability of VTA terminals to gate intrinsic inhibition in the PFC, and under appropriate circumstances, enhance PL neuronal firing. These cellular actions of dopamine may be important for dopamine‐dependent behaviors involving cocaine and cue‐reward associations within cortical–striatal circuits.

Working memory, attention, and salience in active inference

The psychological concepts of working memory and attention are widely used in the cognitive and neuroscientific literatures. Perhaps because of the interdisciplinary appeal of these concepts, the same terms are often used to mean very different things. Drawing on recent advances in theoretical neurobiology, this paper tries to highlight the correspondence between these established psychological constructs and the formal processes implicit in mathematical descriptions of brain function. Here, we consider attention and salience from the perspective offered by active inference. Using variational principles and simulations, we use active inference to demonstrate how attention and salience can be disambiguated in terms of message passing between populations of neurons in cortical and subcortical structures. In brief, we suggest that salience is something that is afforded to actions that realise epistemic affordance, while attention per se is afforded to precise sensory evidence - or beliefs about the causes of sensations.

Accumbens Mechanisms for Cued Sucrose Seeking

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

Genetic moderation of cocaine subjective effects by variation in the TPH1, TPH2, and SLC6A4 serotonin genes

OBJECTIVE

This study investigated variants of tryptophan hydroxylase (TPH)1, TPH2, and SLC6A4 in the moderation of the subjective effects of cocaine.

METHODS

Non-treatment-seeking cocaine-dependent individuals (N=66) were intravenously administered saline and cocaine (40 mg) in a randomized order. Participants self-reported subjective effects of cocaine using a visual analog scale starting before administration of saline or cocaine (-15 min) to up to 20 min after infusion. Self-report ratings on the visual analog scale ranged from 0 (no effect) to 100 (greatest effect). Participants were genotyped for the TPH1 rs1799913, TPH2 rs4290270, and SLC6A4 5-HTTLPR variants. Repeated-measures analysis of covariance was used to examine changes in subjective effect scores over time while controlling for population structure.

RESULTS

Participants carrying the TPH1 rs1799913 A allele reported greater subjective response to cocaine for 'stimulated' and 'access' relative to the CC genotype group. Those carrying the TPH2 rs4290270 A allele reported higher 'good effect' and lower 'depressed' effect relative to the TT genotype group. Those carrying the SLC6A4 5-HTTLPR S' allele reported greater 'desire' and 'access' compared with the L'L' genotype group.

CONCLUSION

These findings indicate that TPH1, TPH2, and SLC6A4 variants moderate the subjective effects of cocaine in non-treatment-seeking cocaine-dependent participants.

Reward Prediction Errors in Drug Addiction and Parkinson's Disease: from Neurophysiology to Neuroimaging

PURPOSE OF REVIEW

Surprises are important sources of learning. Cognitive scientists often refer to surprises as "reward prediction errors," a parameter that captures discrepancies between expectations and actual outcomes. Here, we integrate neurophysiological and functional magnetic resonance imaging (fMRI) results addressing the processing of reward prediction errors and how they might be altered in drug addiction and Parkinson's disease.

RECENT FINDINGS

By increasing phasic dopamine responses, drugs might accentuate prediction error signals, causing increases in fMRI activity in mesolimbic areas in response to drugs. Chronic substance dependence, by contrast, has been linked with compromised dopaminergic function, which might be associated with blunted fMRI responses to pleasant non-drug stimuli in mesocorticolimbic areas. In Parkinson's disease, dopamine replacement therapies seem to induce impairments in learning from negative outcomes. The present review provides a holistic overview of reward prediction errors across different pathologies and might inform future clinical strategies targeting impulsive/compulsive disorders.

Adaptive Value Normalization in the Prefrontal Cortex Is Reduced by Memory Load

Adaptation facilitates neural representation of a wide range of diverse inputs, including reward values. Adaptive value coding typically relies on contextual information either obtained from the environment or retrieved from and maintained in memory. However, it is unknown whether having to retrieve and maintain context information modulates the brain's capacity for value adaptation. To address this issue, we measured hemodynamic responses of the prefrontal cortex (PFC) in two studies on risky decision-making. In each trial, healthy human subjects chose between a risky and a safe alternative; half of the participants had to remember the risky alternatives, whereas for the other half they were presented visually. The value of safe alternatives varied across trials. PFC responses adapted to contextual risk information, with steeper coding of safe alternative value in lower-risk contexts. Importantly, this adaptation depended on working memory load, such that response functions relating PFC activity to safe values were steeper with presented versus remembered risk. An independent second study replicated the findings of the first study and showed that similar slope reductions also arose when memory maintenance demands were increased with a secondary working memory task. Formal model comparison showed that a divisive normalization model fitted effects of both risk context and working memory demands on PFC activity better than alternative models of value adaptation, and revealed that reduced suppression of background activity was the critical parameter impairing normalization with increased memory maintenance demand. Our findings suggest that mnemonic processes can constrain normalization of neural value representations.

