Vector trace cells in the subiculum of the hippocampal formation.

Successfully navigating in physical or semantic space requires a neural representation of allocentric (map-based) vectors to boundaries, objects and goals. Cognitive processes such as path-planning and imagination entail the recall of vector representations, but evidence of neuron-level memory for allocentric vectors has been lacking. Here, we describe a novel neuron type, vector trace cell (VTC), whose firing generates a new vector field when a cue is encountered and a 'trace' version of that field for hours after cue removal. VTCs are concentrated in subiculum, distal to CA1. Compared to non-trace cells, VTCs fire at further distances from cues and exhibit earlier-going shifts in preferred theta phase in response to newly introduced cues, which demonstrates a theta-linked neural substrate for memory encoding. VTCs suggest a vector-based model of computing spatial relationships between an agent and multiple spatial objects, or between different objects, freed from the constraints of direct perception of those objects.

An intermittent hypercaloric diet alters gut microbiota, prefrontal cortical gene expression and social behaviours in rats.

Objectives: Excessive consumption of high fat and high sugar (HFHS) diets alters reward processing, behaviour, and changes gut microbiota profiles. Previous studies in gnotobiotic mice also provide evidence that these gut microorganisms may influence social behaviour. To further investigate these interactions, we examined the impact of the intermittent access to a HFHS diet on social behaviour, gene expression and microbiota composition in adolescent rats. Methods: Male rats were permitted intermittent daily access (2 h / day) to a palatable HFHS chow diet for 28 days across adolescence. Social interaction, social memory and novel object recognition were assessed during this period. Following testing, RT-PCR was conducted on hippocampal and prefrontal cortex (PFC) samples. 16S ribosomal RNA amplicon sequencing was used for identification and relative quantification of bacterial taxa in faecal samples. Results: We observed reduced social interaction behaviours, impaired social memory and novel object recognition in HFHS diet rats compared to chow controls. RT-PCR revealed reduced levels of monoamine oxidase A (Maoa), catechol-O-methyltransferase (Comt) and brain derived neurotrophic factor (Bdnf) mRNA in the PFC of HFHS diet rats. Faecal microbiota analysis demonstrated that the relative abundance of a number of specific bacterial taxa differed significantly between the two diet groups, in particular, Lachnospiraceae and Ruminoccoceae bacteria. Discussion: Intermittent HFHS diet consumption evoked physiological changes to the brain, particularly expression of mRNA associated with reward and neuroplasticity, and gut microbiome. These changes may underpin the observed alterations to social behaviours.

Insoluble Aß overexpression in an App knock-in mouse model alters microstructure and gamma oscillations in the prefrontal cortex, affecting anxiety-related behaviours.

We studied a new amyloid-beta precursor protein (App) knock-in mouse model of Alzheimer's disease (AppNL-G-F ), containing the Swedish KM670/671NL mutation, the Iberian I716F mutation and the Artic E693G mutation, which generates elevated levels of amyloid beta (Aß)40 and Aß42 without the confounds associated with APP overexpression. This enabled us to assess changes in anxiety-related and social behaviours, and neural alterations potentially underlying such changes, driven specifically by Aß accumulation. AppNL-G-F knock-in mice exhibited subtle deficits in tasks assessing social olfaction, but not in social motivation tasks. In anxiety-assessing tasks, AppNL-G-F knock-in mice exhibited: (1) increased thigmotaxis in the open field (OF), yet; (2) reduced closed-arm, and increased open-arm, time in the elevated plus maze (EPM). Their ostensibly anxiogenic OF profile, yet ostensibly anxiolytic EPM profile, could hint at altered cortical mechanisms affecting decision-making (e.g. 'disinhibition'), rather than simple core deficits in emotional motivation. Consistent with this possibility, alterations in microstructure, glutamatergic-dependent gamma oscillations and glutamatergic gene expression were all observed in the prefrontal cortex, but not the amygdala, of AppNL-G-F knock-in mice. Thus, insoluble Aß overexpression drives prefrontal cortical alterations, potentially underlying changes in social and anxiety-related behavioural tasks.This article has an associated First Person interview with the first author of the paper.

En route to delineating hippocampal roles in spatial learning.

