Ultrastructural synaptic correlates of spatial learning in rat hippocampus

Structural Heterogeneity of the GABAergic Tripartite Synapse

The concept of the tripartite synapse describes the close interaction of pre- and postsynaptic elements and the surrounding astrocyte processes. For glutamatergic synapses, it is established that the presence of astrocytic processes and their structural arrangements varies considerably between and within brain regions and between synapses of the same neuron. In contrast, less is known about the organization of astrocytic processes at GABAergic synapses although bi-directional signaling is known to exist at these synapses too. Therefore, we established super-resolution expansion microscopy of GABAergic synapses and nearby astrocytic processes in the stratum radiatum of the mouse hippocampal CA1 region. By visualizing the presynaptic vesicular GABA transporter and the postsynaptic clustering protein gephyrin, we documented the subsynaptic heterogeneity of GABAergic synaptic contacts. We then compared the volume distribution of astrocytic processes near GABAergic synapses between individual synapses and with glutamatergic synapses. We made two novel observations. First, astrocytic processes were more abundant at the GABAergic synapses with large postsynaptic gephyrin clusters. Second, astrocytic processes were less abundant in the vicinity of GABAergic synapses compared to glutamatergic, suggesting that the latter may be selectively approached by astrocytes. Because of the GABA transporter distribution, we also speculate that this specific arrangement enables more efficient re-uptake of GABA into presynaptic terminals.

Experience-dependent plasticity in early stations of sensory processing in mature brains: effects of environmental enrichment on dendrite measures in trigeminal nuclei

Nervous systems respond with structural changes to environmental changes even in adulthood. In recent years, experience-dependent structural plasticity was shown not to be restricted to the cerebral cortex, as it also occurs at subcortical and even peripheral levels. We have previously shown that two populations of trigeminal nuclei neurons, trigeminothalamic barrelette neurons of the principal nucleus (Pr5), and intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus (Sp5C) that project to Pr5 underwent morphometric and topological changes in their dendritic trees after a prolonged total or partial loss of afferent input from the vibrissae. Here we examined whether and what structural alterations could be elicited in the dendritic trees of the same cell populations in young adult rats after being exposed for 2 months to an enriched environment (EE), and how these changes evolved when animals were returned to standard housing for an additional 2 months. Neurons were retrogradely labeled with BDA delivered to, respectively, the ventral posteromedial thalamic nucleus or Pr5. Fully labeled cells were digitally reconstructed with Neurolucida and analyzed with NeuroExplorer. EE gave rise to increases in dendritic length, number of trees and branching nodes, spatial expansion of the trees, and dendritic spines, which were less pronounced in Sp5C than in Pr5 and differed between sides. In Pr5, these parameters returned, but only partially, to control values after EE withdrawal. These results underscore a ubiquity of experience-dependent changes that should not be overlooked when interpreting neuroplasticity and developing plasticity-based therapeutic strategies.

Requirements of Postnatal proBDNF in the Hippocampus for Spatial Memory Consolidation and Neural Function

Mature brain-derived neurotrophic factor (BDNF) and its downstream signaling pathways have been implicated in regulating postnatal development and functioning of rodent brain. However, the biological role of its precursor pro-brain-derived neurotrophic factor (proBDNF) in the postnatal brain remains unknown. The expression of hippocampal proBDNF was blocked in postnatal weeks, and multiple behavioral tests, Western blot and morphological techniques, and neural recordings were employed to investigate how proBDNF played a role in spatial cognition in adults. The peak expression and its crucial effects were found in the fourth but not in the second or eighth postnatal week. Blocking proBDNF expression disrupted spatial memory consolidation rather than learning or memory retrieval. Structurally, blocking proBDNF led to the reduction in spine density and proportion of mature spines. Although blocking proBDNF did not affect N-methyl-D-aspartate (NMDA) receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits, the learning-induced phosphorylation of the GluN2B subunit level declined significantly. Functionally, paired-pulse facilitation, post-low-frequency stimulation (LFS) transiently enhanced depression, and GluN2B-dependent short-lasting long-term depression in the Schaffer collateral-CA1 pathway were weakened. The firing rate of pyramidal neurons was significantly suppressed around the target region during the memory test. Furthermore, the activation of GluN2B-mediated signaling could effectively facilitate neural function and mitigate memory impairment. The findings were consistent with the hypothesis that postnatal proBDNF played an essential role in synaptic and cognitive functions.

Spontaneous Ca2+ Fluctuations Arise in Thin Astrocytic Processes With Real 3D Geometry

Fluctuations of cytosolic Ca²⁺ concentration in astrocytes are regarded as a critical non-neuronal signal to regulate neuronal functions. Although such fluctuations can be evoked by neuronal activity, rhythmic astrocytic Ca²⁺ oscillations may also spontaneously arise. Experimental studies hint that these spontaneous astrocytic Ca²⁺ oscillations may lie behind different kinds of emerging neuronal synchronized activities, like epileptogenic bursts or slow-wave rhythms. Despite the potential importance of spontaneous Ca²⁺ oscillations in astrocytes, the mechanism by which they develop is poorly understood. Using simple 3D synapse models and kinetic data of astrocytic Glu transporters (EAATs) and the Na⁺/Ca²⁺ exchanger (NCX), we have previously shown that NCX activity alone can generate markedly stable, spontaneous Ca²⁺ oscillation in the astrocytic leaflet microdomain. Here, we extend that model by incorporating experimentally determined real 3D geometries of 208 excitatory synapses reconstructed from publicly available ultra-resolution electron microscopy datasets. Our simulations predict that the surface/volume ratio (SVR) of peri-synaptic astrocytic processes prominently dictates whether NCX-mediated spontaneous Ca²⁺ oscillations emerge. We also show that increased levels of intracellular astrocytic Na⁺ concentration facilitate the appearance of Ca²⁺ fluctuations. These results further support the principal role of the dynamical reshaping of astrocyte processes in the generation of intrinsic Ca²⁺ oscillations and their spreading over larger astrocytic compartments.

Lack of change in CA1 dendritic spine density or clustering in rats following training on a radial-arm maze task

Background: Neuronal plasticity is thought to underlie learning and memory formation. The density of dendritic spines in the CA1 region of the hippocampus has been repeatedly linked to mnemonic processes. Both the number and spatial location of the spines, in terms of proximity to nearest neighbour, have been implicated in memory formation. To examine how spatial training impacts synaptic structure in the hippocampus, Lister-Hooded rats were trained on a hippocampal-dependent spatial task in the radial-arm maze.  Methods: One group of rats were trained on a hippocampal-dependent spatial task in the radial arm maze. Two further control groups were included: a yoked group which received the same sensorimotor stimulation in the radial-maze but without a memory load, and home-cage controls. At the end of behavioural training, the brains underwent Golgi staining. Spines on CA1 pyramidal neuron dendrites were imaged and quantitatively assessed to provide measures of density and distance from nearest neighbour.  Results: There was no difference across behavioural groups either in terms of spine density or in the clustering of dendritic spines. Conclusions: Spatial learning is not always accompanied by changes in either the density or clustering of dendritic spines on the basal arbour of CA1 pyramidal neurons when assessed using Golgi imaging.