Nicotine increases anterior insula activation to expected and unexpected outcomes among nonsmokers

RATIONALE

Tobacco has a higher rate of dependence than other drugs of abuse. However, the psychopharmacological effects of nicotine are incongruent with the tenacity of tobacco addiction since nicotine does not produce robust euphoria in humans or self-administration in rodents. A potential explanation is that nicotine amplifies the salience of other stimuli that have some incentive value, which could influence the initiation and persistence of smoking. However, the neural mechanisms of this process are unknown.

OBJECTIVES

One way that nicotine may amplify the salience of other stimuli is by enhancing reward prediction errors. We hypothesized that nicotine would enhance the neural response to unexpected (relative to expected) rewards compared to placebo.

METHODS

Twenty-three nonsmokers underwent two fMRI scans, following nicotine (1 mg) or placebo administration, while performing an outcome expectation task. In the task, a pair of cues was associated with either a subsequent reward (the image of a $100 bill) or a nonreward (the image of a blurry rectangle). On 20% of trials, the cue was followed by an unexpected outcome.

RESULTS

Although nicotine did not affect the magnitude of prediction errors relative to placebo, nicotine did increase BOLD activation in the anterior insula/inferior frontal gyrus and decrease activation in the caudate across all outcome types (including both rewards and nonrewards).

CONCLUSIONS

The insula and caudate could play a role in the initial effects of nicotine in nonsmokers, and these changes in baseline may be the mechanism that underlies how nicotine amplifies the salience of nondrug stimuli.

Striatal Cholinergic Interneurons Modulate Spike-Timing in Striosomes and Matrix by an Amphetamine-Sensitive Mechanism

The striatum is key for action-selection and the motivation to move. Dopamine and acetylcholine release sites are enriched in the striatum and are cross-regulated, possibly to achieve optimal behavior. Drugs of abuse, which promote abnormally high dopamine release, disrupt normal action-selection and drive restricted, repetitive behaviors (stereotypies). Stereotypies occur in a variety of disorders including obsessive-compulsive disorder, autism, schizophrenia and Huntington's disease, as well as in addictive states. The severity of drug-induced stereotypy is correlated with induction of c-Fos expression in striosomes, a striatal compartment that is related to the limbic system and that directly projects to dopamine-producing neurons of the substantia nigra. These characteristics of striosomes contrast with the properties of the extra-striosomal matrix, which has strong sensorimotor and associative circuit inputs and outputs. Disruption of acetylcholine signaling in the striatum blocks the striosome-predominant c-Fos expression pattern induced by drugs of abuse and alters drug-induced stereotypy. The activity of striatal cholinergic interneurons is associated with behaviors related to sensory cues, and cortical inputs to striosomes can bias action-selection in the face of conflicting cues. The neurons and neuropil of striosomes and matrix neurons have observably separate distributions, both at the input level in the striatum and at the output level in the substantia nigra. Notably, cholinergic axons readily cross compartment borders, providing a potential route for local cross-compartment communication to maintain a balance between striosomal and matrix activity. We show here, by slice electrophysiology in transgenic mice, that repetitive evoked firing patterns in striosomal and matrix striatal projection neurons (SPNs) are interrupted by optogenetic activation of cholinergic interneurons either by the addition or the deletion of spikes. We demonstrate that this cholinergic modulation of projection neurons is blocked in brain slices taken from mice exposed to amphetamine and engaged in amphetamine-induced stereotypy, and lacking responsiveness to salient cues. Our findings support a model whereby activity in striosomes is normally under strong regulation by cholinergic interneurons, favoring behavioral flexibility, but that in animals with drug-induced stereotypy, this cholinergic signaling breaks down, resulting in differential modulation of striosomal activity and an inability to bias action-selection according to relevant sensory cues.

