Enhanced attention and impulsive action following NMDA receptor GluN2B-selective antagonist pretreatment

Aberrant glutamatergic systems underlying impulsive behaviors: Insights from clinical and preclinical research

Impulsivity is a broad construct that often refers to one of several distinct behaviors and can be measured with self-report questionnaires and behavioral paradigms. Several psychiatric conditions are characterized by one or more forms of impulsive behavior, most notably the impulsive/hyperactive subtype of attention-deficit/hyperactivity disorder (ADHD), mood disorders, and substance use disorders. Monoaminergic neurotransmitters are known to mediate impulsive behaviors and are implicated in various psychiatric conditions. However, growing evidence suggests that glutamate, the major excitatory neurotransmitter of the mammalian brain, regulates important functions that become dysregulated in conditions like ADHD. The purpose of the current review is to discuss clinical and preclinical evidence linking glutamate to separate aspects of impulsivity, specifically motor impulsivity, impulsive choice, and affective impulsivity. Hyperactive glutamatergic activity in the corticostriatal and the cerebro-cerebellar pathways are major determinants of motor impulsivity. Conversely, hypoactive glutamatergic activity in frontal cortical areas and hippocampus and hyperactive glutamatergic activity in anterior cingulate cortex and nucleus accumbens mediate impulsive choice. Affective impulsivity is controlled by similar glutamatergic dysfunction observed for motor impulsivity, except a hyperactive limbic system is also involved. Loss of glutamate homeostasis in prefrontal and nucleus accumbens may contribute to motor impulsivity/affective impulsivity and impulsive choice, respectively. These results are important as they can lead to novel treatments for those with a condition characterized by increased impulsivity that are resistant to conventional treatments.

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.

The relationship between neurotransmission-related amino acid blood concentrations and neuropsychological performance following acute exercise

Amino acid neurotransmitters, including glutamate, phenylalanine, tyrosine, alanine, and glycine, underlie the majority of the excitatory and inhibitory neurotransmission in the nervous system, and acute exercise has been shown to modulate their concentrations. We aimed to determine whether any correlation exists between the above-mentioned amino acid blood concentrations and the neuropsychological performance after an acute exercise intervention. Sixty basketball players were randomly assigned to one of two experimental conditions: exercise or inactive resting. All participants underwent a comprehensive neuropsychological assessment and blood samples were taken on a Guthrie card before and after the end of the experimental conditions. Amino acid blood concentrations were significantly elevated and cognitive performance significantly improved post-exercise on specific neuropsychological assessments. Significant intervention × group interaction effects were apparent for Trail Making Test part-B [F(1,58) = 20.46, p < .0001, η2 = .26] and Digit Span Backwards [F(1,58) = 15.47, p < .0001, η2 = .21] neuropsychological assessments. Additionally, regression analysis indicated that tyrosine accounted for 38.0% of the variance in the Trail Making Test part-A test. These results suggest that elevated blood concentrations of neurotransmission-related amino acids are associated with improved neuropsychological performance after a single bout of high-intensity exercise.

p-Nrf2/HO-1 Pathway Involved in Methamphetamine-induced Executive Dysfunction through Endoplasmic Reticulum Stress and Apoptosis in the Dorsal Striatum

Methamphetamine (METH) abuse is known to cause executive dysfunction. However, the molecular mechanism underlying METH induced executive dysfunction remains unclear. Go/NoGo experiment was performed in mice to evaluate METH-induced executive dysfunction. Immunoblot analysis of Nuclear factor-E2-related factor 2 (Nrf2), phosphorylated Nrf2 (p-Nrf2), heme-oxygenase-1 (HO-1), Glucose Regulated Protein 78(GRP78), C/EBP homologous protein (CHOP), Bcl-2, Bax and Caspase3 was performed to evaluate the levels of oxidative stress, endoplasmic reticulum (ER) stress and apoptosis in the dorsal striatum (Dstr). Malondialdehyde (MDA) levels and glutathione peroxidase (GSH-Px) activity was conducted to evaluate the level of oxidative stress. TUNEL staining was conducted to detect apoptotic neurons. The animal Go/NoGo testing confirmed that METH abuse impaired the inhibitory control ability of executive function. Meanwhile, METH down-regulated the expression of p-Nrf2, HO-1 and GSH-Px and activated ER stress and apoptosis in the Dstr. Microinjection of Tert-butylhydroxyquinone (TBHQ), an Nrf2 agonist, into the Dstr increased the expression of p-Nrf2, HO-1, and GSH-Px, ameliorated ER stress, apoptosis and executive dysfunction caused by METH. Our results indicated that the p-Nrf2/HO-1 pathway was potentially involved in mediating methamphetamine-induced executive dysfunction by inducing endoplasmic reticulum stress and apoptosis in the dorsal striatum.

Differential effects of lipopolysaccharide on cognition, corticosterone and cytokines in socially-housed vs isolated male rats

Social isolation is an established risk factor for mental illness and impaired immune function. Evidence suggests that neuroinflammatory processes contribute to mental illness, possibly via cytokine-induced modulation of neural activity. We examined the effects of lipopolysaccharide (LPS) administration and social home cage environment on cognitive performance in the 5-Choice Serial Reaction Time Task (5CSRTT), and their effects on corticosterone and cytokines in serum and brain tissue. Male Long-Evans rats were reared in pairs or in isolation before training on the 5CSRTT. The effects of saline and LPS (150 µg/kg i.p.) administration on sickness behaviour and task performance were then assessed. LPS-induced sickness behaviour was augmented in socially-isolated rats, translating to increased omissions and slower response times in the 5CSRTT. Both social isolation and LPS administration reduced impulsive responding, while discriminative accuracy remained unaffected. With the exception of reduced impulsivity in isolated rats, these effects were not observed following a second administration of LPS, revealing behavioural tolerance to repeated LPS injections. In a separate cohort of animals, social isolation potentiated the ability of LPS to increase serum corticosterone and IL-6, which corresponded to increased IL-6 in the orbitofrontal and medial prefrontal cortices and the nucleus accumbens. Basal IL-4 levels in the nucleus accumbens were reduced in socially-isolated rats. These findings are consistent with the adaptive response of reduced motivational drive following immune challenge, and identify social isolation as an exacerbating factor. Enhanced IL-6 signalling may play a role in mediating the potentiated behavioural response to LPS administration in isolated animals.

