Contrasting roles for dopamine D1 and D2 receptor subtypes in the dorsomedial striatum but not the nucleus accumbens core during behavioral inhibition in the stop-signal task in rats

Trait Impulsivity as a Feature of Parkinson's Disease Treatment and Progression

Heightened trait impulsivity in both subclinical and pathological senses is becoming increasingly recognised in Parkinson's disease (PD). Impulsive behaviours and impulse control disorders (ICDs) are a consequence of perturbation to the rewards pathway leading individuals to conduct activities in a repetitive, excessive, and maladaptive fashion. Commonly linked to PD, heightened trait impulsivity has been found to primarily manifest in the forms of hypersexuality, pathological gambling, compulsive shopping, and binge eating, all of which may significantly impact social and financial standing. Subsequent burden to quality of life for both individuals with PD and caregivers are common. Although risk factors and indicators for ICDs in PD are currently lacking, it is recognised that the condition is often precipitated by dopamine replacement therapies, primarily dopamine agonist administration. While this nonmotor symptom is being increasingly diagnosed in PD populations, it remains relatively elusive in comparison to its motor counterparts. Through discussion of impulsivity characteristics, neuroanatomy, and neurochemistry, in addition to reviewing existing research on the potential contributing factors to impulsivity in PD, this review highlights impulsivity as a significant and detrimental PD symptom. Thus, emphasising the imperative need to establish efficacious diagnostic tools and treatments.

The right inferior frontal gyrus as pivotal node and effective regulator of the basal ganglia-thalamocortical response inhibition circuit

Background

The involvement of specific basal ganglia-thalamocortical circuits in response inhibition has been extensively mapped in animal models. However, the pivotal nodes and directed causal regulation within this inhibitory circuit in humans remains controversial.

Objective

The main aim of the present study was to determine the causal information flow and critical nodes in the basal ganglia-thalamocortical inhibitory circuits and also to examine whether these are modulated by biological factors (i.e. sex) and behavioral performance.

Methods

Here, we capitalize on the recent progress in robust and biologically plausible directed causal modeling (DCM-PEB) and a large response inhibition dataset (n = 250) acquired with concomitant functional magnetic resonance imaging to determine key nodes, their causal regulation and modulation via biological variables (sex) and inhibitory performance in the inhibitory circuit encompassing the right inferior frontal gyrus (rIFG), caudate nucleus (rCau), globus pallidum (rGP), and thalamus (rThal).

Results

The entire neural circuit exhibited high intrinsic connectivity and response inhibition critically increased causal projections from the rIFG to both rCau and rThal. Direct comparison further demonstrated that response inhibition induced an increasing rIFG inflow and increased the causal regulation of this region over the rCau and rThal. In addition, sex and performance influenced the functional architecture of the regulatory circuits such that women displayed increased rThal self-inhibition and decreased rThal to GP modulation, while better inhibitory performance was associated with stronger rThal to rIFG communication. Furthermore, control analyses did not reveal a similar key communication in a left lateralized model.

Conclusions

Together, these findings indicate a pivotal role of the rIFG as input and causal regulator of subcortical response inhibition nodes.

Methylphenidate as a treatment option for substance use disorder: a transdiagnostic perspective

A transition in viewing mental disorders from conditions defined as a set of unique characteristics to one of the quantitative variations on a collection of dimensions allows overlap between disorders. The overlap can be utilized to extend to treatment approaches. Here, we consider the overlap between attention-deficit/hyperactivity disorder and substance use disorder to probe the suitability to use methylphenidate as a treatment for substance use disorder. Both disorders are characterized by maladaptive goal-directed behavior, impaired cognitive control, hyperactive phasic dopaminergic neurotransmission in the striatum, prefrontal hypoactivation, and reduced frontal cortex gray matter volume/density. In addition, methylphenidate has been shown to improve cognitive control and normalize associated brain activation in substance use disorder patients and clinical trials have found methylphenidate to improve clinical outcomes. Despite the theoretical basis and promising, but preliminary, outcomes, many questions remain unanswered. Most prominent is whether all patients who are addicted to different substances may equally profit from methylphenidate treatment.

Interval timing relative to response inhibition in the differential reinforcement of low-rate responding in normally developing young adults

With recent proposal suggesting the multifaceted nature of impulsivity, researchers have been intrigued by the question of whether the impulsive behaviour measured in the traditionally psychological paradigms is unitary. One such paradigm, the differential reinforcement of low-rate responding (DRL), has been used to assess response inhibition, but its underlying mechanism has still been debated. In present research, we examined and differentiated the effects of both response inhibition and interval timing on a multisession DRL-10 s (DRL-10 s) in a large sample of normally developing young adults, as well as with three other measures including the stop-signal reaction task (SSRT), time production task-10 s (TPT-10 s), and the Barrett impulsivity scale-11 (BIS-11). The results showed that behavioural changes existed in DRL. As the task sessions progressed, there was an increase in both reinforcement probability and peak time, but a decrease in burst responses. Most importantly, both principal component analysis and generalized multilevel modeling yielded consistent results that as the task progressed, there was an increasing involvement of the TPT in the late sessions of DRL. However, none of the effect of SSRT was found. In sum, the differential degrees of involvement of the timing process, relative to response inhibition, were observed in DRL.

Locus coeruleus integrity correlates with inhibitory functions of the fronto-subthalamic 'hyperdirect' pathway in Parkinson's disease

A long-running debate concerns whether dopamine or noradrenaline deficiency drives response disinhibition in Parkinson's disease (PD). This study aimed to investigate whether damage to the locus coeruleus (LC) or substantia nigra (SN) might impact inhibitory functions of the fronto-subthalamic hyperdirect or fronto-striatal indirect pathway. Patients with PD (n = 29, 13 women) and matched healthy controls (n = 29, 15 women) participated in this cross-sectional study. LC and SN integrity was assessed using neuromelanin-sensitive MRI. Response inhibition was measured using fMRI with a stop-signal task. In healthy controls, LC (but not SN) integrity correlated with the stopping-related activity of the right inferior frontal gyrus (IFG) and right subthalamic nucleus (STN), which further correlated with stop-signal reaction time (SSRT). PD patients showed reduced LC integrity, longer SSRT, and lower stopping-related activity over the right IFG, pre-supplementary motor area, and right caudate nucleus than healthy controls. In PD patients, the relationship between SSRT and the fronto-subthalamic pathway was preserved. However, LC integrity no longer correlated with the stopping-related right IFG or right STN activity. No contribution of SN integrity was found during stopping. In conclusion, LC (but not SN) might modulate inhibitory functions of the right IFG-STN pathway. Damage to the LC might impact the right IFG-STN pathway during stopping, leading to response disinhibition in PD.

Attention-deficit/hyperactive disorder updates

Background

Attention-deficit/hyperactive disorder (ADHD) is a neurodevelopmental disorder that commonly occurs in children with a prevalence ranging from 3.4 to 7.2%. It profoundly affects academic achievement, well-being, and social interactions. As a result, this disorder is of high cost to both individuals and society. Despite the availability of knowledge regarding the mechanisms of ADHD, the pathogenesis is not clear, hence, the existence of many challenges especially in making correct early diagnosis and provision of accurate management.

Objectives

We aimed to review the pathogenic pathways of ADHD in children. The major focus was to provide an update on the reported etiologies in humans, animal models, modulators, therapies, mechanisms, epigenetic changes, and the interaction between genetic and environmental factors.

Methods

References for this review were identified through a systematic search in PubMed by using special keywords for all years until January 2022.

Results

Several genes have been reported to associate with ADHD: DRD1, DRD2, DRD4, DAT1, TPH2, HTR1A, HTR1B, SLC6A4, HTR2A, DBH, NET1, ADRA2A, ADRA2C, CHRNA4, CHRNA7, GAD1, GRM1, GRM5, GRM7, GRM8, TARBP1, ADGRL3, FGF1, MAOA, BDNF, SNAP25, STX1A, ATXN7, and SORCS2. Some of these genes have evidence both from human beings and animal models, while others have evidence in either humans or animal models only. Notably, most of these animal models are knockout and do not generate the genetic alteration of the patients. Besides, some of the gene polymorphisms reported differ according to the ethnic groups. The majority of the available animal models are related to the dopaminergic pathway. Epigenetic changes including SUMOylation, methylation, and acetylation have been reported in genes related to the dopaminergic pathway.

