Interactions between serotonin and dopamine in the control of impulsive choice in rats: therapeutic implications for impulse control disorders

Intracranial self-stimulation as a positive reinforcer to study impulsivity in a probability discounting paradigm

Probability discounting is used to study risky decision-making in humans and rodents. In these paradigms, the subject chooses between a small reward that is always delivered and a large reward that is delivered with varying probabilities. Risk-taking is defined as a preference for the large, uncertain reward. The aversive consequence associated with this task involves choosing the large reward and not obtaining it. To study this form of impulsivity in rodents, food reinforcement is commonly used. Using this reinforcer, and the need to food-deprive rodents to enhance task performance, may be problematic in rodent models that exhibit eating disorders, in pharmacological assessments that alter feeding, and for assessments of the neurocircuitry that is engaged by both feeding and risk-taking. We reveal here that electrical intracranial self-stimulation (ICSS) can be used as the positive reinforcer in risk assessments (i.e., probability discounting). ICSS was selected as it is rapidly acquired, the operant procedures are retained for months, and no tolerance or satiety develops to the reinforcer; thus, ICSS can be used in multiple test sessions in a repeated measures design. We developed an efficient, standardized, six phase ICSS-mediated protocol that allowed for the assessments of risk-taking in a probability discounting task. We demonstrated that the discounting behavior remained stable for several weeks. The value of this protocol is discussed in terms of practical as well as theoretical advantages of using ICSS-mediated reinforcement.

From reinforcement learning models to psychiatric and neurological disorders

Over the last decade and a half, reinforcement learning models have fostered an increasingly sophisticated understanding of the functions of dopamine and cortico-basal ganglia-thalamo-cortical (CBGTC) circuits. More recently, these models, and the insights that they afford, have started to be used to understand important aspects of several psychiatric and neurological disorders that involve disturbances of the dopaminergic system and CBGTC circuits. We review this approach and its existing and potential applications to Parkinson's disease, Tourette's syndrome, attention-deficit/hyperactivity disorder, addiction, schizophrenia and preclinical animal models used to screen new antipsychotic drugs. The approach's proven explanatory and predictive power bodes well for the continued growth of computational psychiatry and computational neurology.

Emotion Knowledge, Emotion Utilization, and Emotion Regulation

This article suggests a way to circumvent some of the problems that follow from the lack of consensus on a definition of emotion (Izard, 2010; Kleinginna & Kleinginna, 1981) and emotion regulation (Cole, Martin, & Dennis, 2004) by adopting a conceptual framework based on discrete emotions theory and focusing on specific emotions. Discrete emotions theories assume that neural, affective, and cognitive processes differ across specific emotions and that each emotion has particular motivational and regulatory functions. Thus, efforts at regulation should target the specific dysregulated emotions. The positive effects of emotion regulation are more likely to be optimized when they result from or lead to emotion utilization—the constructive use of the energy of emotion arousal. Effective processes for regulation differ for basic emotions and emotion schemas. This article identifies neural systems that facilitate emotion experiences and emotion regulation processes. It considers the implications of the developmental change from basic emotions to emotion schemas, and also briefly discusses the effects of interventions on changes in emotion knowledge, emotion regulation, and social and emotional competence.

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.

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.

The roles of dopamine and serotonin in decision making: evidence from pharmacological experiments in humans

Neurophysiological experiments in primates, alongside neuropsychological and functional magnetic resonance investigations in humans, have significantly enhanced our understanding of the neural architecture of decision making. In this review, I consider the more limited database of experiments that have investigated how dopamine and serotonin activity influences the choices of human adults. These include those experiments that have involved the administration of drugs to healthy controls, experiments that have tested genotypic influences upon dopamine and serotonin function, and, finally, some of those experiments that have examined the effects of drugs on the decision making of clinical samples. Pharmacological experiments in humans are few in number and face considerable methodological challenges in terms of drug specificity, uncertainties about pre- vs post-synaptic modes of action, and interactions with baseline cognitive performance. However, the available data are broadly consistent with current computational models of dopamine function in decision making and highlight the dissociable roles of dopamine receptor systems in the learning about outcomes that underpins value-based decision making. Moreover, genotypic influences on (interacting) prefrontal and striatal dopamine activity are associated with changes in choice behavior that might be relevant to understanding exploratory behaviors and vulnerability to addictive disorders. Manipulations of serotonin in laboratory tests of decision making in human participants have provided less consistent results, but the information gathered to date indicates a role for serotonin in learning about bad decision outcomes, non-normative aspects of risk-seeking behavior, and social choices involving affiliation and notions of fairness. Finally, I suggest that the role played by serotonin in the regulation of cognitive biases, and representation of context in learning, point toward a role in the cortically mediated cognitive appraisal of reinforcers when selecting between actions, potentially accounting for its influence upon the processing salient aversive outcomes and social choice.

Nucleus accumbens neurons encode predicted and ongoing reward costs in rats

Efficient decision-making requires that animals consider both the benefits and the costs of potential actions, such as the amount of effort or temporal delay involved in reward seeking. The nucleus accumbens (NAc) has been implicated in the ability to choose between options with different costs and overcome high costs when necessary, but it is not clear how NAc processing contributes to this role. Here, neuronal activity in the rat NAc was monitored using multi-neuron electrophysiology during two cost-based decision tasks in which either reward effort or reward delay was manipulated. In each task, distinct visual cues predicted high-value (low effort/immediate) and low-value (high effort/delayed) rewards. After training, animals exhibited a behavioral preference for high-value rewards, yet overcame high costs when necessary to obtain rewards. Electrophysiological analysis indicated that a subgroup of NAc neurons exhibited phasic increases in firing rate during cue presentations. In the effort-based decision task (but not the delay-based task), this population reflected the cost-discounted value of the future response. In contrast, other subgroups of cells were activated during response initiation or reward delivery, but activity did not differ on the basis of reward cost. Finally, another population of cells exhibited sustained changes in firing rate while animals completed high-effort requirements or waited for delayed rewards. These findings are consistent with previous reports that implicate NAc function in reward prediction and behavioral allocation during reward-seeking behavior, and suggest a mechanism by which NAc activity contributes to both cost-based decisions and actual cost expenditure.

