Methylphenidate Enhances Early-Stage Sensory Processing and Rodent Performance of a Visual Signal Detection Task

The effects of yohimbine and hydrocortisone on selective attention to fearful faces: An fMRI study

INTRODUCTION

Selective attention to salient emotional information can enable an advantage in the face of danger. The present study aims to investigate the influence of the stress neuromodulators, norepinephrine and cortisol, on selective attention processes to fearful faces and its neuronal activation.

METHODS AND MATERIALS

We used a randomized, double-blind, placebo-controlled design. 167 healthy men between 18 and 35 years (mean [SD] age: 25.23 [4.24] years) participated in the study. Participants received either: (A) yohimbine (n= 41), (B) hydrocortisone (n = 41), (C) yohimbine and hydrocortisone (n = 42) or (D) placebo only (n= 43) and participated in a dot-probe task with fearful and neutral faces in an fMRI scanner.

RESULTS

We found an attentional bias toward fearful faces across all groups and related neuronal activation in the left cuneus. We did not find any differences between experimental treatment groups in selective attention and its neuronal activation.

DISCUSSION

Our results provide evidence that fearful faces lead to an attentional bias with related neuronal activation in the left cuneus. We did not replicate formerly reported activation in the amygdala, intraparietal sulcus, dorsal anterior cingulate cortex, and thalamus. Suitability of the dot-probe task for fMRI studies and insignificant treatment effects are discussed.

Perturbations in risk/reward decision making and frontal cortical catecholamine regulation induced by mild traumatic brain injury

Mild traumatic brain injury (mTBI) disrupts cognitive processes that influence risk taking behavior. Little is known regarding the effects of repetitive mild injury (rmTBI) or whether these outcomes are sex specific. Risk/reward decision making is mediated by the prefrontal cortex (PFC), which is densely innervated by catecholaminergic fibers. Aberrant PFC catecholamine activity has been documented following TBI and may underlie TBI-induced risky behavior. The present study characterized the effects of rmTBI on risk/reward decision making behavior and catecholamine transmitter regulatory proteins within the PFC. Rats were exposed to sham, single (smTBI), or three closed-head controlled cortical impact (CH-CCI) injuries and assessed for injury-induced effects on risk/reward decision making using a probabilistic discounting task (PDT). In the first week post-final surgery, mTBI increased risky choice preference. By the fourth week, males exhibited increased latencies to make risky choices following rmTBI, demonstrating a delayed effect on processing speed. When levels of tyrosine hydroxylase (TH) and the norepinephrine reuptake transporter (NET) were measured within subregions of the PFC, females exhibited dramatic increases of TH levels within the orbitofrontal cortex (OFC) following smTBI. However, both males and females demonstrated reduced levels of OFC NET following rmTBI. These results indicate the OFC is susceptible to catecholamine instability after rmTBI and suggests that not all areas of the PFC contribute equally to TBI-induced imbalances. Overall, the CH-CCI model of rmTBI has revealed time-dependent and sex-specific changes in risk/reward decision making and catecholamine regulation following repetitive mild head injuries.

Alterations in hippocampus-centered morphological features and function of the progression from normal cognition to mild cognitive impairment

Mild cognitive impairment (MCI) is a significant precursor to dementia, highlighting the critical need for early identification of individuals at high risk of MCI to prevent cognitive decline. The study aimed to investigate the changes in brain structure and function before the onset of MCI. This study enrolled 19 older adults with progressive normal cognition (pNC) to MCI and 19 older adults with stable normal cognition (sNC). The gray matter (GM) volume and functional connectivity (FC) were estimated via magnetic resonance imaging during their normal cognition state 3 years prior. Additionally, spatial associations between FC maps and neurochemical profiles were examined using JuSpace. Compared to the sNC group, the pNC group showed decreased volume in the left hippocampus and left amygdala. The significantly positive correlation was observed between the GM volume of the left hippocampus and the MMSE scores after 3 years in pNC group. Besides, it showed that the pNC group had increased FC between the left hippocampus and the anterior-posterior cingulate gyrus, which was significantly correlated with the spatial distribution of dopamine D2 and noradrenaline transporter. Taken together, the study identified the abnormal brain characteristics before the onset of MCI, which might provide insight into clinical research.

