Amphetamine decreases behavioral inhibition by stimulation of dopamine D2, but not D3, receptors

Antagonism of D2 receptors via raclopride ameliorates amphetamine-induced associative learning deficits in male mice

Dopamine levels in the dorsomedial striatum (DMS) are highly dynamic and are thought to underly the encoding of action-outcome associations. Although it is known that amphetamine disrupts the learning that is required for goal-directed action, the role of D1 and D2 receptors in this process has not been established. In this study, we examined the role of D1 and D2 receptor antagonists on learning in response to amphetamine. We used the outcome-specific devaluation task to examine goal-directed action in male C57BL6/J mice treated systemically with either a D1 antagonist (SCH-23990; 0.01 mg/kg) or a D2 antagonist (raclopride; 0.5 mg/kg) and then administered amphetamine (1 mg/kg). The mice were injected repeatedly throughout the instrumental training phase of the task to assess the impact on the learning of action-outcomes, and the subsequent choice test assessing performance of goal-directed action was conducted drug free. Effects of chronic drug administration on locomotor behaviour was assessed before and after the choice test. Treatment during learning with either amphetamine, or the D1 or D2 antagonists, impaired the subsequent performance of goal-directed action. The amphetamine-induced impairment in goal-directed action was reversed in mice treated with raclopride, but not when treated with SCH-23990. By contrast, amphetamine-induced hyperactivity was reversed in mice treated with SCH-23990, but not in mice treated with raclopride. Taken together, these data support the role of a balance of dopamine receptor signalling after amphetamine treatment. While overall D1 receptor availability is necessary to promote learning, in a state of elevated dopamine, modifying D2 receptor function can ameliorate learning deficits.

Concurrent measures of impulsive action and choice are partially related and differentially modulated by dopamine D1- and D2-like receptors in a rat model of impulsivity

Impulsivity is a multidimensional construct, but the relationships between its constructs and their respective underlying dopaminergic underpinnings in the general population remain unclear. A cohort of Roman high- (RHA) and low- (RLA) avoidance rats were tested for impulsive action and risky decision-making in the rat gambling task, and then for delay discounting in the delay-discounting task to concurrently measure the relationships among the three constructs of impulsivity using a within-subject design. Then, we evaluated the effects of dopaminergic drugs on the three constructs of impulsivity, considering innate differences in impulsive behaviors at baseline. Risky decision-making and delay-discounting were positively correlated, indicating that both constructs of impulsive choice are related. Impulsive action positively correlated with risky decision-making but not with delay discounting, suggesting partial overlap between impulsive action and impulsive choice. RHAs showed a more impulsive phenotype in the three constructs of impulsivity compared to RLAs, demonstrating the comorbid nature of impulsivity in a population of rats. Amphetamine increased impulsive action and had no effect on risky decision-making regardless of baseline levels of impulsivity, but it decreased delay discounting only in high impulsive RHAs. In contrast, while D₁R and D₃R agonism as well as D_2/3R partial agonism decreased impulsive action regardless of baseline levels of impulsivity, D_2/3R agonism decreased impulsive action exclusively in high impulsive RHAs. Irrespective of baseline levels of impulsivity, risky decision-making was increased by D₁R and D_2/3R agonism but not by D₃R agonism or D_2/3R partial agonism. Finally, while D₁R and D₃R agonism, D_2/3R partial agonism and D₂R blockade increased delay discounting irrespective of baseline levels of impulsivity, D_2/3R agonism decreased it in low impulsive RLAs only. These findings indicate that the acute effects of dopamine drugs were partially overlapping across dimensions of impulsivity, and that only D_2/3R agonism showed baseline-dependent effects on impulsive action and impulsive choice.

Frontal-Sensory Cortical Projections Become Dispensable for Attentional Performance Upon a Reduction of Task Demand in Mice

