Limbic Corticostriatal Systems and Delayed Reinforcement

Dorsal Striatal Functional Connectivity and Repetitive Behavior Dimensions in Children and Youths With Neurodevelopmental Disorders

BACKGROUND

Impairing repetitive behaviors are one of the core diagnostic symptoms in autism spectrum disorder and obsessive-compulsive disorder, but they also manifest in attention-deficit/hyperactivity disorder. Although the dorsal striatal circuit has been implicated in repetitive behaviors, extensive heterogeneity in and cross-diagnostic manifestations of these behaviors have suggested phenotypic and likely neurobiological heterogeneity across neurodevelopmental disorders (NDDs).

METHODS

Intrinsic dorsal striatal functional connectivity was examined in 3 NDDs (autism spectrum disorder, obsessive-compulsive disorder, and attention-deficit/hyperactivity disorder) and typically developing control participants in a large single-cohort sample (N = 412). To learn how diagnostic labels and overlapping behaviors manifest in dorsal striatal functional connectivity measured with functional magnetic resonance imaging, the main and interaction effects of diagnosis and behavior were examined in 8 models (2 seed functional connectivity [caudate and putamen] × 4 sub-behavioral domains [sameness/ritualistic, self-injury, stereotypy, and compulsions]).

RESULTS

The obsessive-compulsive disorder group demonstrated distinctive patterns in visual and visuomotor coordination regions compared with the other diagnostic groups. Lower-order repetitive behaviors (self-injury and stereotypy) manifesting across all participants were implicated in regions involved in motor and cognitive control, although the findings did not survive effects of multiple comparisons, suggesting heterogeneity in these behavioral domains. An interaction between self-injurious behavior and an attention-deficit/hyperactivity disorder diagnosis were observed on caudate-cerebellum functional connectivity.

CONCLUSIONS

These findings confirmed high heterogeneity and overlapping behavioral manifestations in NDDs and their complex underlying neural mechanisms. A call for diagnosis-free symptom measures that can capture not only observable symptoms and severity across NDDs but also the underlying functions and motivations of such behaviors across diagnoses is needed.

Choice impulsivity after repeated social stress is associated with increased perineuronal nets in the medial prefrontal cortex

Repeated stress can predispose to substance abuse. However, behavioral and neurobiological adaptations that link stress to substance abuse remain unclear. This study investigates whether intermittent social defeat (ISD), a stress protocol that promotes drug-seeking behavior, alters intertemporal decision-making and cortical inhibitory function in the medial prefrontal cortex (mPFC). Male long evans rats were trained in a delay discounting task (DDT) where rats make a choice between a fast (1 s) small reward (1 sugar pellet) and a large reward (3 sugar pellets) that comes with a time delay (10 s or 20 s). A decreased preference for delayed rewards was used as an index of choice impulsivity. Rats were exposed to ISD and tested in the DDT 24 h after each stress episode, and one- and two-weeks after the last stress episode. Immunohistochemistry was performed in rat's brains to evaluate perineuronal nets (PNNs) and parvalbumin GABA interneurons (PV) labeling as markers of inhibitory function in mPFC. ISD significantly decreased the preference for delayed large rewards in low impulsive, but not high impulsive, animals. ISD also increased the density of PNNs in the mPFC. These results suggest that increased choice impulsivity and cortical inhibition predispose animals to seek out rewards after stress.

Using rodent data to elucidate dopaminergic mechanisms of ADHD: Implications for human personality

An altered behavioral response to positive reinforcement has been proposed to be a core deficit in attention deficit hyperactivity disorder (ADHD). The spontaneously hypertensive rat (SHR), a congenic animal strain, displays a similarly altered response to reinforcement. The presence of this genetically determined phenotype in a rodent model allows experimental investigation of underlying neural mechanisms. Behaviorally, the SHR displays increased preference for immediate reinforcement, increased sensitivity to individual instances of reinforcement relative to integrated reinforcement history, and a steeper delay of reinforcement gradient compared to other rat strains. The SHR also shows less development of incentive to approach sensory stimuli, or cues, that predict reward after repeated cue-reward pairing. We consider the underlying neural mechanisms for these characteristics. It is well known that midbrain dopamine neurons are initially activated by unexpected reward and gradually transfer their responses to reward-predicting cues. This finding has inspired the dopamine transfer deficit (DTD) hypothesis, which predicts certain behavioral effects that would arise from a deficient transfer of dopamine responses from actual rewards to reward-predicting cues. We argue that the DTD predicts the altered responses to reinforcement seen in the SHR and individuals with ADHD. These altered responses to reinforcement in turn predict core symptoms of ADHD. We also suggest that variations in the degree of dopamine transfer may underlie variations in personality dimensions related to altered reinforcement sensitivity. In doing so, we highlight the value of rodent models to the study of human personality.

Evaluation of neuropsychological characteristics and attention bias in juvenile offenders, juvenile victims, and juveniles who have not participated in the criminal justice system

Introduction

The increasing inclusion of children in the criminal justice system as "juvenile offenders" and "juvenile victims" has recently emerged as a severe and multifaceted problem. This study evaluates whether juvenile offenders differ from juveniles who have not participated in the criminal justice system and juvenile victims regarding executive function skills and attentional bias.

Method

The participant group comprised 85 children aged 12-18, and the study setting was Turkey, utilizing one control group and two treatment groups with open criminal case files in Antalya Courthouse. The first treatment group consisted of 30 juvenile offenders; the second consisted of 30 juvenile victims. The control group consisted of 25 juveniles who were not juvenile offenders or victims. In this context, children's executive functions were measured with the short-form Barratt Impulsivity Scale, the Raven Standard Progressive Matrices Test, the TBAG-form Stroop test, the Wisconsin Card Sorting Test, and the Istanbul 5 Cube Planning Test. Attentional bias was measured using a dot-probe task. Illiteracy, intellectual or developmental disability, and being a non-native Turkish speaker were the exclusion criteria for all three groups.

Results

The study found that the scores of the juvenile offender group on the Barratt Impulsivity Scale were significantly higher than the children in the juvenile victim group and the children in the control group. For other tests measuring executive functions, the control group's scores were significantly higher than juvenile offenders and juvenile victims. Regarding attentional bias, the children in the control group exhibited less attentional bias to negative stimuli than the juvenile offenders and victims.

Discussion

Researchers have generally addressed the reasons that push children to crime and become victims of crime through individual, familial, and environmental reasons. However, the number of studies investigating the neuropsychological characteristics of children dragged into crime is relatively limited in our country. In addition, there is no study comparing the executive functions and attentional bias of children who are dragged into crime, victimized children, and children without a history of being dragged into crime and victimization. In this context, this study can highlight important implications for the judicial system regarding juvenile delinquency interventions.

