The effects of dopaminergic agents on reaction time in rhesus monkeys

Impact of sleep deprivation on neurocognition and inflammation in rhesus macaques

Sleep deprivation in humans is associated with both cognitive impairment and immune dysregulation. An animal model of neuropathogenesis may provide insight to understand the effects of sleep deprivation on the brain. Human neurocognition is more closely mirrored by nonhuman primates (NHP) than other animals. As such, we developed an NHP model to assess the impact of sleep deprivation on neurocognition and markers of systemic immune activation. Six male rhesus macaques underwent three rounds of sleep deprivation (48 h without sleep) at days 0, 14, and 28. We performed domain specific cognitive assessments using the Cambridge Neuropsychological Test Automated Battery (CANTAB) via a touch screen before and after 24 and 48 h of sleep deprivation. Immune activation markers were measured in the blood by multiplex assay and flow cytometry. Although we observed variability in cognitive performance between the three rounds of sleep deprivation, cognitive impairments were identified in all six animals. We noted more cognitive impairments after 48 h than after 24 h of sleep deprivation. Following 48 h of sleep deprivation, elevations in markers of immune activation in the blood were observed in most animals. The observed impairments largely normalized after sleep. The co-occurrence of systemic immune alterations and cognitive impairment establishes this model as useful for studying the impact of sleep deprivation on neurobehavior and immune perturbations in rhesus macaques.

Dissection of neuronal circuits underlying sustained attention with the five-choice serial reaction time task

Attention deficits are common in psychiatric and neurological disorders. The transdiagnostic nature of impaired attention suggests a common set of underlying neural circuits. Yet, there are no circuit-based treatments such as non-invasive brain stimulation currently available due to the lack of sufficiently delineated network targets. Therefore, to better treat attentional deficits, a comprehensive functional dissection of neural circuits underlying attention is imperative. This can be achieved by taking advantage of preclinical animal models and well-designed behavioral assays of attention. The resulting findings in turn can be translated to the development of novel interventions with the goal of advancing them to clinical practice. Here we show that the five-choice serial reaction time task has greatly facilitated the study of the neural circuits underlying attention in a well-controlled setting. We first introduce the task and then focus on its application in preclinical studies on sustained attention, especially in the context of state-of-the-art neuronal perturbations.

Dopamine and norepinephrine differentially mediate the exploration-exploitation tradeoff

The catecholamines dopamine (DA) and norepinephrine (NE) have been repeatedly implicated in neuropsychiatric vulnerability, in part via their roles in mediating the decision making processes. Although the two neuromodulators share a synthesis pathway and are co-activated under states of arousal, they engage in distinct circuits and roles in modulating neural activity across the brain. However, in the computational neuroscience literature, they have been assigned similar roles in modulating the latent cognitive processes of decision making, in particular the exploration-exploitation tradeoff. Revealing how each neuromodulator contributes to this explore-exploit process will be important in guiding mechanistic hypotheses emerging from computational psychiatric approaches. To understand the differences and overlaps of the roles of these two catecholamine systems in regulating exploration and exploitation, a direct comparison using the same dynamic decision making task is needed. Here, we ran mice in a restless two-armed bandit task, which encourages both exploration and exploitation. We systemically administered a nonselective DA receptor antagonist (flupenthixol), a nonselective DA receptor agonist (apomorphine), a NE beta-receptor antagonist (propranolol), and a NE beta-receptor agonist (isoproterenol), and examined changes in exploration within subjects across sessions. We found a bidirectional modulatory effect of dopamine receptor activity on the level of exploration. Increasing dopamine activity decreased exploration and decreasing dopamine activity increased exploration. Beta-noradrenergic receptor activity also modulated exploration, but the modulatory effect was mediated by sex. Reinforcement learning model parameters suggested that dopamine modulation affected exploration via decision noise and norepinephrine modulation affected exploration via outcome sensitivity. Together, these findings suggested that the mechanisms that govern the transition between exploration and exploitation are sensitive to changes in both catecholamine functions and revealed differential roles for NE and DA in mediating exploration.

Systemic D1-R and D2-R antagonists in Non-Human Primates Differentially Impact Learning and Memory While Impairing Motivation and Motor Performance

Dopamine (DA) modulates cognition in part via differential activation of D1 and D2 receptors within the striatum and prefrontal cortex, yet evidence for cognitive impairments stemming from DA blockade or deficiency is inconsistent. Given the predominance of D1 over D2 receptors (R) in the prefrontal cortex of primates, D1-R blockade should more strongly influence frontal executive function (including working memory), while D2-R blockade should impair processes more strongly associated with the dorsal striatum (including cognitive flexibility, and learning). To test how systemic DA blockade disrupts cognition, we administered D1-R and D2-R like antagonists to healthy monkeys while they performed a series of cognitive tasks. Two selective DA receptor antagonist drugs (SCH-23390 hydrochloride: D1/D5-R antagonist; or Eticlopride hydrochloride: D2/D3-R antagonist) or placebo (0.9% saline) were systemically administered. Four tasks were used: (1) 'visually guided reaching', to test response time and accuracy, (2) 'reversal learning', to test association learning and attention, (3) 'self-ordered sequential search' to test spatial working memory, and (4) 'delayed match to sample' to test object working memory. Increased reach response times and decreased motivation to work for liquid reward was observed with both the D1/D5-R and D2/D3-R antagonists at the maximum dosages that still enabled task performance. The D2/D3-R antagonist impaired performance in the reversal learning task, while object and spatial working memory performance was not consistently affected in the tested tasks for either drug. These results are consistent with the theory that systemic D2/D3-R antagonists preferentially influence striatum processes (cognitive flexibility) while systemic D1/D5-R administration is less detrimental to frontal executive function.