Characterization of a differential reinforcement of low rates of responding task in non-deprived male and female rats: Role of Sigma-1 receptors

Impulsive action can be defined as the inability to withhold a response and represents one of the dimensions of the broad construct impulsivity. Here, we characterized a modified differential reinforcement of low rates of responding (DRL) task developed in our laboratory, in which impulsive action is measured in ad libitum fed/watered subjects. Specifically, we first determined the effects of both sex and estrous cycle on impulsive action by systematically comparing male and estrous-synchronized female subjects. In addition, we evaluated the convergent validity of this modified DRL task by testing the effects of the D2R/5HT2AR antagonist, aripiprazole, and the noncompetitive NMDAR antagonist, MK-801. Finally, we tested the effects of the selective antagonist BD-1063 and agonist PRE-084 of Sigma-1 receptor (Sig-1R) on impulsive action using this modified DRL task. We found that female rats showed and increased inability to withhold a response when compared to males, and this effect was driven by the metestrus/diestrus phase of the estrous cycle. In addition, aripiprazole and MK-801 fully retained their capability to reduce and increase impulsive action, respectively. Finally, the selective Sig-1R antagonist, BD-1063 dose-dependently reduced the inability to withhold a response in both sexes, though more potently in female rats. In summary, we show that impulsive action, as measured in a modified DRL task which minimizes energy-homeostatic influences, is a function of both sex and estrous cycle. Furthermore, we validate the convergent validity of the task and provide evidence that Sig-1R antagonism may represent a novel pharmacological strategy to reduce impulsive action.

Effects of the GluN2B-selective antagonist Ro 63-1908 on acquisition and expression of methamphetamine conditioned place preference in male and female rats

BACKGROUND

Methamphetamine abuse has increased significantly in recent years. Currently, there are no FDA-approved pharmacotherapies for the treatment of methamphetamine use disorder. The goal of the current study was to determine if the N-methyl-d-aspartate (NMDA) GluN2B-selective antagonist Ro 63-1908 can block the conditioned rewarding effects of methamphetamine as assessed in conditioned place preference (CPP).

METHODS

Two main experiments were conducted. In the first experiment, male (n = 24) and female (n = 24) rats received either vehicle or Ro 63-1908 (1.0-10.0 mg/kg) 30 min prior to the posttest to determine if blocking the GluN2B subunit attenuates expression of methamphetamine CPP. In the second experiment, male (n = 18) and female (n = 18) rats received either vehicle or Ro 63-1908 (1.0 or 3.0 mg/kg) 30 min prior to each conditioning session to determine if blocking the GluN2B subunit attenuates acquisition of methamphetamine CPP.

RESULTS

Ro 63-1908 (3.0 mg/kg) blocked acquisition of methamphetamine CPP in male rats, but only attenuated CPP in female rats. Ro 63-1908 did not alter expression of CPP in either sex. Increasing the dose of Ro 63-1908 (10.0 mg/kg) failed to block acquisition of CPP in an additional group of female rats (n = 6). A control experiment showed that Ro 63-1908 (3.0 mg/kg) did not produce CPP or conditioned place aversion in male rats (n = 6) or in female rats (n = 6).

CONCLUSIONS

The results of this study show that Ro 63-1908 is able to decrease the conditioned rewarding effects of methamphetamine.

Contrasting effects of d-amphetamine and atomoxetine on measures of impulsive action and choice

Amphetamine (AMP) and atomoxetine (ATX) represent two of the most widely studied drug treatments used in the investigation of impulsive behaviour. While both drugs have relatively well defined effects in tests designed to investigate impulsive action (e.g. 5-choice task; 5-CSRTT), the effects of both drugs in tests of impulsive choice (e.g. delay discounting) are less consistent. In the present study both AMP and ATX were tested in a rodent gambling task (rGT) and delay discounting in rats separately trained to either an ascending or descending delay schedule. Effects of both drugs were compared to measures of impulsive action (premature (PREM) responses) and perseverative (PSV) responses measured in the 5-choice and rGT tasks. Consistent with previous studies, AMP (0.1-1 mg/kg) increased both PREM and PSV responses, and ATX (0.5-2 mg/kg) reduced both measures in the 5-choice and rGT tasks. At equivalent doses ATX had no reliable effect on choice behaviour in either the rGT or delay discounting suggesting a null effect of this drug on impulsive choice and risky decision making. The effects of AMP were more complex, with a subtle shift in preference to a low risk (P1) choice in the rGT, and an effect on discounting that was unrelated to reinforcer value, but instead dependent on delay sequence and baseline choice preference. One aspect to these outcomes is to highlight the importance of multiple methodological factors when assessing drug effects on complex behaviours such as impulsive choice, and question what are the most appropriate test conditions under which to examine these drugs on discounting.