Frequency Specific Optogenetic Stimulation of the Locus Coeruleus Induces Task-Relevant Plasticity in the Motor Cortex

The locus ceruleus (LC) is the primary source of neocortical noradrenaline, which is known to be involved in diverse brain functions including sensory perception, attention, and learning. Previous studies have shown that LC stimulation paired with sensory experience can induce task-dependent plasticity in the sensory neocortex and in the hippocampus. However, it remains unknown whether LC activation similarly impacts neural representations in the agranular motor cortical regions that are responsible for movement planning and production. In this study, we test whether optogenetic stimulation of the LC paired with motor performance is sufficient to induce task-relevant plasticity in the somatotopic cortical motor map. Male and female TH-Cre + rats were trained on a skilled reaching lever-pressing task emphasizing the use of the proximal forelimb musculature, and a viral approach was used to selectively express ChR2 in noradrenergic LC neurons. Once animals reached criterial behavioral performance, they received five training sessions in which correct task performance was paired with optogenetic stimulation of the LC delivered at 3, 10, or 30 Hz. After the last stimulation session, motor cortical mapping was performed using intracortical microstimulation. Our results show that lever pressing paired with LC stimulation at 10 Hz, but not at 3 or 30 Hz, drove the expansion of the motor map representation of the task-relevant proximal FL musculature. These findings demonstrate that phasic, training-paired activation of the LC is sufficient to induce experience-dependent plasticity in the agranular motor cortex and that this LC-driven plasticity is highly dependent on the temporal dynamics of LC activation.

Attention-Deficit/Hyperactivity Disorder Animal Model Presents Retinal Alterations and Methylphenidate Has a Differential Effect in ADHD versus Control Conditions

Attention-Deficit/Hyperactivity Disorder (ADHD) is one of the most prevalent neurodevelopmental disorders. Interestingly, children with ADHD seem to experience more ophthalmologic abnormalities, and the impact of methylphenidate (MPH) use on retinal physiology remains unclear. Thus, we aimed to unravel the retina's structural, functional, and cellular alterations and the impact of MPH in ADHD versus the control conditions. For that, spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) were used as animal models of ADHD and the controls, respectively. Animals were divided into four experimental groups as follows: WKY vehicle (Veh; tap water), WKY MPH (1.5 mg/kg/day), SHR Veh, SHR MPH. Individual administration was performed by gavage between P28-P55. Retinal physiology and structure were evaluated at P56 followed by tissue collection and analysis. The ADHD animal model presents the retinal structural, functional, and neuronal deficits, as well as the microglial reactivity, astrogliosis, blood-retinal barrier (BRB) hyperpermeability and a pro-inflammatory status. In this model, MPH had a beneficial effect on reducing microgliosis, BRB dysfunction, and inflammatory response, but did not correct the neuronal and functional alterations in the retina. Curiously, in the control animals, MPH showed an opposite effect since it impaired the retinal function, neuronal cells, and BRB integrity, and also promoted both microglia reactivity and upregulation of pro-inflammatory mediators. This study unveils the retinal alterations in ADHD and the opposite effects induced by MPH in the retina of ADHD and the control animal models.