Disrupted Small-World Networks in Children with Drug-Naïve Attention-Deficit/Hyperactivity Disorder: A DTI-Based Network Analysis

Attention-deficit/hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, while the potential neurological mechanisms are poorly understood. To explore the alterations in the white matter (WM) structural connectome in children with drug-naïve ADHD, forty-nine ADHD and 51 age- and gender-matched typically developing (TD) children aged 6-14 years were enrolled. WM structural connectivity based on deterministic diffusion tensor imaging (DTI) was constructed in 90 cortical and subcortical regions, and topological parameters of the resulting graphs were calculated. Network metrics were compared between two groups. The concentration index and the total cancellation test scores of digit cancellation test were used to evaluate clinical symptom severity in ADHD. Then, a partial correlation analysis was performed to explore the relationship between significant topologic metrics and clinical symptom severity. Compared to TD group, ADHD showed an increase in the characteristic path length (Lp), normalized clustering coefficient (γ), small worldness (σ), and a decrease in the global efficiency (Eglob) (all p < 0.05). Furthermore, ADHD showed reduced nodal centralities mainly in the regions of default mode network (DMN), central executive network (CEN), basal ganglia, and bilateral thalamus (all p < 0.05). After performing Benjamini-Hochberg's procedure, only the left orbital part of superior frontal gyrus and the left caudate were statistically significant (p < 0.05, FDR-corrected). In addition, the concentration index of ADHD was negatively correlated with the nodal betweenness of the left orbital part of the middle frontal gyrus (r = -0.302, p = 0.042). Our findings revealed an ADHD-related shift of WM network topology toward "regularization" pattern, characterized by decreased global network integration, which is also reflected by changed nodal centralities involving DMN, CEN, basal ganglia, and bilateral thalamus. ADHD could be understood by examining the dysfunction of large-scale spatially distributed neural networks.

Astrotactin 2 (ASTN2) regulates emotional and cognitive functions by affecting neuronal morphogenesis and monoaminergic systems

Astrotactin2 (ASTN2) regulates neuronal migration and synaptic strength through the trafficking and degradation of surface proteins. Deletion of ASTN2 in copy number variants has been identified in patients with schizophrenia, bipolar disorder, and autism spectrum disorder in copy number variant (CNV) analysis. Disruption of ASTN2 is a risk factor for these neurodevelopmental disorders, including schizophrenia, bipolar disorder, autism spectrum disorder, and attention deficit hyperactivity disorder. However, the importance of ASTN2 in physiological functions remains poorly understood. To elucidate the physiological functions of ASTN2, we investigated whether deficiency of ASTN2 affects cognitive and/or emotional behaviors and neurotransmissions using ASTN2-deficient mice. Astn2 knockout (KO) mice produced by CRISPR/Cas9 technique showed no obvious differences in physical characteristics and circadian rhythm. Astn2 KO mice showed increased exploratory activity in a novel environment, social behavior and impulsivity, or decreased despair-, anxiety-like behaviors and exploratory preference for the novel object. Some behavioral abnormalities, such as increased exploratory activity and impulsivity, or decreased exploratory preference were specifically attenuated by risperidone, but not by haloperidol. While, the both drugs did not affect any emotion-related behavioral abnormalities in Astn2 KO mice. Dopamine contents were decreased in the striatum, and serotonin or dopamine turnover were increased in the striatum, nucleus accumbens, and amygdala of Astn2 KO mice. In morphological analyses, thinning of neural cell layers in the hippocampus, reduction of neural cell bodies in the prefrontal cortex, and decrease in spine density and PSD95 protein in both tissues were observed in Astn2 KO mice. The present findings suggest that ASTN2 deficiency develops some emotional or cognitive impairments related to monoaminergic dysfunctions and abnormal neuronal morphogenesis with shrinkage of neuronal soma. ASTN2 protein may contribute to the pathogenic mechanism and symptom onset of mental disorders.

Disrupted waiting behavior in ADHD: exploring the impact of reward availability and predictive cues

Altered motivational processing is purported to contribute to ADHD symptoms. A stronger preference for immediate over delayed reward is well documented in ADHD. However, little attention has been paid to children's capacity to withhold responding until a "better" reward becomes available, and their actions while waiting. Using a novel computer task, we examine the ability of children with and without ADHD to wait to collect a large reward in the presence of a small available reward. The effects of a reward-predicting cue on response times and response choices are also explored. Data from 136 children (6-12 years), 90 with ADHD and 46 typically developing (TD) children, are included. The children could collect a small immediately available reward or wait to access a larger reward after a variable delay, its imminent availability sometimes signaled by a cue. Subsequent probe trials explored the effects of longer waiting times and disruption of the cue-reward association. As expected, children with ADHD collected the small immediately available reward more often than TD children. Importantly, they were more likely to terminate waiting once commenced, collecting the small reward or attempting to collect the large reward early. The cue decreased their response time but disrupted their waiting when it no longer consistently predicted reward. Children with ADHD were more likely to abandon efforts to wait, especially when wait times were extended and when expected rewards failed to appear. Behavioral interventions for ADHD should take into account reduced waiting capacity that extends beyond children's preference for immediate reward.

Childhood trauma exposure and reward processing in healthy adults: A functional neuroimaging study

The association between childhood trauma exposure and risk of developing psychopathology may in part be mediated by the effects of chronic stress on dopaminergic neurotransmission. However, little is known about the differential effects of distinct trauma types on reward processing, particularly in adults without concurrent medical or psychiatric disorders. We examined the association of childhood trauma exposure, including the differential effects of abuse and neglect, with reward processing in healthy adults (n = 114). Functional magnetic resonance imaging during a monetary incentive delay task was used to assess neural activity in the ventral striatum and orbitofrontal cortex in relation to reward anticipation and reward outcome, respectively. Exposure to childhood trauma, including abuse and neglect, was assessed using the Childhood Trauma Questionnaire‐Short Form. We found a significant effect for abuse on ventral striatal activation during reward anticipation, adjusting for age, sex, scanner site, educational level, and household monthly income. There were no effects for abuse or neglect, independently or combined, on orbitofrontal cortex activation during reward outcome. Our findings suggest differential effects of childhood abuse on ventral striatum activation during reward anticipation in healthy adults.

Global Cerebral Ischemia in Male Long Evans Rats Impairs Dopaminergic/ΔFosB Signalling in the Mesocorticolimbic Pathway Without Altering Delay Discounting Rates

Global cerebral ischemia (GCI) in rats has been shown to promote exploration of anxiogenic zones of the Elevated-Plus Maze (EPM) and Open Field Test (OFT). This study investigated changes in impulsive choice and/or defensive responses as possible contributors of heightened anxiogenic exploration observed after ischemia. Impulsivity was assessed using delay discounting (DD) paradigms, while the Predator Odour Test (PO) served to assess changes in defensive responses towards a naturally aversive stimulus. Male Long Evans rats underwent 9 days of autoshaping training and 24 days of DD training prior to GCI or sham surgery (n = 9/group). Post-surgery, rats completed the OFT, EPM, and PO, followed by 6 days of DD sessions. Blood droplets served to evaluate corticosterone secretion associated with PO exposure. With impulsivity being regulated through mesocorticolimbic monoaminergic pathways, we also characterised post-ischemic changes in the expression of dopamine D₂ receptors (DRD₂), dopamine transporters (DAT), and 1FosB in the basolateral amygdala (BLA), nucleus accumbens core (NAcC) and shell (NAcS), and ventromedial prefrontal cortex (vmPFC) using immunohistofluorescence. Our findings revealed no impact of GCI on delay discounting rates, while PO approach behaviours were minimally affected. Nonetheless, GCI significantly reduced DRD₂ and ΔFosB-ir in the NAcS and NAcC, respectively, while DAT-ir was diminished in both NAc subregions. Collectively, our findings refine the understanding of cognitive-behavioural and biochemical responses following stroke or cardiac arrest. They support significant alterations to the dopaminergic mesocorticolimbic pathway after ischemia, which are not associated with altered impulsive choice in a DD task but may influence locomotor exploration of the OFT and EPM.