Control of impulsivity by Gi-protein signalling in layer-5 pyramidal neurons of the anterior cingulate cortex

Pathological impulsivity is a debilitating symptom of multiple psychiatric diseases with few effective treatment options. To identify druggable receptors with anti-impulsive action we developed a systematic target discovery approach combining behavioural chemogenetics and gene expression analysis. Spatially restricted inhibition of three subdivisions of the prefrontal cortex of mice revealed that the anterior cingulate cortex (ACC) regulates premature responding, a form of motor impulsivity. Probing three G-protein cascades with designer receptors, we found that the activation of Gi-signalling in layer-5 pyramidal cells (L5-PCs) of the ACC strongly, reproducibly, and selectively decreased challenge-induced impulsivity. Differential gene expression analysis across murine ACC cell-types and 402 GPCRs revealed that - among Gi-coupled receptor-encoding genes - Grm2 is the most selectively expressed in L5-PCs while alternative targets were scarce. Validating our approach, we confirmed that mGluR2 activation reduced premature responding. These results suggest Gi-coupled receptors in ACC L5-PCs as therapeutic targets for impulse control disorders.

Convergent observations of MK-801-induced impairment in rat 5C-CPT performance across laboratories: reversal with a D1 but not nicotinic agonist

INTRODUCTION

Cognitive function is closely linked to functional outcomes in psychiatric disorders such as schizophrenia, however developing effective treatments for cognitive dysfunction have proven elusive. Potential reasons for this may include the complexity of diseases, the absence of appropriate and translatable animal tests of cognitive dysfunction, and the reproducibility of findings. Attention is a key component of cognitive function traditionally assessed in the clinic using a variant of the continuous performance test (CPT). The 5-choice (5C)-CPT was developed as a translational cross-species version of this task. Given the association between glutamatergic abnormalities and cognitive dysfunction in schizophrenia, we hypothesized that the NMDA receptor antagonist MK-801 would impair 5C-CPT in rats across different laboratories, and determined whether the dopamine D1 receptor agonist SKF38393 or the nonspecific nicotinic agonist nicotine would remediate such deficits.

METHOD

Rats were trained in the 5C-CPT at Beacon Discovery and UCSD. These rats were then treated with MK-801, agonist treatment, and combinations of the two.

RESULTS

MK-801 produced 5C-CPT deficits in the same domains of rats across sites at similar doses. Neither nicotine nor SKF38393 treatment alone improved performance. Importantly, SKF38393, but not nicotine, remediated the MK-801-induced deficits.

CONCLUSION

Convergent observation of MK-801-induced deficits in 5C-CPT was seen across laboratories, resulting in deficits consistent with those seen in people with schizophrenia. Treatment with SKF38393 but not nicotine reversed these deficits. More work is needed, but the 5C-CPT is a reliable method for detecting NMDA receptor disruption-induced deficits in attention.

GluN2B/N-methyl-D-aspartate Receptor Antagonists: Advances in Design, Synthesis, and Pharmacological Evaluation Studies

Selective GluN2B/N-methyl-D-aspartate receptor (NMDAR) antagonists have exposed their clinical effectiveness in a cluster of neurodegenerative diseases, such as epilepsy, Alzheimer's disease, Parkinson's disease, pain, and depression. Hence, GluN2B/NMDARs are considered to be a prospective target for the management of neurodegenerative diseases. Here, we have discussed the current results and significance of subunit selective GluN2B/NMDAR antagonists to pave the way for the establishment of new, safe, and economical drug candidates in the near future. By using summarized data of selective GluN2B/NMDAR antagonists, medicinal chemists are certainly a step closer to the goal of improving the therapeutic and side effect profile of selective antagonists. Outlined summary of designing strategies, synthetic schemes, and pharmacological evaluation studies reinvigorate efforts to identify, modify, and synthesize novel GluN2B/NMDAR antagonists for treating neurodegenerative diseases.

Which came first: Cannabis use or deficits in impulse control?

Impulse control deficits are often found to co-occur with substance use disorders (SUDs). On the one hand, it is well known that chronic intake of drugs of abuse remodels the brain with significant consequences for a range of cognitive behaviors. On the other hand, individual variation in impulse control may contribute to differences in susceptibility to SUDs. Both of these relationships have been described, thus leading to a "chicken or the egg" debate which remains to be fully resolved. Does impulsivity precede drug use or does it manifest as a function of problematic drug usage? The link between impulsivity and SUDs has been most strongly established for cocaine and alcohol use disorders using both preclinical models and clinical data. Much less is known about the potential link between impulsivity and cannabis use disorder (CUD) or the directionality of this relationship. The initiation of cannabis use occurs most often during adolescence prior to the brain's maturation, which is recognized as a critical period of development. The long-term effects of chronic cannabis use on the brain and behavior have started to be explored. In this review we will summarize these observations, especially as they pertain to the relationship between impulsivity and CUD, from both a psychological and biological perspective. We will discuss impulsivity as a multi-dimensional construct and attempt to reconcile the results obtained across modalities. Finally, we will discuss possible avenues for future research with emerging longitudinal data.