Neurocognitive impact of Zika virus infection in adult rhesus macaques

Background

Zika virus (ZIKV) is a mosquito-transmitted flavivirus that affects many regions of the world. Infection, in utero, causes microcephaly and later developmental and neurologic impairments. The impact of ZIKV infection on neurocognition in adults has not been well described. The objective of the study was to assess the neurocognitive impact of ZIKV infection in adult rhesus macaques.

Methods

Neurocognitive assessments were performed using the Cambridge Neuropsychological Test Automated Battery (CANTAB) via a touch screen and modified Brinkman Board before and after subcutaneous ZIKV inoculation. Immune activation markers were measured in the blood and cerebral spinal fluid (CSF) by multiplex assay and flow cytometry.

Results

All animals (N = 8) had detectable ZIKV RNA in plasma at day 1 post-inoculation (PI) that peaked at day 2 PI (median 5.9, IQR 5.6–6.2 log₁₀ genome equivalents/mL). In all eight animals, ZIKV RNA became undetectable in plasma by day 14 PI, but persisted in lymphoid tissues. ZIKV RNA was not detected in the CSF supernatant at days 4, 8, 14 and 28 PI but was detected in the brain of 2 animals at days 8 and 28 PI. Elevations in markers of immune activation in the blood and CSF were accompanied by a reduction in accuracy and reaction speed on the CANTAB in the majority of animals.

Conclusions

The co-occurrence of systemic and CSF immune perturbations and neurocognitive impairment establishes this model as useful for studying the impact of neuroinflammation on neurobehavior in rhesus macaques, as it pertains to ZIKV infection and potentially other pathogens.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02402-4.

Semantic priming and neurobiology in schizophrenia: A theoretical review

In this theoretical review we bridge the cognitive and neurobiological sciences to shed light on the neurocognitive foundations of the semantic priming effect in schizophrenia. We review and theoretically evaluate the neurotransmitter systems (dopaminergic, GABAergic and glutamatergic) and neurobiological underpinnings of behavioural and electrophysiological (N400) semantic priming in the pathology, and the main hypotheses on their geneses: a disinhibition of the semantic spread of activation, a disorganised semantic storage or noisy lexical-semantic associations, a psychomotor artefact, an artefact of relatedness proportions, or an inability to mobilise contextual information. We further assess the literature on the endophenotype of Formal Thought Disorder from multiple standpoints, ranging from neurophysiology to cognition: considerations are weaved on neuronal (PV basket cell, SST, VIP) and receptor deficits (DRD1, NMDA), neurotransmitter imbalances (dopamine), cortical and dopaminergic lateralisation, inter alia. In conclusion, we put forth novel postulates on the underlying causes of controlled hypopriming, automatic hyperpriming, N400 reversals (larger amplitudes for close associations), indirect versus direct hyperpriming, and the endophenotype of lexical-semantic disturbances in schizophrenia.

D1- and D2-like receptors differentially mediate the effects of dopaminergic transmission on cost-benefit evaluation and motivation in monkeys

It has been widely accepted that dopamine (DA) plays a major role in motivation, yet the specific contribution of DA signaling at D₁-like receptor (D₁R) and D₂-like receptor (D₂R) to cost–benefit trade-off remains unclear. Here, by combining pharmacological manipulation of DA receptors (DARs) and positron emission tomography (PET) imaging, we assessed the relationship between the degree of D₁R/D₂R blockade and changes in benefit- and cost-based motivation for goal-directed behavior of macaque monkeys. We found that the degree of blockade of either D₁R or D₂R was associated with a reduction of the positive impact of reward amount and increasing delay discounting. Workload discounting was selectively increased by D₂R antagonism. In addition, blocking both D₁R and D₂R had a synergistic effect on delay discounting but an antagonist effect on workload discounting. These results provide fundamental insight into the distinct mechanisms of DA action in the regulation of the benefit- and cost-based motivation, which have important implications for motivational alterations in both neurological and psychiatric disorders.

Using quantitatively controlled pharmacological manipulations, this study teases apart the role of D1- and D2-like dopamine receptors in motivation and goal-directed behavior in monkeys, revealing complementary roles of two dopamine receptor subtypes in the computation of the cost/benefit trade-off to guide action.

