D-Amphetamine remediates attentional performance in rats with dorsal prefrontal lesions

Parallel Pathways Provide Hippocampal Spatial Information to Prefrontal Cortex

Long-range synaptic connections define how information flows through neuronal networks. Here, we combined retrograde and anterograde trans-synaptic viruses to delineate areas that exert direct and indirect influence over the dorsal and ventral prefrontal cortex (PFC) of the rat (both sexes). Notably, retrograde tracing using pseudorabies virus (PRV) revealed that both dorsal and ventral areas of the PFC receive prominent disynaptic input from the dorsal CA3 (dCA3) region of the hippocampus. The PRV experiments also identified candidate anatomical relays for this disynaptic pathway, namely, the ventral hippocampus, lateral septum, thalamus, amygdala, and basal forebrain. To determine the viability of each of these relays, we performed three additional experiments. In the first, we injected the retrograde monosynaptic tracer Fluoro-Gold into the PFC and the anterograde monosynaptic tracer Fluoro-Ruby into the dCA3 to confirm the first-order connecting areas and revealed several potential relay regions between the PFC and dCA3. In the second, we combined PRV injection in the PFC with polysynaptic anterograde viral tracer (HSV-1) in the dCA3 to reveal colabeled connecting neurons, which were evident only in the ventral hippocampus. In the third, we combined retrograde adeno-associated virus (AAV) injections in the PFC with an anterograde AAV in the dCA3 to reveal anatomical relay neurons in the ventral hippocampus and dorsal lateral septum. Together, these findings reveal parallel disynaptic pathways from the dCA3 to the PFC, illuminating a new anatomical framework for understanding hippocampal-prefrontal interactions. We suggest that the representation of context and space may be a universal feature of prefrontal function.SIGNIFICANCE STATEMENT The known functions of the prefrontal cortex are shaped by input from multiple brain areas. We used transneuronal viral tracing to discover multiple prominent disynaptic pathways through which the dorsal hippocampus (specifically, the dorsal CA3) has the potential to shape the actions of the prefrontal cortex. The demonstration of neuronal relays in the ventral hippocampus and lateral septum presents a new foundation for understanding long-range influences over prefrontal interactions, including the specific contribution of the dorsal CA3 to prefrontal function.

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.

Delayed-matching-to-position working memory in mice relies on NMDA-receptors in prefrontal pyramidal cells

A hypofunction of N-methyl-D-aspartate glutamate receptors (NMDARs) has been implicated in the pathogenesis of schizophrenia by clinical and rodent studies. However, to what extent NMDAR-hypofunction in distinct cell-types across the brain causes different symptoms of this disease is largely unknown. One pharmaco-resistant core symptom of schizophrenia is impaired working memory (WM). NMDARs have been suggested to mediate sustained firing in excitatory neurons of the prefrontal cortex (PFC) that might underlie WM storage. However, if NMDAR-hypofunction in prefrontal excitatory neurons may indeed entail WM impairments is unknown. We here investigated this question in mice, in which NMDARs were genetically-ablated in PFC excitatory cells. This cell type-selective NMDAR-hypofunction caused a specific deficit in a delayed-matching-to-position (DMTP) 5-choice-based operant WM task. In contrast, T-maze rewarded alternation and several psychological functions including attention, spatial short-term habituation, novelty-processing, motivation, sociability, impulsivity, and hedonic valuation remained unimpaired at the level of GluN1-hypofunction caused by our manipulation. Our data suggest that a hypofunction of NMDARs in prefrontal excitatory neurons may indeed cause WM impairments, but are possibly not accounting for most other deficits in schizophrenia.