Lack of change in CA1 dendritic spine density or clustering in rats following training on a radial-arm maze task

Background: Neuronal plasticity is thought to underlie learning and memory formation. The density of dendritic spines in the CA1 region of the hippocampus has been repeatedly linked to mnemonic processes. Both the number and spatial location of the spines, in terms of proximity to nearest neighbour, have been implicated in memory formation. To examine how spatial training impacts synaptic structure in the hippocampus, Lister-Hooded rats were trained on a hippocampal-dependent spatial task in the radial-arm maze.  Methods: One group of rats were trained on a hippocampal-dependent spatial task in the radial arm maze. Two further control groups were included: a yoked group which received the same sensorimotor stimulation in the radial-maze but without a memory load, and home-cage controls. At the end of behavioural training, the brains underwent Golgi staining. Spines on CA1 pyramidal neuron dendrites were imaged and quantitatively assessed to provide measures of density and distance from nearest neighbour.  Results: There was no difference across behavioural groups either in terms of spine density or in the clustering of dendritic spines. Conclusions: Spatial learning is not always accompanied by changes in either the density or clustering of dendritic spines on the basal arbour of CA1 pyramidal neurons when assessed using Golgi imaging.

Bilingualism for Dementia: Neurological Mechanisms Associated With Functional and Structural Changes in the Brain

As the number of older adults increases, the prevalence of dementias, such as Alzheimer’s dementia (AD), vascular dementia, dementia with Lewy bodies, and frontotemporal dementias, also increases. Despite research into pharmacological approaches for treating diverse diseases, there is still no cure. Recently, novel non-pharmacological interventions are attracting attention. Non-pharmacological approaches include cognitive stimulation, alterations in diet, physical activity, and social engagement. Cognitive stimulating activities protect against the negative effects of cognitive decline caused by age-related neurogenerative diseases. Bilingualism is one form of cognitive stimulation that requires multiple aspects of brain activity and has been shown to delay the onset of dementia symptoms in patients by approximately 4–5 years as compared with monolingual patients through cognitive reserve. The purpose of this review was to bilingualism protects against cognitive decline associated with AD and other dementias. We discuss potential underlying neurological mechanisms, including: (1) stimulating adult neurogenesis, (2) enhancing synaptogenesis, (3) strengthening functional connectivity that bilingualism may delay clinical AD symptoms, (4) protecting white matter integrity, and (5) preserving gray matter density.

Sleep Deprivation by Exposure to Novel Objects Increases Synapse Density and Axon-Spine Interface in the Hippocampal CA1 Region of Adolescent Mice

Sleep has been hypothesized to rebalance overall synaptic strength after ongoing learning during waking leads to net synaptic potentiation. If so, because synaptic strength and size are correlated, synapses on average should be larger after wake and smaller after sleep. This prediction was recently confirmed in mouse cerebral cortex using serial block-face electron microscopy (SBEM). However, whether these findings extend to other brain regions is unknown. Moreover, sleep deprivation by gentle handling was reported to produce hippocampal spine loss, raising the question of whether synapse size and number are differentially affected by sleep and waking. Here we applied SBEM to measure axon-spine interface (ASI), the contact area between pre-synapse and post-synapse, and synapse density in CA1 stratum radiatum. Adolescent YFP-H mice were studied after 6-8 h of sleep (S = 6), spontaneous wake at night (W = 4) or wake enforced during the day by novelty exposure (EW = 4; males/females balanced). In each animal ≥425 ASIs were measured and synaptic vesicles were counted in ~100 synapses/mouse. Reconstructed dendrites included many small, nonperforated synapses and fewer large, perforated synapses. Relative to S, ASI sizes in perforated synapses shifted toward higher values after W and more so after EW. ASI sizes in nonperforated synapses grew after EW relative to S and W, and so did their density. ASI size correlated with presynaptic vesicle number but the proportion of readily available vesicles decreased after EW, suggesting presynaptic fatigue. Thus, CA1 synapses undergo changes consistent with sleep-dependent synaptic renormalization and their number increases after extended wake.SIGNIFICANCE STATEMENT Sleep benefits learning, memory consolidation, and the integration of new with old memories, but the underlying mechanisms remain highly debated. One hypothesis suggests that sleep's cognitive benefits stem from its ability to renormalize total synaptic strength, after ongoing learning during wake leads to net synaptic potentiation. Supporting evidence for this hypothesis mainly comes from the cerebral cortex, including the observation that cortical synapses are larger after wake and smaller after sleep. Using serial electron microscopy, we find here that sleep/wake synaptic changes consistent with sleep-dependent synaptic renormalization also occur in the CA1 region. Thus, the role of sleep in maintaining synaptic homeostasis may extend to the hippocampus, a key area for learning and synaptic plasticity.

Oxidative stress and mitochondrial dysfunction contributes to postoperative cognitive dysfunction in elderly rats

Postoperative cognitive dysfunction (POCD) is defined by cognitive impairment determined by neuropsychological tests from before to after surgery. Several mechanisms have been proposed in this bidirectional communication between the immune system and the brain after surgery. We aimed at understanding the mechanisms underlying POCD elderly rats in an experimental tibial fracture model. Elderly male Wistar rats were subjected to tibial fracture (TF) model. Control (sham) and fracture (TF) groups were followed to determine nitrite/nitrate concentration; oxidative damage to lipids and proteins; the activity of antioxidant enzymes (superoxide dismutase-SOD and catalase-CAT), mitochondrial respiratory chain enzymes, and creatine kinase (CK); and BDNF levels in the hippocampus and prefrontal cortex (at 24 h and at seven days) and cognitive function through habituation to the open field task and novel object recognition task (only at seven days). TF group presented increased concentration of nitrite/nitrate, hippocampal lipid peroxidation at seven days, protein oxidative damage in the prefrontal cortex and hippocampus at 24 h, decreased antioxidant activity in both structures on the first postoperative day and compromised function of the mitochondrial respiratory chain complexes as well as the CK enzyme. In addition, the levels of BDNF were reduced and memory function was impaired in the TF group. In conclusion, elderly rats submitted to an experimental model of tibial fracture displayed memory impairment accompanied by an increase in oxidative stress, mitochondrial dysfunction and reduced neurotrophin level.

