Hippocampal Synaptic Expansion Induced by Spatial Experience in Rats Correlates with Improved Information Processing in the Hippocampus

Prolonged and specific spatial training during adolescence reverses adult hippocampal network impairments in a mouse model of fragile X syndrome

The fragile X syndrome (FXS) is the leading monogenetic cause of cognitive impairment and autism. A hallmark of FXS in patients and the FXS mouse model (Fmr1 KO) is an overabundance of immature appearing dendritic spines in the cortex and hippocampus which is associated with behavioral deficits. Spine analysis in the different hippocampal subregions and at different developmental stages revealed that in adult mice, hippocampal spine pathology occurs specifically in the CA3 subregion, which plays a pivotal role in pattern completion processes important for efficient memory recall from parts of the initial memory stimulus. In line with this synaptic defect we document an impairment in memory recall during partially cued reference memory test in the Morris water maze task. This is accompanied by impaired recruitment of engram cells as well as impaired spine structural plasticity in the CA3 region. In order to promote hippocampal network development adolescent mice were either raised in an enriched environment or subjected to specific hippocampus-dependent spatial training. Intriguingly, only specific spatial training alleviated the cognitive symptoms and the spine phenotype shown in adult Fmr1 KO mice suggesting that specific stimulation of hippocampal networks during development might be used in the future as a therapeutic strategy.

Pausing and reorienting behaviors enhance the performance of a spatial working memory task

During spatial working memory tasks, animals need to retain information about a previous trial in order to successfully select their next trajectory. Specifically, the delayed non-match to position task requires rats to follow a cued sample trajectory, then select the opposite route after a delay period. When faced with this choice, rats will occasionally exhibit complex behaviors, such as pausing and sweeping their head back and forth. These behaviors, called vicarious trial and error (VTE), are thought to be a behavioral manifestation of deliberation. However, we identified similarly complex behaviors during sample-phase traversals, despite the fact that these laps do not require a decision. First, we identified that these behaviors occurred more often after incorrect trials than before them, indicating that rats are retaining information between trials. Next, we determined that these pause-and-reorient (PAR) behaviors increased the likelihood of the next choice being selected correctly, suggesting that these behaviors assist the rat in successful task performance. Finally, we identified similarities between PARs and choice-phase VTEs, suggesting that VTEs may not only be reflective of deliberation, but may also contribute to a strategy for successful performance of spatial working memory tasks.

A Feed-Forward Neural Network for Increasing the Hopfield-Network Storage Capacity

In the hippocampal dentate gyrus (DG), pattern separation mainly depends on the concepts of 'expansion recoding', meaning random mixing of different DG input channels. However, recent advances in neurophysiology have challenged the theory of pattern separation based on these concepts. In this study, we propose a novel feed-forward neural network, inspired by the structure of the DG and neural oscillatory analysis, to increase the Hopfield-network storage capacity. Unlike the previously published feed-forward neural networks, our bio-inspired neural network is designed to take advantage of both biological structure and functions of the DG. To better understand the computational principles of pattern separation in the DG, we have established a mouse model of environmental enrichment. We obtained a possible computational model of the DG, associated with better pattern separation ability, by using neural oscillatory analysis. Furthermore, we have developed a new algorithm based on Hebbian learning and coupling direction of neural oscillation to train the proposed neural network. The simulation results show that our proposed network significantly expands the storage capacity of Hopfield network, and more effective pattern separation is achieved. The storage capacity rises from 0.13 for the standard Hopfield network to 0.32 using our model when the overlap in patterns is 10%.

Amyloidosis is associated with thicker myelin and increased oligodendrogenesis in the adult mouse brain

In Alzheimer's disease, amyloid plaque formation is associated with the focal death of oligodendrocytes and soluble amyloid β impairs the survival of oligodendrocytes in vitro. However, the response of oligodendrocyte progenitor cells (OPCs) to early amyloid pathology remains unclear. To explore this, we performed a histological, electrophysiological, and behavioral characterization of transgenic mice expressing a pathological form of human amyloid precursor protein (APP), containing three single point mutations associated with the development of familial Alzheimer's disease (PDGFB‐APP^Sw.Ind, also known as J20 mice). PDGFB‐APP^Sw.Ind transgenic mice had impaired survival from weaning, were hyperactive by 2 months of age, and developed amyloid plaques by 6 months of age, however, their spatial memory remained intact over this time course. Hippocampal OPC density was normal in P60‐P180 PDGFB‐APP^Sw.Ind transgenic mice and, by performing whole‐cell patch‐clamp electrophysiology, we found that their membrane properties, including their response to kainate (100 µM), were largely normal. However, by P100, the response of hippocampal OPCs to GABA was elevated in PDGFB‐APP^Sw.Ind transgenic mice. We also found that the nodes of Ranvier were shorter, the paranodes longer, and the myelin thicker for hippocampal axons in young adult PDGFB‐APP^Sw.Ind transgenic mice compared with wildtype littermates. Additionally, oligodendrogenesis was normal in young adulthood, but increased in the hippocampus, entorhinal cortex, and fimbria of PDGFB‐APP^Sw.Ind transgenic mice as pathology developed. As the new oligodendrocytes were not associated with a change in total oligodendrocyte number, these cells are likely required for cell replacement.

