Chronic treatment with desipramine improves cognitive performance of rats in an attentional set-shifting test

Alcohol, flexible behavior, and the prefrontal cortex: Functional changes underlying impaired cognitive flexibility

Cognitive flexibility enables individuals to alter their behavior in response to changing environmental demands, facilitating optimal behavior in a dynamic world. The inability to do this, called behavioral inflexibility, is a pervasive behavioral phenotype in alcohol use disorder (AUD), driven by disruptions in cognitive flexibility. Research has repeatedly shown that behavioral inflexibility not only results from alcohol exposure across species but can itself be predictive of future drinking. Like many high-level executive functions, flexible behavior requires healthy functioning of the prefrontal cortex (PFC). The scope of this review addresses two primary themes: first, we outline tasks that have been used to investigate flexibility in the context of AUD or AUD models. We characterize these based on the task features and underlying cognitive processes that differentiate them from one another. We highlight the neural basis of flexibility measures, focusing on the PFC, and how acute or chronic alcohol in humans and non-human animal models impacts flexibility. Second, we consolidate findings on the molecular, physiological and functional changes in the PFC elicited by alcohol, that may contribute to cognitive flexibility deficits seen in AUD. Collectively, this approach identifies several key avenues for future research that will facilitate effective treatments to promote flexible behavior in the context of AUD, to reduce the risk of alcohol related harm, and to improve outcomes following AUD. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Effects of chronic stress on cognitive function - From neurobiology to intervention

Exposure to chronic stress contributes considerably to the development of cognitive impairments in psychiatric disorders such as depression, generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and addictive behavior. Unfortunately, unlike mood-related symptoms, cognitive impairments are not effectively treated by available therapies, a situation in part resulting from a still incomplete knowledge of the neurobiological substrates that underly cognitive domains and the difficulty in generating interventions that are both efficacious and safe. In this review, we will present an overview of the cognitive domains affected by stress with a specific focus on cognitive flexibility, behavioral inhibition, and working memory. We will then consider the effects of stress on neuronal correlates of cognitive function and the factors which may modulate the interaction of stress and cognition. Finally, we will discuss intervention strategies for treatment of stress-related disorders and gaps in knowledge with emerging new treatments under development. Understanding how cognitive impairment occurs during exposure to chronic stress is crucial to make progress towards the development of new and effective therapeutic approaches.

Enhanced cognitive flexibility and phasic striatal dopamine dynamics in a mouse model of low striatal tonic dopamine

The catecholamine neuromodulators dopamine and norepinephrine are implicated in motor function, motivation, and cognition. Although roles for striatal dopamine in these aspects of behavior are well established, the specific roles for cortical catecholamines in regulating striatal dopamine dynamics and behavior are less clear. We recently showed that elevating cortical dopamine but not norepinephrine suppresses hyperactivity in dopamine transporter knockout (DAT-KO) mice, which have elevated striatal dopamine levels. In contrast, norepinephrine transporter knockout (NET-KO) mice have a phenotype distinct from DAT-KO mice, as they show elevated extracellular cortical catecholamines but reduced baseline striatal dopamine levels. Here we evaluated the consequences of altered catecholamine levels in NET-KO mice on cognitive flexibility and striatal dopamine dynamics. In a probabilistic reversal learning task, NET-KO mice showed enhanced reversal learning, which was consistent with larger phasic dopamine transients (dLight) in the dorsomedial striatum (DMS) during reward delivery and reward omission, compared to WT controls. Selective depletion of dorsal medial prefrontal cortex (mPFC) norepinephrine in WT mice did not alter performance on the reversal learning task but reduced nestlet shredding. Surprisingly, NET-KO mice did not show altered breakpoints in a progressive ratio task, suggesting intact food motivation. Collectively, these studies show novel roles of cortical catecholamines in the regulation of tonic and phasic striatal dopamine dynamics and cognitive flexibility, updating our current views on dopamine regulation and informing future therapeutic strategies to counter multiple psychiatric disorders.

Prefrontal dynamics and encoding of flexible rule switching

Behavioral flexibility, the ability to adjust behavioral strategies in response to changing environmental contingencies and internal demands, is fundamental to cognitive functions. Despite a large body of pharmacology and lesion studies, the underlying neurophysiological correlates and mechanisms that support flexible rule switching remain elusive. To address this question, we trained mice to distinguish complex sensory cues comprising different perceptual dimensions (set shifting). Endoscopic calcium imaging revealed that medial prefrontal cortex (mPFC) neurons represented multiple task-related events and exhibited pronounced dynamic changes during rule switching. Notably, prominent encoding capacity in the mPFC was associated with switching across, but not within perceptual dimensions. We then showed the involvement of the ascending modulatory input from the locus coeruleus (LC), as inhibiting the LC impaired rule switching behavior and impeded mPFC dynamic processes and encoding. Our results highlight the pivotal role of the mPFC in set shifting processes and demonstrate the profound impact of ascending neuromodulation on shaping prefrontal neural dynamics and behavioral flexibility.

Acute psilocybin enhances cognitive flexibility in rats

Psilocybin has been shown to improve symptoms of depression and anxiety when combined with psychotherapy or other clinician-guided interventions. To understand the neural basis for this pattern of clinical efficacy, experimental and conceptual approaches that are different than traditional laboratory models of anxiety and depression are needed. A potential novel mechanism is that acute psilocybin improves cognitive flexibility, which then enhances the impact of clinician-assisted interventions. Consistent with this idea, we find that acute psilocybin robustly improves cognitive flexibility in male and female rats using a task where animals switched between previously learned strategies in response to uncued changes in the environment. Psilocybin did not influence Pavlovian reversal learning, suggesting that its cognitive effects are selective to enhanced switching between previously learned behavioral strategies. The serotonin (5HT) 2 A receptor antagonist ketanserin blocked psilocybin's effect on set-shifting, while a 5HT2C-selective antagonist did not. Ketanserin alone also improved set-shifting performance, suggesting a complex relationship between psilocybin's pharmacology and its impact on flexibility. Further, the psychedelic drug 2,5-Dimethoxy-4-iodoamphetamine (DOI) impaired cognitive flexibility in the same task, suggesting that this effect of psilocybin does not generalize to all other serotonergic psychedelics. We conclude that the acute impact of psilocybin on cognitive flexibility provides a useful behavioral model to investigate its neuronal effects relevant to its positive clinical outcome.