Conclusion

The dopaminergic pathway remains to be crucial in the pathogenesis of ADHD. It can be affected by environmental factors and other pathways. Nevertheless, it is still unclear how environmental factors relate to all neurotransmitter pathways; thus, more studies are needed. Although several genes have been related to ADHD, there are few animal model studies on the majority of the genes, and they do not generate the genetic alteration of the patients. More animal models and epigenetic studies are required.

Neurobiological mechanisms of control in alcohol use disorder - moving towards mechanism-based non-invasive brain stimulation treatments

Alcohol use disorder (AUD) is characterized by excessive habitual drinking and loss of control over alcohol intake despite negative consequences. Both of these aspects foster uncontrolled drinking and high relapse rates in AUD patients. Yet, common interventions mostly focus on the phenomenological level, and prioritize the reduction of craving and withdrawal symptoms. Our review provides a mechanistic understanding of AUD and suggests alternative therapeutic approaches targeting the mechanisms underlying dysfunctional alcohol-related behaviours. Specifically, we explain how repeated drinking fosters the development of rigid drinking habits and is associated with diminished cognitive control. These behavioural and cognitive effects are then functionally related to the neurobiochemical effects of alcohol abuse. We further explain how alterations in fronto-striatal network activity may constitute the neurobiological correlates of these alcohol-related dysfunctions. Finally, we discuss limitations in current pharmacological AUD therapies and suggest non-invasive brain stimulation (like TMS and tDCS interventions) as a potential addition/alternative for modulating the activation of both cortical and subcortical areas to help re-establish the functional balance between controlled and automatic behaviour.

Reinstatement of synaptic plasticity in the aging brain through specific dopamine transporter inhibition

Aging-related neurological deficits negatively impact mental health, productivity, and social interactions leading to a pronounced socioeconomic burden. Since declining brain dopamine signaling during aging is associated with the onset of neurological impairments, we produced a selective dopamine transporter (DAT) inhibitor to restore endogenous dopamine levels and improve cognitive function. We describe the synthesis and pharmacological profile of (S,S)-CE-158, a highly specific DAT inhibitor, which increases dopamine levels in brain regions associated with cognition. We find both a potentiation of neurotransmission and coincident restoration of dendritic spines in the dorsal hippocampus, indicative of reinstatement of dopamine-induced synaptic plasticity in aging rodents. Treatment with (S,S)-CE-158 significantly improved behavioral flexibility in scopolamine-compromised animals and increased the number of spontaneously active prefrontal cortical neurons, both in young and aging rodents. In addition, (S,S)-CE-158 restored learning and memory recall in aging rats comparable to their young performance in a hippocampus-dependent hole board test. In sum, we present a well-tolerated, highly selective DAT inhibitor that normalizes the age-related decline in cognitive function at a synaptic level through increased dopamine signaling.

D2-Like Receptor Expression in the Hippocampus and Amygdala Informs Performance on the Stop-Signal Task in Parkinson's Disease

The stop-signal task is a well-established assessment of response inhibition, and in humans, proficiency is linked to dorsal striatum D2 receptor availability. Parkinson's disease (PD) is characterized by changes to efficiency of response inhibition. Here, we studied 17 PD patients (6 female and 11 male) using the stop-signal paradigm in a single-blinded d-amphetamine (dAMPH) study. Participants completed [18F]fallypride positron emission topography (PET) imaging in both placebo and dAMPH conditions. A voxel-wise analysis of the relationship between binding potential (BPND) and stop-signal reaction time (SSRT) revealed that faster SSRT is associated with greater D2-like BPND in the amygdala and hippocampus (right cluster qFDR-corr = 0.026, left cluster qFDR-corr = 0.002). A region of interest (ROI) examination confirmed this association in both the amygdala (coefficient = -48.26, p = 0.005) and hippocampus (coefficient = -104.94, p = 0.007). As healthy dopaminergic systems in the dorsal striatum appear to regulate response inhibition, we interpret our findings in PD to indicate either nigrostriatal damage unmasking a mesolimbic contribution to response inhibition, or a compensatory adaptation from the limbic and mesial temporal dopamine systems. These novel results expand the conceptualization of action-control networks, whereby limbic and motor loops may be functionally connected.SIGNIFICANCE STATEMENT While Parkinson's disease (PD) is characteristically recognized for its motor symptoms, some patients develop impulsive and compulsive behaviors (ICBs), manifested as repetitive and excessive participation in reward-driven activities, including sex, gambling, shopping, eating, and hobbyism. Such cognitive alterations compel a consideration of response inhibition in PD. To investigate inhibitory control and assess the brain regions that may participate, we assessed PD patients using a single-blinded d-amphetamine (dAMPH) study, with [18F]fallypride positron emission topography (PET) imaging, and stop-signal task performance. We find a negative relationship between D2-like binding in the mesial temporal region and top-signal reaction time (SSRT), with greater BPND associated with a faster SSRT. These discoveries indicate a novel role for mesolimbic dopamine in response inhibition, and advocate for limbic regulation of action control in this clinical population.

Locus coeruleus integrity and the effect of atomoxetine on response inhibition in Parkinson's disease

Cognitive decline is a common feature of Parkinson's disease, and many of these cognitive deficits fail to respond to dopaminergic therapy. Therefore, targeting other neuromodulatory systems represents an important therapeutic strategy. Among these, the locus coeruleus-noradrenaline system has been extensively implicated in response inhibition deficits. Restoring noradrenaline levels using the noradrenergic reuptake inhibitor atomoxetine can improve response inhibition in some patients with Parkinson's disease, but there is considerable heterogeneity in treatment response. Accurately predicting the patients who would benefit from therapies targeting this neurotransmitter system remains a critical goal, in order to design the necessary clinical trials with stratified patient selection to establish the therapeutic potential of atomoxetine. Here, we test the hypothesis that integrity of the noradrenergic locus coeruleus explains the variation in improvement of response inhibition following atomoxetine. In a double-blind placebo-controlled randomized crossover design, 19 patients with Parkinson's disease completed an acute psychopharmacological challenge with 40 mg of oral atomoxetine or placebo. A stop-signal task was used to measure response inhibition, with stop-signal reaction times obtained through hierarchical Bayesian estimation of an ex-Gaussian race model. Twenty-six control subjects completed the same task without undergoing the drug manipulation. In a separate session, patients and controls underwent ultra-high field 7 T imaging of the locus coeruleus using a neuromelanin-sensitive magnetization transfer sequence. The principal result was that atomoxetine improved stop-signal reaction times in those patients with lower locus coeruleus integrity. This was in the context of a general impairment in response inhibition, as patients on placebo had longer stop-signal reaction times compared to controls. We also found that the caudal portion of the locus coeruleus showed the largest neuromelanin signal decrease in the patients compared to controls. Our results highlight a link between the integrity of the noradrenergic locus coeruleus and response inhibition in patients with Parkinson's disease. Furthermore, they demonstrate the importance of baseline noradrenergic state in determining the response to atomoxetine. We suggest that locus coeruleus neuromelanin imaging offers a marker of noradrenergic capacity that could be used to stratify patients in trials of noradrenergic therapy and to ultimately inform personalized treatment approaches.

