Behavioral models of impulsivity in relation to ADHD: translation between clinical and preclinical studies

The utility of rat models of impulsivity in developing pharmacotherapies for impulse control disorders

High levels of impulsive behaviours are a clinically significant symptom in a range of psychiatric disorders, such as attention deficit hyperactivity disorder, bipolar disorder, personality disorders, pathological gambling and substance abuse. Although often measured using questionnaire assessments, levels of different types of impulsivity can also be determined using behavioural tests. Rodent analogues of these paradigms have been developed, and similar neural circuitry has been implicated in their performance in both humans and rats. In the current review, the methodology underlying the measurement of different aspects of impulsive action and choice are considered from the viewpoint of drug development, with a focus on the continuous performance task (CPT), stop-signal task (SST), go/no-go and delay-discounting paradigms. Current issues impeding translation between animal and human studies are identified, and comparisons drawn between the acute effects of dopaminergic, noradrenergic and serotonergic compounds across species. Although the field could benefit from a more systematic determination of different pharmacological agents across paradigms, there are signs of strong concordance between the animal and human data. However, the type of impulsivity measured appears to play a significant role, with the SST and delay discounting providing more consistent effects for dopaminergic drugs, while the CPT and SST show better predictive validity so far for serotonergic and noradrenergic compounds. Based on the available data, it would appear that these impulsivity models could be used more widely to identify potential pharmacotherapies for impulse control disorders. Novel targets within the glutamatergic and serotonergic system are also suggested.

Animal models to guide clinical drug development in ADHD: lost in translation?

We review strategies for developing animal models for examining and selecting compounds with potential therapeutic benefit in attention-deficit hyperactivity disorder (ADHD). ADHD is a behavioural disorder of unknown aetiology and pathophysiology. Current understanding suggests that genetic factors play an important role in the aetiology of ADHD. The involvement of dopaminergic and noradrenergic systems in the pathophysiology of ADHD is probable. We review the clinical features of ADHD including inattention, hyperactivity and impulsivity and how these are operationalized for laboratory study. Measures of temporal discounting (but not premature responding) appear to predict known drug effects well (treatment validity). Open-field measures of overactivity commonly used do not have treatment validity in human populations. A number of animal models have been proposed that simulate the symptoms of ADHD. The most commonly used are the spontaneously hypertensive rat (SHR) and the 6-hydroxydopamine-lesioned (6-OHDA) animals. To date, however, the SHR lacks treatment validity, and the effects of drugs on symptoms of impulsivity and inattention have not been studied extensively in 6-OHDA-lesioned animals. At the present stage of development, there are no in vivo models of proven effectiveness for examining and selecting compounds with potential therapeutic benefit in ADHD. However, temporal discounting is an emerging theme in theories of ADHD, and there is good evidence of increased value of delayed reward following treatment with stimulant drugs. Therefore, operant behaviour paradigms that measure the effects of drugs in situations of delayed reinforcement, whether in normal rats or selected models, show promise for the future.

Opposite effects of amphetamine on impulsive action with fixed and variable delays to respond

Impulsive action, the failure to withhold an inappropriate response, is treated clinically with dopamine agonists such as amphetamine. Despite the therapeutic efficacy, these drugs have inconsistent effects on impulsive action in rodents, causing improvements or disruptions in different tasks. Thus, we hypothesized that amphetamine is producing an effect by altering distinct cognitive processes in each task. To test this idea, we used the response inhibition (RI) task and trained rats to withhold responding for sucrose until a signal is presented. We then varied the duration that subjects were required to inhibit responding (short=4 s; long=60 s; or variable=1-60 s) and examined whether this influenced the pattern of premature responses. We also tested the effects of amphetamine (0.0, 0.125, 0.25, 0.5, and 1.0 mg/kg) on each task variant. The probability of premature responding varied across the premature interval with a unique pattern of time-dependent errors emerging in each condition. Amphetamine also had distinct effects on each version: the drug promoted premature responding when subjects expected a consistent delay, regardless of its duration, but reduced premature responding when the delay was unpredictable. We propose that the ability to inhibit a motor response is controlled by a different combination of cognitive processes in the three task conditions. These include timing, conditioned avoidance, and attention, which then interact with amphetamine to increase or decrease impulsive action. The effect of amphetamine on impulsive action, therefore, is not universal, but depends on the subject's experience and expectation of the task demands.

Effects of developmental nicotine exposure in rats on decision-making in adulthood

Exposure to tobacco smoke during pregnancy is associated with a range of adverse outcomes in offspring, including cognitive deficits and increased incidence of attention deficit-hyperactivity disorder, but there is a considerable controversy with regard to the causal role of tobacco smoke in these outcomes. To determine whether developmental exposure to the primary psychoactive ingredient in tobacco smoke, nicotine, may cause long-lasting behavioral alterations analogous to those in attention deficit-hyperactivity disorder, male Sprague-Dawley rats underwent a chronic neonatal nicotine administration regimen, which models third-trimester human exposure. Male rat pups were administered nicotine (6 mg/kg/day) by oral gastric intubation on postnatal days 1-7. In adulthood, rats were tested in two decision-making tasks (risky decision-making and delay discounting) as well as in free-operant responding for food reward and the elevated plus maze. Chronic neonatal nicotine attenuated weight gain during nicotine exposure, but there were no effects on performance in the decision-making task, and only a modest decrease in arm entries in the elevated plus maze in one subgroup of rats. These data are consistent with previous findings that developmental nicotine exposure has no effect on delay discounting, and they extend these findings to risky decision-making as well. They further suggest that at least some neurocognitive alterations associated with prenatal tobacco smoke exposure in humans may be due to genetic or other environmental factors, including non-nicotine components of tobacco smoke.

Behavioral toxicology of cognition: extrapolation from experimental animal models to humans: behavioral toxicology symposium overview

A variety of behavioral instruments are available for assessing important aspects of cognition in both animals and humans and, in many cases, the same instruments can be used in both. While nonhuman primates are phylogenetically closest to humans, rodents, pigeons and other animals also offer behaviors worthy of note. Delay Discounting procedures are as useful as any in studies of impulsivity and may have utility in shedding light on processes associated with drug abuse. Specific memory tests such as Visual Paired Comparisons tasks (similar to the Fagan test of infant intelligence) can be modified to allow for assessment of different aspects of memory such as spatial memory. Use of these and other specific memory tasks can be used to directly monitor aspects of cognitive development in infant animals, particularly in nonhuman primates such as monkeys, and children and to draw inferences with respect to possible neuroanatomical substrates sub-serving their functions. Tasks for assessing working memory such as Variable Delayed Response (VDR), modified VDR and Spatial Working Memory tasks are now known to be affected in Parkinson's disease (PD). These and other cognitive function tasks are being used in a monkey model of PD to assess the ability of anti-Parkinson's disease therapies to ameliorate these cognitive deficits without diminishing their therapeutic effects on motor dysfunction. Similarly, in a rat model of the cognitive deficits associated with perinatal exposure to polychlorinated biphenyls (PCBs), clear parallels with children can be seen in at least two areas of executive function: cognitive flexibility and response inhibition. In the rat model, discrimination reversal tasks were utilized to assess cognitive flexibility, a function often assessed in humans using the Wisconsin Card Sorting Task. Response inhibition was assessed using performance in a Differential Reinforcement of Low Response Rates (DRL) task. As the data continue to accumulate, it becomes more clear that our attempts to adapt animal-appropriate tasks for the study of important aspects of human cognition have proven to be very fruitful.

