Psychostimulants as cognitive enhancers: the prefrontal cortex, catecholamines, and attention-deficit/hyperactivity disorder

The impact of synapsin III gene on the neurometabolite level alterations after single-dose methylphenidate in attention-deficit hyperactivity disorder patients

OBJECTIVE

To investigate the neurometabolite level changes according to synapsin III gene rs133945G>A and rs133946C>G polymorphisms by using magnetic resonance spectroscopy (MRS) in patients with attention-deficit hyperactivity disorder (ADHD).

METHODS

Fifty-seven adults diagnosed with ADHD were recruited for the study. The participants were examined by single-voxel (1)H MRS when medication naïve and 30 minutes after oral administration of 10 mg methylphenidate (Mph). Those who had been on a stimulant discontinued the medication 48 hours before MRS imaging. Spectra were taken from the anterior cingulate cortex, dorsolateral prefrontal cortex, striatum, and cerebellum, and N-acetylaspartate (NAA), choline, and creatine levels were examined. For genotyping of the synapsin III gene polymorphisms, DNA was isolated from peripheral blood leukocytes. The effects of age, sex, and ADHD subtypes were controlled in the analyses.

RESULTS

After a single dose of Mph, choline levels increased significantly in the striatum of rs133945G>A polymorphism-GG genotypes (P=0.020) and NAA levels rose in the anterior cingulate cortex of rs133946C>G polymorphism-CG genotypes (P=0.014). Both rs133945G>A and rs133946C>G polymorphisms were found to statistically significantly affect the alteration of NAA levels in response to Mph in dorsolateral prefrontal cortex with two-way repeated measure of analysis of variance. Post hoc comparisons revealed a significant difference between CG and GG genotypes of rs133946C>G polymorphisms after Bonferroni adjustment (P=0.016).

CONCLUSION

Synapsin III gene polymorphisms may be affecting the changes in neurometabolite levels in response to Mph in adult ADHD patients. Future studies are needed to confirm our findings.

Exploring Visual Selective Attention towards Novel Stimuli in Alzheimer's Disease Patients

Background

Alzheimer's disease (AD) is associated with selective attention impairments, which could contribute to cognitive and functional deficits. Selective attention can be explored through examination of novelty preference.

Aims

In this study, we quantified novelty preference in AD patients by measuring visual scanning behaviour using an eye tracking paradigm.

Methods

Mild-to-moderate AD patients and elderly controls viewed slides containing novel and repeated images simultaneously. The outcome measure was time spent on specific images, with novelty preference defined by greater relative fixation time (RFT) on novel versus repeated images. Cognitive status (Standardized Mini-Mental State Examination, SMMSE) and attention (Digit Span, DS) were also measured.

Results

AD patients (age 79.2 ± 6.7 years, SMMSE 22.2 ± 4.0, n = 41) and controls (age 76.2 ± 6.4 years, SMMSE 28.1 ± 2.0, n = 24) were similar in age, education and sex. Compared with controls, AD patients had lower RFT on novel than on repeated images (F_1,63 = 11.18, p = 0.001). Further, reduced RFT was associated with lower scores on SMMSE (r₆₃ = 0.288, p = 0.020) and DS (r₆₃ = 0.269, p = 0.030). Within individuals, novelty preference was detected in 92.3% of patients and in 100% of controls.

Conclusion

These findings suggest that novelty preference, measured by visual scanning behaviour, can differentiate cognitively healthy and impaired people and may offer a nonverbal, less cognitively demanding method of assessing selective attention.

The Role of Dopamine and Its Dysfunction as a Consequence of Oxidative Stress

Dopamine is a neurotransmitter that is produced in the substantia nigra, ventral tegmental area, and hypothalamus of the brain. Dysfunction of the dopamine system has been implicated in different nervous system diseases. The level of dopamine transmission increases in response to any type of reward and by a large number of strongly additive drugs. The role of dopamine dysfunction as a consequence of oxidative stress is involved in health and disease. Introduce new potential targets for the development of therapeutic interventions based on antioxidant compounds. The present review focuses on the therapeutic potential of antioxidant compounds as a coadjuvant treatment to conventional neurological disorders is discussed.

Adolescent methylphenidate treatment differentially alters adult impulsivity and hyperactivity in the Spontaneously Hypertensive Rat model of ADHD

Impulsivity and hyperactivity are two facets of attention deficit/hyperactivity disorder (ADHD). Impulsivity is expressed as reduced response inhibition capacity, an executive control mechanism that prevents premature execution of an intermittently reinforced behavior. During methylphenidate treatment, impulsivity and hyperactivity are decreased in adolescents with ADHD, but there is little information concerning levels of impulsivity and hyperactivity in adulthood after adolescent methylphenidate treatment is discontinued. The current study evaluated impulsivity, hyperactivity as well as cocaine sensitization during adulthood after adolescent methylphenidate treatment was discontinued in the Spontaneously Hypertensive Rat (SHR) model of ADHD. Treatments consisted of oral methylphenidate (1.5mg/kg) or water vehicle provided Monday-Friday from postnatal days 28-55. During adulthood, impulsivity was measured in SHR and control strains (Wistar Kyoto and Wistar rats) using differential reinforcement of low rate (DRL) schedules. Locomotor activity and cocaine sensitization were measured using the open-field assay. Adult SHR exhibited decreased efficiency of reinforcement under the DRL30 schedule and greater levels of locomotor activity and cocaine sensitization compared to control strains. Compared to vehicle, methylphenidate treatment during adolescence reduced hyperactivity in adult SHR, maintained the lower efficiency of reinforcement, and increased burst responding under DRL30. Cocaine sensitization was not altered following adolescent methylphenidate in adult SHR. In conclusion, adolescent treatment with methylphenidate followed by discontinuation in adulthood had a positive benefit by reducing hyperactivity in adult SHR rats; however, increased burst responding under DRL compared to SHR given vehicle, i.e., elevated impulsivity, constituted an adverse consequence associated with increased risk for cocaine abuse liability.