Low Doses of Psilocybin and Ketamine Enhance Motivation and Attention in Poor Performing Rats: Evidence for an Antidepressant Property

Long term benefits following short-term administration of high psychedelic doses of serotonergic and dissociative hallucinogens, typified by psilocybin and ketamine respectively, support their potential as treatments for psychiatric conditions such as major depressive disorder. The high psychedelic doses induce perceptual experiences which are associated with therapeutic benefit. There have also been anecdotal reports of these drugs being used at what are colloquially referred to as "micro" doses to improve mood and cognitive function, although currently there are recognized limitations to their clinical and preclinical investigation. In the present studies we have defined a low dose and plasma exposure range in rats for both ketamine (0.3-3 mg/kg [10-73 ng/ml]) and psilocybin/psilocin (0.05-0.1 mg/kg [7-12 ng/ml]), based on studies which identified these as sub-threshold for the induction of behavioral stereotypies. Tests of efficacy were focused on depression-related endophenotypes of anhedonia, amotivation and cognitive dysfunction using low performing male Long Evans rats trained in two food motivated tasks: a progressive ratio (PR) and serial 5-choice (5-CSRT) task. Both acute doses of ketamine (1-3 mg/kg IP) and psilocybin (0.05-0.1 mg/kg SC) pretreatment increased break point for food (PR task), and improved attentional accuracy and a measure of impulsive action (5-CSRT task). In each case, effect size was modest and largely restricted to test subjects characterized as "low performing". Furthermore, both drugs showed a similar pattern of effect across both tests. The present studies provide a framework for the future study of ketamine and psilocybin at low doses and plasma exposures, and help to establish the use of these lower concentrations of serotonergic and dissociative hallucinogens both as a valid scientific construct, and as having a therapeutic utility.

Differential effects of glutamate N-methyl-D-aspartate receptor antagonists on risky choice as assessed in the risky decision task

RATIONALE

Risky choice can be measured using the risky decision task (RDT). In the RDT, animals choose between a large, risky option that is paired with probabilistic foot shock and a small, safe option that is never paired with shock. To date, studies examining the neurochemical basis of decision-making in the RDT have focused primarily on the dopaminergic system but have not focused on the glutamatergic system, which has been implicated in risky decision-making.

OBJECTIVES

Because glutamate is known to play a critical role in decision-making, we wanted to determine the contribution of the glutamatergic system to performance in the RDT.

METHODS

In the experiment, 32 rats (16 male; 16 female) were tested in the RDT. The probability of receiving a foot shock increased across the session (ascending schedule) for half of the rats but decreased across the session (descending schedule) for half of the rats. Following training, rats received injections of the N-methyl-D-aspartate (NMDA) receptor competitive antagonist CGS 19755 (0, 1.0, 2.5, 5.0 mg/kg; s.c.) and the GluN2B-selective antagonist Ro 63-1908 (0, 0.1, 0.3, 1.0 mg/kg; s.c.).

RESULTS

CGS 19755 (2.5 and 5.0 mg/kg) increased risky choice in males and females trained on the ascending schedule. Ro 63-1908 (1.0 mg/kg) decreased risky choice, but only in male rats trained on the ascending schedule.

CONCLUSIONS

Although NMDA receptor antagonists differentially alter risky choice in the RDT, the current results show that NMDA receptors are an important mediator of decision-making involving probabilistic delivery of positive punishment.

Neural circuitry and mechanisms of waiting impulsivity: relevance to addiction

Impatience-the failure to wait or tolerate delayed rewards (e.g. food, drug and monetary incentives)-is a common behavioural tendency in humans. However, when rigidly and rapidly expressed with limited regard for future, often negative consequences, impatient or impulsive actions underlie and confer susceptibility for such diverse brain disorders as drug addiction, attention-deficit hyperactivity disorder (ADHD) and major depressive disorder. Consequently, 'waiting' impulsivity has emerged as a candidate endophenotype to inform translational research on underlying neurobiological mechanisms and biomarker discovery for many of the so-called impulse-control disorders. Indeed, as reviewed in this article, this research enterprise has revealed a number of unexpected targets and mechanisms for intervention. However, in the context of drug addiction, impulsive decisions that maximize short-term gains (e.g. acute drug consumption) over longer-term punishment (e.g. unemployment, homelessness, personal harm) defines one aspect of impulsivity, which may or may not be related to rapid, unrestrained actions over shorter timescales. We discuss the relevance of this distinction in impulsivity subtypes for drug addiction with reference to translational research in humans and other animals. This article is part of the theme issue 'Risk taking and impulsive behaviour: fundamental discoveries, theoretical perspectives and clinical implications'.

Effects of d-amphetamine and MK-801 on impulsive choice: Modulation by schedule of reinforcement and delay length

Procedural modifications can modulate drug effects in delay discounting, such as signaling the delay to reinforcement and altering the order in which delays are presented. Although the schedule of reinforcement can alter the rate at which animals discount a reinforcer, research has not determined if animals trained on different schedules of reinforcement are differentially affected by pharmacological manipulations. Similarly, research has not determined if using different delays to reinforcement can modulate drug effects in delay discounting. Male Sprague Dawley rats (n = 36) were split into four groups and were trained in a delay-discounting procedure. The schedule of reinforcement (fixed ratio [FR] 1 vs. FR 10) and delays to reinforcement (0, 5, 10, 20, and 50 s vs. 0, 10, 30, 60, 100 s) were manipulated for each group. Following behavioral training, rats were treated with d-amphetamine (0, 0.25, 0.5, and 1.0 mg/kg) and MK-801 (0, 0.03, and 0.06 mg/kg). Results showed that amphetamine decreased impulsive choice when a FR 1 schedule was used, but only when the short delay sequence was used. Conversely, amphetamine decreased impulsive choice when a FR 10 schedule was used, but only when rats were trained on the long delay sequence. MK-801 decreased impulsive choice in rats trained on a FR 1 schedule, regardless of delay sequence, but did not alter choice in rats trained on a FR 10 schedule. These results show that schedule of reinforcement and delay length can modulate drug effects in delay discounting.

