Cortico-hippocampal GluN2B is essential for efficient visual-spatial discrimination learning in a touchscreen paradigm

Mouse Behavior on the Trial-Unique Nonmatching-to-Location (TUNL) Touchscreen Task Reflects a Mixture of Distinct Working Memory Codes and Response Biases

The trial-unique nonmatching to location (TUNL) touchscreen task shows promise as a translational assay of working memory (WM) deficits in rodent models of autism, ADHD, and schizophrenia. However, the low-level neurocognitive processes that drive behavior in the TUNL task have not been fully elucidated. In particular, it is commonly assumed that the TUNL task predominantly measures spatial WM dependent on hippocampal pattern separation, but this proposition has not previously been tested. In this project, we tested this question using computational modeling of behavior from male and female mice performing the TUNL task (N = 163 across three datasets; 158,843 trials). Using this approach, we empirically tested whether TUNL behavior solely measured retrospective WM, or whether it was possible to deconstruct behavior into additional neurocognitive subprocesses. Overall, contrary to common assumptions, modeling analyses revealed that behavior on the TUNL task did not primarily reflect retrospective spatial WM. Instead, behavior was best explained as a mixture of response strategies, including both retrospective WM (remembering the spatial location of a previous stimulus) and prospective WM (remembering an anticipated future behavioral response) as well as animal-specific response biases. These results suggest that retrospective spatial WM is just one of a number of cognitive subprocesses that contribute to choice behavior on the TUNL task. We suggest that findings can be understood within a resource-rational framework, and use computational model simulations to propose several task-design principles that we predict will maximize spatial WM and minimize alternative behavioral strategies in the TUNL task.SIGNIFICANCE STATEMENT Touchscreen tasks represent a paradigm shift for assessment of cognition in nonhuman animals by automating large-scale behavioral data collection. Their main relevance, however, depends on the assumption of functional equivalence to cognitive domains in humans. The trial-unique, delayed nonmatching to location (TUNL) touchscreen task has revolutionized the study of rodent spatial working memory. However, its assumption of functional equivalence to human spatial working memory is untested. We leveraged previously untapped single-trial TUNL data to uncover a novel set of hierarchically ordered cognitive processes that underlie mouse behavior on this task. The strategies used demonstrate multiple cognitive approaches to a single behavioral outcome and the requirement for more precise task design and sophisticated data analysis in interpreting rodent spatial working memory.

Prenatal and postnatal insults differentially contribute to executive function and cognition: Utilizing touchscreen technology for perinatal brain injury research

The use of touchscreen technology to evaluate cognitive deficits in animal models has grown tremendously over the past 20 years. The touchscreen apparatus encompasses many advantages, namely a high level of standardization and translational capability. Improvements in technology in recent years have expanded the versatility of the touchscreen platform, as it is able to test distinct cognitive modalities including working memory, attention, discrimination, and association. Importantly, touchscreen technology has allowed researchers to explore deficits in multiple pillars of cognition in a wide variety of perinatal disorders with neurological sequelae across critical developmental windows. The touchscreen platform has been used to dissect deficits in antenatal CNS injury including fetal alcohol syndrome, prenatal opioid exposure, and chorioamnionitis, to peripartum insults such as term hypoxic-ischemic encephalopathy, to early postnatal insults including infantile traumatic brain injury. Most importantly, touchscreen technology offers the sensitivity necessary to detect subtle injury and treatment-induced changes in cognition and executive function beyond those offered by more rudimentary tests of rodent cognition. Understanding the pathophysiology of these disorders in rodents is paramount to addressing these deficits in human infants and dissecting the neural circuitry essential to perinatal brain injury pathophysiology and responsiveness to novel therapeutics. Touchscreen testing provides an effective, facile, sophisticated technique to accelerate the goal of improving cognitive and behavioral outcomes of children who suffer perinatal brain injury.

Touchscreen Cognitive Deficits, Hyperexcitability, and Hyperactivity in Males and Females Using Two Models of Cdkl5 Deficiency

Many neurodevelopmental disorders (NDDs) are the result of mutations on the X chromosome. One severe NDD resulting from mutations on the X chromosome is CDKL5 deficiency disorder (CDD). CDD is an epigenetic, X-linked NDD characterized by intellectual disability (ID), pervasive seizures and severe sleep disruption, including recurring hospitalizations. CDD occurs at a 4:1 ratio, with a female bias. CDD is driven by the loss of cyclin-dependent kinase-like 5 (CDKL5), a serine/threonine kinase that is essential for typical brain development, synapse formation and signal transmission. Previous studies focused on male subjects from animal models, likely to avoid the complexity of X mosaicism. For the first time, we report translationally relevant behavioral phenotypes in young adult (8–20 weeks) females and males with robust signal size, including impairments in learning and memory, substantial hyperactivity and increased susceptibility to seizures/reduced seizure thresholds, in both sexes, and in two models of CDD preclinical mice, one with a general loss-of-function mutation and one that is a patient-derived mutation.