Differential cognitive actions of norepinephrine a2 and a1 receptor signaling in the prefrontal cortex

The prefrontal cortex (PFC) supports cognitive and behavioral processes that guide goal directed behavior. Moreover, dysregulated prefrontal cognitive dysfunction is associated with multiple psychiatric disorders. Norepinephrine (NE) signaling in the PFC is a critical modulator of prefrontal cognition and is targeted by a variety of drugs used to treat PFC-dependent cognitive dysfunction. Noradrenergic modulation of PFC-dependent cognition is complex, with concentration and receptor-specific actions that are likely dependent on neuronal activity state. Recent studies indicate that within the PFC, noradrenergic α1 and α2 receptors exert unique modulatory actions across distinct cognitive processes that allow for context-dependent modulation of cognition. Specifically, high affinity post-synaptic α2 receptors, engaged at moderate rates of NE release associated with moderate arousal levels, promote working memory. In contrast, lower affinity α1 receptors, engaged at higher rates of release associated with high arousal conditions (e.g. stress), impair working memory performance while promoting flexible attention. While these and other observations were initially interpreted to indicate high rates of NE release promotes the transition from focused to flexible/scanning attention, recent findings indicate that α1 receptors promote both focused and flexible attention. Collectively, these observations indicate that while α2 and α1 receptors in the PFC differentially modulate distinct cognitive processes, this cannot be simply ascribed to differential roles of these receptors in 'focused' vs. 'flexible' cognitive processes. Translationally, this information indicates that: (1) not all tests of prefrontal cognitive function may be appropriate for preclinical programs aimed at specific PFC-dependent disorders and (2) the treatment of specific PFC cognitive deficits may require the differential targeting of noradrenergic receptor subtypes. This article is part of a Special Issue entitled SI: Noradrenergic System.

Doping the Mind: Dopaminergic Modulation of Prefrontal Cortical Cognition

The prefrontal cortex is the center of cognitive control. Processing in prefrontal cortical circuits enables us to direct attention to behaviorally relevant events; to memorize, structure, and categorize information; and to learn new concepts. The prefrontal cortex receives strong projections from midbrain neurons that use dopamine as a transmitter. In this article, we review the crucial role dopamine plays as a modulator of prefrontal cognitive functions, in the primate brain in particular. Following a summary of the anatomy and physiology of the midbrain dopamine system, we focus on recent studies that investigated dopaminergic effects in prefrontal cortex at the cellular level. We then discuss how unregulated prefrontal dopamine signaling could contribute to major disorders of cognition. The studies highlighted in this review demonstrate the powerful influence dopamine exerts on the mind.

Effects of different types of physical activity on the cognitive functions and attention in older people: A randomized controlled study

This study aimed to evaluate the effects of different types of exercise on cognition. Eighty participants, 32 males and 48 females, aged 66.96 ± 11.73, volunteered for this study. The participants were randomly divided into the four following groups: Resistance Group (RG; n=20), involved in high intensity strength training; Cardiovascular Group (CVG; n=20), involved in high intensity cardiovascular training; Postural Group (PG; n=20) involved in low intensity training, based on postural and balance exercises; and Control Group (CG; n=20). Exercises were performed over the course of 12 weeks. All participants were tested for their cognitive functions pre- and post-intervention using the following neurocognitive tests: the Attentive Matrices Test, Raven's Progressive Matrices, Stroop Color and Word Interference Test, Trail Making Test and Drawing Copy Test. Statistical analysis showed that the CVG group improved significantly in the Attentive Matrices Test and Raven's Progressive Matrices (both p=<0.05), whereas the RG group improved in Drawing Copy Test time (p=<0.05). These results confirm that different types of exercise interventions have unique effects on cognition. Cardiovascular training is effective in improving performance attentive and analytic tasks, whereas resistance training is effective in improving praxis. Further investigation is necessary to evaluate the combination of the two exercise types in order to ascertain if their respective effects can be summated when performed together.

Cognitive Improvement of Attention and Inhibition in the Late Afternoon in Children With Attention-Deficit Hyperactivity Disorder (ADHD) Treated With Osmotic-Release Oral System Methylphenidate

Long-acting medications have been developed and approved for use in the treatment of attention-deficit hyperactivity disorder (ADHD). These compounds are intended to optimize and maintain symptoms control throughout the day. We tested prolonged effects of osmotic-release oral system methylphenidate on both attention and inhibition, in the late afternoon. A double-blind, randomized, placebo-controlled study was conducted in 36 boys (7-12 years) with ADHD and 40 typically developing children. The ADHD children received an individualized dose of placebo or osmotic-release oral system methylphenidate. They were tested about 8 hours after taking with 2 continuous performance tests (continuous performance test-X [CPT-X] and continuous performance test-AX [CPT-AX]) and a counting Stroop. A positive effect of osmotic-release oral system methylphenidate was present in CPT-AX with faster and less variable reaction times under osmotic-release oral system methylphenidate than under placebo, and no difference with typically developing children. In the counting Stroop, we found a decreased interference with osmotic-release oral system methylphenidate but no difference between children with ADHD under placebo and typically developing children.

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

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

Methylphenidate has long-lasting metaplastic effects in the prefrontal cortex of adolescent rats

Methylphenidate (MPH) is widely used as a "nootropic" agent and in the treatment of disorders of attention, and has been shown to modulate synaptic plasticity in vitro. Here we present in vivo evidence that this MPH-induced metaplasticity can last long after the end of treatment. MPH (0, 0.2, 1 and 5mg/kg) was administered daily to male rats from postnatal day 42 for 15 days. The animals were tested daily in a radial maze. Long-term potentiation (LTP), a marker of neural plasticity, was induced in vivo in the prefrontal cortex after 2-3h, 15-18 days or 5 months without treatment. The behavioral performance of the 1mg/kg group improved, while that of animals that had received 5mg/kg deteriorated. In the 1 and 5mg/kg groups LTP induced 2-3h after the last MPH treatment was twice as large as in the controls. Further, 15-18 days after the last MPH administration, in groups receiving 1 and 5mg/kg, LTP was about fourfold higher than in controls. However, 5 months later, LTP in the 1mg/kg group was similar to controls and in the 5mg/kg group LTP could not be induced at all. No significant changes of LTP were seen in the low-dose group of animals (0.2mg/kg). Thus, firstly, doses of MPH that improve learning coincide approximately with those that augment LTP. Secondly, MPH-induced increases in LTP can last for several weeks, but these may disappear over longer periods or deteriorate at high doses.

Acute oral administration of low doses of methylphenidate targets calretinin neurons in the rat septal area

Methylphenidate (MPD) is a commonly administered drug to treat children suffering from attention deficit hyperactivity disorder (ADHD). Alterations in septal driven hippocampal theta rhythm may underlie attention deficits observed in these patients. Amongst others, the septo-hippocampal connections have long been acknowledged to be important in preserving hippocampal function. Thus, we wanted to ascertain if MPD administration, which improves attention in patients, could affect septal areas connecting with hippocampus. We used low and orally administered MPD doses (1.3, 2.7 and 5 mg/Kg) to rats what mimics the dosage range in humans. In our model, we observed no effect when using 1.3 mg/Kg MPD; whereas 2.7 and 5 mg/Kg induced a significant increase in c-fos expression specifically in the medial septum (MS), an area intimately connected to the hippocampus. We analyzed dopaminergic areas such as nucleus accumbens and striatum, and found that only 5 mg/Kg induced c-fos levels increase. In these areas tyrosine hydroxylase correlated well with c-fos staining, whereas in the MS the sparse tyrosine hydroxylase fibers did not overlap with c-fos positive neurons. Double immunofluorescence of c-fos with neuronal markers in the septal area revealed that co-localization with choline acethyl transferase, parvalbumin, and calbindin with c-fos did not change with MPD treatment; whereas, calretinin and c-fos double labeled neurons increased after MPD administration. Altogether, these results suggest that low and acute doses of methylphenidate primary target specific populations of caltretinin medial septal neurons.

