Methylphenidate-induced impulsivity: pharmacological antagonism by beta-adrenoreceptor blockade

Noradrenergic regulation of cue-guided decision making and impulsivity is doubly dissociable across frontal brain regions

RATIONALE

Win-paired stimuli can promote risk taking in experimental gambling paradigms in both rats and humans. We previously demonstrated that atomoxetine, a noradrenaline reuptake inhibitor, and guanfacine, a selective α2A adrenergic receptor agonist, reduced risk taking on the cued rat gambling task (crGT), a rodent assay of risky choice in which wins are accompanied by salient cues. Both compounds also decreased impulsive premature responding.

OBJECTIVE

The key neural loci mediating these effects were unknown. The lateral orbitofrontal cortex (lOFC) and the medial prefrontal cortex (mPFC), which are highly implicated in risk assessment, action selection, and impulse control, receive dense noradrenergic innervation. We therefore infused atomoxetine and guanfacine directly into either the lOFC or prelimbic (PrL) mPFC prior to task performance.

RESULTS

When infused into the lOFC, atomoxetine improved decision making score and adaptive lose-shift behaviour in males, but not in females, without altering motor impulsivity. Conversely, intra-PrL atomoxetine improved impulse control in risk preferring animals of both sexes, but did not alter decision making. Guanfacine administered into the PrL, but not lOFC, also altered motor impulsivity in all subjects, though in the opposite direction to atomoxetine.

CONCLUSIONS

These data highlight a double dissociation between the behavioural effects of noradrenergic signaling across frontal regions with respect to risky choice and impulsive action. Given that the influence of noradrenergic manipulations on motor impulsivity could depend on baseline risk preference, these data also suggest that the noradrenaline system may function differently in subjects that are susceptible to the risk-promoting lure of win-associated cues.

Development of a novel rodent rapid serial visual presentation task reveals dissociable effects of stimulant versus nonstimulant treatments on attentional processes

The rapid serial visual presentation (RSVP) task and continuous performance tasks (CPT) are used to assess attentional impairments in patients with psychiatric and neurological conditions. This study developed a novel touchscreen task for rats based on the structure of a human RSVP task and used pharmacological manipulations to investigate their effects on different performance measures. Normal animals were trained to respond to a target image and withhold responding to distractor images presented within a continuous sequence. In a second version of the task, a false-alarm image was included, so performance could be assessed relative to two types of nontarget distractors. The effects of acute administration of stimulant and nonstimulant treatments for ADHD (amphetamine and atomoxetine) were tested in both tasks. Methylphenidate, ketamine, and nicotine were tested in the first task only. Amphetamine made animals more impulsive and decreased overall accuracy but increased accuracy when the target was presented early in the image sequence. Atomoxetine improved accuracy overall with a specific reduction in false-alarm responses and a shift in the attentional curve reflecting improved accuracy for targets later in the image sequence. However, atomoxetine also slowed responding and increased omissions. Ketamine, nicotine, and methylphenidate had no specific effects at the doses tested. These results suggest that stimulant versus nonstimulant treatments have different effects on attention and impulsive behaviour in this rat version of an RSVP task. These results also suggest that RSVP-like tasks have the potential to be used to study attention in rodents.

Assessing attention and impulsivity in the variable stimulus duration and variable intertrial interval rodent continuous performance test schedules using noradrenaline receptor antagonists in female C57BL/6JRj mice

RATIONALE

Noradrenergic dysfunction is associated with disorders of impulsivity and inattention. The rodent continuous performance test (rCPT) quantifies changes in attention and impulsivity.

OBJECTIVE

To use NA receptor antagonists to examine the roles of NA on attention and impulsivity behaviours measured in the rCPT variable stimulus duration (vSD) and the variable intertrial interval (vITI) schedules.

METHODS

Two cohorts of 36 female C57BL/6JRj mice were examined separately in the rCPT vSD and vITI schedules. Both cohorts received antagonists of the following adrenoceptors: α1 (doxazosin, DOX: 1.0, 3.0, 10.0 mg/kg), α2 (yohimbine, YOH: 0.1, 0.3, 1.0 mg/kg), and β1/2 (propranolol, PRO: 1.0, 3.0, 10.0 mg/kg) in consecutive balanced Latin square designs with flanking reference measurements. The antagonists were subsequently examined for effects on locomotor activity.

RESULTS

DOX showed similar effects in both schedules, improving discriminability and accuracy, and reducing responding and impulsivity, and DOX also reduced locomotor activity. YOH showed prominent effects in the vSD schedule to increase responding and impulsivity, while impairing discriminability and accuracy. YOH did not affect locomotor activity. PRO increased responding and impulsivity, decreased accuracy, but did not affect discriminability or locomotor activity.

CONCLUSION

Antagonism of α2 or β1/2 adrenoceptors caused similar increases in responding and impulsivity and worsened attentional performance, while α1 adrenoceptor antagonism showed the opposite effects. Our results suggest that endogenous NA exerts bidirectional control of most behaviours in the rCPT. The parallel vSD and vITI studies showed a substantial overlap in effects, but also some differences that indicate differing sensitivity towards noradrenergic manipulations.

Theory of visual attention (TVA) applied to rats performing the 5-choice serial reaction time task: differential effects of dopaminergic and noradrenergic manipulations

RATIONALE

Attention is compromised in many psychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). While dopamine and noradrenaline systems have been implicated in ADHD, their exact role in attentional processing is yet unknown.

OBJECTIVES

We applied the theory of visual attention (TVA) model, adapted from human research, to the rat 5-choice serial reaction time task (5CSRTT) to investigate catecholaminergic modulation of visual attentional processing in healthy subjects of high- and low-attention phenotypes.

METHODS

Rats trained on the standard 5CSRTT and tested with variable stimulus durations were treated systemically with noradrenergic and/or dopaminergic agents (atomoxetine, methylphenidate, amphetamine, phenylephrine and atipamezole). TVA modelling was applied to estimate visual processing speed for correct and incorrect visual perceptual categorisations, independent of motor reaction times, as measures of attentional capacity.

RESULTS

Atomoxetine and phenylephrine decreased response frequencies, including premature responses, increased omissions and slowed responding. In contrast, methylphenidate, amphetamine and atipamezole sped up responding and increased premature responses. Visual processing speed was also affected differentially. Atomoxetine and phenylephrine slowed, whereas methylphenidate and atipamezole sped up, visual processing, both for correct and incorrect categorisations. Amphetamine selectively improved visual processing for correct, though not incorrect, responses in high-attention rats only, possibly reflecting improved attention.

CONCLUSIONS

These data indicate that the application of TVA to the 5CSRTT provides an enhanced sensitivity to capturing attentional effects. Unexpectedly, we found overall slowing effects, including impaired visual processing, following drugs either increasing extracellular noradrenaline (atomoxetine) or activating the α1-adrenoceptor (phenylephrine), while also ameliorating premature responses (impulsivity). In contrast, amphetamine had potential pro-attentional effects by enhancing visual processing, probably due to central dopamine upregulation.

Adaptive aspects of impulsivity and interactions with effects of catecholaminergic agents in the 5-choice serial reaction time task: implications for ADHD

BACKGROUND

Work in humans has shown that impulsivity can be advantageous in certain settings. However, evidence for so-called functional impulsivity is lacking in experimental animals.

AIMS

This study investigated the contexts in which high impulsive (HI) rats show an advantage in performance compared with mid- (MI) and low impulsive (LI) rats. We also assessed the effects of dopaminergic and noradrenergic agents to investigate underlying neurotransmitter mechanisms.