Serotonin: imaging findings in eating disorders

Anorexia nervosa (AN) and bulimia nervosa (BN) are disorders characterized by aberrant patterns of feeding behavior, weight regulation, and disturbances in attitudes and perceptions toward body weight and shape. Several lines of evidence nominate disturbances of serotonin (5-HT) pathways as playing a role in the pathogenesis and pathophysiology of AN and BN. For example, 5-HT pathways are known to contribute to the modulation of a range of behaviors commonly seen in individuals with AN and BN. New technology using brain imaging with radioligands offers the potential for understanding previously inaccessible brain 5-HT neurotransmitter function and its dynamic relationship with human behaviors. Recent studies using positron emission tomography and single photon emission computed tomography with 5-HT-specific radioligands have consistently shown 5-HT(1A) and 5-HT(2A) receptor and 5-HT transporter alterations in AN and BN in cortical and limbic structures, which may be related to anxiety, behavioral inhibition, and body image distortions. These disturbances are present when subjects are ill and persist after recovery, suggesting that these may be traits that are independent of the state of the illness. Effective treatments for AN and BN have been elusive. A better understanding of neurobiology is likely to be important for developing specific and more powerful therapies for these often chronic and deadly disorders.

Effects of acute and repeated nicotine administration on delay discounting in Lewis and Fischer 344 rats

Biological differences may underlie individual differences in impulsive behavior, such as choice for a smaller, more immediate reinforcer over a larger, more delayed reinforcer. Repeated exposure to drugs of abuse may have different effects on such behavior. To evaluate the acute and repeated effects of nicotine on impulsive choice, two strains of rats that have been shown to differ in impulsive choice were tested in a delay-discounting paradigm. Eight Lewis and eight Fischer 344 rats were allowed to choose between one food pellet delivered immediately and three food pellets delivered after a delay. The delay systematically increased in blocks of trials within each session, and the delay value at which the choice for the two alternatives was equal (i.e. the indifference point) was interpolated. Effects of nicotine (0.1-1.0 mg/kg, subcutaneous) on percent choice and indifference points were determined during the acute-testing phase and during the redetermination of effects of each dose after at least 30 sessions of repeated 1.0 mg/kg nicotine exposure. The Lewis rats had shorter indifference points (i.e. made fewer larger-reinforcer choices) compared with the Fischer 344 rats. Acute nicotine administration increased the mean larger-reinforcer choices at the 0.3 mg/kg dose in the Lewis rats and at the 1.0 mg/kg dose in the Fischer 344 rats. After repeated exposure to nicotine, indifference points returned to near-baseline (predrug) levels for both the strains. Strain differences were observed in the rates of delay discounting, and nicotine may decrease the impulsive choice acutely, but this effect does not seem to be long lasting.

Activation of the central serotonergic system in response to delayed but not omitted rewards

The forebrain serotonergic system is a crucial component in the control of impulsive behaviours. However, there is no direct evidence for natural serotonin activity during behaviours for delayed rewards as opposed to immediate rewards. Herein we show that serotonin efflux is enhanced while rats perform a task that requires waiting for a delayed reward. We simultaneously measured the levels of serotonin and dopamine in the dorsal raphe nucleus using in vivo microdialysis. Rats performed a sequential food–water navigation task under three reward conditions: immediate, delayed and intermittent. During the delayed reward condition, in which the rat had to wait for up to 4 s at the reward sites, the level of serotonin was significantly higher than that during the immediate reward condition, whereas the level of dopamine did not change significantly. By contrast, during the intermittent reward condition, in which food was given on only about one-third of the site visits, the level of dopamine was lower than that during the immediate reward condition, whereas the level of serotonin did not change significantly. Dopamine efflux, but not serotonin efflux, was positively correlated with reward consumption during the task. There was no reciprocal relationship between serotonin and dopamine. This is the first direct evidence that activation of the serotonergic system occurs specifically in relation to waiting for a delayed reward.

Perinatal risk factors interacting with catechol O-methyltransferase and the serotonin transporter gene predict ASD symptoms in children with ADHD

BACKGROUND

Symptoms of autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) often co-occur. Given the previously found familiality of ASD symptoms in children with ADHD, addressing these symptoms may be useful for genetic association studies, especially for candidate gene findings that have not been consistently replicated for ADHD.

METHODS

We studied the association of the catechol O-methyltransferase (COMT) Val158Met polymorphism and the serotonin transporter (SLC6A4/SERT/5-HTT) 5-HTTLPR insertion/deletion polymorphism with ASD symptoms in children with ADHD, and whether these polymorphisms would interact with pre- and perinatal risk factors, i.e., maternal smoking during pregnancy and low birth weight. Analyses were performed using linear regression in 207 Dutch participants with combined type ADHD of the International Multicenter ADHD Genetics (IMAGE) study, and repeated in an independent ADHD sample (n =439) selected from the TRracking Adolescents' Individual Lives Survey (TRAILS). Dependent variables were the total and subscale scores of the Children's Social Behavior Questionnaire (CSBQ).

RESULTS

No significant main effects of COMT Val158Met, 5-HTTLPR, maternal smoking during pregnancy and low birth weight on ASD symptoms were found. However, the COMT Val/Val genotype interacted with maternal smoking during pregnancy in increasing stereotyped behavior in the IMAGE sample (p =.008); this interaction reached significance in the TRAILS sample after correction for confounders (p =.02). In the IMAGE sample, the 5-HTTLPR S/S genotype interacted with maternal smoking during pregnancy, increasing problems in social interaction (p =.02), and also interacted with low birth weight, increasing rigid behavior (p =.03). Findings for 5-HTTLPR in the TRAILS sample were similar, albeit for related CSBQ subscales.

