Automated analysis of spines from confocal laser microscopy images: application to the discrimination of androgen and estrogen effects on spinogenesis

Between neurons and networks: investigating mesoscale brain connectivity in neurological and psychiatric disorders

The study of brain connectivity has been a cornerstone in understanding the complexities of neurological and psychiatric disorders. It has provided invaluable insights into the functional architecture of the brain and how it is perturbed in disorders. However, a persistent challenge has been achieving the proper spatial resolution, and developing computational algorithms to address biological questions at the multi-cellular level, a scale often referred to as the mesoscale. Historically, neuroimaging studies of brain connectivity have predominantly focused on the macroscale, providing insights into inter-regional brain connections but often falling short of resolving the intricacies of neural circuitry at the cellular or mesoscale level. This limitation has hindered our ability to fully comprehend the underlying mechanisms of neurological and psychiatric disorders and to develop targeted interventions. In light of this issue, our review manuscript seeks to bridge this critical gap by delving into the domain of mesoscale neuroimaging. We aim to provide a comprehensive overview of conditions affected by aberrant neural connections, image acquisition techniques, feature extraction, and data analysis methods that are specifically tailored to the mesoscale. We further delineate the potential of brain connectivity research to elucidate complex biological questions, with a particular focus on schizophrenia and epilepsy. This review encompasses topics such as dendritic spine quantification, single neuron morphology, and brain region connectivity. We aim to showcase the applicability and significance of mesoscale neuroimaging techniques in the field of neuroscience, highlighting their potential for gaining insights into the complexities of neurological and psychiatric disorders.

A Deep Learning-Based Workflow for Dendritic Spine Segmentation

The morphological analysis of dendritic spines is an important challenge for the neuroscientific community. Most state-of-the-art techniques rely on user-supervised algorithms to segment the spine surface, especially those designed for light microscopy images. Therefore, processing large dendritic branches is costly and time-consuming. Although deep learning (DL) models have become one of the most commonly used tools in image segmentation, they have not yet been successfully applied to this problem. In this article, we study the feasibility of using DL models to automatize spine segmentation from confocal microscopy images. Supervised learning is the most frequently used method for training DL models. This approach requires large data sets of high-quality segmented images (ground truth). As mentioned above, the segmentation of microscopy images is time-consuming and, therefore, in most cases, neuroanatomists only reconstruct relevant branches of the stack. Additionally, some parts of the dendritic shaft and spines are not segmented due to dyeing problems. In the context of this research, we tested the most successful architectures in the DL biomedical segmentation field. To build the ground truth, we used a large and high-quality data set, according to standards in the field. Nevertheless, this data set is not sufficient to train convolutional neural networks for accurate reconstructions. Therefore, we implemented an automatic preprocessing step and several training strategies to deal with the problems mentioned above. As shown by our results, our system produces a high-quality segmentation in most cases. Finally, we integrated several postprocessing user-supervised algorithms in a graphical user interface application to correct any possible artifacts.

Perinatal Exposure of Bisphenol A Differently Affects Dendritic Spines of Male and Female Grown-Up Adult Hippocampal Neurons

Perinatal exposure to Bisphenol A (BPA) at a very low dose may modulate the development of synapses of the hippocampus during growth to adulthood. Here, we demonstrate that perinatal exposure to 30 μg BPA/kg per mother’s body weight/day significantly altered the dendritic spines of the grownup rat hippocampus. The density of the spine was analyzed by imaging of Lucifer Yellow-injected CA1 glutamatergic neurons in adult hippocampal slices. In offspring 3-month male hippocampus, the total spine density was significantly decreased by BPA exposure from 2.26 spines/μm (control, no BPA exposure) to 1.96 spines/μm (BPA exposure). BPA exposure considerably changed the normal 4-day estrous cycle of offspring 3-month females, resulting in a 4∼5 day estrous cycle with 2-day estrus stages in most of the subjects. In the offspring 3-month female hippocampus, the total spine density was significantly increased by BPA exposure at estrus stage from 2.04 spines/μm (control) to 2.25 spines/μm (BPA exposure). On the other hand, the total spine density at the proestrus stage was moderately decreased from 2.33 spines/μm (control) to 2.19 spines/μm (BPA exposure). Thus, after the perinatal exposure to BPA, the total spine density in males became lower than that in females. Concerning the BPA effect on the morphology of spines, the large-head spine was significantly changed with its significant decrease in males and moderate change in females.

Peroxisome proliferator-activated receptor α as a novel therapeutic target for schizophrenia

BACKGROUND

The pathophysiology of schizophrenia, a major psychiatric disorder, remains elusive. In this study, the role of peroxisome proliferator-activated receptor (PPAR)/retinoid X receptor (RXR) families, belonging to the ligand-activated nuclear receptor superfamily, in schizophrenia, was analyzed.

METHODS

The PPAR/RXR family genes were screened by exploiting molecular inversion probe (MIP)-based targeted next-generation sequencing (NGS) using the samples of 1,200 Japanese patients with schizophrenia. The results were compared with the whole-genome sequencing databases of the Japanese cohort (ToMMo) and the gnomAD. To reveal the relationship between PPAR/RXR dysfunction and schizophrenia, Ppara KO mice and fenofibrate (a clinically used PPARα agonist)-administered mice were assessed by performing behavioral, histological, and RNA-seq analyses.

FINDINGS

Our findings indicate that c.209-2delA, His117Gln, Arg141Cys, and Arg226Trp of the PPARA gene are risk variants for schizophrenia. The c.209-2delA variant generated a premature termination codon. The three missense variants significantly decreased the activity of PPARα as a transcription factor in vitro. The Ppara KO mice exhibited schizophrenia-relevant phenotypes, including behavioral deficits and impaired synaptogenesis in the cerebral cortex. Oral administration of fenofibrate alleviated spine pathology induced by phencyclidine, an N-methyl-d-aspartate (NMDA) receptor antagonist. Furthermore, pre-treatment with fenofibrate suppressed the sensitivity of mice to another NMDA receptor antagonist, MK-801. RNA-seq analysis revealed that PPARα regulates the expression of synaptogenesis signaling pathway-related genes.