Operant Assessment of DMTP Spatial Working Memory in Mice

Working memory (WM) is required to bridge the time between the moment of sensory perception and the usage of the acquired information for subsequent actions. Its frequent and pharmacoresistent impairment in mental health disorders urges the development of rodent paradigms through back-translation of human WM tests, ideally avoiding the confounds of alternation-based assays. Here we show, that mice can acquire a delayed-matching-to-position (DMTP) operant spatial WM (SWM) paradigm that is akin to the combined attention and memory (CAM) task previously developed for rats, and that relies on a 5-choice wall [5-CSWM, 5-choice based operant testing of SWM (5-CSWM)]. Requiring ca. 3 months of daily training with a non-illuminated operant box in the default state, mice could attain a performance level of ≥70% choice accuracy with short (2 s) delays in the DMTP 5-CSWM task. Performance decreased with extended delays, as expected for WM processes. Modafinil (15 and 30 mg/kg) and guanfacine (0.3 and 1 mg/kg) showed no consistent efficacy in enhancing task performance. We also found, that mice did not improve beyond chance level, when trained in the DNMTP-version of the 5-CSWM. Our results outline the methodical possibility and constraints of assessing spatial WM in mice with an operant paradigm that provides high control over potentially confounding variables, such as cue-directed attention, motivation or mediating strategies like body-positioning.

Amphetamine improves mouse and human attention in the 5-choice continuous performance test

Non-medical use of prescription stimulants amongst college students is common, with claims of cognitive and academic benefits. The mechanism, magnitude, and pervasiveness of the cognitive enhancing effects of stimulants in healthy adults remain poorly understood however. The present study determined the effects of dextroamphetamine (D-amp) on the 5-choice continuous performance test (5C-CPT) of attention in healthy young adult humans and mice. A mixed gender sample received placebo (n = 29), 10 (n = 17) or 20 mg D-amp (n = 25) in a double-blind fashion before 5C-CPT testing. In addition, male C57BL/6J mice were trained on a touchscreen adaptation of the 5C-CPT and tested after receiving saline or D-amp (0.1, 0.3, 1.0 mg/kg; n = 8/dose). In humans, D-amp significantly improved 5C-CPT performance. Both doses improved signal detection driven by increased hit rate (reduced omissions). Both doses also improved response accuracy and reduced hit reaction time (HRT) variability. In mice, similar effects (improved signal detection, hit rate, and response accuracy) were observed at the moderate dose (0.3 mg/kg). In contrast to human participants however, no effect on HRT variability was detected in mice, with no effect on HRT in either species. Human 5C-CPT performance was consistent with prior studies and consistent with alternative CPT paradigms. The performance of C57BL/6J mice on the touchscreen 5C-CPT mirrored performance of this strain on 5-hole operant chambers. Importantly, comparable facilitation of attention with D-amp was observed in both species. The 5C-CPT provides a cross-species paradigm by which the cognitive enhancing properties of stimulants and the neural underpinnings of attention can be assessed.

Pharmacological Stimulation of Neuronal Plasticity in Acquired Brain Injury

INTRODUCTION

Brain injuries are one of the leading causes of disability worldwide. It is estimated that nearly half of patients who develop severe sequelae will continue with a chronic severe disability despite having received an appropriate rehabilitation program. For more than 3 decades, there has been a worldwide effort to investigate the possibility of pharmacologically stimulating the neuroplasticity process for enhancing the recovery of these patients.

OBJECTIVE

The objective of this article is to make a critical and updated review of the available evidence that supports the positive effect of different drugs on the recovery from brain injury.

METHOD

To date, there have been several clinical trials that tested different drugs that act on different neurotransmitter systems: catecholaminergic, cholinergic, serotonergic, and glutamatergic. There is both basic and clinical evidence that may support some positive effect of these drugs on motor, cognitive, and language skills; however, only few of the available studies are of sufficient methodological quality (placebo controlled, randomized, blinded, multicenter, etc) to make solid conclusions about their beneficial effects.

CONCLUSIONS

Currently, the pharmacological stimulation of neuroplasticity still does not have enough scientific evidence to make a systematic therapeutic recommendation for all patients, but it certainly is a feasible and very promising field for future research.