Prefrontal and striatal dopamine D2/D3 receptors correlate with fMRI BOLD activation during stopping

D₂-like dopamine receptors in animals and humans have been shown to be linked to impulsive behaviors that are highly relevant for several psychiatric disorders. Here, we investigate the relationship between the fronto-striatal D₂/D₃ dopamine receptor availability and response inhibition in a selected population of healthy OPRM1 G-allele carriers. Twenty-two participants successively underwent blood-oxygen level dependent functional magnetic resonance imaging (fMRI) while performing a stop-signal task and a separate positron emission tomography (PET) scan. Striatal and extrastriatal D₂/D₃ dopamine receptor availability was measured using the radiotracer [¹⁸F]fallypride. Caudate D₂/D₃ dopamine receptor availability positively correlated with stopping-related fronto-striatal fMRI activation. In addition, right prefrontal D₂/D₃ dopamine receptor availability correlated positively with stopping-related striatal fMRI BOLD signal. Our study partially replicates previous findings on correlations between striatal D₂/D₃ dopamine receptor availability and response inhibition in a population selected for its genetic determination of dopamine response to alcohol and as a modulator of impulse control via the endogenous opioid system. We confirm the important role of D₂/D₃ dopamine receptor availability in the fronto-striatal neural circuit for response inhibition. Moreover, we extend previous findings suggesting that dopamine receptor availability in the right inferior frontal cortex, a crucial region of the stopping network, is also strongly associated with stopping-related striatal fMRI activity in healthy OPRM1 G-allele carriers.

Mapping functional gradients of the striatal circuit using simultaneous microelectric stimulation and ultrahigh-field fMRI in non-human primates

Advances in functional magnetic resonance imaging (fMRI) have significantly enhanced our understanding of the striatal system of both humans and non-human primates (NHP) over the last few decades. However, its circuit-level functional anatomy remains poorly understood, partly because in-vivo fMRI cannot directly perturb a brain system and map its casual input-output relationship. Also, routine 3T fMRI has an insufficient spatial resolution. We performed electrical microstimulation (EM) of the striatum in lightly-anesthetized NHPs while simultaneously mapping whole-brain activation, using contrast-enhanced fMRI at ultra-high-field 7T. By stimulating multiple positions along the striatum's main (dorsal-to-ventral) axis, we revealed its complex functional circuit concerning mutually connected subsystems in both cortical and subcortical areas. Indeed, within the striatum, there were distinct brain activation patterns across different stimulation sites. Specifically, dorsal stimulation revealed a medial-to-lateral elongated shape of activation in upper caudate and putamen areas, whereas ventral stimulation evoked areas confined to the medial and lower caudate. Such dorsoventral gradients also appeared in neocortical and thalamic activations, indicating consistent embedding profiles of the striatal system across the whole brain. These findings reflect different forms of within-circuit and inter-regional neuronal connectivity between the dorsal and ventromedial striatum. These patterns both shared and contrasted with previous anatomical tract-tracing and in-vivo resting-state fMRI studies. Our approach of combining microstimulation and whole-brain fMRI mapping in NHPs provides a unique opportunity to integrate our understanding of a targeted brain area's meso- and macro-scale functional systems.

The dopaminergic alterations induced by 4-F-PCP and 4-Keto-PCP may enhance their drug-induced rewarding and reinforcing effects: Implications for abuse

Novel psychoactive substances remain the popular recreational drugs of use over the years. They continue to bypass government restrictions due to their synthesis and modifications. Recent additions to the lists are the 4-F-PCP and 4-Keto-PCP, analogs of the drug phencyclidine (PCP) known to induce adverse effects and abuse potential. However, studies on the abuse potential of 4-F-PCP and 4-Keto-PCP remain scarce. The rewarding and reinforcing effects of the drugs were assessed using conditioned place preference (CPP), self-administration, and locomotor sensitization tests. Dopamine (DA) receptor antagonists (SCH23390 and haloperidol) were administered during CPP to evaluate the involvement of the mesolimbic dopaminergic system. DA-related protein expression in the nucleus accumbens (NAcc) and ventral tegmental area (VTA) was measured. Additionally, phosphorylated cyclic-adenosine monophosphate-activated protein (AMP) response element-binding (p-CREB) protein, deltaFosB (∆FosB), and brain-derived neurotrophic factor (BDNF) protein levels in the NAcc were measured to assess the addiction neural plasticity effect of the drugs. Both 4-F-PCP and 4-Keto-PCP-induced CPP and self-administration; however, only 4-F-PCP elicited locomotor sensitization. Treatment with DA receptor antagonists (SH23390 and haloperidol) inhibited the 4-F- and 4-Keto-induced CPP. Both substances altered the levels of DA receptor D1 (DRD1), thyroxine hydroxylase (TH), DA receptor D2 (DRD2), p-CREB, ∆FosB, and BDNF. The results suggest that 4-F-PCP and 4-Keto-PCP may induce abuse potential in rodents via alterations in dopaminergic system accompanied by addiction neural plasticity.

Segregating domain-general from emotional context-specific inhibitory control systems - ventral striatum and orbitofrontal cortex serve as emotion-cognition integration hubs

Inhibitory control hierarchically regulates cognitive and emotional systems in the service of adaptive goal-directed behavior across changing task demands and environments. While previous studies convergently determined the contribution of prefrontal-striatal systems to general inhibitory control, findings on the specific circuits that mediate emotional context-specific impact on inhibitory control remained inconclusive. Against this background we combined an evaluated emotional Go/No Go task with fMRI in a large cohort of subjects (N=250) to segregate brain systems and circuits that mediate domain-general from emotion-specific inhibitory control. Particularly during a positive emotional context, behavioral results showed a lower accuracy for No Go trials and a faster response time for Go trials. While the dorsal striatum and lateral frontal regions were involved in inhibitory control irrespective of emotional context, activity in the ventral striatum (VS) and medial orbitofrontal cortex (mOFC) varied as a function of emotional context. On the voxel-wise whole-brain network level, limbic and striatal systems generally exhibited highest changes in global brain connectivity during inhibitory control, while global brain connectivity of the left mOFC was less decreased during emotional contexts. Functional connectivity analyses moreover revealed that negative coupling between the VS with inferior frontal gyrus (IFG)/insula and mOFC varied as a function of emotional context. Together these findings indicate separable domain- general as well as emotional context-specific inhibitory brain systems which specifically encompass the VS and its connections with frontal regions.

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

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

Ghrelin modulates dopaminergic neuron formation and attention deficit hyperactivity disorder-like behaviors: From animals to human models

Attention deficit hyperactivity disorder (ADHD) is one of the most prevalent psychiatric disorders in children. The orexigenic hormone ghrelin is important in neuroprotection and neurodevelopment, which may play an important role in psychopathogenesis of ADHD. This study aimed to systematically investigate the genomic and pharmacological manipulations of ghrelin functioning in ADHD-like symptoms in zebrafish models and validated the effects of ghrelin polymorphisms in human subjects with ADHD. We firstly generated ghrelin^Δ/Δ zebrafish mutant, which displayed hyperactive, attention deficit-like and impulsive-like behaviors, as well as endophenotypes, mimicking human ADHD. Ghrelin^Δ/Δ zebrafish exhibited downregulated expression levels of wnt1, wnt3a, wnt5a that are critical for dopaminergic neuron development to possibly regulate their number and spatial organization. Pharmacological blockade of wnt signaling with XAV939 induced a reduced moving activity and less dopaminergic neurons; whereas, wnt agonist SB415286 rescued hyperactivity and dopaminergic neuron loss in ghrelin^Δ/Δ zebrafish. In addition, we further identified and validated a SNP, rs696217, on orexigenic hormone preproghrelin/ghrelin (T408T, Met72Met) to be associated with a higher risk of ADHD in a case-controlled association study with 248 subjects with ADHD and 208 subjects of healthy controls. Together, our results reveal a novel endogenous role for orexigenic hormone ghrelin in ADHD, which provides insights into genetic regulation and drug screens for the identification of novel treatments of ADHD.