METHODS

We tested rats on a variable inter-trial interval (ITI) version of the 5-choice serial reaction time task (5CSRTT). Rats received systemic injections of methylphenidate (MPH, 1 mg/kg and 3 mg/kg), atomoxetine (ATO, 0.3 mg/kg and 1 mg/kg), amphetamine (AMPH, 0.2 mg/kg), the alpha-2a adrenoceptor antagonist atipamezole (ATI, 0.3 mg/kg) and the alpha-1 adrenoceptor agonist phenylephrine (PHEN, 1 mg/kg) prior to behavioural testing.

RESULTS

Unlike LI rats, HI rats exhibited superior performance, earning more reinforcers, on short ITI trials, when the task required rapid responding. MPH, AMPH and ATI improved performance on short ITI trials and increased impulsivity in long ITI trials, recapitulating the behavioural profile of HI. In contrast, ATO and PHEN impaired performance on short ITI trials and decreased impulsivity, thus mimicking the behavioural profile of LI rats. The effects of ATO were greater on MI rats and LI rats.

CONCLUSIONS

These findings indicate that impulsivity can be advantageous when rapid focusing and actions are required, an effect that may depend on increased dopamine neurotransmission. Conversely, activation of the noradrenergic system, with ATO and PHEN, led to a general inhibition of responding.

Noradrenergic contributions to cue-driven risk-taking and impulsivity

RATIONALE

The flashing lights and sounds of modern casinos are alluring and may contribute to the addictive nature of gambling. Such cues can have a profound impact on the noradrenaline (NA) system, which could therefore be a viable therapeutic target for gambling disorder (GD). While there is substantial evidence to support the involvement of NA in the impulsive symptoms of GD, its function in mediating the "pro-addictive" impact of cues is less understood.

OBJECTIVE

We wished to investigate the role of NA in our rodent assay of decision making and impulsivity, the cued rat gambling task (crGT). Given that sex differences are prominent in addiction disorders, and increasingly reported in the monoaminergic regulation of behaviour, we also prioritised evaluating noradrenergic drugs in both sexes.

METHODS

Female and male rats were trained to stability on the crGT and then given intraperitoneal injections of the noradrenaline reuptake inhibitor atomoxetine, the α_2A receptor agonist guanfacine, the beta receptor antagonist propranolol, and the α₂ receptor antagonist yohimbine.

RESULTS

Atomoxetine dose-dependently improved decision-making score. Guanfacine selectively enhanced decision making in risk-preferring males and optimal performing females. Propranolol and yohimbine did not influence decision making. Atomoxetine and guanfacine reduced premature responses, while yohimbine bi-phasically affected this index of motor impulsivity.

CONCLUSIONS

These results support the hypothesis that NA is an important neuromodulator of the cue-induced deficits in decision making observed in laboratory-based gambling paradigms, and suggest that NAergic drugs like atomoxetine and guanfacine may be useful in treating GD.

Relative prevalence of 10 types of pharmacodynamic interactions in psychiatric treatment

OBJECTIVE

To assess the relative prevalence and factors affecting the prescription of medication combinations with a theoretical efficacy limiting pharmacodynamic interaction, defined as two medications with opposing indications and side effects or antagonistic action at the primary receptor of mechanism of action.

METHOD

One hundred sixteen combinations were identified for 10 types of pharmacodynamic interactions. PubMed was searched for each combination to assess the quality of evidence either supporting clinical use or verifying reduced efficacy. Micromedex was searched to determine the presence of warnings to prescribers of reduced efficacy. The prevalence in clinical practice was determined by computer review of the Genoa Healthcare database for all prescribers at 10 participating community mental health centers. The expected prevalence was calculated as the product of the probability of each medication prescribed alone and was compared with the actual prevalence of the combination using the test of proportions.

RESULTS

The frequency of prescription of eight combinations met the Bonferroni corrected level of significance of p < 0.001. Four were combinations of amphetamine and D2 antagonists and each were prescribed less often than chance, p = 0.0001 consistent with epidemiological studies and multiple animal studies verifying an efficacy limiting interaction. Despite epidemiological studies indicating increased risk of accidents, alprazolam and amphetamine were prescribed more often than chance, p = 0.0001. Micromedex generated warnings for efficacy limiting interactions for five other combinations, but with no subsequent change in prescription frequency.

CONCLUSIONS

Neither presence of medical evidence nor warnings from Micromedex consistently affect the prescription of combinations with pharmacodynamic efficacy limiting interactions.

The Neuropharmacology of Impulsive Behaviour, an Update

Neuropharmacological interventions in preclinical translational models of impulsivity have tremendously contributed to a better understanding of the neurochemistry and neural basis of impulsive behaviour. In this regard, much progress has been made over the last years, also due to the introduction of novel techniques in behavioural neuroscience such as optogenetics and chemogenetics. In this chapter, we will provide an update of how the behavioural pharmacology field has progressed and built upon existing data since an earlier review we wrote in 2008. To this aim, we will first give a brief background on preclinical translational models of impulsivity. Next, recent interesting evidence of monoaminergic modulation of impulsivity will be highlighted with a focus on the neurotransmitters dopamine and noradrenaline. Finally, we will close the chapter by discussing some novel directions and drug leads in the neuropharmacological modulation of impulsivity.

Association of 24 h maternal deprivation with a saline injection in the neonatal period alters adult stress response and brain monoamines in a sex-dependent fashion

Maternal deprivation (MD) disinhibits the adrenal glands, rendering them responsive to various stressors, including saline injection, and this increased corticosterone (CORT) response can last for as long as 2 h. In the present study, we tested the hypothesis that association of MD on day 11 with a saline injection would alter emotional behavior, CORT response, and brain monoamine levels, in male and female adult rats. Rats were submitted to the novelty suppressed feeding (NSF), the sucrose negative contrast test (SNCT), social investigation test (SIT), and the elevated plus maze (EPM). One quarter of each group was not tested (providing basal values of CORT and brain monoamines) and the remainder was decapitated 15, 45, or 75 min after the EPM, to assess CORT reactivity. Monoamine levels were determined in the hypothalamus (HPT), frontal cortex (FC), amygdala (AMY), ventral, and dorsal hippocampus (vHPC, dHPC, respectively). MD reduced food intake, in the home-cage, and latency to eat in the NSF in both sexes; females explored less the target animal in the SIT and explored more the open arms of the EPM than males; the CORT response to the EPM was greater in maternally-deprived males and females than in their control counterparts, and this response was further elevated in maternally-deprived females injected with saline. Regarding monoamine levels, females were less affected, showing isolated effects of the stressors, while in males, MD increased 5-HT levels in the HPT and decreased this monoamine in the FC, MD associated with saline reduced dopamine levels in all brain regions, except the HPT. MD at 11 days did not alter emotional behaviors in adult rats, but had an impact in neurobiological parameters associated with this class of behaviors. The impact of MD associated with saline on dopamine levels suggests that males may be vulnerable to motivation-related disorders.

Pharmacological investigations of a yohimbine-impulsivity interaction in rats

Both impulsivity and stress are risk factors for substance abuse, but it is not clear how these two processes interact to alter susceptibility for the disorder. The aim of this project was to examine the pharmacology of a stress-impulsivity interaction in rats. To do so, we tested the effects of yohimbine on impulsive action and then assessed whether behavioural changes could be reduced by antagonists at different receptor subtypes. Male Long-Evans rats were injected with various doses of yohimbine (0-5.0 mg/kg) before testing in the response-inhibition task. In subsequent experiments, yohimbine (2.5 mg/kg) was injected following pretreatment with the following receptor antagonists: corticotropin-releasing factor receptor 1, antalarmin (0-20 mg/kg); glucocorticoid, mifepristone (0-30 mg/kg); noradrenergic (NA) α1, prazosin (0-2 mg/kg); NA α2, guanfacine (0-0.5 mg/kg); NA β2, propranolol (0.5-2.0 mg/kg); dopamine D1/5, SCH 39166 (0-0.0625 mg/kg); μ opioid, naloxone (0-2 mg/kg); or 5-HT2A, M100907 (0.005-0.05 mg/kg). In all experiments, impulsive action was measured as increased premature responding. Yohimbine dose dependently increased impulsive action, but the effect was not reversed by antagonist pretreatment. None of the drugs altered any other behavioural measure. We conclude that stress-impulsivity interactions are likely mediated by a synergy of multiple neurotransmitter systems.