Improvement of attention with amphetamine in low- and high-performing rats

RATIONALE

Attentional deficits occur in a range of neuropsychiatric disorders, such as schizophrenia and attention deficit hyperactivity disorder. Psychostimulants are one of the main treatments for attentional deficits, yet there are limited reports of procognitive effects of amphetamine in preclinical studies. Therefore, task development may be needed to improve predictive validity when measuring attention in rodents.

OBJECTIVES

This study aimed to use a modified signal detection task (SDT) to determine if and at what doses amphetamine could improve attention in rats.

METHODS

Sprague-Dawley rats were trained on the SDT prior to amphetamine challenge (0.1, 0.25, 0.75 and 1.25 mg/kg). This dose range was predicted to enhance and disrupt cognition with the effect differing between individuals depending on baseline performance.

RESULTS

Acute low dose amphetamine (0.1 and 0.25 mg/kg) improved accuracy, while the highest dose (1.25 mg/kg) significantly disrupted performance. The effects differed for low- and high-performing groups across these doses. The effect of amphetamine on accuracy was found to significantly correlate with baseline performance in rats.

CONCLUSIONS

This study demonstrates that improvement in attentional performance with systemic amphetamine is dependent on baseline accuracy in rats. Indicative of the inverted U-shaped relationship between dopamine and cognition, there was a baseline-dependent shift in performance with increasing doses of amphetamine. The SDT may be a useful tool for investigating individual differences in attention and response to psychostimulants in rodents.

Midline thalamic reuniens lesions improve executive behaviors

The role of the thalamus in complex cognitive behavior is a topic of increasing interest. Here we demonstrate that lesions of the nucleus reuniens (NRe), a midline thalamic nucleus interconnected with both hippocampal and prefrontal circuitry, lead to enhancement of executive behaviors typically associated with the prefrontal cortex. Rats were tested on four behavioral tasks: (1) the combined attention-memory (CAM) task, which simultaneously assessed attention to a visual target and memory for that target over a variable delay; (2) spatial memory using a radial arm maze, (3) discrimination and reversal learning using a touchscreen operant platform, and (4) decision-making with delayed outcomes. Following NRe lesions, the animals became more efficient in their performance, responding with shorter reaction times but also less impulsively than controls. This change, combined with a decrease in perseverative responses, led to focused attention in the CAM task and accelerated learning in the visual discrimination task. There were no observed changes in tasks involving either spatial memory or value-based decision making. These data complement ongoing efforts to understand the role of midline thalamic structures in human cognition, including the development of thalamic stimulation as a therapeutic strategy for acquired cognitive disabilities (Schiff, 2008; Mair et al., 2011), and point to the NRe as a potential target for clinical intervention.

Catecholaminergic based therapies for functional recovery after TBI

Among the many pathophysiologic consequences of traumatic brain injury are changes in catecholamines, including dopamine, epinephrine, and norepinephrine. In the context of TBI, dopamine is the one most extensively studied, though some research exploring epinephrine and norepinephrine have also been published. The purpose of this review is to summarize the evidence surrounding use of drugs that target the catecholaminergic system on pathophysiological and functional outcomes of TBI using published evidence from pre-clinical and clinical brain injury studies. Evidence of the effects of specific drugs that target catecholamines as agonists or antagonists will be discussed. Taken together, available evidence suggests that therapies targeting the catecholaminergic system may attenuate functional deficits after TBI. Notably, it is fairly common for TBI patients to be treated with catecholamine agonists for either physiological symptoms of TBI (e.g. altered cerebral perfusion pressures) or a co-occuring condition (e.g. shock), or cognitive symptoms (e.g. attentional and arousal deficits). Previous clinical trials are limited by methodological limitations, failure to replicate findings, challenges translating therapies to clinical practice, the complexity or lack of specificity of catecholamine receptors, as well as potentially counfounding effects of personal and genetic factors. Overall, there is a need for additional research evidence, along with a need for systematic dissemination of important study details and results as outlined in the common data elements published by the National Institute of Neurological Diseases and Stroke. Ultimately, a better understanding of catecholamines in the context of TBI may lead to therapeutic advancements. This article is part of a Special Issue entitled SI:Brain injury and recovery.