The precise role played by the hippocampus in spatial learning tasks, such as the Morris Water Maze (MWM), is not fully understood. One theory is that the hippocampus is not required for 'knowing where' but rather is crucial in 'getting there'. To explore this idea in the MWM, we manipulated 'getting there' variables, such as passive transport or active swimming towards the hidden platform, in rats with and without hippocampal lesions. Our results suggested that for intact rats, self-motion cues enroute to the hidden goal were a necessary component for 'place learning' to progress. Specifically, intact rats could not learn the hidden goal location, when passively transported to it, despite extensive training. However, when rats were either given hippocampal lesions, or placed in a light-tight box during transportation to the hidden goal, passive-placement spatial learning was facilitated. In a subsequent experiment, the 'getting there' component of place navigation was simplified, via the placement of two overhead landmarks, one of which served as a beacon. When 'getting there' was made easier in this way, hippocampal lesions did not induce deficits in 'knowing where' the goal was. In fact, similar to the facilitation observed in passive-placement spatial learning, hippocampal lesions improved landmark learning relative to controls. Finally, demonstrating that our lesions were sufficiently deleterious, hippocampal-lesioned rats were impaired, as predicted, in an environmental-boundary based learning task. We interpret these results in terms of competition between multiple memory systems, and the importance of self-generated motion cues in hippocampal spatial mapping.

The within-subject application of diffusion tensor MRI and CLARITY reveals brain structural changes in Nrxn2 deletion mice.

Background: Of the many genetic mutations known to increase the risk of autism spectrum disorder, a large proportion cluster upon synaptic proteins. One such family of presynaptic proteins are the neurexins (NRXN), and recent genetic and mouse evidence has suggested a causative role for NRXN2 in generating altered social behaviours. Autism has been conceptualised as a disorder of atypical connectivity, yet how single-gene mutations affect such connectivity remains under-explored. To attempt to address this, we have developed a quantitative analysis of microstructure and structural connectivity leveraging diffusion tensor MRI (DTI) with high-resolution 3D imaging in optically cleared (CLARITY) brain tissue in the same mouse, applied here to the Nrxn2α knockout (KO) model. Methods: Fixed brains of Nrxn2α KO mice underwent DTI using 9.4 T MRI, and diffusion properties of socially relevant brain regions were quantified. The same tissue was then subjected to CLARITY to immunolabel axons and cell bodies, which were also quantified. Results: DTI revealed increases in fractional anisotropy in the amygdala (including the basolateral nuclei), the anterior cingulate cortex, the orbitofrontal cortex and the hippocampus. Axial diffusivity of the anterior cingulate cortex and orbitofrontal cortex was significantly increased in Nrxn2α KO mice, as were tracts between the amygdala and the orbitofrontal cortex. Using CLARITY, we find significantly altered axonal orientation in the amygdala, orbitofrontal cortex and the anterior cingulate cortex, which was unrelated to cell density. Conclusions: Our findings demonstrate that deleting a single neurexin gene (Nrxn2α) induces atypical structural connectivity within socially relevant brain regions. More generally, our combined within-subject DTI and CLARITY approach presents a new, more sensitive method of revealing hitherto undetectable differences in the autistic brain.

The developmental transcriptome of the human heart.

The human heart develops through complex mechanisms producing morphological and functional changes during gestation. We have recently demonstrated using diffusion tensor MRI that over the relatively short space of 40 days, between 100-140 days gestational age, the ventricular myocardium transforms from a disorganised tissue to the ordered structure characteristic of mature cardiac tissue. However, the genetic basis underpinning this maturation is unclear. Herein, we have used RNA-Seq to establish the developmentally-regulated transcriptome of gene expression in the developing human heart across three gestational ages in the first and second trimester. By comparing 9 weeks gestational age (WGA) with 12 WGA, we find 288 genes show significant differential expression. 305 genes were significantly altered comparing 12 and 16 WGA, and 806 genes differentially expressed between 9 and 16 WGA. Network analysis was used to identify genetic interactions, node properties and gene ontology categories. In summary, we present a comprehensive transcriptomic analysis of human heart development during early gestation, and identify differentially expressed genes during heart development between 9 and 16 weeks, overlapping the first and early second trimester.

Ventricular myocardium development and the role of connexins in the human fetal heart.

The developmental timeline of the human heart remains elusive. The heart takes on its characteristic four chambered appearance by ~56 days gestational age (DGA). However, owing to the complexities (both technical and logistical) of exploring development in utero, we understand little of how the ventricular walls develop. To address this, we employed diffusion tensor magnetic resonance imaging to explore the architecture and tissue organization of the developing heart aged 95-143 DGA. We show that fractional anisotropy increases (from ~0.1 to ~0.5), diffusion coefficients decrease (from ~1 × 10-3mm2/sec to ~0.4 × 10-3mm2/sec), and fiber paths, extracted by tractography, increase linearly with gestation, indicative of the increasing organization of the ventricular myocytes. By 143 DGA, the developing heart has the classical helical organization observed in mature mammalian tissue. This was accompanied by an increase in connexin 43 and connexin 40 expression levels, suggesting their role in the development of the ventricular conduction system and that electrical propagation across the heart is facilitated in later gestation. Our findings highlight a key developmental window for the structural organization of the fetal heart.