CONCLUSIONS

These findings suggest gene-environment interaction effects on ASD symptoms in children with ADHD.

Differential effects of selective adenosine antagonists on the effort-related impairments induced by dopamine D1 and D2 antagonism

Mesolimbic dopamine (DA) is a critical component of the brain circuitry regulating behavioral activation and effort-related processes. Rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements, and instead select less effortful food-seeking behaviors. Previous work showed that adenosine A(2A) antagonists can reverse the effects of DA D(2) antagonists on effort-related choice. However, less is known about the effects of adenosine A(1) antagonists. Despite anatomical data showing that A(1) and D(1) receptors are co-localized on the same striatal neurons, it is uncertain if A(1) antagonists can reverse the effects DA D(1) antagonists. The present work systematically compared the ability of adenosine A(1) and A(2A) receptor antagonists to reverse the effects of DA D(1) and D(2) antagonists on a concurrent lever pressing/feeding choice task. With this procedure, rats can choose between responding on a fixed ratio 5 lever-pressing schedule for a highly preferred food (i.e. high carbohydrate pellets) vs. approaching and consuming a less preferred rodent chow. The D(1) antagonist ecopipam (0.2 mg/kg i.p.) and the D(2) antagonist eticlopride (0.08 mg/kg i.p.) altered choice behavior, reducing lever pressing and increasing lab chow intake. Co-administration of the adenosine A(1) receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.375, 0.75, and 1.5 mg/kg i.p.), and 8-cyclopentyltheophylline (CPT; 3.0, 6.0, 12.0 mg/kg i.p.) failed to reverse the effects of either the D(1) or D(2) antagonist. In contrast, the adenosine A(2A) antagonist KW-6002 (0.125, 0.25 and 0.5 mg/kg i.p.) was able to produce a robust reversal of the effects of eticlopride, as well as a mild partial reversal of the effects of ecopipam. Adenosine A(2A) and DA D(2) receptors interact to regulate effort-related choice behavior, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, fatigue or anergia that can be observed in depression and other disorders.

The effects of progesterone pretreatment on the response to oral d-amphetamine in Women

Stimulant abuse continues to be a problem, particularly for women. There is increasing preclinical and clinical evidence showing that the hormone progesterone attenuates the behavioral effects of cocaine, and this effect is primarily observed in females. The purpose of the present study was to determine if progesterone would also alter the behavioral effects of another stimulant, oral d-amphetamine (AMPH) in women. Eighteen normal non-drug abusing women completed eight outpatient sessions over two menstrual cycles. During the follicular phase of each cycle, women were administered AMPH (0, 10, 20 mg); in one cycle they were pretreated with oral micronized progesterone (200 mg) and in another cycle they were pretreated with placebo progesterone. Each session, participants completed a range of tasks including subjective measures of abuse liability, cognitive performance tasks, and behavioral measures of impulsivity and risk-taking. AMPH produced dose-related increases in positive subjective effects and these effects were enhanced by progesterone pretreatment. AMPH alone, or in combination with progesterone, had little effect on performance or behavioral measures of impulsivity. These results are in contrast with previous studies showing that progesterone attenuates the subjective response to cocaine and nicotine. Additional studies are needed to explore the modulatory role of progesterone on the effects of AMPH to determine whether progesterone has any clinical utility for AMPH abuse.

Serotonergic modulation of response inhibition and re-engagement? Results of a study in healthy human volunteers

OBJECTIVE

Cognitive functions dependent on the prefrontal cortex, such as the ability to suppress behavior (response inhibition) and initiate a new one (response re-engagement) is important in the activities of daily life. Central serotonin (5-HT) function is thought to be a critical component of these cognitive functions. In recent studies, 5-HT failed to affect stop-signal reaction time (SSRT), a fundamental process in behavioral inhibition. We were interested if response inhibition and re-engagement are influenced through central 5-HT activity as mediated via the 5-HT transporter.

METHODS

Here, using a stop-change task, we investigated the effects of acute and repeated treatment with 10 mg escitalopram, a selective 5-HT reuptake inhibitor, in 36 healthy human volunteers on response inhibition and re-engagement in a randomized, double-blind, placebo-controlled study with cross-over design.

RESULTS

Results do not show an influence of escitalopram on response inhibition or response re-engagement as we did not find differences in SSRT or change reaction time (CRT).

CONCLUSIONS

These findings support the results of previous studies suggesting that 5-HT is not critical in inhibition of already initiated responses and response re-engagement. We hypothesize that results are due to different forms of behavioral inhibition and 5-HT may critical to other forms.

External incentives and internal states guide goal-directed behavior via the differential recruitment of the nucleus accumbens and the medial prefrontal cortex

Goal-directed behavior is governed by internal physiological states and external incentives present in the environment (e.g. hunger and food). While the role of the mesocorticolimbic dopamine (DA) system in behavior guided by environmental incentives has been well studied, the effect of relevant physiological states on the function of this system is less understood. The current study examined the role of the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAcc) in the kind of food-reinforced behaviors known to be sensitive to the internal state produced by food deprivation conditions. Operant lever-press reinforced on fixed ratio 1 (FR1) and progressive ratio (PR) schedules was tested after temporary inactivation of, or DA receptor blockade in, the prelimbic mPFC or NAcc core of rats with differing levels of food deprivation (0, 12 and 36-h). Food deprivation increased PR breakpoints, as well as the number of lever-presses emitted on the FR1 schedule. Both temporary inactivation and DA blockade of NAcc reduced breakpoints across deprivation conditions, while temporary inactivation and DA blockade of mPFC reduced breakpoints only in food-deprived rats. Neither manipulation of mPFC and NAcc had any effect on behavior reinforced on the FR1 schedule. Thus, mPFC and NAcc were differentially relevant to the behaviors tested-NAcc was recruited when the behavioral cost per reinforcer was rising or high regardless of food deprivation conditions, while mPFC was recruited when food-deprived animals behaved through periods of sparse reinforcement density in order to maximize available gain.