INTERPRETATION

The findings of this study indicate that the mechanisms underlying schizophrenia pathogenesis involve PPARα-regulated transcriptional machinery and modulation of synapse physiology. Hence, PPARα can serve as a novel therapeutic target for schizophrenia.

A potential role of fatty acid binding protein 4 in the pathophysiology of autism spectrum disorder

Autism spectrum disorder is a neurodevelopmental disorder characterized by difficulties in social communication and interaction, as well as repetitive and characteristic patterns of behaviour. Although the pathogenesis of autism spectrum disorder is unknown, being overweight or obesity during infancy and low weight at birth are known as risks, suggesting a metabolic aspect. In this study, we investigated adipose tissue development as a pathophysiological factor of autism spectrum disorder by examining the serum levels of adipokines and other metabolic markers in autism spectrum disorder children (n = 123) and typically developing children (n = 92) at 4–12 years of age. Among multiple measures exhibiting age-dependent trajectories, the leptin levels displayed different trajectory patterns between autism spectrum disorder and typically developing children, supporting an adipose tissue-dependent mechanism of autism spectrum disorder. Of particular interest, the levels of fatty acid binding protein 4 (FABP4) were significantly lower in autism spectrum disorder children than in typically developing subjects, at preschool age (4–6 years old: n = 21 for autism spectrum disorder and n = 26 for typically developing). The receiver operating characteristic curve analysis discriminated autism spectrum disorder children from typically developing children with a sensitivity of 94.4% and a specificity of 75.0%. We re-sequenced the exons of the FABP4 gene in a Japanese cohort comprising 659 autism spectrum disorder and 1000 control samples, and identified two rare functional variants in the autism spectrum disorder group. The Trp98Stop, one of the two variants, was transmitted to the proband from his mother with a history of depression. The disruption of the Fabp4 gene in mice evoked autism spectrum disorder-like behavioural phenotypes and increased spine density on apical dendrites of pyramidal neurons, which has been observed in the postmortem brains of autism spectrum disorder subjects. The Fabp4 knockout mice had an altered fatty acid composition in the cortex. Collectively, these results suggest that an ‘adipo-brain axis’ may underlie the pathophysiology of autism spectrum disorder, with FABP4 as a potential molecule for use as a biomarker.

Subanesthetic ketamine exerts antidepressant-like effects in adult rats exposed to juvenile stress

Juvenile stress, like that caused by childhood maltreatment, is a significant risk factor for psychiatric disorders such as depression later in life. Recently, the antidepressant effect of ketamine, a noncompetitive N-methyl-d-aspartate receptor antagonist, has been widely investigated. However, little is known regarding its efficacy against depressive-like alterations caused by juvenile stress, which is clinically relevant in human depression. In the present study, we evaluated the antidepressant-like effect of ketamine in adult rats that had been subjected to juvenile stress. Depressive-like behavior was assessed using the forced swim test (FST), and electrophysiological and morphological alterations in the layer V pyramidal cells of the prelimbic cortex were examined using whole-cell patch-clamp recordings and subsequent recording-cell specific fluorescence imaging. We demonstrated that ketamine (10 mg/kg) attenuated the increased immobility time caused by juvenile stress in the FST, restored the diminished excitatory postsynaptic currents, and caused atrophic changes in the apical dendritic spines. Ketamine's effects reversing impaired excitatory/inhibitory ratio of postsynaptic currents were also revealed. These results indicated that ketamine could be effective in reversing the depression-like alterations caused by juvenile stress.

Sparse Activity of Hippocampal Adult-Born Neurons during REM Sleep Is Necessary for Memory Consolidation

The occurrence of dreaming during rapid eye movement (REM) sleep prompts interest in the role of REM sleep in hippocampal-dependent episodic memory. Within the mammalian hippocampus, the dentate gyrus (DG) has the unique characteristic of exhibiting neurogenesis persisting into adulthood. Despite their small numbers and sparse activity, adult-born neurons (ABNs) in the DG play critical roles in memory; however, their memory function during sleep is unknown. Here, we investigate whether young ABN activity contributes to memory consolidation during sleep using Ca²⁺ imaging in freely moving mice. We found that contextual fear learning recruits a population of young ABNs that are reactivated during subsequent REM sleep against a backdrop of overall reduced ABN activity. Optogenetic silencing of this sparse ABN activity during REM sleep alters the structural remodeling of spines on ABN dendrites and impairs memory consolidation. These findings provide a causal link between ABN activity during REM sleep and memory consolidation.

Automated dendritic spine detection using convolutional neural networks on maximum intensity projected microscopic volumes

BACKGROUND

Dendritic spines are structural correlates of excitatory synapses in the brain. Their density and structure are shaped by experience, pointing to their role in memory encoding. Dendritic spine imaging, followed by manual analysis, is a primary way to study spines. However, an approach that analyses dendritic spines images in an automated and unbiased manner is needed to fully capture how spines change with normal experience, as well as in disease.

NEW METHOD

We propose an approach based on fully convolutional neural networks (FCNs) to detect dendritic spines in two-dimensional maximum-intensity projected images from confocal fluorescent micrographs. We experiment on both fractionally strided convolution and efficient sub-pixel convolutions. Dendritic spines far from the dendritic shaft are pruned by extraction of the shaft to reduce false positives. Performance of the proposed method is evaluated by comparing predicted spine positions to those manually marked by experts.