The role of serotonin in the regulation of patience and impulsivity

Classic theories suggest that central serotonergic neurons are involved in the behavioral inhibition that is associated with the prediction of negative rewards or punishment. Failed behavioral inhibition can cause impulsive behaviors. However, the behavioral inhibition that results from predicting punishment is not sufficient to explain some forms of impulsive behavior. In this article, we propose that the forebrain serotonergic system is involved in “waiting to avoid punishment” for future punishments and “waiting to obtain reward” for future rewards. Recently, we have found that serotonergic neurons increase their tonic firing rate when rats await food and water rewards and conditioned reinforcer tones. The rate of tonic firing during the delay period was significantly higher when rats were waiting for rewards than for tones, and rats were unable to wait as long for tones as for rewards. These results suggest that increased serotonergic neuronal firing facilitates waiting behavior when there is the prospect of a forthcoming reward and that serotonergic activation contributes to the patience that allows rats to wait longer. We propose a working hypothesis to explain how the serotonergic system regulates patience while waiting for future rewards.

On the improvement of inhibitory response control and visuospatial attention by indirect and direct adrenoceptor agonists

RATIONALE

The clinical efficacy of the monoamine and noradrenaline transporter inhibitors methylphenidate and atomoxetine in attention deficit/hyperactivity disorder implicates noradrenergic neurotransmission in modulating inhibitory response control processes. Nonetheless, it is unclear which adrenoceptor subtypes are involved in these effects.

OBJECTIVES

The present study aimed at investigating the effects of adrenoceptor agonists on inhibitory response control as assessed in the rodent 5-choice serial reaction time task, a widely used translational model to measure this executive cognitive function.

RESULTS

Consistent with the previous reported effects of atomoxetine, the noradrenaline transporter inhibitor desipramine improved inhibitory response control, albeit the effect size was smaller compared to that of atomoxetine. Methylphenidate exerted a bimodal effect on inhibitory response control. Interestingly, the preferential β2-adrenoceptor agonist clenbuterol improved inhibitory response control. Moreover, clenbuterol improved visuospatial attention in the task, an effect that was also observed with the preferential β1-adrenoceptor agonist dobutamine. By contrast, although the preferential α1-adrenoceptor and α2-adrenoceptor agonists (phenylephrine and clonidine, respectively) and the non-selective β-adrenoceptor agonist (isoprenaline) were found to alter inhibitory response control, this was probably secondary to the simultaneous increments in response latencies and omissions observed at effective doses.

CONCLUSIONS

Taken together, these findings further strengthen the notion of noradrenergic modulation of inhibitory response control and attentional processes and particularly reveal the involvement of β2-adrenoceptors therein.

Dissociable effects of monoamine reuptake inhibitors on distinct forms of impulsive behavior in rats

RATIONALE

High levels of impulsivity are a core symptom of psychiatric disorders such as ADHD, mania, personality disorders and drug addiction. The effectiveness of drugs targeting dopamine (DA), noradrenaline (NA) and/or serotonin (5-HT) in the treatment of impulse control disorders emphasizes the role of monoaminergic neurotransmission in impulsivity. However, impulsive behavior is behaviorally and neurally heterogeneous, and several caveats remain in our understanding of the role of monoamines in impulse control.

OBJECTIVES

This study aims to investigate the role of DA, NA and 5-HT in two main behavioral dimensions of impulsivity.

METHODS

The effects of selective DA (GBR12909; 2.5-10 mg/kg), NA (atomoxetine; 0.3-3.0 mg/kg) and 5-HT (citalopram; 0.3-3.0 mg/kg) reuptake inhibitors as well as amphetamine (0.25-1.0 mg/kg) were evaluated on impulsive action in the five-choice serial reaction time task (5-CSRTT) and impulsive choice in the delayed reward task (DRT). In the 5-CSRTT, neuropharmacological challenges were performed under baseline and long intertrial interval (ITI) conditions to enhance impulsive behavior in the task.

RESULTS

Amphetamine and GBR12909 increased impulsive action and perseverative responding and decreased accuracy and response latency in the 5-CSRTT. Atomoxetine increased errors of omission and response latency under baseline conditions in the 5-CSRTT. Under a long ITI, atomoxetine also reduced premature and perseverative responding and increased accuracy. Citalopram improved impulse control in the 5-CSRTT. Amphetamine and GBR12909, but not citalopram or atomoxetine, reduced impulsive choice in the DRT.

CONCLUSIONS

Elevation of DA neurotransmission increases impulsive action and reduces impulsive choice. Increasing NA or 5-HT neurotransmission reduces impulsive action.

Linking ADHD, impulsivity, and drug abuse: a neuropsychological perspective

In this chapter, we consider the relevance of impulsivity as both a psychological construct and endophenotype underlying attention-deficit/hyperactivity disorder (ADHD) and drug addiction. The case for executive dysfunction in ADHD and drug addiction is critically reviewed in the context of dissociable cognitive control processes mediated by the dorsolateral prefrontal cortex (DLPFC), the orbital and ventral medial prefrontal cortex (VMPFC). We argue that such neuroanatomical divisions within the prefrontal cortex are likely to account for the multidimensional basis of impulsivity conceptually categorized in terms of "motoric" and "choice" impulsivity. The relevance of this distinction for the etiology of ADHD and drug addiction is integrated within a novel theoretical framework. This scheme embraces animal learning theory to help explain the heterogeneity of impulse control disorders, which are exemplified by ADHD as a vulnerability disorder for drug addiction.

Combining Cognitive and Personality Measures of Impulse Control in the Assessment of Childhood ADHD

Impulse control is measured both with personality questionnaires and in cognitive laboratory tasks, yet previous results concerning the convergence of these two types of instruments are inconsistent. The current study measured impulse control of children with and without ADHD with a personality questionnaire and a go/no-go task as well as with parent ratings of ADHD symptomatology. Scores on the two measures correlate weakly with one another, yet both correlate moderately to strongly with parent ratings – and both explain unique variance of parent ratings. Accordingly, the simple sum of the standardized scores from the two measures outperforms the single measures in diagnostic accuracy and association with ADHD symptomatology. Results show that a conjoint application of personality and cognitive measures of impulse control is useful for an extended assessment of ADHD. The adequacy of personality and cognitive measures for assessing distinct facets of impulse control is discussed with regard to theoretical models of impulsivity and with regard to ADHD symptomatology.

Unidirectional relationship between heroin self-administration and impulsive decision-making in rats

RATIONALE

There is growing clinical evidence for a strong relationship between drug addiction and impulsivity. However, it is not fully clear whether impulsivity is a pre-existing trait or a consequence of drug abuse. Recent observations in the animal models show that pre-existing levels of impulsivity predict cocaine and nicotine seeking. Whether such relationships also exist with respect to non-stimulant drugs is largely unknown.

OBJECTIVE

We studied the relationship between impulsive choice and vulnerability to heroin taking and seeking.

MATERIALS AND METHODS

Rats were selected in the delayed reward task based on individual differences in impulsive choice. Subsequently, heroin intravenous self-administration behaviour was analysed, including acquisition of heroin intake, motivation, extinction and drug- and cue-induced reinstatement. Throughout the entire experiment, changes in impulsive choice were monitored weekly.

RESULTS AND DISCUSSION

High impulsivity did not predict measures of heroin taking. Moreover, high impulsive rats did not differ from low impulsive rats in extinction rates or heroin- and cue-induced reinstatement. However, both groups became more impulsive as heroin self-administration continued. During abstinence, impulsivity levels returned towards baseline (pre-heroin) levels. Our results indicate that, in contrast to psychostimulants, impulsive choice does not predict vulnerability to heroin seeking and taking.

CONCLUSION

These data implicate that different neural mechanisms may underlie the vulnerability to opiate and psychostimulant dependence. Moreover, our data suggest that elevated impulsivity levels as observed in heroin-dependent subjects are a consequence of heroin intake rather than a pre-existing vulnerability trait.

A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT₂A/C and 5-HT₁A receptor agonists

RATIONALE

Existing animal models of impulsivity frequently use food restriction to increase subjects' motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models.

OBJECTIVES

This study was aimed at validating face and convergent validities of a modified adjusting delay task, which assesses impulsive choice between isocaloric reinforcers in ad libitum fed rats.

