Dendritic spines: Morphological building blocks of memory

The gut-brain vagal axis scales hippocampal memory processes and plasticity

The vagus nerve serves as an interoceptive relay between the body and the brain. Despite its well-established role in feeding behaviors, energy metabolism, and cognitive functions, the intricate functional processes linking the vagus nerve to the hippocampus and its contribution to learning and memory dynamics remain still elusive. Here, we investigated whether and how the gut-brain vagal axis contributes to hippocampal learning and memory processes at behavioral, functional, cellular, and molecular levels. Our results indicate that the integrity of the vagal axis is essential for long-term recognition memories, while sparing other forms of memory. In addition, by combing multi-scale approaches, our findings show that the gut-brain vagal tone exerts a permissive role in scaling intracellular signaling events, gene expressions, hippocampal dendritic spines density as well as functional long-term plasticities (LTD and LTP). These results highlight the critical role of the gut-brain vagal axis in maintaining the spontaneous and homeostatic functions of hippocampal ensembles and in regulating their learning and memory functions. In conclusion, our study provides comprehensive insights into the multifaceted involvement of the gut-brain vagal axis in shaping time-dependent hippocampal learning and memory dynamics. Understanding the mechanisms underlying this interoceptive body-brain neuronal communication may pave the way for novel therapeutic approaches in conditions associated with cognitive decline, including neurodegenerative disorders.

Maintenance of Lognormal-Like Skewed Dendritic Spine Size Distributions in Dentate Granule Cells of TNF, TNF-R1, TNF-R2, and TNF-R1/2-Deficient Mice

Dendritic spines are sites of synaptic plasticity and their head size correlates with the strength of the corresponding synapse. We recently showed that the distribution of spine head sizes follows a lognormal-like distribution even after blockage of activity or plasticity induction. As the cytokine tumor necrosis factor (TNF) influences synaptic transmission and constitutive TNF and receptor (TNF-R)-deficiencies cause changes in spine head size distributions, we tested whether these genetic alterations disrupt the lognormality of spine head sizes. Furthermore, we distinguished between spines containing the actin-modulating protein synaptopodin (SP-positive), which is present in large, strong and stable spines and those lacking it (SP-negative). Our analysis revealed that neither TNF-deficiency nor the absence of TNF-R1, TNF-R2 or TNF-R 1 and 2 (TNF-R1/R2) degrades the general lognormal-like, skewed distribution of spine head sizes (all spines, SP-positive spines, SP-negative spines). However, TNF, TNF-R1 and TNF-R2-deficiency affected the width of the lognormal distribution, and TNF-R1/2-deficiency shifted the distribution to the left. Our findings demonstrate the robustness of the lognormal-like, skewed distribution, which is maintained even in the face of genetic manipulations that alter the distribution of spine head sizes. Our observations are in line with homeostatic adaptation mechanisms of neurons regulating the distribution of spines and their head sizes.

Exploring novel insights into the molecular mechanisms underlying Bisphenol A-induced toxicity: A persistent threat to human health

Bisphenol A (BPA) is a ubiquitous industrial chemical used in the production of polycarbonate plastics and epoxy resins, found in numerous consumer products. Despite its widespread use, its potential adverse health effects have raised significant concerns. This review explores the molecular mechanisms and evidence-based literature underlying BPA-induced toxicities and its implications for human health. BPA is an endocrine-disrupting chemical (EDC) which exhibits carcinogenic properties by influencing various receptors, such as ER, AhR, PPARs, LXRs, and RARs. It induces oxidative stress and contributes to cellular dysfunction, inflammation, and DNA damage, ultimately leading to various toxicities including but not limited to reproductive, cardiotoxicity, neurotoxicity, and endocrine toxicity. Moreover, BPA can modify DNA methylation patterns, histone modifications, and non-coding RNA expression, leading to epigenetic changes and contribute to carcinogenesis. Overall, understanding molecular mechanisms of BPA-induced toxicity is crucial for developing effective strategies and policies to mitigate its adverse effects on human health.

Evidence of an active role of dreaming in emotional memory processing shows that we dream to forget

Dreaming is a universal human behavior that has inspired searches for meaning across many disciplines including art, psychology, religion, and politics, yet its function remains poorly understood. Given the suggested role of sleep in emotional memory processing, we investigated whether reported overnight dreaming and dream content are associated with sleep-dependent changes in emotional memory and reactivity, and whether dreaming plays an active or passive role. Participants completed an emotional picture task before and after a full night of sleep and they recorded the presence and content of their dreams upon waking in the morning. The results replicated the emotional memory trade-off (negative images maintained at the cost of neutral memories), but only in those who reported dreaming (Dream-Recallers), and not in Non-Dream-Recallers. Results also replicated sleep-dependent reductions in emotional reactivity, but only in Dream-Recallers, not in Non-Dream-Recallers. Additionally, the more positive the dream report, the more positive the next-day emotional reactivity is compared to the night before. These findings implicate an active role for dreaming in overnight emotional memory processing and suggest a mechanistic framework whereby dreaming may enhance salient emotional experiences via the forgetting of less relevant information.

Metallomics analysis of metal exposure and cognitive function in older adults: A combined epidemiological and bioinformatics study

Dementia is a significant cause of elderly disability and Alzheimer's disease (AD) is the most prevalent form of dementia. As an early stage of AD, the mechanism related to mild cognitive impairment (MCI) and heavy metals is still unclear. This study utilized a cross-sectional design and enrolled 514 older adults in Bejing, China. Cognitive function was assessed by the Mini-Mental State Examination (MMSE) and fourteen blood metals were measured by inductively coupled plasma mass spectrometry (ICP-MS). In the adjusted single-metal models, we observed that copper [Cu, β (95% CI): 3.73 (-6.42, -1.03)] and lead [Pb, β (95% CI): 0.79 (-1.26, -0.32)] demonstrated negative associations with cognitive function, while selenium [Se, β (95% CI): 2.97 (1.23, 4.70)] was beneficial to cognition. Our findings were robust in secondary analysis using multi-metal models, which included generalized linear models (GLM), Bayesian kernel machine regression (BKMR), and quantile g-computation (qgcomp). Moreover, the toxic metal mixture (Cu and Pb) exhibited a significant negative association with MMSE scores and the inclusion of Se in the metal mixture attenuated the neurotoxicity of Cu-Pb mixture. The in silico analysis was used to examine the potential molecular mechanisms (genes, biological processes, pathways, and illnesses) of interaction among metal mixtures. We identified 20 cognition-related genes that are associated with both Cu-Pb and Se. Among these genes, eight (APOE, APP, BAX, BDNF, CASP3, HMOX1, TF, and TPP1) exhibited opposite effects on protein activity, mRNA expression, or protein expression in response to Se and Cu/Pb exposure, which could be the key genes accounting for the anti-neurotoxic effects of Se. Our findings support that Se can attenuate the neurotoxicity of exposure to single Cu or Pb, and Cu-Pb mixture. More research is needed to confirm our findings and gain knowledge about the molecular mechanisms of combined metal exposure on cognitive function.

Transport and localization on dendrite-inspired flat band linear photonic lattices

The capacity of a physical system to transport and localize energy or information is usually linked to its spatial configuration. This is relevant for integration and transmission of signals as performed, for example, by the dendrites of neuronal cells. Inspired by recent works on the organization of spines on the surface of dendrites and how they promote localization or propagation of electrical impulses in neurons, here we propose a linear photonic lattice configuration to study how the geometric features of a dendrite-inspired lattice allows for the localization or propagation of light on a completely linear structure. We show that by increasing the compression of the photonic analogue of spines and thus, by increasing the coupling strength of the spines with the main chain (the "photonic dendrite"), flat band modes become prevalent in the system, allowing spatial localization in the linear - low energy - regime. Furthermore, we study the inclusion of disorder in the distribution of spines and show that the main features of ordered systems persist due to the robustness of the flat band states. Finally, we discuss if the photonic analog, having evanescent interactions, may provide insight into linear morphological mechanisms at work occurring in some biological systems, where interactions are of electric and biochemical origin.

