Sophie Dix

NMDA receptors, cognition and schizophrenia--testing the validity of the NMDA receptor hypofunction hypothesis.

Cognitive dysfunction is core to schizophrenia, and remains poorly treated by existing therapies. A prominent hypothesis suggests that many symptoms arise from N-methyl-d-aspartate receptor (NMDAR) hypofunction. Subsequently, there has emerged a widespread use of many preclinical and clinical NMDAR antagonist models in the search for novel treatments. Clinically, ketamine is broadly purported to induce cognitive symptoms similar to those of schizophrenia. Preclinically, acute, subchronic and neonatal NMDAR antagonist administration models are all utilised in this context, as well as NMDAR transgenic animals. In this review, key strengths and weaknesses of each of these approaches are described with regard to their ability to recapitulate the deficits seen in patients. Given the breadth of literature and vogue for research in this topic, instances of NMDAR antagonist effects in the desired domains can readily be found preclinically. However, it is surprisingly difficult to identify any single aspect of cognitive function that possesses complete translational integrity. That is, there does not seem to be an NMDAR antagonist regimen proven to engage NMDARs equivalently in humans and animals that reliably produces the same cognitive effects in each species. This is likely due to the diverse range of techniques and models used by preclinical researchers, a paucity of research describing pharmacokinetic-pharmacodynamic relationships of NMDAR antagonist regimens, little capability to measure target engagement, and the lack of harmonized procedures between preclinical and clinical studies. Realizing the potential of the NMDAR hypofunction hypothesis to model cognitive impairment in schizophrenia will require some of these issues to be addressed.

A comparison of the effects of ketamine and phencyclidine with other antagonists of the NMDA receptor in rodent assays of attention and working memory

RationaleN-methyl-d-Aspartate receptor (NMDAR) antagonists such as ketamine induce cognitive symptoms in man similar to those of schizophrenia and therefore might be useful as models of the disease in animals. However, it is unclear which NMDAR antagonist(s) offer the best means to produce cognitive deficits in attention and working memory and to what extent those deficits can be measured selectively in rats.ObjectivesThe present study systematically compared the effects of eight different NMDAR antagonists—MK-801, phencyclidine, (S)-(+)-ketamine, memantine, SDZ-220,581, Ro 25-6981, CP 101-606 and NVP-AAM077—in rats using standard tests of visual attention, the five-choice serial reaction time task (5CSRT), and working memory, the delayed matching to position task (DMTP).ResultsDrug-induced responses varied qualitatively and quantitatively in both a compound- and a task-dependent manner. Effects were generally confounded by concomitant motor and motivational disruption, although individual doses of phencyclidine for example appeared to impair selectively cognitive functions. Interestingly, GluN2B selective antagonists were unique in their effects; inducing potential performance benefit in the 5CSRT.ConclusionsOverall, the opportunity to induce a selective cognitive deficit in attention (5CSRT) or working memory (DMTP) in the rat is limited by both the NMDAR antagonist and the dose range used. The importance of a preclinical focus on ketamine, which is used more frequently in clinical settings, is limited by the extent to which cognitive effects can be both detected and quantified using this exposure regimen within these two operant assays.

Diverse and often opposite behavioural effects of NMDA receptor antagonists in rats: implications for “NMDA antagonist modelling” of schizophrenia

RationaleLittle attention has been paid to the relative equivalence of behavioural effects of NMDA receptor antagonists in rodents, with different compounds often used interchangeably to “model” aspects of schizophrenia in preclinical studies.ObjectivesTo further resolve such conjecture, the present study systematically compared eight different NMDA receptor antagonists: MK-801, PCP, ketamine, memantine, SDZ 220,581, Ro 25-6981, CP 101-606 and NVP-AAM077, in a series of variable interval (VI) schedules of reinforcement. Aspects of motivation as indexed in these tasks may well be impaired in schizophrenia and undoubtedly impact on the capacity to perform more complex, explicit tasks of cognition.Methods and resultsAn initial locomotor activity assessment demonstrated that all antagonists tested, except the NR2A-subunit preferring antagonist NVP-AAM077, induced hyperactivity, albeit of greatly differing magnitudes, qualities and temporal profiles. Three distinct patterns of antagonist effect were evident from the VI assays used: a uniform decrease in responding produced by (S)-(+)-ketamine, memantine and NVP-AAM077, a uniform increase in responding caused by the NR2B-subunit preferring antagonists Ro 25-6981 and CP 101-606, and variable bidirectional effects of PCP, SDZ 220,581 and MK-801.ConclusionDespite nominally common mechanisms of action and often presumed biological equivalence, the NMDA antagonists tested produced very diverse effects on the expression of instrumental action. Other aspects of responding were left intact, including switching and matching behaviours, and the ability to respond to conditional stimuli. The implications of such findings with regard to animal modelling of schizophrenic psychotic symptoms are manifold.