Locus coeruleus and dorsal cingulate morphology contributions to slowed processing speed

Slowed information processing speed is a defining feature of cognitive aging. Nucleus locus coeruleus (LC) and medial prefrontal regions are targets for understanding slowed processing speed because these brain regions influence neural and behavioral response latencies through their roles in optimizing task performance. Although structural measures of medial prefrontal cortex have been consistently related to processing speed, it is unclear if 1) declines in LC structure underlie this association because of reciprocal connections between LC and medial prefrontal cortex, or 2) if LC declines provide a separate explanation for age-related changes in processing speed. LC and medial prefrontal structural measures were predicted to explain age-dependent individual differences in processing speed in a cross-sectional sample of 43 adults (19-79 years; 63% female). Higher turbo-spin echo LC contrast, based on a persistent homology measure, and greater dorsal cingulate cortical thickness were significantly and each uniquely related to faster processing speed. However, only dorsal cingulate cortical thickness appeared to statistically mediate age-related differences in processing speed. The results suggest that individual differences in cognitive processing speed can be attributed, in part, to structural variation in nucleus LC and medial prefrontal cortex, with the latter key to understanding why older adults exhibit slowed processing speed.

Theory of visual attention (TVA) applied to rats performing the 5-choice serial reaction time task: differential effects of dopaminergic and noradrenergic manipulations

RATIONALE

Attention is compromised in many psychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). While dopamine and noradrenaline systems have been implicated in ADHD, their exact role in attentional processing is yet unknown.

OBJECTIVES

We applied the theory of visual attention (TVA) model, adapted from human research, to the rat 5-choice serial reaction time task (5CSRTT) to investigate catecholaminergic modulation of visual attentional processing in healthy subjects of high- and low-attention phenotypes.

METHODS

Rats trained on the standard 5CSRTT and tested with variable stimulus durations were treated systemically with noradrenergic and/or dopaminergic agents (atomoxetine, methylphenidate, amphetamine, phenylephrine and atipamezole). TVA modelling was applied to estimate visual processing speed for correct and incorrect visual perceptual categorisations, independent of motor reaction times, as measures of attentional capacity.

RESULTS

Atomoxetine and phenylephrine decreased response frequencies, including premature responses, increased omissions and slowed responding. In contrast, methylphenidate, amphetamine and atipamezole sped up responding and increased premature responses. Visual processing speed was also affected differentially. Atomoxetine and phenylephrine slowed, whereas methylphenidate and atipamezole sped up, visual processing, both for correct and incorrect categorisations. Amphetamine selectively improved visual processing for correct, though not incorrect, responses in high-attention rats only, possibly reflecting improved attention.

CONCLUSIONS

These data indicate that the application of TVA to the 5CSRTT provides an enhanced sensitivity to capturing attentional effects. Unexpectedly, we found overall slowing effects, including impaired visual processing, following drugs either increasing extracellular noradrenaline (atomoxetine) or activating the α1-adrenoceptor (phenylephrine), while also ameliorating premature responses (impulsivity). In contrast, amphetamine had potential pro-attentional effects by enhancing visual processing, probably due to central dopamine upregulation.

Neural resources shift under Methylphenidate: A computational approach to examine anxiety-cognition interplay

The reciprocal interplay between anxiety and cognition is well documented. Anxiety negatively impacts cognition, while cognitive engagement can down-regulate anxiety. The brain mechanisms and dynamics underlying such interplay are not fully understood. To study this question, we experimentally and orthogonally manipulated anxiety (using a threat of shock paradigm) and cognition (using methylphenidate; MPH). The effects of these manipulations on the brain and behavior were evaluated in 50 healthy participants (25 MPH, 25 placebo), using an n-back working memory fMRI task (with low and high load conditions). Behaviorally, improved response accuracy was observed as a main effect of the drug across all conditions. We employed two approaches to understand the neural mechanisms underlying MPH-based cognitive enhancement in safe and threat conditions. First, we performed a hypothesis-driven computational analysis using a mathematical framework to examine how MPH putatively affects cognitive enhancement in the face of induced anxiety across two levels of cognitive load. Second, we performed an exploratory data analysis using Topological Data Analysis (TDA)-based Mapper to examine changes in spatiotemporal brain activity across the entire cortex. Both approaches provided converging evidence that MPH facilitated greater differential engagement of neural resources (brain activity) across low and high working memory load conditions. Furthermore, load-based differential management of neural resources reflects enhanced efficiency that is most powerful during higher load and induced anxiety conditions. Overall, our results provide novel insights regarding brain mechanisms that facilitate cognitive enhancement under MPH and, in future research, may be used to help mitigate anxiety-related cognitive underperformance.