RESULTS

The averaged distance between predicted and manually annotated spines is 2.81 ± 2.63 pixels (0.082 ± 0.076 microns) and 2.87 ± 2.33 pixels (0.084 ± 0.068 microns) based on two different experts. FCN-based detection achieves F scores > 0.80 for both sets of expert annotations.

COMPARISON WITH EXISTING METHODS

Our method significantly outperforms two well-known software, NeuronStudio and Neurolucida (p-value < 0.02).

CONCLUSIONS

FCN architectures used in this work allow for automated dendritic spine detection. Superior outcomes are possible even with small training data-sets. The proposed method may generalize to other datasets on larger scales.

Src Kinase Dependent Rapid Non-genomic Modulation of Hippocampal Spinogenesis Induced by Androgen and Estrogen

Dendritic spine is a small membranous protrusion from a neuron's dendrite that typically receives input from an axon terminal at the synapse. Memories are stored in synapses which consist of spines and presynapses. Rapid modulations of dendritic spines induced by hippocampal sex steroids, including dihydrotestosterone (DHT), testosterone (T), and estradiol (E2), are essential for synaptic plasticity. Molecular mechanisms underlying the rapid non-genomic modulation through synaptic receptors of androgen (AR) and estrogen (ER) as well as its downstream kinase signaling, however, have not been well understood. We investigated the possible involvement of Src tyrosine kinase in rapid changes of dendritic spines in response to androgen and estrogen, including DHT, T, and E2, using hippocampal slices from adult male rats. We found that the treatments with DHT (10 nM), T (10 nM), and E2 (1 nM) increased the total density of spines by ~1.22 to 1.26-fold within 2 h using super resolution confocal imaging of Lucifer Yellow-injected CA1 pyramidal neurons. We examined also morphological changes of spines in order to clarify differences between three sex steroids. From spine head diameter analysis, DHT increased middle- and large-head spines, whereas T increased small- and middle-head spines, and E2 increased small-head spines. Upon application of Src tyrosine kinase inhibitor, the spine increases induced through DHT, T, and E2 treatments were completely blocked. These results imply that Src kinase is essentially involved in sex steroid-induced non-genomic modulation of the spine density and morphology. These results also suggest that rapid effects of exogenously applied androgen and estrogen can occur in steroid-depleted conditions, including “acute” hippocampal slices and the hippocampus of gonadectomized animals.

Quantitative 3-D morphometric analysis of individual dendritic spines

The observation and analysis of dendritic spines morphological changes poses a major challenge in neuroscience studies. The alterations of their density and/or morphology are indicators of the cellular processes involved in neural plasticity underlying learning and memory, and are symptomatic in neuropsychiatric disorders. Despite ongoing intense investigations in imaging approaches, the relationship between changes in spine morphology and synaptic function is still unknown. The existing quantitative analyses are difficult to perform and require extensive user intervention. Here, we propose a new method for (1) the three-dimensional (3-D) segmentation of dendritic spines using a multi-scale opening approach and (2) define 3-D morphological attributes of individual spines for the effective assessment of their structural plasticity. The method was validated using confocal light microscopy images of dendritic spines from dissociated hippocampal cultures and brain slices (1) to evaluate accuracy relative to manually labeled ground-truth annotations and relative to the state-of-the-art Imaris tool, (2) to analyze reproducibility of user-independence of the segmentation method, and (3) to quantitatively analyze morphological changes in individual spines before and after chemically induced long-term potentiation. The method was monitored and used to precisely describe the morphology of individual spines in real-time using consecutive images of the same dendritic fragment.

Rapid nongenomic modulation by neurosteroids of dendritic spines in the hippocampus: Androgen, oestrogen and corticosteroid

Memories are stored in synapses that consist of axon terminals and dendritic spines. Dendritic spines are postsynaptic structures of synapses and are essential for synaptic plasticity and cognition. Therefore, extensive investigations concerning the functions and structures of spines have been performed. Sex steroids and stress steroids have been shown to modulate hippocampal synapses. Although the rapid modulatory action of sex steroids on synapses has been studied in hippocampal neurones over several decades, the essential molecular mechanisms have not been fully understood. Here, a description of kinase-dependent signalling mechanisms is provided that can explain the rapid nongenomic modulation of dendritic spinogenesis in rat and mouse hippocampal slices by the application of sex steroids, including dihydrotestosterone, testosterone, oestradiol and progesterone. We also indicate the role of synaptic (classic) sex steroid receptors that trigger these rapid synaptic modulations. Moreover, we describe rapid nongenomic spine modulation by applying corticosterone, which is an acute stress model of the hippocampus. The explanations for the results obtained are mainly based on the optical imaging of dendritic spines. Comparisons are also performed with results obtained from other types of imaging, including electron microscopic imaging. Relationships between spine modulation and modulation of cognition are discussed. We recognise that most of rapid effects of exogenously applied oestrogen and androgen were observed in steroid-depleted conditions, including acute slices of the hippocampus, castrated male animals and ovariectomised female animals. Therefore, the previously observed effects can be considered as a type of recovery event, which may be essentially similar to hormone replacement therapy under hormone-decreased conditions. On the other hand, in gonadally intact young animals with high levels of endogenous sex hormones, further supplementation of sex hormones might not be effective, whereas the infusion of blockers for steroid receptors or kinases may be effective, with respect to suppressing sex hormone functions, thus providing useful information regarding molecular mechanisms.

Morphological analysis of dendrites and spines by hybridization of ridge detection with twin support vector machine

Dendritic spines are described as neuronal protrusions. The morphology of dendritic spines and dendrites has a strong relationship to its function, as well as playing an important role in understanding brain function. Quantitative analysis of dendrites and dendritic spines is essential to an understanding of the formation and function of the nervous system. However, highly efficient tools for the quantitative analysis of dendrites and dendritic spines are currently undeveloped. In this paper we propose a novel three-step cascaded algorithm–RTSVM— which is composed of ridge detection as the curvature structure identifier for backbone extraction, boundary location based on differences in density, the Hu moment as features and Twin Support Vector Machine (TSVM) classifiers for spine classification. Our data demonstrates that this newly developed algorithm has performed better than other available techniques used to detect accuracy and false alarm rates. This algorithm will be used effectively in neuroscience research.