Remote and Persistent Alterations in Glutamate Receptor Subunit Composition Induced by Spreading Depolarizations in Rat Brain

Spreading depolarizations (SDs) are massive breakdowns of ion homeostasis in the brain's gray matter and are a necessary pathologic mechanism for lesion development in various injury models. However, injury-induced SDs also propagate into remote, healthy tissue where they do not cause cell death, yet their functional long-term effects are unknown. Here we induced SDs in uninjured cortex and hippocampus of Sprague-Dawley rats to study their impact on glutamate receptor subunit expression after three days. We find that both cortical and hippocampal tissue exhibit changes in glutamate receptor subunit expression, including GluA1 and GluN2B, suggesting that SDs in healthy brain tissue may have a role in plasticity. This study is the first to show prolonged effects of SDs on glutamate signaling and has implications for neuroprotection strategies aimed at SD suppression.

New frontiers in translational research: Touchscreens, open science, and the mouse translational research accelerator platform

Many neurodegenerative and neuropsychiatric diseases and other brain disorders are accompanied by impairments in high-level cognitive functions including memory, attention, motivation, and decision-making. Despite several decades of extensive research, neuroscience is little closer to discovering new treatments. Key impediments include the absence of validated and robust cognitive assessment tools for facilitating translation from animal models to humans. In this review, we describe a state-of-the-art platform poised to overcome these impediments and improve the success of translational research, the Mouse Translational Research Accelerator Platform (MouseTRAP), which is centered on the touchscreen cognitive testing system for rodents. It integrates touchscreen-based tests of high-level cognitive assessment with state-of-the art neurotechnology to record and manipulate molecular and circuit level activity in vivo in animal models during human-relevant cognitive performance. The platform also is integrated with two Open Science platforms designed to facilitate knowledge and data-sharing practices within the rodent touchscreen community, touchscreencognition.org and mousebytes.ca. Touchscreencognition.org includes the Wall, showcasing touchscreen news and publications, the Forum, for community discussion, and Training, which includes courses, videos, SOPs, and symposia. To get started, interested researchers simply create user accounts. We describe the origins of the touchscreen testing system, the novel lines of research it has facilitated, and its increasingly widespread use in translational research, which is attributable in part to knowledge-sharing efforts over the past decade. We then identify the unique features of MouseTRAP that stand to potentially revolutionize translational research, and describe new initiatives to partner with similar platforms such as McGill's M3 platform (m3platform.org).

Moderate Prenatal Alcohol Exposure Impairs Visual-Spatial Discrimination in a Sex-Specific Manner: Effects of Testing Order and Difficulty on Learning Performance

BACKGROUND

Exposure to high levels of alcohol during development leads to alterations in neurogenesis and deficits in hippocampal-dependent learning. Evidence suggests that even more moderate alcohol consumption during pregnancy can have negative impacts on the cognitive function of offspring. Methods for assessing impairments differ greatly across species, complicating translation of preclinical findings into potential therapeutics. We have demonstrated the utility of a touchscreen operant measure for assessing hippocampal function in mice.

METHODS

Here, we integrated a well-established "drinking-in-the-dark" exposure model that produces reliable, but more moderate, levels of maternal intoxication with a trial-unique, delayed nonmatching-to-location (TUNL) task to examine the effects of prenatal alcohol exposure (PAE) on hippocampal-sensitive behavior directly analogous to those used in clinical assessment. PAE and SAC offspring mice were trained to touch a single visual stimulus ("sample phase") in one of 10 possible spatial locations (2 × 5 grid) in a touchscreen operant system. After a delay, animals were simultaneously presented with the original stimulus and a rewarded stimulus in a novel location ("choice phase"). PAE and saccharin (SAC) control mice were trained on a series of problems that systematically increased the difficulty by decreasing the separation between the sample and choice stimuli. Next, a separate cohort of PAE and SAC animals were given a brief training and then tested on a challenging variant where both the separation and delay varied with each trial.

RESULTS

We found that PAE mice were generally able to perform at levels similar to SAC control mice at progressively more difficult separations. When tested on the most difficult unpredictable variant immediately, PAE showed a sex-specific deficit with PAE females performing worse during long delays.

CONCLUSIONS

Taken together, these data demonstrate the utility of the TUNL task for examining PAE related alterations in hippocampal function and underline the need to examine sex-by-treatment interactions in these models.