Probing the structure and function of locus coeruleus projections to CNS motor centers

The brainstem nucleus locus coeruleus (LC) sends projections to the forebrain, brainstem, cerebellum and spinal cord and is a source of the neurotransmitter norepinephrine (NE) in these areas. For more than 50 years, LC was considered to be homogeneous in structure and function such that NE would be released uniformly and act simultaneously on the cells and circuits that receive LC projections. However, recent studies have provided evidence that LC is modular in design, with segregated output channels and the potential for differential release and action of NE in its projection fields. These new findings have prompted a radical shift in our thinking about LC operations and demand revision of theoretical constructs regarding impact of the LC-NE system on behavioral outcomes in health and disease. Within this context, a major gap in our knowledge is the relationship between the LC-NE system and CNS motor control centers. While we know much about the organization of the LC-NE system with respect to sensory and cognitive circuitries and the impact of LC output on sensory guided behaviors and executive function, much less is known about the role of the LC-NE pathway in motor network operations and movement control. As a starting point for closing this gap in understanding, we propose using an intersectional recombinase-based viral-genetic strategy TrAC (Tracing Axon Collaterals) as well as established ex vivo electrophysiological assays to characterize efferent connectivity and physiological attributes of mouse LC-motor network projection neurons. The novel hypothesis to be tested is that LC cells with projections to CNS motor centers are scattered throughout the rostral-caudal extent of the nucleus but collectively display a common set of electrophysiological properties. Additionally, we expect to find these LC projection neurons maintain an organized network of axon collaterals capable of supporting selective, synchronous release of NE in motor circuitries for the purpose of coordinately regulating operations across networks that are responsible for balance and movement dynamics. Investigation of this hypothesis will advance our knowledge of the role of the LC-NE system in motor control and provide a basis for treating movement disorders resulting from disease, injury, or normal aging.

Rapid Effects of Vagus Nerve Stimulation on Sensory Processing Through Activation of Neuromodulatory Systems

After sensory information is encoded into neural signals at the periphery, it is processed through multiple brain regions before perception occurs (i.e., sensory processing). Recent work has begun to tease apart how neuromodulatory systems influence sensory processing. Vagus nerve stimulation (VNS) is well-known as an effective and safe method of activating neuromodulatory systems. There is a growing body of studies confirming VNS has immediate effects on sensory processing across multiple sensory modalities. These immediate effects of VNS on sensory processing are distinct from the more well-documented method of inducing lasting neuroplastic changes to the sensory pathways through repeatedly delivering a brief VNS burst paired with a sensory stimulus. Immediate effects occur upon VNS onset, often disappear upon VNS offset, and the modulation is present for all sensory stimuli. Conversely, the neuroplastic effect of pairing sub-second bursts of VNS with a sensory stimulus alters sensory processing only after multiple pairing sessions, this alteration remains after cessation of pairing sessions, and the alteration selectively affects the response properties of neurons encoding the specific paired sensory stimulus. Here, we call attention to the immediate effects VNS has on sensory processing. This review discusses existing studies on this topic, provides an overview of the underlying neuromodulatory systems that likely play a role, and briefly explores the potential translational applications of using VNS to rapidly regulate sensory processing.

Methylphenidate exerts neuroprotective effects through the AMPK signaling pathway

PURPOSE

Cerebral ischemia is the main cause of permanent adult disabilities worldwide. This study investigated the reparative effects and potential mechanisms of methylphenidate (MPH), a medication for the treatment of attention-deficit/hyperactivity disorder.