Deep brain stimulation of the nucleus accumbens shell increases impulsive behavior and tissue levels of dopamine and serotonin

The nucleus accumbens (NAc) is gaining interest as a target for deep brain stimulation (DBS) in refractory neuropsychiatric disorders with impulsivity as core symptom. The nucleus accumbens is composed of two subterritories, core and shell, which have different anatomical connections. In animal models, it has been shown that DBS of the NAc changes impulsive action. Here, we tested the hypothesis that a change in impulsive action by DBS of the NAc is associated with changes in dopamine levels. Rats received stimulating electrodes either in the NAc core or shell, and underwent behavioral testing in a reaction time task. In addition, in a second experiment, the effect of DBS of the NAc core and shell on extracellular dopamine and serotonin levels was assessed in the NAc and medial prefrontal cortex. Control subjects received sham surgery. We have found that DBS of the NAc shell stimulation induced more impulsive action but less perseverative checking. These effects were associated with increased levels of dopamine and serotonin in the NAc, but not in the medial prefrontal cortex. DBS of the NAc core had no effect on impulsive action, but decreased perseverative responses indicative of a better impulse control. In these subjects, no effects were found on neurotransmitter levels. Our data point out that DBS of the NAc shell has negative effects on impulsive action which is accompanied by increases of dopamine and serotonin levels in the NAc, whereas DBS of the NAc core has beneficial behavioral effects.

Contributions of the orbitofrontal cortex to impulsive choice: interactions with basal levels of impulsivity, dopamine signalling, and reward-related cues

RATIONALE

Individual differences in impulsive decision-making may be critical determinants of vulnerability to impulse control disorders and substance abuse, yet little is known of their biological or behavioural basis. The orbitofrontal cortex (OFC) has been heavily implicated in the regulation of impulsive decision-making. However, lesions of the OFC in rats have both increased and decreased impulsivity in delay-discounting paradigms, where impulsive choice is defined as the selection of small immediate over larger delayed rewards.

OBJECTIVES

Reviewing the different methods used, we hypothesized that the effects of OFC inactivation on delay discounting may be critically affected by both subjects' baseline level of impulsive choice and the presence or absence of a cue to bridge the delay between selection and delivery of the large reward.

RESULTS

Here, we show that OFC inactivation increased impulsive choice in less impulsive rats when the delay was cued, but decreased impulsive choice in highly impulsive rats in an uncued condition.

CONCLUSIONS

Providing explicit environmental cues to signal the delay-to-reinforcement appears to change the way in which the OFC is recruited in the decision-making process in a baseline-dependent fashion. This change may reflect activation of the dopamine system, as intra-OFC infusions of dopamine receptor antagonists increased impulsive choice but only when the delay was cued.

The role of the nucleus accumbens core in impulsive choice, timing, and reward processing

The present series of experiments aimed to pinpoint the source of nucleus accumbens core (AcbC) effects on delay discounting. Rats were trained with an impulsive choice procedure between an adjusting smaller sooner reward and a fixed larger later reward. The AcbC-lesioned rats produced appropriate choice behavior when the reward magnitude was equal. An increase in reward magnitude resulted in a failure to increase preference for the larger later reward in the AcbC-lesioned rats, whereas a decrease in the larger later reward duration resulted in normal alterations in choice behavior in AcbC-lesioned rats. Subsequent experiments with a peak timing (Experiments 2 and 3) and a behavioral contrast (Experiment 4) indicated that the AcbC-lesioned rats suffered from decreased incentive motivation during changes in reward magnitude (Experiments 2 and 4) and when expected rewards were omitted (Experiments 2 and 3), but displayed intact anticipatory timing of reward delays (Experiments 2 and 3). The results indicate that the nucleus accumbens core is critical for determining the incentive value of rewards, but does not participate in the timing of reward delays.

Associations between the serotonin-1A receptor C(-1019)G polymorphism and disordered eating symptoms in female adolescents

The purpose of this study was to examine the relationship between the C(-1019)G polymorphism of the serotonin-1A receptor gene and eating behavior in female adolescents. A total of 204 post-menarche, adolescent women, aged 16-17 years, were recruited from two neighboring high schools in Seoul. Polymerase chain reaction (PCR) was used to isolate and examine the C(-1019)G polymorphism in the serotonin-1A receptor genes (rs6295) of all participants. The Bulimia Investigatory Test, Edinburgh (BITE) and the Eating Attitude Test-26 (EAT-26) were administered to all participants. The total score of the EAT-26 differed significantly among the three genotype groups [CC, CG, GG (F = 4.844, p = 0.009)]. Both the EAT-26 (F = 9.69, p = 0.002) and the BITE (F = 5.22, p = 0.023) scores were higher in the participants who were G allele carriers than in the non-carrier group. The dieting subscale of the EAT-26 was higher among the G allele carriers (F = 12.941, p < 0.001), and these results were maintained even after adjusting for depression and anxiety. These findings suggest that the C(-1019)G polymorphism in the 5-HT1A receptor gene is associated with disordered eating symptoms in Korean female adolescents.

Good things come to those who wait: attenuated discounting of delayed rewards in aged Fischer 344 rats

The ability to make advantageous choices among outcomes that differ in magnitude, probability, and delay until their arrival is critical for optimal survival and well-being across the lifespan. Aged individuals are often characterized as less impulsive in their choices than their young adult counterparts, demonstrating an increased ability to forgo immediate in favor of delayed (and often more beneficial) rewards. Such "wisdom" is usually characterized as a consequence of learning and life experience. However, aging is also associated with prefrontal cortical dysfunction and concomitant impairments in advantageous choice behavior. Animal models afford the opportunity to isolate the effects of biological aging on decision-making from experiential factors. To model one critical component of decision-making, young adult and aged Fischer 344 rats were trained on a two-choice delay discounting task in which one choice provided immediate delivery of a small reward and the other provided a large reward delivered after a variable delay period. Whereas young adult rats showed a characteristic pattern of choice behavior (choosing the large reward at short delays and shifting preference to the small reward as delays increased), aged rats maintained a preference for the large reward at all delays (i.e., attenuated "discounting" of delayed rewards). This increased preference for the large reward in aged rats was not due to perceptual, motor, or motivational factors. The data strongly suggest that, independent of life experience, there are underlying neurobiological factors that contribute to age-related changes in decision-making, and particularly the ability to delay gratification.