Top-down attention is a dynamic cognitive process that facilitates the detection of the task-relevant stimuli from our complex sensory environment. A neural mechanism capable of deployment under specific task-demand conditions would be crucial to efficiently control attentional processes and improve promote goal-directed attention performance during fluctuating attentional demand. Previous studies have shown that frontal top-down neurons projecting from the anterior cingulate area (ACA) to the visual cortex (VIS; ACA_VIS) are required for visual attentional behavior during the 5-choice serial reaction time task (5CSRTT) in mice. However, it is unknown whether the contribution of these projecting neurons is dependent on the extent of task demand. Here, we first examined how behavior outcomes depend on the number of locations for mice to pay attention and touch for successful performance, and found that the 2-choice serial reaction time task (2CSRTT) is less task demanding than the 5CSRTT. We then employed optogenetics to demonstrate that suppression ACA_VIS projections immediately before stimulus presentation has no effect during the 2CSRTT in contrast to the impaired performance during the 5CSRTT. These results suggest that ACA_VIS projections are necessary when task demand is high, but once a task demand is lowered, ACA_VIS neuron activity becomes dispensable to adjust attentional performance. These findings support a model that the frontal-sensory ACA_VIS projection regulates visual attention behavior during specific high task demand conditions, pointing to a flexible circuit-based mechanism for promoting attentional behavior.

The Action of TAAR1 Agonist RO5263397 on Executive Functions in Rats

Trace amine-associated receptor 1 (TAAR1) is a widely recognized new perspective target for the neuropsychiatric pharmacological treatment. Despite a growing number of studies investigating TAAR1 role in the animal models of different pathologies, information of TAAR1 agonists impact on executive cognitive functions is limited. The goal of the present study was to evaluate the activity of highly selective partial TAAR1 agonist RO5263397 on various executive cognitive functions. The results of the present study demonstrated that the pretreatment with RO5263397 was able to increase attention and decrease cognitive flexibility in rats. The analysis of the RO5263397 action on impulsivity demonstrated that the TAAR1 activation failed to affect premature responding but was able to slightly modify impulsive choice. Problem solving was resistant to the pharmacological intervention.

Impulsive Action and Impulsive Choice Are Differentially Associated With Gene Expression Variations of the GABAA Receptor Alfa 1 Subunit and the CB1 Receptor in the Lateral and Medial Orbitofrontal Cortices

The orbitofrontal cortex (OFC) is a key brain region for decision-making, action control and impulsivity. Quite notably, previous research has identified a double dissociation regarding the role of this cortical territory in impulsive choice. While medial orbitofrontal lesions increase preference for a large but delayed reward, lateral orbitofrontal lesions have the opposite effect. However, there are no data regarding this anatomical dissociation in impulsive action. The neurochemical basis of impulsivity is still being elucidated, however, in recent years a role for the endocannabinoids and the related glutamatergic and GABAergic neurotransmitter systems has been suggested. Here, we submitted male Wistar rats to a delay-discounting task (DDT) or a two-choice serial reaction time task (2-CSRTT) and classified them as high impulsive or low impulsive in either task using cluster analysis. We then examined the gene expression of several elements of the endocannabinoid system or different subunits of certain glutamatergic or GABAergic ionotropic receptors (AMPA, NMDA, or GABA_A) in the lateral or medial divisions of their orbitofrontal cortices. Our results confirm, at the gene expression level, the dissociation in the participation of the medial, and lateral divisions of the orbitofrontal cortex in impulsivity. While in the 2-CSRTT (inhibitory control) we found that high impulsive animals exhibited lower gene expression levels of the α1 GABA_A receptor subunit in the lateral OFC, no such differences were evident in the medial OFC. When we analyzed DDT performance, we found that high impulsive animals displayed lower levels of CB₁ gene expression in the medial but not in the lateral OFC. We propose that GABAergic dynamics in the lateral OFC might contribute to the inhibitory control mechanisms that are altered in impulsive behavior while endocannabinoid receptor gene transcription in the medial OFC may subserve the delay-discounting processes that participate in certain types of impulsiveness.

Linking ADHD to the Neural Circuitry of Attention

Attention deficit hyperactivity disorder (ADHD) is a complex condition with a heterogeneous presentation. Current diagnosis is primarily based on subjective experience and observer reports of behavioral symptoms - an approach that has significant limitations. Many studies show that individuals with ADHD exhibit poorer performance on cognitive tasks than neurotypical controls, and at least seven main functional domains appear to be implicated in ADHD. We discuss the underlying neural mechanisms of cognitive functions associated with ADHD, with emphasis on the neural basis of selective attention, demonstrating the feasibility of basic research approaches for further understanding cognitive behavioral processes as they relate to human psychopathology. The study of circuit-level mechanisms underlying executive functions in nonhuman primates holds promise for advancing our understanding, and ultimately the treatment, of ADHD.

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.

