Margaret C. Wardle

Amping Up Effort: Effects of d-Amphetamine on Human Effort-Based Decision-Making

Animal studies suggest the neurotransmitter dopamine (DA) plays an important role in decision-making. In rats, DA depletion decreases tolerance for effort and probability costs, while drugs enhancing DA increase tolerance for these costs. However, data regarding the effect of DA manipulations on effort and probability costs in humans remain scarce. The current study examined acute effects of d-amphetamine, an indirect DA agonist, on willingness of healthy human volunteers to exert effort for monetary rewards at varying levels of reward value and reward probability. Based on preclinical research, we predicted amphetamine would increase exertion of effort, particularly when reward probability was low. Over three sessions, 17 healthy normal adults received placebo, d-amphetamine 10 mg, and 20 mg under counterbalanced double-blind conditions and completed the Effort Expenditure for Rewards Task. Consistent with predictions, amphetamine enhanced willingness to exert effort, particularly when reward probability was lower. Amphetamine did not alter effects of reward magnitude on willingness to exert effort. Amphetamine sped task performance, but its psychomotor effects were not strongly related to its effects on decision-making. This is the first demonstration in humans that dopaminergic manipulations alter willingness to exert effort for rewards. These findings help elucidate neurochemical substrates of choice, with implications for neuropsychiatric diseases characterized by dopaminergic dysfunction and motivational deficits.

The Acute Effects of Nicotine on Positive and Negative Affect in Adolescent Smokers

Although adolescent cigarette smoking remains a critical public health concern, little is known about the reinforcing mechanisms governing smoking in this vulnerable population. To assess predictions derived from both positive and negative reinforcement models of drug use, the authors measured the acute effects of nicotine, as administered via tobacco cigarettes, on both positive and negative affect in a group of 15- to 18-year-old smokers. A matched group of nonsmokers served as a comparison group. Findings revealed that whereas adolescents who smoked a cigarette experienced reductions in both positive and negative affect, the observed reductions in negative affect were moderated by nicotine content of the cigarette (high yield vs. denicotinized), level of nicotine dependence, level of baseline craving, and smoking expectancies pertinent to negative affect regulation. Nonsmokers experienced no change in affect over the 10-min assessment period, and no interaction effects were observed for positive affect. Overall, the findings conform to a negative reinforcement model of nicotine effects and strongly suggest that, even among young light smokers, nicotine dependence and resultant withdrawal symptomatology may serve as motivating factors governing smoking behavior.

Effects of MDMA and Intranasal Oxytocin on Social and Emotional Processing

MDMA (±3,4-methylenedioxymethamphetamine, ‘ecstasy’) is used recreationally, reportedly because it increases feelings of empathy, sociability, and interpersonal closeness. One line of evidence suggests that MDMA produces these effects by releasing oxytocin, a peptide involved in social bonding. In the current study, we investigated the acute effects of MDMA and oxytocin on social and emotional processing in healthy human volunteers. MDMA users (N=65) participated in a 4-session, within-between-subjects study in which they received oral MDMA (0.75, 1.5 mg/kg), intranasal oxytocin (20 or 40 IU), or placebo under double-blind conditions. The primary outcomes included measures of emotion recognition and sociability (desire to be with others). Cardiovascular and subjective effects were also assessed. As expected, MDMA dose-dependently increased heart rate and blood pressure and feelings of euphoria (eg, ‘High’ and ‘Like Drug’). On measures of social function, MDMA impaired recognition of angry and fearful facial expressions, and the larger dose (1.5 mg/kg) increased desire to be with others, compared with placebo. Oxytocin produced small but significant increases in feelings of sociability and enhanced recognition of sad facial expressions. Additionally, responses to oxytocin were related to responses to MDMA with subjects on two subjective measures of sociability. Thus, MDMA increased euphoria and feelings of sociability, perhaps by reducing sensitivity to subtle signs of negative emotions in others. The present findings provide only limited support for the idea that oxytocin produces the prosocial effects of MDMA.