Understanding the construct of impulsivity and its relationship to alcohol use disorders

There are well-established links between impulsivity and alcohol use in humans and other model organisms; however, the etiological nature of these associations remains unclear. This is likely due, in part, to the heterogeneous nature of the construct of impulsivity. Many different measures of impulsivity have been employed in human studies, using both questionnaire and laboratory-based tasks. Animal studies also use multiple tasks to assess the construct of impulsivity. In both human and animal studies, different measures of impulsivity often show little correlation and are differentially related to outcome, suggesting that the impulsivity construct may actually consist of a number of more homogeneous (and potentially more meaningful) subfacets. Here, we provide an overview of the different measures of impulsivity used across human and animal studies, evidence that the construct of impulsivity may be better studied in the context of more meaningful subfacets, and recommendations for how research in this direction may provide for better consilience between human and animal studies of the connection between impulsivity and alcohol use.

Enhancement of spatial reversal learning by 5-HT2C receptor antagonism is neuroanatomically specific

We have recently demonstrated that systemic administration of 5-HT(2C) and 5-HT(2A) receptor antagonists significantly enhanced and impaired spatial reversal learning, respectively. In this study, the role of 5-HT(2C) and 5-HT(2A) receptor subtypes in the mediation of these opposing effects was further investigated with respect to neuroanatomical specificity. The roles of 5-HT(2C) and 5-HT(2A) receptors were examined within some of the brain regions implicated in cognitive flexibility, namely the orbitofrontal cortex (OFC), medial prefrontal cortex (mPFC), and nucleus accumbens (NAc), by means of targeted infusions of selective 5-HT(2C) and 5-HT(2A) receptor antagonists (SB 242084 and M100907, respectively). Intra-OFC 5-HT(2C) receptor antagonism produced dose-dependent effects similar to those of systemic administration, i.e., improved spatial reversal learning by reducing the number of trials (all doses: 0.1, 0.3, and 1.0 microg) and perseverative errors to criterion (0.3 and 1.0 microg) compared with controls. However, the highest dose (1.0 microg) showed a nonselective effect, as it also affected retention preceding the reversal phase and decreased learning errors. Intracerebral infusions of SB 242084 into the mPFC or NAc produced no significant effects on any behavioral measures. Similarly, no significant differences were observed with intra-OFC, -mPFC, or -NAc infusions of M100907. These data suggest that the improved performance in reversal learning observed after 5-HT(2C) receptor antagonism is mediated within the OFC. These data also bear on the issue of whether 5-HT(2C) receptor antagonism within the OFC might help elucidate the biological substrate of obsessive-compulsive disorder, offering the potential for therapeutic application.

Interaction of SLC6A4 and DRD2 polymorphisms is associated with a history of delirium tremens

Several genetic polymorphisms have been reported to be associated with alcohol withdrawal seizures (AWS) and delirium tremens (DT). To replicate and further explore these findings, we investigated the effects of 12 previously reported candidate genetic variations in two groups of alcohol-dependent European Americans with a history of withdrawal, which differed according to the presence (n = 112) or absence (n = 92) of AWS and/or DT. Associations of AWS and/or DT with the genomic and clinical characteristics and gene-gene interaction effects were investigated using logistic regression models. None of the polymorphisms were significantly associated with AWS/DT after correction for multiple testing. However, we found a significant interaction effect of the SLC6A4 promoter polymorphism (5-HTTLPR) and DRD2 exon 8 single nucleotide polymorphism rs6276 on AWS and/or DT history (P = 0.009), which became more significant after adjustment for lifetime maximum number of drinks consumed per 24 hours (P < 0.001). Subsequent analysis revealed an even stronger association of the SLC6A4-DRD2 interaction with DT (P < 0.0001), which remained significant after Bonferroni correction. Results reveal decreased likelihood of DT in alcoholics that carry the DRD2 rs6276 G allele and SLC6A4 LL genotype. This study provides the first evidence to implicate the interaction between serotonin and dopamine neurotransmission in the etiology of DT. Replication is necessary to verify this potentially important finding.

Impulsive choice and response in dopamine agonist-related impulse control behaviors

RATIONALE

Dopaminergic medication-related impulse control disorders (ICDs) such as pathological gambling and compulsive shopping have been reported in Parkinson's disease (PD).

HYPOTHESIS

We hypothesized that dopamine agonists (DAs) would be associated with greater impulsive choice or greater discounting of delayed rewards in PD patients with ICDs (PDI).

METHODS

Fourteen PDI patients, 14 PD controls without ICDs, and 16 medication-free matched normal controls were tested on the Experiential Discounting Task (EDT), a feedback-based intertemporal choice task, spatial working memory, and attentional set shifting. The EDT was used to assess choice impulsivity (hyperbolic K value), reaction time (RT), and decision conflict RT (the RT difference between high conflict and low conflict choices). PDI patients and PD controls were tested on and off DA.

RESULTS

On the EDT, there was a group by medication interaction effect [F(1,26) = 5.62; p = 0.03] with pairwise analyses demonstrating that DA status was associated with increased impulsive choice in PDI patients (p = 0.02) but not in PD controls (p = 0.37). PDI patients also had faster RT compared to PD controls [F(1,26) = 7.51, p = 0.01]. DA status was associated with shorter RT [F(3,24) = 8.39, p = 0.001] and decision conflict RT [F(1,26) = 6.16, p = 0.02] in PDI patients but not in PD controls. There were no correlations between different measures of impulsivity. PDI patients on DA had greater spatial working memory impairments compared to PD controls on DA (t = 2.13, df = 26, p = 0.04).

