Structural dynamics of synapses and synaptic components

Emerging Activity Patterns and Synaptogenesis in Dissociated Hippocampal Cultures

Cultures of dissociated hippocampal neurons display a stereotypical development of network activity patterns within the first three weeks of maturation. During this process, network connections develop and the associated spiking patterns range from increasing levels of activity in the first two weeks to regular bursting activity in the third week of maturation. Characterization of network structure is important to examine the mechanisms underlying the emergent functional organization of neural circuits. To accomplish this, confocal microscopy techniques have been used and several automated synapse quantification algorithms based on (co)localization of synaptic structures have been proposed recently. However, these approaches suffer from the arbitrary nature of intensity thresholding and the lack of correction for random-chance colocalization. To address this problem, we developed and validated an automated synapse quantification algorithm that requires minimal operator intervention. Next, we applied our approach to quantify excitatory and inhibitory synaptogenesis using confocal images of dissociated hippocampal neuronal cultures captured at 5, 8, 14 and 20 days in vitro, the time period associated with the development of distinct neuronal activity patterns. As expected, we found that synaptic density increased with maturation, coinciding with increasing spiking activity in the network. Interestingly, the third week of the maturation exhibited a reduction in excitatory synaptic density suggestive of synaptic pruning that coincided with the emergence of regular bursting activity in the network.

Medial Prefrontal Cortex Population Activity Is Plastic Irrespective of Learning

The prefrontal cortex (PFC) is thought to learn the relationships between actions and their outcomes. But little is known about what changes to population activity in PFC are specific to learning these relationships. Here we characterize the plasticity of population activity in the medial PFC (mPFC) of male rats learning rules on a Y-maze. First, we show that the population always changes its patterns of joint activity between the periods of sleep either side of a training session on the maze, regardless of successful rule learning during training. Next, by comparing the structure of population activity in sleep and training, we show that this population plasticity differs between learning and nonlearning sessions. In learning sessions, the changes in population activity in post-training sleep incorporate the changes to the population activity during training on the maze. In nonlearning sessions, the changes in sleep and training are unrelated. Finally, we show evidence that the nonlearning and learning forms of population plasticity are driven by different neuron-level changes, with the nonlearning form entirely accounted for by independent changes to the excitability of individual neurons, and the learning form also including changes to firing rate couplings between neurons. Collectively, our results suggest two different forms of population plasticity in mPFC during the learning of action-outcome relationships: one a persistent change in population activity structure decoupled from overt rule-learning, and the other a directional change driven by feedback during behavior.SIGNIFICANCE STATEMENT The PFC is thought to represent our knowledge about what action is worth doing in which context. But we do not know how the activity of neurons in PFC collectively changes when learning which actions are relevant. Here we show, in a trial-and-error task, that population activity in PFC is persistently changing, regardless of learning. Only during episodes of clear learning of relevant actions are the accompanying changes to population activity carried forward into sleep, suggesting a long-lasting form of neural plasticity. Our results suggest that representations of relevant actions in PFC are acquired by reward imposing a direction onto ongoing population plasticity.

Synaptic size dynamics as an effectively stochastic process

Long-term, repeated measurements of individual synaptic properties have revealed that synapses can undergo significant directed and spontaneous changes over time scales of minutes to weeks. These changes are presumably driven by a large number of activity-dependent and independent molecular processes, yet how these processes integrate to determine the totality of synaptic size remains unknown. Here we propose, as an alternative to detailed, mechanistic descriptions, a statistical approach to synaptic size dynamics. The basic premise of this approach is that the integrated outcome of the myriad of processes that drive synaptic size dynamics are effectively described as a combination of multiplicative and additive processes, both of which are stochastic and taken from distributions parametrically affected by physiological signals. We show that this seemingly simple model, known in probability theory as the Kesten process, can generate rich dynamics which are qualitatively similar to the dynamics of individual glutamatergic synapses recorded in long-term time-lapse experiments in ex-vivo cortical networks. Moreover, we show that this stochastic model, which is insensitive to many of its underlying details, quantitatively captures the distributions of synaptic sizes measured in these experiments, the long-term stability of such distributions and their scaling in response to pharmacological manipulations. Finally, we show that the average kinetics of new postsynaptic density formation measured in such experiments is also faithfully captured by the same model. The model thus provides a useful framework for characterizing synapse size dynamics at steady state, during initial formation of such steady states, and during their convergence to new steady states following perturbations. These findings show the strength of a simple low dimensional statistical model to quantitatively describe synapse size dynamics as the integrated result of many underlying complex processes.