Functional MHCI deficiency induces ADHD-like symptoms with increased dopamine D1 receptor expression

Inappropriate synaptic development has been proposed as a potential mechanism of neurodevelopmental disorders, including attention-deficit hyperactivity disorder (ADHD). Major histocompatibility complex class I (MHCI), an immunity-associated molecule expressed by neurons in the brain, regulates synaptic development; however, the involvement of MHCI in these disorders remains elusive. We evaluated whether functional MHCI deficiency induced by β2m^-/-Tap1^-/- double-knockout in mice leads to abnormalities akin to those seen in neurodevelopmental disorders. We found that functional MHCI deficiency induced locomotor hyperactivity, motor impulsivity, and attention deficits, three major symptoms of ADHD. In contrast, these mice showed normal spatial learning, behavioral flexibility, social behavior, and sensorimotor integration. In the analysis of the dopamine system, upregulation of dopamine D1 receptor (D1R) expression in the nucleus accumbens and a greater locomotor response to D1R agonist SKF 81297 were found in the functional MHCI-deficient mice. Low-dose methylphenidate, used for the treatment of ADHD patients, alleviated the three behavioral symptoms and suppressed c-Fos expression in the D1R-expressing medium spiny neurons of the mice. These findings reveal an unexpected role of MHCI in three major symptoms of ADHD and may provide a novel landmark in the pathogenesis of ADHD.

Reward/Punishment-Based Decision Making in Rodents

Deficits in decision making are at the heart of many psychiatric diseases, such as substance abuse disorders and attention deficit hyperactivity disorder. Consequently, rodent models of decision making are germane to understanding the neural mechanisms underlying adaptive choice behavior and how such mechanisms can become compromised in pathological conditions. A critical factor that must be integrated with reward value to ensure optimal decision making is the occurrence of consequences, which can differ based on probability (risk of punishment) and temporal contiguity (delayed punishment). This article will focus on two models of decision making that involve explicit punishment, both of which recapitulate different aspects of consequences during human decision making. We will discuss each behavioral protocol, the parameters to consider when designing an experiment, and finally how such animal models can be utilized in studies of psychiatric disease. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Behavioral training Support Protocol: Equipment testing Alternate Protocol: Reward discrimination Basic Protocol 2: Risky decision-making task (RDT) Basic Protocol 3: Delayed punishment decision-making task (DPDT).

Peer victimization and its impact on adolescent brain development and psychopathology

Chronic peer victimization has long-term impacts on mental health; however, the biological mediators of this adverse relationship are unknown. We sought to determine whether adolescent brain development is involved in mediating the effect of peer victimization on psychopathology. We included participants (n = 682) from the longitudinal IMAGEN study with both peer victimization and neuroimaging data. Latent profile analysis identified groups of adolescents with different experiential patterns of victimization. We then associated the victimization trajectories and brain volume changes with depression, generalized anxiety, and hyperactivity symptoms at age 19. Repeated measures ANOVA revealed time-by-victimization interactions on left putamen volume (F = 4.38, p = 0.037). Changes in left putamen volume were negatively associated with generalized anxiety (t = -2.32, p = 0.020). Notably, peer victimization was indirectly associated with generalized anxiety via decreases in putamen volume (95% CI = 0.004-0.109). This was also true for the left caudate (95% CI = 0.002-0.099). These data suggest that the experience of chronic peer victimization during adolescence might induce psychopathology-relevant deviations from normative brain development. Early peer victimization interventions could prevent such pathological changes.

Identifying biological markers for improved precision medicine in psychiatry

Mental disorders represent an increasing personal and financial burden and yet treatment development has stagnated in recent decades. Current disease classifications do not reflect psychobiological mechanisms of psychopathology, nor the complex interplay of genetic and environmental factors, likely contributing to this stagnation. Ten years ago, the longitudinal IMAGEN study was designed to comprehensively incorporate neuroimaging, genetics, and environmental factors to investigate the neural basis of reinforcement-related behavior in normal adolescent development and psychopathology. In this article, we describe how insights into the psychobiological mechanisms of clinically relevant symptoms obtained by innovative integrative methodologies applied in IMAGEN have informed our current and future research aims. These aims include the identification of symptom groups that are based on shared psychobiological mechanisms and the development of markers that predict disease course and treatment response in clinical groups. These improvements in precision medicine will be achieved, in part, by employing novel methodological tools that refine the biological systems we target. We will also implement our approach in low- and medium-income countries to understand how distinct environmental, socioeconomic, and cultural conditions influence the development of psychopathology. Together, IMAGEN and related initiatives strive to reduce the burden of mental disorders by developing precision medicine approaches globally.

Abnormal functional network centrality in drug-naïve boys with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed neurodevelopmental disorder in childhood and is characterized by inattention, impulsivity, and hyperactivity. Observations of distributed functional abnormalities in ADHD suggest aberrant large-scale brain network connectivity. However, few studies have measured the voxel-wise network centrality of boys with ADHD, which captures the functional relationships of a given voxel within the entire connectivity matrix of the brain. Here, to examine the network patterns characterizing children with ADHD, we recruited 47 boys with ADHD and 21 matched control boys who underwent resting-state functional imaging scanning in a 3.0 T MRI unit. We measured voxel-wise network centrality, indexing local functional relationships across the entire brain connectome, termed degree centrality (DC). Then, we chose the brain regions with altered DC as seeds to examine the remote functional connectivity (FC) of brain regions. We found that boys with ADHD exhibited (1) decreased centrality in the left superior temporal gyrus (STG) and increased centrality in the left superior occipital lobe (SOL) and right inferior parietal lobe (IPL); (2) decreased FC between the STG and the putamen and thalamus, which belong to the cognitive cortico-striatal-thalamic-cortical (CSTC) loop, and increased FC between the STG and medial/superior frontal gyrus within the affective CSTC loop; and (3) decreased connectivity between the SOL and cuneus within the dorsal attention network. Our results demonstrated that patients with ADHD show a connectivity-based pathophysiological process in the cognitive and affective CSTC loops and attention network.

Optogenetic and chemogenetic approaches to manipulate attention, impulsivity and behavioural flexibility in rodents

Studies manipulating neural activity acutely with optogenetic or chemogenetic intervention in behaving rodents have increased considerably in recent years. More often, these circuit-level neural manipulations are tested within an existing framework of behavioural testing that strives to model complex executive functions or symptomologies relevant to multidimensional psychiatric disorders in humans, such as attentional control deficits, impulsivity or behavioural (in)flexibility. This methods perspective argues in favour of carefully implementing these acute circuit-based approaches to better understand and model cognitive symptomologies or their similar isomorphic animal behaviours, which often arise and persist in overlapping brain circuitries. First, we offer some practical considerations for combining long-term, behavioural paradigms with optogenetic or chemogenetic interventions. Next, we examine how cell-type or projection-specific manipulations to the ascending neuromodulatory systems, local brain region or descending cortical glutamatergic projections influence aspects of cognitive control. For this, we primarily focus on the influence exerted on attentional and motor impulsivity performance in the (3-choice or) 5-choice serial reaction time task, and impulsive, risky or inflexible choice biases during alternative preference, reward discounting or reversal learning tasks.