METHODS

In vitro oxygen-glucose deprivation/reperfusion (OGD/R) and in vivo cerebral ischemia-reperfusion models were established. Sprague-Dawley (SD) rats were randomly divided into four groups (n = 20): Sham, Model, and MPH (0.5 and 1 mg/kg). Rats in MPH groups were treated with 0.5 or 1 mg/kg MPH via intraperitoneal injection for 7 days. Rats in the Sham and Model groups were treated with PBS during the same period. Cell viability was measured using MTT assay. Apoptosis was detected by Annexin V/PI staining. Protein expression was detected by Western blot. The volume of cerebral infarction was detected by triphenyltetrazolium chloride (TTC) staining. The DNA damage in ischemic brain tissues was detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay.

RESULTS

MPH treatment significantly reduced OGD/R-induced cell damage, shown by the increased cell viability and decreased apoptotic rate. p-AMPK and p-ACC protein expression increased in the OGD/R model after MPH treatment. The addition of AMPK inhibitor largely abolished the neuroprotective effects of MPH, evidenced by the reduced cell viability, increased apoptotic rate, and decreased protein expression of p-AMPK as well as p-ACC. Moreover, MPH treatment significantly alleviated the cerebral ischemia-reperfusion injury and decreased apoptosis in brain tissues, which may be associated with the AMPK/ACC pathway.

CONCLUSIONS

MPH exerted protective activities against oxidative stress in the OGD/R model and ameliorated brain damage of rats in the middle cerebral artery occlusion model, at least in part, through activating the AMPK pathway. These data demonstrated neuroprotective properties of MPH and highlighted it as a potential therapeutic agent against cerebral ischemia-reperfusion injury.

Noradrenaline modulates neuronal and perceptual visual detectability via β-adrenergic receptor

RATIONALE

Noradrenaline (NA) is a neuromodulator secreted from noradrenergic neurons in the locus coeruleus to the whole brain depending on the physiological state and behavioral context. It regulates various brain functions including vision via three major adrenergic receptor (AR) subtypes. Previous studies investigating the noradrenergic modulations on vision reported different effects, including improvement and impairment of perceptual visual sensitivity in rodents via β-AR, an AR subtype. Therefore, it remains unknown how NA affects perceptual visual sensitivity via β-AR and what neuronal mechanisms underlie it.

OBJECTIVES

The current study investigated the noradrenergic modulation of perceptual and neuronal visual sensitivity via β-AR in the primary visual cortex (V1).

METHODS

We performed extracellular multi-point recordings from V1 of rats performing a go/no-go visual detection task under the head-fixed condition. A β-AR blocker, propranolol (10 mM), was topically administered onto the V1 surface, and the drug effect on behavioral and neuronal activities was quantified by comparing pre-and post-drug administration.

RESULTS

The topical administration of propranolol onto the V1 surface significantly improved the task performance. An analysis of the multi-unit activity in V1 showed that propranolol significantly suppressed spontaneous activity and facilitated the visual response of the recording sites in V1. We further calculated the signal-to-noise ratio (SNR), finding that the SNR was significantly improved after propranolol administration.

CONCLUSIONS

Pharmacological blockade of β-AR in V1 improves perceptual visual detectability by modifying the SNR of neuronal activity.

The relationship between sensory processing sensitivity and attention deficit hyperactivity disorder traits: A spectrum approach

The aim of the present study was to examine the relationship between sensory processing sensitivity (SPS) and symptoms of Attention Deficit Hyperactivity Disorder (ADHD) in adults. The Highly Sensitive Person Scale (HSPS) scale and the Adult ADHD Self-Report Scale (ASRS) were administered to a non-clinical group of 274 participants recruited from a university volunteers list. We found a highly significant positive correlation between number of self-reported ADHD traits and sensory sensitivity. Furthermore, ADHD traits and age were predictors of SPS and exploratory factor analysis revealed a factor that combined ADHD traits and items from the HSPS. The psychometric properties of the HSPS were also examined supporting the unidimensional nature of the concept. To our knowledge, this is the first study to identify a positive relationship between HSPS and ADHD traits in the general population. Our results further support recent findings suggesting abnormal sensory processing in ADHD.