A simple rule for dendritic spine and axonal bouton formation can account for cortical reorganization after focal retinal lesions

Lasting alterations in sensory input trigger massive structural and functional adaptations in cortical networks. The principles governing these experience-dependent changes are, however, poorly understood. Here, we examine whether a simple rule based on the neurons' need for homeostasis in electrical activity may serve as driving force for cortical reorganization. According to this rule, a neuron creates new spines and boutons when its level of electrical activity is below a homeostatic set-point and decreases the number of spines and boutons when its activity exceeds this set-point. In addition, neurons need a minimum level of activity to form spines and boutons. Spine and bouton formation depends solely on the neuron's own activity level, and synapses are formed by merging spines and boutons independently of activity. Using a novel computational model, we show that this simple growth rule produces neuron and network changes as observed in the visual cortex after focal retinal lesions. In the model, as in the cortex, the turnover of dendritic spines was increased strongest in the center of the lesion projection zone, while axonal boutons displayed a marked overshoot followed by pruning. Moreover, the decrease in external input was compensated for by the formation of new horizontal connections, which caused a retinotopic remapping. Homeostatic regulation may provide a unifying framework for understanding cortical reorganization, including network repair in degenerative diseases or following focal stroke.

The adult brain is less hard-wired than traditionally thought. About ten percent of synapses in the mature visual cortex is continually replaced by new ones (structural plasticity). This percentage greatly increases after lasting changes in visual input. Due to the topographically organized nerve connections from the retina in the eye to the primary visual cortex in the brain, a small circumscribed lesion in the retina leads to a defined area in the cortex that is deprived of input. Recent experimental studies have revealed that axonal sprouting and dendritic spine turnover are massively increased in and around the cortical area that is deprived of input. However, the driving forces for this structural plasticity remain unclear. Using a novel computational model, we examine whether the need for activity homeostasis of individual neurons may drive cortical reorganization after lasting changes in input activity. We show that homeostatic growth rules indeed give rise to structural and functional reorganization of neuronal networks similar to the cortical reorganization observed experimentally. Understanding the principles of structural plasticity may eventually lead to novel treatment strategies for stimulating functional reorganization after brain damage and neurodegeneration.

Simple modification of Oja rule limits L1-norm of weight vector and leads to sparse connectivity

This letter describes a simple modification of the Oja learning rule, which asymptotically constrains the L1-norm of an input weight vector instead of the L2-norm as in the original rule. This constraining is local as opposed to commonly used instant normalizations, which require the knowledge of all input weights of a neuron to update each one of them individually. The proposed rule converges to a weight vector that is sparser (has more zero weights) than the vector learned by the original Oja rule with or without the zero bound, which could explain the developmental synaptic pruning.

Critical dynamics of randomly assembled and diluted threshold networks

The dynamical behavior of a class of randomly assembled networks of binary threshold units subject to random deletion of connections is studied based on the annealed approximation suitable in the thermodynamic limit. The dynamical phase diagram is constructed for several forms of the probability density distribution of nonvanishing connection strengths. The family of power-law distribution functions rho0(x)=(1-alpha)/(2|x|alpha) is found to play a special role in expanding the domain of stable, ordered dynamics at the expense of the disordered, "chaotic" phase. Relationships with other recent studies of the dynamics of complex networks allowing for variable in-degree of the units are explored. The relevance of the pruning of network connections to neural modeling and developmental neurobiology is discussed.