Do cortical plasticity mechanisms differ between males and females?

The difference between male and female behavior and male and female susceptibility to a number of neuropsychiatric conditions is not controversial. From a biological perspective, one might expect to see at least some of these differences underpinned by identifiable physical differences in the brain. This Mini-Review focuses on evidence that plasticity mechanisms differ between males and females and ask at what scale of organization the differences might exist, at the systems level, the circuits level, or the synaptic level. Emerging evidence suggests that plasticity differences may extend to the scale of synaptic mechanisms. In particular, the CaMKK, NOS1 and estrogen receptor pathways show sexual dimorphisms with implications for plasticity in the hippocampus and cerebral cortex. © 2016 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Deficits in social behavioral tests in a mouse model of alternating hemiplegia of childhood.

Social behavioral deficits have been observed in patients diagnosed with alternating hemiplegia of childhood (AHC), rapid-onset dystonia-parkinsonism and CAPOS syndrome, in which specific missense mutations in ATP1A3, encoding the Na(+), K(+)-ATPase α3 subunit, have been identified. To test the hypothesis that social behavioral deficits represent part of the phenotype of Na(+), K(+)-ATPase α3 mutations, we assessed the social behavior of the Myshkin mouse model of AHC, which has an I810N mutation identical to that found in an AHC patient with co-morbid autism. Myshkin mice displayed deficits in three tests of social behavior: nest building, pup retrieval and the three-chamber social approach test. Chronic treatment with the mood stabilizer lithium enhanced nest building in wild-type but not Myshkin mice. In light of previous studies revealing a broad profile of neurobehavioral deficits in the Myshkin model - consistent with the complex clinical profile of AHC - our results suggest that Na(+), K(+)-ATPase α3 dysfunction has a deleterious, but nonspecific, effect on social behavior. By better defining the behavioral profile of Myshkin mice, we identify additional ATP1A3-related symptoms for which the Myshkin model could be used as a tool to advance understanding of the underlying neural mechanisms and develop novel therapeutic strategies.

Missense mutation in DISC1 C-terminal coiled-coil has GSK3ß signaling and sex-dependent behavioral effects in mice.

Disrupted-in-Schizophrenia 1 (DISC1) is a risk factor for schizophrenia and affective disorders. The full-length DISC1 protein consists of an N-terminal 'head' domain and a C-terminal tail domain that contains several predicted coiled-coils, structural motifs involved in protein-protein interactions. To probe the in vivo effects of missense mutation of DISC1's C-terminal tail, we tested mice carrying mutation D453G within a predicted α-helical coiled-coil region. We report that, relative to wild-type littermates, female DISC1(D453G) mice exhibited novelty-induced hyperlocomotion, an anxiogenic profile in the elevated plus-maze and open field tests, and reduced social exploration of unfamiliar mice. Male DISC1(D453G) mice displayed a deficit in passive avoidance, while neither males nor females exhibited any impairment in startle reactivity or prepulse inhibition. Whole brain homogenates showed normal levels of DISC1 protein, but decreased binding of DISC1 to GSK3ß, decreased phospho-inhibition of GSK3ß at serine 9, and decreased levels of ß-catenin in DISC1(D453G) mice of either sex. Interrupted GSK3ß signaling may thus be part of the mechanism underlying the behavioral phenotype associated with D453G, in common with the previously described N-terminal domain mutations Q31L and L100P in mice, and the schizophrenia risk-conferring variant R264Q in humans.

Transgenic rescue of phenotypic deficits in a mouse model of alternating hemiplegia of childhood.

Missense mutations in ATP1A3 encoding Na(+),K(+)-ATPase α3 are the primary cause of alternating hemiplegia of childhood (AHC). Most ATP1A3 mutations in AHC lie within a cluster in or near transmembrane α-helix TM6, including I810N that is also found in the Myshkin mouse model of AHC. These mutations all substantially reduce Na(+),K(+)-ATPase α3 activity. Herein, we show that Myshkin mice carrying a wild-type Atp1a3 transgene that confers a 16 % increase in brain-specific total Na(+),K(+)-ATPase activity show significant phenotypic improvements compared with non-transgenic Myshkin mice. Interventions to increase the activity of wild-type Na(+),K(+)-ATPase α3 in AHC patients should be investigated further.