Critical involvement of 5-HT2C receptor function in amphetamine-induced 50-kHz ultrasonic vocalizations in rats

RATIONALE

Rats emit various distinct types of ultrasonic vocalizations (USV), with high-frequency 50-kHz USV typically occurring in appetitive situations being elicited by administering drugs of abuse, most notably amphetamine (AMPH), possibly reflecting drug wanting/craving and/or liking.

OBJECTIVES

Because 50-kHz USV emission is, at least in part, dopamine (DA) dependent and 5-HT2C agonists inhibit DA neurotransmission, we hypothesized that AMPH-induced 50-kHz USV can be attenuated by pretreatment with a 5-HT2C agonist.

METHODS

In experiments I and II, a dose-response curve for AMPH-induced 50-kHz USV was established, and the partial dependency of AMPH-induced 50-kHz USV on DA neurotransmission was validated by pretreatment with the D2-antagonist eticlopride. In experiment III, rats were pretreated with the 5-HT2C agonist CP 809,101 (0.0, 0.3, 1.0, 3.0, and 10 mg/kg), while in experiment IV, CP 809,101 (3.0 mg/kg), the 5-HT2C antagonist SB 242084 (1.0 mg/kg), or the combination of the two, was applied before AMPH administration (2.0 mg/kg). Finally, in experiment V, rats were treated with SB 242084 (0.0, 0.1, 0.3, and 1.0 mg/kg) only, i.e., in absence of AMPH.

RESULTS

The 5-HT2C agonist CP 809,101 dose-dependently blocked AMPH-induced 50-kHz USV, most notably trills, a call subtype that is considered to exclusively reflect a positive affective state, while the 5-HT2C antagonist SB 242084 induced opposite effects. Moreover, SB 242084 induced 50-kHz USV by its own.

CONCLUSIONS

5-HT2C receptors are critically involved in AMPH-induced 50-kHz USV, with 5-HT2C antagonism resulting in a stimulant-like effect. Attenuation of drug wanting/craving and/or liking by coadministration of a 5-HT2C agonist could be a translational pharmacodynamic biomarker.

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.

Implication of dopamine D3 receptor activation in the reversion of Parkinson's disease-related motivational deficits

In addition to the classical motor symptoms, motivational and affective deficits are core impairments of Parkinson's disease (PD). We recently demonstrated, by lesional approaches in rats, that degeneration of the substantia nigra pars compacta (SNc) dopaminergic (DA) neurons is likely to have a crucial role in the development of these neuropsychiatry symptoms. We have also shown that, as in clinical investigations, chronic treatment with levodopa or the DA D2/D3 receptor (D2/D3R) agonist ropinirole specifically reverses these PD-related motivational deficits. The roles of specific DA receptor subtypes in such reversal effects remain, however, unknown. We therefore investigated here the precise involvement of D1, D2 and D3R in the reversal of the motivational and affective deficits related to SNc DA neuronal loss. Three weeks after bilateral and partial 6-hydroxydopamine (6-OHDA) SNc lesions, rats received 14 daily intraperitoneal administrations of the selective D1R agonist SKF-38393 (2.5 or 3.5 mg kg(-1)), the selective D2R agonist sumanirole (0.1 or 0.15 mg kg(-1)), or the preferring D3R gonist PD-128907 (0.1 or 0.15 mg kg(-1)). Anxiety-, depressive-like and motivated behaviors were assessed in an elevated-plus maze, a forced-swim test, and an operant sucrose self-administration procedure, respectively. All DA agonists attenuated anxiety- and depressive-like behaviors. However, only PD-128907 reversed the motivational deficits induced by 6-OHDA SNc lesions. This effect was blocked by a selective D3R (SB-277011A, 10 mg kg(-1)), but not D2R (L-741,626, 1.5 mg kg(-1)), antagonist. These data provide strong evidence for the role of D3R in motivational processes and identify this receptor as a potentially valuable target for the treatment of PD-related neuropsychiatric symptoms.

Divergent effects of D₂/₃ receptor activation in the nucleus accumbens core and shell on impulsivity and locomotor activity in high and low impulsive rats

RATIONALE

Previously we demonstrated reduced D2/3 receptor availability in the ventral striatum of hyper-impulsive rats on the five-choice serial reaction time task (5-CSRTT). However, the anatomical locus of D2/3 receptor dysfunction in high impulsive (HI) rats is unknown.