Dissociable effects of systemic and orbitofrontal administration of adrenoceptor antagonists on yohimbine-induced motor impulsivity

The α₂-adrenoceptor antagonist, yohimbine, is commonly used as a pharmacological stressor. Its behavioural effects are typically attributed to elevated noradrenaline release via blockade of central, inhibitory autoreceptors. We have previously reported that yohimbine increases motor impulsivity in rats on the five-choice serial reaction time task (5CSRTT), a cognitive behavioural assessment which measures motor impulsivity and visuospatial attention. Furthermore, this effect depended on cyclic adenomonophosphate (cAMP) signalling via cAMP response element binding (CREB) protein in the orbitofrontal cortex (OFC). However, the role of specific adrenoceptors in this effect is not well-characterised. We therefore investigated whether the pro-impulsive effects of systemic yohimbine could be reproduced by direct administration into the OFC, or attenuated by intra-OFC or systemic administration of prazosin and propranolol-antagonists at the α₁- and β-adrenoceptor, respectively. Male Long-Evans rats were trained on the 5CSRTT and implanted with guide cannulae aimed at the OFC. Systemically administered α₁- or β-adrenoceptor antagonists attenuated yohimbine-induced increases in premature responding. In contrast, local infusion of yohimbine into the OFC reduced such impulsive responding, while blockade of α₁- or β-adrenoceptors within the OFC had no effect on either basal or yohimbine-stimulated motor impulsivity. Direct administration of selective antagonists at the α₁-, α₂- or β-adrenoceptor into the OFC therefore produce clearly dissociable effects from systemic administration. Collectively, these data suggest that the pro-impulsivity effect of yohimbine can be modulated by adrenergic signalling in brain areas outside of the OFC, in addition to non-adrenergic signalling pathways within the OFC.

Effects of amphetamine and methylphenidate on attentional performance and impulsivity in the mouse 5-Choice Serial Reaction Time Task

BACKGROUND

Few studies have investigated the effects of conventional attention deficit-hyperactivity disorder (ADHD) medication in the mouse 5-choice serial reaction time task (5-CSRTT), and rat studies have yielded inconsistent results.

OBJECTIVE

We aimed to examine the effects of acute methylphenidate (MPH) and amphetamine (AMPH) treatment in the mouse 5-CSRTT.

METHODS

Trained male C57Bl/6J mice were tested in a variable stimulus duration schedule. Effects of AMPH (0.25, 0.5, and 1 mg/kg) and MPH (0.5, 1.0, and 2.0 mg/kg) on discriminative accuracy, omissions, and premature responses were assessed. Saline treatment data determined high- and low-attentive (LA), and high- and low-impulsive (LI) subgroups according to the upper and lower 30th percentiles, respectively.

RESULTS

In the LA subgroup accuracy was improved by 0.5 mg/kg AMPH and 2 mg/kg MPH, while no effect was found in the high-attentive (HA) subgroup. Premature responses were increased by 1 mg/kg AMPH and 0.5 mg/kg MPH for all animals, and by 1 mg/kg AMPH for the LI subgroup.

CONCLUSIONS

The use of variable stimulus duration, along with the division into high- and LA, and high-and LI subgroups, may improve the sensitivity of the 5-CSRTT when investigating drug effects on attention and impulsivity.

Differential roles for cortical versus sub-cortical noradrenaline and modulation of impulsivity in the rat

RATIONALE

Atomoxetine is a noradrenaline re-uptake inhibitor licensed for the treatment of adult and childhood attention deficit hyperactivity disorder. Although atomoxetine has established efficacy, the mechanisms which mediate its effects are not well understood.

OBJECTIVES

In this study, we investigated the role of cortical versus sub-cortical noradrenaline by using focal dopamine beta hydroxylase-saporin-induced lesions, to the prefrontal cortex (n = 16) or nucleus accumbens shell (n = 18).

METHODS

Healthy animals were tested by using the forced-choice serial reaction time task to assess the impact of the lesion on baseline performance and the response to atomoxetine and the psychostimulant amphetamine.

RESULTS

We observed attenuation in the efficacy of atomoxetine in animals with lesions to the nucleus accumbens shell, but not the prefrontal cortex. Amphetamine-induced increases in premature responses were potentiated in animals with lesions to the prefrontal cortex, but not the nucleus accumbens shell.

CONCLUSIONS

These data suggest that noradrenaline in the nucleus accumbens shell plays an important role in the effects of atomoxetine. Under these conditions, prefrontal cortex noradrenaline did not appear to contribute to atomoxetine's effects suggesting a lack of cortical-mediated "top-down" modulation. Noradrenaline in the prefrontal cortex appears to contribute to the modulation of impulsive responding in amphetamine-treated animals, with a loss of noradrenaline associated with potentiation of its effects. These data demonstrate a potential dissociation between cortical and sub-cortical noradrenergic mechanisms and impulse control in terms of the actions of atomoxetine and amphetamine.

Stimulant drug effects on touchscreen automated paired-associates learning (PAL) in rats

Here we tested in rats effects of the procognitive drugs modafinil and methylphenidate on post-acquisition performance in an object–location paired-associates learning (PAL) task. Modafinil (32; 64 mg/kg) was without effect, while higher (9 mg/kg) but not lower (4.5 mg/kg) doses of methylphenidate impaired PAL performance. Likewise, higher but not lower doses of amphetamine (0.4; 0.8 mg/kg) and MK-801 (0.08; 0.12 mg/kg) decreased PAL performance. Impaired PAL performance induced by methylphenidate, amphetamine, and MK801 most likely reflects compromised cognitive function, e.g., retrieval of learned paired associates. Our data suggest that stimulant drugs such as methylphenidate and modafinil might not facilitate performance in hippocampus-related cognitive tasks.

Enhanced attention and impulsive action following NMDA receptor GluN2B-selective antagonist pretreatment

NMDA GluN2B (NR2B) subtype selective antagonists are currently in clinical development for a variety of indications, including major depression. We previously reported the selective NMDA GluN2B antagonists Ro 63-1908 and traxoprodil, increase premature responding in a 5-choice serial reaction time task (5-CSRTT) suggesting an effect on impulsive action. The present studies extend these investigations to a Go-NoGo and delay discounting task, and the 5-CSRTT under test conditions of both regular (5s) and short (2-5s) multiple ITI (Intertrial interval). Dizocilpine was included for comparison. Both Ro 63-1908 (0.1-1mg/kg SC) and traxoprodil (0.3-3mg/kg SC) increased premature and perseverative responses in both 5-CSRT tasks and improved attention when tested under a short ITI test condition. Ro 63-1908 but not traxoprodil increased motor impulsivity (false alarms) in a Go-NoGo task. Dizocilpine (0.01-0.06mg/kg SC) affected both measures of motor impulsivity and marginally improved attention. In a delay discounting test of impulsive choice, both dizocilpine and Ro 63-1908 decreased impulsive choice (increased choice for the larger, delayed reward), while traxoprodil showed a similar trend. Motor stimulant effects were evident following Ro 63-1908, but not traxoprodil treatment - although no signs of motor stereotypy characteristic of dizocilpine (>0.1mg/kg) were noted. The findings of both NMDA GluN2B antagonists affecting measures of impulsive action and compulsive behavior may underpin emerging evidence to suggest glutamate signaling through the NMDA GluN2B receptor plays an important role in behavioural flexibility. The profiles between Ro 63-1908 and traxoprodil were not identical, perhaps suggesting differences between members of this drug class.