A within-subject cognitive battery in the rat: differential effects of NMDA receptor antagonists

RationaleThe range of cognitive and psychotomimetic effects produced by antagonists of the N-methyl-D-aspartate (NMDA) receptor has lead to widespread usage of these molecules as pharmacological models of cognitive impairment for drug discovery. Historically, NMDA receptor antagonists have been used interchangeably on the assumption that they produce analogous effects.ObjectivesTo profile a subset of these antagonists across a novel within-subject cognitive battery in the rat.MethodsNaïve male Lister Hooded rats were subjected to a series of tests in which they were required to learn a simple visuo-auditory conditional discrimination. They then underwent testing in a delayed discrimination test followed by rule reversal and rule extinction tests.ResultsAll NMDA receptor antagonists tested impaired acquisition performance and, with the exception of ketamine and the GluN2A preferring antagonist, NVP-AAM077, impaired consolidation of extinction. GluN2B antagonism produced a singular profile with potentially enhanced delayed discrimination performance and reduced hit rates in the reversal phase. Only PCP (phencyclidine) and ketamine disrupted performance in the delay phase but did so in a delay-independent manner. MK-801, PCP and memantine all increased the hit rate in the reversal phase; whilst only MK-801 and PCP impaired extinction per se.ConclusionsNMDA receptor-dependent mechanisms are requisite in the acquisition of a simple conditional discrimination and consolidation of extinction. Their role in working memory and reversal tasks appear to be less critical and potentially specific to the paradigm and NMDA receptor antagonist used. It is clearly misleading to generalise across NMDA antagonists with respect to their preclinical cognitive profile.

Using the BOLD MR signal to differentiate the stereoisomers of ketamine in the rat

RATIONALE Ketamine is a chiral molecule that is reported to model aspects of schizophrenia. OBJECTIVES To investigate the stereospecificity of the isomers of ketamine using pharmacological magnetic resonance imaging (phMRI) in order to further understand ketamines pharmacodynamic actions. METHOD Responses to 25 mg kg-1S(+) isomer, R(-) isomer and racemic ketamine in independent groups of Sprague-Dawley rats were investigated using a prepulse inhibition paradigm, locomotor observations, MRI and 2-deoxyglucose techniques. RESULTS Racemic ketamine and the S(+) isomer were both capable of disrupting sensorimotor gating as measured using prepulse inhibition and produced a longer period of hyperlocomotion comparative to the R(-) isomer. In contrast, large alterations in the BOLD MR signal were observed with R(-) isomer, whereas S(+) isomer and racemate precipitated more localized BOLD signal changes predominantly in cortical, hippocampal and hindbrain regions. Glucose utilization rates in conscious animals are in agreement with previously published data and verify the BOLD responses in the racemic group. However, no significant changes in glucose utilization were observed in the anesthetized cohort. CONCLUSIONS Ketamine and its isomers have stereospecific effects on sensorimotor gating and locomotion that correlate with the enantiomers affinity for the NMDA receptor. It would appear that anesthesia, as required for preclinical MRI procedures, may interact with and potentially attenuate the drugs response. Although analysis of the main effect of isomers in comparison to each other or the racemate offers an alternative analysis method that should be less susceptible to anesthetic interactions, only the R(-) isomer comparative to the racemate offers significant differences of interest.

The pharmacological sensitivity of a touchscreen-based visual discrimination task in the rat using simple and perceptually challenging stimuli

RationaleCognitive testing with touchscreen-equipped operant boxes (‘touchscreens’) is becoming increasingly popular. Tasks, such as paired associate learning or reversal learning of visual stimuli, have the discrimination of visual stimuli as a fundamental component. However, the effect of drugs commonly used in the study of cognitive mechanisms has yet to be described in a visual discrimination.ObjectiveThe objective of the study was to profile a range of psychoactive agents (glutamatergic, dopaminergic, and cholinergic agonists and antagonists) known to be important in cognitive processing on visual discrimination performance using a touch sensitive computer monitor.MethodsMale Lister Hooded rats were trained to a stable level of performance in a simple visual discrimination. In Experiment 1, the effect of MK-801, phencyclidine, memantine, dextroamphetamine sulphate (d-amphetamine) and scopolamine was assessed. In Experiment 2, the stimuli were blended together resulting in a perceptually more demanding discrimination and a reduction in accuracy. The rats used in Experiment 1 were then retested with these ‘morphed’ stimuli under the influence of the above compounds.ResultsMK-801, PCP, and d-amphetamine induced selective deficits in accuracy in both versions of the task. In contrast, scopolamine and memantine produced non-selective deficits in accuracy. Morphing the stimuli reduced accuracy, but did not alter the observed behavioural profile after compound administration.ConclusionThese data improve our understanding of the basic neuropharmacology of a visual discrimination in cognitive tests employing touchscreens and will aid in the interpretation of pharmacological studies with more cognitively demanding methodologies.