Inter-individual differences in the impulsive/compulsive dimension: deciphering related dopaminergic and serotonergic metabolisms at rest

Several impulse control disorders such as ADHD, mania, personality disorders or substance abuse share common behavioural traits, like impulsiveness, risk-taking or inflexible behaviour. These disorders are treated with drugs targeting dopamine (DA) and/or serotonin (5-HT). However, the patient's monoamine imbalance that these neurotransmitters compensate is unclear. This study aims to investigate the patterns of DA and 5-HT metabolisms at rest within selected brain regions related to inter-individual variability in six main components of impulsivity/compulsivity (anticipatory hyperactivity, premature responses, delay discounting, risk-taking, perseveration, flexibility). Rats with adaptive and highly inadaptive behaviours were identified in each task and a sensitive biochemical approach allowed mapping of post-mortem endogenous monoamine tissue content in 20 brain areas. Distinct patterns of 5-HT and DA metabolisms were revealed according to the behavioural traits. Except for hyperactive responses, lower control of actions was mainly associated with a lower DA or 5-HT metabolism in prefrontal and/or subcortical areas (i.e. in orbitofrontal cortex (DA), amygdala and anterior cingulate cortex (5-HT) for inflexible and risk-prone rats). Our results reveal the complex nature of behavioural traits related to impulse control disorders through their associated monoaminergic networks at rest, paving the way for understanding the link between mental disorders and drug therapeutic actions.This article is part of the theme issue 'Diverse perspectives on diversity: multi-disciplinary approaches to taxonomies of individual differences'.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

Integrating Preclinical and Clinical Models of Negative Urgency

Overwhelming evidence suggests that negative urgency is robustly associated with rash, ill-advised behavior, and this trait may hamper attempts to treat patients with substance use disorder. Research applying negative urgency to clinical treatment settings has been limited, in part, due to the absence of an objective, behavioral, and translational model of negative urgency. We suggest that development of such a model will allow for determination of prime neurological and physiological treatment targets, the testing of treatment effectiveness in the preclinical and the clinical laboratory, and, ultimately, improvement in negative-urgency-related treatment response and effectiveness. In the current paper, we review the literature on measurement of negative urgency and discuss limitations of current attempts to assess this trait in human models. Then, we review the limited research on animal models of negative urgency and make suggestions for some promising models that could lead to a translational measurement model. Finally, we discuss the importance of applying objective, behavioral, and translational models of negative urgency, especially those that are easily administered in both animals and humans, to treatment development and testing and make suggestions on necessary future work in this field. Given that negative urgency is a transdiagnostic risk factor that impedes treatment success, the impact of this work could be large in reducing client suffering and societal costs.

Development of a peptide targeting dopamine transporter to improve ADHD-like deficits

Attention-deficit hyperactivity disorder (ADHD) is a neurocognitive disorder characterized by hyperactivity, inattention, working memory deficits and impulsivity. Its worldwide prevalence is estimated to be 3–5% in children and adolescents. The mainstay treatment for ADHD is stimulant medications (e.g. methylphenidate), which increase synaptic dopamine by directly blocking dopamine transporter (DAT). Although these pharmacological agents are effective, they are often associated with various side effects including risks for future substance use disorders in ADHD patients. Here, we investigated an interaction between DAT and dopamine D2 receptor (D2R) as a novel target to develop potential therapeutics for the treatment of ADHD by using an interfering peptide (TAT-DAT_NT) to dissociate this protein complex. We found that TAT-DAT_NT promotes locomotor behavior in Sprague-Dawley rats. Furthermore, using in vivo microdialysis and high-performance liquid chromatography, we found that the disruption of D2R-DAT elevates extracellular dopamine level. More importantly, the interfering peptide, TAT-DAT_NT, attenuates hyperactivity and improves spontaneous alternation behavior in spontaneously hypertensive rats (SHR) ------ a common animal model of ADHD. This work presents a different means (i.e. other than direct blockade by a DAT inhibitor) to regulate the activity of DAT and dopaminergic neurotransmission, and a potential target site for future development of ADHD treatments.

Previous cocaine self-administration disrupts reward expectancy encoding in ventral striatum

The nucleus accumbens core (NAc) is important for integrating and providing information to downstream areas about the timing and value of anticipated reward. Although NAc is one of the first brain regions to be affected by drugs of abuse, we still do not know how neural correlates related to reward expectancy are affected by previous cocaine self-administration. To address this issue, we recorded from single neurons in the NAc of rats that had previously self-administered cocaine or sucrose (control). Neural recordings were then taken while rats performed an odor-guided decision-making task in which we independently manipulated value of expected reward by changing the delay to or size of reward across a series of trial blocks. We found that previous cocaine self-administration made rats more impulsive, biasing choice behavior toward more immediate reward. Further, compared to controls, cocaine-exposed rats showed significantly fewer neurons in the NAc that were responsive during odor cues and reward delivery, and in the reward-responsive neurons that remained, diminished directional and value encoding was observed. Lastly, we found that after cocaine exposure, reward-related firing during longer delays was reduced compared to controls. These results demonstrate that prior cocaine self-administration alters reward-expectancy encoding in NAc, which could contribute to poor decision making observed after chronic cocaine use.

Impulse Control Behaviors in Parkinson's Disease: Drugs or Disease? Contribution From Imaging Studies

Impulse control behaviors (ICB) are recognized as non-motor complications of dopaminergic medications in patients with Parkinson's disease (PD). Compelling evidence suggests that ICB are not merely due to the PD-related pathology itself. Several risk factors have been identified, either demographic, clinical, genetic or neuropsychological. Neuroimaging studies have yielded controversial results regarding ICB correlates in PD and still it is not clear whether they can be triggered by the PD biology or the dopaminergic treatment stimulation. We provided an overview of the imaging studies that offered the most relevant insights into the debate about the role of drugs and disease in ICB pathophysiology. Understanding neural correlates and potential predisposing factors of these severe neuropsychiatric symptoms will be crucial to guide clinical practice and to foster preventive strategies.

Drug Cues, Conditioned Reinforcement, and Drug Seeking: The Sequelae of a Collaborative Venture With Athina Markou

Athina Markou spent a research period in my laboratory, then in the Department of Anatomy in Cambridge University, in 1991 to help us establish a cocaine-seeking procedure. Thus we embarked on developing a second-order schedule of intravenous cocaine reinforcement to investigate the neural basis of the pronounced effects of cocaine-associated conditioned stimuli on cocaine seeking. This brief review summarizes the fundamental aspects of cocaine seeking measured using this approach and the importance of the methodology in enabling us to define the neural mechanisms and circuitry underlying conditioned reinforcement and cocaine, heroin, and alcohol seeking. The shift over time and experience of control over drug seeking from a limbic cortical-ventral striatal circuit underlying goal-directed drug seeking to a dorsal striatal system mediating habitual drug seeking are also summarized. The theoretical implications of these data are discussed, thereby revealing the ways in which the outcomes of a collaboration can endure.