Amphetamine Exerts Dose-Dependent Changes in Prefrontal Cortex Attractor Dynamics during Working Memory

Modulation of neural activity by monoamine neurotransmitters is thought to play an essential role in shaping computational neurodynamics in the neocortex, especially in prefrontal regions. Computational theories propose that monoamines may exert bidirectional (concentration-dependent) effects on cognition by altering prefrontal cortical attractor dynamics according to an inverted U-shaped function. To date, this hypothesis has not been addressed directly, in part because of the absence of appropriate statistical methods required to assess attractor-like behavior in vivo. The present study used a combination of advanced multivariate statistical, time series analysis, and machine learning methods to assess dynamic changes in network activity from multiple single-unit recordings from the medial prefrontal cortex (mPFC) of rats while the animals performed a foraging task guided by working memory after pretreatment with different doses of d-amphetamine (AMPH), which increases monoamine efflux in the mPFC. A dose-dependent, bidirectional effect of AMPH on neural dynamics in the mPFC was observed. Specifically, a 1.0 mg/kg dose of AMPH accentuated separation between task-epoch-specific population states and convergence toward these states. In contrast, a 3.3 mg/kg dose diminished separation and convergence toward task-epoch-specific population states, which was paralleled by deficits in cognitive performance. These results support the computationally derived hypothesis that moderate increases in monoamine efflux would enhance attractor stability, whereas high frontal monoamine levels would severely diminish it. Furthermore, they are consistent with the proposed inverted U-shaped and concentration-dependent modulation of cortical efficiency by monoamines.

Withdrawal effect of chronic amphetamine exposure during adolescence on complex maze performance

National survey data suggest a steady increase in the diagnosis and treatment of mental disorders in children, particularly Attention Deficit/Hyperactivity Disorder (ADHD). As nearly all children diagnosed with ADHD are prescribed stimulant drugs, rationale exists to quantitatively characterize behavioral responses following withdrawal from chronic stimulant dosing. These rodent experiments involved chronic administration of 7.5 mg/kg, s.c. amphetamine to subjects throughout adolescence followed by cognitive tests to gauge learning and performance during the withdrawal stage 7 to 14 days past withdrawal. Tests used a complex Stone 14-unit multiple T-maze, which is a robust paradigm for demonstrating age-related differences in rodent models when behavioral cognitive endpoints are used. Results reveal that amphetamine-treated subjects committed fewer major and retracing errors with increased minor errors and a significantly lower mean completion time. These findings suggest that pharmacotherapy aimed at adolescent-phase treatment of ADHD does not provoke spatial memory deficits at times proximal to drug withdrawal and lends support to amphetamine use in the treatment of ADHD children.

The effects of acute pharmacological stimulation of the 5-HT, NA and DA systems on the cognitive judgement bias of rats in the ambiguous-cue interpretation paradigm

In the present study, we investigated the effects of acute pharmacological stimulation of the serotonergic (5-HT), noradrenergic (NA) and dopaminergic (DA) systems on the valence of cognitive judgement bias of rats in the ambiguous-cue interpretation (ACI) paradigm. To accomplish this goal, after initial behavioural training, different groups of rats received single injections of citalopram, desipramine or d-amphetamine and were subsequently tested with the ACI paradigm. Each drug was administered in 3 doses using a fully randomised Latin square design. Citalopram at the dose of 1mg/kg significantly biased animals towards positive interpretation of the ambiguous cue, while at higher doses (5 and 10mg/kg), the animals interpreted the ambiguous cue more negatively. Desipramine at all 3 tested doses (1, 2 and 5mg/kg) significantly biased animals towards negative interpretation of the ambiguous cue, while d-amphetamine at the dose of 1mg/kg induced positive bias, having no effects at lower doses (0.1 and 0.5mg/kg). Our results indicate that cognitive bias in rats can be influenced by acute pharmacological intervention.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