The Human Basal Ganglia Mediate the Interplay between Reactive and Proactive Control of Response through Both Motor Inhibition and Sensory Modulation

The basal ganglia (BG) have long been known for contributing to the regulation of motor behaviour by means of a complex interplay between tonic and phasic inhibitory mechanisms. However, after having focused for a long time on phasic reactive mechanisms, it is only recently that psychological research in healthy humans has modelled tonic proactive mechanisms of control. Mutual calibration between anatomo-functional and psychological models is still needed to better understand the unclear role of the BG in the interplay between proactive and reactive mechanisms of control. Here, we implemented an event-related fMRI design allowing proper analysis of both the brain activity preceding the target-stimulus and the brain activity induced by the target-stimulus during a simple go/nogo task, with a particular interest in the ambiguous role of the basal ganglia. Post-stimulus activity was evoked in the left dorsal striatum, the subthalamus nucleus and internal globus pallidus by any stimulus when the situation was unpredictable, pinpointing its involvement in reactive, non-selective inhibitory mechanisms when action restraint is required. Pre-stimulus activity was detected in the ventral, not the dorsal, striatum, when the situation was unpredictable, and was associated with changes in functional connectivity with the early visual, not the motor, cortex. This suggests that the ventral striatum supports modulatory influence over sensory processing during proactive control.

The role of dopamine in action control: Insights from medication effects in Parkinson's disease

Parkinson's disease (PD) is a neurological disorder associated primarily with overt motor symptoms. Several studies show that PD is additionally accompanied by impairments in covert cognitive processes underlying goal-directed motor functioning (e.g., action planning, conflict adaptation, inhibition), and that dopaminergic medication may modulate these action control components. In this review we aim to leverage findings from studies in this domain to elucidate the role of dopamine (DA) in action control. A qualitative review of studies that investigated the effects of medication status (on vs. off) on action control in PD suggests a component-specific role for DA in action control, although the expression of medication effects depends on characteristics of both the patients and experimental tasks used to measure action control. We discuss these results in the light of findings from other research lines examining the role of DA in action control (e.g., animal research, pharmacology), and recommend that future studies use multi-method, within-subject approaches to model DA effects on action control across different components as well as underlying striatal pathways (ventral vs. dorsal).

Interactive effects of DRD2 rs6277 polymorphism, environment and sex on impulsivity in a population-representative study

Previous research has shown that dopaminergic dysregulation and early life stress interact to impact on aspects of impulse control. This study aimed to explore the potentially interactive effects of the rs6277 polymorphism of the dopamine D2 receptor gene (DRD2), stressful or supportive environment and sex on behavioural and self-reported measures of impulsivity, as well as alcohol use - a condition characterised by a deficit in impulse control. The sample consisted of the younger cohort (n = 583) of the longitudinal Estonian Children Personality, Behaviour and Health Study. The results showed that the CC homozygotes (suggested to have decreased striatal D2 receptor availability) who had experienced stressful life events (SLE) or maltreatment in the family prior to age 15 showed higher self-reported maladaptive impulsivity at age 15. The genotype-SLE interaction and further association with sex was also evident in the frequency of alcohol use at age 15. Lack of warmth in the family contributed to significantly higher levels of thoughtlessness and more frequent alcohol use in CC carriers at age 25, whereas family support was associated with lower thoughtlessness scores in CC males, which may suggest a protective effect of supportive family environment in this group. Together the findings suggest that DRD2 rs6277 polymorphism, in interaction with environmental factors experienced in childhood and youth may affect facets of impulsivity. Future work should aim to further clarify the sex and age-specific effects of stressful and supportive environment on the development of neuronal systems that are compromised in disorders characterised by deficits in impulse control.

Toxoplasma gondii, Suicidal Behavior, and Intermediate Phenotypes for Suicidal Behavior

Within the general literature on infections and suicidal behavior, studies on Toxoplasma gondii (T. gondii) occupy a central position. This is related to the parasite's neurotropism, high prevalence of chronic infection, as well as specific and non-specific behavioral alterations in rodents that lead to increased risk taking, which are recapitulated in humans by T. gondii's associations with suicidal behavior, as well as trait impulsivity and aggression, mental illness and traffic accidents. This paper is a detailed review of the associations between T. gondii serology and suicidal behavior, a field of study that started 15 years ago with our publication of associations between T. gondii IgG serology and suicidal behavior in persons with mood disorders. This "legacy" article presents, chronologically, our primary studies in individuals with mood disorders and schizophrenia in Germany, recent attempters in Sweden, and in a large cohort of mothers in Denmark. Then, it reviews findings from all three meta-analyses published to date, confirming our reported associations and overall consistent in effect size [ranging between 39 and 57% elevation of odds of suicide attempt in T. gondii immunoglobulin (IgG) positives]. Finally, the article introduces certain links between T. gondii and biomarkers previously associated with suicidal behavior (kynurenines, phenylalanine/tyrosine), intermediate phenotypes of suicidal behavior (impulsivity, aggression) and state-dependent suicide risk factors (hopelessness/dysphoria, sleep impairment). In sum, an abundance of evidence supports a positive link between suicide attempts (but not suicidal ideation) and T. gondii IgG (but not IgM) seropositivity and serointensity. Trait impulsivity and aggression, endophenotypes of suicidal behavior have also been positively associated with T. gondii seropositivity in both the psychiatrically healthy as well as in patients with Intermittent Explosive Disorder. Yet, causality has not been demonstrated. Thus, randomized interventional studies are necessary to advance causal inferences and, if causality is confirmed, to provide hope that an etiological treatment for a distinct subgroup of individuals at an increased risk for suicide could emerge.

The Psychopharmacology of Obsessive-Compulsive Disorder: A Preclinical Roadmap

This review evaluates current knowledge about obsessive-compulsive disorder (OCD), with the goal of providing a roadmap for future directions in research on the psychopharmacology of the disorder. It first addresses issues in the description and diagnosis of OCD, including the structure, measurement, and appropriate description of the disorder and issues of differential diagnosis. Current pharmacotherapies for OCD are then reviewed, including monotherapy with serotonin reuptake inhibitors and augmentation with antipsychotic medication and with psychologic treatment. Neuromodulatory therapies for OCD are also described, including psychosurgery, deep brain stimulation, and noninvasive brain stimulation. Psychotherapies for OCD are then reviewed, focusing on behavior therapy, including exposure and response prevention and cognitive therapy, and the efficacy of these interventions is discussed, touching on issues such as the timing of sessions, the adjunctive role of pharmacotherapy, and the underlying mechanisms. Next, current research on the neurobiology of OCD is examined, including work probing the role of various neurotransmitters and other endogenous processes and etiology as clues to the neurobiological fault that may underlie OCD. A new perspective on preclinical research is advanced, using the Research Domain Criteria to propose an adaptationist viewpoint that regards OCD as the dysfunction of a normal motivational system. A systems-design approach introduces the security motivation system (SMS) theory of OCD as a framework for research. Finally, a new perspective on psychopharmacological research for OCD is advanced, exploring three approaches: boosting infrastructure facilities of the brain, facilitating psychotherapeutic relearning, and targeting specific pathways of the SMS network to fix deficient SMS shut-down processes. SIGNIFICANCE STATEMENT: A significant proportion of patients with obsessive-compulsive disorder (OCD) do not achieve remission with current treatments, indicating the need for innovations in psychopharmacology for the disorder. OCD may be conceptualized as the dysfunction of a normal, special motivation system that evolved to manage the prospect of potential danger. This perspective, together with a wide-ranging review of the literature, suggests novel directions for psychopharmacological research, including boosting support systems of the brain, facilitating relearning that occurs in psychotherapy, and targeting specific pathways in the brain that provide deficient stopping processes in OCD.

