A mouse model of fragile X syndrome exhibits heightened arousal and/or emotion following errors or reversal of contingencies

Inhibition of neuronal nitric oxide synthase protects against hippocampal neuronal injuries by increasing neuropeptide Y expression in temporal lobe epilepsy mice

Neuronal nitric oxide synthase (nNOS) plays a pivotal role in the pathological process of neuronal injury in the development of epilepsy. Our previous study has demonstrated that nitric oxide (NO) derived from nNOS in the epileptic brain is neurotoxic due to its reaction with the superoxide radical with the formation of peroxynitrite. Neuropeptide Y (NPY) is widely expressed in the mammalian brain, which has been implicated in energy homeostasis and neuroprotection. Recent studies suggest that nNOS may act as a mediator of NPY signaling. Here in this study, we sought to determine whether NPY expression is regulated by nNOS, and if so, whether the regulation of NPY by nNOS is associated with the neuronal injuries in the hippocampus of epileptic brain. Our results showed that pilocarpine-induced temporal lobe epilepsy (TLE) mice exhibited an increased level of nNOS expression and a decreased level of NPY expression along with hippocampal neuronal injuries and cognition deficit. Genetic deletion of nNOS gene, however, significantly upregulated hippocampal NPY expression and reduced TLE-induced hippocampal neuronal injuries and cognition decline. Knockdown of NPY abolished nNOS depletion-induced neuroprotection and cognitive improvement in the TLE mice, suggesting that inhibition of nNOS protects against hippocampal neuronal injuries by increasing neuropeptide Y expression in TLE mice. Targeting nNOS-NPY signaling pathway in the epileptic brain might provide clinical benefit by attenuating neuronal injuries and preventing cognitive deficits in epilepsy patients.

Disrupted Cortical State Regulation in a Rat Model of Fragile X Syndrome

Children with Fragile X syndrome (FXS) have deficits of attention and arousal. To begin to identify the neural causes of these deficits, we examined juvenile rats lacking the Fragile X mental retardation protein (FMR-KO) for disruption of cortical activity related to attention and arousal. Specifically, we examined the switching of visual cortex between activated and inactivated states that normally occurs during movement and quiet rest, respectively. In both wild-type and FMR-KO rats, during the third and fourth postnatal weeks cortical activity during periods of movement was dominated by an activated state with prominent 18-52 Hz activity. However, during quiet rest, when activity in wild-type rats became dominated by the inactivated state (3-9 Hz activity), FMR-KO rat cortex abnormally remained activated, resulting in increased high-frequency and reduced low-frequency power during rest. Firing rate correlations revealed reduced synchronization in FMR-KO rats, particularly between fast-spiking interneurons, that developmentally precede cortical state defects. Together our data suggest that disrupted inhibitory connectivity impairs the ability of visual cortex to regulate exit from the activated state in a behaviorally appropriate manner, potentially contributing to disrupted attention and sensory processing observed in children with FXS by making it more difficult to decrease cortical drive by unattended stimuli.

Potential Involvement of Impaired BKCa Channel Function in Sensory Defensiveness and Some Behavioral Disturbances Induced by Unfamiliar Environment in a Mouse Model of Fragile X Syndrome

In fragile X syndrome (FXS), sensory hypersensitivity and impaired habituation is thought to result in attention overload and various behavioral abnormalities in reaction to the excessive and remanent salience of environment features that would normally be ignored. This phenomenon, termed sensory defensiveness, has been proposed as the potential cause of hyperactivity, hyperarousal, and negative reactions to changes in routine that are often deleterious for FXS patients. However, the lack of tools for manipulating sensory hypersensitivity has not allowed the experimental testing required to evaluate the relevance of this hypothesis. Recent work has shown that BMS-204352, a BK_Ca channel agonist, was efficient to reverse cortical hyperexcitability and related sensory hypersensitivity in the Fmr1-KO mouse model of FXS. In the present study, we report that exposing Fmr1-KO mice to novel or unfamiliar environments resulted in multiple behavioral perturbations, such as hyperactivity, impaired nest building and excessive grooming of the back. Reversing sensory hypersensitivity with the BK_Ca channel agonist BMS-204352 prevented these behavioral abnormalities in Fmr1-KO mice. These results are in support of the sensory defensiveness hypothesis, and confirm BK_Ca as a potentially relevant molecular target for the development of drug medication against FXS/ASD.