SpineTool is an open-source software for analysis of morphology of dendritic spines

Dendritic spines form most excitatory synaptic inputs in neurons and these spines are altered in many neurodevelopmental and neurodegenerative disorders. Reliable methods to assess and quantify dendritic spines morphology are needed, but most existing methods are subjective and labor intensive. To solve this problem, we developed an open-source software that allows segmentation of dendritic spines from 3D images, extraction of their key morphological features, and their classification and clustering. Instead of commonly used spine descriptors based on numerical metrics we used chord length distribution histogram (CLDH) approach. CLDH method depends on distribution of lengths of chords randomly generated within dendritic spines volume. To achieve less biased analysis, we developed a classification procedure that uses machine-learning algorithm based on experts' consensus and machine-guided clustering tool. These approaches to unbiased and automated measurements, classification and clustering of synaptic spines that we developed should provide a useful resource for a variety of neuroscience and neurodegenerative research applications.

Could theropod dinosaurs have evolved to a human level of intelligence?

Noting that some theropod dinosaurs had large brains, large grasping hands, and likely binocular vision, paleontologist Dale Russell suggested that a branch of these dinosaurs might have evolved to a human intelligence level, had dinosaurs not become extinct. I offer reasons why the likely pallial organization in dinosaurs would have made this improbable, based on four assumptions. First, it is assumed that achieving human intelligence requires evolving an equivalent of the about 200 functionally specialized cortical areas characteristic of humans. Second, it is assumed that dinosaurs had an avian nuclear type of pallial organization, in contrast to the mammalian cortical organization. Third, it is assumed that the interactions between the different neuron types making up an information processing unit within pallium are critical to its role in analyzing information. Finally, it is assumed that increasing axonal length between the neuron sets carrying out this operation impairs its efficacy. Based on these assumptions, I present two main reasons why dinosaur pallium might have been unable to add the equivalent of 200 efficiently functioning cortical areas. First, a nuclear pattern of pallial organization would require increasing distances between the neuron groups corresponding to the separate layers of any given mammalian cortical area, as more sets of nuclei equivalent to a cortical area are interposed between the existing sets, increasing axon length and thereby impairing processing efficiency. Second, because of its nuclear organization, dinosaur pallium could not reduce axon length by folding to bring adjacent areas closer together, as occurs in cerebral cortex.

Dendritic Spines in Learning and Memory: From First Discoveries to Current Insights

The central nervous system is composed of neural ensembles, and their activity patterns are neural correlates of cognitive functions. Those ensembles are networks of neurons connected to each other by synapses. Most neurons integrate synaptic signal through a remarkable subcellular structure called spine. Dendritic spines are protrusions whose diverse shapes make them appear as a specific neuronal compartment, and they have been the focus of studies for more than a century. Soon after their first description by Ramón y Cajal, it has been hypothesized that spine morphological changes could modify neuronal connectivity and sustain cognitive abilities. Later studies demonstrated that changes in spine density and morphology occurred in experience-dependent plasticity during development, and in clinical cases of mental retardation. This gave ground for the assumption that dendritic spines are the particular locus of cerebral plasticity. With the discovery of synaptic long-term potentiation, a research program emerged with the aim to establish whether dendritic spine plasticity could explain learning and memory. The development of live imaging methods revealed on the one hand that dendritic spine remodeling is compatible with learning process and, on the other hand, that their long-term stability is compatible with lifelong memories. Furthermore, the study of the mechanisms of spine growth and maintenance shed new light on the rules of plasticity. In behavioral paradigms of memory, spine formation or elimination and morphological changes were found to correlate with learning. In a last critical step, recent experiments have provided evidence that dendritic spines play a causal role in learning and memory.

c-Abl kinase at the crossroads of healthy synaptic remodeling and synaptic dysfunction in neurodegenerative diseases

Our ability to learn and remember depends on the active formation, remodeling, and elimination of synapses. Thus, the development and growth of synapses as well as their weakening and elimination are essential for neuronal rewiring. The structural reorganization of synaptic complexes, changes in actin cytoskeleton and organelle dynamics, as well as modulation of gene expression, determine synaptic plasticity. It has been proposed that dysregulation of these key synaptic homeostatic processes underlies the synaptic dysfunction observed in many neurodegenerative diseases. Much is known about downstream signaling of activated N-methyl-D-aspartate and α-amino-3-hydroxy-5-methyl-4-isoazolepropionate receptors; however, other signaling pathways can also contribute to synaptic plasticity and long-lasting changes in learning and memory. The non-receptor tyrosine kinase c-Abl (ABL1) is a key signal transducer of intra and extracellular signals, and it shuttles between the cytoplasm and the nucleus. This review focuses on c-Abl and its synaptic and neuronal functions. Here, we discuss the evidence showing that the activation of c-Abl can be detrimental to neurons, promoting the development of neurodegenerative diseases. Nevertheless, c-Abl activity seems to be in a pivotal balance between healthy synaptic plasticity, regulating dendritic spines remodeling and gene expression after cognitive training, and synaptic dysfunction and loss in neurodegenerative diseases. Thus, c-Abl genetic ablation not only improves learning and memory and modulates the brain genetic program of trained mice, but its absence provides dendritic spines resiliency against damage. Therefore, the present review has been designed to elucidate the common links between c-Abl regulation of structural changes that involve the actin cytoskeleton and organelles dynamics, and the transcriptional program activated during synaptic plasticity. By summarizing the recent discoveries on c-Abl functions, we aim to provide an overview of how its inhibition could be a potentially fruitful treatment to improve degenerative outcomes and delay memory loss.

Dendritic Spines: Synaptogenesis and Synaptic Pruning for the Developmental Organization of Brain Circuits

Synaptic overproduction and elimination is a regular developmental event in the mammalian brain. In the cerebral cortex, synaptic overproduction is almost exclusively correlated with glutamatergic synapses located on dendritic spines. Therefore, analysis of changes in spine density on different parts of the dendritic tree in identified classes of principal neurons could provide insight into developmental reorganization of specific microcircuits.The activity-dependent stabilization and selective elimination of the initially overproduced synapses is a major mechanism for generating diversity of neural connections beyond their genetic determination. The largest number of overproduced synapses was found in the monkey and human cerebral cortex. The highest (exceeding adult values by two- to threefold) and most protracted overproduction (up to third decade of life) was described for associative layer IIIC pyramidal neurons in the human dorsolateral prefrontal cortex.Therefore, the highest proportion and extraordinarily extended phase of synaptic spine overproduction is a hallmark of neural circuitry in human higher-order associative areas. This indicates that microcircuits processing the most complex human cognitive functions have the highest level of developmental plasticity. This finding is the backbone for understanding the effect of environmental impact on the development of the most complex, human-specific cognitive and emotional capacities, and on the late onset of human-specific neuropsychiatric disorders, such as autism and schizophrenia.

Pentylenetetrazol-induced seizures are followed by a reduction in the multiunitary activity of hippocampal CA1 pyramidal neurons in adult rats

Pentylenetetrazol (PTZ) blocks the inhibitory action of GABA, triggering a Glu-mediated hyperexcitation of the dendritic spines in hippocampal CA1 pyramidal neurons that leads to the generation of epileptiform seizures. The aim of this work was to determine the effect of PTZ on the electrical activity of the hippocampal pyramidal neurons in male rats. Bipolar electrodes were implanted stereotaxically in the right and left hippocampal CA1 fields of adults, and PTZ (65 mg/kg) was administered i.p. Simultaneous recordings of the field activity and the firing rate (multiunitary activity, MUA) were analyzed at 10, 20, and 30 min post-administration of PTZ. Only rats that presented tonic-clonic seizures during the first 1-5 min after PTZ treatment were included in the study. The recordings of the field activity were analyzed in 4 frequency bands. In both the right and left hippocampal CA1 fields, the relative power corresponding to the slow waves (4-7 Hz) increased, while in the bands 13-30 Hz and 31-50 Hz, it decreased at 10, 20, and 30 min post-PTZ. MUA recordings were analyzed at four levels. The highest levels corresponded to larger amplitudes of the action potentials in the pyramidal neurons. The firing rates of the PTZ-treated rats did not differ from baseline but presented a significant decrement at 10, 20, and 30 min post-PTZ. The decreased firing rate of the hippocampal CA1 pyramidal neurons after PTZ treatment could be associated with plastic changes of dendritic spines along with some microenvironmental adaptations at synaptic level, after neuronal PTZ-mediated hyperexcitation.