Examining the neurochemical underpinnings of animal models of risky choice: Methodological and analytic considerations

Because risky choice is associated with several psychiatric conditions, recent research has focused on examining the underlying neurochemical processes that control risk-based decision-making. Not surprisingly, several tasks have been developed to study the neural mechanisms involved in risky choice. The current review will briefly discuss the major tasks used to measure risky choice and will summarize the contribution of several major neurotransmitter systems to this behavior. To date, the most common measures of risky choice are the probability discounting task, the risky decision task, and the rat gambling task. Across these three tasks, the contribution of the dopaminergic system has been most studied, although the effects of serotonergic, adrenergic, cholinergic, and glutamatergic ligands will be discussed. Drug effects across these tasks have been inconsistent, which makes determining the precise role of neurotransmitter systems in risky choice somewhat difficult. Furthermore, procedural differences can modulate drug effects in these procedures, and the way data are analyzed can alter the interpretations one makes concerning pharmacological manipulations. By taking these methodological/analytic considerations into account, we may better elucidate the neurochemistry of risky decision-making. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Can N-Methyl-D-Aspartate Receptor Hypofunction in Schizophrenia Be Localized to an Individual Cell Type?

Hypofunction of N-methyl-D-aspartate glutamate receptors (NMDARs), whether caused by endogenous factors like auto-antibodies or mutations, or by pharmacological or genetic manipulations, produces a wide variety of deficits which overlap with-but do not precisely match-the symptom spectrum of schizophrenia. In order to understand how NMDAR hypofunction leads to different components of the syndrome, it is necessary to take into account which neuronal subtypes are particularly affected by it in terms of detrimental functional alterations. We provide a comprehensive overview detailing findings in rodent models with cell type-specific knockout of NMDARs. Regarding inhibitory cortical cells, an emerging model suggests that NMDAR hypofunction in parvalbumin (PV) positive interneurons is a potential risk factor for this disease. PV interneurons display a selective vulnerability resulting from a combination of genetic, cellular, and environmental factors that produce pathological multi-level positive feedback loops. Central to this are two antioxidant mechanisms-NMDAR activity and perineuronal nets-which are themselves impaired by oxidative stress, amplifying disinhibition. However, NMDAR hypofunction in excitatory pyramidal cells also produces a range of schizophrenia-related deficits, in particular maladaptive learning and memory recall. Furthermore, NMDAR blockade in the thalamus disturbs thalamocortical communication, and NMDAR ablation in dopaminergic neurons may provoke over-generalization in associative learning, which could relate to the positive symptom domain. Therefore, NMDAR hypofunction can produce schizophrenia-related effects through an action on various different circuits and cell types.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

3,4-Dihydroxyphenylethanol Assuages Cognitive Impulsivity in Alzheimer's Disease by Attuning HPA-Axis via Differential Crosstalk of α7 nAChR with MicroRNA-124 and HDAC6

Cognitive impulsivity, a form of suboptimal cost-benefit decision making, is an illustrious attribute of an array of neurodegenerative diseases including Alzheimer's disease (AD). In this study, a delay discounting paradigm was used to assess the effect of 3,4-dihydroxyphenylethanol (DOPET) on cognitive impulsivity, in an oA42i (oligomeric amyloid β_1-42 plus ibotenic acid) induced AD mouse model, using a nonspatial T-maze task. The results depicted that oA42i administration elevated cognitive impulsivity, whereas DOPET treatment attenuated the impulsive behavior and matched the choice of the sham-operated controls. In addition, DOPET treatment has ameliorated the anxiety-like behavior in the oA42i-challenged mice. Probing the molecular signaling cascades underpinning these functional ramifications in the oA42i-challenged mice revealed reduced cholinergic (α7 nAChR; alpha 7 nicotinic acetylcholine receptor) function, dysregulated hypothalamic-pituitary-adrenal (HPA) axis (manifested by amplified glucocorticoid receptor expression and plasma corticosterone levels), and also aberrations in the neuroepigenetic (microRNA-124, HDAC6 (histone deacetylase 6), and HSP90 (heat-shock protein 90) expressions) as well as nucleocytoplasmic (importin-α1 expression and nuclear ultra-architecture) continuum. Nonetheless, DOPET administration ameliorated these perturbations and the observations were in line with that of the sham-operated mice. Further validation of the results with organotypic hippocampal slice cultures (OHSCs) confirmed the in vivo findings. We opine that HPA-axis attunement by DOPET might be orchestrated through the α7 nAChR-mediated pathway. Based on these outcomes, we posit that 3,4-dihydroxyphenylethanol might be a potential multimodal agent for the management of cognitive impulsivity and neuromolecular quagmire in AD.

Effects of GluN2B-selective antagonists on delay and probability discounting in male rats: Modulation by delay/probability presentation order

The contribution of the GluN2B subunit of the NMDA receptor to impulsivity has recently been examined. Ro 63-1908, a highly selective antagonist for the GluN2B, decreases impulsive choice. Because the order in which delays are presented modulates drug effects in discounting procedures, one goal of the current study was to determine the effects of Ro 63-1908 in delay discounting procedures in which the delays to obtaining the large reinforcer either increase or decrease across the session. We also determined if Ro 63-1908 differentially alters risky choice in probability discounting procedures that use ascending/descending schedules. Male rats were trained in either delay (n = 24) or probability (n = 24) discounting in which the delay to/odds against reinforcement were presented in either ascending or descending order (n = 12 each schedule). Following training, rats received the GluN2B antagonists Ro 63-1908 (0-1.0 mg/kg) and CP-101,606 (0-3.0 mg/kg). In delay discounting, Ro 63-1908 (1.0 mg/kg), but not CP-101,606, decreased choice for the large reinforcer, but only when the delays decreased across the session. In probability discounting, Ro 63-1908 (0.3 mg/kg)/CP-101,606 (1.0 mg/kg) increased choice for the large reinforcer when the probability of obtaining this alternative decreased across the session, but Ro 63-1908 (1.0 mg/kg)/CP-101,606 (3.0 mg/kg) decreased choice when the probabilities increased. These results show that the GluN2B is a mediator of impulsive/risky choice, but the effects of GluN2B antagonists are dependent on the order in which delays/probabilities are presented. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Cognitive decline in Tg2576 mice shows sex-specific differences and correlates with cerebral amyloid-beta