METHODS

Male Wistar rats (n = 18) were used to assess the preferredness and reinforcing efficacy of a "supersaccharin" solution (1.5% glucose/0.4% saccharin) over a 1.5% glucose solution. A separate group of rats (n = 24) was trained in a modified adjusting delay task, which involved repeated choice between the glucose solution delivered immediately and the supersaccharin solution delivered after a variable delay. To pharmacologically validate the task, the effects of the 5-HT(2A/C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)-DOI] and the 5-HT(1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide [(±)-8-OH-DPAT] on impulsive choice were then evaluated.

RESULTS

Supersaccharin was highly reinforcing and uniformly preferred over the glucose solution by all subjects. Rats quickly learned the task, and impulsivity was a very stable and consistent trait. DOI and 8-OH-DPAT significantly and dose dependently increased impulsive choice in this modified adjusting delay task.

CONCLUSIONS

We validated a rodent task of impulsive choice, which eliminates typical energy-homeostasis limitations and, therefore, opens new avenues in the study of impulsivity in preclinical feeding and obesity research.

Chronic atomoxetine treatment during adolescence decreases impulsive choice, but not impulsive action, in adult rats and alters markers of synaptic plasticity in the orbitofrontal cortex

RATIONALE

Impulsivity is a key symptom of attention-deficit hyperactivity disorder (ADHD). The use of the norepinephrine reuptake inhibitor, atomoxetine, to treat ADHD suggests that the activity of the norepinephrine transporter (NET) may be important in regulating impulsive behavior. Many ADHD patients receive chronic drug treatment during adolescence, a time when frontal brain regions important for impulse control are undergoing extensive development.

OBJECTIVES

The current study aimed to determine the effects of chronic atomoxetine treatment during adolescence in rats on two distinct forms of impulsivity in adulthood and whether any behavioral changes were accompanied by alterations in mRNA or protein levels within the frontal cortices.

METHODS

Rats received daily injections of saline or atomoxetine (1 mg/kg) during adolescence (postnatal days 40-54). Two weeks later, animals were trained to perform either the delay-discounting test or the five-choice serial reaction time task (5CSRT).

RESULTS

Adolescent atomoxetine treatment caused a stable decrease in selection of small immediate rewards over larger delayed rewards (impulsive choice) in adulthood, but did not affect premature responding (impulsive action) in the 5CSRT. Chronic atomoxetine treatment also altered the ability of acute atomoxetine to modulate aspects of impulsivity, but did not change the response to d-amphetamine. Ex vivo analysis of brain tissue indicated that chronic atomoxetine decreased phosphorylation of CREB and ERK in the orbitofrontal cortex and decreased mRNA for BDNF and cdk5.

CONCLUSIONS

These data suggest that repeated administration of atomoxetine in adolescence can lead to stable decreases in impulsive choice during adulthood, potentially via modulating development of the orbitofrontal cortex.

Pre-treatment with the mGlu2/3 receptor agonist LY379268 attenuates DOI-induced impulsive responding and regional c-Fos protein expression

RATIONALE

Overactivation of serotonin (5-hydroxytryptamine, 5-HT)(2A) receptors causes impulsivity and attentional deficits. Since 5-HT(2A) receptors are known to entertain antagonistic interactions with metabotropic glutamate (mGlu)2/3 receptors, this interaction may provide an alternative target for a novel class of antipsychotics.

OBJECTIVES/METHODS

The study characterizes interactions between 5-HT(2A) and mGlu2/3 receptors implicated in impulse control. Hooded Lister rats were trained in a 5-choice serial reaction time task (5-CSRTT) and treated with the 5-HT(2A/2C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride (DOI, 0.1 mg/kg) and the mGlu2/3 receptor agonist LY379268 (1 mg/kg). In addition, associated drug-induced changes in neuronal activity were assessed via c-Fos immunoreactivity (Fos IR), and co-localization of c-Fos and GABAergic markers was detected using double immunofluorescence labeling.

RESULTS

Systemic DOI caused impulsive overresponding that was attenuated in animals pre-treated with LY379268. LY379268 itself had no significant effect on the rats' performance in the 5-CSRTT. DOI enhanced Fos IR within fronto-cortical and limbic brain structures, and this effect was blocked by LY379268 pre-treatment. Double immunofluorescence labeling showed a specific co-localization of DOI-elicited Fos IR with GABAergic (GAD(67)-positive) cells lacking the calcium-binding protein parvalbumin while LY379268 increased Fos IR in GABAergic and non-GABAergic cells.

CONCLUSION

Our results suggest that impulsivity is possibly due to a primary increase in Glu transmission mediated via 5-HT(2A) receptor activation. Thus, mGlu2/3 receptor agonists might have some potential for treating motor impulsivity-related impairments while their cognitive enhancing effects were not confirmed in this study.

Functional themes from psychiatric genome-wide screens

Technological advances and a greater degree of inter-laboratory co-operation mean that genome-wide analyses can now be used to identify genetic variants that are robustly associated with the risk of developing psychiatric and neurological disorders. In contrast to the candidate gene approach, such screens may identify variants within genes which have a hitherto unappreciated role in disorder pathogenesis, and whose brain function is obscure. In this Perspective, I discuss how the behavioral functions of such genes may be investigated using model systems, drawing attention to the potential caveats and limitations with such approaches. The power of focused cross-species studies needs to be effectively exploited to enable useful insights into the molecular pathogenesis of common and disabling disorders, and ultimately to provide better clinical outcomes for patients.

Effects of acute administration of nicotine, amphetamine, diazepam, morphine, and ethanol on risky decision-making in rats

RATIONALE

Most individuals can accurately assess the risks and rewards associated with choice alternatives and decide accordingly; however, drug users often display maladaptive decision-making, such that choices are biased toward excessively risky options.

OBJECTIVE

The purpose of this study was to investigate the effects of a range of drugs of abuse on risky decision-making.

METHODS

Male Long-Evans rats were trained in the Risky Decision-Making Task, in which they chose between two levers, one which produced a small, "safe" food reward and the other which produced a large, "risky" food reward. The large reward was accompanied by the risk of a mild footshock, the probability of which increased over the course of each test session (0%, 25%, 50%, 75%, and 100%).

RESULTS

Nicotine (0.6 mg/kg) and amphetamine (1.5 mg/kg) caused a significant decrease in choice of the large risky reward (decreased risk taking). Diazepam (1.0 mg/kg) caused a significant increase in choice of the large risky reward (increased risk taking), whereas morphine (3.0 mg/kg) caused only a trend toward increased choice of the large risky reward. Ethanol had no effect on choice behavior.

CONCLUSIONS

These results show that acute administration of drugs of abuse can modulate risk taking in a drug-specific manner, either increasing or decreasing preference for highly rewarding, but risky, options.

Deficits in attentional control: cholinergic mechanisms and circuitry-based treatment approaches

The cognitive control of attention involves maintaining task rules in working memory (or "online"), monitoring reward and error rates, filtering distractors, and suppressing prepotent, and competitive responses. Weak attentional control increases distractibility and causes attentional lapses, impulsivity, and attentional fatigue. Levels of tonic cholinergic activity (changes over tens of seconds or minutes) modulate cortical circuitry as a function of the demands on cognitive control. Increased cholinergic modulation enhances the representation of cues, by augmenting cue-evoked activity in thalamic glutamatergic afferents, thereby increasing the rate of detection. Such cholinergic modulation is mediated primarily via α4β2* nicotinic acetylcholine receptors. Animal experiments and clinical trials in adult patients with ADHD indicate that attentional symptoms and disorders may benefit from drugs that stimulate this receptor. Tonic cholinergic modulation of cue-evoked glutamatergic transients in prefrontal regions is an essential component of the brain's executive circuitry. This circuitry model guides the development of treatments of deficits in attentional control.