Differential effects of the pharmacological stressor yohimbine on impulsive decision making and response inhibition

RATIONALE

High levels of impulsivity have been associated with psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and substance abuse. In addition, acute stress is known to exacerbate many psychiatric symptoms in impulse control disorders.

OBJECTIVES

The purpose of the current study was to investigate the acute effects of the pharmacological stressor yohimbine on response inhibition and impulsive choice.

METHODS

A group of male rats (n = 12) was trained in the delayed reward task (DRT) to assess impulsive choice. A separate group (n = 10) was trained in the stop-signal task (SST) to measure response inhibition. Upon stable responding, the effects of yohimbine (0, 1.25, 2.5, and 5 mg/kg i.p.) were tested in a Latin square design.

RESULTS

Acute yohimbine significantly increased the preference for the large and delayed reinforcer in the DRT, indicating a decrease in impulsive choice. On the contrary, the effect size of 1.25 mg/kg yohimbine on stop-signal reaction times correlated negatively with baseline performance, suggesting a baseline-dependent effect on response inhibition as measured in the SST.

CONCLUSIONS

The current data suggest that the effects of the pharmacological stressor yohimbine on impulse control strongly depend on the type of impulsive behavior. Pharmacological stress decreased impulsive decision making, an observation that is in line with previously published rodent studies. By contrast, the lowest dose of yohimbine revealed a baseline-dependent effect on response inhibition. As such, the effects of yohimbine are largely comparable to the effects of psychostimulants on impulsivity and may support the notion of cross sensitization of stress and psychostimulants.

Beta-adrenoreceptor blockade abolishes atomoxetine-induced risk taking

RATIONALE

Clinical studies have shown that patients with exaggerated risk-taking tendencies have high baseline levels of norepinephrine. In this work, we systemically manipulated norepinephrine levels in rats and studied their behavioral changes in a probabilistic discounting task, which is a paradigm for gauging risk taking.

METHODS

This study aims to explore the effects of the selective norepinephrine reuptake inhibitor (atomoxetine at doses of 0.6, 1.0 and 1.8 mg/kg), and receptor selective antagonists (propranolol at a single dose of 1.0/kg, and prazosin at a single dose of 0.1 mg/kg), on risk taking using a probabilistic discounting task. In this task, there were two levers available to rats: pressing the 'small/certain' lever guaranteed a single food pellet, and pressing the 'large/risky' lever yielded either four pellets or none. The probability of receiving four food pellets decreased across the four experimental blocks from 100% to 12.5%.

RESULTS

Atomoxetine increased the tendency to choose the large/risky lever. It significantly reduced the lose-shift effect (i.e. pressing a different lever after losing a trial), but did not affect the win-stay effect (i.e. pressing the same lever after winning a trial). Furthermore, co-administration of beta-adrenoreceptor antagonist, propranolol, eliminated the effects of atomoxetine on risk taking and the lose-shift effect; but co-administration of alpha1-adrenoreceptor antagonist, prazosin, did not.

CONCLUSIONS

Atomoxetine boosted NE levels and increased risk taking. This was because atomoxetine decreased rats' sensitivity to losses. These effects were likely mediated by beta-adrenoreceptor.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

Waiting Impulsivity: The Influence of Acute Methylphenidate and Feedback

BACKGROUND

The ability to wait and to weigh evidence is critical to behavioral regulation. These behaviors are known as waiting and reflection impulsivity. In Study 1, we examined the effects of methylphenidate, a dopamine and norepinephrine reuptake inhibitor, on waiting and reflection impulsivity in healthy young individuals. In study 2, we assessed the role of learning from feedback in disorders of addiction.

METHODS

We used the recently developed 4-Choice Serial Reaction Time task and the Beads task. Twenty-eight healthy volunteers were tested twice in a randomized, double-blind, placebo-controlled cross-over trial with 20mg methylphenidate. In the second study, we analyzed premature responses as a function of prior feedback in disorders of addiction.

RESULTS

Study 1: Methylphenidate was associated with greater waiting impulsivity to a cue predicting reward along with faster responding to target onset without a generalized effect on reaction time or attention. Methylphenidate influenced reflection impulsivity based on baseline impulsivity. Study 2: More premature responses occurred after premature responses in stimulant-dependent subjects.

CONCLUSIONS

We show that methylphenidate has dissociable effects on waiting and reflection impulsivity. Chronic stimulant exposure impairs learning from prior premature responses, suggesting a failure to learn that premature responding is suboptimal. These findings provide a greater mechanistic understanding of waiting impulsivity.

The cognition-enhancing effects of psychostimulants involve direct action in the prefrontal cortex

Psychostimulants are highly effective in the treatment of attention-deficit/hyperactivity disorder. The clinical efficacy of these drugs is strongly linked to their ability to improve cognition dependent on the prefrontal cortex (PFC) and extended frontostriatal circuit. The procognitive actions of psychostimulants are only associated with low doses. Surprisingly, despite nearly 80 years of clinical use, the neurobiology of the procognitive actions of psychostimulants has only recently been systematically investigated. Findings from this research unambiguously demonstrate that the cognition-enhancing effects of psychostimulants involve the preferential elevation of catecholamines in the PFC and the subsequent activation of norepinephrine α2 and dopamine D1 receptors. In contrast, while the striatum is a critical participant in PFC-dependent cognition, where examined, psychostimulant action within the striatum is not sufficient to enhance cognition. At doses that moderately exceed the clinical range, psychostimulants appear to improve PFC-dependent attentional processes at the expense of other PFC-dependent processes (e.g., working memory, response inhibition). This differential modulation of PFC-dependent processes across dose appears to be associated with the differential involvement of noradrenergic α2 versus α1 receptors. Collectively, this evidence indicates that at low, clinically relevant doses, psychostimulants are devoid of the behavioral and neurochemical actions that define this class of drugs and instead act largely as cognitive enhancers (improving PFC-dependent function). This information has potentially important clinical implications as well as relevance for public health policy regarding the widespread clinical use of psychostimulants and for the development of novel pharmacologic treatments for attention-deficit/hyperactivity disorder and other conditions associated with PFC dysregulation.

Milnacipran remediates impulsive deficits in rats with lesions of the ventromedial prefrontal cortex

BACKGROUND

Deficits in impulse control are often observed in psychiatric disorders in which abnormalities of the prefrontal cortex are observed, including attention-deficit/hyperactivity disorder and bipolar disorder. We recently found that milnacipran, a serotonin/noradrenaline reuptake inhibitor, could suppress impulsive action in normal rats. However, whether milnacipran could suppress elevated impulsive action in rats with lesions of the ventromedial prefrontal cortex, which is functionally comparable with the human prefrontal cortex, remains unknown.

METHODS

Selective lesions of the ventromedial prefrontal cortex were made using quinolinic acid in rats previously trained on a 3-choice serial reaction time task. Sham rats received phosphate buffered saline. Following a period of recovery, milnacipran (0 or 10mg/kg/d × 14 days) was orally administered 60 minutes prior to testing on the 3-choice task. After 7 days of drug cessation, Western blotting, immunohistochemistry, electrophysiological analysis, and morphological analysis were conducted.

RESULTS

Lesions of the ventromedial prefrontal cortex induced impulsive deficits, and repeated milnacipran ameliorated the impulsive deficit both during the dosing period and after the cessation of the drug. Repeated milnacipran remediated the protein levels of mature brain-derived neurotrophic factor and postsynaptic density-95, dendritic spine density, and excitatory currents in the few surviving neurons in the ventromedial prefrontal cortex of ventromedial prefrontal cortex-lesioned rats.

CONCLUSIONS

The findings of this study suggest that milnacipran treatment could be a novel strategy for the treatment of psychiatric disorders that are associated with a lack of impulse control.