Subcircuit-specific neuromodulation in the prefrontal cortex

During goal-directed behavior, the prefrontal cortex (PFC) exerts top-down control over numerous cortical and subcortical regions. PFC dysfunction has been linked to many disorders that involve deficits in cognitive performance, attention, motivation, and/or impulse control. A common theme among these disorders is that neuromodulation of the PFC is disrupted. Anatomically, the PFC is reciprocally connected with virtually all neuromodulatory centers. Recent studies of PFC neurons, both in vivo and ex vivo, have found that subpopulations of prefrontal projection neurons can be segregated into distinct subcircuits based on their long-range projection targets. These subpopulations differ in their connectivity, intrinsic properties, and responses to neuromodulators. In this review we outline the evidence for subcircuit-specific neuromodulation in the PFC, and describe some of the functional consequences of selective neuromodulation on behavioral states during goal-directed behavior.

Co-segregation of hyperactivity, active coping styles, and cognitive dysfunction in mice selectively bred for low levels of anxiety

We established mouse models of extremes in trait anxiety, which are based on selective breeding for low vs. normal vs. high open-arm exploration on the elevated plus-maze. Genetically selected low anxiety-related behavior (LAB) coincided with hyperactivity in the home cage. Given the fact that several psychiatric disorders such as schizophrenia, mania, and attention deficit hyperactivity disorder (ADHD) share hyperactivity symptom, we systematically examined LAB mice with respect to unique and overlapping endophenotypes of the three diseases. To this end Venn diagrams were used as an instrument for discrimination of possible models. We arranged the endophenotypes in Venn diagrams and translated them into different behavioral tests. LAB mice showed elevated levels of locomotion in the open field (OF) test with deficits in habituation, compared to mice bred for normal (NAB) and high anxiety-related behavior (HAB). Cross-breeding of hypoactive HAB and hyperactive LAB mice resulted in offspring showing a low level of locomotion comparable to HAB mice, indicating that the HAB alleles are dominant over LAB alleles in determining the level of locomotion. In a holeboard test, LAB mice spent less time in hole exploration, as shown in patients with schizophrenia and ADHD; however, LAB mice displayed no impairments in social interaction and prepulse inhibition (PPI), implying a unlikelihood of LAB as an animal model of schizophrenia. Although LAB mice displayed hyperarousal, active coping styles, and cognitive deficits, symptoms shared by mania and ADHD, they failed to reveal the classic manic endophenotypes, such as increased hedonia and object interaction. The neuroleptic haloperidol reduced locomotor activity in all mouse lines. The mood stabilizer lithium and the psychostimulant amphetamine, in contrast, selectively reduced hyperactivity in LAB mice. Based on the behavioral and pharmacological profiles, LAB mice are suggested as a novel rodent model of ADHD-like symptoms.

Medial prefrontal lesions in mice impair sustained attention but spare maintenance of information in working memory

Working memory and attention are complex cognitive functions that are disrupted in several neuropsychiatric disorders. Mouse models of such human diseases are commonly subjected to maze-based tests that can neither distinguish between these cognitive functions nor isolate specific aspects of either function. Here, we have adapted a simple visual discrimination task, and by varying only the timing of events within the same task construct, we are able to measure independently the behavioral response to increasing attentional demand and increasing length of time that information must be maintained in working memory. We determined that mPFC lesions in mice impair attention but not working memory maintenance.

Neural and behavioral endophenotypes in ADHD

In recent years, descriptive symptom-based approaches of attention deficit hyperactivity disorder (ADHD) have been increasingly replaced by more sophisticated endophenotype-based strategies, better suited to investigate its pathophysiological basis, which is inherently heterogeneous. Measurements derived from neuroimaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI) constitute endophenotypes of growing interest, capable of providing unprecedented windows on neurochemical and neuroanatomical components of psychiatric conditions. This chapter reviews the current state of knowledge regarding putative neural and behavioral endophenotypes of ADHD, across the lifespan. To this end, recent evidence drawn from molecular and structural neuroimaging studies are discussed in the light of widely accepted neuropsychological and pharmacological models of ADHD.