2dSpAn: semiautomated 2-d segmentation, classification and analysis of hippocampal dendritic spine plasticity

MOTIVATION

Accurate and effective dendritic spine segmentation from the dendrites remains as a challenge for current neuroimaging research community. In this article, we present a new method (2dSpAn) for 2-d segmentation, classification and analysis of structural/plastic changes of hippocampal dendritic spines. A user interactive segmentation method with convolution kernels is designed to segment the spines from the dendrites. Formal morphological definitions are presented to describe key attributes related to the shape of segmented spines. Spines are automatically classified into one of four classes: Stubby, Filopodia, Mushroom and Spine-head Protrusions.

RESULTS

The developed method is validated using confocal light microscopy images of dendritic spines from dissociated hippocampal cultures for: (i) quantitative analysis of spine morphological changes, (ii) reproducibility analysis for assessment of user-independence of the developed software and (iii) accuracy analysis with respect to the manually labeled ground truth images, and also with respect to the available state of the art. The developed method is monitored and used to precisely describe the morphology of individual spines in real-time experiments, i.e. consequent images of the same dendritic fragment.

AVAILABILITY AND IMPLEMENTATION

The software and the source code are available at https://sites.google.com/site/2dspan/ under open-source license for non-commercial use.

CONTACT

subhadip@cse.jdvu.ac.in or j.wlodarczyk@nencki.gov.pl

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Prenatal Exposure to Arsenic Impairs Behavioral Flexibility and Cortical Structure in Mice

Exposure to arsenic from well water in developing countries is suspected to cause developmental neurotoxicity. Although, it has been demonstrated that exposure to sodium arsenite (NaAsO₂) suppresses neurite outgrowth of cortical neurons in vitro, it is largely unknown how developmental exposure to NaAsO₂ impairs higher brain function and affects cortical histology. Here, we investigated the effect of prenatal NaAsO₂ exposure on the behavior of mice in adulthood, and evaluated histological changes in the prelimbic cortex (PrL), which is a part of the medial prefrontal cortex that is critically involved in cognition. Drinking water with or without NaAsO₂ (85 ppm) was provided to pregnant C3H mice from gestational days 8 to 18, and offspring of both sexes were subjected to cognitive behavioral analyses at 60 weeks of age. The brains of female offspring were subsequently harvested and used for morphometrical analyses. We found that both male and female mice prenatally exposed to NaAsO₂ displayed an impaired adaptation to repetitive reversal tasks. In morphometrical analyses of Nissl- or Golgi-stained tissue sections, we found that NaAsO₂ exposure was associated with a significant increase in the number of pyramidal neurons in layers V and VI of the PrL, but not other layers of the PrL. More strikingly, prenatal NaAsO₂ exposure was associated with a significant decrease in neurite length but not dendrite spine density in all layers of the PrL. Taken together, our results indicate that prenatal exposure to NaAsO₂ leads to behavioral inflexibility in adulthood and cortical disarrangement in the PrL might contribute to this behavioral impairment.

Hippocampal spine changes across the sleep-wake cycle: corticosterone and kinases

The corticosterone (CORT) level changes along the circadian rhythm. Hippocampus is sensitive to CORT, since glucocorticoid receptors are highly expressed. In rat hippocampus fixed in a living state every 3 h, we found that the dendritic spine density of CA1 pyramidal neurons increased upon waking (within 3 h), as compared with the spine density in the sleep state. Particularly, the large-head spines increased. The observed change in the spine density may be due to the change in the hippocampal CORT level, since the CORT level at awake state (∼30 nM) in cerebrospinal fluid was higher than that at sleep state (∼3 nM), as observed from our earlier study. In adrenalectomized (ADX) rats, such a wake-induced increase of the spine density disappeared. S.c. administration of CORT into ADX rats rescued the decreased spine density. By using isolated hippocampal slices, we found that the application of 30 nM CORT increased the spine density within 1 h and that the spine increase was mediated via PKA, PKC, ERK MAPK, and LIMK signaling pathways. These findings suggest that the moderately rapid increase of the spine density on waking might mainly be caused by the CORT-driven kinase networks.

Estradiol rapidly modulates spinogenesis in hippocampal dentate gyrus: Involvement of kinase networks

This article is part of a Special Issue "Estradiol and cognition". Estradiol (E2) is locally synthesized within the hippocampus and the gonads. Rapid modulation of hippocampal synaptic plasticity by E2 is essential for synaptic regulation. The molecular mechanisms of modulation through the synaptic estrogen receptor (ER) and its downstream signaling, however, are largely unknown in the dentate gyrus (DG). We investigated the E2-induced modulation of dendritic spines in male adult rat hippocampal slices by imaging Lucifer Yellow-injected DG granule cells. Treatments with 1 nM E2 increased the density of spines by approximately 1.4-fold within 2h. Spine head diameter analysis showed that the density of middle-head spines (0.4-0.5 μm) was significantly increased. The E2-induced spine density increase was suppressed by blocking Erk MAPK, PKA, PKC and LIMK. These suppressive effects by kinase inhibitors are not non-specific ones because the GSK-3β antagonist did not inhibit E2-induced spine increase. The ER antagonist ICI 182,780 also blocked the E2-induced spine increase. Taken together, these results suggest that E2 rapidly increases the density of spines through kinase networks that are driven by synaptic ER.