Heterozygous deletion of α-neurexin I or α-neurexin II results in behaviors relevant to autism and schizophrenia.

The neurexins are a family of presynaptic cell adhesion molecules. Human genetic studies have found heterozygous deletions affecting NRXN1 and NRXN2, encoding α-neurexin I (Nrxn1α) and α-neurexin II (Nrxn2α), in individuals with autism spectrum disorders and schizophrenia. However, the link between α-neurexin deficiency and the manifestation of psychiatric disorders remain unclear. To assess whether the heterozygous loss of neurexins results in behaviors relevant to autism or schizophrenia, we used mice with heterozygous (HET) deletion of Nrxn1α or Nrxn2α. We found that in a test of social approach, Nrxn1α HET mice show no social memory for familiar versus novel conspecifics. In a passive avoidance test, female Nrxn1α HET mice cross to the conditioned chamber sooner than female wild-type and Nrxn2α HET mice. Nrxn2α HET mice also express a lack of long-term object discrimination, indicating a deficit in cognition. The observed Nrxn1α and Nrxn2α genotypic effects were specific, as neither HET deletion had effects on a wide range of other behavioral measures, including several measures of anxiety. Our findings demonstrate that the heterozygous loss of α-neurexin I and α-neurexin II in mice leads to phenotypes relevant to autism and schizophrenia.

Characterization of cognitive deficits in mice with an alternating hemiplegia-linked mutation.

Cognitive impairment is a prominent feature in a range of different movement disorders. Children with Alternating Hemiplegia of Childhood are prone to developmental delay, with deficits in cognitive functioning becoming progressively more evident as they grow older. Heterozygous mutations of the ATP1A3 gene, encoding the Na+,K+-ATPase α3 subunit, have been identified as the primary cause of Alternating Hemiplegia. Heterozygous Myshkin mice have an amino acid change (I810N) in Na+,K+-ATPase α3 that is also found in Alternating Hemiplegia. To investigate whether Myshkin mice exhibit learning and memory deficits resembling the cognitive impairments of patients with Alternating Hemiplegia, we subjected them to a range of behavioral tests that interrogate various cognitive domains. Myshkin mice showed impairments in spatial memory, spatial habituation, locomotor habituation, object recognition, social recognition, and trace fear conditioning, as well as in the visible platform version of the Morris water maze. Increasing the duration of training ameliorated the deficit in social recognition but not in spatial habituation. The deficits of Myshkin mice in all of the learning and memory tests used are consistent with the cognitive impairment of the vast majority of AHC patients. These mice could thus help advance our understanding of the underlying neural mechanisms influencing cognitive impairment in patients with ATP1A3-related disorders.

The role of nitric oxide in pre-synaptic plasticity and homeostasis.

Since the observation that nitric oxide (NO) can act as an intercellular messenger in the brain, the past 25 years have witnessed the steady accumulation of evidence that it acts pre-synaptically at both glutamatergic and GABAergic synapses to alter release-probability in synaptic plasticity. NO does so by acting on the synaptic machinery involved in transmitter release and, in a coordinated fashion, on vesicular recycling mechanisms. In this review, we examine the body of evidence for NO acting as a retrograde factor at synapses, and the evidence from in vivo and in vitro studies that specifically establish NOS1 (neuronal nitric oxide synthase) as the important isoform of NO synthase in this process. The NOS1 isoform is found at two very different locations and at two different spatial scales both in the cortex and hippocampus. On the one hand it is located diffusely in the cytoplasm of a small population of GABAergic neurons and on the other hand the alpha isoform is located discretely at the post-synaptic density (PSD) in spines of pyramidal cells. The present evidence is that the number of NOS1 molecules that exist at the PSD are so low that a spine can only give rise to modest concentrations of NO and therefore only exert a very local action. The NO receptor guanylate cyclase is located both pre- and post-synaptically and this suggests a role for NO in the coordination of local pre- and post-synaptic function during plasticity at individual synapses. Recent evidence shows that NOS1 is also located post-synaptic to GABAergic synapses and plays a pre-synaptic role in GABAergic plasticity as well as glutamatergic plasticity. Studies on the function of NO in plasticity at the cellular level are corroborated by evidence that NO is also involved in experience-dependent plasticity in the cerebral cortex.

The role of nitric oxide synthase in cortical plasticity is sex specific.