Impact of pharmacological treatments on cognitive and behavioral outcome in the postacute stages of adult traumatic brain injury: a meta-analysis

Pharmacological treatments that are administered to adults in the postacute stage after a traumatic brain injury (TBI) (≥4 weeks after injury) have the potential to reduce persistent cognitive and behavioral problems. While a variety of treatments have been examined, the findings have yet to be consolidated, hampering advances in the treatment of TBI. A meta-analysis of research that has investigated the cognitive and behavioral effects of pharmacological treatments administered in the later stage after TBI was therefore conducted. The PubMed and PsycINFO databases were searched, and Cohen d effect sizes, percent overlap, and failsafe N statistics were calculated for each treatment. Both randomized controlled trials and open-label studies (prospective and retrospective) were included. Nineteen treatments were investigated by 30 independent studies, comprising 395 participants with TBI in the treatment groups and 137 control subjects. When treated in the postacute period, 1 dopaminergic agent (methylphenidate) improved behavior (anger/aggression, psychosocial function) and 1 cholinergic agent (donepezil) improved cognition (memory, attention). In addition, when the injury-to-treatment interval was broadened to include studies that administered treatment just before the postacute period, 2 dopaminergic agents (methylphenidate, amantadine) showed clinically useful treatment benefits for behavior, whereas 1 serotonergic agent (sertraline) markedly impaired cognition and psychomotor speed.

The effects of amphetamine on recovery of function in animal models of cerebral injury: a critical appraisal

Therapeutic strategies to promote recovery from stroke are now beginning to utilize current knowledge of neural plasticity and the neuromodulatory role of physical rehabilitation. Current interests are also focused on adjuvant therapies that may enhance plasticity associated with recovery and rehabilitation. Amphetamine was one of the earliest pharmacological interventions and continues to show promising results as an adjuvant treatment for recovery of function in pre-clinical animal studies. This drug is a potent modulator of neurological function and cortical excitation, acting primarily through norepinephrine and dopamine mechanisms to enhance arousal and attention, and thus, to facilitate learning of motor skills. Although the results from the pre-clinical studies have been primarily positive, they have not translated well to clinical trials, which have yielded mixed results. This review addresses some of the conflicting evidence from pre-clinical studies conducted between 1982 and 2008 in order to better understand how to optimize the clinical application of amphetamine as an adjuvant therapy for stroke recovery. Among many of the factors that relate to differences in outcome, it is likely that both amphetamine dose and the timing of the intervention with respect to the time of injury affected the outcome.

Dopamine release in dissociable striatal subregions predicts the different effects of oral methylphenidate on reversal learning and spatial working memory

Previous data suggest that methylphenidate can have variable effects on different cognitive tasks both within and between individuals. This is thought to be underpinned by inverted U-shaped relationships between cognitive performance and dopaminergic activity in relatively separate fronto-striatal circuits and reflected by individual differences in trait impulsivity. Direct evidence for this is currently lacking. In this study, we demonstrate for the first time that therapeutic doses of oral methylphenidate administered to young healthy subjects result in different sized changes in D(2)/D(3) receptor availability in different regions of the human striatum and that the change in receptor availability within an individual subregion predicts cognitive performance on a particular task. Methylphenidate produced significantly different effects on reversal learning and spatial working memory tasks within individuals. Performance on the reversal learning task was predicted by the drug-induced change in D(2)/D(3) receptor availability in postcommissural caudate, measured using [(11)C]-raclopride radioligand PET imaging, whereas performance on the spatial working memory task was predicted by changes in receptor availability in the ventral striatum. Reversal learning performance was also predicted by subjects' trait impulsivity, such that the most impulsive individuals benefited more from methylphenidate, consistent with this drug's beneficial effects on cognition in attention deficit hyperactivity disorder.