Acute psilocybin enhances cognitive flexibility in rats

Psilocybin has been shown to improve symptoms of depression and anxiety when combined with psychotherapy or other clinician-guided interventions. To understand the neural basis for this pattern of clinical efficacy, experimental and conceptual approaches that are different than traditional laboratory models of anxiety and depression are needed. A potential novel mechanism is that acute psilocybin improves cognitive flexibility, which then enhances the impact of clinician-assisted interventions. Consistent with this idea, we find that acute psilocybin robustly improves cognitive flexibility in male and female rats using a task where animals switched between previously learned strategies in response to uncued changes in the environment. Psilocybin did not influence Pavlovian reversal learning, suggesting that its cognitive effects are selective to enhanced switching between previously learned behavioral strategies. The serotonin (5HT) 2A receptor antagonist ketanserin blocked psilocybin's effect on set-shifting, while a 5HT2C-selective antagonist did not. Ketanserin alone also improved set-shifting performance, suggesting a complex relationship between psilocybin's pharmacology and its impact on flexibility. Further, the psychedelic drug 2,5-Dimethoxy-4-iodoamphetamine (DOI) impaired cognitive flexibility in the same task, suggesting that this effect of psilocybin does not generalize to all other serotonergic psychedelics. We conclude that the acute impact of psilocybin on cognitive flexibility provides a useful behavioral model to investigate its neuronal effects relevant to its positive clinical outcome.

A combined therapeutic regimen of citalopram and environmental enrichment ameliorates attentional set-shifting performance after brain trauma

Traumatic brain injuries (TBI) have led to lasting deficits for an estimated 5.3 million American patients. Effective therapies for these patients remain scarce and each of the clinical trials stemming from success in experimental models has failed. We believe that the failures may be, in part, due to the lack of preclinical assessment of cognitive domains that widely affect clinical TBI. Specifically, the behavioral tasks in the TBI literature often do not focus on common executive impairments related to the frontal lobe such as cognitive flexibility. In previous work, we have demonstrated that the attentional set-shifting test (AST), a task analogous to the clinically-employed Wisconsin Card Sorting Test (WCST), could be used to identify cognitive flexibility impairments following controlled cortical impact (CCI) injury. In this study, we hypothesized that both the administration of the antidepressant drug citalopram (CIT) and exposure to a preclinical model of neurorehabilitation, environmental enrichment (EE), would attenuate cognitive performance deficits on AST when provided alone and lead to greater benefits when administered in combination. Adult male rats were subjected to a moderate-severe CCI or sham injury. Rats were randomly divided into experimental groups that included surgical injury, drug therapy, and housing condition. We observed that both CIT and EE provided significant cognitive recovery when administered alone and reversal learning performance recovery increased the most when the therapies were combined (p < 0.05). Ongoing studies continue to evaluate novel ways of assessing more clinically relevant measurements of high order cognitive TBI-related impairments in the rat model.

Lack of Cannabinoid Receptor Type-1 Leads to Enhanced Age-Related Neuronal Loss in the Locus Coeruleus

Our laboratory and others have previously shown that cannabinoid receptor type-1 (CB1r) activity is neuroprotective and a modulator of brain ageing; a genetic disruption of CB1r signaling accelerates brain ageing, whereas the pharmacological stimulation of CB1r activity had the opposite effect. In this study, we have investigated if the lack of CB1r affects noradrenergic neurons in the locus coeruleus (LC), which are vulnerable to age-related changes; their numbers are reduced in patients with neurodegenerative diseases and probably also in healthy aged individuals. Thus, we compared LC neuronal numbers between cannabinoid 1 receptor knockout (Cnr1^−/−) mice and their wild-type littermates. Our results reveal that old Cnr1^−/− mice have less noradrenergic neurons compared to their age-matched wild-type controls. This result was also confirmed by the analysis of the density of noradrenergic terminals which proved that Cnr1^−/− mice had less compared to the wild-type controls. Additionally, we assessed pro-inflammatory glial activity in the LC. Although the density of microglia in Cnr1^−/− mice was enhanced, they did not show enhanced inflammatory profile. We hypothesize that CB1r activity is necessary for the protection of noradrenergic neurons, but its anti-inflammatory effect probably only plays a minor role in it.

Impaired Phasic Discharge of Locus Coeruleus Neurons Based on Persistent High Tonic Discharge-A New Hypothesis With Potential Implications for Neurodegenerative Diseases

The locus coeruleus (LC) is a small brainstem nucleus with widely distributed noradrenergic projections to the whole brain, and loss of LC neurons is a prominent feature of age-related neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). This article discusses the hypothesis that in early stages of neurodegenerative diseases, the discharge mode of LC neurons could be changed to a persistent high tonic discharge, which in turn might impair phasic discharge. Since phasic discharge of LC neurons is required for the release of high amounts of norepinephrine (NE) in the brain to promote anti-inflammatory and neuroprotective effects, persistent high tonic discharge of LC neurons could be a key factor in the progression of neurodegenerative diseases. Transcutaneous vagal stimulation (t-VNS), a non-invasive technique that potentially increases phasic discharge of LC neurons, could therefore provide a non-pharmacological treatment approach in specific disease stages. This article focuses on LC vulnerability in neurodegenerative diseases, discusses the hypothesis that a persistent high tonic discharge of LC neurons might affect neurodegenerative processes, and finally reflects on t-VNS as a potentially useful clinical tool in specific stages of AD and PD.

Catecholaminergic modulation of meta-learning

The remarkable expedience of human learning is thought to be underpinned by meta-learning, whereby slow accumulative learning processes are rapidly adjusted to the current learning environment. To date, the neurobiological implementation of meta-learning remains unclear. A burgeoning literature argues for an important role for the catecholamines dopamine and noradrenaline in meta-learning. Here, we tested the hypothesis that enhancing catecholamine function modulates the ability to optimise a meta-learning parameter (learning rate) as a function of environmental volatility. 102 participants completed a task which required learning in stable phases, where the probability of reinforcement was constant, and volatile phases, where probabilities changed every 10-30 trials. The catecholamine transporter blocker methylphenidate enhanced participants' ability to adapt learning rate: Under methylphenidate, compared with placebo, participants exhibited higher learning rates in volatile relative to stable phases. Furthermore, this effect was significant only with respect to direct learning based on the participants' own experience, there was no significant effect on inferred-value learning where stimulus values had to be inferred. These data demonstrate a causal link between catecholaminergic modulation and the adjustment of the meta-learning parameter learning rate.