CONCLUSION

Greater impulsive choice, faster RT, faster decision conflict RT, and executive dysfunction may contribute to ICDs in PD.

Prefrontal cortical contribution to risk-based decision making

Damage to various regions of the prefrontal cortex (PFC) impairs decision making involving evaluations about risks and rewards. However, the specific contributions that different PFC subregions make to risk-based decision making are unclear. We investigated the effects of reversible inactivation of 4 subregions of the rat PFC (prelimbic medial PFC, orbitofrontal cortex [OFC], anterior cingulate, and insular cortex) on probabilistic (or risk) discounting. Rats were well trained to choose between either a "Small/Certain" lever that always delivered 1 food pellet, or another, "Large/Risky" lever, which delivered 4 pellets, but the probability of receiving reward decreased across 4 trial blocks (100%, 50%, 25%, and 12.5%). Infusions of gama-aminobutyric acid agonists muscimol/baclofen into the medial PFC increased risky choice. However, similar medial PFC inactivations decreased risky choice when the Large/Risky reward probability increased over a session. OFC inactivation increased response latencies in the latter trial blocks without affecting choice. Anterior cingulate or insular inactivations were without effect. The effects of prelimbic inactivations were not attributable to disruptions in response flexibility or judgments about the relative value of probabilistic rewards. Thus, the prelimbic, but not other PFC regions, plays a critical role in risk discounting, integrating information about changing reward probabilities to update value representations that facilitate efficient decision making.

Positron emission tomography measures of endogenous opioid neurotransmission and impulsiveness traits in humans

CONTEXT

The endogenous opioid system and opioid mu receptors (mu-receptors) are known to interface environmental events, positive (eg, relevant emotional stimuli) and negative (eg, stressors), with pertinent behavioral responses and to regulate motivated behavior.

OBJECTIVE

To examine the degree to which trait impulsiveness (the tendency to act on cravings and urges rather than to delay gratification) is predicted by baseline mu-receptor availability or the response of this system to a standardized, experientially matched stressor.

DESIGN, SETTING, AND PATIENTS

Nineteen young healthy male volunteers completed a personality questionnaire (NEO Personality Inventory, Revised) and underwent positron emission tomography scans with the mu-receptor-selective radiotracer carfentanil labeled with carbon 11. Measures of receptor concentrations were obtained at rest and during receipt of an experimentally maintained pain stressor of matched intensity between subjects.

MAIN OUTCOME MEASURES

Baseline receptor levels and stress-induced activation of mu-opioid system neurotransmission compared between subjects scoring above and below the population median on the NEO Personality Inventory, Revised, impulsiveness subscale and the orthogonal dimension (deliberation) expected to interact with it.

RESULTS

High impulsiveness and low deliberation scores were associated with significantly higher regional mu-receptor concentrations and greater stress-induced endogenous opioid system activation. Effects were obtained in the prefrontal and orbitofrontal cortices, anterior cingulate, thalamus, nucleus accumbens, and basolateral amygdala-all regions involved in motivated behavior and the effects of drugs of abuse. Availability of the mu-receptor and the magnitude of stress-induced endogenous opioid activation in these regions accounted for 17% to 49% of the variance in these personality traits.

CONCLUSIONS

Individual differences in the function of the endogenous mu-receptor system predict personality traits that confer vulnerability to or resiliency against risky behaviors such as the predisposition to develop substance use disorders. These personality traits are also implicated in psychopathological states (eg, personality disorders) in which variations in the function of this neurotransmitter system also may play a role.

DeltaFosB induction in orbitofrontal cortex potentiates locomotor sensitization despite attenuating the cognitive dysfunction caused by cocaine

The effects of addictive drugs change with repeated use: many individuals become tolerant of their pleasurable effects but also more sensitive to negative sequelae (e.g., anxiety, paranoia, and drug craving). Understanding the mechanisms underlying such tolerance and sensitization may provide valuable insight into the basis of drug dependency and addiction. We have recently shown that chronic cocaine administration reduces the ability of an acute injection of cocaine to affect impulsivity in rats. However, animals become more impulsive during withdrawal from cocaine self-administration. We have also shown that chronic administration of cocaine increases expression of the transcription factor DeltaFosB in the orbitofrontal cortex (OFC). Mimicking this drug-induced elevation in OFC DeltaFosB through viral-mediated gene transfer mimics these behavioural changes: DeltaFosB over-expression in OFC induces tolerance to the effects of an acute cocaine challenge but sensitizes rats to the cognitive sequelae of withdrawal. Here we report novel data demonstrating that increasing DeltaFosB in the OFC also sensitizes animals to the locomotor-stimulant properties of cocaine. Analysis of nucleus accumbens tissue taken from rats over-expressing DeltaFosB in the OFC and treated chronically with saline or cocaine does not provide support for the hypothesis that increasing OFC DeltaFosB potentiates sensitization via the nucleus accumbens. These data suggest that both tolerance and sensitization to cocaine's many effects, although seemingly opposing processes, can be induced in parallel via the same biological mechanism within the same brain region, and that drug-induced changes in gene expression within the OFC play an important role in multiple aspects of addiction.

Acute effects of morphine on distinct forms of impulsive behavior in rats

RATIONALE

Disturbances in impulse control are key features of substance abuse disorders, and conversely, many drugs of abuse are known to elicit impulsive behavior both clinically and preclinically. To date, little is known with respect to the involvement of the opioid system in impulsive behavior, although recent findings have demonstrated its involvement in delay discounting processes. The aim of the present study was to further investigate the role of the opioid system in varieties of impulsivity.

MATERIALS AND METHODS

To this end, groups of rats were trained in the five-choice serial reaction time task (5-CSRTT) and stop-signal task (SST), operant paradigms that provide measures of inhibitory control and response inhibition, respectively. In addition, another group of rats was trained in the delayed reward paradigm, which measures the sensitivity towards delay of gratification and as such assesses impulsive choice.