The mediating role of phosphodiesterase type 4 in the dopaminergic modulation of motor impulsivity

The current study investigated the mediating role of phosphodiesterase type 4 (PDE4) regulated cAMP in the dopaminergic modulation of premature responding (action restraint) in rats. Response inhibition, which includes action restraint, finds its neurobiological origin in cortico-striatal-thalamic circuitry and can be modulated by dopamine. Intracellularly, the effect of dopamine is largely mediated through the cAMP/PKA signaling cascade. Areas in the prefrontal cortex are very sensitive to their neurochemical environment, including catecholamine levels. As a result, we investigated the effects of intracellular modulation of the dopamine cascade by means of PDE4 inhibition by roflumilast on premature responding in a hypo, normal and hyper dopaminergic state of the brain. As a hypo dopaminergic model we induced a 6-OHDA lesion in the (rat) prefrontal cortex, more specifically the infralimbic cortex. For the hyper dopaminergic state we also turned to a well-established model of impaired action restraint, namely the systemic administration of d-amphetamine. In line with the notion of a U-shaped relation between dopamine and impulsive responding, we found that both increasing and decreasing dopamine levels resulted in an increase in premature responding in the choice serial reaction time task (CSRTT). The PDE4 inhibitor roflumilast increased premature responses in combination with d-amphetamine, whereas a decrease in premature responding after roflumilast treatment was found in the 6-OHDA lesioned animals. As a result, it would be interesting to test the effects of PDE4 inhibition in disorders affected by disrupted impulse control related to cortico-striatal-thalamic hypodopaminergia including attention deficit hyperactivity disorder (ADHD).

Setting the occasion for adolescent inhibitory control

During adolescence, individuals experience a broad range of dynamic environments as they strive to establish independence. Learning to respond appropriately in both new and previously encountered environments requires that an individual identify and learn the meaning of cues indicating that a behavior is appropriate, or alternatively, that it should be altered or inhibited. Although the ability to regulate goal-directed behavior continues to develop across adolescence, the specific circumstances under which adolescents experience difficulty with inhibitory control remain unclear. Here we review recent findings in our laboratory that address how adolescents learn to proactively inhibit a response. Much of our research has utilized a negative occasion setting paradigm, in which one cue (a feature) gates the meaning of a second cue (a target). The feature provides information that resolves the ambiguity of the target and indicates the appropriate behavioral response to the target. As such, we have been able to determine how adolescents learn about ambiguous stimuli, such as those whose meaning changes in accordance with other features of the surrounding environment. We consider why adolescents in particular exhibit difficulty in negative occasion setting compared to either pre-adolescents or adults. In addition, we review findings indicating that a balance in neural activity between orbitofrontal cortex and nucleus accumbens is necessary to support normal negative occasion setting. Finally, we consider aspects of associative learning that may contribute to adolescent inhibitory control, as well as provide insight into adolescent behavior as a whole.

Dynamic interaction between fetal adversity and a genetic score reflecting dopamine function on developmental outcomes at 36 months

Background

Fetal adversity, evidenced by poor fetal growth for instance, is associated with increased risk for several diseases later in life. Classical cut-offs to characterize small (SGA) and large for gestational age (LGA) newborns are used to define long term vulnerability. We aimed at exploring the possible dynamism of different birth weight cut-offs in defining vulnerability in developmental outcomes (through the Bayley Scales of Infant and Toddler Development), using the example of a gene vs. fetal adversity interaction considering gene choices based on functional relevance to the studied outcome.

Methods

36-month-old children from an established prospective birth cohort (Maternal Adversity, Vulnerability, and Neurodevelopment) were classified according to birth weight ratio (BWR) (SGA ≤0.85, LGA >1.15, exploring a wide range of other cut-offs) and genotyped for polymorphisms associated with dopamine signaling (TaqIA-A1 allele, DRD2-141C Ins/Ins, DRD4 7-repeat, DAT1-10- repeat, Met/Met-COMT), composing a score based on the described function, in which hypofunctional variants received lower scores.

Results

There were 251 children (123 girls and 128 boys). Using the classic cut-offs (0.85 and 1.15), there were no statistically significant interactions between the neonatal groups and the dopamine genetic score. However, when changing the cut-offs, it is possible to see ranges of BWR that could be associated with vulnerability to poorer development according to the variation in the dopamine function.

Conclusion

The classic birth weight cut-offs to define SGA and LGA newborns should be seen with caution, as depending on the outcome in question, the protocols for long-term follow up could be either too inclusive—therefore most costly, or unable to screen true vulnerabilities—and therefore ineffective to establish early interventions and primary prevention.

Strength of Structural and Functional Frontostriatal Connectivity Predicts Self-Control in the Healthy Elderly

Self-regulation refers to the successful use of executive functions and initiation of top-down processes to control one's thoughts, behavior, and emotions, and it is crucial to perform self-control. Self-control is needed to overcome impulses and can be assessed by delay of gratification (DoG) and delay discounting (DD) paradigms. In children/adolescents, good DoG/DD ability depends on the maturity of frontostriatal connectivity, and its decline in strength with advancing age might adversely affect self-control because prefrontal brain regions are more prone to normal age-related atrophy than other regions. Here, we aimed at highlighting the relationship between frontostriatal connectivity strength and DoG performance in advanced age. We recruited 40 healthy elderly individuals (mean age 74.0 ± 7.7 years) and assessed the DoG ability using the German version of the DoG test for adults in addition to the delay discounting (DD) paradigm. Based on diffusion-weighted and resting-state functional magnetic resonance imaging data, respectively, the structural and functional whole-brain connectome were reconstructed based on 90 different brain regions of interest in addition to a 12-node frontostriatal DoG-specific network and the resulting connectivity matrices were subjected to network-based statistics. The 90-nodes whole-brain connectome analyses revealed subnetworks significantly associated with DoG and DD with a preponderance of frontostriatal nodes involved suggesting a high specificity of the findings. Structural and functional connectivity strengths between the putamen, caudate nucleus, and nucleus accumbens on the one hand and orbitofrontal, dorsal, and ventral lateral prefrontal cortices on the other hand showed strong positive correlations with DoG and negative correlations with DD corrected for age, sex, intracranial volume, and head motion parameters. These associations cannot be explained by differences in impulsivity and executive functioning. This pattern of correlations between structural or functional frontostriatal connectivity strength and self-control suggests that, in addition to the importance of the frontostriatal nodes itself, the structural and functional properties of different connections within the frontostriatal network are crucial for self-controlled behaviors in the healthy elderly. Because high DoG/low DD is a significant predictor of willpower and wellbeing in the elderly population, interventions aiming at strengthening frontostriatal connectivity to strengthen self-controlled behavior are needed in the future.

Modelling ADHD: A review of ADHD theories through their predictions for computational models of decision-making and reinforcement learning

Attention deficit hyperactivity disorder (ADHD) is characterized by altered decision-making (DM) and reinforcement learning (RL), for which competing theories propose alternative explanations. Computational modelling contributes to understanding DM and RL by integrating behavioural and neurobiological findings, and could elucidate pathogenic mechanisms behind ADHD. This review of neurobiological theories of ADHD describes predictions for the effect of ADHD on DM and RL as described by the drift-diffusion model of DM (DDM) and a basic RL model. Empirical studies employing these models are also reviewed. While theories often agree on how ADHD should be reflected in model parameters, each theory implies a unique combination of predictions. Empirical studies agree with the theories' assumptions of a lowered DDM drift rate in ADHD, while findings are less conclusive for boundary separation. The few studies employing RL models support a lower choice sensitivity in ADHD, but not an altered learning rate. The discussion outlines research areas for further theoretical refinement in the ADHD field.