Combined DiI and Antibody Labeling Reveals Complex Dysgenesis of Hippocampal Dendritic Spines in a Mouse Model of Fragile X Syndrome

Structural, functional, and molecular alterations in excitatory spines are a common hallmark of many neurodevelopmental disorders including intellectual disability and autism. Here, we describe an optimized methodology, based on combined use of DiI and immunofluorescence, for rapid and sensitive characterization of the structure and composition of spines in native brain tissue. We successfully demonstrate the applicability of this approach by examining the properties of hippocampal spines in juvenile Fmr1 KO mice, a mouse model of Fragile X Syndrome. We find that mutant mice display pervasive dysgenesis of spines evidenced by an overabundance of both abnormally elongated thin spines and cup-shaped spines, in combination with reduced density of mushroom spines. We further find that mushroom spines expressing the actin-binding protein Synaptopodin-a marker for spine apparatus-are more prevalent in mutant mice. Previous work identified spines with Synaptopodin/spine apparatus as the locus of mGluR-LTD, which is abnormally elevated in Fmr1 KO mice. Altogether, our data suggest this enhancement may be linked to the preponderance of this subset of spines in the mutant. Overall, these findings demonstrate the sensitivity and versatility of the optimized methodology by uncovering a novel facet of spine dysgenesis in Fmr1 KO mice.

Cranial irradiation induces cognitive decline associated with altered dendritic spine morphology in the young rat hippocampus

OBJECTIVE

Therapeutic irradiation is commonly used to treat brain cancers but can induce cognitive dysfunction, especially in children. The mechanism is unknown but likely involves alterations in dendritic spine number and structure.

METHODS

To explore the impact of radiation exposure on the alteration of dendritic spine morphology in the hippocampus of young brains, 21-day-old Sprague-Dawley rats received cranial irradiation (10 Gy), and changes in spine density and morphology in dentate gyrus (DG) granules and CA1 pyramidal neurons were detected 1 and 3 months later by using Golgi staining. Moreover, we analyzed synapse-associated proteins within dendritic spines after irradiation.

RESULT

Our data showed that cognitive deficits were detected in young rats at both time points postirradiation, accompanied by morphological changes in dendritic spines. Our results revealed significant reductions in spine density in the DG at both 1 month (40.58%) and 3 months (28.92%) postirradiation. However, there was a decrease in spine density only at 1 month (33.29%) postirradiation in the basal dendrites of CA1 neurons and no significant changes in the apical dendrites of CA1 neurons at either time point. Notably, among our findings were the significant dynamic changes in spine morphology that persisted 3 months following cranial irradiation. Meanwhile, we found that depletion of the synapse-associated proteins PSD95 and Drebrin coincided with alterations in dendritic spines.

CONCLUSION

These data suggest that the decreased levels of PSD95 and Drebrin after ionizing radiation may cause changes in synaptic plasticity by affecting the morphological structure of dendritic spines, blocking the functional connectivity pathways of the brain and leading to cognitive impairment. Although the mechanism involved is unclear, understanding how ionizing radiation affects young brain hippocampal tissue may be useful to gain new mechanistic insights into radiation-induced cognitive dysfunction.

Evaluation of Neurotoxicity in BALB/c Mice following Chronic Exposure to Polystyrene Microplastics

BACKGROUND

The toxicity of microplastics (MPs) has attracted wide attention from researchers. Previous studies have indicated that MPs produce toxic effects on a variety of organs in aquatic organisms and mammals. However, the exact neurotoxicity of MPs in mammals is still unclear.

OBJECTIVES

We aimed to confirm the neurotoxicity of chronic exposure to polystyrene MPs (PS-MPs) at environmental pollution concentrations.

METHODS

In the present study, mice were provided drinking water containing 100μg/L and 1,000μg/L PS-MPs with diameters of 0.5, 4, and 10μm for 180 consecutive days. After the exposure period, the mice were anesthetized to gain brain tissues. The accumulation of PS-MPs in brain tissues, integrity of the blood-brain barrier, inflammation, and spine density were detected. We evaluated learning and memory ability by the Morris water maze and novel object recognition tests.

RESULTS

We observed the accumulation of PS-MPs with various particle diameters (0.5, 4, and 10μm) in the brains of exposed mice. Meanwhile, exposed mice also exhibited disruption of the blood-brain barrier, higher level of dendritic spine density, and an inflammatory response in the hippocampus. In addition, exposed mice exhibited cognitive and memory deficits compared with control mice as determined using the Morris water maze and novel object recognition tests, respectively. There was a concentration-dependent trend, but no particle size-dependent differences were seen in the neurotoxicity of MPs.

CONCLUSIONS

Collectively, our results suggested that PS-MPs exposure can lead to learning and memory dysfunctions and induce neurotoxic effects in mice, findings which have wide-ranging implications for the public regarding the potential risks of MPs. https://doi.org/10.1289/EHP10255.

Bioinspired micro- and nano-structured neural interfaces

The development of a functional nervous system requires neurons to interact with and promptly respond to a wealth of biochemical, mechanical and topographical cues found in the neural extracellular matrix (ECM). Among these, ECM topographical cues have been found to strongly influence neuronal function and behavior. Here, we discuss how the blueprint of the architectural organization of the brain ECM has been tremendously useful as a source of inspiration to design biomimetic substrates to enhance neural interfaces and dictate neuronal behavior at the cell-material interface. In particular, we focus on different strategies to recapitulate cell-ECM and cell-cell interactions. In order to mimic cell-ECM interactions, we introduce roughness as a first approach to provide informative topographical biomimetic cues to neurons. We then examine 3D scaffolds and hydrogels, as softer 3D platforms for neural interfaces. Moreover, we will discuss how anisotropic features such as grooves and fibers, recapitulating both ECM fibrils and axonal tracts, may provide recognizable paths and tracks that neuron can follow as they develop and establish functional connections. Finally, we show how isotropic topographical cues, recapitulating shapes, and geometries of filopodia- and mushroom-like dendritic spines, have been instrumental to better reproduce neuron-neuron interactions for applications in bioelectronics and neural repair strategies. The high complexity of the brain architecture makes the quest for the fabrication of create more biologically relevant biomimetic architectures in continuous and fast development. Here, we discuss how recent advancements in two-photon polymerization and remotely reconfigurable dynamic interfaces are paving the way towards to a new class of smart biointerfaces forin vitroapplications spanning from neural tissue engineering as well as neural repair strategies.

Pridopidine Promotes Synaptogenesis and Reduces Spatial Memory Deficits in the Alzheimer's Disease APP/PS1 Mouse Model

Sigma-1 receptor agonists have recently gained a great deal of interest due to their anti-amnesic, neuroprotective, and neurorestorative properties. Compounds such as PRE-084 or pridopidine (ACR16) are being studied as a potential treatment against cognitive decline associated with neurodegenerative disease, also to include Alzheimer's disease. Here, we performed in vitro experiments using primary neuronal cell cultures from rats to evaluate the abilities of ACR16 and PRE-084 to induce new synapses and spines formation, analyzing the expression of the possible genes and proteins involved. We additionally examined their neuroprotective properties against neuronal death mediated by oxidative stress and excitotoxicity. Both ACR16 and PRE-084 exhibited a concentration-dependent neuroprotective effect against NMDA- and H2O2-related toxicity, in addition to promoting the formation of new synapses and dendritic spines. However, only ACR16 generated dendritic spines involved in new synapse establishment, maintaining a more expanded activation of MAPK/ERK and PI3K/Akt signaling cascades. Consequently, ACR16 was also evaluated in vivo, and a dose of 1.5 mg/kg/day was administered intraperitoneally in APP/PS1 mice before performing the Morris water maze. ACR16 diminished the spatial learning and memory deficits observed in APP/PS1 transgenic mice via PI3K/Akt pathway activation. These data point to ACR16 as a pharmacological tool to prevent synapse loss and memory deficits associated with Alzheimer's disease, due to its neuroprotective properties against oxidative stress and excitotoxicity, as well as the promotion of new synapses and spines through a mechanism that involves AKT and ERK signaling pathways.