Novel Antimuscarinic Antidepressant-like Compounds with Reduced Effects on Cognition

The cholinergic nervous system has been implicated in mood disorders, evident in the fast-onset antidepressant effects of scopolamine, a potent muscarinic antagonist, in clinical studies. One prominent disadvantage of the use of scopolamine in the treatment of depression is its detrimental effects on cognition, especially as such effects might aggravate cognitive deficits that occur with depression itself. Thus, the identification of antimuscarinic drugs that are free of such detrimental effects may provide an important avenue for the development of novel therapeutics for the management of depression. The present data in rats indicate that a historical muscarinic antagonist, L-687,306, and a muscarinic antagonist of our own design, CJ2100, were as or more effective than scopolamine in antagonizing both the bradycardic effects of the muscarinic agonist arecoline in cardiovascular studies and its discriminative stimulus and rate-decreasing effects in behavioral studies. Additionally, both novel muscarinic antagonists were as effective as scopolamine in decreasing immobility in the forced swim test, a preclinical indicator of potential antidepressant activity. However, at equieffective or even larger doses, they were considerably less disruptive than scopolamine in assays of cognition-related behavior. All three drugs displayed high specificity for the mAChRs with few off-target binding sites, and CJ2100 showed modest affinity across the mAChRs when compared with L-687,306 and scopolamine. These data emphasize the dissimilar pharmacological profiles that are evident across antimuscarinic compounds and the potential utility of novel antagonists for the improved treatment of depression. SIGNIFICANCE STATEMENT: Some clinical studies with the muscarinic antagonist scopolamine document its ability to produce antidepressant effects in patients with mood disorders; however, scopolamine also has well known adverse effects on both autonomic and centrally mediated physiological functions that limit its therapeutic use. This study characterizes the cardiovascular and discriminative stimulus effects of two novel muscarinic antagonists, L-687,306 and CJ2100, that produce antidepressant-like effects in a rodent model (forced swim test) without affecting touchscreen-based cognitive performance (titrating psychomotor vigilance and delayed matching-to-position).

Dopaminergic Modulation of Human Intertemporal Choice: A Diffusion Model Analysis Using the D2-Receptor Antagonist Haloperidol

The neurotransmitter dopamine is implicated in diverse functions, including reward processing, reinforcement learning, and cognitive control. The tendency to discount future rewards over time has long been discussed in the context of potential dopaminergic modulation. Here we examined the effect of a single dose of the D2 receptor antagonist haloperidol (2 mg) on temporal discounting in healthy female and male human participants. Our approach extends previous pharmacological studies in two ways. First, we applied combined temporal discounting drift diffusion models to examine choice dynamics. Second, we examined dopaminergic modulation of reward magnitude effects on temporal discounting. Hierarchical Bayesian parameter estimation revealed that the data were best accounted for by a temporal discounting drift diffusion model with nonlinear trialwise drift rate scaling. This model showed good parameter recovery, and posterior predictive checks revealed that it accurately reproduced the relationship between decision conflict and response times in individual participants. We observed reduced temporal discounting and substantially faster nondecision times under haloperidol compared with placebo. Discounting was steeper for low versus high reward magnitudes, but this effect was largely unaffected by haloperidol. Results were corroborated by model-free analyses and modeling via more standard approaches. We previously reported elevated caudate activation under haloperidol in this sample of participants, supporting the idea that haloperidol elevated dopamine neurotransmission (e.g., by blocking inhibitory feedback via presynaptic D2 auto-receptors). The present results reveal that this is associated with an augmentation of both lower-level (nondecision time) and higher-level (temporal discounting) components of the decision process.SIGNIFICANCE STATEMENT Dopamine is implicated in reward processing, reinforcement learning, and cognitive control. Here we examined the effects of a single dose of the D2 receptor antagonist haloperidol on temporal discounting and choice dynamics during the decision process. We extend previous studies by applying computational modeling using the drift diffusion model, which revealed that haloperidol reduced the nondecision time and reduced impulsive choice compared with placebo. These findings are compatible with a haloperidol-induced increase in striatal dopamine (e.g., because of a presynaptic mechanism). Our data provide novel insights into the contributions of dopamine to value-based decision-making and highlight how comprehensive model-based analyses using sequential sampling models can inform the effects of pharmacological modulation on choice processes.

Differential effects of D1 and D2 dopamine agonists on memory, motivation, learning and response time in non-human primates

Dopamine (DA) plays a critical role in cognition, motivation and information processing. DA action has been shown to both improve and/or impair cognition across different receptor types, species, subjects and tasks. This complex relationship has been described as an inverted U-shaped function and may be due to the differential effects of DA receptor activation in the striatum and prefrontal cortex. We have investigated the effects of selective DA agonists on cognitive performance in healthy monkeys using a touch screen running tasks from the CAmbridge Neuropsychological Test Automated Battery (CANTAB). One of two DA agonist drugs or placebo was administered prior to each daily CANTAB session: Dihydrexidine hydrochloride (selective D1 agonist, 0.4-0.9 mg/kg), or sumanirole maleate (selective D2 agonist 0.05-0.3 mg/kg). Three CANTAB tasks were tested: (a) "self-ordered sequential search task" which tested spatial working memory, (b) "reversal learning task," which tested association learning, cognitive flexibility and attention and (c) "visually guided reaching task," which tested reaction time and accuracy. At high dosages, the D2 agonist improved spatial working memory performance, while impairing reversal learning and slowing reach response latency. No consistent cognitive effects were observed with the D1 agonist across the dosages tested. A significant decrease in trial completion rate was observed at the higher dosages of both the D1 and D2 agonists which were consistent with decreased motivation. These results are consistent with task-specific effects of a D2 agonist as well as dose specific insensitivities of a D1 agonist on cognitive and motor behaviors in a healthy monkey.