Persistent cognitive dysfunction after traumatic brain injury: A dopamine hypothesis

Traumatic brain injury (TBI) represents a significant cause of death and disability in industrialized countries. Of particular importance to patients the chronic effect that TBI has on cognitive function. Therapeutic strategies have been difficult to evaluate because of the complexity of injuries and variety of patient presentations within a TBI population. However, pharmacotherapies targeting dopamine (DA) have consistently shown benefits in attention, behavioral outcome, executive function, and memory. Still it remains unclear what aspect of TBI pathology is targeted by DA therapies and what time-course of treatment is most beneficial for patient outcomes. Fortunately, ongoing research in animal models has begun to elucidate the pathophysiology of DA alterations after TBI. The purpose of this review is to discuss clinical and experimental research examining DAergic therapies after TBI, which will in turn elucidate the importance of DA for cognitive function/dysfunction after TBI as well as highlight the areas that require further study.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Remediation of attentional dysfunction in rats with lesions of the medial prefrontal cortex by intra-accumbens administration of the dopamine D(2/3) receptor antagonist sulpiride

RATIONALE

Anti-psychotic drugs are widely recognised to produce beneficial effects on impaired cognition in schizophrenia but their mechanism of action is poorly understood. The prefrontal cortex (PFC) and nucleus accumbens (NAC) are key brain loci considered to mediate many of the cognitive deficits associated with schizophrenia and related disorders.

OBJECTIVES

To investigate (1) the effects of selective damage to the PFC on visuo-spatial attention and cognition in the rat and (2) the ability of the anti-psychotic drug sulpiride after its intra-NAC administration to ameliorate cognitive and behavioural deficits produced by lesions of the PFC.

METHODS

Selective lesions of the medial PFC were made using quinolinic acid in rats previously trained on a five-choice serial reaction time task of sustained visual attention (n = 7). Sham rats received phosphate-buffered saline infusions (n = 7). Following a period of recovery, low doses of sulpiride (0.5 ng or 1 ng) were infused into the core sub-region of the NAC of sham and lesioned rats immediately prior to testing on the five-choice task.

RESULTS

Lesions of the medial PFC produced a range of impairments on the five-choice task, including decreased attentional accuracy, slower latencies to respond correctly and increased omissions and premature responses, the latter an operational measure of impulsivity. Intra-NAC sulpiride dose-dependently ameliorated the increased impulsivity and attentional impairment present in PFC-lesioned rats.

CONCLUSIONS

These findings suggest that attentional and cognitive impairment in schizophrenia may be determined in part by a dysregulation of the subcortical dopamine systems occurring as a consequence of damage to the PFC.

Noradrenergic genotype predicts lapses in sustained attention

Sustained attention is modulated by the neurotransmitter noradrenaline. The balance of dopamine and noradrenaline in the cortex is controlled by the DBH gene. The principal variant in this gene is a C/T change at position -1021, and the T allele at this locus is hypothesised to result in a slower rate of dopamine to noradrenaline conversion than the C allele. Two hundred participants who were genotyped for the DBH C-1021T marker performed the Sustained Attention to Response Task (SART). DBH genotype was found to significantly predict performance; participants with more copies of the T allele made more errors of commission, indicative of lapses in sustained attention. A significant negative correlation was also observed for all participants between errors of commission and mean reaction time. The decrease in noradrenaline occasioned by the T allele may impair sustained attention by reducing participants' ability to remain alert throughout the task and by increasing their susceptibility to distractors.