Prevention of age-associated neuronal hyperexcitability with improved learning and attention upon knockout or antagonism of LPAR2

Recent studies suggest that synaptic lysophosphatidic acids (LPAs) augment glutamate-dependent cortical excitability and sensory information processing in mice and humans via presynaptic LPAR2 activation. Here, we studied the consequences of LPAR2 deletion or antagonism on various aspects of cognition using a set of behavioral and electrophysiological analyses. Hippocampal neuronal network activity was decreased in middle-aged LPAR2^−/− mice, whereas hippocampal long-term potentiation (LTP) was increased suggesting cognitive advantages of LPAR2^−/− mice. In line with the lower excitability, RNAseq studies revealed reduced transcription of neuronal activity markers in the dentate gyrus of the hippocampus in naïve LPAR2^−/− mice, including ARC, FOS, FOSB, NR4A, NPAS4 and EGR2. LPAR2^−/− mice behaved similarly to wild-type controls in maze tests of spatial or social learning and memory but showed faster and accurate responses in a 5-choice serial reaction touchscreen task requiring high attention and fast spatial discrimination. In IntelliCage learning experiments, LPAR2^−/− were less active during daytime but normally active at night, and showed higher accuracy and attention to LED cues during active times. Overall, they maintained equal or superior licking success with fewer trials. Pharmacological block of the LPAR2 receptor recapitulated the LPAR2^−/− phenotype, which was characterized by economic corner usage, stronger daytime resting behavior and higher proportions of correct trials. We conclude that LPAR2 stabilizes neuronal network excitability upon aging and allows for more efficient use of resting periods, better memory consolidation and better  performance in tasks requiring high selective attention. Therapeutic LPAR2 antagonism may alleviate aging-associated cognitive dysfunctions.

Bidirectional pharmacological perturbations of the noradrenergic system differentially affect tactile detection

The brain neuromodulatory systems heavily influence behavioral and cognitive processes. Previous work has shown that norepinephrine (NE), a classic neuromodulator mainly derived from the locus coeruleus (LC), enhances neuronal responses to sensory stimuli. However, the role of the LC-NE system in modulating perceptual task performance is not well understood. In addition, systemic perturbation of NE signaling has often been proposed to specifically target the LC in functional studies, yet the assumption that localized (specific) and systemic (nonspecific) perturbations of LC-NE have the same behavioral impact remains largely untested. In this study, we trained mice to perform a head-fixed, quantitative tactile detection task, and administered an α2 adrenergic receptor agonist or antagonist to pharmacologically down- or up-regulate LC-NE activity, respectively. We addressed the outstanding question of how bidirectional perturbations of LC-NE activity affect tactile detection, and tested whether localized and systemic drug treatments exert the same behavioral effects. We found that both localized and systemic suppression of LC-NE impaired tactile detection by reducing motivation. Surprisingly, while locally activating LC-NE enabled mice to perform in a near-optimal regime, systemic activation impaired behavior by promoting impulsivity. Our results demonstrate that localized silencing and activation of LC-NE differentially affect tactile detection, and that localized and systemic NE activation induce distinct behavioral changes.

Effects of methamphetamine on neural responses to visual stimuli

RATIONALE

The behavioral and reward-related effects of stimulant drugs have been studied extensively; yet the effect of stimulants on sensory processing is still relatively unknown. Prior brain imaging studies have shown that single doses of stimulant drugs increase neural function during cognitive and attentional processes. However, it is not clear if stimulant drugs such as methamphetamine (MA) affect neural responses to novel sensory stimuli, and whether these effects depend on the visual features of the stimuli.

OBJECTIVE

In this study, we examined the effects of a single dose of MA (20 mg oral) on neural activation in response to visual stimuli that varied on "non-straight edges" (NSE), a low-level visual feature that quantifies curved/fragmented edges and is related to perceived image complexity.

METHODS

Healthy adult participants (n = 18) completed two sessions in which they received MA and placebo in counterbalanced order before an fMRI scan where they viewed both high and low NSE images. Participants also completed measures of subjective drug effects throughout both sessions.