Nitric oxide synthase-1 (NOS1) is involved in several forms of plasticity including hippocampal-dependent learning and memory, experience-dependent plasticity in the barrel cortex, and long-term potentiation (LTP) in the hippocampus and neocortex. NOS1 also contributes to ischemic damage during stroke and has a stronger deleterious effect in males than females. We therefore investigated whether the role of NOS1 in plasticity might also be sex specific. We tested LTP in the layer IV-II/III pathway between barrel columns and experience-dependent plasticity in the barrel cortex of αNOS1 knock-out mice and their wild-type littermates. We found that LTP was absent in male αNOS1 knock-out mice but not in females and that the residual LTP in females was not NO dependent. We also found that experience-dependent potentiation due to single whisker experience was significantly reduced in male αNOS1 knockouts but was unaffected in females. The αNOS1 knockout had a small effect on the development of the barrels, which were reduced in size by 20% compared with wild types, but this effect was not sex specific. We therefore conclude that neocortical plasticity mechanisms differ between males and females at the synaptic level, either in their basic plasticity induction pathways or in their ability to compensate for loss of αNOS1.

Experience-dependent plasticity acts via GluR1 and a novel neuronal nitric oxide synthase-dependent synaptic mechanism in adult cortex.

Synaptic plasticity directs development of the nervous system and is thought to underlie memory storage in adult animals. A great deal of our current understanding of the role of AMPA receptors in synaptic plasticity comes from studies on developing cortex and cell cultures. In the present study, we instead focus on plasticity in mature neurons in the neocortex of adult animals. We find that the glutamate receptor 1 (GluR1) subunit of the AMPA receptor is involved in experience-dependent plasticity in adult cortex in vivo and that it acts in addition to neuronal nitric oxide synthase (αNOS1), an enzyme that produces the rapid synaptic signaling molecule nitric oxide (NO). Potentiation of the spared whisker response, following single whisker experience, is ∼33% less in GluR1-null mutants than in wild types. We found that the remaining plasticity depended on αNOS1. Potentiation was reduced by >42% in the single αNOS1-null mutants and completely abolished in GluR1/αNOS1 double-knock-out mice. However, potentiation in GluR1/NOS3 double knock-outs occurred at similar levels to that seen in GluR1 single knock-outs. Synaptic plasticity in the layer IV to II/III pathway in vitro mirrored the results in vivo, in that LTP was present in GluR1/NOS3 double-knock-out mice but not in the GluR1/αNOS1 animals. While basal levels of NO in cortical slices depended on both αNOS1 and NOS3, NMDA receptor-dependent NO release only depended on αNOS1 and not on NOS3. These findings demonstrate that αNOS1 acts in concert with GluR1 to produce experience-dependent plasticity in the neocortex.

Gender specific requirement of GluR1 receptors in contextual conditioning but not spatial learning.

The GluR1 subunit of the AMPA receptor is required for hippocampal-dependent memory formation, emotional learning and synaptic plasticity. Recent work has shown that GluR1-independent synaptic plasticity is mediated by nitric oxide. Nitric oxide activity is influenced by estrogen. It is unknown whether this gender-dependent effect conveys a gender dimorphic requirement of GluR1 for learning. This hypothesis was tested in two behavioral paradigms. In Experiment 1, the retention of contextual fear conditioning was impaired in male but not female GluR1 knockout mice. In Experiment 2, GluR1 knockout mice made significantly more arm entry errors during acquisition of a radial-arm watermaze task. This deficit was independent of gender. These results indicate that some forms of learning are gender dimorphic in GluR1 knockout mice. The results are discussed with reference to task and gender-specific interactions between GluR1 receptor intracellular signalling pathways.

Sensory deprivation unmasks a PKA-dependent synaptic plasticity mechanism that operates in parallel with CaMKII.

Calcium/calmodulin kinase II (CaMKII) is required for LTP and experience-dependent potentiation in the barrel cortex. Here, we find that whisker deprivation increases LTP in the layer IV to II/III pathway and that PKA antagonists block the additional LTP. No LTP was seen in undeprived CaMKII-T286A mice, but whisker deprivation again unmasked PKA-sensitive LTP. Infusion of a PKA agonist potentiated EPSPs in deprived wild-types and deprived CaMKII-T286A point mutants but not in undeprived animals of either genotype. The PKA-dependent potentiation mechanism was not present in GluR1 knockouts. Infusion of a PKA antagonist caused depression of EPSPs in undeprived but not deprived cortex. LTD was occluded by whisker deprivation and blocked by PKA manipulation, but not blocked by cannabinoid antagonists. NMDA receptor currents were unaffected by sensory deprivation. These results suggest that sensory deprivation causes synaptic depression by reversing a PKA-dependent process that may act via GluR1.