Randomly fluctuating neural connections may implement a consolidation mechanism that explains classic memory laws

How can we reconcile the massive fluctuations in neural connections with a stable long-term memory? Two-photon microscopy studies have revealed that large portions of neural connections (spines, synapses) are unexpectedly active, changing unpredictably over time. This appears to invalidate the main assumption underlying the majority of memory models in cognitive neuroscience, which rely on stable connections that retain information over time. Here, we show that such random fluctuations may in fact implement a type of memory consolidation mechanism with a stable very long-term memory that offers novel explanations for several classic memory 'laws', namely Jost's Law (1897: superiority of spaced learning) and Ribot's Law (1881: loss of recent memories in retrograde amnesia), for which a common neural basis has been postulated but not established, as well as other general 'laws' of learning and forgetting. We show how these phenomena emerge naturally from massively fluctuating neural connections.

Persistent up-regulation of polyribosomes at synapses during long-term memory, reconsolidation, and extinction of associative memory

Local protein synthesis at synapses can provide a rapid supply of proteins to support synaptic changes during consolidation of new memories, but its role in the maintenance or updating of established memories is unknown. Consolidation requires new protein synthesis in the period immediately following learning, whereas established memories are resistant to protein synthesis inhibitors. We have previously reported that polyribosomes are up-regulated in the lateral amygdala (LA) during consolidation of aversive-cued Pavlovian conditioning. In this study, we used serial section electron microscopy reconstructions to determine whether the distribution of dendritic polyribosomes returns to baseline during the long-term memory phase. Relative to control groups, long-term memory was associated with up-regulation of polyribosomes throughout dendrites, including in dendritic spines of all sizes. Retrieval of a consolidated memory by presentation of a small number of cues induces a new, transient requirement for protein synthesis to maintain the memory, while presentation of a large number of cues results in extinction learning, forming a new memory. One hour after retrieval or extinction training, the distribution of dendritic polyribosomes was similar except in the smallest spines, which had more polyribosomes in the extinction group. Our results demonstrate that the effects of learning on dendritic polyribosomes are not restricted to the transient translation-dependent phase of memory formation. Cued Pavlovian conditioning induces persistent synapse strengthening in the LA that is not reversed by retrieval or extinction, and dendritic polyribosomes may therefore correlate generally with synapse strength as opposed to recent activity or transient translational processes.

The Intriguing Contribution of Hippocampal Long-Term Depression to Spatial Learning and Long-Term Memory

Long-term potentiation (LTP) and long-term depression (LTD) comprise the principal cellular mechanisms that fulfill established criteria for the physiological correlates of learning and memory. Traditionally LTP, that increases synaptic weights, has been ascribed a prominent role in learning and memory whereas LTD, that decreases them, has often been relegated to the category of "counterpart to LTP" that serves to prevent saturation of synapses. In contradiction of these assumptions, studies over the last several years have provided functional evidence for distinct roles of LTD in specific aspects of hippocampus-dependent associative learning and information encoding. Furthermore, evidence of the experience-dependent "pruning" of excitatory synapses, the majority of which are located on dendritic spines, by means of LTD has been provided. In addition, reports exist of the temporal and physical restriction of LTP in dendritic compartments by means of LTD. Here, we discuss the role of LTD and LTP in experience-dependent information encoding based on empirical evidence derived from conjoint behavioral and electrophysiological studies conducted in behaving rodents. We pinpoint the close interrelation between structural modifications of dendritic spines and the occurrence of LTP and LTD. We report on findings that support that whereas LTP serves to acquire the general scheme of a spatial representation, LTD enables retention of content details. We argue that LTD contributes to learning by engaging in a functional interplay with LTP, rather than serving as its simple counterpart, or negator. We propose that similar spatial experiences that share elements of neuronal representations can be modified by means of LTD to enable pattern separation. Therewith, LTD plays a crucial role in the disambiguation of similar spatial representations and the prevention of generalization.

Calcium modeling of spine apparatus-containing human dendritic spines demonstrates an “all-or-nothing” communication switch between the spine head and dendrite

Dendritic spines are highly dynamic neuronal compartments that control the synaptic transmission between neurons. Spines form ultrastructural units, coupling synaptic contact sites to the dendritic shaft and often harbor a spine apparatus organelle, composed of smooth endoplasmic reticulum, which is responsible for calcium sequestration and release into the spine head and neck. The spine apparatus has recently been linked to synaptic plasticity in adult human cortical neurons. While the morphological heterogeneity of spines and their intracellular organization has been extensively demonstrated in animal models, the influence of spine apparatus organelles on critical signaling pathways, such as calcium-mediated dynamics, is less well known in human dendritic spines. In this study we used serial transmission electron microscopy to anatomically reconstruct nine human cortical spines in detail as a basis for modeling and simulation of the calcium dynamics between spine and dendrite. The anatomical study of reconstructed human dendritic spines revealed that the size of the postsynaptic density correlates with spine head volume and that the spine apparatus volume is proportional to the spine volume. Using a newly developed simulation pipeline, we have linked these findings to spine-to-dendrite calcium communication. While the absence of a spine apparatus, or the presence of a purely passive spine apparatus did not enable any of the reconstructed spines to relay a calcium signal to the dendritic shaft, the calcium-induced calcium release from this intracellular organelle allowed for finely tuned “all-or-nothing” spine-to-dendrite calcium coupling; controlled by spine morphology, neck plasticity, and ryanodine receptors. Our results suggest that spine apparatus organelles are strategically positioned in the neck of human dendritic spines and demonstrate their potential relevance to the maintenance and regulation of spine-to-dendrite calcium communication.

During the past decade it has become increasingly clear that abnormal synaptic plasticity is a major hallmark of neurological and cognitive disorders. Developing a better understanding of the synaptic plasticity process, which describes the ability of neurons to adapt their contacts in an activity-dependent manner, will lead to improved treatment of many neurological and cognitive disorders. It is known that calcium-dependent events such as synaptic transmission, intracellular calcium release, and calcium wave propagation, are required for many types of synaptic plasticity expression. However, the biological significance of these processes in neurons of the adult human cortex remains unknown. Due to technical limitations and ethical concerns, experimental data addressing this biologically and clinically relevant topic are not available. Therefore, we have implemented a computational model to study the intracellular calcium dynamics in realistic human dendritic spines based on detailed morphological reconstructions. With our model and simulations, we have established the morphological and biological requirements for the propagation of calcium from spines into the dendrites. Our results suggest a critical role for the calcium-storing spine apparatus organelle in regulating calcium homeostasis and propagation in human dendritic spines.

Microanatomical study of pyramidal neurons in the contralesional somatosensory cortex after experimental ischemic stroke

At present, many studies support the notion that after stroke, remote regions connected to the infarcted area are also affected and may contribute to functional outcome. In the present study, we have analyzed possible microanatomical alterations in pyramidal neurons from the contralesional hemisphere after induced stroke. We performed intracellular injections of Lucifer yellow in pyramidal neurons from layer III in the somatosensory cortex of the contralesional hemisphere in an ischemic stroke mouse model. A detailed 3-dimensional analysis of the neuronal complexity and morphological alterations of dendritic spines was then performed. Our results demonstrate that pyramidal neurons from layer III in the somatosensory cortex of the contralesional hemisphere show selective changes in their dendritic arbors, namely, less dendritic complexity of the apical dendritic arbor-but no changes in the basal dendritic arbor. In addition, we found differences in spine morphology in both apical and basal dendrites comparing the contralesional hemisphere with the lesional hemisphere. Our results show that pyramidal neurons of remote areas connected to the infarct zone exhibit a series of selective changes in neuronal complexity and morphological distribution of dendritic spines, supporting the hypothesis that remote regions connected to the peri-infarcted area are also affected after stroke.