Deficiencies of microglia and TNFα in the mPFC-mediated cognitive inflexibility induced by social stress during adolescence

The crucial roles played by microglia and their release of cytokines in the regulation of brain maturation are increasingly being recognized. Adolescence is a unique period characterized by continued brain maturation, especially in the area of the prefrontal cortex. Our previous studies showed that adolescent social stress induced impairment in extradimensional set-shifting (EDS), a core component of cognitive flexibility mediated by the medial prefrontal cortex (mPFC) in adult mice. The present study further determined the role of microglia and the inflammatory cytokine tumor necrosis factor alpha (TNFα) in cognitive dysfunction. Accompanied by a deficit in EDS in adulthood, previously stressed mice showed significant reductions in the expression of the microglial molecular biomarker Iba1, cell numbers, and the levels of TNFα mRNA and protein in the mPFC. Pharmacological inhibition of TNFα signaling by direct injection of a neutralizer into the mPFC also specifically impaired EDS performance. Moreover, the cognitive and immune alterations in previously stressed adult mice were ameliorated by both acute LPS and chronic antidepressant treatment. Together, our data suggest that microglia and TNFα play important roles in cognitive flexibility and can provide attractive therapeutic targets for the treatment of cognitive deficits in psychiatric disorders.

A Neuroeconomic Framework for Creative Cognition

Neuroeconomics is the study of the neurobiological bases of subjective preferences and choices. We present a novel framework that synthesizes findings from the literatures on neuroeconomics and creativity to provide a neurobiological description of creative cognition. We propose that value-based decision-making processes and activity in the locus ceruleus-norepinephrine (LC-NE) neuromodulatory system underlie creative cognition, as well as the large-scale brain network dynamics shown to be associated with creativity. This reconceptualization leads to several falsifiable hypotheses that can further understanding of creativity, decision making, and brain network dynamics.

Noradrenergic and Cholinergic Modulation of Belief Updating

To make optimal predictions in a dynamic environment, the impact of new observations on existing beliefs-that is, the learning rate-should be guided by ongoing estimates of change and uncertainty. Theoretical work has proposed specific computational roles for various neuromodulatory systems in the control of learning rate, but empirical evidence is still sparse. The aim of the current research was to examine the role of the noradrenergic and cholinergic systems in learning rate regulation. First, we replicated our recent findings that the centroparietal P3 component of the EEG-an index of phasic catecholamine release in the cortex-predicts trial-to-trial variability in learning rate and mediates the effects of surprise and belief uncertainty on learning rate (Study 1, n = 17). Second, we found that pharmacological suppression of either norepinephrine or acetylcholine activity produced baseline-dependent effects on learning rate following nonobvious changes in an outcome-generating process (Study 1). Third, we identified two genes, coding for α2A receptor sensitivity (ADRA2A) and norepinephrine reuptake (NET), as promising targets for future research on the genetic basis of individual differences in learning rate (Study 2, n = 137). Our findings suggest a role for the noradrenergic and cholinergic systems in belief updating and underline the importance of studying interactions between different neuromodulatory systems.

Prefrontal cortex executive processes affected by stress in health and disease

Prefrontal cortical executive functions comprise a number of cognitive capabilities necessary for goal directed behavior and adaptation to a changing environment. Executive dysfunction that leads to maladaptive behavior and is a symptom of psychiatric pathology can be instigated or exacerbated by stress. In this review we survey research addressing the impact of stress on executive function, with specific focus on working memory, attention, response inhibition, and cognitive flexibility. We then consider the neurochemical pathways underlying these cognitive capabilities and, where known, how stress alters them. Finally, we review work exploring potential pharmacological and non-pharmacological approaches that can ameliorate deficits in executive function. Both preclinical and clinical literature indicates that chronic stress negatively affects executive function. Although some of the circuitry and neurochemical processes underlying executive function have been characterized, a great deal is still unknown regarding how stress affects these processes. Additional work focusing on this question is needed in order to make progress on developing interventions that ameliorate executive dysfunction.

Assessment of intradimensional/extradimensional attentional set-shifting in rats

The rat intradimensional/extradimensional (ID/ED) task, first described by Birrell and Brown 18 years ago, has become the predominant means by which attentional set-shifting is investigated in rodents: the use of rats in the task has been described in over 135 publications by researchers from nearly 90 universities and pharmaceutical companies. There is variation in the protocols used by different groups, including differences in apparatus, stimuli (both stimulus dimensions and exemplars within), and also the methodology. Nevertheless, most of these variations seem to be of little consequence: there is remarkable similarity in the profile of published data, with consistency of learning rates and in the size and reliability of the set-shifting and reversal 'costs'. However, we suspect that there may be inconsistent data that is unpublished or perhaps 'failed experiments' that may have been caused by unintended deviations from effective protocols. The purpose of this review is to describe our approach and the rationale behind certain aspects of the protocol, including common pitfalls that are encountered when establishing an effective local protocol.

Acute and Chronic Noradrenergic Effects on Cortical Excitability in Healthy Humans

Background

Noradrenaline is a major neuromodulator in the central nervous system, and it is involved in the pathophysiology of diverse neuropsychiatric diseases. Previous transcranial magnetic stimulation studies suggested that acute application of selective noradrenaline reuptake inhibitors enhances cortical excitability in the human brain. However, other, such like clinical effects, usually require prolonged noradrenaline reuptake inhibitor treatment, which might go along with different physiological effects.

Methods

The purpose of this study was to investigate the acute and chronic effects of the selective noradrenaline reuptake inhibitor reboxetine on cortical excitability in healthy humans in a double-blind, placebo-controlled, randomized crossover study. Sixteen subjects were assessed with different transcranial magnetic stimulation measurements: motor thresholds, input-output curve, short-latency intracortical inhibition and intracortical facilitation, I-wave facilitation, and short-interval afferent inhibition before and after placebo or reboxetine (8 mg) single-dose administration. Afterwards, the same subjects took reboxetine (8 mg/d) consecutively for 21 days. During this period (subjects underwent 2 experimental sessions with identical transcranial magnetic stimulation measures under placebo or reboxetine), transcranial magnetic stimulation measurements were assessed before and after drug intake.

Results

Both single-dose and chronic administration of reboxetine increased cortical excitability; increased the slope of the input-output curve, intracortical facilitation, and I-wave facilitation; but decreased short-latency intracortical inhibition and short-interval afferent inhibition. Moreover, chronic reboxetine showed a larger enhancement of intracortical facilitation and I-wave facilitation compared with single-dose application.

Conclusions

The results show physiological mechanisms of noradrenergic enhancement possibly underlying the functional effects of reboxetine regarding acute and chronic application.