Patients suffering from Alzheimer's disease show a sex-dependent decline of cognitive function. The aim of this investigation was to show these differences in an animal model for Alzheimer's disease and to determine whether this effect is correlated to amyloid-beta-induced pathophysiological changes. Therefore, we assessed cognitive performance with the modified hole-board test in female and male Tg2576 and wild type mice at the age of 6, 8, 10, 12, 14, and 16 months and correlated these findings to the total amount of soluble amyloid-beta and insoluble amyloid deposits in the brain. Tg2576 mice perform worse than wild types. Female Tg2576 mice develop an accentuated cognitive impairment (wrong choice total) beginning at the age of 12 months compared to their male littermates. Alterations in the mice's behaviour do not show interference with these deficits. Cognitive impairment is correlated to the amount of soluble amyloid-beta and insoluble amyloid deposits in the brain in a sex-dependent manner.

Effects of N-methyl-d-aspartate receptor (NMDAr) uncompetitive antagonists in a delay discounting paradigm using a concurrent-chains procedure

Impulsive choice is often assessed in rodents using a delay discounting (DD) paradigm in which the delay to a large reinforcer (LR) increases across the session. This procedure allows one to test the effects of pharmacological manipulations within a single session. Because discounting is influenced by sensitivity to reinforcer magnitude (SRM) and sensitivity to delayed reinforcement (SDR), applying quantitative analyses (e.g., fitting hyperbolic function) is important for determining the precise behavioral mechanisms being altered following drug administration. One caveat to this approach is that observing increases in SMR/SDR can be difficult (e.g., most rats choose the LR when its delivery is immediate, whereas some rats may show exclusive preference for the small reinforcer [SR] when a delay on the LR is imposed). We utilized a variant of a concurrent-chains procedure in which rats (n = 8) could not show exclusive preference for either reinforcer, thus allowing one to observe increases/decreases in responding at each delay. The NMDAr antagonists MK-801 (0, 0.003, 0.01, 0.03 mg/kg), ketamine (0, 1.0, 5.0, 10.0 mg/kg), and memantine (0, 2.5, 5.0, 7.5 mg/kg) were administered following baseline training because this receptor has recently been implicated in DD. MK-801 (0.03 mg/kg) decreased SRM and SDR. Memantine (7.5 mg/kg) decreased SRM only. These results show that this variant of the concurrent-chains procedure can be used to study the effects of pharmacological manipulations on distinct aspects of DD.

Effects of the NMDA receptor antagonists dizocilpine and Ro 63-1908 on delay-discounting and risky decision-making in a gambling task

Previous studies demonstrated that NMDA receptor antagonists such as dizocilpine (MK801) and the GluN2B NMDA antagonist Ro 63-1908 promote impulsive action (motor impulsivity). The effects of these treatments on impulsive choice and decision-making is less well characterized. Two experiments were undertaken. In the first experiment, given evidence for delay order as a factor in choice selection, the effect of dizocilpine was examined in a delay discounting task with separate groups of male Long-Evans rats trained to a schedule of either ascending (i.e. 0-40 s), or descending delays (i.e. 40-0 s). Under the ascending-delay schedule, dizocilpine (0.03-0.06 mg/kg SC) reduced discounting, yet on the descending-delay schedule discounting was increased. Subgrouping rats according to discounting rate under vehicle pretreatment were consistent with a treatment-induced choice perseveration. In a second experiment, male Long-Evans rats were trained to a gambling task (rGT). Neither dizocilpine (0.01-0.06 mg/kg SC) nor Ro 63-1908 (0.1-1 mg/kg SC) shifted choice from the advantageous to the disadvantageous options. However dizocilpine, and marginally Ro 63-1908, increased choice of the least risky, but suboptimal option. This effect was most evident in rats that initially preferred the disadvantageous options. Consistent with previous studies, both treatments increased measures of motor impulsivity. These results demonstrate that dizocilpine has effects on discounting dependent on delay order and likely reflective of perseveration. On the rGT task, neither dizocilpine nor Ro 63-1908 promoted risky choice, yet both NMDA receptor antagonists increased impulsive action.

Dissecting drug effects in preclinical models of impulsive choice: emphasis on glutamatergic compounds

RATIONALE

Impulsive choice is often measured with delay discounting paradigms. Because there are multiple discounting procedures, as well as different statistical analyses that can be applied to data generated from these paradigms, there are some inconsistencies in the literature regarding drug effects on impulsive choice.

OBJECTIVES

The goal of the current paper is to review the methodological and analytic approaches used to measure discounting and to discuss how these differences can account for differential drug effects observed across studies.

RESULTS

Because some procedures/analyses use a single data point as the dependent variable, changes in this value following pharmacological treatment may be interpreted as alterations in sensitivity to delayed reinforcement, but when other procedures/analyses are used, no changes in behavior are observed. Even when multiple data points are included, some studies show that the statistical analysis (e.g., ANOVA on raw proportion of responses vs. using hyperbolic/exponential functions) can lead to different interpretations. Finally, procedural differences (e.g., delay presentation order, signaling the delay to reinforcement, etc.) in the same discounting paradigm can alter how drugs affect sensitivity to delayed reinforcement.

CONCLUSIONS

Future studies should utilize paradigms that allow one to observe alterations in responding at each delay (e.g., concurrent-chains schedules). Concerning statistical analyses, using parameter estimates derived from nonlinear functions or incorporating the generalized matching law can allow one to determine if drugs affect sensitivity to delayed reinforcement or impair discrimination of the large and small magnitude reinforcers. Using these approaches can help further our understanding of the neurochemical underpinnings of delay discounting.