Personality traits in adults with attention-deficit hyperactivity disorder and their unaffected first-degree relatives

BACKGROUND

Attention-deficit hyperactivity disorder (ADHD) shows clear, albeit heterogeneous, cognitive dysfunctions. However, personality traits are not well understood in adults with ADHD, and it is unclear whether they are predisposing factors or phenotypical facets of the condition.

AIMS

To assess whether personality traits of impulsivity, sensation seeking and sensitivity to punishment and reward are predisposing factors for ADHD or aspects of the clinical phenotype.

METHOD

Twenty adults with ADHD, 20 unaffected first-degree relatives and 20 controls completed rating scales assessing traits of impulsivity, sensation seeking and sensitivity to punishment/reward.

RESULTS

Compared with relatives and controls, individuals with ADHD showed increased impulsive personality traits, were more susceptible to boredom and presented hypersensitivity to reward but normal sensitivity to punishment.

CONCLUSIONS

High impulsivity traits, heightened sensitivity to reward and boredom are associated with the phenotype of ADHD, rather than being predisposing factors, as these traits were not shared between ADHD probands and their relatives.

DECLARATION OF INTEREST

E.T.B. is employed part-time by GSK and part-time by the University of Cambridge; he holds stock in GSK. B.J.S. consults for Cambridge Cognition, Servier and Lundbeck; she holds a grant from Janssen/J&J. U.M. has received honoraria for consultancy and speaking at conferences and travel expenses from Bristol-Myers Squibb, Eli Lilly, Janssen-Cilag, Lundbeck, Pharmacia-Upjohn and UCB Pharma.

COPYRIGHT AND USAGE

© The Royal College of Psychiatrists 2016. This is an open access article distributed under the terms of the Creative Commons Non-Commercial, No Derivatives (CC BY-NC-ND) license.

Negative occasion setting in juvenile rats

Prior findings indicate that adolescent rats exhibit difficulty using negative occasion setters to guide behavior compared to adult rats (Meyer and Bucci, 2014). Here, additional groups of juvenile rats were trained in the same negative occasion setting procedure to further define the development of negative occasion setting. Beginning on either postnatal day (PND) 30, 40, or 50, rats received daily training sessions in which a tone was paired with food reinforcement on some trials, while on other trials a light preceded the tone and no reinforcement was delivered. We found that rats that began training on PND 50 required 10 training sessions to discriminate between the two types of trials, consistent with prior findings with young adult rats. Interestingly, rats in the PND 30 group (pre-adolescents) also required just 10 training sessions, in stark contrast to adolescent rats that began training on PND 35 (adolescents) and required 18 sessions (Meyer and Bucci, 2014). Rats that began training on PND 40 (adolescents) also required more sessions than the PND 30 group. These data indicate that the development of negative occasion setting is non-linear and have direct bearing on understanding the behavioral and neural substrates that underlie suboptimal behavioral control in adolescents.

Age-related changes in prefrontal norepinephrine transporter density: The basis for improved cognitive flexibility after low doses of atomoxetine in adolescent rats

Adolescence is a period of major behavioral and brain reorganization. As diagnoses and treatment of disorders like attention deficit hyperactivity disorder (ADHD) often occur during adolescence, it is important to understand how the prefrontal cortices change and how these changes may influence the response to drugs during development. The current study uses an adolescent rat model to study the effect of standard ADHD treatments, atomoxetine and methylphenidate on attentional set shifting and reversal learning. While both of these drugs act as norepinephrine reuptake inhibitors, higher doses of atomoxetine and all doses of methylphenidate also block dopamine transporters (DAT). Low doses of atomoxetine, were effective at remediating cognitive rigidity found in adolescents. In contrast, methylphenidate improved performance in rats unable to form an attentional set due to distractibility but was without effect in normal subjects. We also assessed the effects of GBR 12909, a selective DAT inhibitor, but found no effect of any dose on behavior. A second study in adolescent rats investigated changes in norepinephrine transporter (NET) and dopamine beta hydroxylase (DBH) density in five functionally distinct sub-regions of the prefrontal cortex: infralimbic, prelimbic, anterior cingulate, medial and lateral orbitofrontal cortices. These regions are implicated in impulsivity and distractibility. We found that NET, but not DBH, changed across adolescence in a regionally selective manner. The prelimbic cortex, which is critical to cognitive rigidity, and the lateral orbitofrontal cortex, critical to reversal learning and some forms of response inhibition, showed higher levels of NET at early than mid- to late adolescence. This article is part of a Special Issue entitled SI: Noradrenergic System.

Neonatal handling causes impulsive behavior and decreased pharmacological response to methylphenidate in male adult wistar rats

Neonatal handling has an impact on adult behavior of experimental animals and is associated with rapid and increased palatable food ingestion, impaired behavioral flexibility, and fearless behavior to novel environments. These symptoms are characteristic features of impulsive trait, being controlled by the medial prefrontal cortex (mPFC). Impulsive behavior is a key component of many psychiatric disorders such as attention deficit hyperactivity disorder (ADHD), manic behavior, and schizophrenia. Others have reported a methylphenidate (MPH)-induced enhancement of mPFC functioning and improvements in behavioral core symptoms of ADHD patients. The aims of the present study were: (i) to find in vivo evidence for an association between neonatal handling and the development of impulsive behavior in adult Wistar rats and (ii) to test whether neonatal handling could have an impact on monoamine levels in the mPFC and the pharmacological response to MPH in vivo. Therefore, experimental animals (litters) were classified as: "non-handled" and "handled" (10[Formula: see text]min/day, postnatal days 1-10). After puberty, they were exposed to either a larger and delayed or smaller and immediate reward (tolerance to delay of reward task). Acute MPH (3[Formula: see text]mg/Kg. i.p.) was used to suppress and/or regulate impulsive behavior. Our results show that only neonatally handled male adult Wistar rats exhibit impulsive behavior with no significant differences in monoamine levels in the medial prefrontal cortex, together with a decreased response to MPH. On this basis, we postulate that early life interventions may have long-term effects on inhibitory control mechanisms and affect the later response to pharmacological agents during adulthood.

Dissociable effects of mGluR5 allosteric modulation on distinct forms of impulsivity in rats: interaction with NMDA receptor antagonism

RATIONALE

Impaired N-methyl-D-aspartate (NMDA) receptor signalling underlies several psychiatric disorders that express high levels of impulsivity. Although synergistic interactions exist between NMDA receptors and metabotropic glutamate receptor 5 (mGluR5), the significance of this interaction for impulsivity is unknown.

OBJECTIVE

This study aims to investigate the effects of negative and positive allosteric mGluR5 modulation (NAM/PAM) on trait impulsivity and impulsivity evoked by NMDA receptor antagonism in rats.

METHODS

Motor and choice impulsivity were assessed using the five-choice serial reaction time task (5-CSRTT) and delayed-discounting task (DDT), respectively. The effects of RO4917523 and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) (NAMs) and ADX47273 (PAM) were investigated in non-impulsive rats and in trait high- and low-impulsive rats. The effects of these compounds on impulsivity induced by NMDA receptor antagonism (MK801) in the 5-CSRTT were also investigated.

RESULTS

RO4917523 (0.1-1 mg/kg) decreased premature responding and increased omissions but had no effect on locomotor activity up to 0.1 mg/kg. MTEP significantly increased omissions, decreased accuracy and slowed responding but had no effect on premature responding. ADX47273 decreased premature responding at doses that had no effect on locomotor activity. MK801 increased premature responding and impaired attentional accuracy; these deficits were dose dependently rescued by ADX47273 pre-treatment. Allosteric modulation of mGluR5 had no significant effect on choice impulsivity, nor did it modulate general task performance.