Genome-Wide Association Study of d -Amphetamine Response in Healthy Volunteers Identifies Putative Associations, Including Cadherin 13 ( CDH13 )

Both the subjective response to d-amphetamine and the risk for amphetamine addiction are known to be heritable traits. Because subjective responses to drugs may predict drug addiction, identifying alleles that influence acute response may also provide insight into the genetic risk factors for drug abuse. We performed a Genome Wide Association Study (GWAS) for the subjective responses to amphetamine in 381 non-drug abusing healthy volunteers. Responses to amphetamine were measured using a double-blind, placebo-controlled, within-subjects design. We used sparse factor analysis to reduce the dimensionality of the data to ten factors. We identified several putative associations; the strongest was between a positive subjective drug-response factor and a SNP (rs3784943) in the 8th intron of cadherin 13 (CDH13; P = 4.58×10−8), a gene previously associated with a number of psychiatric traits including methamphetamine dependence. Additionally, we observed a putative association between a factor representing the degree of positive affect at baseline and a SNP (rs472402) in the 1st intron of steroid-5-alpha-reductase-α-polypeptide-1 (SRD5A1; P = 2.53×10−7), a gene whose protein product catalyzes the rate-limiting step in synthesis of the neurosteroid allopregnanolone. This SNP belongs to an LD-block that has been previously associated with the expression of SRD5A1 and differences in SRD5A1 enzymatic activity. The purpose of this study was to begin to explore the genetic basis of subjective responses to stimulant drugs using a GWAS approach in a modestly sized sample. Our approach provides a case study for analysis of high-dimensional intermediate pharmacogenomic phenotypes, which may be more tractable than clinical diagnoses.

Lack of Association Between COMT and Working Memory in a Population-Based Cohort of Healthy Young Adults

The Val158Met polymorphism of the catechol-O-methyltransferase (COMT) gene is an important regulator of dopamine in the prefrontal cortex, an area critical to working memory. Working memory deficits are present in several psychiatric disorders, and there is wide variation in working memory capacity in the normal population. Association studies of COMT and working memory in healthy volunteers have yielded inconsistent results, possibly because of small sample sizes. Here we examine COMT in relation to N-Back working memory task performance in a large population-based cohort of young adults. We predicted individuals with one or two copies of the Met allele would perform better, and that this relationship would be more evident in males than females. Participants (N=1857–2659) tested at 18 years of age, were enrolled in the Avon Longitudinal Study of Parents and Children (ALSPAC). We used multiple regression to examine effects of sex and COMT genotype on N-Back hits, false positives, discriminability (d’), and reaction time while controlling for important covariates. COMT genotype did not predict hits or d’. There was a nominally significant interaction between COMT and sex on false positives, but this was not in the predicted direction, and was not significant after controlling for covariates. COMT genotype was not related to working memory in this large population-based cohort. It is possible COMT is not meaningfully associated with working memory in healthy young adults, or that COMT effects are detectable only in assessments reflecting neural processes underlying cognition, such as fMRI, rather than in behavioral performance.

Iowa Gambling Task performance and emotional distress interact to predict risky sexual behavior in individuals with dual substance and HIV diagnoses

HIV+ substance-dependent individuals (SDIs) show emotional distress and executive impairment, but in isolation these poorly predict sexual risk. We hypothesized that an executive measure sensitive to emotional aspects of judgment (Iowa Gambling Task; IGT) would identify HIV+ SDIs whose sexual risks were influenced by emotional distress. We assessed emotional distress and performance on several executive tasks in 190 HIV+ SDIs. IGT performance interacted significantly with emotional distress, such that only in better performers were distress and risk related. Our results are interpreted using the somatic marker hypothesis and indicate that the IGT identifies HIV+ SDIs for whom psychological distress influences HIV risk.

Negative Affect and Addiction

Publisher Summary Once an individual has crossed the line at which withdrawal symptomatology emerges in the absence of drug self-administration, that person is deemed addicted; his/her drug use is then believed to be motivated predominantly by the need to stave off the uncomfortable symptoms of withdrawal, which almost universally—across virtually all drugs of abuse and even nonpharmacological addictive behaviors—include various forms of negative affect (NA). Noting that virtually all drug users attribute their use to its purported ability to assuage NA, this chapter reviews the empirical literature in an effort to try to address the veracity of this claim. First, it explains what comprises NA and reviews several influential theoretical models of drug addiction, all of which make a strong case for the important role played by negative affect in promoting addictive substance use. It then considers the potentially important role played by expectancies in shaping and influencing the relationship between affect and drug use. Next, it argues that a meaningful delineation of the complex relationship between NA and drug addiction is a must.