Differences in trait impulsivity do not bias the response to pharmacological drug challenge in the rat five-choice serial reaction time task

RATIONALE

Maladaptive impulsivity is symptomatic of several neuropsychiatric disorders including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and substance abuse disorders; paradigms designed to assess the underlying neurobiology of this behavior are essential for the discovery of novel therapeutic agents. Various models may be used to assess impulsivity as measured by the five-choice serial reaction time task (5-CSRTT), including variable inter-trial interval (ITI) sessions, the selection of extreme high and low impulsivity phenotypes from a large outbred population of rats, as well as pharmacological challenges.

OBJECTIVES

The aim of this study is to evaluate if pharmacological challenge models for impulsivity are biased by underlying differences in impulsivity phenotype.

METHODS

Extreme high and low impulsivity phenotypes were selected in the 5-CSRTT, and dose-dependent effects of various pharmacological challenges, namely MK-801, yohimbine, and cocaine, were evaluated on task performance, specifically accuracy and premature responses.

RESULTS

All three compounds increased premature responding, while a decrease in attentional performance occurred following MK-801 and yohimbine administration. No differences in drug-induced impulsivity between rats selected for high or low impulsivity or in parameters indicative of attentional performance could be determined.

CONCLUSIONS

Our findings indicate that different pharmacological challenges increase impulsivity on the 5-CSRTT, with modest effects on attention. These effects were not influenced by underlying differences in impulsivity phenotype, which is an important prerequisite to reliably use these challenge models to screen and profile compounds with putative anti-impulsive characteristics.

Effects of intra-accumbal administration of dopamine and ionotropic glutamate receptor drugs on delay discounting performance in rats

Nucleus accumbens core (NAcc) has been implicated in impulsive choice, as measured in delay discounting. The role of dopamine (DA) in impulsive choice has received considerable attention, whereas glutamate (Glu) has recently been shown to be an important mediator of discounting. However, research has not examined how DA or Glu receptors in NAcc mediate different aspects of delay discounting performance, that is, (a) sensitivity to reinforcer magnitude and (b) sensitivity to delayed reinforcement. Adult male Sprague-Dawley rats were first trained in a delay discounting task, in which the delay to a large magnitude food reinforcer increased across blocks of trials. Following behavioral training, rats received bilateral implantation of guide cannulas into NAcc. Half of the rats (n = 12) received infusions of the DA-selective ligands SKF 38393 (D1-like agonist: 0.03 or 0.1 μg), SCH 23390 (D1-like antagonist: 0.3 or 1.0 μg), quinpirole (D2-like agonist: 0.3 or 1.0 μg), and eticlopride (D2-like antagonist: 0.3 or 1.0 μg). The other half received infusions of the ionotropic Glu ligands MK-801 (NMDA uncompetitive antagonist: 0.3 or 1.0 μg), AP-5 (NMDA competitive antagonist: 0.3 or 1.0 μg), ifenprodil (noncompetitive antagonist at NR2B-containing NMDA receptors: 0.3 or 1.0 μg), and CNQX (AMPA competitive antagonist: 0.2 or 0.5 μg). Results showed that SCH 23390 (0.3 μg) decreased sensitivity to reinforcer magnitude without altering impulsive choice, whereas ifenprodil (1.0 μg) decreased sensitivity to delayed reinforcement (i.e., impulsive choice). The current results show that DA and NMDA receptors in NAcc mediate distinct aspects of discounting performance. (PsycINFO Database Record

Rodent Test of Attention and Impulsivity: The 5-Choice Serial Reaction Time Task

The 5-choice serial reaction time task (5-CSRTT) is employed extensively to measure attention in rodents. The assay involves animals trained to respond to a brief, unpredictable visual stimulus presented in one of five locations. The effects of experimental manipulations on response speed and choice accuracy are measured, and each related to attentional performance. The 5-CSRTT is also used to measure motor impulsivity. Adapted from a human task, the 5-CSRTT can be employed with rodents or primates, highlighting its translational value. Another strength of this procedure is its adaptability to task modification. An example is the 5-choice continuous performance task, which has both target and non-target trial types. Overall, the 5-CSRTT has proven to be valuable for drug discovery efforts aimed at identifying new agents for the treatment of central nervous system disorders and for further understanding the neurobiological processes of attention and impulsivity. Its flexibility offers considerable scope to the experimenter, and in this respect the task continues to evolve. © 2017 by John Wiley & Sons, Inc.

Effects of N-methyl-D-aspartate receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure

RATIONALE

The N-methyl-D-aspartate (NMDA) receptor has been recently identified as an important mediator of impulsive choice, as assessed in delay discounting. Although discounting is independently influenced by sensitivity to reinforcer magnitude and delayed reinforcement, few studies have examined how NMDA receptor ligands differentially affect these parameters.

OBJECTIVES

The current study examined the effects of various NMDA receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure.

METHODS

Following behavioral training, rats received treatments of the following NMDA receptor ligands: the uncompetitive antagonists ketamine (0, 1.0, 5.0, or 10.0 mg/kg; i.p.), MK-801 (0, 0.003, 0.01, or 0.03 mg/kg; s.c.), and memantine (0, 2.5, 5.0, or 10.0 mg/kg; i.p.), the competitive antagonist CGS 19755 (0, 5.0, 10.0, or 20.0 mg/kg; s.c.), the non-competitive NR2B subunit-selective antagonist ifenprodil (0, 1.0, 3.0, or 10.0 mg/kg; i.p), and the partial agonist D-cycloserine (0, 3.25, 15.0, or 30.0 mg/kg; s.c.).