Altered subjective reward valuation among female heavy marijuana users

Maladaptive decision-making is a cardinal feature of drug use, contributing to ongoing use, and reflecting alterations in how drug users assess uncertain reward value. Accumulating evidence indicates the consequences of heavy marijuana use are worse for female versus male animals and humans, but research assessing sex differences in reward-related decision-making among marijuana users remains scarce. We examined sex differences in the subjective valuation of certain and uncertain rewards among heavy marijuana users (52; 26 male and 26 female) and controls (52; 26 male and 26 female). We offered male and female heavy marijuana users and controls monetary rewards of certain and uncertain (probabilistic) values. We measured how preferences for uncertain rewards varied by the objective value of those rewards, moderators of reward uncertainty, Marijuana Group and Sex. Men were more sensitive to changes in the objective value of uncertain rewards than women. However, this effect of Sex differed by Marijuana Group. Female heavy marijuana users were more sensitive to changes in uncertain reward value, particularly when the "stakes" were high (i.e., greater difference between potential uncertain rewards), than female controls. Female heavy marijuana users' sensitivity to changes in the value of high stakes uncertain rewards was comparable to male marijuana users and controls. In contrast, male marijuana users' sensitivity to changes in the value of high stakes uncertain rewards did not differ from male controls. These results suggest sex differences in sensitivity to high risk rewards may be one pathway contributing to severer consequences of heavy marijuana use among women. (PsycINFO Database Record

Striatal Activity and Reward Relativity: Neural Signals Encoding Dynamic Outcome Valuation

The striatum is a key brain region involved in reward processing. Striatal activity has been linked to encoding reward magnitude and integrating diverse reward outcome information. Recent work has supported the involvement of striatum in the valuation of outcomes. The present work extends this idea by examining striatal activity during dynamic shifts in value that include different levels and directions of magnitude disparity. A novel task was used to produce diverse relative reward effects on a chain of instrumental action. Rats (Rattus norvegicus) were trained to respond to cues associated with specific outcomes varying by food pellet magnitude. Animals were exposed to single-outcome sessions followed by mixed-outcome sessions, and neural activity was compared among identical outcome trials from the different behavioral contexts. Results recording striatal activity show that neural responses to different task elements reflect incentive contrast as well as other relative effects that involve generalization between outcomes or possible influences of outcome variety. The activity that was most prevalent was linked to food consumption and post-food consumption periods. Relative encoding was sensitive to magnitude disparity. A within-session analysis showed strong contrast effects that were dependent upon the outcome received in the immediately preceding trial. Significantly higher numbers of responses were found in ventral striatum linked to relative outcome effects. Our results support the idea that relative value can incorporate diverse relationships, including comparisons from specific individual outcomes to general behavioral contexts. The striatum contains these diverse relative processes, possibly enabling both a higher information yield concerning value shifts and a greater behavioral flexibility.

Brain Stimulation in Addiction

Localized stimulation of the human brain to treat neuropsychiatric disorders has been in place for over 20 years. Although these methods have been used to a greater extent for mood and movement disorders, recent work has explored brain stimulation methods as potential treatments for addiction. The rationale behind stimulation therapy in addiction involves reestablishing normal brain function in target regions in an effort to dampen addictive behaviors. In this review, we present the rationale and studies investigating brain stimulation in addiction, including transcranial magnetic stimulation, transcranial direct current stimulation, and deep brain stimulation. Overall, these studies indicate that brain stimulation has an acute effect on craving for drugs and alcohol, but few studies have investigated the effect of brain stimulation on actual drug and alcohol use or relapse. Stimulation therapies may achieve their effect through direct or indirect modulation of brain regions involved in addiction, either acutely or through plastic changes in neuronal transmission. Although these mechanisms are not well understood, further identification of the underlying neurobiology of addiction and rigorous evaluation of brain stimulation methods has the potential for unlocking an effective, long-term treatment of addiction.

Glutamate neurons are intermixed with midbrain dopamine neurons in nonhuman primates and humans

The rodent ventral tegmental area (VTA) and substantia nigra pars compacta (SNC) contain dopamine neurons intermixed with glutamate neurons (expressing vesicular glutamate transporter 2; VGluT2), which play roles in reward and aversion. However, identifying the neuronal compositions of the VTA and SNC in higher mammals has remained challenging. Here, we revealed VGluT2 neurons within the VTA and SNC of nonhuman primates and humans by simultaneous detection of VGluT2 mRNA and tyrosine hydroxylase (TH; for identification of dopamine neurons). We found that several VTA subdivisions share similar cellular compositions in nonhuman primates and humans; their rostral linear nuclei have a high prevalence of VGluT2 neurons lacking TH; their paranigral and parabrachial pigmented nuclei have mostly TH neurons, and their parabrachial pigmented nuclei have dual VGluT2-TH neurons. Within nonhuman primates and humans SNC, the vast majority of neurons are TH neurons but VGluT2 neurons were detected in the pars lateralis subdivision. The demonstration that midbrain dopamine neurons are intermixed with glutamate or glutamate-dopamine neurons from rodents to humans offers new opportunities for translational studies towards analyzing the roles that each of these neurons play in human behavior and in midbrain-associated illnesses such as addiction, depression, schizophrenia, and Parkinson's disease.