Effect of methylphenidate treatment during adolescence on norepinephrine transporter function in orbitofrontal cortex in a rat model of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is associated with hypofunctional medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC). Methylphenidate (MPH) remediates ADHD, in part, by inhibiting the norepinephrine transporter (NET). MPH also reduces ADHD-like symptoms in spontaneously hypertensive rats (SHRs), a model of ADHD. However, effects of chronic MPH treatment on NET function in mPFC and OFC in SHR have not been reported. In the current study, long-term effects of repeated treatment with a therapeutically relevant oral dose of MPH during adolescence on NET function in subregions of mPFC (cingulate gyrus, prelimbic cortex and infralimbic cortex) and in the OFC of adult SHR, Wistar-Kyoto (WKY, inbred control) and Wistar (WIS, outbred control) rats were determined using in vivo voltammetry. Following local ejection of norepinephrine (NE), uptake rate was determined as peak amplitude (Amax)× first-order rate constant (k-1). In mPFC subregions, no strain or treatment effects were found in NE uptake rate. In OFC, NE uptake rate in vehicle-treated adult SHR was greater than in adult WKY and WIS administered vehicle. MPH treatment during adolescence normalized NE uptake rate in OFC in SHR. Thus, the current study implicates increased NET function in OFC as an underlying mechanism for reduced noradrenergic transmission in OFC, and consequently, the behavioral deficits associated with ADHD. MPH treatment during adolescence normalized NET function in OFC in adulthood, suggesting that the therapeutic action of MPH persists long after treatment cessation and may contribute to lasting reductions in deficits associated with ADHD.

Improved prefrontal activity in AD/HD children treated with atomoxetine: a NIRS study

BACKGROUND/AIMS

Atomoxetine (ATX), a selective norepinephrine reuptake inhibitor, is the first approved non-stimulant drug for treatment of attention deficit/hyperactivity disorder (AD/HD). The present study examined the effects of long-term treatment with ATX on prefrontal hemodynamic activity in AD/HD children during a continuous performance task (CPT) using near-infrared spectroscopy (NIRS).

METHODS

Prefrontal hemodynamic activity was measured in 12 children with AD/HD during experimental sessions conducted before and 6 months or more after starting ATX treatment. The average maintenance dose of ATX was 1.6 mg/kg/day. Fourteen age-matched typically developing children participated as a control group.

RESULTS

In the control group, the CPT induced a significant increase in oxygenated hemoglobin (oxy-Hb) concentration in the bilateral dorsolateral prefrontal cortex (DLPFC). In the AD/HD group in the pre-ATX condition, the CPT did not induce a significant increase in oxy-Hb concentration in any of the NIRS channels, but induced a significant decrease in oxy-Hb concentration in the left ventrolateral prefrontal cortex (VLPFC). In the AD/HD group in the post-ATX condition, significant activation was observed in the right DLPFC and the decrease in oxy-Hb concentration in the left VLPFC disappeared.

CONCLUSIONS

These results suggest that long-term treatment with ATX improved prefrontal hemodynamic activity in AD/HD children, and NIRS may be useful for assessment of the prefrontal hemodynamic response to ATX treatment.

Attention enhancing effects of methylphenidate are age-dependent

The psychostimulant methylphenidate (MPH, Ritalin®) is used to treat a variety of cognitive disorders. MPH is also popular among healthy individuals, including the elderly, for its ability to focus attention and improve concentration, but these effects have not been shown to be comparable between aged and adult subjects. Thus, we tested whether MPH would improve performance in sustained attention in both adult and aged rats. In addition, we tested the impact of visual distraction on performance in this task and the ability of MPH to mitigate the effects of distraction. Adult (6-12 months) and aged (18-22 months) male Sprague-Dawley rats were given oral MPH, and their cognitive and motor abilities were tested. Results suggest that while MPH improves task performance in adults; there is no improvement in the aged animals. These outcomes suggest that the use of MPH for cognitive enhancement in elderly individuals may be ineffective.

Amphetamine-induced release of dopamine in primate prefrontal cortex and striatum: striking differences in magnitude and timecourse

The psychostimulant amphetamine (AMPH) is frequently used to increase catecholamine levels in attention disorders and positron emission tomography imaging studies. Despite the fact that most radiotracers for positron emission tomography studies are characterized in non-human primates (NHPs), data on regional differences of the effect of AMPH in NHPs are very limited. This study examined the impact of AMPH on extracellular dopamine (DA) levels in the medial prefrontal cortex and the caudate of NHPs using microdialysis. In addition to differences in magnitude, we observed striking differences in the temporal profile of extracellular DA levels between these regions that can likely be attributed to differences in the regulation of dopamine uptake and biosynthesis. The present data suggest that cortical DA levels may remain elevated longer than in the caudate which may contribute to the clinical profile of the actions of AMPH. Using microdialysis probes implanted in the cortex and caudate region of non-human primate brains, we observed in vivo differences in the magnitude and temporal profile of extracellular dopamine levels in response to intravenous amphetamine administration.

The attention deficit hyperactivity disorder phenotype as a summation of deficits in executive functioning and reward sensitivity: does this explain its relationship with oppositional defiant disorder?

OBJECTIVE

Attention deficit hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) commonly coexist but the relationship between them has not been well explained. We present a view of the ADHD phenotype as a summation of deficits in executive functioning and reward sensitivity and apply this to explain its co-occurrence with ODD.

MODEL

ADHD relates to difficulty in sustaining concentration for completing tasks. This is associated with lack of reward due to lack of achievement. The unfavorable balance of effort to reward in ADHD is exacerbated in ODD, a condition in which lack of reward is responsible for an adverse effect on mood resulting in a negative attitude and oppositional behavior. We have called this model the "Mental effort-reward imbalances model".

CONCLUSION

This model predicts that oppositional symptoms would be highly prevalent in ADHD, to the extent that ODD would be a frequent co-morbidity.

Illicit dopamine transients: reconciling actions of abused drugs

Phasic increases in brain dopamine are required for cue-directed reward seeking. Although compelling within the framework of appetitive behavior, the view that illicit drugs hijack reward circuits by hyperactivating these dopamine transients is inconsistent with established psychostimulant pharmacology. However, recent work reclassifying amphetamine (AMPH), cocaine, and other addictive dopamine-transporter inhibitors (DAT-Is) supports transient hyperactivation as a unifying hypothesis of abused drugs. We argue here that reclassification also identifies generating burst firing by dopamine neurons as a keystone action. Unlike natural rewards, which are processed by sensory systems, drugs act directly on the brain. Consequently, to mimic natural rewards and exploit reward circuits, dopamine transients must be elicited de novo. Of available drug targets, only burst firing achieves this essential outcome.