Does Bisphenol A Confer Risk of Neurodevelopmental Disorders? What We Have Learned from Developmental Neurotoxicity Studies in Animal Models

Substantial evidence indicates that bisphenol A (BPA), a ubiquitous environmental chemical used in the synthesis of polycarbonate plastics and epoxy resins, can impair brain development. Clinical and epidemiological studies exploring potential connections between BPA and neurodevelopmental disorders in humans have repeatedly identified correlations between early BPA exposure and developmental disorders, such as attention deficit/hyperactivity disorder and autism spectrum disorder. Investigations using invertebrate and vertebrate animal models have revealed that developmental exposure to BPA can impair multiple aspects of neuronal development, including neural stem cell proliferation and differentiation, synapse formation, and synaptic plasticity-neuronal phenotypes that are thought to underpin the fundamental changes in behavior-associated neurodevelopmental disorders. Consistent with neuronal phenotypes caused by BPA, behavioral analyses of BPA-treated animals have shown significant impacts on behavioral endophenotypes related to neurodevelopmental disorders, including altered locomotor activity, learning and memory deficits, and anxiety-like behavior. To contextualize the correlations between BPA and neurodevelopmental disorders in humans, this review summarizes the current literature on the developmental neurotoxicity of BPA in laboratory animals with an emphasis on neuronal phenotypes, molecular mechanisms, and behavioral outcomes. The collective works described here predominantly support the notion that gestational exposure to BPA should be regarded as a risk factor for neurodevelopmental disorders.

Iron Chelation Remits Memory Deficits Caused by the High-Fat Diet in a Mouse Model of Alzheimer’s Disease

Background: Obesity is a worldwide health problem that has been implicated in many diseases, including Alzheimer’s disease (AD). AD is one of the most common neurodegenerative disorders and is characterized by two pathologies, including extracellular senior plaques composed of amyloid-β (Aβ) and intracellular neurofibrillary tangles (NFTs) consisting of abnormally hyperphosphorylated tau. According to current research, a high-fat diet (HFD) could exacerbate Aβ accumulation, oxidative damage, and cognitive defects in AD mice. However, the accurate role of HFD in the pathogenesis of AD is far more unclear. Objective: To explore the accurate role of HFD in the pathogenesis of AD. Methods: Open Field, Barns Maze, Elevated zero-maze, Contextual fear condition, Tail suspension test, western blotting, immunofluorescence, Fluoro-Jade C Labeling, Perls’ Prussian blue staining, and ELISA were used. Results: HFD caused nonheme iron overload in the brains of APPswe/PS1dE9 (APP/PS1) mice. Furthermore, the administration of M30 (0.5 mg/kg) for iron chelation once every 2 days per os (p.o.) for 1 month remitted memory deficits caused by HFD in APP/PS1 mice. Notably, a variety of hematological parameters in whole blood had no difference after iron chelation. In addition, iron chelation effectively reduced synaptic impairment in hippocampus and neuronal degeneration in cortex in the HFD-fed APP/PS1 mice. Meanwhile, iron chelation decreased Aβ 1–40 and Aβ 1–42 level as well as neuroinflammation in HFD-fed APP/PS1 mice. Conclusion: These data enhance our understanding of how HFD aggravates AD pathology and cognitive impairments and might shed light on future preclinical studies.

Tangeretin Inhibits BACE1 Activity and Attenuates Cognitive Impairments in AD Model Mice

Tangeretin (TAN) exhibits many bioactivities, including neuroprotective effects. However, the efficacy of TAN in Alzheimer's disease (AD) has not been sufficiently investigated. In the present study, we integrated behavioral tests, pathology assessment, and biochemical analyses to elucidate the antidementia activity of TAN in APPswe/PSEN1dE9 transgenic (Tg) mice. At supplementation levels of 100 mg/kg body weight per day, TAN significantly attenuated the cognitive impairment of Tg mice in behavioral tests. These effects were associated with less synaptic impairments and fewer β-amyloid accumulations after TAN administration. Furthermore, our study revealed that TAN possessed powerful inhibitory activity against β-secretase both in vitro and in vivo, which played a crucial role in the process of Aβ generation. These findings indicate that TAN is a potential drug for preventing AD pathology. The key mechanism underlying the antidementia effect of TAN may include its inhibitory activity against β-secretase.

Ayurvedic formulations amalaki rasayana and rasa sindoor improve age-associated memory deficits in mice by modulating dendritic spine densities

BACKGROUND

Emerging reports indicate that age-associated cognitive decline begins with the transition from young to middle-aged, and this neurological condition manifests mainly due to the progressive impairment in the adaptive homeostasis process. Moreover, cognitive decline is associated with neurodegenerative changes in older adults.

OBJECTIVE

Previous studies have shown that the administration of Ayurvedic formulations restores the homeostatic pathways and ameliorates neurodegeneration in animal models of neurodegenerative diseases. Therefore, we wanted to check whether Ayurvedic formulations can rescue or delay the age-associated cognitive decline in middle-aged mice.

MATERIAL AND METHODS

We fed two-month-old mice with amalaki aasayana (AR, 1025 mg/kg per day) or rasa sindoor (RS, 41 mg/kg per day) mixed in a gelatin-based jelly for six months. Mice eating regular chow or blank jelly served as control. Subsequently, we looked at the improvements in the cognitive and behavioural traits of the treated animals. We have also analysed the effect of these formulations on the dendritic processes of neurons, glial activation, and the formation of corpora amylacea.

RESULTS

We found a significant improvement in episodic, working- and reference-spatiotemporal memory in animals fed on AR or RS. Microscopic analyses revealed a significant increase in the dendritic spine density in the apical dendrites of the hippocampal pyramidal neurons. The treatment, however, did not significantly affect gliosis and corpora amylacea in the brains.

CONCLUSIONS

Both AR and RS showed beneficial effects on memory functions of the middle-aged mice, possibly due to their effect on the dendritic spine densities. Our findings provide strong evidence to conclude that formulations AR and RS can prevent or delay the onset of age-associated cognitive decline.

Unique properties of dually innervated dendritic spines in pyramidal neurons of the somatosensory cortex uncovered by 3D correlative light and electron microscopy

Pyramidal neurons (PNs) are covered by thousands of dendritic spines receiving excitatory synaptic inputs. The ultrastructure of dendritic spines shapes signal compartmentalization, but ultrastructural diversity is rarely taken into account in computational models of synaptic integration. Here, we developed a 3D correlative light-electron microscopy (3D-CLEM) approach allowing the analysis of specific populations of synapses in genetically defined neuronal types in intact brain circuits. We used it to reconstruct segments of basal dendrites of layer 2/3 PNs of adult mouse somatosensory cortex and quantify spine ultrastructural diversity. We found that 10% of spines were dually innervated and 38% of inhibitory synapses localized to spines. Using our morphometric data to constrain a model of synaptic signal compartmentalization, we assessed the impact of spinous versus dendritic shaft inhibition. Our results indicate that spinous inhibition is locally more efficient than shaft inhibition and that it can decouple voltage and calcium signaling, potentially impacting synaptic plasticity.

Fear Memory Retrieval Is Associated With a Reduction in AMPA Receptor Density at Thalamic to Amygdala Intercalated Cell Synapses

The amygdala plays a crucial role in attaching emotional significance to environmental cues. Its intercalated cell masses (ITC) are tight clusters of GABAergic neurons, which are distributed around the basolateral amygdala complex. Distinct ITC clusters are involved in the acquisition and extinction of conditioned fear responses. Previously, we have shown that fear memory retrieval reduces the AMPA/NMDA ratio at thalamic afferents to ITC neurons within the dorsal medio-paracapsular cluster. Here, we investigate the molecular mechanisms underlying the fear-mediated reduction in the AMPA/NMDA ratio at these synapses and, in particular, whether specific changes in the synaptic density of AMPA receptors underlie the observed change. To this aim, we used a detergent-digested freeze-fracture replica immunolabeling technique (FRIL) approach that enables to visualize the spatial distribution of intrasynaptic AMPA receptors at high resolution. AMPA receptors were detected using an antibody raised against an epitope common to all AMPA subunits. To visualize thalamic inputs, we virally transduced the posterior thalamic complex with Channelrhodopsin 2-YFP, which is anterogradely transported along axons. Using face-matched replica, we confirmed that the postsynaptic elements were ITC neurons due to their prominent expression of μ-opioid receptors. With this approach, we show that, following auditory fear conditioning in mice, the formation and retrieval of fear memory is linked to a significant reduction in the density of AMPA receptors, particularly at spine synapses formed by inputs of the posterior intralaminar thalamic and medial geniculate nuclei onto identified ITC neurons. Our study is one of the few that has directly linked the regulation of AMPA receptor trafficking to memory processes in identified neuronal networks, by showing that fear-memory induced reduction in AMPA/NMDA ratio at thalamic-ITC synapses is associated with a reduced postsynaptic AMPA receptor density.