Hippocampal mechanisms in impaired spatial learning and memory in male offspring of rats fed a low-protein isocaloric diet in pregnancy and/or lactation

Maternal nutritional challenges during fetal and neonatal development result in developmental programming of multiple offspring organ systems including brain maturation and function. A maternal low-protein diet during pregnancy and lactation impairs associative learning and motivation. We evaluated effects of a maternal low-protein diet during gestation and/or lactation on male offspring spatial learning and hippocampal neural structure. Control mothers (C) ate 20% casein and restricted mothers (R) 10% casein, providing four groups: CC, RR, CR, and RC (first letter pregnancy, second lactation diet). We evaluated the behavior of young adult male offspring around postnatal day 110. Corticosterone and ACTH were measured. Males were tested for 2 days in the Morris water maze (MWM). Stratum lucidum mossy fiber (MF) area, total and spine type in basal dendrites of stratum oriens in the hippocampal CA3 field were measured. Corticosterone and ACTH were higher in RR vs. CC. In the MWM acquisition test CC offspring required two, RC three, and CR seven sessions to learn the maze. RR did not learn in eight trials. In a retention test 24 h later, RR, CR, and RC spent more time locating the platform and performed fewer target zone entries than CC. RR and RC offspring spent less time in the target zone than CC. MF area, total, and thin spines were lower in RR, CR, and RC than CC. Mushroom spines were lower in RR and RC than CC. Stubby spines were higher in RR, CR, and RC than CC. We conclude that maternal low-protein diet impairs spatial acquisition and memory retention in male offspring, and that alterations in hippocampal presynaptic (MF), postsynaptic (spines) elements and higher glucocorticoid levels are potential mechanisms to explain these learning and memory deficits.

Developing and applying the adverse outcome pathway concept for understanding and predicting neurotoxicity

The Adverse Outcome Pathway (AOP) concept has recently been proposed to support a paradigm shift in regulatory toxicology testing and risk assessment. This concept is similar to the Mode of Action (MOA), in that it describes a sequence of measurable key events triggered by a molecular initiating event in which a stressor interacts with a biological target. The resulting cascade of key events includes molecular, cellular, structural and functional changes in biological systems, resulting in a measurable adverse outcome. Thereby, an AOP ideally provides information relevant to chemical structure-activity relationships as a basis for predicting effects of structurally similar compounds. AOPs could potentially also form the basis for qualitative and quantitative predictive modeling of the human adverse outcome resulting from molecular initiating or other key events for which higher-throughput testing methods are available or can be developed. A variety of cellular and molecular processes are known to be critical for normal function of the central (CNS) and peripheral nervous systems (PNS). Because of the biological and functional complexity of the CNS and PNS, it has been challenging to establish causative links and quantitative relationships between key events that comprise the pathways leading from chemical exposure to an adverse outcome in the nervous system. Following introduction of the principles of MOA and AOPs, examples of potential or putative adverse outcome pathways specific for developmental or adult neurotoxicity are summarized and aspects of their assessment considered. Their possible application in developing mechanistically informed Integrated Approaches to Testing and Assessment (IATA) is also discussed.

Experience Modulates the Effects of Histone Deacetylase Inhibitors on Gene and Protein Expression in the Hippocampus: Impaired Plasticity in Aging

The therapeutic potential of histone deacetylase inhibitor (HDACi) treatment has attracted considerable attention in the emerging area of cognitive neuroepigenetics. The possibility that ongoing cognitive experience importantly regulates the cell biological effects of HDACi administration, however, has not been systematically examined. In an initial experiment addressing this issue, we tested whether water maze training influences the gene expression response to acute systemic HDACi administration in the young adult rat hippocampus. Training powerfully modulated the response to HDACi treatment, increasing the total number of genes regulated to nearly 3000, including many not typically linked to neural plasticity, compared with <300 following HDACi administration alone. Although water maze training itself also regulated nearly 1800 genes, the specific mRNAs, gene networks, and biological pathways involved were largely distinct when the same experience was provided together with HDACi administration. Next, we tested whether the synaptic protein response to HDACi treatment is similarly dependent on recent cognitive experience, and whether this plasticity is altered in aged rats with memory impairment. Whereas synaptic protein labeling in the young hippocampus was selectively increased when HDACi administration was provided in conjunction with water maze training, combined treatment had no effect on synaptic proteins in the aged hippocampus. Our findings indicate that ongoing experience potently regulates the molecular consequences of HDACi treatment and that the interaction of recent cognitive experience with histone acetylation dynamics is disrupted in the aged hippocampus.

SIGNIFICANCE STATEMENT

The possibility that interventions targeting epigenetic regulation could be effective in treating a range of neurodegenerative disorders has attracted considerable interest. Here we demonstrate in the rat hippocampus that ongoing experience powerfully modifies the molecular response to one such intervention, histone deacetylase inhibitor (HDACi) administration. A single learning episode dramatically shifts the gene expression profile induced by acute HDACi treatment, yielding a qualitatively distinct hippocampal transcriptome compared with the influence of behavioral training alone. The downstream synaptic protein response to HDACi administration is similarly experience-dependent, and we report that this plasticity is disrupted in the aged hippocampus. The findings highlight that accommodating the modulatory influence of ongoing experience represents a challenge for therapeutic development in the area of cognitive neuroepigenetics.

Structural synaptic plasticity in the hippocampus induced by spatial experience and its implications in information processing

INTRODUCTION

Long-lasting memory formation requires that groups of neurons processing new information develop the ability to reproduce the patterns of neural activity acquired by experience.

DEVELOPMENT

Changes in synaptic efficiency let neurons organise to form ensembles that repeat certain activity patterns again and again. Among other changes in synaptic plasticity, structural modifications tend to be long-lasting which suggests that they underlie long-term memory. There is a large body of evidence supporting that experience promotes changes in the synaptic structure, particularly in the hippocampus.

CONCLUSION

Structural changes to the hippocampus may be functionally implicated in stabilising acquired memories and encoding new information.

Long-lasting plasticity of hippocampal adult-born neurons

Adult neurogenesis occurs in the dentate gyrus of the hippocampus, which is a key structure in learning and memory. It is believed that adult-born neurons exert their unique role in information processing due to their high plasticity during immature stage that renders them malleable in response to environmental demands. Here, we demonstrate that, in rats, there is no critical time window for experience-induced dendritic plasticity of adult-born neurons as spatial learning in the water maze sculpts the dendritic arbor of adult-born neurons even when they are several months of age. By ablating neurogenesis within a specific period of time, we found that learning was disrupted when the delay between ablation and learning was extended to several months. Together, these results show that mature adult-born neurons are still plastic when they are functionally integrated into dentate network. Our results suggest a new perspective with regard to the role of neo-neurons by highlighting that even mature ones can provide an additional source of plasticity to the brain to process memory information.