Visual processing of biological motion in children and adolescents with attention-deficit/hyperactivity disorder: an event related potential-study

Attention-deficit/hyperactivity disorder (ADHD) is often accompanied by problems in social behaviour, which are sometimes similar to some symptoms of autism-spectrum disorders (ASD). However, neuronal mechanisms of ASD-like deficits in ADHD have rarely been studied. The processing of biological motion-recently discussed as a marker of social cognition-was found to be disrupted in ASD in several studies. Thus in the present study we tested if biological motion processing is disrupted in ADHD. We used 64-channel EEG and spatio-temporal source analysis to assess event-related potentials associated with human motion processing in 21 children and adolescents with ADHD and 21 matched typically developing controls. On the behavioural level, all subjects were able to differentiate between human and scrambled motion. But in response to both scrambled and biological motion, the N200 amplitude was decreased in subjects with ADHD. After a spatio-temporal dipole analysis, a human motion specific activation was observable in occipital-temporal regions with a reduced and more diffuse activation in ADHD subjects. These results point towards neuronal determined alterations in the processing of biological motion in ADHD.

Effect of methylphenidate on attention in apathetic AD patients in a randomized, placebo-controlled trial

BACKGROUND

Little is known about the effect of methylphenidate (MPH) on attention in Alzheimer's disease (AD). MPH has shown to improve apathy in AD, and both apathy and attention have been related to dopaminergic function. The goal was to investigate MPH effects on attention in AD and assess the relationship between attention and apathy responses.

METHODS

MPH (10 mg PO twice daily) or placebo was administered for six weeks in a randomized, double-blind trial in mild-to-moderate AD outpatients with apathy (Neuropsychiatric Inventory (NPI) Apathy ≥ 4). Attention was measured with the Wechsler Adult Intelligence Scale--Digit Span (DS) subtest (DS forward, selective attention) and apathy with the Apathy Evaluation Scale (AES). A mixed effects linear regression estimated the difference in change from baseline between treatment groups, defined as δ (MPH (DS week 6-DS baseline)) - (placebo (DS week 6-DS baseline)).

RESULTS

In 60 patients (37 females, age = 76 ± 8, Mini-Mental State Examination (MMSE) = 20 ± 5, NPI Apathy = 7 ± 2), the change in DS forward (δ = 0.87 (95% CI: 0.06-1.68), p = 0.03) and DS total (δ = 1.01 (95% CI: 0.09-1.93), p = 0.03) favored MPH over placebo. Of 57 completers, 17 patients had improved apathy (≥3.3 points on the AES from baseline to end point) and 40 did not. There were no significant associations between AES and NPI Apathy with DS change scores in the MPH, placebo, AES responder, or non-responder groups. DS scores did not predict apathy response to MPH treatment.

CONCLUSION

These results suggest MPH can improve attention and apathy in AD; however, the effects appear independent in this population.

Animal model of methylphenidate's long-term memory-enhancing effects

Methylphenidate (MPH), introduced more than 60 years ago, accounts for two-thirds of current prescriptions for attention deficit hyperactivity disorder (ADHD). Although many studies have modeled MPH's effect on executive function, almost none have directly modeled its effect on long-term memory (LTM), even though improvement in LTM is a critical target of therapeutic intervention in ADHD. We examined the effects of a wide range of doses of MPH (0.01–10 mg/kg, i.p.) on Pavlovian fear learning, a leading model of memory. MPH's effects were then compared to those of atomoxetine (0.1–10 mg/kg, i.p.), bupropion (0.5–20 mg/kg, i.p.), and citalopram (0.01–10 mg/kg, i.p.). At low, clinically relevant doses, MPH enhanced fear memory; at high doses it impaired memory. MPH's memory-enhancing effects were not confounded by its effects on locomotion or anxiety. Further, MPH-induced memory enhancement seemed to require both dopamine and norepinephrine transporter inhibition. Finally, the addictive potential of MPH (1 mg/kg and 10 mg/kg) was compared to those of two other psychostimulants, amphetamine (0.005 mg/kg and 1.5 mg/kg) and cocaine (0.15 mg/kg and 15 mg/kg), using a conditioned place preference and behavioral sensitization paradigm. We found that memory-enhancing effects of psychostimulants observed at low doses are readily dissociable from their reinforcing and locomotor activating effects at high doses. Together, our data suggest that fear conditioning will be an especially fruitful platform for modeling the effects of psychostimulants on LTM in drug development.

Psychostimulants in moderate to severe affective disorder: a systematic review of randomized controlled trials

BACKGROUND

Despite antidepressant therapy of appropriate trial duration and dose optimization, 50-60% of depressed patients have an adequate treatment response, whereas only 35-40% achieve remission. Psychostimulants have been suggested as potential candidates to promote acceleration of response and to alleviate residual symptoms of depression.

AIMS

In this review results from randomized clinical trials (RCTs) exploring the efficacy of psychostimulants in the treatment of major depressive disorder (MDD) were analyzed to clarify the current empirically founded evidence for clinical approaches involving psychostimulants.

METHOD

Literature research via PubMed retrieved 846 articles. 18 RCTs reporting on the use of psychostimulants in the treatment of adult patient populations, suffering from moderate-severe depression and having no other concomitant medical illnesses, were included in this review. 14 articles provided results for unipolar depression, two for bipolar depression, whereas two articles presented mixed samples of unipolar and bipolar patients.

RESULTS

Five different psychostimulants were evaluated: modafinil, methylphenidate, dexamphetamine, methylamphetamine and pemoline. Two studies examining modafinil demonstrated significant ameliorating characteristics pertaining to symptoms of depression. No clear evidence for the effectiveness of traditional psychostimulants in the therapeutic management of MDD was found. In general the quality of included trials was poor since the majority was of short-term duration, comprising relatively small sample sizes and some, especially older studies, were methodologically flawed.

CONCLUSION

Clearly larger well designed placebo-controlled studies with longer follow-up accompanied by evaluations of tolerance/dependence are warranted before psychostimulants can be recommended in routine clinical practice for the treatment of MDD.

Methylphenidate and the juvenile brain: enhancement of attention at the expense of cortical plasticity?