Catecholaminergic Regulation of Learning Rate in a Dynamic Environment

Adaptive behavior in a changing world requires flexibly adapting one's rate of learning to the rate of environmental change. Recent studies have examined the computational mechanisms by which various environmental factors determine the impact of new outcomes on existing beliefs (i.e., the 'learning rate'). However, the brain mechanisms, and in particular the neuromodulators, involved in this process are still largely unknown. The brain-wide neurophysiological effects of the catecholamines norepinephrine and dopamine on stimulus-evoked cortical responses suggest that the catecholamine systems are well positioned to regulate learning about environmental change, but more direct evidence for a role of this system is scant. Here, we report evidence from a study employing pharmacology, scalp electrophysiology and computational modeling (N = 32) that suggests an important role for catecholamines in learning rate regulation. We found that the P3 component of the EEG-an electrophysiological index of outcome-evoked phasic catecholamine release in the cortex-predicted learning rate, and formally mediated the effect of prediction-error magnitude on learning rate. P3 amplitude also mediated the effects of two computational variables-capturing the unexpectedness of an outcome and the uncertainty of a preexisting belief-on learning rate. Furthermore, a pharmacological manipulation of catecholamine activity affected learning rate following unanticipated task changes, in a way that depended on participants' baseline learning rate. Our findings provide converging evidence for a causal role of the human catecholamine systems in learning-rate regulation as a function of environmental change.

The effects of prolonged administration of norepinephrine reuptake inhibitors on long-term potentiation in dentate gyrus, and on tests of spatial and object recognition memory in rats

Phasic norepinephrine (NE) release events are involved in arousal, novelty detection and in plasticity processes underlying learning and memory in mammalian systems. Although the effects of phasic NE release events on plasticity and memory are prevalently documented, it is less understood what effects chronic NE reuptake inhibition and sustained increases in noradrenergic tone, might have on plasticity and cognitive processes in rodent models of learning and memory. This study investigates the effects of chronic NE reuptake inhibition on hippocampal plasticity and memory in rats. Rats were administered NE reuptake inhibitors (NRIs) desipramine (DMI; 0, 3, or 7.5mg/kg/day) or nortriptyline (NTP; 0, 10 or 20mg/kg/day) in drinking water. Long-term potentiation (LTP; 200 Hz) of the perforant path-dentate gyrus evoked potential was examined in urethane anesthetized rats after 30-32 days of DMI treatment. Short- (4-h) and long-term (24-h) spatial memory was tested in separate rats administered 0 or 7.5mg/kg/day DMI (25-30 days) using a two-trial spatial memory test. Additionally, the effects of chronically administered DMI and NTP were tested in rats using a two-trial, Object Recognition Test (ORT) at 2- and 24-h after 45 and 60 days of drug administration. Rats administered 3 or 7.5mg/kg/day DMI had attenuated LTP of the EPSP slope but not the population spike at the perforant path-dentate gyrus synapse. Short- and long-term memory for objects is differentially disrupted in rats after prolonged administration of DMI and NTP. Rats that were administered 7.5mg/kg/day DMI showed decreased memory for a two-trial spatial task when tested at 4-h. In the novel ORT, rats receiving 0 or 7.5mg/kg/day DMI showed a preference for the arm containing a Novel object when tested at both 2- and 24-h demonstrating both short- and long-term memory retention of the Familiar object. Rats that received either dose of NTP or 3mg/kg/day DMI showed impaired memory at 2-h, however this impairment was largely reversed at 24-h. Animals in the high-dose NTP (20mg/kg/day) group were impaired at both short- and long-term intervals. Activity levels, used as an index of location memory during the ORT, demonstrated that rats receiving DMI were again impaired at retaining memory for location. DMI dose-dependently disrupts LTP in the dentate gyrus of anesthetized rats and also disrupts memory for tests of spatial memory when administered for long periods.

Optogenetic silencing of locus coeruleus activity in mice impairs cognitive flexibility in an attentional set-shifting task

The locus coeruleus (LC) is the sole source of noradrenergic projections to the cortex and essential for attention-dependent cognitive processes. In this study we used unilateral optogenetic silencing of the LC in an attentional set-shifting task (ASST) to evaluate the influence of the LC on prefrontal cortex-dependent functions in mice. We expressed the halorhodopsin eNpHR 3.0 to reversibly silence LC activity during task performance, and found that silencing selectively impaired learning of those parts of the ASST that most strongly rely on cognitive flexibility. In particular, extra-dimensional set-shifting (EDS) and reversal learning was impaired, suggesting an involvement of the medial prefrontal cortex (mPFC) and the orbitofrontal cortex. In contrast, those parts of the task that are less dependent on cognitive flexibility, i.e., compound discrimination (CD) and the intra-dimensional shifts (IDS) were not affected. Furthermore, attentional set formation was unaffected by LC silencing. Our results therefore suggest a modulatory influence of the LC on cognitive flexibility, mediated by different frontal networks.

Atomoxetine accelerates attentional set shifting without affecting learning rate in the rat

RATIONALE

Shifting to a new rule is a form of behavioral flexibility that is impaired in numerous psychiatric and neurological illnesses. Animal studies have revealed that this form of flexibility depends upon norepinephrine (NE) neurotransmission. Atomoxetine, a NE reuptake inhibitor, improves performance of humans in set shifting tasks.

OBJECTIVE

Our objective was to validate its effects in a rodent set shifting task.

METHODS

We tested the drug effect using an operant task that required a shift from a visual cue-guided behavior to a novel location-guided rule.

RESULTS

A 1.0-mg/kg dose significantly accelerated rule shifting without affecting learning strategies, such as win-stay or lose-shift. Fitting behavioral performance with a learning function provided a measure of learning rate.

CONCLUSION

This novel analysis revealed that atomoxetine accelerated shifting to the new rule without affecting learning rate.

Executive function

Components of human executive function, like rule generation and selection in response to stimuli (attention set-shifting) or overcoming a habit (reversal learning), can be reliably modelled in rodents. The rodent paradigms are based upon tasks that assess cognitive flexibility in clinical populations and have been effective in distinguishing the neurobiological substrates and the underlying neurotransmitter systems relevant to executive function. A review of the literature on the attentional set-shifting task highlights a prominent role for the medial region of the prefrontal cortex in the ability to adapt to a new rule (extradimensional shift) while the orbitofrontal cortex has been associated with the reversal learning component of the task. In other paradigms specifically developed to examine reversal learning in rodents, the orbitofrontal cortex also plays a prominent role. Modulation of dopamine, serotonin, and glutamatergic receptors can disrupt executive function, a feature commonly exploited to develop concepts underlying psychiatric disorders. While these paradigms do have excellent translational construct validity, they have been less effective as predictive preclinical models for cognitive enhancers, especially for cognition in health subjects. Accordingly, a more diverse battery of tasks may be necessary to model normal human executive function in the rodent for drug development.