Extinction procedure induces pruning of dendritic spines in CA1 hippocampal field depending on strength of training in rats

Numerous reports indicate that learning and memory of conditioned responses are accompanied by genesis of dendritic spines in the hippocampus, although there is a conspicuous lack of information regarding spine modifications after behavioral extinction. There is ample evidence that treatments that typically produce amnesia become innocuous when animals are submitted to a procedure of enhanced training. We now report that extinction of inhibitory avoidance (IA), trained with relatively low foot-shock intensities, induces pruning of dendritic spines along the length of the apical dendrites of hippocampal CA1 neurons. When animals are trained with a relatively high foot-shock there is a high resistance to extinction, and pruning in the proximal and medial segments of the apical dendrite are seen, while spine count in the distal dendrite remains normal. These results indicate that pruning is involved in behavioral extinction, while maintenance of spines is a probable mechanism that mediates the protecting effect against amnesic treatments produced by enhanced training.

Network, cellular, and molecular mechanisms underlying long-term memory formation

The neural network stores information through activity-dependent synaptic plasticity that occurs in populations of neurons. Persistent forms of synaptic plasticity may account for long-term memory storage, and the most salient forms are the changes in the structure of synapses. The theory proposes that encoding should use a sparse code and evidence suggests that this can be achieved through offline reactivation or by sparse initial recruitment of the network units. This idea implies that in some cases the neurons that underwent structural synaptic plasticity might be a subpopulation of those originally recruited; However, it is not yet clear whether all the neurons recruited during acquisition are the ones that underwent persistent forms of synaptic plasticity and responsible for memory retrieval. To determine which neural units underlie long-term memory storage, we need to characterize which are the persistent forms of synaptic plasticity occurring in these neural ensembles and the best hints so far are the molecular signals underlying structural modifications of the synapses. Structural synaptic plasticity can be achieved by the activity of various signal transduction pathways, including the NMDA-CaMKII and ACh-MAPK. These pathways converge with the Rho family of GTPases and the consequent ERK 1/2 activation, which regulates multiple cellular functions such as protein translation, protein trafficking, and gene transcription. The most detailed explanation may come from models that allow us to determine the contribution of each piece of this fascinating puzzle that is the neuron and the neural network.

Nanoscale analysis of structural synaptic plasticity

Structural plasticity of dendritic spines and synapses is an essential mechanism to sustain long lasting changes in the brain with learning and experience. The use of electron microscopy over the last several decades has advanced our understanding of the magnitude and extent of structural plasticity at a nanoscale resolution. In particular, serial section electron microscopy (ssEM) provides accurate measurements of plasticity-related changes in synaptic size and density and distribution of key cellular resources such as polyribosomes, smooth endoplasmic reticulum, and synaptic vesicles. Careful attention to experimental and analytical approaches ensures correct interpretation of ultrastructural data and has begun to reveal the degree to which synapses undergo structural remodeling in response to physiological plasticity.

Increase of mushroom spine density in CA1 apical dendrites produced by water maze training is prevented by ovariectomy

Dendritic spine density increases after spatial learning in hippocampal CA1 pyramidal neurons. Gonadal activity also regulates spine density, and abnormally low levels of circulating estrogens are associated with deficits in hippocampus-dependent tasks. To determine if gonadal activity influences behaviorally induced structural changes in CA1, we performed a morphometric analysis on rapid Golgi-stained tissue from ovariectomized (Ovx) and sham-operated (Sham) female rats 7 days after they were given a single water maze (WM) training session (hidden platform procedure) or a swimming session in the tank containing no platform (SC). We evaluated the density of different dendritic spine types (stubby, thin, and mushroom) in three segments (distal, medial, and proximal) of the principal apical dendrite from hippocampal CA1 pyramidal neurons. Performance in the WM task was impaired in Ovx animals compared to Sham controls. Total spine density increased after WM in Sham animals in the proximal and distal CA1 apical dendrite segments but not in the medial. Interestingly, mushroom spine density consistently increased in all CA1 segments after WM. As compared to the Sham group, SC-Ovx rats showed spine pruning in all the segments, but mushroom spine density did not change significantly. In Ovx rats, WM training increased the density of stubby and thin, but not mushroom spines. Thus, ovariectomy alone produces spine pruning, while spatial learning increases spine density in spite of ovariectomy. Finally, the results suggest that mushroom spine production in CA1 after spatial learning requires gonadal activity, whereas this activity is not required for mushroom spine maintenance.

[A new chapter in the field of memory: hippocampal neo-neurogenesis]

The dogma according to which "once the development of the central nervous system ended, generation of neurons was impossible" has been challenged by the discovery that new neurons are created in specific regions of the adult mammalian brain. This discovery has been one of the most controversial of modern neuroscience. One of these regions is the dentate gyrus of the hippocampal formation, a key structure in memory. Here we will review our current knowledge on the role of adult hippocampal neurogenesis in memory and in the pathophysiology of memory. In particular we will review evidence showing that adult-born neurons are required for learning and memory and that an alteration of their production rate leads to memory impairments. We also discuss how neurogenesis is finely shaped by learning for the purpose of mnemonic information processing.

Hippocampal mossy fiber sprouting induced by forced and voluntary physical exercise

Alterations in the function and organization of synapses have been proposed to induce learning and memory. Previous studies have demonstrated that mossy fiber induced by overtraining in a spatial learning task can be related with spatial long-term memory formation. In this work we analyzed whether physical exercise could induce mossy fiber sprouting by using a zinc-detecting histologic technique (Timm). Rats were submitted to 3 and 5days of forced or voluntary exercise. Rat brains were processed for Timm's staining to analyze mossy fiber projection at 7, 12 and 30days after the last physical exercise session. A significant increase of mossy fiber terminals in the CA3 stratum oriens region was observed after 5days of forced or voluntary exercise. Interestingly, the pattern of Timm's staining in CA3 mossy fibers was significantly altered when analyzed 12days after exercise but not at 7days post-exercise. In contrast, animals trained for only 3days did not show increments of mossy fiber terminals in the stratum oriens. Altogether, these results demonstrate that sustained or programmed exercise can alter mossy fiber sprouting. Further Investigations are necessary to determine whether mossy fiber sprouting induced by exercise is also involved in learning and memory processes.