Methylphenidate (Ritalin) is the most commonly prescribed psychoactive drug for juveniles and adolescents. Used to treat attention-deficit/hyperactivity disorder (ADHD) and for cognitive enhancement in healthy individuals, it has been regarded as a relatively safe medication for the past several decades. However, a thorough review of the literature reveals that the age-dependent activities of the drug, as well as potential developmental effects, are largely ignored. In addition, the diagnosis of ADHD is subjective, leaving open the possibility of misdiagnosis and excessive prescription of the drug. Recent studies have suggested that early life exposure of healthy rodent models to methylphenidate resulted in altered sleep/wake cycle, heightened stress reactivity, and, in fact, a dosage previously thought of as therapeutic depressed neuronal function in juvenile rats. Furthermore, juvenile rats exposed to low-dose methylphenidate displayed alterations in neural markers of plasticity, indicating that the drug might alter the basic properties of prefrontal cortical circuits. In this review of the current literature, we propose that juvenile exposure to methylphenidate may cause abnormal prefrontal function and impaired plasticity in the healthy brain, strengthening the case for developing a more thorough understanding of methylphenidate's actions on the developing, juvenile brain, as well as better diagnostic measures for ADHD.

The effect of modafinil on fatigue, cognitive functioning, and mood in primary brain tumor patients: a multicenter randomized controlled trial

BACKGROUND

Fatigue, cognitive deficits, and depression are frequently reported but often undertreated symptoms that can profoundly affect daily life in patients with primary brain tumors (PBTs). To evaluate the effects of the psychostimulant modafinil on fatigue, depression, health-related quality of life (HRQOL), and cognitive functioning in PBT patients, we performed a multicenter, double-blind placebo-controlled crossover trial.

METHODS

Patients randomly received either 6 weeks of treatment with modafinil (up to 400 mg/day) or 6 weeks with placebo. After a 1-week washout period, the opposite treatment was provided. Assessments took place at baseline and immediately after the first and second condition. Patients completed self-report questionnaires on fatigue (Checklist Individual Strength [CIS]), depression (Center for Epidemiologic Studies Depression Scale [CES-D]), HRQOL (Short-Form Health Survey [SF-36]), and self-perceived cognitive functioning (Medical Outcomes Study [MOS]). They also underwent comprehensive neurocognitive testing.

RESULTS

In total, 37 patients participated. Relative to baseline, patients reported lower fatigue severity (CIS) and better motivation (CIS) in both the modafinil (P = .010 and P = .021, respectively) and the placebo condition (P < .001 and P = .027, respectively). The same held for physical health (SF-36 Physical Component Summary score; P = .001 and P = .008, respectively), working memory (P = .040 and P = .043), and information processing capacity (P = .036 and P = .040). No improvement in depressive symptoms was found in either condition.

CONCLUSIONS

Modafinil did not exceed the effects of placebo with respect to symptom management. Patient accrual was slow, and relatively many patients dropped out during the trial, due mostly to side effects. Other, preferably nonpharmacologic intervention studies should be considered to improve symptom management of PBT patients.

The neurobiology of thought: the groundbreaking discoveries of Patricia Goldman-Rakic 1937-2003

Patricia S. Goldman-Rakic (1937–2003) transformed the study of the prefrontal cortex (PFC) and the neural basis of mental representation, the basic building block of abstract thought. Her pioneering research first identified the dorsolateral PFC (dlPFC) region essential for spatial working memory, and the extensive circuits of spatial cognition. She discovered the cellular basis of working memory, illuminating the dlPFC microcircuitry underlying spatially tuned, persistent firing, whereby precise information can be held “in mind”: persistent firing arises from recurrent excitation within glutamatergic pyramidal cell circuits in deep layer III, while tuning arises from GABAergic lateral inhibition. She was the first to discover that dopamine is essential for dlPFC function, particularly through D1 receptor actions. She applied a host of technical approaches, providing a new paradigm for scientific inquiry. Goldman-Rakic's work has allowed the perplexing complexities of mental illness to begun to be understood at the cellular level, including atrophy of the dlPFC microcircuits subserving mental representation. She correctly predicted that impairments in dlPFC working memory activity would contribute to thought disorder, a cardinal symptom of schizophrenia. Ten years following her death, we look back to see how she inspired an entire field, fundamentally changing our view of cognition and cognitive disorders.

Neurocircuitry underlying the preferential sensitivity of prefrontal catecholamines to low-dose psychostimulants

Low doses of psychostimulants, including methylphenidate (MPH), are highly effective in the treatment of attention-deficit/hyperactivity disorder (ADHD). At these doses, psychostimulants improve prefrontal cortex (PFC)-dependent function. Recent evidence indicates that low and clinically relevant doses of psychostimulants target norepinephrine (NE) and dopamine (DA) signaling preferentially in the PFC. To better understand the neural mechanisms responsible for the regional selectivity of low-dose psychostimulant action, it is important to first identify the underlying neurocircuitry. The current study used reverse microdialysis to test the hypothesis that the preferential targeting of PFC catecholamines by low-dose psychostimulants involves direct action within the PFC, reflecting an intrinsic property of this region. For these studies, the effects of varying concentrations of MPH (0.25, 1.0, and 4.0 μM) on NE and DA efflux were examined within the PFC and select subcortical fields in unanesthetized rats. Low concentrations of MPH elicited significantly larger increases in extracellular levels of NE and DA in the PFC than in subcortical regions linked to motor-activating and arousal-promoting actions of psychostimulants (nucleus accumbens and medial septal area, respectively). The differential action of MPH across regions disappeared at higher concentrations. The enhanced sensitivity of PFC catecholamines to low and clinically relevant doses of psychostimulants, at least in part, reflects a unique sensitivity of this region to NE/DA transporter blockade. Available evidence suggests that the increased sensitivity of PFC catecholamines likely involves DA clearance through the NE transporter within the PFC.

Exercise: applications to childhood ADHD

ADHD is the most common neurobehavioral disorder of childhood, presenting with pervasive and impairing symptoms of inattention, hyperactivity, impulsivity, or a combination. The leading hypothesis of the underlying physiology of this disorder of inattention and/or hyperactivity-impulsivity is based on catecholamine dysfunction. Pharmacotherapy research indicates that psychostimulants, which are catecholamine agonists, show the greatest efficacy for treating the core symptoms of ADHD. Exercise affects the same dopaminergic and noradrenergic systems that stimulant medications target and is a stressor, which elicits measurable physiological changes. The magnitude of these peripheral alterations is posited as a potential biomarker of ADHD. The hypothesis that exercise training alters the underlying physiology present in ADHD and other medical conditions as well as conceptual issues behind its potential clinical utility is reviewed.