RESULTS

When an exponential model was used to describe discounting, CGS 19755 (5.0 mg/kg) increased impulsive choice without altering sensitivity to reinforcer magnitude. Conversely, ketamine (10.0 mg/kg), memantine (5.0 mg/kg), and ifenprodil (10.0 mg/kg) decreased sensitivity to reinforcer magnitude without altering impulsive choice. MK-801 and D-cycloserine did not alter delay-discounting performance, although two-way ANOVA analyses indicated D-cycloserine (15.0 mg/kg) decreased impulsive choice.

CONCLUSIONS

The behavioral changes observed in delay discounting following administration of NMDA receptor antagonists do not always reflect an alteration in impulsive choice. These results emphasize the utility in employing quantitative methods to assess drug effects in delay discounting.

Effects of Group I metabotropic glutamate receptor antagonists on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting task in rats: Contribution of delay presentation order

Metabotropic glutamate receptor 1 (mGluR₁) blockade has been shown to decrease impulsive choice, as measured in delay discounting. However, several variables are known to influence an animal's discounting, including sensitivity to delayed reinforcement and sensitivity to reinforcer magnitude. The goal of this experiment was to determine the effects of mGluR₁, as well as mGluR₅, antagonism on these parameters. Forty Sprague Dawley rats were trained in delay discounting, in which consistently choosing a small, immediate reward reflects impulsive choice. For half of the rats, the delay to the large reinforcer increased across blocks of trials, whereas the delay decreased across the session for half of the rats. Following training, half of the rats received injections of the mGluR₁ antagonist JNJ 16259685 (JNJ; 0, 0.1, 0.3, or 1.0mg/kg; i.p), and half received injections of the mGluR₅ antagonist MPEP (0, 1.0, 3.0, or 10.0mg/kg; i.p.). Administration of JNJ increased sensitivity to delayed reinforcement (i.e., promoted impulsive choice), regardless of which schedule was used. However, the order in which delays were presented modulated the effects of JNJ on sensitivity to reinforcer magnitude. Specifically, JNJ decreased sensitivity to reinforcer magnitude in rats trained on the descending schedule only. MPEP did not alter sensitivity to reinforcer magnitude or sensitivity to delayed reinforcement. These results show that mGluR₁ is an important mediator of impulsive choice, and they provide further evidence that delay order presentation is an important variable that influences drug effects in delay discounting.

Toluene inhalation in adolescent rats reduces flexible behaviour in adulthood and alters glutamatergic and GABAergic signalling

Toluene is a commonly abused inhalant that is easily accessible to adolescents. Despite the increasing incidence of use, our understanding of its long-term impact remains limited. Here, we used a range of techniques to examine the acute and chronic effects of toluene exposure on glutameteric and GABAergic function, and on indices of psychological function in adult rats after adolescent exposure. Metabolomics conducted on cortical tissue established that acute exposure to toluene produces alterations in cellular metabolism indicative of a glutamatergic and GABAergic profile. Similarly, in vitro electrophysiology in Xenopus oocytes found that acute toluene exposure reduced NMDA receptor signalling. Finally, in an adolescent rodent model of chronic intermittent exposure to toluene (10 000 ppm), we found that, while toluene exposure did not affect initial learning, it induced a deficit in updating that learning when response-outcome relationships were reversed or degraded in an instrumental conditioning paradigm. There were also group differences when more effort was required to obtain the reward; toluene-exposed animals were less sensitive to progressive ratio schedules and to delayed discounting. These behavioural deficits were accompanied by changes in subunit expression of both NMDA and GABA receptors in adulthood, up to 10 weeks after the final exposure to toluene in the hippocampus, prefrontal cortex and ventromedial striatum; regions with recognized roles in behavioural flexibility and decision-making. Collectively, our data suggest that exposure to toluene is sufficient to induce adaptive changes in glutamatergic and GABAergic systems and in adaptive behaviour that may underlie the deficits observed following adolescent inhalant abuse, including susceptibility to further drug-use.

Effects of NMDA receptor antagonists on probability discounting depend on the order of probability presentation

Risky decision making can be measured using a probability-discounting procedure, in which animals choose between a small, certain reinforcer and a large, uncertain reinforcer. Recent evidence has identified glutamate as a mediator of risky decision making, as blocking the N-methyl-d-aspartate (NMDA) receptor with MK-801 increases preference for a large, uncertain reinforcer. Because the order in which probabilities associated with the large reinforcer can modulate the effects of drugs on choice, the current study determined if NMDA receptor ligands alter probability discounting using ascending and descending schedules. Sixteen rats were trained in a probability-discounting procedure in which the odds against obtaining the large reinforcer increased (n=8) or decreased (n=8) across blocks of trials. Following behavioral training, rats received treatments of the NMDA receptor ligands MK-801 (uncompetitive antagonist; 0, 0.003, 0.01, or 0.03mg/kg), ketamine (uncompetitive antagonist; 0, 1.0, 5.0, or 10.0mg/kg), and ifenprodil (NR2B-selective non-competitive antagonist; 0, 1.0, 3.0, or 10.0mg/kg). Results showed discounting was steeper (indicating increased risk aversion) for rats on an ascending schedule relative to rats on the descending schedule. Furthermore, the effects of MK-801, ketamine, and ifenprodil on discounting were dependent on the schedule used. Specifically, the highest dose of each drug decreased risk taking in rats in the descending schedule, but only MK-801 (0.03mg/kg) increased risk taking in rats on an ascending schedule. These results show that probability presentation order modulates the effects of NMDA receptor ligands on risky decision making.