Bright light exposure induces dynamic changes of spatial memory in nocturnal rodents

Bright light has been reported to improve spatial memory of diurnal rodents, yet how it will influence the spatial memory of nocturnal rodents is unknown. Here, we found that dynamic changes in spatial memory and anxiety were induced at different time point after bright light treatment. Mice maintained in brighter light exhibited impaired memory in Y maze at one day after bright light exposure, but showed significantly improved spatial memory in the Y maze and Morris water maze at four weeks after bright light exposure. We also found increased anxiety one day after bright light exposure, which could be the reason of impaired memory. However, no change of anxiety was detected after four weeks. Thus, we further explore the underlying mechanism of the beneficial effects of long term bright light on spatial memory. Golgi staining indicated that the structure of dendritic spines changed, accompanied by increased expression of synaptophysin and postsynaptic density 95 in the hippocampus. Further research has found that bright light treatment leads to elevated CaMKII/CREB phosphorylation levels in the hippocampus, which are associated with synaptic function. Moreover, higher expression of brain-derived neurotrophic factor (BDNF) was followed by increased phosphorylated TrkB levels in the hippocampus, indicating that BDNF/TrkB signaling is also activated during this process. Taken together, these findings revealed that bright light exposure with different duration exert different effects on spatial memory in nocturnal rodents, and the potential molecular mechanism by which long term bright light regulates spatial memory was also demonstrated.

High fat diet aggravates AD-related pathogenic processes in APP/PS1 mice

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disorder and negative life-style factors may contribute to its etiopathogenesis. Substantial evidence from humans and murine models reveal that Insulin Resistance (IR) associated with high fat diet (HFD) increase the risk of developing AD and age-related amyloidogenesis.

OBJECTIVE

To corroborate and clarify the influence of HFD on amyloidogenesis and cognitive deficits in AD model mice.

RESULTS

We here show that a four months HFD-feeding increases IR in both the periphery and brain of APP/PS1 mice, which are used as AD model. Meanwhile, long term HFD exacerbates cognitive defects and impairs dendritic integrity and expressions of synaptic proteins in APP/PS1 mice. Furthermore, HFD induces an increase in β-secretase (BACE1) expression and a decrease in insulin degrading enzyme (IDE) expression, resulting in β-amyloid (Aβ) accumulation.

CONCLUSION

Our data suggests that long term HFD, with the accompanying IR, promotes Aβ toxicity and cognitive deficits, indicating that modifiable life-style hazards such as HFD-induced IR might contribute to AD pathogenesis.

Neuroprotective Effects of Tripeptides-Epigenetic Regulators in Mouse Model of Alzheimer's Disease

KED and EDR peptides prevent dendritic spines loss in amyloid synaptotoxicity in in vitro model of Alzheimer’s disease (AD). The objective of this paper was to study epigenetic mechanisms of EDR and KED peptides’ neuroprotective effects on neuroplasticity and dendritic spine morphology in an AD mouse model. Daily intraperitoneal administration of the KED peptide in 5xFAD mice from 2 to 4 months of age at a concentration of 400 μg/kg tended to increase neuroplasticity. KED and EDR peptides prevented dendritic spine loss in 5xFAD-M mice. Their action’s possible molecular mechanisms were investigated by molecular modeling and docking of peptides in dsDNA, containing all possible combinations of hexanucleotide sequences. Similar DNA sequences were found in the lowest-energy complexes of the studied peptides with DNA in the classical B-form. EDR peptide has binding sites in the promoter region of CASP3, NES, GAP43, APOE, SOD2, PPARA, PPARG, GDX1 genes. Protein products of these genes are involved in AD pathogenesis. The neuroprotective effect of EDR and KED peptides in AD can be defined by their ability to prevent dendritic spine elimination and neuroplasticity impairments at the molecular epigenetic level.

Synaptic modifications in learning and memory - A dendritic spine story

Synapses are specialized sites where neurons connect and communicate with each other. Activity-dependent modification of synaptic structure and function provides a mechanism for learning and memory. The advent of high-resolution time-lapse imaging in conjunction with fluorescent biosensors and actuators enables researchers to monitor and manipulate the structure and function of synapses both in vitro and in vivo. This review focuses on recent imaging studies on the synaptic modification underlying learning and memory.

Upregulation of eIF4E, but not other translation initiation factors, in dendritic spines during memory formation

Local translation can provide a rapid, spatially targeted supply of new proteins in distal dendrites to support synaptic changes that underlie learning. Learning and memory are especially sensitive to manipulations of translational control mechanisms, particularly those that target the initiation step, and translation initiation at synapses could be a means of maintaining synapse specificity during plasticity. Initiation predominantly occurs via recruitment of ribosomes to the 5' mRNA cap by complexes of eukaryotic initiation factors (eIFs), and the interaction between eIF4E and eIF4G1 is a particularly important target of translational control pathways. Pharmacological inhibition of eIF4E-eIF4G1 binding impairs formation of memory for aversive Pavlovian conditioning as well as the accompanying increase in polyribosomes in the heads of dendritic spines in the lateral amygdala (LA). This is consistent with a role for initiation at synapses in memory formation, but whether eIFs are even present near synapses is unknown. To determine whether dendritic spines contain eIFs and whether eIF distribution is affected by learning, we combined immunolabeling with serial section transmission electron microscopy (ssTEM) volume reconstructions of LA dendrites after Pavlovian conditioning. Labeling for eIF4E, eIF4G1, and eIF2α-another key target of regulation-occurred in roughly half of dendritic spines, but learning effects were only found for eIF4E, which was upregulated in the heads of dendritic spines. Our results support the possibility of regulated translation initiation as a means of synapse-specific protein targeting during learning and are consistent with the model of eIF4E availability as a central point of control.

The role of nuclear Ca2+ in maintaining neuronal homeostasis and brain health

Nuclear Ca2+ has emerged as one of the most potent mediators of the dialogue between neuronal synapses and the nucleus that regulates heterochromatin states, transcription factor activity, nuclear morphology and neuronal gene expression induced by synaptic activity. Recent studies underline the importance of nuclear Ca2+ signaling in long-lasting, activity-induced adaptation and maintenance of proper brain function. Diverse forms of neuroadaptation require transient nuclear Ca2+ signaling and cyclic AMP-responsive element-binding protein (CREB1, referred to here as CREB) as its prime target, which works as a tunable switch to drive and modulate specific gene expression profiles associated with memory, pain, addiction and neuroprotection. Furthermore, a reduction of nuclear Ca2+ levels has been shown to be neurotoxic and a causal factor driving the progression of neurodegenerative disorders, as well as affecting neuronal autophagy. Because of its central role in the brain, deficits in nuclear Ca2+ signaling may underlie a continuous loss of neuroprotection in the aging brain, contributing to the pathophysiology of Alzheimer's disease. In this Review, we discuss the principles of the 'nuclear calcium hypothesis' in the context of human brain function and its role in controlling diverse forms of neuroadaptation and neuroprotection. Furthermore, we present the most relevant and promising perspectives for future studies.

Prolonged anesthesia alters brain synaptic architecture

Prolonged medically induced coma (pMIC) is carried out routinely in intensive care medicine. pMIC leads to cognitive impairment, yet the underlying neuromorphological correlates are still unknown, as no direct studies of MIC exceeding ∼6 h on neural circuits exist. Here, we establish pMIC (up to 24 h) in adolescent and mature mice, and combine longitudinal two-photon imaging of cortical synapses with repeated behavioral object recognition assessments. We find that pMIC affects object recognition, and that it is associated with enhanced synaptic turnover, generated by enhanced synapse formation during pMIC, while the postanesthetic period is dominated by synaptic loss. Our results demonstrate major side effects of prolonged anesthesia on neural circuit structure.