A behavioral neuroenergetics theory of ADHD

Energetic insufficiency in neurons due to inadequate lactate supply is implicated in several neuropathologies, including attention-deficit/hyperactivity disorder (ADHD). By formalizing the mechanism and implications of such constraints on function, the behavioral Neuroenergetics Theory (NeT) predicts the results of many neuropsychological tasks involving individuals with ADHD and kindred dysfunctions, and entails many novel predictions. The associated diffusion model predicts that response times will follow a mixture of Wald distributions from the attentive state, and ex-Wald distributions after attentional lapses. It is inferred from the model that ADHD participants can bring only 75-85% of the neurocognitive energy to bear on tasks, and allocate only about 85% of the cognitive resources of comparison groups. Parameters derived from the model in specific tasks predict performance in other tasks, and in clinical conditions often associated with ADHD. The primary action of therapeutic stimulants is to increase norepinephrine in active regions of the brain. This activates glial adrenoceptors, increasing the release of lactate from astrocytes to fuel depleted neurons. The theory is aligned with other approaches and integrated with more general theories of ADHD. Therapeutic implications are explored.

The effects of methylphenidate on cognitive function in children with attention-deficit/hyperactivity disorder

Focusing on behavioral criteria for attention-deficit/hyperactivity disorder (ADHD) diagnosis leads to considerable neuropsychological profile heterogeneity among diagnosed children, as well as variable response to methylphenidate (MPH) treatment. Documenting "cold" executive working memory (EWM) or "hot" self-regulation (SR) neuropsychological impairments could aid in the differential diagnosis of ADHD subtypes and may help to determine the optimal MPH treatment dose. In this study, children with ADHD inattentive type (n = 19), combined type (n = 33), and hyperactive-impulse type (n = 4) underwent randomized controlled MPH trials; neuropsychological, behavioral, and observational data were collected to evaluate the children's responses. Those with moderate or significant baseline EWM/SR impairment showed robust MPH response, whereas response for those with lower baseline impairment was equivocal. Implications for medication use and titration, academic achievement, and long-term treatment efficacy are examined.

Memory consolidation — Mechanisms and opportunities for enhancement

Memory consolidation is the process by which relevant information is selected and transferred from a short-term, fragile state, into a stable, longer term domain from which it can be recalled. Effective memory underpins our ability to carry out everyday activities. When memory consolidation fails, such as in Alzheimer’s disease, the consequences can be devastating. Understanding the neurobiology of memory will help develop treatments for patients with memory loss. Here we describe the myriad processes involved in memory consolidation, including cholinergic and dopaminergic neurotransmission predominantly in hippocampal networks. We discuss established therapies as well as potential novel strategies for boosting cognition. Future approaches to enhancement of memory consolidation include not only pharmacological and neurosurgical treatments, but also lifestyle interventions — for example, modifications to sleep, exercise and diet.

Effects of atomoxetine on locomotor activity and impulsivity in the spontaneously hypertensive rat

Atomoxetine (ATX) is a commonly used non-stimulant treatment for Attention deficit hyperactivity disorder (ADHD). It primarily acts to increase noradrenalin levels; however, at higher doses it can increase dopamine levels. To date there has been no investigations into the effects of orally-administered ATX in the most commonly used model of ADHD, the spontaneously hypertensive rat (SHR). The aim of this study was to describe the effects of doses thought to be selective (0.15 mg/kg) and non-selective (0.3 mg/kg) for noradrenalin on behavioural measures in the SHR. Firstly, we examined the effects of acute and chronic ATX on locomotor activity including sensitisation and cross-sensitisation to amphetamine. Secondly, we measured drug effects on impulsivity using a T-maze delay discounting paradigm. We found no effect of ATX on locomotor activity and no evidence for sensitisation or cross-sensitisation. Furthermore, there were no differences in T-maze performance, indicating no effects on impulsivity at these doses. The absence of behavioural sensitisation supports previous claims of superior safety relative to psychostimulants for the doses administered. There was also no effect on impulsivity; however, we suggest that was confounded by stress specific to SHRs. Implications for future studies, behavioural assessment of SHRs and their use as a model of ADHD are discussed.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

Review of recent studies on interventions for cognitive deficits in patients with cancer

Research has demonstrated that patients with cancer experience cognitive deficits, often due to aggressive anticancer treatments. In this article, we critically review the interventional studies that have been conducted to investigate beneficial effects on cognitive function in cancer patients. Pharmacological agents that have been studied include psychostimulants, such as methylphenidate and modafinil, erythropoietin, and hormonal (supplement) treatments for patients who receive hormonal suppression therapy. In addition, several cognitive rehabilitation programs have been evaluated in cancer patients. Recently, the approach of physical exercise to treat cognitive deficits has received great interest, and findings from novel studies are keenly anticipated. Although, in general, the studies reviewed were well designed, future studies may wish to include larger sample sizes and pay more attention to the accurate assessment of cognitive function.

A sensitive and selective quantification of catecholamine neurotransmitters in rat microdialysates by pre-column dansyl chloride derivatization using liquid chromatography-tandem mass spectrometry

A rapid and sensitive liquid chromatography tandem mass spectrometry method for simultaneous quantification of catecholamine neurotransmitters in microdialysates was developed. The catecholamine neurotransmitters dopamine (DA) and norepinephrine (NE) were pre-column derivatized with dansyl chloride and analyzed. A gradient elution method was used to separate the analytes from the interferences on an Agilent Poroshell 120 EC-C18 outer porous micro particulate column. The method was robust and sensitive to determine with the lower limit of quantification value of 0.068pmol/mL and 0.059pmol/mL for DA and NE, respectively. It has acceptable precision and accuracy for concentrations over the standard curve range. The method was successfully applied for simultaneous quantitation of DA and NE in the prefrontal cortex (PFC) dialysates of rats obtained from a microdialysis study dosed with vehicle and atomoxetine through intra peritoneal (i.p.) route at a dose of 3mg/kg to monitor the change in extracellular concentrations. Thus, accomplishment of this method would facilitate the neurochemical monitoring for discovery of new chemical entities targeted for the treatment of attention deficit hyperactivity disorder (ADHD).

Psychostimulants act within the prefrontal cortex to improve cognitive function

BACKGROUND

At low and clinically relevant doses, psychostimulants enhance cognitive and behavioral function dependent on the prefrontal cortex (PFC) and extended frontostriatal circuitry. These actions are observed in individuals with attention-deficit/hyperactivity disorder, as well as in normal human and animal subjects. Despite the widespread use of these drugs, the sites of action involved in their cognition-enhancing and therapeutic effects are poorly understood. Indirect and/or correlative evidence suggests the cognition-enhancing/therapeutic effects of psychostimulants may involve actions directly within the PFC or extended frontostriatal circuitry. The current studies examined the degree to which methylphenidate (MPH) (Ritalin) acts within distinct frontostriatal subfields to improve PFC-dependent cognition as measured in a delayed-response test of spatial working memory.

METHODS

Working memory performance was assessed following microinfusion of vehicle or varying doses of MPH (.03-8.0 μg/500 nL) directly into the dorsomedial PFC (dorsal prelimbic and dorsal anterior cingulate cortex), the ventromedial PFC (infralimbic), and the dorsomedial striatum of rats (n = 69).