A NEUROECONOMIC MODELING OF ATTENTION-DEFICIT/HYPERACTIVITY DISORDER (ADHD)

In this paper we present a new neuroeconomics model for decision-making applied to the Attention-Deficit/Hyperactivity Disorder (ADHD). The model is based on the hypothesis that decision-making is dependent on the evaluation of expected rewards and risks assessed simultaneously in two decision spaces: the personal (PDS) and the interpersonal emotional spaces (IDS). Motivation to act is triggered by necessities identified in PDS or IDS. The adequacy of an action in fulfilling a given necessity is assumed to be dependent on the expected reward and risk evaluated in the decision spaces. Conflict generated by expected reward and risk influences the easiness (cognitive effort) and the future perspective of the decision-making. Finally, the willingness (not) to act is proposed to be a function of the expected reward (or risk), adequacy, easiness and future perspective. The two most frequent clinical forms are ADHD hyperactive(AD/HDhyp) and ADHD inattentive(AD/HDdin). AD/HDhyp behavior is hypothesized to be a consequence of experiencing high rewarding expectancies for short periods of time, low risk evaluation, and short future perspective for decision-making. AD/HDin is hypothesized to be a consequence of experiencing high rewarding expectancies for long periods of time, low risk evaluation, and long future perspective for decision-making.

Effects of selective dopaminergic compounds on a delay-discounting task

Impulsivity is widely regarded as a multidimensional trait that encompasses two or more distinct patterns of behavior, and dopaminergic systems are implicated in the expression of impulsive behavior in both humans and animal subjects. Impulsive choice, or the tendency to choose rewards associated with relatively little or no delay, has been extensively studied in humans and animal subjects using delay-discounting tasks. Here, delay-discounting procedures were used to assess the effects of receptor-selective dopaminergic agonists, antagonists, and dopamine transporter ligands on choices of immediate versus delayed sucrose pellets. The effects of d-amphetamine, GBR 12909, apomorphine, SKF 81297, sumanirole, pramipexole, ABT-724, SCH 23390, L-741,626, PG01037, and L-745,870 were assessed in 24 Sprague-Dawley rats. The only drugs to affect impulsive choice selectively without altering undelayed choice were the D1-like antagonist, SCH 23390 (0.01 mg/kg), and the D4 partial agonist, ABT-724 (3.2 mg/kg), which both increased impulsive choice. The shared effects of these compounds may be explained by their localization within the prefrontal cortex on different groups of neurons. None of the selective agonists and antagonists tested reduced impulsive choice, so further research is needed to determine if direct dopaminergic agonists or antagonists may be therapeutically useful in the treatment of impulse-control disorders.

Cannabinoid CB1 receptor activation mediates the opposing effects of amphetamine on impulsive action and impulsive choice

It is well known that acute challenges with psychostimulants such as amphetamine affect impulsive behavior. We here studied the pharmacology underlying the effects of amphetamine in two rat models of impulsivity, the 5-choice serial reaction time task (5-CSRTT) and the delayed reward task (DRT), providing measures of inhibitory control, an aspect of impulsive action, and impulsive choice, respectively. We focused on the role of cannabinoid CB1 receptor activation in amphetamine-induced impulsivity as there is evidence that acute challenges with psychostimulants activate the endogenous cannabinoid system, and CB1 receptor activity modulates impulsivity in both rodents and humans. Results showed that pretreatment with either the CB1 receptor antagonist/inverse agonist SR141716A or the neutral CB1 receptor antagonist O-2050 dose-dependently improved baseline inhibitory control in the 5-CSRTT. Moreover, both compounds similarly attenuated amphetamine-induced inhibitory control deficits, suggesting that CB1 receptor activation by endogenously released cannabinoids mediates this aspect of impulsive action. Direct CB1 receptor activation by Δ9-Tetrahydrocannabinol (Δ9-THC) did, however, not affect inhibitory control. Although neither SR141716A nor O-2050 affected baseline impulsive choice in the DRT, both ligands completely prevented amphetamine-induced reductions in impulsive decision making, indicating that CB1 receptor activity may decrease this form of impulsivity. Indeed, acute Δ9-THC was found to reduce impulsive choice in a CB1 receptor-dependent way. Together, these results indicate an important, though complex role for cannabinoid CB1 receptor activity in the regulation of impulsive action and impulsive choice as well as the opposite effects amphetamine has on both forms of impulsive behavior.

The influence of sex-linked genetic mechanisms on attention and impulsivity

It is now generally agreed that there are inherent sex differences in healthy individuals across a number of neurobiological domains (including brain structure, neurochemistry, and cognition). Moreover, there is a burgeoning body of evidence highlighting sex differences within neuropsychiatric populations (in terms of the rates of incidence, clinical features/progression, neurobiology and pathology). Here, we consider the extent to which attention and impulsivity are sexually dimorphic in healthy populations and the extent to which sex might modulate the expression of disorders characterised by abnormalities in attention and/or impulsivity such as attention deficit hyperactivity disorder (ADHD), autism and addiction. We then discuss general genetic mechanisms that might underlie sex differences in attention and impulsivity before focussing on specific positional and functional candidate sex-linked genes that are likely to influence these cognitive processes. Identifying novel sex-modulated molecular targets should ultimately enable us to develop more effective therapies in disorders associated with attentional/impulsive dysfunction.

Acute nicotine increases both impulsive choice and behavioural disinhibition in rats

RATIONALE

Heavy smokers exhibit greater levels of impulsive choice and behavioural disinhibition than non-smokers. To date, however, the relationship between nicotine use and differing dimensions of impulsivity has not been systematically assessed.

OBJECTIVES

A series of studies was designed to assess the acute dose-response effects of nicotine and the nicotinic receptor antagonist mecamylamine alone, and in combination with nicotine, on impulsive choice and behavioural disinhibition in rats.

METHODS

Separate groups of rats were trained on a symmetrically reinforced go/no-go task to measure levels of disinhibition and a systematic delayed reward task to measure levels of impulsive choice. Once trained, all animals in each task were treated acutely with nicotine (0.125, 0.25, 0.5 and 1.0 mg/kg), mecamylamine (0.1, 0.3 and 1.0 mg/kg) and varying doses of mecamylamine (0.1, 0.3 and 1.0 mg/kg) prior to nicotine (0.5 mg/kg). An additional experiment assessed the effects of alterations in primary motivation (presatiation and fasting) on performance in both tasks.

RESULTS

Acute nicotine increased both impulsive choice and behavioural disinhibition, effects that were blocked by pre-treatment with mecamylamine. Mecamylamine when administered alone did not alter impulsive behaviour. The lack of effect of presatiation on performance measures suggests that the observed nicotine-induced impulsivity cannot be attributed to the anorectic activity of the compound.

CONCLUSIONS

Present findings support the hypothesis that heightened impulsivity in smokers may in part be a consequence of the direct acute effects of nicotine. As such, drug-induced changes in impulsivity may play a critical role in the transition to and maintenance of nicotine dependence.

ADHD in adolescents with borderline personality disorder

BACKGROUND

The aims of this study were to assess the prevalence of a comorbid Attention Deficit Hyperactivity Disorder (ADHD) diagnosis in Borderline Personality Disorder (BPD), and its impact on the clinical presentation of BPD in adolescents, and to determine which type of impulsivity specifically characterizes adolescents with BPD-ADHD.

METHODS

ADHD diagnoses were sought in a sample of 85 DSM-IV BPD adolescents drawn from the EURNET BPD. Axis-I and -II disorders were determined with the K-SADS-PL and the SIDP-IV, respectively. Impulsivity was assessed with the BIS-11.

RESULTS

11% (N = 9) of BPD participants had a current ADHD diagnosis. BPD-ADHD adolescents showed higher prevalence of Disruptive disorders (Chi2 = 9.09, p = 0.01) and a non-significant trend for a higher prevalence of other cluster B personality disorders (Chi2 = 2.70, p = 0.08). Regression analyses revealed a significant association between Attentional/Cognitive impulsivity scores and ADHD (Wald Z = 6.69; p = 0.01; Exp(B) = 2.02, CI 95% 1.19-3.45).

CONCLUSIONS

Comorbid ADHD influences the clinical presentation of adolescents with BPD and is associated with higher rates of disruptive disorders, with a trend towards a greater likelihood of cluster B personality disorders and with higher levels of impulsivity, especially of the attentional/cognitive type. A subgroup of BPD patients may exhibit developmentally driven impairments of the inhibitory system persisting since childhood. Specific interventions should be recommended for this subsample of BPD adolescents.