Voluntary adolescent drinking enhances excitation by low levels of alcohol in a subset of dopaminergic neurons in the ventral tegmental area

Enhanced dopamine (DA) neurotransmission from the ventral tegmental area (VTA) to the ventral striatum is thought to drive drug self-administration and mediate positive reinforcement. We examined neuronal firing rates in slices of mouse midbrain following adolescent binge-like alcohol drinking and find that prior alcohol experience greatly enhanced the sensitivity to excitation by ethanol itself (10-50 mM) in a subset of ventral midbrain DA neurons located in the medial VTA. This enhanced response after drinking was not associated with alterations of firing rate or other measures of intrinsic excitability. In addition, the phenomenon appears to be specific to adolescent drinking, as mice that established a drinking preference only after the onset of adulthood showed no change in alcohol sensitivity. Here we demonstrate not only that drinking during adolescence induces enhanced alcohol sensitivity, but also that this DA neuronal response occurs over a range of alcohol concentrations associated with social drinking in humans.

Epigenetic mechanisms and associated brain circuits in the regulation of positive emotions: A role for transposable elements

Epigenetic programming and reprogramming are at the heart of cellular differentiation and represent developmental and evolutionary mechanisms in both germline and somatic cell lines. Only about 2% of our genome is composed of protein-coding genes, while the remaining 98%, once considered "junk" DNA, codes for regulatory/epigenetic elements that control how genes are expressed in different tissues and across time from conception to death. While we already know that epigenetic mechanisms are at play in cancer development and in regulating metabolism (cellular and whole body), the role of epigenetics in the developing prenatal and postnatal brain, and in maintaining a proper brain activity throughout the various stages of life, in addition to having played a critical role in human evolution, is a relatively new domain of knowledge. Here we present the current state-of-the-art techniques and results of these studies within the domain of emotions, and then speculate on how genomic and epigenetic mechanisms can modify and potentially alter our emotional (limbic) brain and affect our social interactions. J. Comp. Neurol. 524:2944-2954, 2016. © 2016 Wiley Periodicals, Inc.

The Emergence of a Circuit Model for Addiction

Addiction is a disease of altered behavior. Addicts use drugs compulsively and will continue to do so despite negative consequences. Even after prolonged periods of abstinence, addicts are at risk of relapse, particularly when cues evoke memories that are associated with drug use. Rodent models mimic many of the core components of addiction, from the initial drug reinforcement to cue-associated relapse and continued drug intake despite negative consequences. Rodent models have also enabled unprecedented mechanistic insight into addiction, revealing plasticity of glutamatergic synaptic transmission evoked by the strong activation of mesolimbic dopamine-a defining feature of all addictive drugs-as a neural substrate for these drug-adaptive behaviors. Cell type-specific optogenetic manipulations have allowed both identification of the relevant circuits and design of protocols to reverse drug-evoked plasticity and to establish links of causality with drug-adaptive behaviors. The emergence of a circuit model for addiction will open the door for novel therapies, such as deep brain stimulation.

Amphetamine alters neural response to sucrose in healthy women

Amphetamine, likely via action on the brain's dopaminergic systems, induces anorectic eating behavior and blunts dopaminergic midbrain activation to rewards. Past work has hypothesized that this blunted reward responsivity is a result of increasing tonic over phasic DA activity. We sought to extend past findings to sweet taste during fMRI following single-blind administration of dextroamphetamine and placebo in 11 healthy women. We hypothesized that neural response in both limbic and cognitive sweet taste circuits would mirror past work with monetary rewards by effectively blunting sweet taste reward, and 'equalizing' it's rewarding taste with receipt of water. Behavioral results showed that amphetamine reduced self-reported hunger (supporting the existence of amphetamine anorexia) and increased self-report euphoria. In addition, region of Interest analysis revealed significant treatment by taste interactions in the middle insula and dorsal anterior cingulate confirming the 'equalizing' hypothesis in the cingulate, but unlike monetary reinforcers, the insula actually evinced enhanced separation between tastes on the amphetamine day. These results suggest a divergence from prior research using monetary reinforcers when extended to primary reinforcers, and may hint that altering dopaminergic signaling in the insula and anterior cingulate may be a target for pharmacological manipulation of appetite, and the treatment of obesity.