Structural plasticity and hippocampal function

The hippocampus is a region of the mammalian brain that shows an impressive capacity for structural reorganization. Preexisting neural circuits undergo modifications in dendritic complexity and synapse number, and entirely novel neural connections are formed through the process of neurogenesis. These types of structural change were once thought to be restricted to development. However, it is now generally accepted that the hippocampus remains structurally plastic throughout life. This article reviews structural plasticity in the hippocampus over the lifespan, including how it is investigated experimentally. The modulation of structural plasticity by various experiential factors as well as the possible role it may have in hippocampal functions such as learning and memory, anxiety, and stress regulation are also considered. Although significant progress has been made in many of these areas, we highlight some of the outstanding issues that remain.

Developmental periods of choline sensitivity provide an ontogenetic mechanism for regulating memory capacity and age-related dementia

In order to determine brain and behavioral sensitivity of nutrients that may serve as inductive signals during early development, we altered choline availability to rats during 7 time frames spanning embryonic day (ED) 6 through postnatal day (PD) 75 and examined spatial memory ability in the perinatally-treated adults. Two sensitive periods were identified, ED 12–17 and PD 16–30, during which choline supplementation facilitated spatial memory and produced increases in dendritic spine density in CA1 and dentate gyrus (DG) regions of the hippocampus while also changing the dendritic fields of DG granule cells. Moreover, choline supplementation during ED 12–17 only, prevented the memory decline normally observed in aged rats. These behavioral changes were strongly correlated with the acetylcholine (ACh) content of hippocampal slices following stimulated release. Our data demonstrate that the availability of choline during critical periods of brain development influences cognitive performance in adulthood and old age, and emphasize the importance of perinatal nutrition for successful cognitive aging.

Prenatal stress induces learning deficits and is associated with a decrease in granules and CA3 cell dendritic tree size in rat hippocampus

Exposure to gestational stress impairs hippocampal-dependent learning in offspring. In spite of the known decisive role of hippocampal dendritic architecture in learning and memory, there has been no study to date that examines the effect of prenatal stress on the morphology of the hippocampal neurons. Therefore we performed a quantitative morphological analysis of the dendritic architecture of Golgi-impregnated hippocampal neurons in prenatally stressed rats. Subjects included male rat offspring (2 months old) for which the mothers had been restrained for 1 h/day during the last week of gestation. Spatial learning performance levels using Morris water maze and changes in the morphology of hippocampal dendritic trees were studied. Results indicated that the study group had lower spatial learning capabilities along with decreased length and number of dendritic segments, branching of granules and cornu ammonis (CA)3 pyramidal cells. There was no change in the dendritic morphology of CA1 pyramidal cells. These results suggest that prenatal stress in rat results in spatial learning deficits and profound alterations in the neurites of the hippocampal cells of male offspring.

Brain plasticity mechanisms and memory: a party of four

A defining characteristic of the brain is its remarkable capacity to undergo activity-dependent functional and morphological remodeling via mechanisms of plasticity that form the basis of our capacity to encode and retain memories. Today, it is generally accepted that the neurobiological substrate of memories resides in activity-driven modifications of synaptic strength and structural remodeling of neural networks activated during learning. Since the discovery of long-term potentiation, the role of synaptic strengthening in learning and memory has been the subject of considerable investigation, and numerous studies have provided new insights into how this form of plasticity can subserve memory function. At the same time, other studies have explored the contribution of synaptic elimination or weakening; synaptogenesis, the growth of new synaptic connections and synapse remodeling; and more recently, neurogenesis, the birth and growth of new neurons in the adult brain. In this review, based on work in the hippocampus, the authors briefly outline recent advances in their understanding of the mechanisms and functional role of these four types of brain plasticity in the context of learning and memory. While they have long been considered as alternative mechanisms of plasticity underlying the storage of long-term memories, recent evidence suggests that they are functionally linked, suggesting the mechanisms underlying plasticity in the brain required for the formation and retention of memories are multifaceted.

The astrocytes number in different subfield of rat's hippocampus in reference memory learning method

In this study with usage of morris water maze and reference memory technique, we used 10 male albino wistar rats. Five rats in control group and 5 rats in Reference memory group. After histological preparation, the slides were stained with PTAH staining for showing the Astrocytes. Present results showed significant difference in astrocytes number in CA1, CA2 and CA3 area of hippocampus between control and reference memory group. The number of astrocytes is increased in reference memory group. Then we divided the hippocampus to three parts: Anterior, middle and posterior and with compare of different area (CA1, CA2 and CA3) of hippocampus, we found that the increase of astrocytes number in posterior two-third of CA2 and CA3 is more than of it's number in the anterior one-third.

Ultrastructural plasticity associated with hippocampal-dependent learning: A meta-analysis

In order to develop a profile of how individual synapses in the hippocampal formation alter their structure following learning experience, a meta-analysis synthesized the available literature on morphological change following hippocampal-dependent learning. Analysis of the 132 calculated effect sizes suggest a consistent profile of morphological change in the hippocampus following learning experience. Across the hippocampal formation, dendritic complexity, spine density, and the size of perforated postsynaptic densities showed consistent increases following training. Both the density of synapses in general and perforated synapses in particular showed unique responses to training, depending on the duration of training and/or different cell layers of the hippocampal formation. Most importantly, it seems that this profile, while consistent, is small and specific--only a select few of the morphological parameters typically measured in anatomical studies of plasticity showed significant change following training. Collectively, these data suggest that the distinct electrophysiological properties of neocortical versus hippocampal synapses may be at least partially mediated by distinct morphological cascades. That is, on the basis of theory, and with the support of the current data, it seems that synaptogenesis correlates with enduring neocortical plasticity, while structural changes correlate with more transient hippocampal plasticity. To be able to state these conclusions with conviction, however, more data are needed in several key areas for continued pursuit of the morphological correlates of hippocampal-dependent learning.

Associative Pavlovian conditioning leads to an increase in spinophilin‐immunoreactive dendritic spines in the lateral amygdala

Changes in dendritic spine number and shape are believed to reflect structural plasticity consequent to learning. Previous studies have strongly suggested that the dorsal subnucleus of the lateral amygdala is an important site of physiological plasticity in Pavlovian fear conditioning. In the present study, we examined the effect of auditory fear conditioning on dendritic spine numbers in the dorsal subnucleus of the lateral amygdala using an immunolabelling procedure to visualize the spine-associated protein spinophilin. Associatively conditioned rats that received paired tone and shock presentations had 35% more total spinophilin-immunoreactive spines than animals that had unpaired stimulation, consistent with the idea that changes in the number of dendritic spines occur during learning and account in part for memory.