Altered tryptophan and alanine transport in fibroblasts from boys with attention-deficit/hyperactivity disorder (ADHD): an in vitro study

Background

The catecholaminergic and serotonergic neurotransmitter systems are implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). The amino acid tyrosine is the precursor for synthesis of the catecholamines dopamine and norepinephrine, while tryptophan is the precursor of serotonin. A disturbed transport of tyrosine, as well as other amino acids, has been found in a number of other psychiatric disorders, such as schizophrenia, bipolar disorder and autism, when using the fibroblast cell model. Hence, the aim of this study was to explore whether children with ADHD may have disturbed amino acid transport.

Methods

Fibroblast cells were cultured from skin biopsies obtained from 14 boys diagnosed with ADHD and from 13 matching boys without a diagnosis of a developmental disorder. Transport of the amino acids tyrosine, tryptophan and alanine across the cell membrane was measured by the cluster tray method. The kinetic parameters, maximal transport capacity (V_max) and affinity constant (K_m) were determined. Any difference between the two groups was analyzed by Student's unpaired t-test or the Mann Whitney U test.

Results

The ADHD group had significantly decreased V_max (p = 0.039) and K_m (increased affinity) (p = 0.010) of tryptophan transport in comparison to controls. They also had a significantly higher V_maxof alanine transport (p = 0.031), but the Km of alanine transport did not differ significantly. There were no significant differences in any of the kinetic parameters regarding tyrosine transport in fibroblasts for the ADHD group.

Conclusions

Tryptophan uses the same transport systems in both fibroblasts and at the blood brain barrier (BBB). Hence, a decreased transport capacity of tryptophan implies that less tryptophan is being transported across the BBB in the ADHD group. This could lead to deficient serotonin access in the brain that might cause disturbances in both the serotonergic and the catecholaminergic neurotransmitter systems, since these systems are highly interconnected. The physiological importance of an elevated transport capacity of alanine to the brain is not known to date.

Adults with a family history of alcohol related problems are more impulsive on measures of response initiation and response inhibition

BACKGROUND

Previous studies have found individuals with family histories of alcohol use disorders are more impulsive on some but not all laboratory behavioral measures, suggesting deficits on specific forms of impulse control. However, drawing conclusions is tenuous because these different measures have not been administered together in the same group of participants.

METHODS

In the present study, we compared healthy 21-35 year old adults with family histories of alcohol related problems (FHAP+) or without such histories (FHAP-) on behavioral measures of response inhibition, response initiation, and consequence sensitivity impulsivity. FHAP+ (n=36) and FHAP- (n=36) participants were compared on performance on the Immediate Memory Task (IMT, response initiation), GoStop Impulsivity Paradigm (GoStop, response inhibition), Two Choice Impulsivity Paradigm (TCIP, consequence sensitivity) and Single Key Impulsivity Paradigm (SKIP, consequence sensitivity).

RESULTS

FHAP+ individuals were more impulsive on the IMT and GoStop but not on the TCIP or SKIP.

CONCLUSIONS

These results suggest that response initiation and response inhibition impulsivity are increased in individuals with family histories of alcohol related problems despite not having alcohol or drug use disorders themselves. In contrast, increased consequence sensitivity impulsivity may be associated with additional risk factors such as more severe family histories of alcohol use disorders, or it may be increased as a consequence of heavy drug or alcohol use.

Dopamine D2/D3 receptor availability and venturesomeness

The construct of impulsivity is considered as a major trait of personality. There is growing evidence that the mesolimbic dopamine system plays an important role in the modulation of impulsivity and venturesomeness, the two key components within the impulsivity-construct. The aim of the present study was to explore an association between trait impulsivity measured with self-assessment and the dopaminergic neurotransmission as measured by positron emission tomography (PET) in a cohort of healthy male subjects. In vivo D2/D3 receptor availability was determined with [(18)F]fallypride PET in 18 non-smoking healthy subjects. The character trait impulsivity was measured using the Impulsiveness-Venturesomeness-Empathy questionnaire (I7). Image processing and statistical analysis was performed on a voxel-by-voxel basis using statistical parametric mapping (SPM) software. The I7 subscale venturesomeness correlated positively with the D2/D3 receptor availability within the left temporal cortex and the thalamus. Measures on the I7 subscale impulsiveness and empathy did not correlate with the D2/D3 receptor availability in any brain region investigated. Our results suggest the involvement of extrastriatal dopaminergic neurotransmission in venturesomeness, a component of impulsivity.

Neuroanatomical, sensorimotor and cognitive deficits in adult rats with white matter injury following prenatal ischemia

Perinatal brain injury including white matter damage (WMD) is highly related to sensory, motor or cognitive impairments in humans born prematurely. Our aim was to examine the neuroanatomical, functional and behavioral changes in adult rats that experienced prenatal ischemia (PI), thereby inducing WMD. PI was induced by unilateral uterine artery ligation at E17 in pregnant rats. We assessed performances in gait, cognitive abilities and topographical organization of maps, and neuronal and glial density in primary motor and somatosensory cortices, the hippocampus and prefrontal cortex, as well as axonal degeneration and astrogliosis in white matter tracts. We found WMD in corpus callosum and brainstem, and associated with the hippocampus and somatosensory cortex, but not the motor cortex after PI. PI rats exhibited mild locomotor impairments associated with minor signs of spasticity. Motor map organization and neuronal density were normal in PI rats, contrasting with major somatosensory map disorganization, reduced neuronal density, and a marked reduction of inhibitory interneurons. PI rats exhibited spontaneous hyperactivity in open-field test and short-term memory deficits associated with abnormal neuronal density in related brain areas. Thus, this model reproduces in adult PI rats the main deficits observed in infants with a perinatal history of hypoxia-ischemia and WMD.

Serotonin (5-hydroxytryptamine) 5-HT(2A) receptor: association with inherent and cocaine-evoked behavioral disinhibition in rats

Alterations in the balance of functional activity within the serotonin [5-hydroxytryptamine (5-HT)] system are hypothesized to underlie impulse control. Cocaine-dependent subjects consistently show greater impulsivity relative to nondrug using control subjects. Preclinical studies suggest that the 5-HT(2A) receptor (5-HT(2A)R) contributes to the regulation of impulsive behavior and also mediates some of the behavioral effects of cocaine. We hypothesized that the selective 5-HT(2A)R antagonist M100907 would reduce inherent levels of impulsivity and attenuate impulsive responding induced by cocaine in two animal models of impulsivity, the differential reinforcement of low rate (DRL) task and the one-choice serial reaction time (1-CSRT) task. M100907 reduced rates of responding in the DRL task and premature responding in the 1-CSRT task. Conversely, cocaine disrupted rates of responding in the DRL task and increased premature responding in the 1-CSRT task. M100907 attenuated cocaine-induced increases in specific markers of behavioral disinhibition in the DRL and 1-CSRT tasks. These results suggest that the 5-HT(2A)R regulates inherent impulsivity, and that blockade of the 5-HT(2A)R alleviates specific aspects of elevated levels of impulsivity induced by cocaine exposure. These data point to the 5-HT(2A)R as an important regulatory substrate in impulse control.

What have positron emission tomography and 'Zippy' told us about the neuropharmacology of drug addiction?