Milnacipran remediates impulsive deficits in rats with lesions of the ventromedial prefrontal cortex

BACKGROUND

Deficits in impulse control are often observed in psychiatric disorders in which abnormalities of the prefrontal cortex are observed, including attention-deficit/hyperactivity disorder and bipolar disorder. We recently found that milnacipran, a serotonin/noradrenaline reuptake inhibitor, could suppress impulsive action in normal rats. However, whether milnacipran could suppress elevated impulsive action in rats with lesions of the ventromedial prefrontal cortex, which is functionally comparable with the human prefrontal cortex, remains unknown.

METHODS

Selective lesions of the ventromedial prefrontal cortex were made using quinolinic acid in rats previously trained on a 3-choice serial reaction time task. Sham rats received phosphate buffered saline. Following a period of recovery, milnacipran (0 or 10mg/kg/d × 14 days) was orally administered 60 minutes prior to testing on the 3-choice task. After 7 days of drug cessation, Western blotting, immunohistochemistry, electrophysiological analysis, and morphological analysis were conducted.

RESULTS

Lesions of the ventromedial prefrontal cortex induced impulsive deficits, and repeated milnacipran ameliorated the impulsive deficit both during the dosing period and after the cessation of the drug. Repeated milnacipran remediated the protein levels of mature brain-derived neurotrophic factor and postsynaptic density-95, dendritic spine density, and excitatory currents in the few surviving neurons in the ventromedial prefrontal cortex of ventromedial prefrontal cortex-lesioned rats.

CONCLUSIONS

The findings of this study suggest that milnacipran treatment could be a novel strategy for the treatment of psychiatric disorders that are associated with a lack of impulse control.

Estradiol rapidly modulates synaptic plasticity of hippocampal neurons: Involvement of kinase networks

Estradiol (E2) is locally synthesized within the hippocampus in addition to the gonads. Rapid modulation of hippocampal synaptic plasticity by E2 is essential for synaptic regulation. Molecular mechanisms of modulation through synaptic estrogen receptor (ER) and its downstream signaling, however, have been still unknown. We investigated induction of LTP by the presence of E2 upon weak theta burst stimulation (weak-TBS) in CA1 region of adult male hippocampus. Since only weak-TBS did not induce full-LTP, weak-TBS was sub-threshold stimulation. We observed LTP induction by the presence of E2, after incubation of hippocampal slices with 10nM E2 for 30 min, upon weak-TBS. This E2-induced LTP was blocked by ICI, an ER antagonist. This E2-LTP induction was inhibited by blocking Erk MAPK, PKA, PKC, PI3K, NR2B and CaMKII, individually, suggesting that Erk MAPK, PKA, PKC, PI3K and CaMKII may be involved in downstream signaling for activation of NMDA receptors. Interestingly, dihydrotestosterone suppressed the E2-LTP. We also investigated rapid changes of dendritic spines (=postsynapses) in response to E2, using hippocampal slices from adult male rats. We found 1nM E2 increased the density of spines by approximately 1.3-fold within 2h by imaging Lucifer Yellow-injected CA1 pyramidal neurons. The E2-induced spine increase was blocked by ICI. The increase in spines was suppressed by blocking PI3K, Erk MAPK, p38 MAPK, PKA, PKC, LIMK, CaMKII or calcineurin, individually. On the other hand, blocking JNK did not inhibit the E2-induced spine increase. Taken together, these results suggest that E2 rapidly induced LTP and also increased the spine density through kinase networks that are driven by synaptic ER. This article is part of a Special Issue entitled SI: Brain and Memory.

Rapid increase of spines by dihydrotestosterone and testosterone in hippocampal neurons: Dependence on synaptic androgen receptor and kinase networks

Rapid modulation of hippocampal synaptic plasticity by locally synthesized androgen is important in addition to circulating androgen. Here, we investigated the rapid changes of dendritic spines in response to the elevation of dihydrotestosterone (DHT) and testosterone (T), by using hippocampal slices from adult male rats, in order to clarify whether these signaling processes include synaptic/extranuclear androgen receptor (AR) and activation of kinases. We found that the application of 10nM DHT and 10nM T increased the total density of spines by approximately 1.3-fold within 2h, by imaging Lucifer Yellow-injected CA1 pyramidal neurons. Interestingly, DHT and T increased different head-sized spines. While DHT increased middle- and large-head spines, T increased small-head spines. Androgen-induced spinogenesis was suppressed by individually blocking Erk MAPK, PKA, PKC, p38 MAPK, LIMK or calcineurin. On the other hand, blocking CaMKII did not inhibit spinogenesis. Blocking PI3K altered the spine head diameter distribution, but did not change the total spine density. Blocking mRNA and protein synthesis did not suppress the enhancing effects induced by DHT or T. The enhanced spinogenesis by androgens was blocked by AR antagonist, which AR was localized postsynaptically. Taken together, these results imply that enhanced spinogenesis by DHT and T is mediated by synaptic/extranuclear AR which rapidly drives the kinase networks. This article is part of a Special Issue entitled SI: Brain and Memory.

Estrogen receptor KO mice study on rapid modulation of spines and long-term depression in the hippocampus

Rapid modulation of hippocampal synaptic plasticity through synaptic estrogen receptors is an essential topic. We analyzed estradiol-induced modulation of CA1 dendritic spines using adult male ERαKO and ERβKO mice. A 2h treatment of estradiol particularly increased the density of middle-head spines (diameter 0.3-0.4 µm) in wild type mouse hippocampal slices. The enhancement of spinogenesis was completely suppressed by MAP kinase inhibitor. Estradiol-induced increase in middle-head spines was observed in ERβKO mice (which express ERα), but not in ERαKO, indicating that ERα is necessary for the spinogenesis. Direct observation of the dynamic estradiol-induced enhancing effect on rapid spinogenesis was performed using time-lapse imaging of spines in hippocampal live slices from yellow fluorescent protein expressed mice. Both appearance and disappearance of spines occurred, and the number of newly appeared spines was significantly greater than that of disappeared spines, resulting in the net increase of the spine density within 2h. As another type of synaptic modulation, we observed that estradiol rapidly enhanced N-methyl-D-aspartate (NMDA)-induced long-term depression (LTD) in CA1 of the wild type mouse hippocampus. In contrast, estradiol did not enhance NMDA-LTD in ERαKO mice, indicating the involvement of ERα in the estrogen signaling. This article is part of a Special Issue entitled SI: Brain and Memory.