An autonomous, automated and mobile device to concurrently assess several cognitive functions in group-living non-human primates

Research methods in cognitive neuroscience using non-human primates have undergone notable changes over the last decades. Recently, several research groups have described freely accessible devices equipped with a touchscreen interface. Two characteristics of such systems are of particular interest: some apparatuses include automated identification of subjects, while others are mobile. Here, we designed, tested and validated an experimental system that, for the first time, combine automatization and mobility. Moreover, our system allows autonomous learning and testing of cognitive performance in group-living subjects, including follow-up assessments. The mobile apparatus is designed to be available 24h a day, 7days a week, in a typical confined primate breeding and housing facility. Here we present as proof of concept, the results of two pilot studies. We report that rhesus macaques (Macaca mulatta) learned the tasks rapidly and achieved high-level of stable performance. Approaches of this kind should be developed for future pharmacological and biomedical studies in non-human primates.

Brain Structures Implicated in Inflammation-Associated Depression

Systemic inflammation rapidly impairs mood, motivation, and cognition inducing a stereotyped cluster of symptoms collectively known as "sickness behaviors." When inflammation is severe or chronic, these behavioral changes can appear indistinguishable from major depressive disorder (MDD). Human and rodent neuroimaging combined with experimental inflammatory challenges has clarified the neural circuitry associated with many of the key features of inflammation-induced-sickness behavior, and in so doing revealed often-remarkable commonalities with circuit abnormalities observed in MDD. This review aims to provide the first synthesis of this work illustrating areas of convergence and divergence with the MDD literature as well as highlighting areas for future study.

Comparisons of Δ9-Tetrahydrocannabinol and Anandamide on a Battery of Cognition-Related Behavior in Nonhuman Primates

The primary psychoactive ingredient of marijuana, Δ(9)-tetrahydrocannabinol (Δ(9)-THC), has medicinal value but also produces unwanted deleterious effects on cognitive function, promoting the search for improved cannabinergic therapeutics. The present studies used a battery of touchscreen procedures in squirrel monkeys to compare the effects of different types of cannabinergic drugs on several measures of performance including learning (repeated acquisition), cognitive flexibility (discrimination reversal), short-term memory (delayed matching-to-sample), attention (psychomotor vigilance), and motivation (progressive ratio). Drugs studied included the cannabinoid agonist Δ(9)-THC, fatty acid amide hydrolase (FAAH) inhibitor cyclohexylcarbamic acid 3-carbamoylbiphenyl-3-yl ester (URB597), and endocannabinoid anandamide and its stable synthetic analog methanandamide [(R)-(+)-arachidonyl-1'-hydroxy-2'-propylamide]. The effects of Δ(9)-THC and anandamide after treatment with the cannabinoid receptor type 1 inverse agonist/antagonist rimonabant [5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-N-(piperidin-1-yl)-1Hpyrazole-3-carboxamide] and the FAAH inhibitor URB597, respectively, also were examined. The results showed the following: 1) Δ(9)-THC produced dose-related impairments of discrimination-based cognitive behavior with potency that varied across tasks (discriminative capability < learning < flexibility < short-term memory); 2) anandamide alone and URB597 alone were without effect on all endpoints; 3) anandamide following URB597 pretreatment and methanandamide had negligible effects on discriminative capability, learning, and reversal, but following large doses affected delayed matching-to-sample performance in some subjects; 4) all drugs, except anandamide and URB597, disrupted attention; and 5) progressive ratio breakpoints were generally unaffected by all drugs tested, suggesting little to no effect on motivation. Taken together, these data indicate that metabolically stable forms of anandamide may have lesser adverse effects on cognitive functions than Δ(9)-THC, possibly offering a therapeutic advantage in clinical settings.

Asymmetric generalization and interaction profiles in rhesus monkeys discriminating intravenous cocaine or intravenous heroin from vehicle

Many polydrug abusers combine cocaine with heroin in the form of a "speedball." This study investigated the discriminative stimulus (DS) effects of speedballs in rhesus monkeys trained to discriminate either intravenous cocaine or intravenous heroin from vehicle. Initial substitution tests revealed an asymmetry in the generalization profile of dopamine and opioid agonists such that mu agonists partially substituted for cocaine, but direct and indirect dopamine agonists did not substitute for heroin. Subsequent speedball tests in which drug mixtures were administered by coinjecting the component drugs while keeping the dose-ratio constant revealed an additional asymmetry. In cocaine-trained monkeys, coadministration of cocaine and heroin produced leftward shifts in the cocaine dose-response function. Heroin's cocaine-enhancing effects were mimicked by the mu agonists fentanyl and methadone and less consistently by the delta agonist (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC 80) and reversed by the mu antagonist naltrexone and the delta antagonist naltrindole. In heroin-trained monkeys, coadministration of cocaine and heroin attenuated the DS effects of heroin. Cocaine's heroin-attenuating effects were mimicked by the D1-like agonist 6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine (SKF 81297) and the D2-like agonist R-(-)-propylnorapomorphine and reversed by the D1-like antagonist (6aS-trans)-11-chloro-6,6a,7,8,9,13b-hexahydro-7-methyl-5H- benzo[d] aphtha[2,1-b]azepin-12-ol hydrobromide (SCH 39166) and the D2-like antagonist raclopride. Attenuation of the effects of heroin was accompanied by decreases in response rate. These results suggest that heroin enhances the DS effects of cocaine via mu, and to a lesser extent delta, receptor mechanisms; whereas cocaine-induced inhibition of the DS effects of heroin probably was due at least in part to masking of the heroin DS presumably via stimulation of both D1- and D2-like receptors.