Translational molecular imaging with positron emission tomography (PET) and allied technologies offer unrivalled applications in the discovery of biomarkers and aetiological mechanisms relevant to human disease. Foremost among clinical PET findings during the past two decades of addiction research is the seminal discovery of reduced dopamine D(2/3) receptor expression in the striatum of drug addicts, which could indicate a predisposing factor and/or compensatory reaction to the chronic abuse of stimulant drugs. In parallel, recent years have witnessed significant improvements in the performance of small animal tomographs (microPET) and a refinement of animal models of addiction based on clinically relevant diagnostic criteria. This review surveys the utility of PET in the elucidation of neuropharmacological mechanisms underlying drug addiction. It considers the consequences of chronic drug exposure on regional brain metabolism and neurotransmitter function and identifies those areas where further research is needed, especially concerning the implementation of PET tracers targeting neurotransmitter systems other than dopamine, which increasingly have been implicated in the pathophysiology of drug addiction. In addition, this review considers the causal effects of behavioural traits such as impulsivity and novelty/sensation-seeking on the emergence of compulsive drug-taking. Previous research indicates that spontaneously high-impulsive rats--as exemplified by 'Zippy'--are pre-disposed to escalate intravenous cocaine self-administration, and subsequently to develop compulsive drug taking tendencies that endure despite concurrent adverse consequences of such behaviour, just as in human addiction. The discovery using microPET of pre-existing differences in dopamine D(2/3) receptor expression in the striatum of high-impulsive rats suggests a neural endophenotype that may likewise pre-dispose to stimulant addiction in humans.

Internal consistency and discriminant validity of a delay-discounting task with an adult self-reported ADHD sample

OBJECTIVE

The purpose of this research was to evaluate the reliability and discriminant validity of a behavioral task of impulsivity, the delay-discounting task.

METHOD

Young adults were asked to report ADHD diagnostic status and to complete the delay-discounting task. The internal consistency of the task was determined, task performances of individuals with and without self-reported ADHD were compared, and the ability of this task to discriminate between participants with and without self-reported ADHD was assessed.

RESULTS

The delay-discounting task showed very good to excellent internal consistency. Furthermore, participants with self-reported ADHD responded to the task in a more impulsive manner than did non-ADHD participants. Finally, moderate discriminant validity for detecting self-reported ADHD using this task was found.

CONCLUSION

Findings showed that the delay-discounting task is a promising task for both clinical research and practice as it has strong reliability overall and has moderate discriminant ability for self-reported ADHD in young adults.

Epigenetics in Developmental Disorder: ADHD and Endophenotypes

Heterogeneity in attention-deficit/hyperactivity disorder (ADHD), with complex interactive operations of genetic and environmental factors, is expressed in a variety of disorder manifestations: severity, co-morbidities of symptoms, and the effects of genes on phenotypes. Neurodevelopmental influences of genomic imprinting have set the stage for the structural-physiological variations that modulate the cognitive, affective, and pathophysiological domains of ADHD. The relative contributions of genetic and environmental factors provide rapidly proliferating insights into the developmental trajectory of the condition, both structurally and functionally. Parent-of-origin effects seem to support the notion that genetic risks for disease process debut often interact with the social environment, i.e., the parental environment in infants and young children. The notion of endophenotypes, markers of an underlying liability to the disorder, may facilitate detection of genetic risks relative to a complex clinical disorder. Simple genetic association has proven insufficient to explain the spectrum of ADHD. At a primary level of analysis, the consideration of epigenetic regulation of brain signalling mechanisms, dopamine, serotonin, and noradrenaline is examined. Neurotrophic factors that participate in the neurogenesis, survival, and functional maintenance of brain systems, are involved in neuroplasticity alterations underlying brain disorders, and are implicated in the genetic predisposition to ADHD, but not obviously, nor in a simple or straightforward fashion. In the context of intervention, genetic linkage studies of ADHD pharmacological intervention have demonstrated that associations have fitted the "drug response phenotype," rather than the disorder diagnosis. Despite conflicting evidence for the existence, or not, of genetic associations between disorder diagnosis and genes regulating the structure and function of neurotransmitters and brain-derived neurotrophic factor (BDNF), associations between symptoms-profiles endophenotypes and single nucleotide polymorphisms appear reassuring.

Prefrontal cortex and impulsive decision making

Impulsivity refers to a set of heterogeneous behaviors that are tuned suboptimally along certain temporal dimensions. Impulsive intertemporal choice refers to the tendency to forego a large but delayed reward and to seek an inferior but more immediate reward, whereas impulsive motor responses also result when the subjects fail to suppress inappropriate automatic behaviors. In addition, impulsive actions can be produced when too much emphasis is placed on speed rather than accuracy in a wide range of behaviors, including perceptual decision making. Despite this heterogeneous nature, the prefrontal cortex and its connected areas, such as the basal ganglia, play an important role in gating impulsive actions in a variety of behavioral tasks. Here, we describe key features of computations necessary for optimal decision making and how their failures can lead to impulsive behaviors. We also review the recent findings from neuroimaging and single-neuron recording studies on the neural mechanisms related to impulsive behaviors. Converging approaches in economics, psychology, and neuroscience provide a unique vista for better understanding the nature of behavioral impairments associated with impulsivity.

Translational approaches to frontostriatal dysfunction in attention-deficit/hyperactivity disorder using a computerized neuropsychological battery

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent condition associated with cognitive dysfunction. The Cambridge Neuropsychological Test Automated Battery is a computerized set of tests that has been widely used in ADHD and in translation/back-translation. Following a survey of translational research relevant to ADHD in experimental animals, a comprehensive literature review was conducted of studies that had used core Cambridge Neuropsychological Test Automated Battery tests 1) to evaluate cognitive dysfunction in ADHD and 2) to evaluate effects of salient drugs in patients and in volunteers. Meta-analysis was conducted where four or more independent datasets were available. Meta-analysis revealed medium-large decrements in ADHD for response inhibition (d = .790, p < .001), working memory (d = .883, p < .001), executive planning (d = .491, p < .001), and a small decrement in attentional set shifting (d = .160, p = .040). Qualitative review of the literature showed some consistent patterns. In ADHD, methylphenidate improved working memory, modafinil improved planning, and methylphenidate, modafinil, and atomoxetine improved inhibition. Meta-analysis of modafinil healthy volunteer studies showed no effects on sustained attention or set shifting. Results were paralleled by findings in experimental animals on comparable tests, enabling further analysis of drug mechanisms. Substantial cognitive deficits are present in ADHD, which can be remediated somewhat with current medications and which can readily be modeled in experimental animals using back-translational methodology. The findings suggest overlapping but also distinct early cognitive effects of ADHD medications and have important implications for understanding the pathophysiology of ADHD and for future trials.

Genetic dissection of behavioral flexibility: reversal learning in mice

BACKGROUND

Behavioral inflexibility is a feature of schizophrenia, attention-deficit/hyperactivity disorder, and behavior addictions that likely results from heritable deficits in the inhibitory control over behavior. Here, we investigate the genetic basis of individual differences in flexibility, measured using an operant reversal learning task.

METHODS

We quantified discrimination acquisition and subsequent reversal learning in a cohort of 51 BXD strains of mice (2-5 mice/strain, n = 176) for which we have matched data on sequence, gene expression in key central nervous system regions, and neuroreceptor levels.

RESULTS

Strain variation in trials to criterion on acquisition and reversal was high, with moderate heritability (∼.3). Acquisition and reversal learning phenotypes did not covary at the strain level, suggesting that these traits are effectively under independent genetic control. Reversal performance did covary with dopamine D2 receptor levels in the ventral midbrain, consistent with a similar observed relationship between impulsivity and D2 receptors in humans. Reversal, but not acquisition, is linked to a locus on mouse chromosome 10 with a peak likelihood ratio statistic at 86.2 megabase (p < .05 genome-wide). Variance in messenger RNA levels of select transcripts expressed in neocortex, hippocampus, and striatum correlated with the reversal learning phenotype, including Syn3, Nt5dc3, and Hcfc2.

CONCLUSIONS

This work demonstrates the clear trait independence between, and genetic control of, discrimination acquisition and reversal and illustrates how globally coherent data sets for a single panel of highly related strains can be interrogated and integrated to uncover genetic sources and molecular and neuropharmacological candidates of complex behavioral traits relevant to human psychopathology.

