Effect of an acute d-amphetamine administration on context information memory in healthy volunteers: evidence from a source memory task

Systematic reviews of the acute effects of amphetamine on working memory and other cognitive performances in healthy individuals, with a focus on the potential influence of personality traits

OBJECTIVES

This research aimed to systematically review the acute effects of amphetamine (AMP), a dopamine-releasing agent, on working memory (WM) and other cognitive performances. The investigation also aimed to review the impact of personality traits on the subjective and objective effects of AMP and possible links between personality traits and effects of AMP.

METHODS

Previous double-blind controlled studies assessing the main effects of AMP on WM and other cognitive performances in healthy volunteers were systematically reviewed. An electronic search was performed in the PUBMED and SCOPUS databases. Narrative reviews of the influence of personality traits on the subjective and objective effects of AMP were included.

RESULTS

Nineteen WM studies were included in the current review. Seven studies found effects of AMP on spatial WM, but only one study found the effect of AMP on verbal WM. Thirty-seven independent studies on other aspects of cognitive performance were identified. Twenty-two reported effects of AMP on cognitive functions. Studies also showed that personality traits are associated with the subjective effects of AMP. However, few studies reported the impacts of personality traits on the objective (such as WM) effects of AMP.

CONCLUSION

Overall, findings indicate that AMP has mixed-effects on spatial WM and other cognitive functions, but it lacks effects on verbal WM. Although there are insufficient studies on objective measures, studies also indicated that the subjective effects of AMP administration are linked to between-person variations in personality traits.

Retrieval practice improves item memory but not source memory in the context of stress

Smith, Floerke, and Thomas (2016) demonstrated that learning by repeated testing, or retrieval practice, reduced stress-related memory impairment when compared to learning by repeatedly studying material. In the present experiment, we tested whether, relative to study practice, retrieval practice would improve post-stress memory by increasing access to both item and source information. Participants learned two wordlists, which were temporally segregated to facilitate distinction between the two lists. Participants returned one week later for stress induction and two memory tests. Each test featured a recognition test that was given to assess item memory accessibility, and a list-discrimination task that was given to assess source memory. Relative to study practice, successful retrieval practice during learning reduced false alarms but did not improve source memory on the post-stress test. Results are discussed as they relate to current theories surrounding stress effects and retrieval practice effects.

Prescription Stimulants' Effects on Healthy Inhibitory Control, Working Memory, and Episodic Memory: A Meta-analysis

The use of prescription stimulants to enhance healthy cognition has significant social, ethical, and public health implications. The large number of enhancement users across various ages and occupations emphasizes the importance of examining these drugs' efficacy in a nonclinical sample. The present meta-analysis was conducted to estimate the magnitude of the effects of methylphenidate and amphetamine on cognitive functions central to academic and occupational functioning, including inhibitory control, working memory, short-term episodic memory, and delayed episodic memory. In addition, we examined the evidence for publication bias. Forty-eight studies (total of 1,409 participants) were included in the analyses. We found evidence for small but significant stimulant enhancement effects on inhibitory control and short-term episodic memory. Small effects on working memory reached significance, based on one of our two analytical approaches. Effects on delayed episodic memory were medium in size. However, because the effects on long-term and working memory were qualified by evidence for publication bias, we conclude that the effect of amphetamine and methylphenidate on the examined facets of healthy cognition is probably modest overall. In some situations, a small advantage may be valuable, although it is also possible that healthy users resort to stimulants to enhance their energy and motivation more than their cognition.