Impaired cognitive discrimination and discoordination of coupled theta-gamma oscillations in Fmr1 knockout mice

Fragile X syndrome (FXS) patients do not make the fragile X mental retardation protein (FMRP). The absence of FMRP causes dysregulated translation, abnormal synaptic plasticity and the most common form of inherited intellectual disability. But FMRP loss has minimal effects on memory itself, making it difficult to understand why the absence of FMRP impairs memory discrimination and increases risk of autistic symptoms in patients, such as exaggerated responses to environmental changes. While Fmr1 knockout (KO) and wild-type (WT) mice perform cognitive discrimination tasks, we find abnormal patterns of coupling between theta and gamma oscillations in perisomatic and dendritic hippocampal CA1 local field potentials of the KO. Perisomatic CA1 theta-gamma phase-amplitude coupling (PAC) decreases with familiarity in both the WT and KO, but activating an invisible shock zone, subsequently changing its location, or turning it off, changes the pattern of oscillatory events in the LFPs recorded along the somato-dendritic axis of CA1. The cognition-dependent changes of this pattern of neural activity are relatively constrained in WT mice compared to KO mice, which exhibit abnormally weak changes during the cognitive challenge caused by changing the location of the shock zone and exaggerated patterns of change when the shock zone is turned off. Such pathophysiology might explain how dysregulated translation leads to intellectual disability in FXS. These findings demonstrate major functional abnormalities after the loss of FMRP in the dynamics of neural oscillations and that these impairments would be difficult to detect by steady-state measurements with the subject at rest or in steady conditions.

Learning and behavioral deficits associated with the absence of the fragile X mental retardation protein: what a fly and mouse model can teach us

The Fragile X syndrome (FXS) is the most frequent form of inherited mental disability and is considered a monogenic cause of autism spectrum disorder. FXS is caused by a triplet expansion that inhibits the expression of the FMR1 gene. The gene product, the Fragile X Mental Retardation Protein (FMRP), regulates mRNA metabolism in brain and nonneuronal cells. During brain development, FMRP controls the expression of key molecules involved in receptor signaling, cytoskeleton remodeling, protein synthesis and, ultimately, spine morphology. Symptoms associated with FXS include neurodevelopmental delay, cognitive impairment, anxiety, hyperactivity, and autistic-like behavior. Twenty years ago the first Fmr1 KO mouse to study FXS was generated, and several years later other key models including the mutant Drosophila melanogaster, dFmr1, have further helped the understanding of the cellular and molecular causes behind this complex syndrome. Here, we review to which extent these biological models are affected by the absence of FMRP, pointing out the similarities with the observed human dysfunction. Additionally, we discuss several potential treatments under study in animal models that are able to partially revert some of the FXS abnormalities.

Hyperactivity, perseveration and increased responding during attentional rule acquisition in the Fragile X mouse model

Attentional deficits and executive function impairments are common to many neurodevelopmental disorders of intellectual disability and autism, including Fragile X syndrome (FXS). In the knockout mouse model for FXS, significant changes in synaptic plasticity and connectivity are found in the prefrontal cortex (PFC)-a prominent region for attentional processing and executive control. Given these alterations in PFC synaptic function, we tested whether adult Fragile X knockout mice exhibited corresponding impairments in inhibitory control, perseveration, and sustained attention. Furthermore, we investigated individual performance during attentional rule acquisition. Using the 5-choice serial reaction time task, our results show no impairments in inhibitory control and sustained attention. Fragile X knockout mice exhibited enhanced levels of correct and incorrect responding, as well as perseveration of responding during initial phases of rule acquisition, that normalized with training. For both knockout and wild type mice, pharmacological attenuation of metabotropic glutamate receptor 5 signaling did not affect response accuracy but reduced impulsive responses and increased omission errors. Upon rule reversal, Fragile X knockout mice made more correct and incorrect responses, similar to the initial phases of rule acquisition. Analogous to heightened activity upon novel rule acquisition, Fragile X knockout mice were transiently hyperactive in both a novel open field (OF) arena and novel home cage. Hyperactivity ceased with familiarization to the environment. Our findings demonstrate normal inhibitory control and sustained attention but heightened perseveration, responding, and hyperactivity during novel rule acquisition and during exposure to novel environments in Fragile X knockout mice. We therefore provide evidence for subtle but significant differences in the processing of novel stimuli in the mouse model for the FXS.