Animal models of obsessive-compulsive spectrum disorders

Obsessive-compulsive disorder (OCD) and related conditions (trichotillomania, pathological skin-picking, pathological nail-biting) are common and disabling. Current treatment approaches fail to help a significant proportion of patients. Multiple tiers of evidence link these conditions with underlying dysregulation of particular cortico-subcortical circuitry and monoamine systems, which represent targets for treatment. Animal models designed to capture aspects of these conditions are critical for several reasons. First, they help in furthering our understanding of neuroanatomical and neurochemical underpinnings of the obsessive-compulsive (OC) spectrum. Second, they help to account for the brain mechanisms by which existing treatments (pharmacotherapy, psychotherapy, deep brain stimulation) exert their beneficial effects on patients. Third, they inform the search for novel treatments. This article provides a critique of key animal models for selected OC spectrum disorders, beginning with initial work relating to anxiety, but moving on to recent developments in domains of genetic, pharmacological, cognitive, and ethological models. We find that there is a burgeoning literature in these areas with important ramifications, which are considered, along with salient future lines of research.

The neural and genetic basis of executive function: attention, cognitive flexibility, and response inhibition

Executive function is a collection of cognitive processes essential for higher order mental function. Processes involved in executive function include, but are not limited to, working memory, attention, cognitive flexibility, and impulse control. These complex behaviors are largely mediated by prefrontal cortical function but are modulated by dopaminergic, noradrenergic, serotonergic, and cholinergic input. The ability of these neurotransmitter systems to modulate executive function allows for adaptation in cognitive behavior in response to changes in the environment. Because of the important role these neurotransmitter systems play in regulating executive function, changes in these systems can also have a grave impact on executive function. In addition, polymorphisms in genes associated with these neurotransmitters are associated with phenotypic differences in executive function. Understanding how these naturally occurring polymorphisms contribute to different executive function phenotypes will advance basic knowledge of cognition and potentially further understanding and treatment of mental illness that involve changes in executive function. In this review, we will examine the influence of dopamine, norepinephrine, serotonin, and acetylcholine on the following measures of executive function: attention, cognitive flexibility, and impulse control. We will also review the effects of polymorphisms in genes associated with these neurotransmitter systems on these measures of executive function.

Noradrenergic modulation of cognition: therapeutic implications

The noradrenaline (norepinephrine) system exerts profound influences on cognition via ascending projections to the forebrain, mostly originating from the locus coeruleus. This paper provides an overview of available infrahuman and healthy human studies, exploring the effects of specific noradrenergic manipulations on dissociable cognitive functions, including attention, working memory, cognitive flexibility, response inhibition and emotional memory. Remarkable parallels across species have been reported which may account for the mechanisms by which noradrenergic medications exert their beneficial effects in disorders such as depression and attention-deficit hyperactivity disorder (ADHD). The literature is discussed in relation to prevailing models of noradrenergic influences over cognition and novel therapeutic directions, including in relation to investigating the effects of noradrenergic manipulations on other disorders characterized by impulsivity, and dementias. Unanswered questions are also highlighted, along with key avenues for future research, both proof-of-concept and clinical.

Evidence for a regional specificity in the density and distribution of noradrenergic varicosities in rat cortex

The brainstem nucleus locus coeruleus (LC) is the sole source of norepinephrine (NE)-containing fibers in the mammalian cortex. Previous studies suggest that the density of noradrenergic fibers in rat is relatively uniform across cortical regions and that cells in the nucleus discharge en masse. This implies that activation of the LC results in equivalent release of NE throughout the cortex. However, it is possible that there could be differences in the density of axonal varicosities across regions, and that these differences, rather than a difference in fiber density, may contribute to the regulation of NE efflux. Quantification of dopamine β-hydroxylase (DβH)-immunostained varicosities was performed on several cortical regions and in the ventral posterior medial (VPM) thalamus by using unbiased sampling methods. The density of DβH varicosities is greater in the prefrontal cortex than in motor, somatosensory, or piriform cortices, greater in superficial than in deep layers of cortex, and greater in the VPM than in the somatosensory cortex. Our results provide anatomical evidence for non-uniform release of NE across functionally discrete cortical regions. This morphology may account for a differential, region-specific, impact of LC output on different cortical areas.

Atomoxetine facilitates attentional set shifting in adolescent rats

Adolescent rats show immaturities in executive function and are less able than adult rats to learn reinforcement reversals and shift attentional set. These two forms of executive function rely on the functional integrity of the orbitofrontal and prelimbic cortices respectively. Drugs used to treat attention deficit disorder, such as atomoxetine, that increase cortical catecholamine levels improve executive functions in humans, non-human primates and adult rats with prefrontal lesions. Cortical noradrenergic systems are some of the last to mature in primates and rats. Moreover, norepinephrine transporters (NET) are higher in juvenile rats than adults. The underdeveloped cortical noradrenergic system and higher number of NET are hypothesized to underlie the immaturities in executive function found in adolescents. We assessed executive function in male Long-Evans rats using an intra-dimensional/extradimensional set shifting task. We administered the NET blocker, atomoxetine (0.0, 0.1, 0.9 mg/kg/ml; i.p.), prior to the test of attentional set shift and a reinforcement reversal. The lowest dose of drug facilitated attentional set shifting but had no effect on reversal learning. These data demonstrate that NET blockade allows adolescent rats to more easily perform attentional set shifting.

5-HT2A receptors in the orbitofrontal cortex facilitate reversal learning and contribute to the beneficial cognitive effects of chronic citalopram treatment in rats

Chronic stress is a risk factor for depression, and chronic stress can induce cognitive impairments associated with prefrontal cortical dysfunction, which are also major components of depression. We have previously shown that 5 wk chronic intermittent cold (CIC) stress induced a reversal-learning deficit in rats, associated with reduced serotonergic transmission in the orbitofrontal cortex (OFC) that was restored by chronic treatment with a selective serotonin reuptake inhibitor (SSRI). However, the mechanisms underlying the beneficial cognitive effects of chronic SSRI treatment are currently unknown. Thus, the purpose of this study was to investigate the potential modulatory influence specifically of 5-HT2A receptors (5-HT2ARs) in the OFC on reversal learning, and their potential contribution to the beneficial cognitive effects of chronic SSRI treatment. Bilateral microinjections of the selective 5-HT2AR antagonist, MDL 100,907 into OFC (0.02-2.0 nmol) had a dose-dependent detrimental effect on a reversal-learning task, suggesting a facilitatory influence of 5-HT2ARs in the OFC. In the next experiment, rats were exposed to 5 wk CIC stress, which compromised reversal learning, and treated chronically with the SSRI, citalopram (20 mg/kg.d) during the final 3 wk of chronic stress. Chronic citalopram treatment improved reversal learning in the CIC-stressed rats, and bilateral microinjection of MDL 100,907 (0.20 nmol, the optimal dose from the preceding experiment) into OFC once again had a detrimental effect on reversal learning, opposing the beneficial effect of citalopram. We conclude that 5-HT2ARs in the OFC facilitate reversal learning, and potentially contribute to the beneficial cognitive effects of chronic SSRI treatment.