Declarative memory

Declarative Memory consists of memory for events (episodic memory) and facts (semantic memory). Methods to test declarative memory are key in investigating effects of potential cognition-enhancing substances--medicinal drugs or nutrients. A number of cognitive performance tests assessing declarative episodic memory tapping verbal learning, logical memory, pattern recognition memory, and paired associates learning are described. These tests have been used as outcome variables in 34 studies in humans that have been described in the literature in the past 10 years. Also, the use of episodic tests in animal research is discussed also in relation to the drug effects in these tasks. The results show that nutritional supplementation of polyunsaturated fatty acids has been investigated most abundantly and, in a number of cases, but not all, show indications of positive effects on declarative memory, more so in elderly than in young subjects. Studies investigating effects of registered anti-Alzheimer drugs, cholinesterase inhibitors in mild cognitive impairment, show positive and negative effects on declarative memory. Studies mainly carried out in healthy volunteers investigating the effects of acute dopamine stimulation indicate enhanced memory consolidation as manifested specifically by better delayed recall, especially at time points long after learning and more so when drug is administered after learning and if word lists are longer. The animal studies reveal a different picture with respect to the effects of different drugs on memory performance. This suggests that at least for episodic memory tasks, the translational value is rather poor. For the human studies, detailed parameters of the compositions of word lists for declarative memory tests are discussed and it is concluded that tailored adaptations of tests to fit the hypothesis under study, rather than "off-the-shelf" use of existing tests, are recommended.

Influence of methylphenidate treatment assumptions on cognitive function in healthy young adults in a double-blind, placebo-controlled trial

Background

Increasing numbers of students use stimulants such as methylphenidate (MPH) to improve their study capacity, making them prone to subsequent prolonged drug abuse. This study explored the cognitive effects of MPH in students who either assumed they received MPH or assumed they received a placebo.

Methods

In a double-blind, randomized, placebo-controlled trial with a between-subjects design, 21 students were subjected to partial sleep deprivation, receiving no more than 4 hours sleep the night before they were tested. In the morning, they were given either a placebo or 20 mg of MPH. They then performed free recall verbal tests and Go/No-Go tasks repeatedly, their moods were evaluated using Profile of Mood States and their tiredness was assessed using a visual analog scale, with evaluation of vigilance.

Results

No significant differences were found between those subjects who received MPH and those who received a placebo. However, significant differences were found between subjects who assumed they had received MPH or had no opinion, and those who assumed they had received a placebo. At three minutes, one hour, and one day after memorizing ten lists of 20 words, those who assumed they had received MPH recalled 54%, 58%, and 54% of the words, respectively, whereas those who assumed they had received placebo only recalled 35%, 37%, and 34%.

Conclusion

Healthy, partially sleep-deprived young students who assume they have received 20 mg of MPH experience a substantial placebo effect that improves consolidation of information into long-term memory. This is independent of any pharmacologic effects of MPH, which had no significant effects on verbal memory in this study. This information may be used to dissuade students from taking stimulants such as MPH during examination periods, thus avoiding subsequent abuse and addiction.

Cognitive enhancement drug use among future physicians: findings from a multi-institutional census of medical students

BACKGROUND

Nonmedical use of prescription psychostimulants such as methylphenidate and amphetamine salts for the purpose of cognitive enhancement is a growing trend, particularly in educational environments. To our knowledge, no recent studies have evaluated the use of these psychostimulants in a medical academic setting.

OBJECTIVE

To conduct an online census of psychostimulant use among medical students.

DESIGN

In 2011, we conducted a multi-institutional census using a 31-48 item online survey regarding use of prescription psychostimulants.

PARTICIPANTS

2,732 actively enrolled medical students at four private and public medical schools in the greater Chicago area.

MAIN MEASURES

Prevalence and correlates of psychostimulant use

KEY RESULTS

1,115 (41 %) of students responded to the web-based questionnaire (range 26-47 % among schools). On average, students were 25.1 years of age (SD = 2.7, range 20-49), and single (70 %). Overall, 18 % (198/1,115) of this medical student sample had used prescription psychostimulants at least once in their lifetime, with first use most often in college. Of these, 11 % (117/1,115) of students reported use during medical school (range 7-16 % among schools). Psychostimulant use was significantly correlated with use of barbiturates, ecstasy, and tranquilizers (Pearson's correlation r > 0.5, Student's t-test p < 0.01); male gender (21 % male versus 15 % female, Chi squared p = 0.007); and training at a medical school which by student self-report determined class rank (68 % versus 51 %, Chi-squared p = 0.018). Non-users were more likely to be first year students (Chi-squared p = 0.048) or to have grown up outside of the United States (Chi-squared p = 0.013).

CONCLUSIONS

Use of psychostimulants, including use without a prescription, is common among medical students. Further study of the side effects, medical implications, and use during post-graduate medical training and medical practice is needed to inform evidence-based policy.