Neurobehavioral effects of head-only gamma-radiation exposure in rats

The present report describes initial steps in the development of an animal model for assessing the effects of low levels of radiation encountered in the space environment on human cognitive function by examining the effects of radiation on a range of neurobehavioral functions in rodents that are similar to a number of basic human cognitive functions. The present report presents baseline data on the effects of gamma radiation on neurobehavioral functions in rodents (psychomotor speed, discrimination accuracy and inhibitory control) that are similar to those in humans. Two groups of eight Long-Evans rats were trained to perform a reaction-time task that required them to depress a lever for 1-3 s and to release the lever within 1.5 s of a release stimulus (correct trial) to receive a reward. Releasing the lever prior to the release stimulus (error) terminated the trial. One group was exposed to head-only gamma radiation (5 Gy at a dose rate of 1 Gy/min), while the second group was sham-irradiated using the same anesthesia protocol. The irradiated group showed significant deficits in both performance accuracy (percentage correct scores) and performance reliability (false alarm scores) from 1 to 4 months after irradiation, indicating clear performance impairments. The increase in false alarm scores is consistent with reduced inhibitory control and a shift toward increased anticipatory responses at the cost of decreased accuracy. The nonirradiated group showed no such changes over the same period.

Peripheral inflammation is associated with altered substantia nigra activity and psychomotor slowing in humans

BACKGROUND

Systemic infections commonly cause sickness symptoms including psychomotor retardation. Inflammatory cytokines released during the innate immune response are implicated in the communication of peripheral inflammatory signals to the brain.

METHODS

We used functional magnetic resonance brain imaging (fMRI) to investigate neural effects of peripheral inflammation following typhoid vaccination in 16 healthy men, using a double-blind, randomized, crossover-controlled design.

RESULTS

Vaccination had no global effect on neurovascular coupling but markedly perturbed neural reactivity within substantia nigra during low-level visual stimulation. During a cognitive task, individuals in whom typhoid vaccination engendered higher levels of circulating interleukin-6 had significantly slower reaction time responses. Prolonged reaction times and larger interleukin-6 responses were associated with evoked neural activity within substantia nigra.

CONCLUSIONS

Our findings provide mechanistic insights into the interaction between inflammation and neurocognitive performance, specifically implicating circulating cytokines and midbrain dopaminergic nuclei in mediating the psychomotor consequences of systemic infection.

Haloperidol and clozapine have dissociable effects in a model of attentional performance deficits induced by blockade of NMDA receptors in the mPFC

RATIONALE

Cognitive impairment in schizophrenia is particularly evident in the domains of attention and executive functions. Atypical antipsychotics are somewhat more effective than conventional antipsychotics in improving cognitive functioning in these patients.

OBJECTIVE

The aim of this study was to compare the effects of conventional and atypical antipsychotics in a model of attentional performance deficit of schizophrenia induced by blockade of N-methyl-D: -aspartate (NMDA) receptors in the medial prefrontal cortex.

MATERIALS AND METHODS

Attentional performance was assessed using the five-choice serial reaction time task. The task provides indices of attentional functioning (% correct responses), executive control (measured by anticipatory and perseverative responding), decision time (measured by correct response latency), and omissions. Haloperidol and clozapine were given intraperitoneally (IP) to animals that had received vehicle or a competitive NMDA receptor antagonist, 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP), directly into the medial prefrontal cortex.

RESULTS

Fifty nanograms/side of CPP reduced accuracy (% correct responses) and increased anticipatory and perseverative responding. Haloperidol (0.03 mg/kg IP) reduced the CPP-induced anticipatory and perseverative overresponding but not the impairment in accuracy. In contrast, clozapine (2.5 mg/kg IP) reversed the decrease in accuracy and impulsivity (anticipatory responding) but not perseverative overresponding. CPP increased decision time and omissions, but these effects were not affected by either haloperidol or clozapine.

CONCLUSIONS

The effects on "impulsivity" and "compulsive perseveration" in a rat model of attentional and executive deficit of schizophrenia might differentiate conventional and atypical antipsychotics. Antagonistic activity at 5-HT(2A) receptors may best explain the facilitatory effects of clozapine on cognition.

Differential contributions of dopaminergic D1- and D2-like receptors to cognitive function in rhesus monkeys

RATIONALE

Dopaminergic neurotransmission is critically involved in many aspects of complex behavior and cognition beyond reward/reinforcement and motor function. Mental and behavioral disorders associated with major disruptions of dopamine neurotransmission, including schizophrenia, attention deficit/hyperactivity disorder, Parkinson's disease, Huntington's disease, and substance abuse produce constellations of neuropsychological deficits in learning, memory, and attention in addition to other defining symptoms.

OBJECTIVE

To delineate the role dopaminergic D1- and D2-like receptor subtypes play in complex brain functions.

MATERIALS AND METHODS

Monkeys (N = 6) were trained on cognitive tests adapted from a human neuropsychological assessment battery (CAmbridge Neuropsychological Test Automated Battery). The battery included tests of spatial working memory (self-ordered spatial search task), visuo-spatial associative memory and learning (visuo-spatial paired associates learning task, vsPAL) and motivation (progressive ratio task, PR). Tests of motor function (bimanual motor skill task, BMS; rotating turntable task, RTT) were also included. The effects of the dopamine D2-like antagonist raclopride (10-56 microg/kg, i.m.) and the D1-like antagonist SCH23390 (SCH, 3.2-56 microg/kg, i.m.) on cognitive performance were then determined.