RESULTS AND DISCUSSION

Results demonstrated that morphine, a selective micro-opioid receptor agonist, primarily impaired inhibitory control in the 5-CSRTT by increasing premature responding. In addition, in keeping with previous data, morphine decreased the preference for the large over small reward in the delayed reward paradigm. The effects of morphine on measures of impulsivity in both the 5-CSRTT and delayed reward paradigm were blocked by naloxone, a micro-opioid receptor antagonist. Naloxone by itself did not alter impulsive behavior, suggesting limited involvement of an endogenous opioid tone in impulsivity. Response inhibition measured in the SST was neither altered by morphine nor naloxone, although some baseline-dependent effects of morphine on response inhibition were observed.

CONCLUSION

In conclusion, the present data demonstrate that acute challenges with morphine modulate distinct forms of impulsive behavior, thereby suggesting a role for the opioid system in impulsivity.

Perturbations in different forms of cost/benefit decision making induced by repeated amphetamine exposure

RATIONALE

Psychostimulant abuse has been linked to impairments in cost-benefit decision making.

OBJECTIVE

We assessed the effects of repeated amphetamine (AMPH) treatment in rodents on two distinct forms of decision making.

MATERIALS AND METHODS

Separate groups of rats were trained for 26 days on either a probabilistic (risk) or effort-discounting task, each consisting of four discrete blocks of ten choice trials. One lever always delivered a smaller reward (one or two pellets), whereas another lever delivered a four-pellet reward. For risk-discounting, the probability of receiving the larger reward decreased across trial blocks (100-12.5%), whereas on the effort task, four pellets could be obtained after a ratio of presses that increased across blocks (2-20). After training, rats received 15 saline or AMPH injections (escalating from 1 to 5 mg/kg) and were then retested during acute and long-term withdrawal.

RESULTS

Repeated AMPH administration increased risky choice 2-3 weeks after drug exposure, whereas these treatments did not alter effort-based decision making in a separate group of animals. However, prior AMPH exposure sensitized the effects of acute AMPH on both forms of decision making, whereby lower doses were effective at inducing "risky" and "lazy" patterns of choice.

CONCLUSIONS

Repeated AMPH exposure leads to relatively long-lasting increases in risky choice, as well as sensitization to the effects of acute AMPH on different forms of cost/benefit decision making. These findings suggest that maladaptive decision-making processes exhibited by psychostimulant abusers may be caused in part by repeated drug exposure.

Gender differences in delay-discounting under mild food restriction

Impulsivity, a core symptom of attention-deficit hyperactivity disorder (ADHD), is tested in animal models by delay-discounting tasks. So far, mainly male subjects have been used in this paradigm at severe levels of food restriction. Here we studied the impulsive behaviour of CD-1 adult male and female mice at mild levels of food restriction. Mice maintained at 90 +/- 5% of ad libitum bodyweight, were tested in operant chambers provided with nose-poking holes. Nose poking in one hole resulted in the immediate delivery of one food pellet (small-soon, SS), whereas nose poking in the other hole delivered five food pellets after a delay (large-late, LL), which was increased progressively each day (0-150 s). Two subgroups emerged: individuals that shifted at short delays ("steep") and individuals that did not shift, even at the highest delays ("flat"). Analysis showed that "steep" females shifted at shorter delays than "steep" males, while no difference existed between males and females within the "flat" sub-population. In home-cage circadian activity as well as in a novelty-seeking test, females were more active than males. It can be concluded from these results that female mice are more impulsive than male mice under mild food restriction. This is in contrast with findings in earlier studies with more severe food restriction. Therefore, an alternative explanation is that females are more explorative, and that different features might be tested in delay-discounting paradigms, depending on restriction levels.

Comparing the role of the anterior cingulate cortex and 6-hydroxydopamine nucleus accumbens lesions on operant effort-based decision making

Both the anterior cingulate cortex (ACC) and mesolimbic dopamine, particularly in the nucleus accumbens (NAc), have been implicated in allowing an animal to overcome effort constraints to obtain greater benefits. However, their exact contribution to such decisions has, to date, never been directly compared. To investigate this issue we tested rats on an operant effort-related cost-benefit decision-making task where animals selected between two response alternatives, one of which involved investing effort by lever pressing on a high fixed-ratio (FR) schedule to gain high reward [four food pellets (HR)], whereas the other led to a small amount of food on an FR schedule entailing less energetic cost [two food pellets, low reward (LR)]. All animals initially preferred to put in work to gain the HR. Systemic administration of a D2 antagonist caused a significant switch in choices towards the LR option. Similarly, post-operatively, excitotoxic ACC lesions caused a significant bias away from HR choices compared with sham-lesioned animals. There was no slowing in the speed of lever pressing and no correlation between time to complete the FR requirement and choice performance. Unexpectedly, no such alteration in choice allocation was observed in animals following 6-hydroxydopamine NAc lesions. However, these rats were consistently slower to initiate responding when cued to commence each trial and also showed a reduction in food hoarding on a species-typical foraging task. Taken together, this implies that only ACC lesions, and not 6-hydroxydopamine NAc lesions as performed here, cause a bias away from investing effort for greater reward when choosing between competing options

Cortico-limbic-striatal circuits subserving different forms of cost-benefit decision making

Research on the neural basis that underlies decision making in humans has revealed that these processes are mediated by distributed neural networks that incorporate different regions of the frontal lobes, the amygdala, the ventral striatum, and the dopamine system. In the present article, we review recent studies in rodents investigating the contribution of these systems to different forms of cost-benefit decision making and focus on evaluations related to delays, effort, or risks associated with certain rewards. Anatomically distinct regions of the medial and orbital prefrontal cortex make dissociable contributions to different forms of decision making, although lesions of these regions can induce variable effects, depending on the type of tasks used to assess these functions. The basolateral amygdala and the nucleus accumbens play a more fundamental role in these evaluations, helping an organism overcome different costs to obtain better rewards. Dopamine activity biases behavior toward more costly yet larger rewards, although abnormal increases in dopamine transmission can exert opposing actions on different types of decision making. The fact that similar neural circuits are recruited to solve these types of problems in both humans and animals suggests that animal models of decision making will prove useful in elucidating the mechanisms mediating these processes.