CONCLUSIONS

These findings demonstrate that mGluR5 allosteric modulation selectively dissociates motor and choice impulsivity. We further show that mGluR5 PAMs may have therapeutic utility in selectively targeting specific aspects of impulsivity and executive dysfunction.

Impulsivity and sensitivity to amount and delay of reinforcement in an animal model of ADHD

Previous research has been inconclusive about the degree of impulsivity displayed by spontaneously hypertensive rats (SHR), an animal model of Attention Deficit Hyperactivity Disorder (ADHD). In the present set of experiments, concurrent-chains schedules were employed in order to explore SHR's impulsivity, sensitivity to delay, and sensitivity to amount of reinforcement; Wistar rats (WIS) were used as comparison group. In the three experiments - performed with different subjects - non-independent variable interval 30s schedules were presented in the initial links; the difference between experiments was in the terminal links. For exploring impulsivity, one of the terminal links (SS) was associated to a short delay (2s) and a small reinforcer (1 pellet), whereas the other terminal link (LL) was associated to a longer delay (28s) and a larger reinforcer (4 pellets). The results indicated a remarkably higher impulsivity in SHR. Because this impulsivity may have as potential mechanisms an increased sensitivity to delay and/or a decreased sensitivity to the amount of reinforcement, in experiments 2 and 3 these possibilities were examined. For assessing sensitivity to delay, the following pairs of fixed interval (FI) schedules were used in the terminal links in five conditions: 2-28, 6-24, 15-15, 24-6, 28-2s; the magnitude of reinforcement was 1 pellet in all conditions for both alternatives. For assessing sensitivity to amount, in five conditions the alternatives were associated with different magnitudes of reinforcement: 1-5 pellets, 2-4, 3-3, 4-2 and 5-1 in left-right alternatives, respectively; the delay to reinforcement was controlled by a FI 15s in all conditions and for both alternatives. The sensitivity to delay and the sensitivity to amount were calculated according to the Generalized Matching Law. The results indicated a higher sensitivity to delay in SHR, and the same sensitivity to amount in SHR and WIS rats. These results suggest that the increased sensitivity to delay influences the high level of impulsivity observed in SHR.

Investing in the future: stimulation of the medial prefrontal cortex reduces discounting of delayed rewards

Generally, rewards that are received sooner are often preferred over future rewards, and the time between the choice and the reception of the reward is an important factor that influences our decisions, a phenomenon called delay discounting (DD). In DD, the medial prefrontal cortex (MePFC) and striatal dopamine neurotransmission both play an important role. We used repetitive transcranial magnetic stimulation (rTMS) to transiently activate the MePFC to evaluate its behavioral effect on the DD paradigm, and subsequently to measure its effect on striatal dopamine. Twenty-four right-handed young healthy subjects (11 females; age: 22.1±2.9 years) underwent DD following 10 Hz-rTMS of the MePFC and vertex stimulation (control condition). Thereafter, 11 subjects (5 females; age: 22.2±2.87 years) completed the PET study at rest using [(11)C]-(+)-PHNO following 10 Hz-rTMS of the MePFC and vertex. Modulation of the MePFC excitability influenced the subjective level of DD for delayed rewards and interfered with synaptic dopamine level in the striatum. The present study yielded findings that might reconcile the role of these areas in inter-temporal decision making and dopamine modulation, suggesting that the subjective sense of time and value of reward are critically controlled by these important regions.

Endocannabinoids and striatal function: implications for addiction-related behaviours

Since the identification and cloning of the major cannabinoid receptor expressed in the brain almost 25 years ago research has highlighted the potential of drugs that target the endocannabinoid system for treating addiction. The endocannabinoids, anandamide and 2-arachidonoyl glycerol, are lipid-derived metabolites found in abundance in the basal ganglia and other brain areas innervated by the mesocorticolimbic dopamine systems. Cannabinoid CB1 receptor antagonists/inverse agonists reduce reinstatement of responding for cocaine, alcohol and opiates in rodents. However, compounds acting on the endocannabinoid system may have broader application in treating drug addiction by ameliorating associated traits and symptoms such as impulsivity and anxiety that perpetuate drug use and interfere with rehabilitation. As a trait, impulsivity is known to predispose to addiction and facilitate the emergence of addiction to stimulant drugs. In contrast, anxiety and elevated stress responses accompany extended drug use and may underlie the persistence of drug intake in dependent individuals. In this article we integrate and discuss recent findings in rodents showing selective pharmacological modulation of impulsivity and anxiety by cannabinoid agents. We highlight the potential of selective inhibitors of endocannabinoid metabolism, directed at fatty acid amide hydrolase and monoacylglycerol lipase, to reduce anxiety and stress responses, and discuss novel mechanisms underlying the modulation of the endocannabinoid system, including the attenuation of impulsivity, anxiety, and drug reward by selective CB2 receptor agonists.

Dopaminergic function and intertemporal choice

The discounting of delayed rewards, also known as temporal or delay discounting, is intrinsic to everyday decisions and can be impaired in pathological states such as addiction disorders. Preclinical and human studies suggest a role for dopaminergic function in temporal discounting but this relationship has not yet been verified using molecular imaging of the living human brain. Here, we evaluated dopaminergic function in temporal discounting using positron emission tomography (PET) with two different dopaminergic ligands assessing three populations in whom temporal discounting has been shown to be impaired. First, we show using [11C]raclopride PET that in pathological gamblers, greater temporal discounting correlates with decreased ventral striatal binding potential, convergent with translational findings of lower nucleus accumbens D2/D3 receptor density in high-impulsive rodents. Temporal discounting also correlates with lower ventral striatal dopamine release in response to high-reward magnitude suggesting that dopamine-mediated devaluation of larger delayed rewards may drive choice preferences. Second, we show using [18F]fluorodopa PET that in Parkinson's disease, temporal discounting correlates with greater left caudate dopaminergic terminal function. Finally, in subjects with Parkinson's disease and dopamine medication-induced behavioral addictions, temporal discounting is further correlated with greater dopaminergic terminal function in the anterior putamen. These findings provide insights into the relationship between striatal dopamine function and temporal discounting, and its potential role in pathological disorders and mechanisms underlying treatment interventions.

Translatable and Back-Translatable Measurement of Impulsivity and Compulsivity: Convergent and Divergent Processes

Impulsivity and compulsivity have emerged as important dimensional constructs that challenge traditional psychiatric classification systems. Both are present in normal healthy populations where the need to act quickly and repeatedly without hesitation can be highly advantageous. However, when excessively expressed, impulsive and compulsive behavior can lead to adverse consequences and spectrum disorders exemplified by attention-deficit/hyperactivity disorder (ADHD), obsessive compulsive disorder (OCD), autism, and drug addiction. Impulsive individuals have difficulty in deferring gratification and are inclined to 'jump the gun' and respond prematurely before sufficient information is gathered. Compulsivity involves repetitive behavior often motivated by the need to reduce or prevent anxiety, thus leading to the maladaptive perseveration of behavior. Defined in this way, impulsivity and compulsivity could be viewed as separate entities or 'traits' but overwhelming evidence indicates that both may be present in the same disorder, either concurrently or even separately at different time points. Herein we discuss the neural and cognitive heterogeneity of impulsive and compulsive endophenotypes. These constructs map onto distinct fronto-striatal neural and neurochemical structures interacting both at nodal convergent points and as opponent processes highlighting both the heterogeneity and the commonalities of function. We focus on discoveries made using both translational research methodologies and studies exclusively in humans, and implications for treatment intervention in disorders in which impulsive and compulsive symptoms prevail. We emphasize the relevance of these constructs for understanding dimensional psychiatry.