Nicotine-induced impulsive action: sensitization and attenuation by mecamylamine

A conjunctive variable-interval differential-reinforcement-of-low-rate (VI-DRL, n=18) responding schedule and a stop-signal task (n=18) were used to evaluate the disinhibiting effects of nicotine on response withholding in rats. Sucrose solution was used to reinforce responding, and after a stable baseline was achieved under saline-administration conditions, 0.3 mg/kg nicotine was delivered before each session. Experiment 1 showed that repeated, but not the initial, administration of nicotine decreased performance on both tasks, and the effect of sensitization followed a similar timeline; 10 consecutive doses resulted in poorer proportion-correct VI-DRL trials and percent correct stop trials than the initial dose of nicotine. Furthermore, sensitization to 0.3 mg/kg nicotine decreased performance regardless of whether a spaced or consecutive-dosing regimen was followed. Experiment 2 was designed to test whether mecamylamine hydrochloride (0.1-1.0 mg/kg) could attenuate the effects of repeated 0.3 mg/kg nicotine administration, and the degree to which mecamylamine attenuation of the effect of nicotine to produce impulsive action was relative to dose. Results from experiment 2 showed that response disinhibition, as evaluated using the VI-DRL and stop-signal tasks, is related in a systematic manner to nicotinic-acetylcholine receptor activation.

High impulsivity predicting vulnerability to cocaine addiction in rats: some relationship with novelty preference but not novelty reactivity, anxiety or stress

RATIONALE

Impulsivity is a vulnerability marker for drug addiction in which other behavioural traits such as anxiety and novelty seeking ('sensation seeking') are also widely present. However, inter-relationships between impulsivity, novelty seeking and anxiety traits are poorly understood.

OBJECTIVE

The objective of this paper was to investigate the contribution of novelty seeking and anxiety traits to the expression of behavioural impulsivity in rats.

METHODS

Rats were screened on the five-choice serial reaction time task (5-CSRTT) for spontaneously high impulsivity (SHI) and low impulsivity (SLI) and subsequently tested for novelty reactivity and preference, assessed by open-field locomotor activity (OF), novelty place preference (NPP), and novel object recognition (OR). Anxiety was assessed on the elevated plus maze (EPM) both prior to and following the administration of the anxiolytic drug diazepam, and by blood corticosterone levels following forced novelty exposure. Finally, the effects of diazepam on impulsivity and visual attention were assessed in SHI and SLI rats.

RESULTS

SHI rats were significantly faster to enter an open arm on the EPM and exhibited preference for novelty in the OR and NPP tests, unlike SLI rats. However, there was no dimensional relationship between impulsivity and either novelty-seeking behaviour, anxiety levels, OF activity or novelty-induced changes in blood corticosterone levels. By contrast, diazepam (0.3-3 mg/kg), whilst not significantly increasing or decreasing impulsivity in SHI and SLI rats, did reduce the contrast in impulsivity between these two groups of animals.

CONCLUSIONS

This investigation indicates that behavioural impulsivity in rats on the 5-CSRTT, which predicts vulnerability for cocaine addiction, is distinct from anxiety, novelty reactivity and novelty-induced stress responses, and thus has relevance for the aetiology of drug addiction.

Family routine moderates the relation between child impulsivity and oppositional defiant disorder symptoms

Although child impulsivity is associated with oppositional defiant disorder (ODD) symptoms, few studies have examined whether family processes moderate this association. To address this gap, we tested whether child-reported family routine moderated the relation between child hyperactivity/impulsivity (HI) and ODD symptoms among a sample of low-income, urban, ethnic-minority children (N = 87, 51% male). Child HI and ODD symptoms were assessed using parent and teacher reports. HI also was indexed by a laboratory task. Family routine was assessed using child self-report. Hierarchical regression analyses indicated that family routine moderated child HI. Among children with higher levels of teacher-reported HI symptoms, lower levels of family routine were associated with higher levels of teacher-reported ODD symptoms compared to children with lower levels of teacher-reported HI symptoms. Children who self-reported higher levels of family routine were rated as low on teacher-reported ODD symptoms, regardless of teacher-reported HI levels. Parent report and laboratory measures of child HI did not produce significant interactions. Lower levels of family routine may confer risk for ODD symptoms among low-income, urban, ethnic-minority children experiencing higher levels of HI.

Greater discounting of delayed rewards in young adults with family histories of alcohol and drug use disorders: studies from the Oklahoma family health patterns project

BACKGROUND

Increased discounting of delayed rewards may be a premorbid characteristic and possible risk factor for alcohol and other drug use disorders; however, previous studies have found no or minimal differences in delay discounting in individuals at risk for substance use disorders based on family history. It is possible that increased delay discounting may be more closely associated with antisocial traits, evident in a subset of individuals with positive family histories of alcohol and drug use disorders, and that previous studies were underpowered for detecting subtle to modest overall group differences.

METHODS

In this study, we compared 143 young adults with family histories of alcohol and other drug use disorders (FH+) and 155 young adults with no such histories (FH-) on delay discounting and subsequently examined how delay discounting was related to antisocial traits and other selected psychological and demographic variables.

RESULTS

The FH+ group discounted delayed rewards more than the FH- group. Subsequent analyses revealed that increased delay discounting was correlated with having more parents and grandparents with alcohol and drug use disorders, more antisocial traits, more depressive tendencies and lower IQs, and lower income. After controlling for all these relationships, more antisocial traits and lower IQ still predicted greater delay discounting, and subsequent analysis revealed that the greater delay discounting in the FH+ group was mediated by this group's greater number of individuals with antisocial traits.

CONCLUSION

FH+ individuals who discount delayed rewards more may be at increased risk for developing alcohol and other drug use disorders; however, additional descriptive studies and longitudinal studies are needed.

Schedule-induced polydipsia in the spontaneously hypertensive rat and its relation to impulsive behaviour

Eight Spontaneously Hypertensive Rat (SHR), 8 Wistar-Kyoto (WKY) and 8 Wistar rats, all male, maintained at 80-85% of their free-feeding weight by controlled access to food, were exposed to a series of fixed time (FT) schedules whereby food pellets were regularly delivered regardless of the animals' behaviour. The FT values used were 9, 15, 30, 60, 120 and 180 s, with the order of presentation of the schedules among the animals being counterbalanced (except under the FT 120-s and 180-s schedules, which were successively presented as the last two of the series). Due to freely available access to water, the animals developed schedule-induced drinking under all FT schedules, marked by the characteristic bitonic function that relates the number of licks and amount of water drunk to the length of the inter-food interval. Wistar and WKY rats displayed maximum drinking under an FT 15-s schedule, with WKY rats registering lower quantities across all FT values. Among SHR rats, maximum schedule-induced polydipsia was observed under the FT 30-s schedule, with a rightward shift in the bitonic function compared to controls. For long FT values, the temporal distribution of licks within inter-food intervals was shifted slightly towards the right in the SHR rats. In a subsequent study, only the SHR and Wistar rats were used, and the animals were exposed to a delay-discounting procedure. The rats were faced with successive choices, in which they could choose between an immediate reward of one food pellet and another of four food pellets at a delay of 3, 6, 12 or 24s. In the case of the longer delays, SHR rats chose the immediate reward of lower magnitude more often than did their Wistar counterparts, and also committed a greater number of omissions during the forced-choice trials of the procedure. The results indicate that differences in schedule-induced polydipsia are related to indexes of cognitive rather than motor impulsivity, a finding in line with the theoretical idea that adjunctive behaviour is linked to operant reinforcement processes.