Ropinirole, a dopamine agonist with high D3 affinity, reduces proactive inhibition: A double-blind, placebo-controlled study in healthy adults

Response inhibition describes the cognitive processes mediating the suppression of unwanted actions. A network involving the basal ganglia mediates two forms of response inhibition: reactive and proactive inhibition. Reactive inhibition serves to abruptly stop motor activity, whereas proactive inhibition is goal-orientated and results in slowing of motor activity in anticipation of stopping. Due to its impairment in several psychiatric disorders, the neurochemistry of response inhibition has become of recent interest. Dopamine has been posed as a candidate mediator of response inhibition due to its role in functioning of the basal ganglia and the observation that patients with Parkinson's disease on dopamine agonists develop impulse control disorders. Although the effects of dopamine on reactive inhibition have been studied, substantial literature on the role of dopamine on proactive inhibition is lacking. To fill this gap, we devised a double-blind, placebo-controlled study of 1 mg ropinirole (a dopamine agonist) on response inhibition in healthy volunteers. We found that whilst reactive inhibition was unchanged, proactive inhibition was impaired when participants were on ropinirole relative to when on placebo. To investigate how ropinirole mediated this effect on proactive inhibition, we used hierarchical drift-diffusion modelling. We found that ropinirole impaired the ability to raise the decision threshold when proactive inhibition was called upon. Our results provide novel evidence that an acute dose of ropinirole selectively reduces proactive inhibition in healthy participants. These results may help explain how ropinirole induces impulse control disorders in susceptible patients with Parkinson's disease.

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Proactive but not reactive inhibition is impaired under the influence of ropinirole vs placebo.

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Ropinirole impairs the ability to raise the decision threshold when proactive inhibition is called upon.

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We provide novel evidence that an acute dose of ropinirole selectively reduces proactive inhibition in healthy participants.

A Single 30 Minutes Bout of Combination Physical Exercises Improved Inhibition and Vigor-Mood in Middle-Aged and Older Females: Evidence From a Randomized Controlled Trial

Background

Long-term combination of physical exercises has reported benefits for cognitive functions and mood states. However, it remains unclear whether a single bout of combination exercise training has acute positive effects on cognitive functions and mood states in middle-aged and older women. It is important to investigate acute effect of physical exercise because it would help to understand a mechanism of benefits of physical exercise. The purpose of this study was to investigate 30 min of a single bout of combination exercise training on cognition and mood states in middle-aged and older females.

Methods

In this single-blinded randomized control trial (RCT), middle-aged and older females were assigned randomly to two groups: a combination exercise group and a no-exercise control group. The former group did the combination exercise training (aerobic, strength, and stretching exercises) for 30 min. Meanwhile, the latter group did not do any exercise and waited for 30 min. We measured cognitive functions and mood performance states before and after the exercise or control interventions.

Results

Our main results demonstrated that, compared to the control group, the combination exercise improved inhibition (reverse Stroop and Stroop) and increased vigor–activity mood scores in both middle-aged and older groups. We also found that the only combination exercise group showed the significant positive correlations between improved inhibition performance and improved vigor–activity mood.

Discussion

This randomized controlled trial revealed the acute benefits of combination exercise on inhibition in executive functions and vigor–activity in the healthy middle-aged and older females. Our results provided the scientific evidence related to acute effects of the single bout of the combination exercise training. It suggests that we would be better to do the 30 min physical exercise for our health.

Clinical Trial Registration

This trial was registered in the University Hospital Medical Information Network Clinical Trials Registry (UMIN000029681). Registered 24 October 2017, https://upload.umin.ac.jp/cgi-bin/ctr/ctr_view_reg.cgi?recptno=R000033922.

Dorsal and ventral striatal dopamine D1 and D2 receptors differentially modulate distinct phases of serial visual reversal learning

Impaired cognitive flexibility in visual reversal-learning tasks has been observed in a wide range of neurological and neuropsychiatric disorders. Although both human and animal studies have implicated striatal D2-like and D1-like receptors (D2R; D1R) in this form of flexibility, less is known about the contribution they make within distinct sub-regions of the striatum and the different phases of visual reversal learning. The present study investigated the involvement of D2R and D1R during the early (perseverative) phase of reversal learning as well as in the intermediate and late stages (new learning) after microinfusions of D2R and D1R antagonists into the nucleus accumbens core and shell (NAcC; NAcS), the anterior and posterior dorsomedial striatum (DMS) and the dorsolateral striatum (DLS) on a touchscreen visual serial reversal-learning task. Reversal learning was improved after dopamine receptor blockade in the nucleus accumbens; the D1R antagonist, SCH23390, in the NAcS and the D2R antagonist, raclopride, in the NAcC selectively reduced early, perseverative errors. In contrast, reversal learning was impaired by D2R antagonism, but not D1R antagonism, in the dorsal striatum: raclopride increased errors in the intermediate phase after DMS infusions, and increased errors across phases after DLS infusions. These findings indicate that D1R and D2R modulate different stages of reversal learning through effects localised to different sub-regions of the striatum. Thus, deficits in behavioral flexibility observed in disorders linked to dopamine perturbations may be attributable to specific D1R and D2R dysfunction in distinct striatal sub-regions.

Neural correlates and role of medication in reactive motor impulsivity in Tourette disorder

Abnormality of inhibitory control is considered to be a potential cognitive marker of tics in Tourette disorder (TD), attention deficit hyperactivity disorder (ADHD), and impulse control disorders. The results of the studies on inhibitory control in TD showed discrepant results. The aim of the present study was to assess reactive inhibitory control in adult TD patients with and without antipsychotic medication, and under emotional stimulation (visual images with positive, neutral and negative content). We assessed 31 unmedicated and 19 medicated TD patients and 26 matched healthy controls using the stop signal task as an index of reactive motor impulsivity and emotional stimulation with the aim to increase impulsivity. We performed a multimodal neuroimaging analysis using a regions of interest approach on grey matter signal, resting-state spontaneous brain activity and functional connectivity analyses. We found a higher reactive motor impulsivity in TD patients medicated with antipsychotics compared to unmedicated TD patients and controls. This propensity for reactive motor impulsivity in medicated TD patients was not influenced by ADHD or emotional stimulation. Neuroimaging results in medicated TD patients suggested that reactive motor impulsivity was underpinned by an increased grey matter signal from the right supplementary motor area and inferior frontal gyrus; decreased resting-state spontaneous activity of the left putamen; higher functional connectivity between the inferior frontal gyrus and the superior temporal gyri (bilaterally); lower functional connectivity between the cerebellum and the right subthalamic nucleus. Taken together, our data suggested (i) a deficit in reactive motor impulsivity in TD patients medicated with atypical antipsychotics that was unrelated to ADHD and (ii) that motor impulsivity was underpinned by structures and by functional connectivity of the fronto-temporo-basal ganglia-cerebellar pathway.

Human Brain Imaging Links Dopaminergic Systems to Impulsivity

Molecular and functional imaging techniques have been used and combined with pharmacological probes to evaluate the role of dopamine in impulsivity. Overall, strong evidence links striatal dopaminergic function with impulsivity, measured by self-reports and laboratory tests of cognitive control and reward-based decision-making. The combination of molecular imaging using positron emission tomography (PET) with functional magnetic resonance imaging (fMRI) specifically implicates striatal D2-type dopamine receptors (i.e., D2 and D3) and corticostriatal connectivity in cognitive control. Low levels of striatal and midbrain D2-type receptor availability correlate with self-reported impulsivity, whereas striatal D2-type receptor availability shows positive correlation with motor response inhibition and cognitive flexibility. Impulsive choice on reward-based decision-making tasks also is related to deficits in striatal D2-type dopamine receptor availability, and there is evidence for an inverted U-shaped function in this relationship, reflecting an optimum of striatal dopaminergic activity. Findings from studies of clinical populations that present striatal dopamine D2-type receptor deficits as well as healthy control research participants identify D2-type receptors as therapeutic targets to improve cognitive control.

Abnormalities of striatal morphology in gambling disorder and at-risk gambling

OBJECTIVE

The clinical phenotype of gambling disorder (GD) is suggestive of changes in brain regions involved in reward and impulse suppression, notably the striatum. Studies have yet to characterize striatal morphology (shape) in GD and whether this may be a vulnerability marker.

AIMS

To characterize the morphology of the striatum in those with disordered gambling (at-risk gambling and GD) versus controls.