Mechanisms of neuronal dysfunction in HIV-associated neurocognitive disorders

HIV-associated neurocognitive disorder (HAND) is characterized by cognitive and behavioral deficits in people living with HIV. HAND is still common in patients that take antiretroviral therapies, although they tend to present with less severe symptoms. The continued prevalence of HAND in treated patients is a major therapeutic challenge, as even minor cognitive impairment decreases patient’s quality of life. Therefore, modern HAND research aims to broaden our understanding of the mechanisms that drive cognitive impairment in people with HIV and identify promising molecular pathways and targets that could be exploited therapeutically. Recent studies suggest that HAND in treated patients is at least partially induced by subtle synaptodendritic damage and disruption of neuronal networks in brain areas that mediate learning, memory, and executive functions. Although the causes of subtle neuronal dysfunction are varied, reversing synaptodendritic damage in animal models restores cognitive function and thus highlights a promising therapeutic approach. In this review, we examine evidence of synaptodendritic damage and disrupted neuronal connectivity in HAND from clinical neuroimaging and neuropathology studies and discuss studies in HAND models that define structural and functional impairment of neurotransmission. Then, we report molecular pathways, mechanisms, and comorbidities involved in this neuronal dysfunction, discuss new approaches to reverse neuronal damage, and highlight current gaps in knowledge. Continued research on the manifestation and mechanisms of synaptic injury and network dysfunction in HAND patients and experimental models will be critical if we are to develop safe and effective therapies that reverse subtle neuropathology and cognitive impairment.

Distinct Transcriptomic Profiles in the Dorsal Hippocampus and Prelimbic Cortex Are Transiently Regulated following Episodic Learning

A fundamental, evolutionarily conserved biological mechanism required for long-term memory formation is rapid induction of gene transcription upon learning in relevant brain areas. For episodic types of memories, two regions undergoing this transcription are the dorsal hippocampus (dHC) and prelimbic (PL) cortex. Whether and to what extent these regions regulate similar or distinct transcriptomic profiles upon learning remain to be understood. Here, we used RNA sequencing in the dHC and PL cortex of male rats to profile their transcriptomes in untrained conditions (baseline) and at 1 h and 6 d after inhibitory avoidance learning. We found that, of 33,713 transcripts, >14,000 were significantly expressed at baseline in both regions and ∼3000 were selectively enriched in each region. Gene Ontology biological pathway analyses indicated that commonly expressed pathways included synapse organization, regulation of membrane potential, and vesicle localization. The enriched pathways in the dHC were gliogenesis, axon development, and lipid modification, while in the PL cortex included vesicle localization and synaptic vesicle cycle. At 1 h after learning, 135 transcripts changed significantly in the dHC and 478 in the PL cortex; of these, only 34 were shared. Biological pathways most significantly regulated by learning in the dHC were protein dephosphorylation, glycogen and glucan metabolism, while in the PL cortex were axon development and axonogenesis. The transcriptome profiles returned to baseline by 6 d after training. Thus, a significant portion of dHC and PL cortex transcriptomic profiles is divergent, and their regulation upon learning is largely distinct and transient.SIGNIFICANCE STATEMENT Long-term episodic memory formation requires gene transcription in several brain regions, including the hippocampus and PFC. The comprehensive profiles of the dynamic mRNA changes that occur in these regions following learning are not well understood. Here, we performed RNA sequencing in the dorsal hippocampus and prelimbic cortex, a PFC subregion, at baseline, 1 h, and 6 d after episodic learning in rats. We found that, at baseline, dorsal hippocampus and prelimbic cortex differentially express a significant portion of mRNAs. Moreover, learning produces a transient regulation of region-specific profiles of mRNA, indicating that unique biological programs in different brain regions underlie memory formation.

Increase in serum prolactin levels in females improves the performance of spatial learning by promoting changes in the circuital dynamics of the hippocampus

Beyond the direct physiological functions associated with motherhood in mammals, previous studies have suggested the potential role of prolactin (Prl) in distinct brain processes such as neuroprotection, neurogenesis, and stress responses. However, the cognitive influence of Prl remains unclear, particularly regarding the mechanisms of acquisition, consolidation and retrieval of information in the brain. Using chronic implanted electrodes in freely moving female mice combined with behavioral tests, we investigated the rhythmic activity changes induced by Prl in a model of hippocampus-dependent learning and memory. Our results show that Prl improves the learning of a spatial memory task in the acquisition stage. The main variations at the circuitry level were in the theta frequency band (4-8 Hz and 8-12 Hz), marked by a faster change in oscillatory activity with no modifications to higher frequencies. These results show that Prl plays a significant role in the acquisition of information during learning of a spatial memory task, suggesting that an increase in Prl levels may induce changes in circuital network plasticity.

Sculpting Dendritic Spines during Initiation and Maintenance of Neuropathic Pain

Accumulating evidence has established a firm role for synaptic plasticity in the pathogenesis of neuropathic pain. Recent advances have highlighted the importance of dendritic spine remodeling in driving synaptic plasticity within the CNS. Identifying the molecular players underlying neuropathic pain induced structural and functional maladaptation is therefore critical to understanding its pathophysiology. This process of dynamic reorganization happens in unique phases that have diverse pathologic underpinnings in the initiation and maintenance of neuropathic pain. Recent evidence suggests that pharmacological targeting of specific proteins during distinct phases of neuropathic pain development produces enhanced antinociception. These findings outline a potential new paradigm for targeted treatment and the development of novel therapies for neuropathic pain. We present a concise review of the role of dendritic spines in neuropathic pain and outline the potential for modulation of spine dynamics by targeting two proteins, srGAP3 and Rac1, critically involved in the regulation of the actin cytoskeleton.

Commonalities between Copper Neurotoxicity and Alzheimer's Disease

Alzheimer's disease, a highly prevalent form of dementia, targets neuron function beginning from the hippocampal region and expanding outwards. Alzheimer's disease is caused by elevated levels of heavy metals, such as lead, zinc, and copper. Copper is found in many areas of daily life, raising a concern as to how this metal and Alzheimer's disease are related. Previous studies have not identified the common pathways between excess copper and Alzheimer's disease etiology. Our review corroborates that both copper and Alzheimer's disease target the hippocampus, cerebral cortex, cerebellum, and brainstem, affecting motor skills and critical thinking. Additionally, Aβ plaque formation was analyzed beginning from synthesis at the APP parent protein site until Aβ plaque formation was completed. Structural changes were also noted. Further analysis revealed a relationship between amyloid-beta plaques and copper ion concentration. As copper ion levels increased, it bound to the Aβ monomer, expediting the plaque formation process, and furthering neurodegeneration. These conclusions can be utilized in the medical community to further research on the etiology of Alzheimer's disease and its relationships to copper and other metal-induced neurotoxicity.

Spastin interacts with collapsin response mediator protein 3 to regulate neurite growth and branching

Cytoskeletal microtubule rearrangement and movement are crucial in the repair of spinal cord injury. Spastin plays an important role in the regulation of microtubule severing. Both spastin and collapsin response mediator proteins can regulate neurite growth and branching; however, whether spastin interacts with collapsin response mediator protein 3 (CRMP3) during this process remains unclear, as is the mechanism by which CRMP3 participates in the repair of spinal cord injury. In this study, we used a proteomics approach to identify key proteins associated with spinal cord injury repair. We then employed liquid chromatography-mass spectrometry to identify proteins that were able to interact with glutathione S-transferase-spastin. Then, co-immunoprecipitation and staining approaches were used to evaluate potential interactions between spastin and CRMP3. Finally, we co-transfected primary hippocampal neurons with CRMP3 and spastin to evaluate their role in neurite outgrowth. Mass spectrometry identified the role of CRMP3 in the spinal cord injury repair process. Liquid chromatography-mass spectrometry pulldown assays identified three CRMP3 peptides that were able to interact with spastin. CRMP3 and spastin were co-expressed in the spinal cord and were able to interact with one another in vitro and in vivo. Lastly, CRMP3 overexpression was able to enhance the ability of spastin to promote neurite growth and branching. Therefore, our results confirm that spastin and CRMP3 play roles in spinal cord injury repair by regulating neurite growth and branching. These proteins may therefore be novel targets for spinal cord injury repair. The Institutional Animal Care and Use Committee of Jinan University, China approved this study (approval No. IACUS-20181008-03) on October 8, 2018.