Effects of stimulus salience on touchscreen serial reversal learning in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability in males and the most common genetic cause of autism. Although executive dysfunction is consistently found in humans with FXS, evidence of executive dysfunction in Fmr1 KO mice, a mouse model of FXS, has been inconsistent. One possible explanation for this is that executive dysfunction in Fmr1 KO mice, similar to humans with FXS, is only evident when cognitive demands are high. Using touchscreen operant conditioning chambers, male Fmr1 KO mice and their male wildtype littermates were tested on the acquisition of a pairwise visual discrimination followed by four serial reversals of the response rule. We assessed reversal learning performance under two different conditions. In the first, the correct stimulus was salient and the incorrect stimulus was non-salient. In the second and more challenging condition, the incorrect stimulus was salient and the correct stimulus was non-salient; this increased cognitive load by introducing conflict between sensory-driven (i.e., bottom-up) and task-dependent (i.e., top-down) signals. Fmr1 KOs displayed two distinct impairments relative to wildtype littermates. First, Fmr1 KOs committed significantly more learning-type errors during the second reversal stage, but only under high cognitive load. Second, during the first reversal stage, Fmr1 KOs committed significantly more attempts to collect a reward during the timeout following an incorrect response. These findings indicate that Fmr1 KO mice display executive dysfunction that, in some cases, is only evident under high cognitive load.

Dietary and genetic manipulations of folate metabolism differentially affect neocortical functions in mice

Converging evidence suggests that folate-mediated one-carbon metabolism may modulate cognitive functioning throughout the lifespan, but few studies have directly tested this hypothesis. This study examined the separate and combined effects of dietary and genetic manipulations of folate metabolism on neocortical functions in mice, modeling a common genetic variant in the MTHFD1 gene in humans. Mutant (Mthfd1(gt/+)) and wildtype (WT) male mice were assigned to a folate sufficient or deficient diet at weaning and continued on these diets throughout testing on a series of visual attention tasks adapted from the 5-choice serial reaction time task. WT mice on a deficient diet exhibited impulsive responding immediately following a change in task parameters that increased demands on attention and impulse control, and on trials following an error. This pattern of findings indicates a heightened affective response to stress and/or an inability to regulate negative emotions. In contrast, Mthfd1(gt/+) mice (regardless of diet) exhibited attentional dysfunction and a blunted affective response to committing an error. The Mthfd1(gt/+) mice also showed significantly decreased expression levels for genes encoding choline dehydrogenase and the alpha 7 nicotinic cholinergic receptor. The effects of the MTHFD1 mutation were less pronounced when combined with a deficient diet, suggesting a compensatory mechanism to the combined genetic and dietary perturbation of folate metabolism. These data demonstrate that common alterations in folate metabolism can produce functionally distinct cognitive and affective changes, and highlight the importance of considering genotype when making dietary folate recommendations.

Learning and memory deficits consequent to reduction of the fragile X mental retardation protein result from metabotropic glutamate receptor-mediated inhibition of cAMP signaling in Drosophila

Loss of the RNA-binding fragile X protein [fragile X mental retardation protein (FMRP)] results in a spectrum of cognitive deficits, the fragile X syndrome (FXS), while aging individuals with decreased protein levels present with a subset of these symptoms and tremor. The broad range of behavioral deficits likely reflects the ubiquitous distribution and multiple functions of the protein. FMRP loss is expected to affect multiple neuronal proteins and intracellular signaling pathways, whose identity and interactions are essential in understanding and ameliorating FXS symptoms. We used heterozygous mutants and targeted RNA interference-mediated abrogation in Drosophila to uncover molecular pathways affected by FMRP reduction. We present evidence that FMRP loss results in excess metabotropic glutamate receptor (mGluR) activity, attributable at least in part to elevation of the protein in affected neurons. Using high-resolution behavioral, genetic, and biochemical analyses, we present evidence that excess mGluR upon FMRP attenuation is linked to the cAMP decrement reported in patients and models, and underlies olfactory associative learning and memory deficits. Furthermore, our data indicate positive transcriptional regulation of the fly fmr1 gene by cAMP, via protein kinase A, likely through the transcription factor CREB. Because the human Fmr1 gene also contains CREB binding sites, the interaction of mGluR excess and cAMP signaling defects we present suggests novel combinatorial pharmaceutical approaches to symptom amelioration upon FMRP attenuation.