RESULTS

During both sessions, high NSE images activated primary visual cortex to a greater extent than low NSE images. Further, MA increased activation only for low NSE images in three areas of visual association cortex: left fusiform, right cingulate/precuneus, and posterior right middle temporal gyrus. This interaction was unrelated to subjective drug effects.

CONCLUSIONS

These findings suggest that stimulant drugs may change the relative sensitivity of higher order sensory processing to increase visual attention when viewing less complex stimuli. Moreover, MA-induced alterations in this type of sensory processing appear to be independent of the drugs' ability to increase feelings of well-being.

Locus coeruleus activation accelerates perceptual learning

Neural representations of the external world are constructed and updated in a manner that depends on behavioral context. For neocortical networks, this contextual information is relayed by a diverse range of neuromodulatory systems, which govern attention and signal the value of internal state variables such as arousal, motivation, and stress. Neuromodulators enable cortical circuits to differentially process specific stimuli and modify synaptic strengths in order to maintain short- or long-term memory traces of significant perceptual events and behavioral episodes. One of the most important subcortical neuromodulatory systems for attention and arousal is the noradrenergic locus coeruleus. Here we report that the noradrenergic system can enhance behavior in rats performing a self-initiated auditory recognition task, and optogenetic stimulation of noradrenergic locus coeruleus neurons accelerated the rate at which trained rats began correctly responding to a change in reward contingency. Animals successively progressed through distinct behavioral epochs, including periods of perseverance and exploration that occurred much more rapidly when animals received locus coeruleus stimulation. In parallel, we made recordings from primary auditory cortex and found that pairing tones with locus coeruleus stimulation led to a similar set of changes to cortical tuning profiles. Thus both behavioral and neural responses go through phases of adjustment for exploring and exploiting environmental reward contingencies. Furthermore, behavioral engagement does not necessarily recruit optimal locus coeruleus activity.

Psychostimulants As Cognitive Enhancers in Adolescents: More Risk than Reward?

Methylphenidate and other psychostimulants, originally developed to treat attention deficit-hyperactivity disorder, are increasingly abused by healthy adolescents and adults seeking an advantage in scholastic performance and work productivity. However, how these drugs may affect cognitive performance, especially in the young brain, remains unclear. Here, we review recent literature and emphasize the risks of abuse of psychostimulants in healthy adolescents and young adults. We conclude that while the desire for cognitive enhancement, particularly with rising costs of education and increasingly competitive nature of scholarship programs, is unlikely to diminish in the near future, it is crucial for the scientific community to thoroughly examine the efficacy and safety of these stimulants in healthy populations across development. The current dearth of knowledge on the dose–response curve, metabolism, and cognitive outcomes in adolescents following methylphenidate or other psychostimulant exposure may be perpetuating a perception of these drugs as “safe” when that might not be true for developing brains.

Noradrenergic Modulation of Cognition in Health and Disease

Norepinephrine released by the locus coeruleus modulates cellular processes and synaptic transmission in the central nervous system through its actions at a number of pre- and postsynaptic receptors. This transmitter system facilitates sensory signal detection and promotes waking and arousal, processes which are necessary for navigating a complex and dynamic sensory environment. In addition to its effects on sensory processing and waking behavior, norepinephrine is now recognized as a contributor to various aspects of cognition, including attention, behavioral flexibility, working memory, and long-term mnemonic processes. Two areas of dense noradrenergic innervation, the prefrontal cortex and the hippocampus, are particularly important with regard to these functions. Due to its role in mediating normal cognitive function, it is reasonable to expect that noradrenergic transmission becomes dysfunctional in a number of neuropsychiatric and neurodegenerative diseases characterized by cognitive deficits. In this review, we summarize the unique role that norepinephrine plays in prefrontal cortical and hippocampal function and how its interaction with its various receptors contribute to cognitive behaviors. We further assess the changes that occur in the noradrenergic system in Alzheimer's disease, Parkinson's disease, attention-deficit/hyperactivity disorder, and schizophrenia and how these changes contribute to cognitive decline in these pathologies.