Reversible changes in hippocampal CA1 synapses associated with water maze training in rats

Long-term memories seem to require protein synthesis to be established. This process can be related with synaptogenesis resulting in changes in the form or even in the number or proportion of synaptic contacts. Results from behavioral studies assessing quantitative changes associated with different learning tasks are controversial. The aim of our work was to assess whether the number of CA1 hippocampal synaptic contacts can be modified after training in different tasks in the Morris water maze (MWM). We found transient changes in the synaptic density of the symmetric synapses associated with place learning. A reduced synaptic density of the symmetric synapses in the stratum radiatum of CA1 was found at 48 h posttraining, returning to control levels 72 h posttraining. The same effect was observed 1 h after training in a nonspatial task. Synaptic changes found in the CA1 shortly after water maze training suggest a possible participation of the hippocampus in the acquisition of nonspatial tasks together with a role in the short-term consolidation of spatial memory. As no changes were found in the total number of synapses counted, it is likely that subtle changes in synaptic efficacy than new synapse generation may be sufficient to support the acquisition and maintenance of new memories.

Simultaneous olfactory discrimination elicits a strain-specific increase in dendritic spines in the hippocampus of inbred mice

This study examines the extent to which simultaneous olfactory discrimination learning increases spine density on hippocampal CA1 pyramidal neurons in C57BL/6J (C57) and DBA/2J (DBA) inbred mice, characterized by spontaneous differences in hippocampal plasticity and hippocampus-related learning. The behavioral data first showed a clear-cut difference in performance between the two strains. C57 mice learned to identify the positively reinforced olfactory cue whereas DBA did not. Both strains, however, similarly acquired the procedural aspects of the task. The morphological analysis performed 24 h post-training revealed that spine density was significantly increased along apical, oblique, and basal dendrites in trained C57 mice compared to trained DBA mice, and to pseudotrained as well as to control cage mice of both strains. These findings confirm the ability of C57 mice to solve hippocampal-dependent tasks and provide the first evidence that simultaneous olfactory discrimination learning elicits spine growth in the mouse hippocampus. In addition, the finding that DBA mice failed to discriminate between the two olfactory cues but were as efficient as C57 mice in learning the procedural aspects of the task outlines that the structural changes observed in the latter strain were independent from any procedural learning component.

Influence of predator stress on the consolidation versus retrieval of long-term spatial memory and hippocampal spinogenesis

We have studied the influence of predator stress (30 min of cat exposure) on long-term (24 h) spatial memory and the density of spines in basilar dendrites of CA1 neurons. Predator stress occurred either immediately before water maze training (Stress Pre-Training) or before the 24 h memory test (Stress Pre-Retrieval). The Control (nonstress) group exhibited excellent long-term spatial memory and a robust increase in the density of stubby, but not mushroom, shaped spines. The Stress Pre-Training group had impaired long-term memory and did not exhibit any changes in spine density. The Stress Pre-Retrieval group was also impaired in long-term memory performance, but this group exhibited an increase in the density of stubby, but not mushroom, shaped spines, which was indistinguishable from the control group. These findings indicate that: (1) A single day of water maze training under control conditions produced intact long-term memory and an increase in the density of stubby spines in CA1; (2) Stress before training interfered with the consolidation of information into long-term memory and suppressed the training-induced increase in spine density; and (3) Stress immediately before the 24 h memory test trial impaired the retrieval of the stored memory, but did not reverse the training-induced increase in CA1 spine density. Overall, this work provides evidence of structural plasticity in dendrites of CA1 neurons which may be involved in the consolidation process, and how spinogenesis and memory are modulated by stress.

Modulation of dendritic spines in epilepsy: cellular mechanisms and functional implications

Epilepsy patients often suffer from significant neurological deficits, including memory impairment, behavioral problems, and psychiatric disorders. While the causes of neuropsychological dysfunction in epilepsy are multifactorial, accumulating evidence indicates that seizures themselves may directly cause brain injury. Although seizures sometimes result in neuronal death, they may also cause more subtle pathological changes in neuronal structure and function, including abnormalities in synaptic transmission. Dendritic spines receive a majority of the excitatory synaptic inputs to cortical neurons and are critically involved in synaptic plasticity and learning. Studies of human epilepsy and experimental animal models demonstrate that seizures may directly affect the morphological and functional properties of dendritic spines, suggesting that seizure-related changes in spines may represent a mechanistic basis for cognitive deficits in epilepsy. Novel therapeutic strategies directed at modulation of spine motility may prevent the detrimental effects of seizures on cognitive function in epilepsy.

The environmental estrogen bisphenol a inhibits estradiol-induced hippocampal synaptogenesis

Bisphenol A (BPA) is an estrogenic chemical that is widely used in the manufacture of plastics and epoxy resins. Because BPA leaches out of plastic food and drink containers, as well as the BPA-containing plastics used in dental prostheses and sealants, considerable potential exists for human exposure to this compound. In this article we show that treatment of ovariectomized rats with BPA dose-dependently inhibits the estrogen-induced formation of dendritic spine synapses on pyramidal neurons in the CA1 area of the hippocampus. Significant inhibitory effects of BPA were observed at a dose of only 40 microg/kg, below the current U.S. Environmental Protection Agency reference daily limit for human exposure. Because synaptic remodeling has been postulated to contribute to the rapid effects of estrogen on hippocampus-dependent memory, these data suggest that environmental BPA exposure may interfere with the development and expression of normal sex differences in cognitive function, via inhibition of estrogen-dependent hippocampal synapse formation. It may also exacerbate the impairment of hippocampal function observed during normal aging, as endogenous estrogen production declines.

Evaluation of DOI, 8-OH-DPAT, eticlopride and amphetamine on impulsive responding in a reaction time task in rats

We examined the effects of DOI (2,5-dimethoxy-4-iodoamphetamine), 8-OH-DPAT (8-hydroxy-2-(N,N-dipropylamino)tetralin, eticlopride and amphetamine in a reaction time (RT) task. In this task a trial is initiated after a rat pushes a panel. Rats have to wait (0.5-1.5 s) until a tone is presented before making a response. The number of premature responses, releasing the panel before tone was switched on, was taken as a measure of motor impulsivity. A group of 10 Lewis rats was tested in the RT task after treatment with different doses of drugs which have been shown previously to affect impulsive responding: DOI (0.1, 0.2 mg/kg), 8-OH-DPAT (0.1, 0.3 mg/kg), eticlopride (0.01, 0.03 mg/kg) and D-amphetamine (0.3, 1 mg/kg). A progressive ratio test was used to control for drug effects on food motivation. DOI (0.1 mg/kg) and D-amphetamine (0.3 mg/kg) increased impulsive responding in the RT task. Conversely, 8-OH-DPAT decreased impulsive responding in the RT task. These effects of DOI, D-amphetamine and 8-OH-DPAT on impulsive responding were not associated with changes in food motivation, as assessed by performance in the progressive ratio task. Eticlopride did not affect impulsive responding. The present data suggest that 5-HT2A receptors and dopamine (but not D2 receptors) are associated with motor impulsivity.