RESULTS

Methylphenidate infusion into the dorsomedial PFC, but not ventromedial PFC, elicited an inverted U-shaped facilitation of PFC-dependent cognition as measured in this task. The magnitude of this improvement was comparable with that seen with systemic administration. Additional studies demonstrated that although the dorsomedial striatum is necessary for accurate performance in this task, MPH infusion into this region did not affect working memory performance.

CONCLUSIONS

These observations provide the first definitive evidence that the PFC is a site of action in the cognition-enhancing and presumably therapeutic actions of low-dose psychostimulants.

Atomoxetine modulates spontaneous and sensory-evoked discharge of locus coeruleus noradrenergic neurons

Atomoxetine (ATM) is a potent norepinephrine (NE) uptake inhibitor and increases both NE and dopamine synaptic levels in prefrontal cortex, where it is thought to exert its beneficial effects on attention and impulsivity. At the behavioral level, ATM has been shown to cause improvements on the measures of executive functions, such as response inhibition, working memory and attentional set shifting across different species. However, the exact mechanism of action for ATM's effects on cognition is still not clear. One possible target for the cognitive enhancing effects of ATM is the noradrenergic locus coeruleus (LC), the only source of NE to key forebrain areas such as cerebral cortex and hippocampus. Although it is known that ATM increases NE availability overall by blocking reuptake of NE, the effects of this agent on impulse activity of LC neurons have not been reported. Here, the effect of ATM (0.1-1 mg/kg, ip) on NE-LC neurons was investigated by recording extracellular activity of LC neurons in isoflurane-anesthetized rats. ATM caused a significant decrease of the tonic activity of LC single-units, although leaving intact the sensory-evoked excitatory component of LC phasic response. Moreover, the magnitude of the inhibitory component of LC response to paw stimulation was increased after 1 mg/kg of ATM and its duration was prolonged at 0.3 mg/kg. Together, these effects of ATM produced an increase in the phasic-to-tonic ratio of LC phasic response to sensory stimulation. ATM also modulated the average sensory-evoked local field potential (LFP) and spike-field coherence in LC depending on the dose tested. The lower dose (0.1 mg/kg) significantly decreased early positive and negative components of the sensory-evoked LFP response. Higher doses (0.3-1 mg/kg) initially increased and then decreased the amplitude of components of the evoked fields, whereas the spike-field coherence was enhanced by 1 mg/kg ATM across frequency bands. Finally, coherence between LC fields and EEG signals was generally increased by 1 mg/kg ATM, whereas 0.1 and 0.3 mg/kg respectively decreased and increased coherence values in specific frequency bands. Taken together these results suggest that ATM effects on LC neuronal activity are dose-dependent, with different doses affecting different aspects of LC firing. This modulation of activity of LC-NE neurons may play a role in the cognitive effects of ATM. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

A selective dopamine reuptake inhibitor improves prefrontal cortex-dependent cognitive function: potential relevance to attention deficit hyperactivity disorder

Drugs used to treat attention deficit hyperactivity disorder (ADHD) improve prefrontal cortex (PFC)-dependent cognitive function. The majority of ADHD-related treatments act either as dual norepinephrine (NE) and dopamine (DA) reuptake inhibitors (psychostimulants) or selective NE reuptake inhibitors (SNRIs). Certain benztropine analogs act as highly selective DA reuptake inhibitors while lacking the reinforcing actions, and thus abuse potential, of psychostimulants. To assess the potential use of these compounds in the treatment of ADHD, we examined the effects of a well-characterized benztropine analog, AHN 2-005, on performance of rats in a PFC-dependent delayed-alternation task of spatial working memory. Similar to that seen with all drugs currently approved for ADHD, AHN 2-005 dose-dependently improved performance in this task. Clinically-relevant doses of psychostimulants and SNRIs elevate NE and DA preferentially in the PFC. Despite the selectivity of this compound for the DA transporter, additional microdialysis studies demonstrated that a cognition-enhancing dose of AHN 2-005 that lacked locomotor activating effects increased extracellular levels of both DA and NE in the PFC. AHN 2-005 produced a larger increase in extracellular DA in the nucleus accumbens, although the magnitude of this was well below that seen with motor activating doses of psychostimulants. Collectively, these observations suggest that benztropine analogs may be efficacious in the treatment of ADHD or other disorders associated with PFC dysfunction. These studies provide a strong rationale for future research focused on the neural mechanisms contributing to the cognition-enhancing actions and the potential clinical utility of AHN 2-005 and related compounds. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Pharmacotherapy for attention-deficit/hyperactivity disorder: from cells to circuits

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent disorder of childhood and adulthood, with a considerable impact on public health. There is a substantial pharmacopoeia available for safe and effective treatment of ADHD, and newly available agents diversify the treatment options. With the burgeoning scientific literature addressing the genetic, neurochemical, and neural systems basis for this condition, increasing attention is directed at establishing the neural basis for the efficacy of existing treatments. ADHD remains the only highly prevalent, nondegenerative neuropsychiatric disorder for which effective medications remediate the principal cognitive disturbances in concert with clinical efficacy. Therefore, deeper insight into the neural mechanisms of cognitive remediation may serve to advance treatment development not only in ADHD, but across a wide range of neuropsychiatric disorders in which cognitive dysfunction is a cardinal feature and a strong predictor of clinical outcome. To date, all effective medications for ADHD act on 1 or both of the major catecholamine neurotransmitter systems in the brain. These 2 systems, which arise from subcortical nuclei and use norepinephrine (NE) or dopamine (DA) as transmitters, exert strong modulatory effects on widely distributed cortical-subcortical neural circuits, with important effects on cognition, mood, and behavior, in both health and illness. The present review outlines the actions of ADHD medications from subcellular effects to effects on neural systems and cognition in ADHD patients. This is a very active area of investigation at all phases of the translational cycle, and near-term work is poised to firmly link cellular neuropharmacology to large-scale effects, and point the way toward advances in treatment.

The effects of stimulant medication on working memory functional connectivity in attention-deficit/hyperactivity disorder

BACKGROUND

Working memory impairments are commonly found in attention-deficit/hyperactivity disorder (ADHD) and often improve with psychostimulant treatment. Little is known about how these medications affect the function of frontoparietal brain regions engaged for working memory. This study used functional magnetic resonance imaging (fMRI) to examine medication-related changes in brain activation and functional connectivity in ADHD.

METHODS

Eighteen ADHD-combined subtype youths (ages 11-17) twice completed a Sternberg working memory fMRI task in a randomized, double-blind, placebo-controlled design. Medications were individualized as patients' standard, clinically effective psychostimulant (e.g., methylphenidate or dextroamphetamine/amphetamine combination) dose. Brain activity and functional connectivity were characterized using group independent component analysis. SPM5 repeated-measures t tests compared ADHD patients' network engagement and regional functional connectivity on and off medication.