METHOD

Individuals aged 18-29 years were classified a priori into those with some degree of GD symptoms (at-risk gambling and GD) or controls. Exclusion criteria were a current mental disorder (apart from GD), history of brain injury, or taking psychoactive medication within 6 weeks of enrollment. History of any substance use disorder was exclusionary. Participants completed an impulsivity questionnaire and structural brain scan. Group differences in volumes and morphology were characterized in subcortical regions of interest, focusing on the striatum.

RESULTS

Thirty-two people with GD symptoms (14 at-risk and 18 GD participants) and 22 controls completed the study. GD symptoms were significantly associated with higher impulsivity and morphological alterations in the bilateral pallidum and left putamen. Localized contraction in the right pallidum strongly correlated with trait impulsivity in those with GD symptoms.

CONCLUSIONS

Morphologic abnormalities of the striatum appear to exist early in the disease trajectory from subsyndromal gambling to GD and thus constitute candidate biological vulnerability markers, which may reflect differences in brain development associated with trait impulsivity. Striatal morphology and associated impulsivity might predispose to a range of problematic repetitive behaviors.

The Fyn kinase inhibitor, AZD0530, suppresses mouse alcohol self-administration and seeking

Fyn is a member of the Src family of protein tyrosine kinases (PTKs) that plays an important role not only in normal synaptic functions but also in brain pathologies including alcohol use disorder. We previously reported that repeated cycles of binge drinking and withdrawal activate Fyn in the dorsomedial striatum (DMS) of rodents, and that Fyn signaling in the DMS contributes to rat alcohol intake and relapse. Here, we used AZD0530, a CNS penetrable inhibitor of Src PTKs developed for the treatment of Alzheimer disease and cancer and tested its efficacy to suppress alcohol-dependent molecular and behavioral effects. We show that systemic administration of AZD0530 prevents alcohol-induced Fyn activation and GluN2B phosphorylation in the DMS of mice. We further report that a single dose of AZD0530 reduces alcohol operant self-administration and promotes extinction of alcohol self-administration without altering basal and dopamine D1 receptor-dependent locomotion. Together, our findings suggest that AZD0530, through its inhibitory actions on Fyn kinase, dampens alcohol seeking and drinking.

Dopamine D2 receptor stimulation modulates the balance between ignoring and updating according to baseline working memory ability

BACKGROUND

Working memory (WM) deficits in neuropsychiatric disorders have often been attributed to altered dopaminergic signalling. Specifically, D₂ receptor stimulation is thought to affect the ease with which items can be gated into and out of WM. In addition, this effect has been hypothesised to vary according to baseline WM ability, a putative index of dopamine synthesis levels. Moreover, whether D₂ stimulation affects WM vicariously through modulating relatively WM-free cognitive control processes has not been explored.

AIMS

We examined the effect of administering a dopamine agonist on the ability to ignore or update information in WM.

METHOD

A single dose of cabergoline (1 mg) was administered to healthy older adult humans in a within-subject, double-blind, placebo-controlled study. In addition, we obtained measures of baseline WM ability and relatively WM-free cognitive control (overcoming response conflict).

RESULTS

Consistent with predictions, baseline WM ability significantly modulated the effect that drug administration had on the proficiency of ignoring and updating. High-WM individuals were relatively better at ignoring compared to updating after drug administration. Whereas the opposite occurred in low-WM individuals. Although the ability to overcome response conflict was not affected by cabergoline, a negative relationship between the effect the drug had on response conflict performance and ignoring was observed. Thus, both response conflict and ignoring are coupled to dopaminergic stimulation levels.

CONCLUSIONS

Cumulatively, these results provide evidence that dopamine affects subcomponents of cognitive control in a diverse, antagonistic fashion and that the direction of these effects is dependent upon baseline WM.

MR imaging central thalamic deep brain stimulation restored autistic-like social deficits in the rat

BACKGROUND

Social deficit is a core symptom in autism spectrum disorder (ASD). Although deep brain stimulation (DBS) has been proposed as a potential treatment for ASD, an ideal target nucleus is yet to be identified. DBS at the central thalamic nucleus (CTN) is known to alter corticostriatal and limbic circuits, and subsequently increase the exploratory motor behaviors, cognitive performance, and skill learning in neuropsychiatric and neurodegenerative disorders.

OBJECTIVE

We first investigated the ability of CTN-DBS to selectively engage distinct brain circuits and compared the spatial distribution of evoked network activity and modulation. Second, we investigated whether CTN-DBS intervention improves social interaction in a valproic acid-exposed ASD rat offspring model.

METHODS

Brain regions activated through CTN-DBS by using a magnetic resonance (MR)-compatible neural probe, which is capable of inducing site-selective microstimulations during functional MRI (fMRI), were investigated. We then performed functional connectivity MRI, the three-chamber social interaction test, and Western blotting analyses to evaluate the therapeutic efficacy of CTN-DBS in an ASD rat offspring model.

RESULTS

The DBS-evoked fMRI results indicated that the activated brain regions were mainly located in cortical areas, limbic-related areas, and the dorsal striatum. We observed restoration of brain functional connectivity (FC) in corticostriatal and corticolimbic circuits after CTN-DBS, accompanied with increased social interaction and decreased social avoidance in the three-chamber social interaction test. The dopamine D2 receptor decreased significantly after CTN-DBS treatment, suggesting changes in synaptic plasticity and alterations in the brain circuits.

CONCLUSIONS

Applying CTN-DBS to ASD rat offspring increased FC and altered the synaptic plasticity in the corticolimbic and the corticostriatal circuits. This suggests that CTN-DBS could be an effective treatment for improving the social behaviors of individuals with ASD.

Neural evidence for automatic value-modulated approach behaviour

Reward learning has the ability to bias both attention and behaviour. The current study presents behavioural and neural evidence that irrelevant responses evoked by previously reward-associated stimuli are more robustly represented in the motor system using a combined go/no-go and flankers task. Following a colour-reward association training, participants were instructed to respond to a central target only in a response-relevant context, while ignoring flankers that appeared either in a high-value or low-value colour. The motor cortex and cerebellum exhibited reduced activation to low-value flankers in a response-irrelevant context, consistent with goal-directed response suppression. However, these same regions exhibited similar activation to high-value flankers regardless of their response relevance, indicating less effective suppression, and the resulting interaction in motor cortex activation was strongly predicted by the influence of the flankers on behaviour. These findings suggest that associative reward learning produces a general approach bias, which is particularly evident when it conflicts with task goals, extending the principle of value-driven attention to stimulus-evoked responses in the motor system.

Controllability affects endocrine response of adolescent male rats to stress as well as impulsivity and behavioral flexibility during adulthood

Exposure to stress during adolescence exerts a long-term impact on behavior and might contribute to the development of several neuropsychiatric disorders. In adults, control over stress has been found to protect from the negative consequences of stress, but the influence of controllability at early ages has not been extensively studied. Here, we evaluated in a rodent model the effects of repeated exposure in adolescent male rats to controllable versus uncontrollable foot-shock stress (CST or UST, respectively). Rats were assigned to three groups: non-stress (stress-naïve), CST (exposed to 8 sessions of a two-way shuttle active avoidance task over a period of 22 days) and UST (receiving the same amount of shocks as CST, regardless of their actual behavior). During adulthood, different cohorts were tested in several tasks evaluating inhibitory control and cognitive flexibility: 5-choice serial reaction time, delay-discounting, gambling test and probabilistic reversal learning. Results showed that the hypothalamic-pituitary-adrenal response to the first shock session was similar in CST and UST animals, but the response to the 8^th session was lower in CST animals. In adulthood, the UST animals presented impaired motor (but not cognitive) impulsivity and more perseverative behavior. The behavioral effects of UST were associated with increased number of D2 dopamine receptors in dorsomedial striatum, but not in other striatal regions. In summary, UST exposure during adolescence induced long-term impairments in impulsivity and compulsivity, whereas CST had only minor effects. These data support a critical role of stress uncontrollability on the long-lasting consequences of stress, as a risk factor for mental illnesses.