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

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

Impulsive actions and choices in laboratory animals and humans: effects of high vs. low dopamine states produced by systemic treatments given to neurologically intact subjects

Increases and decreases in dopamine (DA) transmission have both been suggested to influence reward-related impulse-control. The present literature review suggests that, in laboratory animals, the systemic administration of DA augmenters preferentially increases susceptibility to premature responding; with continued DA transmission, reward approach behaviors are sustained. Decreases in DA transmission, in comparison, diminish the appeal of distal and difficult to obtain rewards, thereby increasing susceptibility to temporal discounting and other forms of impulsive choice. The evidence available in humans is not incompatible with this model but is less extensive.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Impaired flexibility in decision making in rats after administration of the pharmacological stressor yohimbine

RATIONALE

Stress-induced disruption of decision making has been hypothesized to contribute to drug-seeking behaviors and addiction. Noradrenergic signaling plays a central role in mediating stress responses. However, the effects of acute stress on decision making, and the role of noradrenergic signaling in regulating these effects, have not been well characterized.

OBJECTIVE

To characterize changes in decision making caused by acute pharmacological stress, the effects of yohimbine (an α2-adrenergic antagonist) were examined in a delay discounting task. Noradrenergic contributions to decision making were further characterized by examining the effects of propranolol (a β antagonist), prazosin (an α1 antagonist), and guanfacine (an α2 agonist).

METHODS

Sprague-Dawley rats were administered drugs prior to performance on a delay discounting task, in which the delay preceding the large reward increased within each session (ascending delays). To dissociate drug-induced changes in delay sensitivity from behavioral inflexibility, drug effects were subsequently tested in a modified version of the discounting task, in which the delay preceding the large reward decreased within each session (descending delays).

RESULTS

Yohimbine increased choice of the large reward when tested with ascending delays but decreased choice of the same large reward when tested with descending delays, suggesting that drug effects could be attributed to perseverative choice of the lever preferred at the beginning of the session. Propranolol increased choice of the large reward when tested with ascending delays. Prazosin and guanfacine had no effect on reward choice.

CONCLUSIONS

The stress-like effects of yohimbine administration may impair decision making by causing inflexible, perseverative behavior.

Atomoxetine reduces anticipatory responding in a 5-choice serial reaction time task for adult zebrafish

Deficits in impulse control are related to a number of psychiatric diagnoses, including attention deficit hyperactivity disorder, addiction, and pathological gambling. Despite increases in our knowledge about the underlying neurochemical and neuroanatomical correlates, understanding of the molecular and cellular mechanisms is less well established. Understanding these mechanisms is essential in order to move towards individualized treatment programs and increase efficacy of interventions. Zebrafish are a very useful vertebrate model for exploring molecular processes underlying disease owing to their small size and genetic tractability. Their utility in terms of behavioral neuroscience, however, hinges on the validation and publication of reliable assays with adequate translational relevance. Here, we report an initial pharmacological validation of a fully automated zebrafish version of the commonly used five-choice serial reaction time task using a variable interval pre-stimulus interval. We found that atomoxetine reduced anticipatory responses (0.6 mg/kg), whereas a high-dose (4 mg/kg) methylphenidate increased anticipatory responses and the number of trials completed in a session. On the basis of these results, we argue that similar neurochemical processes in fish as in mammals may control impulsivity, as operationally defined by anticipatory responses on a continuous performance task such as this, making zebrafish potentially a good model for exploring the molecular basis of impulse control disorders and for first-round drug screening.

A selective role for dopamine D₄ receptors in modulating reward expectancy in a rodent slot machine task

BACKGROUND

Cognitive distortions regarding gambling outcomes confer vulnerability to pathological gambling. Using a rat slot machine task (rSMT), we previously demonstrated that the nonspecific D₂ agonist quinpirole enhances erroneous expectations of reward on near-miss trials, suggesting a pivotal role for the D₂ receptor family in mediating the near-miss effect. Identifying which receptor subtype is involved could facilitate treatment development for compulsive slot machine play.

METHODS

Thirty-two male Long Evans rats learned the rSMT. Three flashing lights could be set to on or off. A win was signaled if all three lights were set to on, whereas any other light pattern indicated a loss. Rats then chose between responding on the collect lever, which delivered 10 sugar pellets on win trials but a 10-second time penalty on loss trials, or to start a new trial instead. Performance was assessed following systemic administration of selective D₂, D₃, and D₄ receptor ligands.

RESULTS

The selective D₂ antagonist L-741,626, the D₃ antagonist SB-277011-A, and the D₃ agonist PD128,907 had no effect. In contrast, the selective D₄ agonist PD168077 partially mimicked quinpirole's effects, increasing erroneous collect responses on nonwin trials, whereas the D₄ antagonist L-745,870 improved the error rate. L-745,870 was also the only antagonist that could attenuate the deleterious effects of quinpirole.

CONCLUSIONS

The dopamine D₄ receptor is critically involved in signaling reward expectancy in the rSMT. The ability of L-745,870 to reduce the classification of losses as wins suggests that D₄ antagonists could be effective in treating problematic slot machine play.

Investigating the roles of different monoamine transmitters and impulse control using the 5-choice serial reaction time task

Previous studies have shown that drugs which block the reuptake of catecholamine neurotransmitters improve impulse control in diseases such as attention deficit hyperactivity disorder (ADHD). Serotonin-specific reuptake inhibitors (SSRI) lack efficacy in ADHD and have been linked to increased suicide risk. The present study investigated drugs with affinity for one or more of the monoamine reuptake transporters using the 5-choice serial reaction time task, a model of attention and impulsivity in rodents. We also tested the effects of the alpha(2)-adreoceptor antagonist, idazoxan and novel antidepressant, agomelatine, which both increase cortical noradrenaline concentrations through non-reuptake mechanisms. Improvements in impulse control were observed with venlafaxine, a serotonin and noradrenaline re-uptake inhibitor (SNRI) but not bupropion (dopamine and noradrenaline re-uptake inhibitor). Sibutramine (SNRI) reduced premature responses by ~50% at the highest dose tested but this was not significant. All three of the SSRIs tested reduced premature responding in a dose-dependent manner, although also slowed response and collection latencies. Neither idazoxan nor agomelatine significantly reduced premature responding, suggesting a lack of efficacy at the doses tested. None of the drugs tested improved attention in this task but sibutramine (SNRI), fluoxetine (SSRI) and paroxetine (SSRI) all increased omissions at the highest dose tested. These data suggest that the SNRIs and SSRIs reduce premature responding but tend to be less specific than noradrenaline specific reuptake inhibitors in this model. SSRIs did not induce any specific impairment in impulse control in this model.

Noradrenergic versus dopaminergic modulation of impulsivity, attention and monitoring behaviour in rats performing the stop-signal task: possible relevance to ADHD

RATIONALE

Deficient response inhibition is a prominent feature of many pathological conditions characterised by impulsive and compulsive behaviour. Clinically effective doses of catecholamine reuptake inhibitors are able to improve such inhibitory deficits as measured by the stop-signal task (SST) in humans and other animals. However, the precise therapeutic mode of action of these compounds in terms of their relative effects on dopamine (DA) and noradrenaline (NA) systems in prefrontal cortical and striatal regions mediating attention and cognitive control remains unclear.

OBJECTIVES

We sought to fractionate the effects of global catecholaminergic manipulations on SST performance by using receptor-specific compounds for NA or DA. The results are described in terms of the effects of modulating specific receptor subtypes on various behavioural measures such as response inhibition, perseveration, sustained attention, error monitoring and motivation.

RESULTS

Blockade of α2-adrenoceptors improved sustained attention and response inhibition, whereas α1 and β1/2 adrenergic receptor antagonists disrupted go performance and sustained attention, respectively. No relevant effects were obtained after targeting DA D1, D2 or D4 receptors, while both a D3 receptor agonist and antagonist improved post-error slowing and compulsive nose-poke behaviour, though generally impairing other task measures.