RESULTS

Independent component analysis identified six frontoparietal networks/components with hemodynamic responses to encoding/maintenance or retrieval phases of the Sternberg fMRI task. On medication, three of these networks significantly increased activation. Functional connectivity analyses found medication led to recruitment of additional brain regions that were not engaged into the networks when participants were on placebo. Also, medication strengthened connectivity of some frontoparietal regions. Many connectivity changes were directly related to improved working memory reaction time. Overall, there was strong evidence for regional functional connectivity changes following medication in structures previously implicated as abnormal in ADHD, such as anterior cingulate, ventrolateral prefrontal cortex, and precuneus.

CONCLUSIONS

Stimulant medication has widespread effects on the functional connectivity of frontoparietal brain networks, which might be a mechanism that underlies their beneficial effects on working memory performance.

Differential sensitivity to psychostimulants across prefrontal cognitive tasks: differential involvement of noradrenergic α₁ - and α₂-receptors

BACKGROUND

Psychostimulants improve a variety of cognitive and behavioral processes in patients with attention-deficit/hyperactivity disorder (ADHD). Limited observations suggest a potentially different dose-sensitivity of prefrontal cortex (PFC)-dependent function (narrow inverted-U-shaped dose-response curves) versus classroom/overt behavior (broad inverted U) in children with ADHD. Recent work in rodents demonstrates that methylphenidate (MPH; Ritalin) elicits a narrow inverted-U-shaped improvement in performance in PFC-dependent tests of working memory. The current studies first tested the hypothesis that PFC-dependent tasks, in general, display narrow dose sensitivity to the beneficial actions of MPH.

METHODS

The effects of varying doses of MPH were examined on performance of rats in two tests of PFC-dependent cognition, sustained attention and attentional set shifting. Additionally, the effect of pretreatment with the α₁-antagonist prazosin (.5 mg/kg) on MPH-induced improvement in sustained attention was examined.

RESULTS

MPH produced a broad inverted-U-shaped facilitation of sustained attention and attentional set shifting. Prior research indicates α₁-receptors impair, whereas α₂-receptors improve, working memory. In contrast, attentional set shifting is improved with α₁-receptor activation, whereas α₂-receptors exert minimal effects in this task. Given the similar dose sensitivity of sustained attention and attentional set-shifting tasks, additional studies examined whether α₁-receptors promote sustained attention, similar to attentional set shifting. In these studies, MPH-induced improvement in sustained attention was abolished by α₁-receptor blockade.

CONCLUSIONS

PFC-dependent processes display differential sensitivity to the cognition-enhancing actions of psychostimulants that are linked to the differential involvement of α₁- versus α₂-receptors in these processes. These observations have significant preclinical and clinical implications.

A randomized trial on the efficacy of methylphenidate and modafinil for improving cognitive functioning and symptoms in patients with a primary brain tumor

Limited research is available regarding the efficacy of psychostimulants in treating cognitive function in primary brain tumor patients. An open-label, randomized, pilot trial examined both the general and differential efficacy of 4 weeks of methylphenidate (MPH) and modafinil (MOD) in 24 brain tumor patients. Participants completed cognitive tests and self-report measures of fatigue, sleep disturbance, mood and quality of life at baseline and after 4 weeks.Following stimulant treatment, there was evidence of a beneficial effect on test performance in speed of processing and executive function requiring divided attention. Patients with the greatest deficit in executive function at baseline appeared to derive the greatest benefit following stimulant therapy. Inconsistent, differential effects were found on a measure of attention in favor of MPH and on a measure of processing speed in favor of MOD. There was also evidence of a general beneficial effect on patient-reported measures of fatigue, mood, and quality of life, with no statistically significant differences between treatment arms in these measures over time. The results from this small pilot study should be interpreted with caution, but appear to warrant additional research, in larger study samples, targeting fatigue, processing speed and executive function, and exploring different doses of stimulants. Future studies may also wish to explore the specific patient factors that may be associated with responsiveness to psychostimulant treatment.

The molecular genetics of executive function: role of monoamine system genes

Executive control processes, such as sustained attention, response inhibition, and error monitoring, allow humans to guide behavior in appropriate, flexible, and adaptive ways. The consequences of executive dysfunction for humans can be dramatic, as exemplified by the large range of both neurologic and neuropsychiatric disorders in which such deficits negatively affect outcome and quality of life. Much evidence suggests that many clinical disorders marked by executive deficits are highly heritable and that individual differences in quantitative measures of executive function are strongly driven by genetic differences. Accordingly, intense research effort has recently been directed toward mapping the genetic architecture of executive control processes in both clinical (e.g., attention-deficit/hyperactivity disorder) and nonclinical populations. Here we review the extant literature on the molecular genetic correlates of three exemplar but dissociable executive functions: sustained attention, response inhibition, and error processing. Our review focuses on monoaminergic gene variants given the strong body of evidence from cognitive neuroscience and pharmacology implicating dopamine, noradrenaline, and serotonin as neuromodulators of executive function. Associations between DNA variants of the dopamine beta hydroxylase gene and measures of sustained attention accord well with cognitive-neuroanatomical models of sustained attention. Equally, functional variants of the dopamine D2 receptor gene are reliably associated with performance monitoring, error processing, and reinforcement learning. Emerging evidence suggests that variants of the dopamine transporter gene (DAT1) and dopamine D4 receptor gene (DRD4) show promise for explaining significant variance in individual differences in both behavioral and neural measures of inhibitory control.

Catecholamine influences on dorsolateral prefrontal cortical networks

The symptoms of attention-deficit/hyperactivity disorder (ADHD) involve impairments in prefrontal cortical top-down regulation of attention and behavior. All current pharmacological treatments for ADHD facilitate catecholamine transmission, and basic research suggests that these compounds have prominent actions in the prefrontal cortex (PFC). The dorsolateral PFC is especially sensitive to levels of norepinephrine and dopamine, whereby either too little or too much markedly impairs PFC function. Recent physiological studies have shown that norepinephrine strengthens PFC network connectivity and maintains persistent firing during a working memory task through stimulation of postsynaptic α(2A)-adrenoceptors on PFC neurons. Conversely, dopamine acts at D1 receptors to narrow spatial tuning, sculpting network inputs to decrease noise (i.e., stabilization of the representation). The stimulant medications and atomoxetine appear to enhance PFC function by indirectly increasing these catecholamine actions through blockade of norepinephrine and/or dopamine transporters. In contrast, guanfacine mimics the enhancing effects of norepinephrine at postsynaptic α(2A)-receptors in the PFC, strengthening network connectivity. Stronger PFC regulation of attention, behavior, and emotion likely contributes to the therapeutic effects of these medications for the treatment of ADHD.