Caffeine increases psychomotor performance on the effort expenditure for rewards task

Preclinical studies suggest that cost/benefit decision-making involves interactions between adenosine and dopamine (DA). In rats, DA depletion decreases willingness to incur effort costs, while adenosine antagonism reverses these effects, likely by increasing DA transmission. Caffeine is a non-selective adenosine antagonist commonly used to facilitate effortful tasks, and thus may affect decisions involving effort costs in humans. The current study examined acute effects of 200 mg of caffeine on willingness to exert effort for monetary rewards at varying levels of reward value and reward probability, in young adult light caffeine users. Based on previous findings with amphetamine, we predicted that caffeine would increase willingness to exert effort. At separate sessions, 23 healthy normal adults received placebo or 200 mg caffeine under counterbalanced double-blind conditions, then completed the effort expenditure for rewards task (EEfRT). Measures of subjective and cardiovascular effects were obtained at regular intervals. Caffeine produced small but significant subjective and cardiovascular effects, and sped psychomotor performance on the EEfRT. Caffeine did not alter willingness to exert effort, except in high cardiovascular responders to caffeine, in whom it decreased willingness to exert effort. These results were contrary to our predictions, but consistent with rodent studies suggesting that moderate doses of caffeine alone do not affect effort, but rather only influence effort in the context of DA antagonism. Our results demonstrate that psychomotor speeding and decisional effects on the EEfRT are dissociable, providing additional evidence for the EEfRT as a specific measure of effort-based decision-making. This study provides a starting point for exploring contributions of the adenosine system to motivation in humans.

Effect of social stress during acute nicotine abstinence.

RationaleRelapse to smoking is often precipitated by stress, yet little is known about the effects of nicotine withdrawal on responses to acute stress, or whether nicotine replacement reverses withdrawal-induced changes in stress response.ObjectivesThe aim of the present study is to use an effective social stressor, the Trier Social Stress Test (TSST), to study subjective, cardiovascular and hormonal responses to stress during withdrawal, and examine whether nicotine replacement moderates responses to stress during withdrawal.MethodsForty-nine current regular smokers were randomly assigned to smoke as normal (SM), 12-h abstention with placebo patch (PL), or 12-h abstention with nicotine patch (NIC). They participated in a single session using the TSST, during which subjective affect, heart rate (HR), mean arterial blood pressure (MAP) and salivary cortisol were measured.ResultsThe TSST produced expected increases in subjective negative affect, HR, MAP, and cortisol. Groups did not differ in subjective or cardiovascular responses, but the PL group exhibited larger stress-induced increase in cortisol than the other groups.ConclusionsThe increased cortisol response might indicate a greater hormonal stress response during nicotine withdrawal. Alternatively, considering that cortisol also provides negative feedback to the stress system, and blunted cortisol responses are predictive of smoking relapse, the lower cortisol responses in the NIC and SM groups might indicate chronic dysregulation of the stress system. In this case, restoration of cortisol response by nicotine treatment to the lower levels seen during regular smoking may actually represent an undesired side effect of nicotine replacement.

Does COMT genotype influence the effects of d‐amphetamine on executive functioning?

In a widely cited study, Mattay et al. reported that amphetamine (0.25 mg/kg oral, or 17 mg for a 68 kg individual) impaired behavioral and brain indices of executive functioning, measured using the Wisconsin Card Sorting Task (WCST) and N‐Back working memory task, in 6 individuals homozygous for the met allele of the val158met polymorphism in the catechol‐O‐methyltransferase (COMT) gene, whereas it improved executive functioning in 10 individuals homozygous for the more active val allele. We attempted to replicate their behavioral findings in a larger sample, using similar executive functioning tasks and a broader range of amphetamine doses. Over four sessions, n = 200 healthy normal adults received oral placebo, d‐amphetamine 5, 10, and 20 mg (average of 0.07, 0.15 and 0.29 mg/kg), under counterbalanced double‐blind conditions and completed WCST and N‐back tests of executive functioning. Amphetamine had typical effects on blood pressure and processing speed but did not affect executive functioning. COMT genotype (val158met) was not related to executive functioning under placebo or amphetamine conditions, even when we compared only the homozygous val/val and met/met genotypes at the highest dose of amphetamine (20 mg). Thus, we were not able to replicate the behavioral interaction between COMT and amphetamine seen in Mattay et al. We discuss possible differences between the studies and the implications of our findings for the use of COMT genotyping to predict clinical responses to dopaminergic drugs, and the use of intermediate phenotypes in genetic research.