Prolactin enhances hippocampal synaptic plasticity in female mice of reproductive age

Dynamic signaling between the endocrine system (ES) and the nervous system (NS) is essential for brain and body homeostasis. In particular, reciprocal interaction occurs during pregnancy and motherhood that may involve changes in some brain plasticity processes. Prolactin (PRL), a hormone with pleiotropic effects on the NS, promotes maternal behavior and has been linked to modifications in brain circuits during motherhood; however, it is unclear whether PRL may regulate synaptic plasticity. Therefore, the main aim of the present work was to determine the cellular and molecular mechanisms triggered by PRL that regulate synaptic plasticity in the hippocampus. By analyzing extracellular recordings in CA3‐CA1 synapses of hippocampal slices, we report that PRL modifies short and long‐term synaptic plasticity in female mice of reproductive age, but not in sexually immature females or adult males. This effect is carried out through mechanisms that include participation of GABA_A receptors and activation of the JAK2‐mediated signaling pathway. These findings show for the first time how PRL enhances the synaptic strength in hippocampal circuits and that this effect is sexually dimorphic, which would influence complex brain processes in physiological conditions like pregnancy and lactation.

The actin-modulating protein synaptopodin mediates long-term survival of dendritic spines

Large spines are stable and important for memory trace formation. The majority of large spines also contains synaptopodin (SP), an actin-modulating and plasticity-related protein. Since SP stabilizes F-actin, we speculated that the presence of SP within large spines could explain their long lifetime. Indeed, using 2-photon time-lapse imaging of SP-transgenic granule cells in mouse organotypic tissue cultures we found that spines containing SP survived considerably longer than spines of equal size without SP. Of note, SP-positive (SP+) spines that underwent pruning first lost SP before disappearing. Whereas the survival time courses of SP+ spines followed conditional two-stage decay functions, SP-negative (SP-) spines and all spines of SP-deficient animals showed single-phase exponential decays. This was also the case following afferent denervation. These results implicate SP as a major regulator of long-term spine stability: SP clusters stabilize spines, and the presence of SP indicates spines of high stability.

HERC1 Ubiquitin Ligase Is Required for Hippocampal Learning and Memory

Mutations in the human HERC1 E3 ubiquitin ligase protein develop intellectual disability. The tambaleante (tbl) mouse carries a HERC1 mutation characterized by cerebellar ataxia due of adult cerebellar Purkinje cells death by extensive autophagy. Our previous studies demonstrated that both the neuromuscular junction and the peripheral nerve myelin sheaths are also affected in this mutant. Moreover, there are signs of dysregulated autophagy in the central nervous system in the tbl mouse, affecting spinal cord motor neurons, and pyramidal neurons of the neocortex and the hippocampal CA3 region. The tbl mutation affects associative learning, with absence of short- and long-term potentiation in the lateral amygdala, altered spinogenesis in their neurons, and a dramatic decrease in their glutamatergic input. To assess whether other brain areas engaged in learning processes might be affected by the tbl mutation, we have studied the tbl hippocampus using behavioral tests, ex vivo electrophysiological recordings, immunohistochemistry, the Golgi-Cox method and transmission electron microscopy. The tbl mice performed poorly in the novel-object recognition, T-maze and Morris water maze tests. In addition, there was a decrease in glutamatergic input while the GABAergic one remains unaltered in the hippocampal CA1 region of tbl mice, accompanied by changes in the dendritic spines, and signs of cellular damage. Moreover, the proportions of immature and mature neurons in the dentate gyrus of the tbl hippocampus differ relative to the control mice. Together, these observations demonstrate the important role of HERC1 in regulating synaptic activity during learning.

Dendritic Spines Shape Analysis-Classification or Clusterization? Perspective

Dendritic spines are small protrusions from the dendrite membrane, where contact with neighboring axons is formed in order to receive synaptic input. Changes in size, shape, and density of synaptic spines are associated with learning and memory, and observed after drug abuse in a variety of neurodegenerative, neurodevelopmental, and psychiatric disorders. Due to the preeminent importance of synaptic spines, there have been major efforts into developing techniques that enable visualization and analysis of dendritic spines in cultured neurons, in fixed slices and in intact brain tissue. The classification of synaptic spines into predefined morphological groups is a standard approach in neuroscience research, where spines are divided into fixed categories such as thin, mushroom, and stubby subclasses. This study examines accumulated evidence that supports the existence of dendritic spine shapes as a continuum rather than separated classes. Using new approaches and software tools we reflect on complex dendritic spine shapes, positing that understanding of their highly dynamic nature is required to perform analysis of their morphology. The study discusses and compares recently developed algorithms that rely on clusterization rather than classification, therefore enabling new levels of spine shape analysis. We reason that improved methods of analysis may help to investigate a link between dendritic spine shape and its function, facilitating future studies of learning and memory as well as studies of brain disorders.

Systematic Quantification of Synapses in Primary Neuronal Culture

Most neurological disorders display impaired synaptic connectivity. Hence, modulation of synapse formation may have therapeutic relevance. However, the high density and small size of synapses complicate their quantification. To improve synapse-oriented screens, we analyzed the labeling performance of synapse-targeting antibodies on neuronal cell cultures using segmentation-independent image analysis based on sliding window correlation. When assessing pairwise colocalization, a common readout for mature synapses, overlap was incomplete and confounded by spurious signals. To circumvent this, we implemented a proximity ligation-based approach that only leads to a signal when two markers are sufficiently close. We applied this approach to different marker combinations and demonstrate its utility for detecting synapse density changes in healthy and compromised cultures. Thus, segmentation-independent analysis and exploitation of resident protein proximity increases the sensitivity of synapse quantifications in neuronal cultures and represents a valuable extension to the analytical toolset for in vitro synapse screens.

Dendritic Spine Plasticity: Function and Mechanisms

Dendritic spines are small protrusions studding neuronal dendrites, first described in 1888 by Ramón y Cajal using his famous Golgi stainings. Around 50 years later the advance of electron microscopy (EM) confirmed Cajal's intuition that spines constitute the postsynaptic site of most excitatory synapses in the mammalian brain. The finding that spine density decreases between young and adult ages in fixed tissues suggested that spines are dynamic. It is only a decade ago that two-photon microscopy (TPM) has unambiguously proven the dynamic nature of spines, through the repeated imaging of single spines in live animals. Spine dynamics comprise formation, disappearance, and stabilization of spines and are modulated by neuronal activity and developmental age. Here, we review several emerging concepts in the field that start to answer the following key questions: What are the external signals triggering spine dynamics and the molecular mechanisms involved? What is, in return, the role of spine dynamics in circuit-rewiring, learning, and neuropsychiatric disorders?

Structural LTP: from synaptogenesis to regulated synapse enlargement and clustering

Nature teaches us that form precedes function, yet structure and function are intertwined. Such is the case with synapse structure, function, and plasticity underlying learning, especially in the hippocampus, a crucial brain region for memory formation. As the hippocampus matures, enduring changes in synapse structure produced by long-term potentiation (LTP) shift from synaptogenesis to synapse enlargement that is homeostatically balanced by stalled spine outgrowth and local spine clustering. Production of LTP leads to silent spine outgrowth at P15, and silent synapse enlargement in adult hippocampus at 2hours, but not at 5 or 30min following induction. Here we consider structural LTP in the context of developmental stage and variation in the availability of local resources of endosomes, smooth endoplasmic reticulum and polyribosomes. The emerging evidence supports a need for more nuanced analysis of synaptic plasticity in the context of subcellular resource availability and developmental stage.