Cognitive impairment in major depression and the mGlu2 receptor as a therapeutic target

Cognitive impairment, in particular of attention and memory, is often reported by patients suffering from major depressive disorder (MDD) and deficits in attention are part of the current diagnostic criteria of MDD. Objectively measured cognitive deficits associated with MDD have been described in many studies. They have been conceptualized as an integral facet and epiphenomenon of MDD. However, evidence accumulated in recent years has challenged this notion and demonstrated that in a subset of patients the degree of cognitive deficits cannot be accounted for by the severity of depression. In addition, in some patients cognitive deficits persist despite resolution of depressive symptomatology. It is plausible to assume that cognitive deficits contribute to functional impairment even though supportive data for such a relationship are lacking. However, the exact association between cognitive deficits and major depression and the clinical and neurobiological characteristics of patients with MDD in whom cognitive deficits seem partially or fully independent of the clinical manifestation of depressive symptoms remain poorly understood. This review focuses on objective measures of non-emotional cognitive deficits in MDD and discusses the presence of a subgroup of patients in whom these symptoms can be defined independently and in dissociation from the rest of the depressive symptomatology. The current understanding of brain circuits and molecular events implicated in cognitive impairment in MDD are discussed with an emphasis on the missing elements that could further define the specificity of cognitive impairment in MDD and lead to new therapeutics. Furthermore, this article presents in detail observations made in behavioral studies in rodents with potential novel therapeutic agents, such as negative allosteric modulators at the metabotropic glutamate receptor type 2/3 (mGlu2/3 NAM) which exhibit both cognitive enhancing and antidepressant properties. Such a compound, RO4432717, was tested in tests of short term memory (delayed match to position), cognitive flexibility (Morris water maze, reversal protocol), impulsivity and compulsivity (5-choice serial reaction time) and spontaneous object recognition in rodents, providing first evidence of a profile potentially relevant to address cognitive impairment in MDD. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The emerging role of norepinephrine in cognitive dysfunctions of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1% of the population over age 60. In those patients cognitive dysfunction is a persistent issue that impairs quality of life and productivity. Neuropathological studies demonstrate significant damage in brain regions outside the nigral dopamine (DA) system, including early degeneration of locus coeruleus norepinephrine (LC-NE) neurons, yet discussion of PD and treatment focus has remained dopaminergic-based. Motor symptoms benefit from DA replacement for many years, but other symptoms including several cognitive deficits continue unabated. Recent interest in non-DA substrates of PD highlights early involvement of LC-NE neurons and provides evidence for a prodromal phase, with cognitive disturbance, even in sporadic PD. We outline insights from basic research in LC-NE function to clinical and pathological evidence highlighting a role for NE in PD cognitive dysfunction. We propose that loss of LC-NE regulation, particularly in higher cortical regions, critically underlies certain cognitive dysfunctions in early PD. As a major unmet need for patients, research and use of NE drugs in PD may provide significant benefits for cognitive processing.

Nicotine improves performance in an attentional set shifting task in rats

A large number of studies in both humans and experimental animals have demonstrated nicotine-induced improvements in various aspects of cognitive function, including attention and memory. The prefrontal cortex (PFC) is thought to be critically involved in the modulation of executive function and these attentional processes are enhanced by nicotine acting at nicotinic acetylcholine receptors. The involvement of nicotinic processes on cognitive flexibility in particular has not been specifically investigated. The effects of nicotine on attentional flexibility were therefore evaluated using the rodent attentional set shifting task in rats. Nicotine injected both acutely and following repeated pre-exposure significantly improved both intradimensional and extradimensional set shifting performance in the task. Further investigation of the acute effects of nicotine demonstrated this improvement in attentional flexibility to be dose-dependent. These results implicate the nicotinic receptor system in the mediation of processes underlying cognitive flexibility and suggest that nicotine improves attentional flexibility in rats, both within and between perceptual dimensions of a compound stimulus. Nicotine-induced alterations in prefrontal circuitry may underlie these effects on cognitive flexibility. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

On the improvement of inhibitory response control and visuospatial attention by indirect and direct adrenoceptor agonists

RATIONALE

The clinical efficacy of the monoamine and noradrenaline transporter inhibitors methylphenidate and atomoxetine in attention deficit/hyperactivity disorder implicates noradrenergic neurotransmission in modulating inhibitory response control processes. Nonetheless, it is unclear which adrenoceptor subtypes are involved in these effects.

OBJECTIVES

The present study aimed at investigating the effects of adrenoceptor agonists on inhibitory response control as assessed in the rodent 5-choice serial reaction time task, a widely used translational model to measure this executive cognitive function.

RESULTS

Consistent with the previous reported effects of atomoxetine, the noradrenaline transporter inhibitor desipramine improved inhibitory response control, albeit the effect size was smaller compared to that of atomoxetine. Methylphenidate exerted a bimodal effect on inhibitory response control. Interestingly, the preferential β2-adrenoceptor agonist clenbuterol improved inhibitory response control. Moreover, clenbuterol improved visuospatial attention in the task, an effect that was also observed with the preferential β1-adrenoceptor agonist dobutamine. By contrast, although the preferential α1-adrenoceptor and α2-adrenoceptor agonists (phenylephrine and clonidine, respectively) and the non-selective β-adrenoceptor agonist (isoprenaline) were found to alter inhibitory response control, this was probably secondary to the simultaneous increments in response latencies and omissions observed at effective doses.

CONCLUSIONS

Taken together, these findings further strengthen the notion of noradrenergic modulation of inhibitory response control and attentional processes and particularly reveal the involvement of β2-adrenoceptors therein.