Selective serotonin reuptake inhibitor treatment of early postnatal mice reverses their prenatal stress-induced brain dysfunction

Prenatal stress has long-lasting effects on cognitive function and on the hypothalamic-pituitary-adrenal response to stress. We previously reported that the serotonin concentration and synaptic density in the hippocampus were reduced following prenatal stress [Int J Dev Neurosci 16 (1998) 209]. Since serotonin plays a role in the formation and maintenance of synapses, we hypothesized that a neonatal reduction in hippocampal serotonin levels may lead to learning disabilities in prenatally stressed mice. To test this hypothesis, we treated prenatally stressed mice with a selective serotonin reuptake inhibitor in order to normalize their postnatal serotonin turnover levels. What we found was that the oral administration of a selective serotonin reuptake inhibitor to prenatally stressed mice during postnatal weeks 1-3 but not 6-8 normalized their corticosterone response to stress, serotonin turnover in the hippocampus, and density of dendritic spines and synapses in the hippocampal CA3 region. Concomitantly, such treatment partially restored their ability to learn spatial information.

New spines, new memories

For more than a century dendritic spines have been a source of fascination and speculation. The long-held belief that these anatomical structures are involved in learning and memory are addressed. Specifically, two lines of evidence that support this claim are reviewed. In the first, we review evidence that experimental manipulations that affect dendritic spine number in the hippocampus also affect learning processes of various sorts. In the second, we review evidence that learning itself affects the presence of dendritic spines in the hippocampus. Based on these observations, we propose that the presence of spines enhances synaptic efficacy and thereby the excitability of the network involved in the learning process. With this scheme, learning is not dependent on changes in spine density but rather changes in the presence of dendritic spines provide anatomical support for the processing of novel information used in memory formation.

Long-term synaptic morphometry changes after induction of long-term potentiation and long-term depression in the dentate gyrus of awake rats are not simply mirror phenomena

Mechanisms of expression of long-term synaptic plasticity are believed to involve morphological changes of the activated synapses and remodelling of connectivity. Here, we investigated changes in synaptic and neuronal parameters in the dentate gyrus 24 h after induction of long-term potentiation (LTP) and long-term depression (LTD) in awake rats. In dentate granule cells, tetanization of the medial or lateral perforant paths induces LTP in specific synaptic bands along the dendrites in the middle and outer molecular layers, respectively, and tetanization of the lateral path induces robust LTD heterosynaptically in the middle molecular layer. This functional segregation allowed us to assess morphological changes associated with LTP and LTD in each pathway in the same population of neurons. Electron microscopy and unbiased counting methods were used to estimate neuronal density, axospinous, axodendritic and perforated synapse density, multiple synapse bouton density and postsynaptic density (PSD) area. Whereas there was no change in neuronal density, PSD area and multiple synapse boutons 24 h after either LTP or LTD, there was a noninput-specific increase in unperforated axospinous synapses after both LTP and LTD. However, we found that LTP of the medial, but not lateral, perforant path is associated with a specific increase in perforated axospinous synapses in the potentiated area. We also show that heterosynaptic LTD is associated with an input-specific increase in axodendritic synapse density. These results suggest that each perforant pathway may differ with respect to the nature of LTP-induced long-term changes and show that morphologically LTD is not simply the converse of LTP.

Rapid reversal of stress induced loss of synapses in CA3 of rat hippocampus following water maze training

The impact was examined of exposing rats to two life experiences of a very different nature (stress and learning) on synaptic structures in hippocampal area CA3. Rats were subjected to either (i) chronic restraint stress for 21 days, and/or (ii) spatial training in a Morris water maze. At the behavioural level, restraint stress induced an impairment of acquisition of the spatial response. Moreover, restraint stress and water maze training had contrasting impacts on CA3 synaptic morphometry. Chronic stress induced a loss of simple asymmetric synapses [those with an unperforated postsynaptic density (PSD)], whilst water maze learning reversed this effect, promoting a rapid recovery of stress-induced synaptic loss within 2-3 days following stress. In addition, in unstressed animals a correlation was found between learning efficiency and the density of synapses with an unperforated PSD: the better the performance in the water maze, the lower the synaptic density. Water maze training increased the number of perforated synapses (those with a segmented PSD) in CA3, both in stressed and, more notably, in unstressed rats. The distinct effects of stress and learning on CA3 synapses reported here provide a neuroanatomical basis for the reported divergent effects of these experiences on hippocampal synaptic activity, i.e. stress as a suppressor and learning as a promoter of synaptic plasticity.

Morphological changes in hippocampal dentate gyrus synapses following spatial learning in rats are transient

The hippocampus is believed to play a crucial role in the formation of memory for spatial tasks. In the present study quantitative electron microscopy was used to investigate morphological changes in the hippocampal dentate gyrus of 3-month-old male rats at 3, 9 and 24 h after training to find a hidden platform in a Morris water maze. Average escape latency (time taken to reach the platform) in all trained groups decreased progressively with increased training but data from a probe trial (quadrant analysis test) at the end of training indicated that only animals in the 9- and 24-h groups, not the 3-h group, displayed significant retention of platform location. Unbiased stereological methods were used to estimate synapse and neuronal density at each time point after training. The majority of synapses had unperforated postsynaptic densities, were localized on small dendritic spines and were classed as axo-spinous. In comparison to age-matched untrained rats, significant but transient increases were observed in axo-spinous synapse density and synapse-to-neuron ratio 9 h after the start of training, but not at earlier (3 h) or later (24 h) times. These changes at 9 h post-training were accompanied by transient decreases in both mean synaptic height and area of postsynaptic density. No such changes were observed in an exercise-matched control group of rats, indicating that the transient synaptic changes in the dentate gyrus are most likely to be specifically related to processes involved in memory formation for the spatial learning task.

Role of the medial septum diagonal band of Broca cholinergic neurons in oestrogen-induced spine synapse formation on hippocampal CA1 pyramidal cells of female rats

Oestrogen is known to influence pyramidal cell spine synapse plasticity in the CA1 subfield of the hippocampus. Apart from direct oestrogen action on the hippocampus, oestrogen effects mediated by subcortical structures are known to be important. The purpose of this study was to investigate whether the medial septum diagonal band of Broca (MSDB) takes part in mediating oestrogen effects to the hippocampus. Special attention was given to the role of cholinergic MSDB neurons that project to the hippocampus, as a rather large population of them contains oestrogen receptors and, consequently, may be sensitive to oestrogen signals. Adult female rats were ovariectomized. Oestradiol- and cholesterol-filled cannulae (control) were implanted into the MSDB. To selectively eliminate the cholinergic population of MSDB neurons of oestrogen-treated animals, a group of rats was injected with 192 IgG-saporin (SAP) into the lateral ventricle 1 week before the cannula implant. Immunostaining with anti-choline acetyltransferase and parvalbumin (PA) showed that cholinergic but not PA-containing GABAergic neurons were substantially reduced in the MSDB of SAP rats. Comparative electron microscopic unbiased stereological analysis on the spine synapse density of CA1 area pyramidal cells was performed between all animal groups. Rats that received oestradiol-filled cannulae showed a higher (30%) spine synapse density than control animals. Oestrogen-treated rats that had received SAP treatment showed no significant difference to controls. Thus, this observation indicates that septo-hippocampal cholinergic neurons are involved in mediating oestrogen effects to the hippocampus. The relevance of this observation to mnemonic functions and Alzheimer's disease is discussed.