Source discrimination in adults with attention deficit hyperactivity disorder

Objectives

The context of memory experiences is referred to as source memory and can be distinguished from the content of episodic item memory. Source memory represents a crucial part of biographic events and elaborate memory experiences. Whereas individuals with attention deficit hyperactivity disorder (ADHD) were shown to have inefficient item memory, little is known about the context of memory experiences.

Methods

The present study compared 37 adult patients with a diagnosed ADHD with 40 matched healthy participants on a word list paradigm. Memory functions of encoding, retention and source discrimination were assessed. Furthermore, standardized measures of memory and executive control were applied in order to explore a qualitative differentiation of memory components.

Results

Adult patients with ADHD showed impaired performance in encoding of new information whereas the retention of encoded items was found to be preserved. The most pronounced impairment of patients with ADHD was observed in source discrimination. Regression models of cognitive functions on memory components supported some qualitative differentiation.

Conclusions

Data analysis suggests a differential pattern of memory impairment in adults suffering from ADHD with a particular deficit in source discrimination. Inefficient source discrimination in adults with ADHD can affect daily functioning by limiting biographic awareness and disturbing general cognitive processes.

The atypical stimulant and nootropic modafinil interacts with the dopamine transporter in a different manner than classical cocaine-like inhibitors

Modafinil is a mild psychostimulant with pro-cognitive and antidepressant effects. Unlike many conventional stimulants, modafinil has little appreciable potential for abuse, making it a promising therapeutic agent for cocaine addiction. The chief molecular target of modafinil is the dopamine transporter (DAT); however, the mechanistic details underlying modafinil's unique effects remain unknown. Recent studies suggest that the conformational effects of a given DAT ligand influence the magnitude of the ligand's reinforcing properties. For example, the atypical DAT inhibitors benztropine and GBR12909 do not share cocaine's notorious addictive liability, despite having greater binding affinity. Here, we show that the binding mechanism of modafinil is different than cocaine and similar to other atypical inhibitors. We previously established two mutations (W84L and D313N) that increase the likelihood that the DAT will adopt an outward-facing conformational state—these mutations increase the affinity of cocaine-like inhibitors considerably, but have little or opposite effect on atypical inhibitor binding. Thus, a compound's WT/mutant affinity ratio can indicate whether the compound preferentially interacts with a more outward- or inward-facing conformational state. Modafinil displayed affinity ratios similar to those of benztropine, GBR12909 and bupropion (which lack cocaine-like effects in humans), but far different than those of cocaine, β-CFT or methylphenidate. Whereas treatment with zinc (known to stabilize an outward-facing transporter state) increased the affinity of cocaine and methylphenidate two-fold, it had little or no effect on the binding of modafinil, benztropine, bupropion or GBR12909. Additionally, computational modeling of inhibitor binding indicated that while β-CFT and methylphenidate stabilize an “open-to-out” conformation, binding of either modafinil or bupropion gives rise to a more closed conformation. Our findings highlight a mechanistic difference between modafinil and cocaine-like stimulants and further demonstrate that the conformational effects of a given DAT inhibitor influence its phenomenological effects.

Verbal memory improved by D-amphetamine: influence of the testing effect

OBJECTIVE

The improvement of long-term retention of verbal memory after an acute administration of D-amphetamine in recall and recognition tasks has been ascribed to an influence of the drug on memory consolidation. Because recent research has demonstrated that intermediate testing is of overriding importance for retention, we investigated whether D-amphetamine modulates the repeated testing effect in verbal long-term recognition.

METHOD

Forty men participated in two double blind placebo controlled studies. In Experiment 1, we manipulated the number of recognition tests and in Experiment 2, we compared repeated with nonrepeated testing of the same items.

RESULTS

Drug effects were observed on delayed tests only, leaving immediate recognition unaffected. Number of intermediate recognition tests and repeated testing of the same items were not affected by D-amphetamine.

CONCLUSIONS

We conclude that the D-amphetamine memory enhancement is not related to the testing effect. This result supports that D-amphetamine modulates other aspects of the consolidation process, probably related to context effects.