Haptically assisted connection procedure for the reconstruction of dendritic spines

Dendritic spines are thin protrusions that cover the dendritic surface of numerous neurons in the brain and whose function seems to play a key role in neural circuits. The correct segmentation of those structures is difficult due to their small size and the resulting spines can appear incomplete. This paper presents a four-step procedure for the complete reconstruction of dendritic spines. The haptically driven procedure is intended to work as an image processing stage before the automatic segmentation step giving the final representation of the dendritic spines. The procedure is designed to allow both the navigation and the volume image editing to be carried out using a haptic device. A use case employing our procedure together with a commercial software package for the segmentation stage is illustrated. Finally, the haptic editing is evaluated in two experiments; the first experiment concerns the benefits of the force feedback and the second checks the suitability of the use of a haptic device as input. In both cases, the results shows that the procedure improves the editing accuracy.

Acute modulation of synaptic plasticity of pyramidal neurons by activin in adult hippocampus

Activin A is known as a neuroprotective factor produced upon acute excitotoxic injury of the hippocampus (in pathological states). We attempt to reveal the role of activin as a neuromodulator in the adult male hippocampus under physiological conditions (in healthy states), which remains largely unknown. We showed endogenous/basal expression of activin in the hippocampal neurons. Localization of activin receptors in dendritic spines (=postsynapses) was demonstrated by immunoelectron microscopy. The incubation of hippocampal acute slices with activin A (10 ng/mL, 0.4 nM) for 2 h altered the density and morphology of spines in CA1 pyramidal neurons. The total spine density increased by 1.2-fold upon activin treatments. Activin selectively increased the density of large-head spines, without affecting middle-head and small-head spines. Blocking Erk/MAPK, PKA, or PKC prevented the activin-induced spinogenesis by reducing the density of large-head spines, independent of Smad-induced gene transcription which usually takes more than several hours. Incubation of acute slices with activin for 2 h induced the moderate early long-term potentiation (moderate LTP) upon weak theta burst stimuli. This moderate LTP induction was blocked by follistatin, MAPK inhibitor (PD98059) and inhibitor of NR2B subunit of NMDA receptors (Ro25-6981). It should be noted that the weak theta burst stimuli alone cannot induce moderate LTP. These results suggest that MAPK-induced phosphorylation of NMDA receptors (including NR2B) may play an important role for activin-induced moderate LTP. Taken together, the current results reveal interesting physiological roles of endogenous activin as a rapid synaptic modulator in the adult hippocampus.

Corticosterone rapidly increases thorns of CA3 neurons via synaptic/extranuclear glucocorticoid receptor in rat hippocampus

Modulation of synapses under acute stress is attracting much attention. Exposure to acute stress induces corticosterone (CORT) secretion from the adrenal cortex, resulting in rapid increase of CORT levels in plasma and the hippocampus. We tried to test whether rapid CORT effects involve activation of essential kinases as non-genomic processes. We demonstrated rapid effects (~1 h) of CORT on the density of thorns, by imaging Lucifer Yellow-injected neurons in adult male rat hippocampal slices. Thorns of thorny excrescences of CA3 hippocampal neurons are post-synaptic regions whose presynaptic partners are mossy fiber terminals. The application of CORT at 100, 500, and 1000 nM induced a rapid increase in the density of thorns in the stratum lucidum of CA3 pyramidal neurons. Co-administration of RU486, an antagonist of glucocorticoid receptor (GR), abolished the effect of CORT. Blocking a single kinase, including MAPK, PKA, or PKC, suppressed CORT-induced enhancement of thorn-genesis. On the other hand, GSK-3β was not involved in the signaling of thorn-genesis. Blocking AMPA receptors suppressed the CORT effect. Expression of CA3 synaptic/extranuclear GR was demonstrated by immunogold electron microscopic analysis. From these results, stress levels of CORT (100-1000 nM) might drive the rapid thorn-genesis via synaptic/extranuclear GR and multiple kinase pathways, although a role of nuclear GRs cannot be completely excluded.

Female hippocampal estrogens have a significant correlation with cyclic fluctuation of hippocampal spines

Synaptic plasticity of the female hippocampus may cyclically fluctuate across the estrous cycle. The spine density fluctuation had been explained by fluctuation of plasma estradiol (E2) and progesterone (PROG), with the assumption that these steroids penetrate into the hippocampus. Recently, however, we demonstrated that male hippocampal levels of sex steroids are much higher than those in plasma, suggesting a weak contribution of plasma steroids to the spine density. By combination of mass-spectrometric analysis with HPLC-purification and picolinoyl-derivatization of hippocampal sex steroids, we determined the accurate concentration of E2 and PROG at four stages of plasma estrous cycle including Proestrus (Pro), Estrus (Est), Diestrus 1 (D1), and Diestrus 2 (D2). Hippocampal levels of E2 and PROG showed cyclic fluctuation with a peak at Pro for E2 and at D1 for PROG, having a positive correlation with the plasma estrous cycle. All these sex steroid levels are much higher in the hippocampus than in plasma. Even after ovariectomy a significant levels of E2 and PROG were observed in the hippocampus. The total spine density showed higher values at Pro and D1, and lower values at Est and D2, having a good correlation with the peak levels of hippocampal E2 or PROG. We also examined fluctuation of the head diameter of spines. Interestingly, mRNA expression level of steroidogenic enzymes (P450arom and 17β-HSD, etc.) and sex-steroid receptors did not significantly change across the estrous cycle. Therefore, the fluctuation of total hippocampal PROG (equal to sum of hippocampus-synthesized PROG and plasma PROG) may be originated from the contribution of cyclic change in plasma PROG, which can induce the fluctuation of total hippocampal E2, since steroid conversion activity of hippocampus might be nearly the same across the estrus cycle.