Gonadal hormones affect spine synaptic density in the CA1 hippocampal subfield of male rats

The effects of androgen on the density of spine synapses on pyramidal neurons in the CA1 area of the hippocampus were studied in male rats. Gonadectomy (GDNX) had no significant effect on the number of CA1 pyramidal cells but reduced CA1 spine synapse density by almost 50% (to 0.468 +/- 0.018 spine synapses/microm(3)) compared with sham-operated controls (0.917 +/- 0.06 spine synapses/microm(3)). Treatment of GDNX rats with testosterone propionate (500 microg/d, s.c., 2 d) increased spine synapse density to levels (1.01 +/- 0.026 spine synapses/microm(3)) comparable with intact males. A similar increase in synapse density (1.013 +/- 0.05 spine synapses/microm(3)) was observed in GDNX animals after treatment with dihydrotestosterone (DHT) (500 microg/d, s.c., 2 d) but not after estradiol (10 microg/d, s.c., 2 d; 0.455 +/- 0.02 spine synapse/microm(3)). These data indicate that testosterone is important for maintenance of normal spine synapse density in the CA1 region of the male rat hippocampus. The comparable responses to testosterone and the non-aromatizable androgen DHT, coupled with the lack of response to estradiol, suggest that testosterone acts directly on hippocampal androgen receptors rather than indirectly via local estrogen biosynthesis.

Associative memory formation increases the observation of dendritic spines in the hippocampus

Dendritic spines are sources of synaptic contact that can be altered by experience and, as such, may be involved in memories for that experience. Here we tested whether the acquisition of new memories is associated with changes in the density of dendritic spines. Adult male rats were trained using the trace eyeblink conditioning paradigm, an associative learning task that requires the hippocampus for acquisition. Additional groups were exposed to the same number of stimuli presented in an explicitly unpaired manner or were naive. Twenty-four hours later, the density of dendritic spines was measured using Golgi impregnation. Trace conditioning was associated with an increase in the density of dendritic spines on the pyramidal cells of area CA1 of the hippocampus, an effect that was prevented by blocking acquisition of the learned response with a competitive NMDA receptor antagonist. Training with delay conditioning, a similar task that does not require the hippocampus, also produced an increase in spine density. The learning-induced increase in dendritic spine density was specific to basal dendrites of pyramidal cells in the CA1 region of the hippocampus. Changes did not occur on their apical dendrites or on cells in the dentate gyrus or somatosensory cortex. These results suggest that the formation and expression of associative memories increase the availability of dendritic spines and the potential for synaptic contact.

FK960, a potential anti-dementia drug, increases synaptic density in the hippocampal CA3 region of aged rats

There is accumulating evidence suggesting that synapse formation in the adult brain is dynamically regulated, and that this regulation plays a role in cognitive function. A decrease in synaptic density is reportedly related to memory deficits in aged animals as well as in Alzheimer's patients. FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel anti-dementia drug, has been shown to ameliorate experimental amnesia in rats and monkeys through activation of the somatostatinergic nervous system in the hippocampus. Furthermore, FK960 has been shown to be considerably more effective in a model of spontaneous amnesia in aged rats than cholinesterase inhibitors. In the present electron microscopy study, we demonstrated that the density of axodendritic and axosomatic synapses in the hippocampal CA3 region of aged rats was reduced compared to young rats, and that repeated treatment with FK960 for either 3 or 21 days dose-dependently reversed these deficits in aged rats. This FK960-induced increase in synaptic density was transient and density returned to basal levels at 8 days after the final dose. In contrast, FK960 did not alter synaptic density in the cingulate cortex or hippocampal CA1 region in aged rats, nor the CA3 region of young rats. Collectively, these results suggest that FK960 can selectively and reversibly increase synaptic density in the hippocampal CA3 region of aged rats, and that this activity may play a role in its cognitive-enhancing action.

Gonadal hormones are responsible for maintaining the integrity of spine synapses in the CA1 hippocampal subfield of female nonhuman primates

It is well established that gonadal hormonal manipulation results in morphologic changes in the rat hippocampus. The great similarities in the hippocampal formation between nonhuman primates and humans, as well as the differences in this structure between humans and rats, led to this investigation of whether hormonal manipulation in female subhuman primates influences pyramidal cell spine density in the CA1 hippocampal subfield, as it does in rats. African green monkeys (Cercopithecus aethiops sabaeus) were ovariectomized, and half of the animals received estrogen replacement therapy. One month later, the monkeys were killed. In the first group of experiments, pyramidal cell spines were analyzed on Golgi-impregnated material taken from the CA1 hippocampal subfield. In the second experiment, unbiased electron microscopic stereologic calculations were performed to estimate the volumetric density of spine synapses in the same hippocampal subfield. Analysis of the Golgi-impregnated material showed that the spine density of CA1 pyramidal cells is much lower in the ovariectomized animals than in ovariectomized and estrogen-replaced monkeys. The unbiased, electron microscopic, stereologic calculation confirmed the light microscopic observation. The volumetric density (number of spine synapses/microm(3)) of spine synapses was significantly lower (43.33%) in the ovariectomized animals than in ovariectomized and estrogen-replaced monkeys. Because the hippocampus is involved in specific mnemonic functions, this observation highlights the importance of hormone replacement therapy in postmenopausal conditions.

Structure and function of dendritic spines

Spines are neuronal protrusions, each of which receives input typically from one excitatory synapse. They contain neurotransmitter receptors, organelles, and signaling systems essential for synaptic function and plasticity. Numerous brain disorders are associated with abnormal dendritic spines. Spine formation, plasticity, and maintenance depend on synaptic activity and can be modulated by sensory experience. Studies of compartmentalization have shown that spines serve primarily as biochemical, rather than electrical, compartments. In particular, recent work has highlighted that spines are highly specialized compartments for rapid large-amplitude Ca(2+) signals underlying the induction of synaptic plasticity.