OBJECTIVE

In the present study, we investigated whether D2/3 receptor dysfunction in HI rats is localised to the core or shell sub-regions of the nucleus accumbens (NAcb).

METHODS

Rats were selected for low (low impulsive, LI) and high impulsivity on the 5-CSRTT and implanted with guide cannulae targeting the NAcb core and shell. The D2/3 receptor agonist quinpirole was locally injected in the NAcb (0.1, 0.3 and 1 μg per infusion) and its effects investigated on the performance of LI and HI rats on the 5-CSRTT as well as spontaneous locomotor activity in an open field.

RESULTS

Intra-NAcb core quinpirole increased premature responding in HI rats but not in LI rats. In contrast, intra-NAcb shell quinpirole strongly increased locomotor activity in HI rats, unlike LI rats. This effect was blocked by intra-NAcb shell infusions of the D2/3 receptor antagonist nafadotride (0.03 μg). However, nafadotride was ineffective in blocking the effects of intra-NAcb core quinpirole on premature responding in HI rats.

CONCLUSIONS

These findings indicate that impulsivity and hyperactivity are separately regulated by core and shell sub-regions of the NAcb and that HI rats show an enhanced response to D2/3 receptor activation in these regions. These results suggest that the symptom clusters of hyperactivity and impulsivity in attention-deficit hyperactivity disorder may be neurally dissociable at the level of the NAcb.

Pramipexole-induced increased probabilistic discounting: comparison between a rodent model of Parkinson's disease and controls

The dopamine agonist pramipexole (PPX) can increase impulsiveness, and PPX therapy for neurological diseases (Parkinson's disease (PD) and restless leg syndrome) is associated with impulse control disorders (ICDs) in subpopulations of treated patients. A commonly reported ICD is pathological gambling of which risk taking is a prominent feature. Probability discounting is a measurable aspect of risk taking. We recently developed a probability discounting paradigm wherein intracranial self-stimulation (ICSS) serves as the positive reinforcer. Here we used this paradigm to determine the effects of PPX on discounting. We included assessments of a rodent model of PD, wherein 6-OHDA was injected into the dorsolateral striatum of both hemispheres, which produced persistent PD-like deficits in posture adjustment. Rats were trained to perform ICSS-mediated probability discounting, in which PD-like and control groups exhibited similar profiles. Rats were treated twice daily for 2 weeks with 2 mg/kg (±)PPX (ie, 1 mg/kg of the active form), a dose that improved lesion-induced motor deficits. In both groups, (±)PPX increased discounting; preference for the large reinforcer was enhanced 30-45% at the most uncertain probabilities. Tolerance did not develop with repeated treatments. Increased discounting subsided within 2 weeks of (±)PPX cessation, and re-exposure to (±)PPX reinstated heightened discounting. Such findings emulate the clinical scenario; therefore, ICSS for discounting assessments in rats exhibited high face validity. This model should prove useful in medication development where assessment of the propensity of a putative therapy to induce risk-taking behaviors is of interest.

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.

Amphetamine modifies ethanol intake of psychosocially stressed male rats

Studies of socially housed rodents have provided significant information regarding the consequences of exposure to stressors. Psychosocial stressors are known to alter the ingestion of ethanol and the activity of the dopaminergic neuronal system. Since both stressors and ethanol are known to affect the function of dopaminergic neurons, we employed amphetamine to assess the role of this neural system on the ingestion of ethanol by psychosocially stressed male rats. Male rats housed two per cage were designated as dominant or subdominant rats based on evaluations of agonistic behavior and body weight changes. The dyad-housed rats and a group of single-housed rats were sequentially assessed for ethanol intake after injections of saline or amphetamine (0.3, 0.9 or 2.7 mg/kg i.p.) both prior to dyad housing and subsequently again during dyad-housing. Prior to dyad housing ethanol intake of future subdominant rats was higher than that of future dominant rats. Dyad-housing significantly increased ethanol intake of dominant rats. Pre-dyad the highest dose of amphetamine potently depressed ethanol ingestion. Sensitivity to amphetamine's depressant effect on ethanol intake was higher at the dyad test in all subjects, most prominently in single-housed rats. In contrast to the single-housed rats, the dyad-housed rats displayed saccharin anhedonia. It can be concluded that dopaminergic system modulates, at least partially, the psychosocial stress-induced changes in ethanol intake. Furthermore, the level of ethanol ingestion at the pre-dyad test was predictive of future hierarchical status.