An Integrated Model of Action Selection: Distinct Modes of Cortical Control of Striatal Decision Making

Making decisions in environments with few choice options is easy. We select the action that results in the most valued outcome. Making decisions in more complex environments, where the same action can produce different outcomes in different conditions, is much harder. In such circumstances, we propose that accurate action selection relies on top-down control from the prelimbic and orbitofrontal cortices over striatal activity through distinct thalamostriatal circuits. We suggest that the prelimbic cortex exerts direct influence over medium spiny neurons in the dorsomedial striatum to represent the state space relevant to the current environment. Conversely, the orbitofrontal cortex is argued to track a subject's position within that state space, likely through modulation of cholinergic interneurons.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

Examining the link between reward and response inhibition in individuals with substance abuse tendencies

BACKGROUND

Substance use problems are often characterized by dysregulation in reward sensitivity and inhibitory control. In line with this representation, the goal of this investigation was to determine how substance abuse tendencies among university students affect incentivized response inhibition. Additionally, this study examined whether striatal dopamine moderates the impact of substance use on response inhibition performance.

METHODS

The sample included ninety-eight university students. Participants completed this prospective experimental study at an on-campus laboratory. All participants completed substance abuse and disinhibition subscales of the Externalizing Spectrum Inventory-Brief Form. Using a within-subjects design, participants then performed the Stop Signal Task under both neutral (unrewarded) and reward conditions, in which correct response cancellations resulted in a monetary reward. Striatal tonic dopamine levels were operationalized using spontaneous eyeblink rate.

RESULTS

The outcome measures were Stop Signal Reaction Time (SSRT) performance in the unrewarded and rewarded phases of the task. A hierarchical linear regression analysis, controlling for trait disinhibition, age, gender, and cigarette smoking status, identified an interactive effect of substance use and striatal dopamine levels on incentivized SSRT. Substance abuse tendencies were associated with slower SSRT and thus poorer inhibitory control under reward conditions among individuals with low levels of striatal dopamine (F = 7.613, p = .007).

CONCLUSIONS

This work has implications for research examining advanced drug use trajectories. In situations in which rewards are at stake, drug users with low tonic dopamine may be more motivated to seek those rewards at the expense of regulating inhibitory control.

Moderation of the relationship between Toxoplasma gondii seropositivity and trait impulsivity in younger men by the phenylalanine-tyrosine ratio

Previously, we reported that Toxoplasma gondii (T. gondii)-seropositivity is associated with higher impulsive sensation seeking in younger men. As dopaminergic and serotonergic signaling regulate impulsivity, and as T. gondii directly and indirectly affects dopaminergic signaling and induces activation of the kynurenine pathway leading to the diversion of tryptophan from serotonin production, we investigated if dopamine and serotonin precursors or the tryptophan metabolite kynurenine interact with the T. gondii-impulsivity association. In 950 psychiatrically healthy participants, trait impulsivity scores were related to T. gondii IgG seropositivity. Interactions were also identified between categorized levels of phenylalanine (Phe), tyrosine (Tyr), Phe:Tyr ratio, kynurenine (Kyn), tryptophan (Trp) and Kyn:Trp ratio, and age and gender. Only younger T. gondii-positive men with a high Phe:Tyr ratio, were found to have significantly higher impulsivity scores. There were no significant associations in other demographic groups, including women and older men. No significant effects or interactions were identified for Phe, Tyr, Kyn, Trp, or Kyn:Trp ratio. Phe:Tyr ratio, therefore, may play a moderating role in the association between T. gondii seropositivity and impulsivity in younger men. These results could potentially lead to individualized approaches to reduce impulsivity, based on combined demographic, biochemical and serological factors.

A comparative study on the neurophysiological mechanisms underlying effects of methylphenidate and neurofeedback on inhibitory control in attention deficit hyperactivity disorder

In Attention Deficit Hyperactivity Disorder (AD(H)D), treatments using methylphenidate (MPH) and behavioral interventions like neurofeedback (NF) reflect major therapeutic options. These treatments also ameliorate executive dysfunctions in AD(H)D. However, the mechanisms underlying effects of MPH and NF on executive functions in AD(H)D (e.g. the ability to inhibit prepotent responses) are far from understood. It is particularly unclear whether these interventions affect similar or dissociable neural mechanisms and associated functional neuroanatomical structures. This, however, is important when aiming to further improve these treatments. We compared the neurophysiological mechanisms of MPH and theta/beta NF treatments on inhibitory control on the basis of EEG recordings and source localization analyses. The data show that MPH and theta/beta NF both increase the ability to inhibit pre-potent responses to a similar extent. However, the data suggest that MPH and NF target different neurophysiological mechanisms, especially when it comes to functional neuroanatomical structures associated with these effects. Both treatments seem to affect neurophysiological correlates of a ‘braking function’ in medial frontal areas. However, in case of the NF intervention, inferior parietal areas are also involved. This likely reflects the updating and stabilisation of efficient internal representations in order to initiate appropriate actions. No effects were seen in correlates of perceptual and attentional selection processes. Notably, reliable effects were only obtained after accounting for intra-individual variability in the neurophysiological data, which may also explain the diversity of findings in studies on treatment effects in AD(H)D, especially concerning neurofeedback.

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Neurophysiological mechanisms of methylphenidate (MPH) and neurofeedback (NF) in ADHD are compared.

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Both treatments improve inhibition, but have different underlying neurophysiological mechanisms.

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Both treatments affect the neurophysiological correlates of a ‘breaking function’ in medial frontal areas.

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NF also affects inferior parietal areas and likely the updating of internal representations.

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Distinctive effects are only seen when accounting for intra-individual variability.

Prefrontal cortex executive processes affected by stress in health and disease

Prefrontal cortical executive functions comprise a number of cognitive capabilities necessary for goal directed behavior and adaptation to a changing environment. Executive dysfunction that leads to maladaptive behavior and is a symptom of psychiatric pathology can be instigated or exacerbated by stress. In this review we survey research addressing the impact of stress on executive function, with specific focus on working memory, attention, response inhibition, and cognitive flexibility. We then consider the neurochemical pathways underlying these cognitive capabilities and, where known, how stress alters them. Finally, we review work exploring potential pharmacological and non-pharmacological approaches that can ameliorate deficits in executive function. Both preclinical and clinical literature indicates that chronic stress negatively affects executive function. Although some of the circuitry and neurochemical processes underlying executive function have been characterized, a great deal is still unknown regarding how stress affects these processes. Additional work focusing on this question is needed in order to make progress on developing interventions that ameliorate executive dysfunction.

Opinion on monoaminergic contributions to traits and temperament

This article critically reviews evidence relating temperamental traits and personality factors to the monoamine neurotransmitters, especially dopamine and serotonin. The genetic evidence is not yet considered to be conclusive and it is argued that basic neuroscience research on the neural basis of behaviour in experimental animals should be taken more into account. While questionnaire and lexical methodology including the 'Five Factor' theory has been informative (mostly for the traits relevant to social functioning, i.e. personality), biologically oriented approaches should be employed with more objective, theoretically grounded measures of cognition and behaviour, combined with neuroimaging and psychopharmacology, where appropriate. This strategy will enable specific functions of monoamines and other neuromodulators such as acetylcholine and neuropeptides (such as orexin) to be defined with respect to their roles in modulating activity in specific neural networks-leading to a more realistic definition of their interactive roles in complex, biologically based traits (i.e. temperament).This article is part of the theme issue 'Diverse perspectives on diversity: multi-disciplinary approaches to taxonomies of individual differences'.