Dopaminergic modulation of risk-based decision making

Psychopharmacological studies have implicated the mesolimbic dopamine (DA) system in the mediation of cost/benefit evaluations about delay or effort-related costs associated with larger rewards. However, the role of DA in risk-based decision making remains relatively unexplored. The present study investigated the effects of systemic manipulations of DA transmission on risky choice using a probabilistic discounting task. Over discrete trials, rats chose between two levers; a press on the 'small/certain' lever always delivered one reward pellet, whereas a press on the other, 'large/risky' lever delivered four pellets, but the probability of receiving reward decreased across the four trial blocks (100, 50, 25, 12.5%). In separate groups of well-trained rats we assessed the effects of the DA releaser amphetamine, as well as receptor selective agonists and antagonists. Amphetamine consistently increased preference for the large/risky lever; an effect that was blocked or attenuated by co-administration of either D(1) (SCH23390) or D(2) (eticlopride) receptor antagonists. Blockade of either of these receptors alone induced risk aversion. Conversely, stimulation of D(1) (SKF81297) or D(2) (bromocriptine) receptors also increased risky choice. In contrast, activation of D(3) receptors with PD128,907 reduced choice of the large/risky lever. Likewise, D(3) antagonism with nafadotride potentiated the amphetamine-induced increase in risky choice. Blockade or stimulation of D(4) receptors did not reliably alter behavior. These findings indicate that DA has a critical role in mediating risk-based decision making, with increased activation of D(1) and D(2) receptors biasing choice toward larger, probabilistic rewards, whereas D(3) receptors appear to exert opposing effects on this form of decision making.

Early life social isolation alters corticotropin-releasing factor responses in adult rats

Stress induced by early life social isolation leads to long-lasting alterations in stress responses and serotonergic activity. Corticotropin-releasing factor (CRF) is a neurotransmitter that mediates stress responses and alters serotonergic activity. We tested the hypothesis that the stress of early life isolation enhances responses to CRF in adulthood by determining the effect of CRF infusions into the dorsal raphe nucleus (dRN) on 5-HT release in the nucleus accumbens (NAc) of adult rats using in vivo microdialysis. Juvenile male rats were either isolated or housed in groups of three for a 3-week period beginning on postnatal day 21 after which, all rats were group-reared for an additional 2 weeks. Following the isolation/re-socialization procedure, infusion of 100 ng CRF into the dRN decreased 5-HT release in the NAc of group-reared rats. This treatment did not significantly affect 5-HT release in the NAc of isolation-reared animals. In contrast, infusion of 500 ng CRF into the dRN transiently increased 5-HT release in the NAc of both group-reared and isolated animals with isolated animals showing a more prolonged serotonergic response. Western blot and immunofluorescent staining for CRF receptors in the dRN showed that CRF(2) receptor levels were increased in the dRN of isolation-reared animals when compared with group-reared rats. Taken together, the results suggest that isolation during the early part of development causes alterations in both CRF receptor levels and CRF-mediated serotonergic activity. These effects may underlie the increased sensitivity to stress observed in isolates.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

Neurobehavioral effects of head-only gamma-radiation exposure in rats

The present report describes initial steps in the development of an animal model for assessing the effects of low levels of radiation encountered in the space environment on human cognitive function by examining the effects of radiation on a range of neurobehavioral functions in rodents that are similar to a number of basic human cognitive functions. The present report presents baseline data on the effects of gamma radiation on neurobehavioral functions in rodents (psychomotor speed, discrimination accuracy and inhibitory control) that are similar to those in humans. Two groups of eight Long-Evans rats were trained to perform a reaction-time task that required them to depress a lever for 1-3 s and to release the lever within 1.5 s of a release stimulus (correct trial) to receive a reward. Releasing the lever prior to the release stimulus (error) terminated the trial. One group was exposed to head-only gamma radiation (5 Gy at a dose rate of 1 Gy/min), while the second group was sham-irradiated using the same anesthesia protocol. The irradiated group showed significant deficits in both performance accuracy (percentage correct scores) and performance reliability (false alarm scores) from 1 to 4 months after irradiation, indicating clear performance impairments. The increase in false alarm scores is consistent with reduced inhibitory control and a shift toward increased anticipatory responses at the cost of decreased accuracy. The nonirradiated group showed no such changes over the same period.

Nucleus accumbens adenosine A2A receptors regulate exertion of effort by acting on the ventral striatopallidal pathway

Goal-directed actions are sensitive to work-related response costs, and dopamine in nucleus accumbens is thought to modulate the exertion of effort in motivated behavior. Dopamine-rich striatal areas such as nucleus accumbens also contain high numbers of adenosine A(2A) receptors, and, for that reason, the behavioral and neurochemical effects of the adenosine A(2A) receptor agonist CGS 21680 [2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine] were investigated. Stimulation of accumbens adenosine A(2A) receptors disrupted performance of an instrumental task with high work demands (i.e., an interval lever-pressing schedule with a ratio requirement attached) but had little effect on a task with a lower work requirement. Immunohistochemical studies revealed that accumbens neurons that project to the ventral pallidum showed adenosine A(2A) receptors immunoreactivity. Moreover, activation of accumbens A(2A) receptors by local injections of CGS 21680 increased extracellular GABA levels in the ventral pallidum. Combined contralateral injections of CGS 21680 into the accumbens and the GABA(A) agonist muscimol into ventral pallidum (i.e., "disconnection" methods) also impaired response output, indicating that these structures are part of a common neural circuitry regulating the exertion of effort. Thus, accumbens adenosine A(2A) receptors appear to regulate behavioral activation and effort-related processes by modulating the activity of the ventral striatopallidal pathway. Research on the effort-related functions of these forebrain systems may lead to a greater understanding of pathological features of motivation, such as psychomotor slowing, anergia, and fatigue in depression.