RESULTS

Deficits on PR, RTT, and BMS performance were observed after both raclopride and SCH23390. Spatial working memory accuracy was reduced to a greater extent by raclopride than by SCH, which was unexpected, given prior reports on the involvement of D1 signaling for spatial working memory in monkeys. Deficits were observed on vsPAL performance after raclopride, but not after SCH23390.

CONCLUSIONS

The intriguing results suggest a greater contribution of D2- over D1-like receptors to both spatial working memory and object-location associative memory.

Synthesizing schizophrenia: a bottom-up, symptomatic approach

The cognitive deficits in schizophrenia are discussed in terms of the other symptoms of the disorder, as well as according to a translational approach that involves using similar or analogous tests for humans and experimental animals. This approach, it is argued, will enable the testing of novel drugs and the development of adequate etiological models of schizophrenia.

Effects of nicotine and mecamylamine on cognition in rhesus monkeys

RATIONALE

Nicotine and other agonists of nicotinic cholinergic receptors (nAChR) have been shown to improve performance in specific memory domains in rodents and monkeys. Such beneficial effects are observed in preclinical models of age-related cognitive decline, stimulating interest in nAChR ligands as possible therapeutics. Prior work has typically focused on assays of spatial working memory in rodent studies and visual recognition memory in monkey studies.

OBJECTIVE

The current study was conducted to determine the role played by nAChRs in multiple types of memory in monkeys.

METHODS

Rhesus monkeys (n=6) were trained to perform a battery of six behavioral tasks and then serially challenged with acute doses of nicotine (3.2-56 microg/kg, i.m.) and the nAChR antagonist mecamylamine (0.32-1.78 mg/kg, i.m.).

RESULTS

Nicotine improved performance on tests designed to assay visual recognition memory, spatial working memory and visuo-spatial associative memory, while mecamylamine impaired visuo-spatial associative memory. Ballistic and fine motor performance was not significantly improved by nicotine but fine motor performance was impaired by mecamylamine.

CONCLUSIONS

Although nicotine may improve performance in multiple domains, effects on visuo-spatial associative memory is the most specifically attributable to nAChR signaling.

Effects of parametric feeding manipulations on behavioral performance in macaques

Early experimental psychologists made broad use of knowledge that is undoubtedly as old as animal domestication, i.e., that the power of appetitive reinforcement is enhanced by restricting the subjects' access to food. This has led to the nearly universal practice of restricting common laboratory rodent and avian subjects to 85% of free-feeding weight for operant experiments. Appetitive operant procedures in nonhuman primates (NHPs) vary more widely, in part because of the time required for such animals to reach mature weight and greater individual variability in body size compared with inbred laboratory species. In addition, many NHPs will grow obese under true ad-libitum feeding. Therefore, food restriction protocols for monkeys tend to be highly individualized and conducted on the basis of laboratory experience within a given model. The present study was undertaken to determine to what extent short-term, ad-libitum food consumption in rhesus macaques would impair performance on an established neuropsychological testing battery. A second part of the study was to formalize food-restriction parameters to determine what degree of restriction was required to produce consistent behavioral performance. Results show clearly that behavioral performance on a range of tasks is detrimentally affected by short-term, ad-libitum chow feeding, even when the reinforcer is highly preferred or the tasks are well trained. Furthermore, it is shown that maintenance of weekly chow intake in the range of 70-85% of National Research Council recommendations for metabolizable energy is necessary for consistent behavioral responding.

Psychopharmacological approaches to modulating attention in the five-choice serial reaction time task: implications for schizophrenia

RATIONALE

In schizophrenia, attentional disturbance is a core feature which may not only accompany the disorder, but may precede the onset of psychiatric symptoms.

OBJECTIVES

The five-choice serial reaction time task (5CSRTT) is a test of visuo-spatial attention that has been used extensively in rats for measuring the effects of systemic and central neurochemical manipulations on various aspects of attentional performance, including selective attention, vigilance and executive control. These findings are relevant to our understanding of the neural systems that may be compromised in patients with schizophrenia.

METHODS

The 5CSRTT is conducted in an operant chamber that has multiple response locations, in which brief visual stimuli can be presented randomly. Performance is maintained using food reinforcers to criterion levels of accuracy. Various aspects of performance are measured, including attentional accuracy and premature responding, especially under different attentional challenges.

RESULTS

The effects of systemic and intra-cerebral infusions of selective dopamine, serotonin and cholinergic receptor agents on the 5CSRTT are reviewed with a view to identifying attention-enhancing effects that may be relevant to the treatment of cognitive deficits in schizophrenia. In addition, some novel agents such as modafinil and histamine receptor agents are also considered. Examining the effects of selective neurochemical lesions helped define the neural locus of attentional effects. Similarly, findings from microdialysis studies helped identify the extracellular changes in neurotransmitters and their metabolites in freely moving rats during performance of the 5CSRTT.

CONCLUSIONS

The monoaminergic and cholinergic systems have independent but complementary roles in attentional function, as measured by the 5CSRTT. These functions are predominantly under the control of the prefrontal cortex and striatum. These conclusions are considered in the context of their application towards therapeutic approaches for attentional disturbances that are typically observed in schizophrenic patients.