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.

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.

Orbitofrontal connectivity with resting-state networks is associated with midbrain dopamine D3 receptor availability

Animal research and human postmortem evidence highlight the importance of brain dopamine D3 receptor (D3R) function in multiple neuropsychiatric disorders, including addiction. Separate anatomical and functional neuroimaging findings implicate disrupted frontal cortical connectivity with distributed brain networks in processes relevant for these diseases. This potential conjunction between molecular and functional markers has not, however, been tested directly. Here, we used a novel combination of [(11)C]-(+)-PHNO positron emission tomography and resting-state functional magnetic resonance imaging in the same healthy individuals to investigate whether differences in midbrain D3R availability are associated with functional interactions between large-scale networks and regions involved in reward processing and cognition. High midbrain D3R availability was associated with reduced functional connectivity between orbitofrontal cortex (OFC) and networks implicated in cognitive control and salience processing. The opposite pattern was observed in subcortical reward circuitry and the "default mode" network, which showed greater connectivity with OFC in individuals with high D3R availability. These findings demonstrate that differential interactions between OFC and networks implicated in cognitive control and reward are associated with midbrain D3R availability, consistent with the hypothesis that dopamine D3R signaling is an important molecular pathway underlying goal-directed behavior.

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.

Rats prone to attribute incentive salience to reward cues are also prone to impulsive action

Animals vary considerably in the degree to which they attribute incentive salience to cues predictive of reward. When a discrete cue (conditional stimulus) is repeatedly paired with delivery of a food reward (unconditional stimulus) only some rats ("sign-trackers"; STs) come to find the cue itself an attractive and desirable incentive stimulus. For other rats ("goal-trackers"; GTs) the cue is an effective conditional stimulus - it evokes a conditional response - but it is less attractive and less desirable. Given that STs have particular difficulty resisting reward cues, and are thought to have poor inhibitory control over their behavior, we hypothesized that they may also be more impulsive. There are, however, multiple forms of impulsivity; therefore, we compared STs and GTs on two tests of so-called impulsive action - a 2-choice serial reaction time task and a differential reinforcement of low rates of responding task, and one test of impulsive choice - a delay discounting choice procedure. We found that relative to GTs, STs were more impulsive on the two tests of impulsive action, but not on the test of impulsive choice. We speculate that when these two traits combine, that is, when an individual is not only prone to attribute incentive salience to reward cues but also prone to impulsive action, they may be especially susceptible to impulse control disorders, including addiction.

μ-Opioid receptors in the nucleus accumbens shell region mediate the effects of amphetamine on inhibitory control but not impulsive choice

Acute challenges with psychostimulants such as amphetamine affect impulsive behavior in both animals and humans. With regard to amphetamine, it is important to unravel how this drug affects impulsivity since it is not only a widely abused recreational drug but also regularly prescribed to ameliorate maladaptive impulsivity. Therefore, we studied the effects of amphetamine in two rat models of impulsivity, the five-choice serial reaction time task and the delayed-reward task, providing measures of inhibitory control and impulsive choice, respectively. We focused on the role of opioid receptor activation in amphetamine-induced impulsivity as there is ample evidence indicating an important role for endogenous opioids in several behavioral and neurochemical effects of amphetamine. Results showed that amphetamine-induced inhibitory control deficits were dose-dependently attenuated by the preferential μ-opioid receptor antagonist naloxone, but not by the selective δ-opioid receptor antagonist naltrindole or κ-opioid receptor antagonist nor-BNI (nor-binaltorphimine dihydrochloride). In contrast, naloxone did not affect amphetamine-induced improvements in impulsive decision making. Naloxone also completely prevented inhibitory control deficits induced by GBR 12909 [1-(2-[bis(4-fluorophenyl)methoxy] ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride], a selective dopamine transporter inhibitor. Intracranial infusions of naloxone, the selective μ-opioid receptor antagonist CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)), morphine, and the selective μ-opioid receptor agonist DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin acetate salt) revealed that μ-opioid receptor activation in the shell rather than the core subregion of the nucleus accumbens (NAc) modulates inhibitory control and subserves the effect of amphetamine thereon. Together, these results indicate an important role for NAc shell μ-opioid receptors in the regulation of inhibitory control, probably via an interaction between these receptors and the mesolimbic dopamine system.