Neurobiology of attention deficit/hyperactivity disorder

Attention deficit/hyperactivity disorder (ADHD), a prevalent neurodevelopmental disorder, has been associated with various structural and functional CNS abnormalities but findings about neurobiological mechanisms linking genes to brain phenotypes are just beginning to emerge. Despite the high heritability of the disorder and its main symptom dimensions, common individual genetic variants are likely to account for a small proportion of the phenotype's variance. Recent findings have drawn attention to the involvement of rare genetic variants in the pathophysiology of ADHD, some being shared with other neurodevelopmental disorders. Traditionally, neurobiological research on ADHD has focused on catecholaminergic pathways, the main target of pharmacological treatments. However, more distal and basic neuronal processes in relation with cell architecture and function might also play a role, possibly accounting for the coexistence of both diffuse and specific alterations of brain structure and activation patterns. This article aims to provide an overview of recent findings in the rapidly evolving field of ADHD neurobiology with a focus on novel strategies regarding pathophysiological analyses.

Modifying behavioral phenotypes in Fmr1KO mice: genetic background differences reveal autistic-like responses

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability in humans. In addition to cognitive impairment, patients may exhibit hyperactivity, attention deficits, social difficulties and anxiety, and autistic-like behaviors. The degree to which patients display these behaviors varies considerably and is influenced by family history, suggesting that genetic modifiers play a role in the expression of behaviors in FXS. Several studies have examined behavior in a mouse model of FXS in which the Fmr1 gene has been ablated. Most of those studies were done in Fmr1 knockout mice on a pure C57BL/6 or FVB strain background. To gain a better understanding of the effects of genetic background on behaviors resulting from the loss of Fmr1 gene expression, we generated F1 hybrid lines from female Fmr1 heterozygous mice on a pure C57BL/6J background bred with male Fmr1 wild-type (WT) mice of various background strains (A/J, DBA/2J, FVB/NJ, 129S1/SvImJ and CD-1). Male Fmr1 knockout and WT littermates from each line were examined in an extensive behavioral test battery. Results clearly indicate that multiple behavioral responses are dependent on genetic background, including autistic-like traits that are present on limited genetic backgrounds. This approach has allowed us to identify improved models for different behavioral symptoms present in FXS including autistic-like traits.

Emerging pharmacotherapies for neurodevelopmental disorders

A growing and interdisciplinary translational neuroscience research effort for neurodevelopmental disorders (NDDs) is investigating the mechanisms of dysfunction and testing effective treatment strategies in animal models and, when possible, in the clinic. NDDs with a genetic basis have received particular attention. Transgenic animals that mimic genetic insults responsible for disease in man have provided insight about mechanisms of dysfunction, and, surprisingly, have shown that cognitive deficits can be addressed in adult animals. This review will present recent translational research based on animal models of genetic NDDs, as well as pharmacotherapeutic strategies under development to address deficits of brain function for Down syndrome, fragile X syndrome, Rett syndrome, neurofibromatosis-1, tuberous sclerosis, and autism. Although these disorders vary in underlying causes and clinical presentation, common pathways and mechanisms for dysfunction have been observed. These include abnormal gene dosage, imbalance among neurotransmitter systems, and deficits in the development, maintenance and plasticity of neuronal circuits. NDDs affect multiple brain systems and behaviors that may be amenable to drug therapies that target distinct deficits. A primary goal of translational research is to replace symptomatic and supportive drug therapies with pharmacotherapies based on a principled understanding of the causes of dysfunction. Based on this principle, several recently developed therapeutic strategies offer clear promise for clinical development in man.