Contemporary approaches to neural circuit manipulation and mapping: focus on reward and addiction

Tying complex psychological processes to precisely defined neural circuits is a major goal of systems and behavioural neuroscience. This is critical for understanding adaptive behaviour, and also how neural systems are altered in states of psychopathology, such as addiction. Efforts to relate psychological processes relevant to addiction to activity within defined neural circuits have been complicated by neural heterogeneity. Recent advances in technology allow for manipulation and mapping of genetically and anatomically defined neurons, which when used in concert with sophisticated behavioural models, have the potential to provide great insight into neural circuit bases of behaviour. Here we discuss contemporary approaches for understanding reward and addiction, with a focus on midbrain dopamine and cortico-striato-pallidal circuits.

Reward Sensitivity of ACC as an Intermediate Phenotype between DRD4-521T and Substance Misuse

The development and expression of the midbrain dopamine system is determined in part by genetic factors that vary across individuals such that dopamine-related genes are partly responsible for addiction vulnerability. However, a complete account of how dopamine-related genes predispose individuals to drug addiction remains to be developed. Adopting an intermediate phenotype approach, we investigated whether reward-related electrophysiological activity of ACC—a cortical region said to utilize dopamine reward signals to learn the value of extended, context-specific sequences of goal-directed behaviors—mediates the influence of multiple dopamine-related functional polymorphisms over substance use. We used structural equation modeling to examine whether two related electrophysiological phenomena associated with the control and reinforcement learning functions of ACC—theta power and the reward positivity—mediated the relationship between the degree of substance misuse and genetic polymorphisms that regulate dopamine processing in frontal cortex. Substance use data were collected from 812 undergraduate students. One hundred ninety-six returned on a subsequent day to participate in an electrophysiological experiment and to provide saliva samples for DNA analysis. We found that these electrophysiological signals mediated a relationship between the DRD4-521T dopamine receptor genotype and substance misuse. Our results provide a theoretical framework that bridges the gap between genes and behavior in drug addiction and illustrate how future interventions might be individually tailored for specific genetic and neurocognitive profiles.

Annual Research Review: On the developmental neuropsychology of substance use disorders

BACKGROUND

Adolescence represents a period of development during which critical biological, as well as social and cognitive, changes occur that are necessary for the transition into adulthood. A number of researchers have suggested that the pattern of normative brain changes that occurs during this period not only predisposes adolescents to engage in risk behaviours, such as experimentation with drugs, but that they additionally make the adolescent brain more vulnerable to the direct pharmacological impact of substances of abuse. The neural circuits that we examine in this review involve cortico-basal-ganglia/limbic networks implicated in the processing of rewards, emotion regulation, and the control of behaviour, emotion and cognition.

FINDINGS AND CONCLUSIONS

We identify certain neurocognitive and personality/comorbidity-based risk factors for the onset of substance misuse during adolescence, and summarise the evidence suggesting that these risk factors may be further impacted by the direct effect of drugs on the underlying neural circuits implicated in substance misuse vulnerability.

Altered subjective reward valuation among drug-deprived heavy marijuana users: Aversion to uncertainty

Marijuana is the most commonly used illicit drug in the United States and its use is rising. Nonetheless, scientific efforts to clarify the risk for addiction and other harm associated with marijuana use have been lacking. Maladaptive decision-making is a cardinal feature of addiction that is likely to emerge in heavy users. In particular, distorted subjective reward valuation related to homeostatic or allostatic processes has been implicated for many drugs of abuse. Selective changes in responses to uncertainty have been observed in response to intoxication and deprivation from various drugs of abuse. To assess for these potential neuroadaptive changes in reward valuation associated with marijuana deprivation, we examined the subjective value of uncertain and certain rewards among deprived and nondeprived heavy marijuana users in a behavioral economics decision-making task. Deprived users displayed reduced valuation of uncertain rewards, particularly when these rewards were more objectively valuable. This uncertainty aversion increased with increasing quantity of marijuana use. These results suggest comparable decision-making vulnerability from marijuana use as other drugs of abuse, and highlights targets for intervention.