Are BDNF and glucocorticoid activities calibrated?

One hypothesis to account for the onset and severity of neurological disorders is the loss of trophic support. Indeed, changes in the levels and activities of brain-derived neurotrophic factor (BDNF) occur in numerous neurodegenerative and neuropsychiatric diseases. A deficit promotes vulnerability whereas a gain of function facilitates recovery by enhancing survival, synapse formation and synaptic plasticity. Implementation of 'BDNF therapies', however, faces numerous methodological and pharmacokinetic issues. Identifying BDNF mimetics that activate the BDNF receptor or downstream targets of BDNF signaling represent an alternative approach. One mechanism that shows great promise is to study the interplay of BDNF and glucocorticoid hormones, a major class of natural steroid secreted during stress reactions and in synchrony with circadian rhythms. While small amounts of glucocorticoids support normal brain function, excess stimulation by these steroid hormones precipitates stress-related affective disorders. To date, however, because of the paucity of knowledge of underlying cellular mechanisms, deleterious effects of glucocorticoids are not prevented following extreme stress. In the present review, we will discuss the complementary roles shared by BDNF and glucocorticoids in synaptic plasticity, and delineate possible signaling mechanisms mediating these effects.

Modulation of synaptic plasticity in the hippocampus by hippocampus-derived estrogen and androgen

The hippocampus synthesizes estrogen and androgen in addition to the circulating sex steroids. Synaptic modulation by hippocampus-derived estrogen or androgen is essential to maintain healthy memory processes. Rapid actions (1-2h) of 17β-estradiol (17β-E2) occur via synapse-localized receptors (ERα or ERβ), while slow genomic E2 actions (6-48h) occur via classical nuclear receptors (ERα or ERβ). The long-term potentiation (LTP), induced by strong tetanus or theta-burst stimulation, is not further enhanced by E2 perfusion in adult rats. Interestingly, E2 perfusion can rescue corticosterone (stress hormone)-induced suppression of LTP. The long-term depression is modulated rapidly by E2 perfusion. Elevation of the E2 concentration changes rapidly the density and head structure of spines in neurons. ERα, but not ERβ, drives this enhancement of spinogenesis. Kinase networks are involved downstream of ERα. Testosterone (T) or dihydrotestosterone (DHT) also rapidly modulates spinogenesis. Newly developed Spiso-3D mathematical analysis is used to distinguish these complex effects by sex steroids and kinases. It has been doubted that the level of hippocampus-derived estrogen and androgen may not be high enough to modulate synaptic plasticity. Determination of the accurate concentration of E2, T or DHT in the hippocampus is enabled by mass-spectrometric analysis in combination with new steroid-derivatization methods. The E2 level in the hippocampus is approximately 8nM for the male and 0.5-2nM for the female, which is much higher than that in circulation. The level of T and DHT is also higher than that in circulation. Taken together, hippocampus-derived E2, T, and DHT play a major role in modulation of synaptic plasticity.

Mild exercise increases dihydrotestosterone in hippocampus providing evidence for androgenic mediation of neurogenesis

Mild exercise activates hippocampal neurons through the glutamatergic pathway and also promotes adult hippocampal neurogenesis (AHN). We hypothesized that such exercise could enhance local androgen synthesis and cause AHN because hippocampal steroid synthesis is facilitated by activated neurons via N-methyl-D-aspartate receptors. Here we addressed this question using a mild-intense treadmill running model that has been shown to be a potent AHN stimulator. A mass-spectrometric analysis demonstrated that hippocampal dihydrotestosterone increased significantly, whereas testosterone levels did not increase significantly after 2 wk of treadmill running in both orchidectomized (ORX) and sham castrated (Sham) male rats. Furthermore, analysis of mRNA expression for the two isoforms of 5α-reductases (srd5a1, srd5a2) and for androgen receptor (AR) revealed that both increased in the hippocampus after exercise, even in ORX rats. All rats were injected twice with 5'-bromo-2'deoxyuridine (50 mg/kg body weight, i.p.) on the day before training. Mild exercise significantly increased AHN in both ORX and Sham rats. Moreover, the increase of doublecortin or 5'-bromo-2'deoxyuridine/NeuN-positive cells in ORX rats was blocked by s.c. flutamide, an AR antagonist. It was also found that application of an estrogen receptor antagonist, tamoxifen, did not suppress exercise-induced AHN. These results support the hypothesis that, in male animals, mild exercise enhances hippocampal synthesis of dihydrotestosterone and increases AHN via androgenenic mediation.

Corticosterone induces rapid spinogenesis via synaptic glucocorticoid receptors and kinase networks in hippocampus

Background

Modulation of dendritic spines under acute stress is attracting much attention. Exposure to acute stress induces corticosterone (CORT) secretion from the adrenal cortex, resulting in rapid increase of CORT levels in plasma and the hippocampus.