Prevention of age-associated neuronal hyperexcitability with improved learning and attention upon knockout or antagonism of LPAR2

Recent studies suggest that synaptic lysophosphatidic acids (LPAs) augment glutamate-dependent cortical excitability and sensory information processing in mice and humans via presynaptic LPAR2 activation. Here, we studied the consequences of LPAR2 deletion or antagonism on various aspects of cognition using a set of behavioral and electrophysiological analyses. Hippocampal neuronal network activity was decreased in middle-aged LPAR2^−/− mice, whereas hippocampal long-term potentiation (LTP) was increased suggesting cognitive advantages of LPAR2^−/− mice. In line with the lower excitability, RNAseq studies revealed reduced transcription of neuronal activity markers in the dentate gyrus of the hippocampus in naïve LPAR2^−/− mice, including ARC, FOS, FOSB, NR4A, NPAS4 and EGR2. LPAR2^−/− mice behaved similarly to wild-type controls in maze tests of spatial or social learning and memory but showed faster and accurate responses in a 5-choice serial reaction touchscreen task requiring high attention and fast spatial discrimination. In IntelliCage learning experiments, LPAR2^−/− were less active during daytime but normally active at night, and showed higher accuracy and attention to LED cues during active times. Overall, they maintained equal or superior licking success with fewer trials. Pharmacological block of the LPAR2 receptor recapitulated the LPAR2^−/− phenotype, which was characterized by economic corner usage, stronger daytime resting behavior and higher proportions of correct trials. We conclude that LPAR2 stabilizes neuronal network excitability upon aging and allows for more efficient use of resting periods, better memory consolidation and better  performance in tasks requiring high selective attention. Therapeutic LPAR2 antagonism may alleviate aging-associated cognitive dysfunctions.

Prenatal immune activation induces age-related alterations in rat offspring: Effects upon NMDA receptors and behaviors

Prenatal exposure to polyriboinosinic-polyribocytidylic acid (poly I:C) results in psychotic-like behavior in mature rat offspring as well as enduring modifications of glutamatergic excitatory synaptic transmission. However, little is known about the dynamic behavioral and glutamate N-methyl-D-aspartate (NMDA) receptor changes in rat offspring following poly I:C treatment of pregnant dams. In this study, poly I:C was administered to rats intravenously at a dose of 10 mg/kg on gestational day 9 in order to assess changes in behavior and NMDA receptors in offspring over time. Results demonstrate progressive worsening behaviors in adolescents and adults that manifest as increased anxiety, cognitive impairment, and pre-pulse inhibition deficits. Age-related alteration of NMDA receptors in the prefrontal cortex and hippocampus, either total number or distribution, were observed from weaning to adulthood. These results suggest that abnormalities of NMDA receptors occur prior to obvious schizophrenia-like behavioral manifestations. Hence, NMDA receptors may be potential therapeutic targets to prevent disease development during asymptomatic periods of schizophrenia, and may serve as targets for preventive and/or therapeutic strategies for schizophrenia. Further, PSD95, a scaffolding protein that is a component of the NMDA receptor signaling complex, is increased in the hippocampus of adult offspring, when serious behavioral abnormalities emerge. This result suggests that PSD95 may be involved in behavioral abnormalities of schizophrenia.

Genetic inactivation of hypoxia inducible factor 1-alpha (HIF-1α) in adult hippocampal progenitors impairs neurogenesis and pattern discrimination learning

HIF-1α is a hypoxia-inducible protein that regulates many cellular processes, including neural stem cell maintenance. Previous work demonstrated constitutive stabilization of HIF-1α in neural stem cells (NSCs) of the adult mouse subventricular zone (SVZ) and hippocampal subgranular zone (SGZ). Genetic inactivation of NSC-encoded HIF-1α in the adult SVZ results in gradual loss of NSCs, but whether HIF-1α is required for the maintenance of SGZ hippocampal progenitors and adult hippocampal neurogenesis has not been determined. Here we tested the hypothesis that HIF-1α plays an essential role in the maintenance of adult hippocampal neurogenesis using Nestin-CreER^T2/R26R-YFP/Hif1a^fl/fl triple transgenic mice, in which HIF-1α was genetically inactivated in nestin⁺ hippocampal progenitors and their downstream progeny following tamoxifen exposure. We found that disruption of HIF-1α gene expression resulted in a marked 50% reduction of adult-generated dentate granule cells (DGCs) that was highly correlated with impaired hippocampal function, as assessed using two behavioral assays of pattern discrimination. These behavioral tests included the A-B contextual fear-conditioning task and the trial-unique, delayed nonmatching-to-location (TUNL) touch-screen operant chamber task. Our findings identify HIF-1α as a novel regulator of adult hippocampal neurogenesis under non-pathological conditions, and underscore the importance of neurogenesis for pattern discrimination learning.