Striatum and entorhinal cortex atrophy in AD mouse models: MRI comprehensive analysis

Alzheimers disease is experimentally modeled in transgenic (Tg) mice overexpressing mutated forms of the human amyloid precursor protein either alone or combined with mutated presenilins and tau. In the present study, we developed a systematic approach to compare double (TASTPM) and triple (APP/PS2/Tau) Tg mice by serial magnetic resonance imaging and spectroscopy analysis from 4 to 26 months of age to define homologous biomarkers between mice and humans. Hippocampal atrophy was found in Tg mice compared with WT. In APP/PS2/Tau the effect was age-dependent, whereas in TASTPM it was detectable from the first investigated time point. Importantly, both mice displayed an age-related entorhinal cortex thinning and robust striatal atrophy, the latter associated with a significant loss of synaptophysin. Hippocampal magnetic resonance spectroscopy revealed lower glutamate levels in both Tg mice and a selective myo-inositol increase in TASTPM. This noninvasive magnetic resonance imaging analysis, revealed common biomarkers between humans and mice, and could, thus, be promoted as a fully translational tool to be adopted in the preclinical investigation of therapeutic approaches.

Deficits of psychomotor and mnesic functions across aging in mouse lemur primates

Owing to a similar cerebral neuro-anatomy, non-human primates are viewed as the most valid models for understanding cognitive deficits. This study evaluated psychomotor and mnesic functions of 41 young to old mouse lemurs (Microcebus murinus). Psychomotor capacities and anxiety-related behaviors decreased abruptly from middle to late adulthood. However, mnesic functions were not affected in the same way with increasing age. While results of the spontaneous alternation task point to a progressive and widespread age-related decline of spatial working memory, both spatial reference and novel object recognition (NOR) memory tasks did not reveal any tendency due to large inter-individual variability in the middle-aged and old animals. Indeed, some of the aged animals performed as well as younger ones, whereas some others had bad performances in the Barnes maze and in the object recognition test. Hierarchical cluster analysis revealed that declarative-like memory was strongly impaired only in 7 out of 25 middle-aged/old animals. These results suggest that this analysis allows to distinguish elder populations of good and bad performers in this non-human primate model and to closely compare this to human aging.

A Review of the Effects of Hypoxia, Sleep Deprivation and Transcranial Magnetic Stimulation on EEG Activity in Humans: Challenges for Drug Discovery for Alzheimer's Disease

Different kinds of challenge can alter cognitive process and electroencephalographic (EEG) rhythms in humans. This can provide an alternative paradigms to evaluate treatment effects in drug discovery. Here, we report recent findings on the effects of challenges represented by sleep deprivation (SD), transient hypoxia, and transcranial magnetic stimulation (TMS) in healthy volunteers on cognitive processes and EEG rhythms to build a knowledge platform for novel research for drug discovery in AD Alzheimers disease (AD). Sleep pressure enhanced frontal delta rhythms (< 4 Hz) during the night, while SD increased slow rhythms in the theta range (4-7 Hz), and reduced resting state alpha rhythms (8-12 Hz) after the following day. Furthermore, SD transiently affected cognitive performance. In contrast, transient experimental hypoxia induced abnormal posterior resting state delta and alpha rhythms in healthy volunteers that resemble the abnormal EEG rhythms typically recorded in AD patients. However, the relationship between the cognitive and EEG effects of such challenges is poorly understood. TMS reversibly interfered with higher brain functions during EEG recordings, but few studies have investigated the relationship between the cognitive and EEG effects of TMS. In conclusion, SD is the most mature challenge model for testing new drugs for AD. Future investigation is needed to better understand the opportunities offered by TMS and hypoxia challenges.

An automated maze task for assessing hippocampus-sensitive memory in mice

Highlights • Alternation procedures in rodents are highly sensitive to manipulations of the hippocampus.• However as they require hand testing, they are low throughput and stressful for the animal.• An automated maze was developed for assessing alternation performance in mice.• Alternation performance was shown to be impaired in mice with lesions to the hippocampus.