Modeling a task that is sensitive to dementia of the Alzheimer's type: individual differences in acquisition of a visuo-spatial paired-associate learning task in rhesus monkeys

Early detection of progressive diseases such as Alzheimer's Disease (AD) is crucial for both the treatment and study of the disease. Performance on a visuo-spatial paired-associates learning (vsPAL) task was recently shown to reliably predict a diagnosis of AD in aged populations. The present study reports the development of this vsPAL task for use in nonhuman primates. Translation of vsPAL to a nonhuman model may provide improved preclinical tools for study of the etiology and treatment of dementia. Twelve young adult male rhesus monkeys were trained to perform the vsPAL task concurrently with tests comprising a nonhuman primate neuropsychological test battery. Monkeys successfully learned to perform vsPAL and did so in a task-difficulty ranked fashion. Despite significant individual differences in capability in the acquisition of the recognition memory aspects of the task, all monkeys evidenced the ability to learn within-trial, i.e. to improve with repeated stimulus-location pairings. These results support the use of vsPAL performance under various challenge conditions to investigate the possible substrates of early cognitive decline in AD. Comparison of performance on vsPAL with performance on other memory tasks in the battery will be of more general use in differentiating mechanisms involved in various aspects of mnemonic function.

Performance of the marmoset monkey on computerized tasks of attention and working memory

The CAmbridge Neuropsychological Test Automated Battery (CANTAB) is a computerised battery of neuropsychological tests presented as stimuli on a touch-sensitive computer screen that has been used to assess a wide range of cognitive functions in neuropsychiatric patients, healthy volunteers, and species of non-human primate, primarily the rhesus macaque. The common marmoset is a small-bodied, tractable simian primate that breeds well under laboratory conditions. This primate has been quite extensively studied in terms of its abilities and limitations with respect to appetitive cognitive conditioning. However, the CANTAB versions of sustained/divided attention and working memory tasks have to-date not been studied in the marmoset. Here we describe adult marmoset performance on the CANTAB five-choice serial reaction time task, a delayed match-to-position task, and a task derived from the CANTAB visuo-spatial paired associates learning task that constituted two, concurrent delayed match-to-position tasks. The acquisition and stable longitudinal performance of these tasks provide strong evidence that the marmoset, in addition to the macaque, can be the species of choice for CANTAB-based drug and lesion studies of cognitive function, using tasks similar to those deployed in the study of human cognition and diagnosis of neuropsychiatric disorders.

Impaired performance on a rhesus monkey neuropsychological testing battery following simian immunodeficiency virus infection

Infection with simian immunodeficiency virus (SIV) in macaques provides an excellent model of AIDS including HIV-induced central nervous system (CNS) pathology and cognitive/behavioral impairment. Recently a behavioral test battery has been developed for macaques based on the CANTAB human neuropsychological testing battery. As with human neuropsychological batteries, different tasks are thought to involve different neural substrates, and therefore performance profiles may assess function in particular brain regions. Ten rhesus monkeys were infected with SIV after being trained on two or more of the battery tasks addressing memory (delayed nonmatching to sample, DNMS), spatial working memory (using a self-ordered spatial search task, SOSS), motivation (progressive-ratio, PR), reaction time (RT), and/or fine motor skills (bimanual motor skill, BMS). Performance was compared to that of 9 uninfected monkeys. Overall, some aspect of performance was impaired in all 10 monkeys following infection. Consistent with results in human AIDS patients, individual performance was impaired most often on battery tasks thought to be sensitive to frontostriatal dopaminergic functioning such as SOSS, RT, and BMS. These results further demonstrate the similarity of behavioral impairment produced by SIV and HIV on homologous behavioral tests, and establish the utility of the testing battery for further investigations into the CNS mechanisms of the reported behavioral changes.

Ketamine impairs multiple cognitive domains in rhesus monkeys

Available evidence suggests that recreational use and abuse of the dissociative anaesthetic ketamine is increasing. Characterization of the cognitive risks of ketamine exposure contributes substantially to understanding this growing public health threat. Although prior human studies demonstrate that ketamine impairs a range of cognitive skills, investigation in nonhuman models permits more precise exploration of neurochemical mechanisms which may underlie detrimental behavioral effects. Adult male rhesus monkeys (N=7) were trained on a neuropsychological battery including tests of memory (delayed match-to-sample, DMS; self-ordered spatial search, SOSS), reaction time (RT), reinforcer efficacy and sustained attention (progressive ratio, PR) and fine motor coordination (bimanual motor skill, BMS). Battery performance was then serially challenged with acute doses of ketamine (0.3, 1.0, 1.78 mg/kg IM). Ketamine impaired DMS and SOSS in a dose x difficulty dependent manner with the most difficult task conditions disrupted at the 1.0 and 1.78 mg/kg doses. Thus, both visual recognition memory and working memory indices were affected. Ketamine also slowed RT and BMS performance and interfered with PR performance at the 1.78 mg/kg dose. Overall the present findings confirm that ketamine interferes with multiple aspects of cognition at subanesthetic doses in monkeys.