3,5-T2 and 3,3',5-T3 Regulate Cerebellar Thyroid Hormone Signalling and Myelin Molecular Dynamics in Tilapia

In contrast to mammalian adults, myelination in teleosts occurs throughout their lifespan and most of the progenitor cells are originated in the cerebellum. To understand the role that thyroid hormones (THs) play in juvenile cerebellar myelination in teleosts, we identified and localised the expression of genes involved in TH signalling (mct8, oatp1c1, dio2, dio3, thraa and l-thrb1) and analysed the effects of the two bioactive THs, T2 and T3, upon their regulation, as well as upon some structural components of the myelination process. Ex vivo approaches using organotypic cerebellar cultures followed by FISH and qPCR showed gene-specific localisation and regulation of TH signalling genes in the cerebellar nuclei. In vivo approaches using methimazole (MMI)-treated juvenile tilapias replaced with low doses of T3 and T2 showed by immunofluorescence that myelin fibres in the cerebellum are more abundant in the granular layer and that their visible size is reduced after MMI treatment but partially restored with TH replacement, suggesting that low doses of TH promote the re-myelination process in an altered condition. Together, our data support the idea that T2 and T3 promote myelination via different pathways and prompt T2 as a target for further analysis as a promising therapy for hypomyelination.

Locus Coeruleus Phasic, But Not Tonic, Activation Initiates Global Remapping in a Familiar Environment

Locus coeruleus (LC) neurons, the source of hippocampal norepinephrine (NE), are activated by novelty and changes in environmental contingencies. Based on the role of monoamines in reconfiguring invertebrate networks, and data from mammalian systems, a network reset hypothesis for the effects of LC activation has been proposed. We used the cellular compartmental analysis of temporal FISH technique based on the cellular distribution of immediate early genes to examine the effect of LC activation and inactivation, on regional hippocampal maps in male rats, when LC activity was manipulated just before placement in a second familiar (A/A) and/or novel environment (A/B). We found that bilateral phasic, but not tonic, activation of LC reset hippocampal maps in the A/A condition, whereas silencing the LC with clonidine before placement in the A/B condition blocked map reset and a familiar map emerged in the dentate gyrus, proximal and distal CA1, and CA3c. However, CA3a and CA3b encoded the novel environment. These results support a role for phasic LC responses in generating novel hippocampal sequences during memory encoding and, potentially, memory updating. The silencing experiments suggest that novel environments may not be recognized as different by dentate gyrus and CA1 without LC input. The functional distinction between phasic and tonic LC activity argues that these parameters are critical for determining network changes. These data are consistent with the hippocampus activating internal network representations to encode novel experiential episodes and suggest LC input is critical for this role.SIGNIFICANCE STATEMENT Burst activation of the broadly projecting novelty signaling system of the locus coeruleus initiates new network representations throughout the hippocampus despite unchanged external environments. Tonic activation does not alter network representations in the same condition. This suggests differences in the temporal parameters of neuromodulator network activation are critical for neuromodulator function. Silencing this novelty signaling system prevented the appearance of new network representations in a novel environment. Instead, familiar representations were expressed in a subset of hippocampal areas, with another subset encoding the novel environment. This "being in two places at once" argues for independent functional regions within the hippocampus. These experiments strengthen the view that internal states are major determinants of the brain's construction of environmental representations.

Behavior-associated Neuronal Activation After Kainic Acid-induced Hippocampal Neurotoxicity is Modulated in Time

Kainic acid-induced (KA) hippocampal damage leads to neuronal death and further synaptic plasticity. Formation of aberrant as well as of functional connections after such procedure has been documented. However, the impact of such structural plasticity on cell activation along time after damage and in face of a behavioral demand has not been explored. We evaluated if the mRNA and protein levels of plasticity-related protein synaptophysin (Syp and SYP, respectively) and activity-regulated cytoskeleton-associated protein mRNA and protein levels (Arc and Arc, respectively) in the dentate gyrus were differentially modulated in time in response to a spatial-exploratory task after KA-induced hippocampal damage. In addition, we analyzed Arc+/NeuN+ immunopositive cells in the different experimental conditions. We infused KA intrahippocampally to young-adult rats and 10 or 30 days post-lesion (dpl) animals performed a hippocampus-activating spatial-exploratory task. Our results show that Syp mRNA levels significantly increase at 10dpl and return to control levels after 30dpl, whereas SYP protein levels are diminished at 10dpl, but significantly increase at 30dpl, as compared to 10dpl. Arc mRNA and protein levels are both increased at 30dpl as compared to sham. Also the number of NeuN+/Arc+ cells significantly increases at 30dpl in the group with a spatial-exploratory demand. These results provide information on the long-term modifications associated to structural plasticity and neuronal activation in the dentate gyrus after excitotoxic damage and in face of a spatial-exploratory behavior. Anat Rec, 300:425-432, 2017. © 2016 Wiley Periodicals, Inc.