CONCLUSIONS

Our results suggest that the use of specific pharmacological agents targeting α2 and β noradrenergic receptors may improve existing treatments for attentional deficits and impulsivity, whereas DA D3 receptors may modulate error monitoring and perseverative behaviour.

Milnacipran enhances the control of impulsive action by activating D₁-like receptors in the infralimbic cortex

RATIONALE

Elevated impulsivity is often observed in patients with depression. We recently found that milnacipran, an antidepressant and a serotonin/noradrenaline reuptake inhibitor, could enhance impulse control in rats. However, the neural mechanisms underlying the effects of milnacipran on impulsive action remain unclear. Milnacipran increases not only extracellular serotonin and noradrenaline but also dopamine specifically in the medial prefrontal cortex, which is one of the brain regions responsible for impulsive action.

OBJECTIVES

Our goal was to identify whether D(1)- and/or D(2)-like receptors in the infralimbic cortex (IL), the ventral portion of the medial prefrontal cortex, mediates the milnacipran-enhanced impulse control in a three-choice serial reaction time task.

METHODS

The rats were bilaterally injected with SCH23390, a selective D(1)-like receptor antagonist (0.3 or 3 ng/side) or eticlopride, a selective D(2)-like receptor antagonist (0.3 or 1 μg/side) into the IL after acute intraperitoneal administration of milnacipran (10 mg/kg).

RESULTS

Intra-IL SCH23390 injections reversed the milnacipran-enhanced impulse control, whereas injections of eticlopride into the IL failed to block the effects of milnacipran on impulsive action.

CONCLUSIONS

This is the first report that demonstrates a critical role for D(1)-like receptors of the IL in milnacipran-enhanced control of impulsive action.

Impulsive action and impulsive choice are mediated by distinct neuropharmacological substrates in rat

Impulsivity is a heterogeneous construct according to clinical and preclinical behavioural measures and there is some preliminary evidence indicating distinct neurobiological substrates underlying the sub-components of impulsivity. Two preclinical assays, the five-choice serial reaction time task (5-CSRTT) and the delayed discounting task (DDT), are hypothesized to provide measures of impulsive action (premature responding) and impulsive choice (percent choice for delayed reward), respectively. In the present studies, we show that the norepinephrine reuptake inhibitor atomoxetine attenuated premature responding in the 5-CSRTT, but was ineffective in the DDT. The mixed dopamine/norepinephrine reuptake inhibitor methylphenidate exhibited an opposite profile of effects. In addition, blockade of 5-HT2A/C receptors via ketanserin decreased premature responding but had no effects on percent choice for delayed reward; blockade of 5-HT2C receptors via SB 242084 had opposite effects. Follow-up studies provided some limited evidence of additive effects of 5-HT2A/C receptor blockade on the effects of atomoxetine on impulsive action. These studies demonstrate dissociable profiles of stimulant vs. non-stimulant attention deficit hyperactivity disorder medications and 5-HT subtype-selective ligands, in the 5-CSRTT and DDT assays. Thus, the present findings support the sub-categorization of impulsivity and suggest that 5-HT receptor subtype-selective antagonists may provide therapeutic targets for disorders characterized by different forms of impulsivity.

Antagonism of L-type Ca(v) channels with nifedipine differentially affects performance of wildtype and NK1R-/- mice in the 5-Choice Serial Reaction-Time Task

Mice with functional ablation of the substance P-preferring receptor gene ('Nk1r' in mice ('NK1R-/-'), 'TACR1' in humans) display deficits in cognitive performance that resemble those seen in patients with Attention Deficit Hyperactivity Disorder (ADHD): namely, inattentiveness, impulsivity and perseveration. A recent report suggested that the L-type Ca(v) channel blocker, nifedipine, can ameliorate behavioral abnormalities of this type in humans. In light of evidence that NK1R antagonists modulate the opening of these L-type channels, we investigated whether nifedipine modifies %premature responses (impulsivity), perseveration or %omissions (inattentiveness) in the 5-Choice Serial Reaction-Time Task (5-CSRTT) and whether the response differs in NK1R-/- and wildtype mice. %Premature responses and perseveration were reduced in both genotypes, although wildtype mice were more sensitive to the effects of nifedipine than NK1R-/- mice. By contrast, nifedipine greatly increased %omissions but, again, was more potent in wildtypes. %Accuracy and locomotor activity were unaffected in either genotype. We infer that behavior of mice in the 5-CSRTT depends on the regulation of striato-cortical networks by L-type Ca(v) channels and NK1R. We further suggest that disruption of NK1R signaling in patients with ADHD, especially those with polymorphisms of the TACR1 gene, could lead to compensatory changes in the activity of L-type channels that underlie or exacerbate their problems. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

α₂- and β-adrenoceptors involvement in nortriptyline modulation of auditory sustained attention and impulsivity

RATIONALE

The catecholamine innervation of the prefrontal cortex controls attentional focus and inhibits inappropriate behavioral responses. The mechanism of action with which norepinephrine (NE) reuptake inhibitors modulate these cognitive functions has not been fully investigated.

OBJECTIVE

We investigated the effect of systemic administration of the NE reuptake blocker nortriptyline (NT) on attention and impulsivity using an auditory sustained attention task. The task was designed to assess impulsive behavior and the maintenance of attentional focus to an auditory stimulus presented at interresponse time durations (IRT) between 5 and 80 s.

RESULTS

NT (2.0 but not 3.0 mg/kg) improved sustained attention and decreased the percentage of premature responses without changing their latency. To better understand the adrenergic component of NT action, we tested the effect of noradrenergic receptor antagonists alone or together with NT. The α(2)-receptor antagonist yohimbine, the α(1)-receptor antagonist prazosin, or the β-receptor antagonist propranolol alone did not significantly affect attentive performance or premature responses. However, the beneficial effects of NT on sustained attention and premature responses were attenuated by pretreatment with either yohimbine or propranolol. On the contrary, prazosin did not affect the NT-mediated improvement in sustained attention.

CONCLUSIONS

We conclude that sustained attention displays an inverse U-shaped dependence on NT, mediated-at least in part-by α(2)- and β-adrenoceptors. We speculate that low doses of NT improve performance by maximizing the phasic release of NE, while higher doses of NT would elevate tonic levels of NE, thus producing suboptimal levels of phasically released NE.

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.

Blockade of noradrenaline re-uptake sites improves accuracy and impulse control in rats performing a five-choice serial reaction time tasks

RATIONALE

Atomoxetine, reboxetine and methylphenidate all act at the noradrenaline transporter (NAT) and atomoxetine and methylphenidate are licensed for the treatment of ADHD. The five-choice serial reaction time task (5CSRTT) provides a valid model to study attention and impulsivity in rodents. Studies using this task have largely failed to demonstrate improvements in attention with atomoxetine and methylphenidate and reboxetine has not been investigated previously.

OBJECTIVES

The present study used modifications to the standard rat 5CSRTT and demonstrated that blockade of NAT improves attention and reduces premature responding.

METHODS

Rats were trained in a fixed inter-trial interval (ITI), 5CSRTT then tested at baseline and under conditions to acutely challenge attention and/or impulse control following vehicle or atomoxetine (0.3 mg/kg, i.p.).

RESULTS

Atomoxetine (0.3 mg/kg, i.p.) significantly improved impulse control under all conditions (p < 0.05) but had no significant effects on accuracy. To increase the attentional demands of the task, rats were re-baselined in a non-paced, variable ITI task where presentations of the stimuli were unpredictable. In the VITI task, atomoxetine (0.0-0.3 mg/kg, i.p.) induced a dose-dependent improvement in accuracy (p < 0.05) and reduction in premature responses (p < 0.05). Reboxetine (0.0-1.0 mg/kg, i.p.) and methylphenidate (1-10 mg/kg, p.o.) did not significantly improve accuracy in the whole population but median split analysis revealed a significant improvement with both drugs, as well as atomoxetine, in the poor performing animals (p < 0.05).