Corticotropin-releasing factor in the norepinephrine nucleus, locus coeruleus, facilitates behavioral flexibility

Corticotropin-releasing factor (CRF), the stress-related neuropeptide, acts as a neurotransmitter in the brain norepinephrine nucleus, locus coeruleus (LC), to activate this system during stress. CRF shifts the mode of LC discharge from a phasic to a high tonic state that is thought to promote behavioral flexibility. To investigate this, the effects of CRF administered either intracerebroventricularly (30-300 ng, i.c.v.) or directly into the LC (intra-LC; 2-20 ng) were examined in a rat model of attentional set shifting. CRF differentially affected components of the task depending on dose and route of administration. Intracerebroventricular CRF impaired intradimensional set shifting, reversal learning, and extradimensional set shifting (EDS) at different doses. In contrast, intra-LC CRF did not impair any aspect of the task. The highest dose of CRF (20 ng) facilitated reversal learning and the lowest dose (2 ng) improved EDS. The dose-response relationship for CRF on EDS performance resembled an inverted U-shaped curve with the highest dose having no effect. Intra-LC CRF also elicited c-fos expression in prefrontal cortical neurons with an inverted U-shaped dose-response relationship. The number of c-fos profiles was positively correlated with EDS performance. Given that CRF excites LC neurons, the ability of intra-LC CRF to activate prefrontal cortical neurons and facilitate EDS is consistent with findings implicating LC-norepinephrine projections to medial prefrontal cortex in this process. Importantly, the results suggest that CRF release in the LC during stress facilitates shifting of attention between diverse stimuli in a dynamic environment so that the organism can adapt an optimal strategy for coping with the challenge.

Reversal learning as a measure of impulsive and compulsive behavior in addictions

BACKGROUND

Our ability to measure the cognitive components of complex decision-making across species has greatly facilitated our understanding of its neurobiological mechanisms. One task in particular, reversal learning, has proven valuable in assessing the inhibitory processes that are central to executive control. Reversal learning measures the ability to actively suppress reward-related responding and to disengage from ongoing behavior, phenomena that are biologically and descriptively related to impulsivity and compulsivity. Consequently, reversal learning could index vulnerability for disorders characterized by impulsivity such as proclivity for initial substance abuse as well as the compulsive aspects of dependence.

OBJECTIVE

Though we describe common variants and similar tasks, we pay particular attention to discrimination reversal learning, its supporting neural circuitry, neuropharmacology and genetic determinants. We also review the utility of this task in measuring impulsivity and compulsivity in addictions.

METHODS

We restrict our review to instrumental, reward-related reversal learning studies as they are most germane to addiction.

CONCLUSION

The research reviewed here suggests that discrimination reversal learning may be used as a diagnostic tool for investigating the neural mechanisms that mediate impulsive and compulsive aspects of pathological reward-seeking and -taking behaviors. Two interrelated mechanisms are posited for the neuroadaptations in addiction that often translate to poor reversal learning: frontocorticostriatal circuitry dysregulation and poor dopamine (D2 receptor) modulation of this circuitry. These data suggest new approaches to targeting inhibitory control mechanisms in addictions.

Chronic desipramine treatment alters tyrosine hydroxylase but not norepinephrine transporter immunoreactivity in norepinephrine axons in the rat prefrontal cortex

Pharmacological blockade of norepinephrine (NE) reuptake is clinically effective in treating several mental disorders. Drugs that bind to the NE transporter (NET) alter both protein levels and activity of NET and also the catecholamine synthetic enzyme tyrosine hydroxylase (TH). We examined the rat prefrontal cortex (PFC) by electron microscopy to determine whether the density and subcellular distribution of immunolabelling for NET and co-localization of NET with TH within individual NE axons were altered by chronic treatment with the selective NE uptake inhibitor desipramine (DMI). Following DMI treatment (21 d, 15 mg/kg.d), NET-immunoreactive (ir) axons were significantly less likely to co-localize TH. This finding is consistent with reports of reduced TH levels and activity in the locus coeruleus after chronic DMI and indicates a reduction of NE synthetic capacity in the PFC. Measures of NET expression and membrane localization, including the number of NET-ir profiles per tissue area sampled, the number of gold particles per NET-ir profile area, and the proportion of gold particles associated with the plasma membrane, were similar in DMI- and vehicle-treated rats. These findings were verified using two different antibodies directed against distinct epitopes of the NET protein. The results suggest that chronic DMI treatment does not reduce NET expression within individual NE axons in vivo or induce an overall translocation of NET protein away from the plasma membrane in the PFC as measured by ultrastructural immunogold labelling. Our findings encourage consideration of possible post-translational mechanisms for regulating NET activity in antidepressant-induced modulation of NE clearance.

Stress modulation of cognitive and affective processes

This review summarizes the major discussion points of a symposium on stress modulation of cognitive and affective processes, which was held during the 2010 workshop on the neurobiology of stress (Boulder, CO, USA). The four discussants addressed a number of specific cognitive and affective factors that are modulated by exposure to acute or repeated stress. Dr David Morilak discussed the effects of various repeated stress situations on cognitive flexibility, as assessed with a rodent model of attentional set-shifting task, and how performance on slightly different aspects of this test is modulated by different prefrontal regions through monoaminergic neurotransmission. Dr Serge Campeau summarized the findings of several studies exploring a number of factors and brain regions that regulate habituation of various autonomic and neuroendocrine responses to repeated audiogenic stress exposures. Dr Kerry Ressler discussed a body of work exploring the modulation and extinction of fear memories in rodents and humans, especially focusing on the role of key neurotransmitter systems including excitatory amino acids and brain-derived neurotrophic factor. Dr Israel Liberzon presented recent results on human decision-making processes in response to exogenous glucocorticoid hormone administration. Overall, these discussions are casting a wider framework on the cognitive/affective processes that are distinctly regulated by the experience of stress and some of the brain regions and neurotransmitter systems associated with these effects.

Factor analysis of attentional set-shifting performance in young and aged mice

Background

Executive dysfunction may play a major role in cognitive decline with aging because frontal lobe structures are particularly vulnerable to advancing age. Lesion studies in rats and mice have suggested that intradimensional shifts (IDSs), extradimensional shifts (EDSs), and reversal learning are mediated by the anterior cingulate cortex, the medial prefrontal cortex, and the orbitofrontal cortex, respectively. We hypothesized that the latent structure of cognitive performance would reflect functional localization in the brain and would be altered by aging.

Methods

Young (4 months, n = 16) and aged (23 months, n = 18) C57BL/6N mice performed an attentional set-shifting task (ASST) that evaluates simple discrimination (SD), compound discrimination (CD), IDS, EDS, and reversal learning. The performance data were subjected to an exploratory factor analysis to extract the latent structures of ASST performance in young and aged mice.

Results

The factor analysis extracted two- and three-factor models. In the two-factor model, the factor associated with SD and CD was clearly separated from the factor associated with the rest of the ASST stages in the young mice only. In the three-factor model, the SD and CD loaded on distinct factors. The three-factor model also showed a separation of factors associated with IDS, EDS, and CD reversal. However, the other reversal learning variables, ID reversal and ED reversal, had somewhat inconsistent factor loadings.