Mechanisms of impulsivity in bipolar disorder and related illness

Aims – Impulsivity is a multifaceted aspect of behavior that is prominent in psychiatric disorders and has serious behavioral consequences. This paper reviews studies integrating behavioral and physiological mechanisms in impulsivity and their role in severity and course of bipolar and related disorders. Methods – This is a review of work that used questionnaire, human behavioral laboratory, and neurophysiological measurements of impulsivity or related aspects of behavior. Subjects included individuals with bipolar disorder, substance-use disorders, antisocial personality disorder, and healthy controls. Results – Models of impulsivity include rapid-response impulsivity, with inability to reflect or to evaluate a stimulus adequately before responding, and reward-based impulsivity, with inability to delay response for a reward. In normal subjects, rapid-response impulsivity is increased by yohimbine, which increases norepinephrine release. Impulsivity is increased in bipolar disorder, whether measured by questionnaire, by measures of rapid-response impulsivity, or by measures of ability to delay reward. While affective state has differential effects on impulsivity, impulsivity is increased in bipolar disorder regardless of affective state or treatment. Impulsivity, especially rapid-response, is more severe with a highly recurrent course of illness or with comorbid substance-use disorder, and with history of medically severe suicide attempt. In antisocial personality disorder, rapid-response impulsivity is increased, but rewardbased impulsivity is not. In general, impulsivity is increased more in bipolar disorder than in antisocial personality disorder. In combined bipolar disorder and antisocial personality disorder, increased impulsivity is associated with substance-use disorders and suicide attempts. Conclusions – Impulsivity is associated with severe behavioral complications of bipolar disorder, antisocial personality disorder, and substance-use disorders.

Impulsivity, compulsivity, and top-down cognitive control

Impulsivity is the tendency to act prematurely without foresight. Behavioral and neurobiological analysis of this construct, with evidence from both animal and human studies, defines several dissociable forms depending on distinct cortico-striatal substrates. One form of impulsivity depends on the temporal discounting of reward, another on motor or response disinhibition. Impulsivity is commonly associated with addiction to drugs from different pharmacological classes, but its causal role in human addiction is unclear. We characterize in neurobehavioral and neurochemical terms a rodent model of impulsivity based on premature responding in an attentional task. Evidence is surveyed that high impulsivity on this task precedes the escalation subsequently of cocaine self-administration behavior, and also a tendency toward compulsive cocaine-seeking and to relapse. These results indicate that the vulnerability to stimulant addiction may depend on an impulsivity endophenotype. Implications of these findings for the etiology, development, and treatment of drug addiction are considered.

Contributions of serotonin in addiction vulnerability

The serotonin (5-hydroxytryptamine; 5-HT) system has long been associated with mood and its dysregulation implicated in the pathophysiology of mood and anxiety disorders. While modulation of 5-HT neurotransmission by drugs of abuse is also recognized, its role in drug addiction and vulnerability to drug relapse is a more recent focus of investigation. First, we review preclinical data supporting the serotonergic raphe nuclei and their forebrain projections as targets of drugs of abuse, with emphasis on the effects of psychostimulants, opioids and ethanol. Next, we examine the role of 5-HT receptors in impulsivity, a core behavior that contributes to the vulnerability to addiction and relapse. Finally, we discuss evidence for serotonergic dysregulation in comorbid mood and addictive disorders and suggest novel serotonergic targets for the treatment of addiction and the prevention of drug relapse.

Prenatal exposure to nicotine impairs performance of the 5-choice serial reaction time task in adult rats

Cigarette smoking is associated with a wide variety of adverse reproductive outcomes, including increased infant mortality and decreased birth weight. Prenatal exposure to tobacco smoke, of which nicotine is a major teratogenic component, has also been linked to the acceleration of the risk for different psychiatric disorders, including conduct disorder and attention deficit hyperactivity disorder (ADHD). Whether this increased risk is influenced by the direct effects of gestational nicotine exposure on the developing fetus remains uncertain. In this study we provide experimental evidence for the effects of prenatal nicotine exposure on measures of attention and impulsivity in adult male rats. Offspring of females exposed during pregnancy to 0.06 mg/ml nicotine solution as the only source of water (daily consumption: 69.6±1.4 ml/kg; nicotine blood level: 96.0±31.9 ng/ml) had lower birth weight and delayed sensorimotor development measured by negative geotaxis, righting reflex, and grip strength. In the 5-choice serial reaction time test, adult rats showed increased numbers of anticipatory responses and omissions errors, more variable response times, and lower accuracy with evidence of delayed learning of the task demands when the 1 s stimulus duration was introduced. In contrast, prenatal nicotine exposure had no effect on exploratory locomotion or delay-discounting test. Prenatal nicotine exposure increased expression of the D5 dopamine receptor gene in the striatum, but did not change expression of other dopamine-related genes (DRD4, DAT1, NR4A2, and TH) in either the striatum or the prefrontal cortex. These data suggest a direct effect of prenatal nicotine exposure on important aspects of attention, inhibitory control, or learning later in life.

Repeated exposure to chlorpyrifos alters the performance of adolescent male rats in animal models of depression and anxiety

Chlorpyrifos (CPF) is a broad spectrum, highly effective organophosphorus (OP) pesticide that has been largely used worldwide. Over the past decades, numerous studies have assessed the potential neurotoxic effects of either acute or chronic exposure to CPF on developing brain. Despite being an acetylcholinersterase inhibitor, the effects of CPF are not only confined to cholinergic system, but are involved in a wide variety of neurotransmitter systems, especially the serotonin (5-HT) system, which leads to long-lasting changes in 5-HT-related emotional behaviors. In our present study, 4-week-old adolescent male Sprague-Dawley rats were repeatedly exposed to CPF at daily doses of 10, 20, 40, 80, and 160 mg/kg/day (s.c., 7 days), and then subjected to a battery of emotional behavioral tests that related to serotonergic function in order to determine CPF effects in adolescent rats. Results in behavioral tests demonstrated CPF significantly increased the entries to and time spent in the open arms in the elevated plus-maze test at the dose of 40-160 mg/kg, the number of shocks in the Vogel's conflict test at the dose of 20-160 mg/kg, and significantly decreased the latency to feed in the novelty-suppressed feeding test in both dose range. Interestingly, in the forced swimming test, at the dose of 10mg/kg, CPF significantly increased the immobility time, whereas it significantly decreased the immobility time at the dose of 160 mg/kg. Our data suggest that repeated exposure to CPF elicits alterations of the emotional behaviors related to serotonergic nervous system in adolescent male rats. However, the underlying mechanism needs further investigations.

Performance deficits of NK1 receptor knockout mice in the 5-choice serial reaction-time task: effects of d-amphetamine, stress and time of day

Background

The neurochemical status and hyperactivity of mice lacking functional substance P-preferring NK1 receptors (NK1R-/-) resemble abnormalities in Attention Deficit Hyperactivity Disorder (ADHD). Here we tested whether NK1R-/- mice express other core features of ADHD (impulsivity and inattentiveness) and, if so, whether they are diminished by d-amphetamine, as in ADHD. Prompted by evidence that circadian rhythms are disrupted in ADHD, we also compared the performance of mice that were trained and tested in the morning or afternoon.

Methods and Results

The 5-Choice Serial Reaction-Time Task (5-CSRTT) was used to evaluate the cognitive performance of NK1R-/- mice and their wildtypes. After training, animals were tested using a long (LITI) and a variable (VITI) inter-trial interval: these tests were carried out with, and without, d-amphetamine pretreatment (0.3 or 1 mg/kg i.p.). NK1R-/- mice expressed greater omissions (inattentiveness), perseveration and premature responses (impulsivity) in the 5-CSRTT. In NK1R-/- mice, perseveration in the LITI was increased by injection-stress but reduced by d-amphetamine. Omissions by NK1R-/- mice in the VITI were unaffected by d-amphetamine, but premature responses were exacerbated by this psychostimulant. Omissions in the VITI were higher, overall, in the morning than the afternoon but, in the LITI, premature responses of NK1R-/- mice were higher in the afternoon than the morning.

Conclusion

In addition to locomotor hyperactivity, NK1R-/- mice express inattentiveness, perseveration and impulsivity in the 5-CSRTT, thereby matching core criteria for a model of ADHD. Because d-amphetamine reduced perseveration in NK1R-/- mice, this action does not require functional NK1R. However, the lack of any improvement of omissions and premature responses in NK1R-/- mice given d-amphetamine suggests that beneficial effects of this psychostimulant in other rodent models, and ADHD patients, need functional NK1R. Finally, our results reveal experimental variables (stimulus parameters, stress and time of day) that could influence translational studies.