CONCLUSIONS

These data suggest that blockade of noradrenaline re-uptake sites is an important target in terms of enhancing both attention and impulse control.

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.

Modulation of high impulsivity and attentional performance in rats by selective direct and indirect dopaminergic and noradrenergic receptor agonists

RATIONALE

Impulsivity is associated with a number of psychiatric disorders, most notably attention deficit/hyperactivity disorder (ADHD). Drugs that augment catecholamine function (e.g. methylphenidate and the selective noradrenaline reuptake inhibitor atomoxetine) have clinical efficacy in ADHD, but their precise mechanism of action is unclear.

OBJECTIVE

The objective of this study is to investigate the relative contribution of dopamine (DA) and noradrenaline (NA) to the therapeutic effects of clinically effective drugs in ADHD using rats selected for high impulsivity on the five-choice serial reaction time task (5CSRTT).

METHODS

We examined the effects of direct and indirect DA and NA receptor agonists and selective DA and NA reuptake inhibitors in rats showing high and low levels of impulsivity on the 5CSRTT (designated high impulsive 'HI' and low impulsive 'LI', respectively). Drugs were administered by systemic injection in a randomized, counterbalanced manner.

RESULTS

Low doses of quinpirole (a D2/D3 agonist) and sumanirole (a D2 agonist) selectively reduced impulsivity on the 5CSRTT, whilst higher doses resulted in increased omissions and slower response latencies. The NA reuptake inhibitor, atomoxetine, and the alpha-2 adrenoreceptor agonist, guanfacine, dose dependently decreased premature responding. The dopaminergic reuptake inhibitor GBR-12909 increased impulsivity, whereas the nonselective DA and NA reuptake inhibitor methylphenidate had no significant effect on impulsive responses in HI and LI rats.

CONCLUSIONS

These findings indicate that high impulsivity can be ameliorated in rats by drugs that mimic the effects of DA and NA, just as in ADHD, and that activation of D2/3 receptors selectively decreases high impulsivity on the 5CSRTT.

High impulsivity in rats predicts amphetamine conditioned place preference

Stimulants such as d-amphetamine (AMPH) are used commonly to treat attention-deficit hyperactivity disorder (ADHD), but concerns have been raised regarding the use of AMPH due to its reinforcing and potentially addictive properties. The current study examined if individual differences in impulsive choice predict AMPH-induced hyperactivity and conditioned place preference (CPP). Rats were first tested in delay discounting using an adjusting delay procedure to measure impulsive choice and then were subsequently tested for AMPH CPP. High impulsive (HiI) and low impulsive (LoI) rats were conditioned across four sessions with 0.1, 0.5, or 1.5 mg/kg of AMPH. AMPH increased locomotor activity for HiI and LoI rats following 0.5 mg/kg but failed to increase activity following 0.1 and 1.5 mg/kg. CPP was established for HiI rats with both 0.5 and 1.5 mg/kg of AMPH, whereas LoI rats did not develop CPP following any dose of AMPH; HiI and LoI groups differed significantly following 0.5 mg/kg of AMPH. These results indicate that HiI rats are more sensitive to the rewarding effects of AMPH compared to LoI rats, which is consistent with research showing that high impulsive individuals may be more vulnerable to stimulant abuse.

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.

Methylphenidate and fluphenazine, but not amphetamine, differentially affect impulsive choice in spontaneously hypertensive, Wistar-Kyoto and Sprague-Dawley rats

Impulsivity is one of the core symptoms of attention-deficit/hyperactivity disorder (ADHD). The spontaneously hypertensive rat (SHR), a putative animal model of ADHD, has been used to investigate the neurobiology of impulsivity, although this model has been questioned over concerns that use of Wistar-Kyoto rats (WKY) as a comparison strain may exaggerate effects. The present study compared SHR, WKY and standard, outbred Sprague-Dawley (SD) rats on a delay discounting task where the primary measure was mean adjusted delay (MAD), or the indifference point (in sec) between choice of an immediate delivery of 1 grain-based pellet versus 3 pellets delivered after varying delays. The acute dose effects of the ADHD medications amphetamine (0.1-1.0 mg/kg) and methylphenidate (1.0-10 mg/kg) were then determined; in addition, the effect of the dopamine receptor antagonist fluphenazine (0.1-1.0 mg/kg) was also assessed for comparison with the indirect agonists. While there were no strain differences in the rate of task acquisition or stabilization of baseline MAD scores, SHR had significantly lower MAD scores than WKY but not SD due to the greater individual variability of MAD scores in SD. Although amphetamine did not alter MAD scores in any strain, methylphenidate selectively increased MAD scores in WKY and fluphenazine selectively increased MAD scores in SHR. WKY were also more sensitive than SHR and SD to the response-impairing effects of each drug. The finding that SHR showed a decrease in impulsivity following fluphenazine, but not following either amphetamine or methylphenidate, suggests that delay discounting in SHR may not represent a valid predictive model for screening effective ADHD medications in humans.

A neurochemical yin and yang: does serotonin activate and norepinephrine deactivate the prefrontal cortex?

INTRODUCTION

The prefrontal cortex (PFC) receives serotonergic input from the dorsal raphe nucleus of the brainstem, as well as noradrenergic input from another brainstem nucleus, the locus coeruleus. A large number of studies have shown that these two neurotransmitter systems, and drugs that affect them, modulate the functional properties of the PFC in both humans and animal models.

RESULTS

Here I examine the hypothesis that serotonin (5-HT) plays a general role in activating the PFC, whereas norepinephrine (NE) plays a general role in deactivating this brain region. In this manner, the two neurotransmitter systems may have opposing effects on PFC-influenced behavior. To assess this hypothesis, three primary lines of evidence are examined comprising the effects of 5-HT and NE on impulsivity, cognitive flexibility, and working memory.

DISCUSSION

While all of the existing data do not unequivocally support the activation/deactivation hypothesis, there is a large body of support for it.

Sub-optimal performance in the 5-choice serial reaction time task in rats was sensitive to methylphenidate, atomoxetine and d-amphetamine, but unaffected by the COMT inhibitor tolcapone

Prefrontal cortical dopamine (DA) and norepinephrine (NE) are implicated in multiple aspects of cognitive function assessed via the 5-choice serial reaction time task (5-CSRTT) in rodents. The present studies assessed the effects of the NE reuptake inhibitor atomoxetine (0.5-2.0 mg/kg), the mixed DA/NE reuptake inhibitor methylphenidate (0.1-2.0 mg/kg), the catecholamine releaser D-amphetamine (0.1-1.0 mg/kg) and the catecholamine-o-methyl-transferase (COMT) inhibitor tolcapone (3.0-30.0 mg/kg) in rats that exhibited sub-optimal performance (reduced accuracy: <70% correct) in the 5-CSRTT. Increased ITI durations were associated with increased premature responding. Decreased ITI durations resulted in increased percent omissions, increased perseverative responses and increased response latencies, but had no effects on magazine latencies or percent correct. Atomoxetine decreased premature responding at prolonged ITI durations and methylphenidate decreased percent omissions at low doses (0.1 and 0.5 mg/kg). By contrast, D-amphetamine increased premature and perseverative responding in a dose-dependent manner (0.3-1.0 mg/kg). Finally, tolcapone had no effects on sub-optimal performance in the variable ITI 5-CSRTT. These results suggest minimal potential of tolcapone as a therapeutic agent for ADHD and implicate cortical NE, not DA, in impulsive action.