Methodology/Principal Findings

Here we demonstrated the mechanisms of rapid effect (∼1 h) of CORT on the density and morphology of spines by imaging neurons in adult male rat hippocampal slices. The application of CORT at 100–1000 nM induced a rapid increase in the density of spines of CA1 pyramidal neurons. The density of small-head spines (0.2–0.4 µm) was increased even at low CORT levels (100–200 nM). The density of middle-head spines (0.4–0.5 µm) was increased at high CORT levels between 400–1000 nM. The density of large-head spines (0.5–1.0 µm) was increased only at 1000 nM CORT. Co-administration of RU486, an antagonist of glucocorticoid receptor (GR), abolished the effect of CORT. Blocking a single kinase, such as MAPK, PKA, PKC or PI3K, suppressed CORT-induced enhancement of spinogenesis. Blocking NMDA receptors suppressed the CORT effect.

Conclusions/Significance

These results imply that stress levels of CORT (100–1000 nM) drive the spinogenesis via synaptic GR and multiple kinase pathways.

Nanomolar dose of bisphenol A rapidly modulates spinogenesis in adult hippocampal neurons

We demonstrated the rapid effects of 10nM bisphenol A (BPA) on the spinogenesis of adult rat hippocampal slices. The density of spines was analyzed by imaging Lucifer Yellow-injected CA1 neurons in slices. Not only the total spine density but also the head diameter distribution of spine was quantitatively analyzed. Spinogenesis was significantly enhanced by BPA within 2h. In particular, the density of middle-head spine (with head diameter of 0.4-0.5μm) was significantly increased. Hydroxytamoxifen, an antagonist of both estrogen-related receptor gamma (ERRγ) and estrogen receptors (ERα/ERβ), blocked the BPA-induced enhancement of the spine density. However, ICI 182,780, an antagonist of ERα/ERβ, did not suppress the BPA effects. Therefore, ERRγ is deduced to be a high affinity receptor of BPA, responsible for modulation of spinogenesis. The BPA-induced enhancement of spinogenesis was also suppressed by MAP kinase inhibitor, PD98059, and the blocker of NMDA receptors, MK-801. Washout of BPA for additional 2h after 2h BPA treatment abolished the BPA-induced enhancement of spinogenesis, suggesting that the BPA effect was reversible. ERRγ was localized at synapses as well as cell bodies of principal neurons. ERRγ at synapses may contribute to the observed rapid effect. The level of BPA in the hippocampal slices was determined by mass-spectrometric analysis.

Automated 4D analysis of dendritic spine morphology: applications to stimulus-induced spine remodeling and pharmacological rescue in a disease model

Uncovering the mechanisms that regulate dendritic spine morphology has been limited, in part, by the lack of efficient and unbiased methods for analyzing spines. Here, we describe an automated 3D spine morphometry method and its application to spine remodeling in live neurons and spine abnormalities in a disease model. We anticipate that this approach will advance studies of synapse structure and function in brain development, plasticity, and disease.

Endogenous synthesis of corticosteroids in the hippocampus

BACKGROUND

Brain synthesis of steroids including sex-steroids is attracting much attention. The endogenous synthesis of corticosteroids in the hippocampus, however, has been doubted because of the inability to detect deoxycorticosterone (DOC) synthase, cytochrome P450(c21).

METHODOLOGY/PRINCIPAL FINDINGS

The expression of P450(c21) was demonstrated using mRNA analysis and immmunogold electron microscopic analysis in the adult male rat hippocampus. DOC production from progesterone (PROG) was demonstrated by metabolism analysis of (3)H-steroids. All the enzymes required for corticosteroid synthesis including P450(c21), P450(2D4), P450(11β1) and 3β-hydroxysteroid dehydrogenase (3β-HSD) were localized in the hippocampal principal neurons as shown via in situ hybridization and immunoelectron microscopic analysis. Accurate corticosteroid concentrations in rat hippocampus were determined by liquid chromatography-tandem mass spectrometry. In adrenalectomized rats, net hippocampus-synthesized corticosterone (CORT) and DOC were determined to 6.9 and 5.8 nM, respectively. Enhanced spinogenesis was observed in the hippocampus following application of low nanomolar (10 nM) doses of CORT for 1 h.

CONCLUSIONS/SIGNIFICANCE

These results imply the complete pathway of corticosteroid synthesis of 'pregnenolone →PROG→DOC→CORT' in the hippocampal neurons. Both P450(c21) and P450(2D4) can catalyze conversion of PROG to DOC. The low nanomolar level of CORT synthesized in hippocampal neurons may play a role in modulation of synaptic plasticity, in contrast to the stress effects by micromolar CORT from adrenal glands.

Hippocampal synthesis of sex steroids and corticosteroids: essential for modulation of synaptic plasticity

Sex steroids play essential roles in the modulation of synaptic plasticity and neuroprotection in the hippocampus. Accumulating evidence shows that hippocampal neurons synthesize both estrogen and androgen. Recently, we also revealed the hippocampal synthesis of corticosteroids. The accurate concentrations of these hippocampus-synthesized steroids are determined by liquid chromatography-tandem mass-spectrometry in combination with novel derivatization. The hippocampal levels of 17β-estradiol (E2), testosterone (T), dihydrotestosterone (DHT), and corticosterone (CORT), are 5-15 nM, and these levels are sufficient to modulate synaptic plasticity. Hippocampal E2 modulates memory-related synaptic plasticity not only slowly/genomically but also rapidly/non-genomically. Slow actions of E2 occur via classical nuclear receptors (ERα or ERβ), while rapid E2 actions occur via synapse-localized or extranuclear ERα or ERβ. Nanomolar concentrations of E2 change rapidly the density and morphology of spines in hippocampal neurons. ERα, but not ERβ, drives this enhancement/suppression of spinogenesis in adult animals. Nanomolar concentrations of androgens (T and DHT) and CORT also increase the spine density. Kinase networks are involved downstream of ERα and androgen receptor. Newly developed Spiso-3D mathematical analysis is useful to distinguish these complex effects by sex steroids and kinases. Significant advance has been achieved in investigations of rapid modulation by E2 of the long-term depression or the long-term potentiation.