An insulin resistance associated neural correlate of impulsivity in type 2 diabetes mellitus

Central insulin resistance (IR) influences striatal dopamine (DA) tone, an important determinant of behavioral self-regulation. We hypothesized that an association exists between the degree of peripheral IR and impulse control, mediated by the impact of IR on brain circuits controlling the speed of executing “go” and/or “stop” responses. We measured brain activation and associated performance on a stop signal task (SST) in obese adults with type 2 diabetes (age, 48.1 ± 6.9 yrs (mean ± SD); BMI, 36.5 ± 4.0 kg/m²; HOMA-IR, 7.2 ± 4.1; 12 male, 18 female). Increasing IR, but not BMI, was a predictor of shorter critical stop signal delay (cSSD), a measure of the time window during which a go response can be successfully countermanded (R² = 0.12). This decline was explained by an IR-associated increase in go speed (R² = 0.13) with little impact of IR or BMI on stop speed. Greater striatal fMRI activation contrast in stop error (SE) compared with stop success (SS) trials (CON_SE>SS) was a significant predictor of faster go speeds (R² = 0.33, p = 0.002), and was itself predicted by greater IR (CON_SE>SS vs HOMA-IR: R² = 0.10, p = 0.04). Furthermore, this impact of IR on striatal activation was a significant mediator of the faster go speeds and greater impulsivity observed with greater IR. These findings suggest a neural mechanism by which IR may increase impulsivity and degrade behavioral self-regulation.

Hierarchical investigation of genetic influences on response inhibition in healthy young adults

Poor inhibitory control is a known risk factor for substance use disorders, making it a priority to identify the determinants of these deficits. The aim of the current study was to identify genetic associations with inhibitory control using the stop signal task in a large sample (n = 934) of healthy young adults of European ancestry. We genotyped the subjects genome-wide and then used a hierarchical approach in which we tested seven a priori single nucleotide polymorphisms (SNPs) previously associated with stop signal task performance, approximately 9,000 SNPs designated as high-value addiction (HVA) markers by the SmokeScreen array, and approximately five million genotyped and imputed SNPs, followed by a gene-based association analysis using the resultant p values. A priori SNP analyses revealed nominally significant associations between response inhibition and one locus in HTR2A (rs6313; p = .04, dominance model, uncorrected) in the same direction as prior findings. A nominally significant association was also found in one locus in ANKK1 (rs1800497; p = .03, uncorrected), although in the opposite direction of previous reports. After accounting for multiple comparisons, the HVA, genome-wide, and gene-based analyses yielded no significant findings. This study implicates variation in serotonergic and dopaminergic genes while underscoring the difficulty of detecting the influence of individual SNPs, even when biological information is used to prioritize testing. Although such small effect sizes suggest limited utility of individual SNPs in predicting risk for addiction or other impulse control disorders, they may nonetheless shed light on complex biological processes underlying poor inhibitory control. (PsycINFO Database Record

Cross-species studies of cognition relevant to drug discovery: a translational approach

This review advances the case that bidirectional, cross-species translation of findings from experimental animals to and from humans is an important strategy for drug discovery. Animal models of mental disorders require appropriate behavioural or cognitive outcome variables that can be generalized cross-species. One example is the treatment of impulsive behaviour in attention deficit hyperactivity disorder (ADHD) with stimulant drugs. Performance on the stop signal reaction task as an index of impulsivity is improved both in healthy human volunteers and in patients with adult ADHD by stimulant drugs and also by the selective noradrenaline reuptake blocker atomoxetine. Functional neuroimaging evidence suggests a modulation of circuitry including the inferior prefrontal cortex by this drug. Parallel work in rats had shown that atomoxetine improves stop signal performance by affecting possibly homologous regions of the rodent prefrontal cortex. This parallel effect of atomoxetine in rodents and humans could potentially be exploited in other disorders in which impulsivity plays a role, such as stimulant abuse and Parkinson's disease. A contrasting relative lack of involvement of 5-HT mechanisms in the stop signal reaction time task will also be described. Research in humans and experimental animals that demonstrate effects of serotoninergic agents such as the selective serotonin (5-HT) reuptake inhibitor citalopram on probabilistic learning and reversal (upon which atomoxetine has little effect) will also be reviewed, possibly relevant to the treatment of clinical depression, Finally, other promising examples of parallel studies of behavioural effects of CNS-active drugs in animals and humans will also be described. Linked Articles This article is part of a themed section on Pharmacology of Cognition: a Panacea for Neuropsychiatric Disease? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.19/issuetoc.

Age and impulsive behavior in drug addiction: A review of past research and future directions

Impulsive behavior is implicated in the initiation, maintenance, and relapse of drug-seeking behaviors involved in drug addiction. Research shows that changes in impulsive behavior across the lifespan contribute to drug use and addiction. The goal of this review is to examine existing research on the relationship between impulsive behavior and drug use across the lifespan and to recommend directions for future research. Three domains of impulsive behavior are explored in this review: impulsive behavior-related personality traits, delay discounting, and prepotent response inhibition. First, we present previous research on these three domains of impulsive behavior and drug use across developmental stages. Then, we discuss how changes in impulsive behavior across the lifespan are implicated in the progression of drug use and addiction. Finally, we discuss the relatively limited attention given to middle-to-older adults in the current literature, consider the validity of the measures used to assess impulsive behavior in middle-to-older adulthood, and suggest recommendations for future research.

Fractionating impulsivity: neuropsychiatric implications

The ability to make decisions and act quickly without hesitation can be advantageous in many settings. However, when persistently expressed, impulsive decisions and actions are considered risky, maladaptive and symptomatic of such diverse brain disorders as attention-deficit hyperactivity disorder, drug addiction and affective disorders. Over the past decade, rapid progress has been made in the identification of discrete neural networks that underlie different forms of impulsivity - from impaired response inhibition and risky decision making to a profound intolerance of delayed rewards. Herein, we review what is currently known about the neural and psychological mechanisms of impulsivity, and discuss the relevance and application of these new insights to various neuropsychiatric disorders.

Low Cognitive Impulsivity Is Associated with Better Gain and Loss Learning in a Probabilistic Decision-Making Task

Many advances have been made over the last decades in describing, on the one hand, the link between reward-based learning and decision-making, and on the other hand, the link between impulsivity and decision-making. However, the association between reward-based learning and impulsivity remains poorly understood. In this study, we evaluated the association between individual differences in loss-minimizing and gain-maximizing behavior in a learning-based probabilistic decision-making task and individual differences in cognitive impulsivity. We found that low cognitive impulsivity was associated both with a better performance minimizing losses and maximizing gains during the task. These associations remained significant after controlling for mathematical skills and gender as potential confounders. We discuss potential mechanisms through which cognitive impulsivity might interact with reward-based learning and decision-making.

Haloperidol 2 mg impairs inhibition but not visuospatial attention

RATIONALE

The dopaminergic system has been implicated in visuospatial attention and inhibition, but the exact role has yet to be elucidated. Scarce literature suggests that attenuation of dopaminergic neurotransmission negatively affects attentional focusing and inhibition. To the best of our knowledge, this is the first study that evaluated the effect of dopaminergic antagonism on stopping performance.

METHODS

Dopaminergic neurotransmission was attenuated in 28 healthy male participants by using 2 mg haloperidol. A repeated-measures placebo-controlled crossover design was implemented, and performance indices of attention and inhibition were assessed in the visual spatial cueing task (VSC) and stop signal task (SST). Additionally, the effect of haloperidol on motoric parameters was assessed. It was expected that haloperidol as contrasted to placebo would result in a reduction of the "validity effect," the benefit of valid cueing as opposed to invalid cueing of a target in terms of reaction time. Furthermore, an increase in stop signal reaction time (SSRT) in the SST was expected.

RESULTS AND CONCLUSION

Results partially confirmed the hypothesis. Haloperidol negatively affected inhibitory motor control in the SST as indexed by SSRT, but there were no indications that haloperidol affected bias or disengagement in the VSC task as indicated by a lack of an effect on RTs. Pertaining to secondary parameters, motor activity increased significantly under haloperidol. Haloperidol negatively affected reaction time variability and errors in both tasks, as well as omissions in the SST, indicating a decreased sustained attention, an increase in premature responses, and an increase in lapses of attention, respectively.