Prefrontal cortex and drug abuse vulnerability: translation to prevention and treatment interventions

Vulnerability to drug abuse is related to both reward seeking and impulsivity, two constructs thought to have a biological basis in the prefrontal cortex (PFC). This review addresses similarities and differences in neuroanatomy, neurochemistry and behavior associated with PFC function in rodents and humans. Emphasis is placed on monoamine and amino acid neurotransmitter systems located in anatomically distinct subregions: medial prefrontal cortex (mPFC); lateral prefrontal cortex (lPFC); anterior cingulate cortex (ACC); and orbitofrontal cortex (OFC). While there are complex interconnections and overlapping functions among these regions, each is thought to be involved in various functions related to health-related risk behaviors and drug abuse vulnerability. Among the various functions implicated, evidence suggests that mPFC is involved in reward processing, attention and drug reinstatement; lPFC is involved in decision-making, behavioral inhibition and attentional gating; ACC is involved in attention, emotional processing and self-monitoring; and OFC is involved in behavioral inhibition, signaling of expected outcomes and reward/punishment sensitivity. Individual differences (e.g., age and sex) influence functioning of these regions, which, in turn, impacts drug abuse vulnerability. Implications for the development of drug abuse prevention and treatment strategies aimed at engaging PFC inhibitory processes that may reduce risk-related behaviors are discussed, including the design of effective public service announcements, cognitive exercises, physical activity, direct current stimulation, feedback control training and pharmacotherapies. A major challenge in drug abuse prevention and treatment rests with improving intervention strategies aimed at strengthening PFC inhibitory systems among at-risk individuals.

Yohimbine increases impulsivity through activation of cAMP response element binding in the orbitofrontal cortex

BACKGROUND

Stress can increase impulsivity and has a negative impact on psychiatric outcome. Norepinephrine is heavily implicated in responses to stress, and the alpha(2) antagonist yohimbine is used clinically to study this aspect of the stress response. Yohimbine induces mild anxiety and increases impulsivity in healthy volunteers but has more detrimental effects in some psychiatric populations, triggering mania in bipolar patients and drug craving in substance-dependent individuals. Understanding the mechanism by which yohimbine affects brain function could provide insight into the heightened reaction to stress in these patients.

METHODS

Yohimbine's effects were assessed in rats using the five-choice serial reaction time test of attention and impulse control. We then examined whether yohimbine altered activity of cyclic adenosine monophosphate response element binding (CREB) protein-a transcription factor implicated in the stress response-in brain areas that regulate impulsivity. The behavioral consequences of any changes in CREB activity were subsequently assessed using viral-mediated gene transfer to regionally overexpress CREB or the dominant negative antagonist mCREB.

RESULTS

Yohimbine increased impulsive responding in rats and selectively increased CREB phosphorylation within the orbitofrontal cortex but not medial prefrontal cortex or nucleus accumbens. Overexpressing mCREB within the orbitofrontal cortex blocked yohimbine's effects on impulsivity, whereas overexpressing CREB in this region increased impulsive responding and potentiated the proimpulsive actions of yohimbine.

DISCUSSION

These data suggest a novel molecular mechanism contributing to impulsivity that may be sensitive to stress. Such findings may improve our understanding of the neurobiological pathways linking the response to stress and impulsivity in both healthy and psychiatric populations.

Functional interactions between 5-HT2A and presynaptic 5-HT1A receptor-based responses in mice genetically deficient in the serotonin 5-HT transporter (SERT)

BACKGROUND AND PURPOSE

Despite decreased presynaptic 5-HT(1A) and altered 5-HT(2A) receptor function in genetically-deficient serotonin (5-HT) transporter (SERT) mice, the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100635) still induced head twitches in these mice, a well-established 5-HT(2A) receptor-mediated response.

EXPERIMENTAL APPROACH

Interactions between 5-HT(1A) and 5-HT(2A) receptors were assessed using the head-twitch response following 5-HT(1A) and 5-HT(2A) receptor agonists and antagonists in SERT wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The role of brain 5-HT availability in WAY 100635 induced head twitches was also examined.

KEY RESULTS

WAY 100635 induced head twitches in a SERT gene-dose dependent manner, inducing 5-fold more head twitches in SERT -/- versus SERT +/+ mice. In SERT -/- mice, inhibition of 5-HT synthesis with p-chlorophenylalanine (PCPA) markedly depleted tissue 5-HT in all five brain areas examined and abolished WAY 100635 induced head twitches. Further, the selective 5-HT reuptake inhibitor fluvoxamine increased WAY 100635 induced head twitches in SERT +/+ and +/- mice. Head twitches following the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) were robust in SERT +/+ and +/- mice but much reduced in SERT -/- mice. DOI-induced head twitches were decreased by the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in SERT +/+ and +/- mice. All drug-induced head twitches were blocked by the 5-HT(2A) receptor antagonist a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939).

CONCLUSIONS AND IMPLICATIONS

These data show that indirect activation of 5-HT(2A) receptors via blockade of presynaptic 5-HT(1A) receptors potentiated head-twitch responses, suggesting functional interactions between these receptors, interactions affected by altered 5-HT availability. Our findings strongly support the correlation of WAY 100635 induced head twitches with increased 5-HT availability, induced genetically or pharmacologically.

Is there an inhibitory-response-control system in the rat? Evidence from anatomical and pharmacological studies of behavioral inhibition

Many common psychiatric conditions, such as attention deficit/hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), Parkinson's disease, addiction and pathological gambling are linked by a failure in the mechanisms that control, or inhibit, inappropriate behavior. Models of rat behavioral inhibition permit us to study in detail the anatomical and pharmacological bases of inhibitory failure, using methods that translate directly with patient assessment in the clinic. This review updates current ideas relating to behavioral inhibition based on two significant lines of evidence from rat studies: (1) To integrate new findings from the stop-signal task into existing models of behavioral inhibition, in particular relating to 'impulsive action' control. The stop-signal task has been used for a number of years to evaluate psychiatric conditions and has recently been translated for use in the rat, bringing a wealth of new information to behavioral inhibition research. (2) To consider the importance of the subthalamic nucleus (STN) in the neural circuitry of behavioral inhibition. This function of this nucleus is central to a number of 'disinhibitory' disorders such as Parkinson's disease and OCD, and their therapies, but its role in behavioral inhibition is still undervalued, and often not considered in preclinical models of behavioral control. Integration of these findings has pinpointed the orbitofrontal cortex (OF), dorsomedial striatum (DMStr) and STN within a network that normally inhibits many forms of behavior, including both impulsive and compulsive forms. However, there are distinct differences between behavioral subtypes in their neurochemical modulation. This review brings new light to the classical view of the mechanisms that inhibit behavior, in particular suggesting a far more prominent role for the STN, a structure that is usually omitted from conventional behavioral-inhibition networks. The OF-DMStr-STN circuitry may form the basis of a control network that defines behavioral inhibition and that acts to suppress or countermand many forms of inappropriate or maladaptive behavior.

Differences in impulsivity on a delay-discounting task predict self-administration of a low unit dose of methylphenidate in rats

There is controversy about the abuse liability of methylphenidate (MPH) in humans, and MPH has yet to be established fully as a reinforcer in rats. The present experiment examined whether intravenous MPH served as a reinforcer in rats, and how individual differences in impulsivity impacted MPH self-administration. Rats were exposed to a delay-discounting procedure, and then were implanted with an intravenous catheter to assess self-administration of 0.56 mg/kg/infusion MPH at different fixed ratio values. Self-administration rates of different MPH doses (0.03-1.0 mg/kg/infusion) were also examined. Both high and low impulsive rats acquired MPH self-administration at the same rate. All rats pressed more on the active lever than the inactive lever regardless of MPH dose, and pressed more for MPH than for saline. High impulsive rats self-administered more MPH than low impulsive rats at a low unit dose (0.1 mg/kg/infusion), though not at higher doses, indicating that individual differences in impulsive choice influence the dose-dependent reinforcing effects of MPH.