The contribution of medial prefrontal cortical regions to conditioned inhibition

Few studies have considered the process by which individuals learn to omit a response, which is an essential aspect of adaptive behavior. Several lines of evidence indicate that two regions of the medial prefrontal cortex have disparate roles in behavioral flexibility. In particular, the prelimbic cortex (PL) is thought to facilitate the generation of a strategy to inhibit a prepotent response, whereas the infralimbic cortex (IL) appears to be more important for maintaining extensively trained inhibitory behaviors. The present experiments were designed to elucidate the contributions of PL and IL to the acquisition and maintenance of Pavlovian conditioned inhibition. In Experiment 1, damage to PL before training in a compound feature negative discrimination task impaired inhibitory learning. By comparison, lesions of IL had little effect. In Experiment 2, lesions of PL or IL occurred after overtraining, and damage to IL significantly impaired subsequent performance in the task, suggesting that this region is involved in the continued expression of Pavlovian conditioned inhibition after thorough training. PL may also be involved in maintaining inhibition, as evidenced by a marginally significant lesion-induced performance deficit. These data support the notion that PL and IL have distinguishable roles in modulating inhibition, while contributing important information about the specific role for PL in acquisition of an inhibitory response and IL in performance.

Models of Impulsivity with a Focus on Waiting Impulsivity: Translational Potential for Neuropsychiatric Disorders

Waiting impulsivity, also known as premature or anticipatory responding, is well established in preclinical studies through the 5-Choice Serial Reaction Time (5-CSRT) task. Waiting impulsivity is important in disorders of addiction. Preclinical studies suggest a role both as a predictor, and as a consequence, in disorders of addiction. Here we discuss the relationship between the preclinical 5-CSRT and translational fidelity in newly developed translational tasks. Preclinical and clinical literature relevant to premature responding and disorders of addiction are reviewed. Understanding which processes are critical to premature responding is important in understanding the nature of premature responding. Premature responding may also have overlaps with motivational processes, proactive response inhibition, tonic inhibitory processes, and delay discounting.

Neural antecedents of self-initiated actions in secondary motor cortex

The neural origins of spontaneous or self-initiated actions are not well understood and their interpretation is controversial. To address these issues, we used a task in which rats decide when to abort waiting for a delayed tone. We recorded neurons in the secondary motor cortex (M2) and interpreted our findings in light of an integration-to-bound decision model. A first population of M2 neurons ramped to a constant threshold at rates proportional to waiting time, strongly resembling integrator output. A second population, which we propose provide input to the integrator, fired in sequences and showed trial-to-trial rate fluctuations correlated with waiting times. An integration model fit to these data also quantitatively predicted the observed inter-neuronal correlations. Together, these results reinforce the generality of the integration-to-bound model of decision-making. These models identify the initial intention to act as the moment of threshold crossing while explaining how antecedent subthreshold neural activity can influence an action without implying a decision.

Neural and psychological mechanisms underlying compulsive drug seeking habits and drug memories--indications for novel treatments of addiction

This review discusses the evidence for the hypothesis that the development of drug addiction can be understood in terms of interactions between Pavlovian and instrumental learning and memory mechanisms in the brain that underlie the seeking and taking of drugs. It is argued that these behaviours initially are goal-directed, but increasingly become elicited as stimulus-response habits by drug-associated conditioned stimuli that are established by Pavlovian conditioning. It is further argued that compulsive drug use emerges as the result of a loss of prefrontal cortical inhibitory control over drug seeking habits. Data are reviewed that indicate these transitions from use to abuse to addiction depend upon shifts from ventral to dorsal striatal control over behaviour, mediated in part by serial connectivity between the striatum and midbrain dopamine systems. Only some individuals lose control over their drug use, and the importance of behavioural impulsivity as a vulnerability trait predicting stimulant abuse and addiction in animals and humans, together with consideration of an emerging neuroendophenotype for addiction are discussed. Finally, the potential for developing treatments for addiction is considered in light of the neuropsychological advances that are reviewed, including the possibility of targeting drug memory reconsolidation and extinction to reduce Pavlovian influences on drug seeking as a means of promoting abstinence and preventing relapse.

MR imaging-detectable metabolic alterations in attention deficit/hyperactivity disorder: from preclinical to clinical studies

MR spectroscopy represents one of the most suitable in vivo tool to assess neurochemical dysfunction in several brain disorders, including attention deficit/hyperactivity disorder. This is the most common neuropsychiatric disorder in childhood and adolescence, which persists into adulthood (in approximately 30%-50% of cases). In past years, many studies have applied different MR spectroscopy techniques to investigate the pathogenesis and effect of conventional treatments. In this article, we review the most recent clinical and preclinical MR spectroscopy results on subjects with attention deficit/hyperactivity disorder and animal models, from childhood to adulthood. We found that the most investigated brain regions were the (pre)frontal lobes and striatum, both involved in the frontostriatal circuits and networks that are known to be impaired in this pathology. Neurometabolite alterations were detected in several regions: the NAA, choline, and glutamatergic compounds. The creatine pool was also altered when an absolute quantitative protocol was adopted. In particular, glutamate was increased in children with attention deficit/hyperactivity disorder, and this can apparently be reversed by methylphenidate treatment. The main difficulties in reviewing MR spectroscopy studies were in the nonhomogeneity of the analyzed subjects, the variety of the investigated brain regions, and also the use of different MR spectroscopy techniques. As for possible improvements in future studies, we recommend the use of standardized protocols and the analysis of other brain regions of particular interest for attention deficit hyperactivity disorder, like the hippocampus, limbic structures, thalamus, and cerebellum.

The ontogeny of learned inhibition

Previous studies have examined the maturation of learning and memory abilities during early stages of development. By comparison, much less is known about the ontogeny of learning and memory during later stages of development, including adolescence. In Experiment 1, we tested the ability of adolescent and adult rats to learn a Pavlovian negative occasion setting task. This procedure involves learning to inhibit a behavioral response when signaled by a cue in the environment. During reinforced trials, a target stimulus (a tone) was presented and immediately followed by a food reward. On nonreinforced trials, a feature stimulus (a light) was presented 5 sec prior to the tone and indicated the absence of reward following presentation of the tone. Both adult and adolescent rats learned to discriminate between two different trial types and withhold responding when the light preceded the tone. However, adolescent rats required more sessions than adults to discriminate between reinforced and nonreinforced trials. The results of Experiment 2 revealed that adolescents could learn the task rules but were specifically impaired in expressing that learning in the form of withholding behavior on nonreinforced trials. In Experiment 3, we found that adolescents were also impaired in learning a different version of the task in which the light and tone were presented simultaneously during the nonreinforced trials. These findings add to existing literature by indicating that impairments in inhibitory behavior during adolescence do not reflect an inability to learn to inhibit a response, but instead reflect a specific deficit in expressing that learning.