Conclusions

The separation of performance factors in aged mice was less clear than in young mice, which suggests that aged mice utilize neuronal networks more broadly for specific cognitive functions. The result that the factors associated with SD and CD were separated in the three-factor model may suggest that the introduction of an irrelevant or distracting dimension results in the use of a new/orthogonal strategy for better discrimination.

A neurochemical yin and yang: does serotonin activate and norepinephrine deactivate the prefrontal cortex?

INTRODUCTION

The prefrontal cortex (PFC) receives serotonergic input from the dorsal raphe nucleus of the brainstem, as well as noradrenergic input from another brainstem nucleus, the locus coeruleus. A large number of studies have shown that these two neurotransmitter systems, and drugs that affect them, modulate the functional properties of the PFC in both humans and animal models.

RESULTS

Here I examine the hypothesis that serotonin (5-HT) plays a general role in activating the PFC, whereas norepinephrine (NE) plays a general role in deactivating this brain region. In this manner, the two neurotransmitter systems may have opposing effects on PFC-influenced behavior. To assess this hypothesis, three primary lines of evidence are examined comprising the effects of 5-HT and NE on impulsivity, cognitive flexibility, and working memory.

DISCUSSION

While all of the existing data do not unequivocally support the activation/deactivation hypothesis, there is a large body of support for it.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Long-lasting cognitive deficit induced by stress is alleviated by acute administration of antidepressants

Deficits in executive control associated with frontal lobe dysfunction have been reported in affective disorder, which is often precipitated by stressful life events. Here we examined the impact of repeated restraint stress (1h daily for 7 days) on rats' performance in the attentional set-shifting task (ASST). To evaluate the persistence of cognitive deficits, the performance of separate groups of rats was assessed on the 4th, 7th, 14th and 21st day following stress cessation. Stressed rats exhibited unusually long-lasting extra-dimensional (ED) set-shifting impairments, since these deficits were demonstrated even 3 weeks following stress termination. An inhibitor of corticosterone synthesis, the drug metyrapone (50mg/kg, IP) protected rats from the cognitive impairment suggesting an involvement of endogenous adrenal steroids in the debilitating effects of stress. Acute intraperitoneal administration of four different antidepressants (desipramine, nomifensine, fluoxetine and escitalopram) at the minimum effective doses of 3, 0.3, 1 and 1mg/kg, respectively, reversed the deficits of ED set-shifting in stressed animals. Desipramine, nomifensine, fluoxetine and escitalopram at the minimum effective doses of 6, 1, 1 and 1mg/kg, IP, respectively, promoted also cognitive flexibility in unstressed groups. We conclude that stress-induced long-term set-shifting impairment may represent a useful model mimicking clinically relevant aspects of depression, i.e., the persistence of executive dysfunction. The potential utility of antidepressants in treating frontal-like cognitive impairments is suggested.

Human category learning 2.0

During the 1990s and early 2000s, cognitive neuroscience investigations of human category learning focused on the primary goal of showing that humans have multiple category-learning systems and on the secondary goals of identifying key qualitative properties of each system and of roughly mapping out the neural networks that mediate each system. Many researchers now accept the strength of the evidence supporting multiple systems, and as a result, during the past few years, work has begun on the second generation of research questions-that is, on questions that begin with the assumption that humans have multiple category-learning systems. This article reviews much of this second generation of research. Topics covered include (1) How do the various systems interact? (2) Are there different neural systems for categorization and category representation? (3) How does automaticity develop in each system? and (4) Exactly how does each system learn?

A cognitive deficit induced in rats by chronic intermittent cold stress is reversed by chronic antidepressant treatment

We have previously reported that 14-d chronic intermittent cold (CIC) stress induced a cognitive deficit in reversal learning on the rat attentional set-shifting test. This effect may be related to dysregulation of 5-HT function in orbitofrontal cortex, as a model of cognitive dysfunction in depression. To test the ability of chronic antidepressant drug treatment to reverse the cognitive deficit induced by CIC, it was first necessary to assess the temporal characteristics of the CIC-induced cognitive deficit. Thus, in the first experiment, we assessed the duration of the cognitive deficit following 2-wk CIC stress. Replicating previous experiments, CIC induced a reversal learning deficit tested 3 d after the last cold exposure. However, cognitive performance of CIC-stressed rats was no different from unstressed controls when tested 7, 14 or 21 d after termination of the stress treatment. We next compared behaviour 3 d after 2-wk CIC to that seen 3 d after 5-wk CIC, and found similar deficits in reversal learning. Thus, in the final experiment, antidepressant drug treatment was initiated after 2-wk CIC stress, and was maintained for 3 wk, concurrent with the continuation of CIC stress. Both chronic and acute treatment with the selective serotonin reuptake inhibitor, citalopram, but not the norepinephrine reuptake blocker, desipramine, reversed the cognitive deficit induced by CIC stress. Thus, this stress-induced cognitive deficit may be a useful model for cognitive deficits related to prefrontal cortical hypoactivity in depression, and for investigating neurobiological mechanisms underlying the beneficial effects of chronic antidepressant drug treatment.

The role of the noradrenergic system in the exploration-exploitation trade-off: a psychopharmacological study

Animal research and computational modeling have indicated an important role for the neuromodulatory locus coeruleus–norepinephrine (LC–NE) system in the control of behavior. According to the adaptive gain theory, the LC–NE system is critical for optimizing behavioral performance by regulating the balance between exploitative and exploratory control states. However, crucial direct empirical tests of this theory in human subjects have been lacking. We used a pharmacological manipulation of the LC–NE system to test predictions of this theory in humans. In a double-blind parallel-groups design (N = 52), participants received 4 mg reboxetine (a selective norepinephrine reuptake inhibitor), 30 mg citalopram (a selective serotonin reuptake inhibitor), or placebo. The adaptive gain theory predicted that the increased tonic NE levels induced by reboxetine would promote task disengagement and exploratory behavior. We assessed the effects of reboxetine on performance in two cognitive tasks designed to examine task (dis)engagement and exploitative versus exploratory behavior: a diminishing-utility task and a gambling task with a non-stationary pay-off structure. In contrast to predictions of the adaptive gain theory, we did not find differences in task (dis)engagement or exploratory behavior between the three experimental groups, despite demonstrable effects of the two drugs on non-specific central and autonomic nervous system parameters. Our findings suggest that the LC–NE system may not be involved in the regulation of the exploration–exploitation trade-off in humans, at least not within the context of a single task. It remains to be examined whether the LC–NE system is involved in random exploration exceeding the current task context.