Animal models concerning the role of dopamine in attention-deficit hyperactivity disorder

Several models of attention-deficit hyperactivity disorder (ADHD) have been proposed, ranging from administration of neurotoxins to genetically manipulated models. These models are used to gain insight into ADHD as a disorder and assist in the discovery of new therapeutic strategies. However, the information gained from these models differs, depending to a large extent on the validity (or otherwise) of the model. Thus the insights gained from these models with respect to the pathophysiology and aetiology of ADHD remains inconclusive. No animal model resembles the clinical situation of ADHD perfectly but good animal models of ADHD should mimic its characteristics, confirm to an underlying theory of ADHD and ultimately make predictions of future therapies. While the involvement of dopamine (DA) in ADHD has been established, the evaluation of rodent models of ADHD particularly with respect to dopaminergic systems is attempted here. It is concluded that the neonatal 6-hydroxy-dopamine lesioned rat and DA transporter knockout/knockdown mice have the highest degree of validity for ADHD.

A shift of paradigm: from noradrenergic to dopaminergic modulation of learning?

d-Amphetamine coupled with behavioral training has been effective for improving functional recovery after stroke. d-amphetamine acts on multiple brain transmitter systems, but the recovery enhancing effect has been attributed to its noradrenergic actions. Another potent modulator of learning is dopamine, which may also enhance stroke recovery in humans. Based on data from previous studies of our group, we compared the learning enhancing effects of d-amphetamine with a more selective dopaminergic substance (levodopa) in identical protocols. Using a prospective, randomized, double-blind, placebo-controlled design, we had taught 60 male healthy subjects a miniature lexicon of 50 concrete nouns over the course of five consecutive training days using an associative learning principle. Subjects had received either d-amphetamine (0.25 mg/kg), levodopa/carbidopa (fixed dose of 100/25 mg), or placebo 90 min prior to training on each of the 5 days. Novel word learning was significantly enhanced in both the d-amphetamine and levodopa groups as compared to the placebo group. The learning superiority was maintained at the two re-assessments (1 week and 1 month post training). Both d-amphetamine and levodopa are thus potent drugs in enhancing learning in humans. We here discuss why the efficiency of both d-amphetamine and levodopa may be related to dopaminergic rather than noradrenergic actions.

Auditory inhibitory gating in medial prefrontal cortex: Single unit and local field potential analysis

Medial prefrontal cortex is a crucial region involved in inhibitory processes. Damage to the medial prefrontal cortex can lead to loss of normal inhibitory control over motor, sensory, emotional and cognitive functions. The goal of the present study was to examine the basic properties of inhibitory gating in this brain region in rats. Inhibitory gating has recently been proposed as a neurophysiological assay for sensory filters in higher brain regions that potentially enable or disable information throughput. This perspective has important clinical relevance due to the findings that gating is dramatically impaired in individuals with emotional and cognitive impairments (i.e. schizophrenia). We used the standard inhibitory gating two-tone paradigm with a 500 ms interval between tones and a 10 s interval between tone pairs. We recorded both single unit and local field potentials from chronic microwire arrays implanted in the medial prefrontal cortex. We investigated short-term (within session) and long-term (between session) variability of auditory gating and additionally examined how altering the interval between the tones influenced the potency of the inhibition. The local field potentials displayed greater variability with a reduction in the amplitudes of the tone responses over both the short and long-term time windows. The decrease across sessions was most intense for the second tone response (test tone) leading to a more robust gating (lower T/C ratio). Surprisingly, single unit responses of different varieties retained similar levels of auditory responsiveness and inhibition in both the short and long-term analysis. Neural inhibition decreased monotonically related to the increase in intertone interval. This change in gating was most consistent in the local field potentials. Subsets of single unit responses did not show the lack of inhibition even for the longer intertone intervals tested (4 s interval). These findings support the idea that the medial prefrontal cortex is an important site where early inhibitory functions reside and potentially mediate psychological processes.

Arousal Mechanisms: Speedy Flies Don’t Sleep at Night

Alertness and behavioral performance depend on an animal's level of arousal. In vertebrates, reinforcement and maintenance of arousal in the cortex are ensured by diffuse inputs from neurons releasing biogenic amine neuromodulators. Fruit flies similarly use dopamine for arousal control, indicating an ancient evolutionary origin of this essential feature of the functioning brain.