Cocaine's effects on the discrimination of simple and complex auditory stimuli by baboons

The effects of cocaine on tone frequency discriminations by baboons were examined and compared with previous data for more complex acoustic stimuli (speech sounds) to see if cocaine's perceptual effects on these discriminations depends upon the type of stimulus employed (i.e., tones vs. speech sounds). Baboons pressed a lever to produce one repeating "standard" tone and released the lever only when one of four other "comparison" tones occasionally occurred in place of the standard tone. Cocaine's effects were assessed once or twice weekly by giving an intramuscular injection of cocaine hydrochloride (0.01-0.56 mg/kg) immediately prior to performing the task and by examining correct detections and reaction times for each tone following drug administration. Cocaine impaired tone discriminability, with greater impairments occurring for those tones that were more similar in frequency to the standard tone. Cocaine's perceptual effects occurred within 20-70 min following drug administration. Cocaine also impaired or facilitated the speed of responding to auditory stimuli, depending upon the drug dose and subject. The results demonstrate that cocaine can impair auditory discriminations involving simple tones, as well as speech sounds, and further supports the suggestion that cocaine's effects are focused on CNS mechanisms related to the use of pitch cues.

Performance norms for a rhesus monkey neuropsychological testing battery: acquisition and long-term performance

A computerized behavioral battery based upon human neuropsychological tests (CANTAB, CeNeS, Cambridge, UK) has been developed to assess cognitive behaviors of rhesus monkeys. Monkeys reliably performed multiple tasks, providing long-term assessment of changes in a number of behaviors for a given animal. The overall goal of the test battery is to characterize changes in cognitive behaviors following central nervous system (CNS) manipulations. The battery addresses memory (delayed non-matching to sample, DNMS; spatial working memory, using a self-ordered spatial search task, SOSS), attention (intra-/extra-dimensional shift, ID/ED), motivation (progressive-ratio, PR), reaction time (RT) and motor coordination (bimanual task). As with human neuropsychological batteries, different tasks are thought to involve different neural substrates, and therefore performance profiles should assess function in particular brain regions. Monkeys were tested in transport cages, and responding on a touch sensitive computer monitor was maintained by food reinforcement. Parametric manipulations of several tasks demonstrated the sensitivity of performance to increases in task difficulty. Furthermore, the factors influencing difficulty for rhesus monkeys were the same as those shown to affect human performance. Data from this study represent performance of a population of healthy normal monkeys that will be used for comparison in subsequent studies of performance following CNS manipulations such as infection with simian immunodeficiency virus (NeuroAIDS) or drug administration.

Scopolamine alters rhesus monkey performance on a novel neuropsychological test battery

Rhesus monkeys (6) were trained on a test battery including cognitive tests adapted from a human neuropsychological assessment battery (CANTAB; CeNeS, Cambridge, UK) as well as a bimanual motor skill task. The complete battery included tests of memory (delayed non-match to sample, DNMS; self-ordered spatial search, SOSS), reaction time (RT), motivation (progressive ratio; PR) and fine motor coordination (bimanual). The animals were trained to asymptotic performance in all tasks and then were administered two of the four CANTAB tasks on alternate weekdays (PR/SWM; DNMS/RT) with the bimanual task being administered on each weekday. The effect of acute administration of scopolamine (3-24 microg/kg, i.m.) on performance was then determined. Although performance on DNMS was impaired there was no interaction of drug treatment with retention interval, suggesting that scopolamine does not increase the rate of forgetting in this task. Scopolamine administration produced a decrement in SOSS performance that was dependent on task difficulty as well as dose. Scopolamine also impaired motor responses, resulting in increased time required to complete the bimanual motor task and increased movement time in the RT task. Performance in the PR task was decreased in a dose-dependent fashion by scopolamine. The results suggest that scopolamine interferes with memory storage and motor responses but not memory retention/retrieval or vigilance. The findings demonstrate that the test battery is useful for distinguishing the effects of neuropharmacological manipulation on various aspects of cognitive performance in monkeys.

Delayed motor response correlates with striatal degeneration in Parkinson's disease

OBJECTIVE

Studies on reaction time have suggested a selective deficit of slowness in motor readiness and motor programming in Parkinson's disease (PD). Objective of this study was the putative relation between delayed initiation and execution of movement and the striatal dopamine deficiency in PD.

MATERIAL AND METHODS

We investigated 32 idiopathic, previously untreated parkinsonian patients to evaluate dopaminergic nigrostriatal degeneration by single photon emission tomography in combination with the radiotracer [123I]-beta-CIT and performed a simple reaction time paradigm on the same day.

RESULTS

Significant relations between the [123I]-beta-CIT-SPECT-ratio striatum/cerebellum and reaction--and movement time appeared. Reaction time and movement time of parkinsonian patients were significantly longer compared to age-matched controls.

CONCLUSIONS

We conclude that reaction- and movement-time is related to the dopaminergic nigrostriatal degeneration in untreated parkinsonian subjects.

Longitudinal analysis of behavioral, neurophysiological, viral and immunological effects of SIV infection in rhesus monkeys

A model is proposed in which a neurovirulent, microglial-passaged, simian immunodeficiency virus (SIV) is used to produce central nervous system (CNS) pathology and behavioral deficits in rhesus monkeys reminiscent of those seen in humans infected with human immunodeficiency virus (HIV). The time course of disease progression was characterized by using functional measures of cognition and motor skill, as well as neurophysiologic monitoring. Concomitant assessment of immunological and virological parameters illustrated correspondence between impaired behavioral performance and viral pathogenesis. Convergent results were obtained from neuropathological findings indicative of significant CNS disease. In ongoing studies, this SIV model is being used to explore the behavioral sequelae of immunodeficiency virus infection, the viral and host factors leading to neurologic dysfunction, and to begin testing potential therapeutic agents.