Alterations of Synaptic Turnover Rate in Aging May Trigger Senile Plaque Formation and Neurodegeneration

The changes of synaptic ultrastructure were investigated by morphometry in the frontal (FC) and temporal (TC) cortex of adult and aged monkeys, to assess the potential role of age-related synaptic deterioration in neurodegeneration. The average synaptic size (S), the synaptic numeric density (Nv: number of synapses/microm(3) of tissue), the synaptic surface density (Sv: overall area of synaptic junctional zones/microm(3) of tissue), and the number of synapses/neuron (Syn/Neur) were calculated. In FC, significant differences of Nv and Sv due to age were not revealed, while the S value was significantly increased in the aged animals. In TC, Sv did not change in relation to age, whereas Nv was significantly decreased and S significantly increased in aged monkeys. A percent distribution of S showed that the fraction of enlarged synapses (>0.20 microm(2)) was higher in TC than in FC, regardless of the age of the animals (21.3% versus 16.9% in adult and 33.9% versus 26.0% in aged monkeys, respectively). In aged animals, Syn/Neur was not significantly decreased in TC and not significantly increased in FC (4.4%). The above morphometric parameters account for the ongoing rearrangements of synaptic ultrastructure, reacting to the environmental stimuli. Our findings provide evidence of an age-related decline of synaptic plasticity in the brain of aged monkeys that is statistically significant in TC. According to current literature data on synaptic structural dynamics, this decay may represent an early and subtle alteration able to trigger the development of senile plaques and neurodegenerative events.

Synaptic and mitochondrial physiopathologic changes in the aging nervous system and the role of zinc ion homeostasis

Brain performances, e.g. learning and memory, decay during aging. Deterioration of synaptic junctions, as structural correlates of these key functions of the central nervous system, may play a central role in this impairment. Current research on the age-related changes of synapses is documenting that the numeric loss of contacts appears to trigger a compensatory reaction by the old CNS, i.e. the surviving junctional areas in old individuals are larger than in adult subjects. The final outcome of the balanced changes in synaptic number and size is that the overall synaptic junctional area per cubic micron of neuropil is also reduced in aging and this may account for the age-associated functional decay of CNS performances. Among the suggested determinants of synaptic deterioration in aging, a considerable number of recent studies support an early and pivotal role of the progressive decline of the mitochondrial metabolic competence, i.e. the capacity of select pools of organelles to provide adequate amounts of adenosine triphosphate. Quantitative ultrastructural studies together with cytochemistry of key enzymes of the respiratory chain (cytochrome oxidase and succinic dehydrogenase) have shown that mitochondrial dysfunctions play an early and central role in synaptic deterioration events associated with aging and neurodegenerative diseases. Among the various causes, the multiple mechanisms and molecules involved in zinc ion homeostasis have been supposed to be less efficient in the aging brain. Thus, a transient imbalance of free zinc ion concentration in the cytosol ([Zn2+]i) can be considered an unfavourable trigger of subtle mitochondrial damage and synaptic pathology.

A simulation model for compensatory plasticity in the prefrontal cortex inducing a cortico-cortical dysconnection in early brain development

In the present work, an abstract prefrontal cortex simulation model is used to predict compensatory structural alterations of the cortico-cortical connectivity pattern in the normal and pathologic forebrain maturation. The simulation network shows different representative courses of morphogenesis when developing undisturbed or when suffering from disturbing excitatory afferences. The simulative results could be affirmed by an immuno-histochemical study, revealing a qualitatively comparable development of the glutamatergic projection fibre density in gerbils (Meriones unguiculatus) after juvenile and adult methamphetamine intoxication. The simulation model further allows to consider different rearing conditions (enriched-environment model), and claims contradictory effects of an equal disturbance after enriched or impoverished rearing which are in accordance with the experimental findings.

Storage Capacity Diverges With Synaptic Efficiency in an Associative Memory Model With Synaptic Delay and Pruning

It is known that storage capacity per synapse increases by synaptic pruning in the case of a correlation-type associative memory model. However, the storage capacity of the entire network then decreases. To overcome this difficulty, we propose decreasing the connectivity while keeping the total number of synapses constant by introducing delayed synapses. In this paper, a discrete synchronous-type model with both delayed synapses and their prunings is discussed as a concrete example of the proposal. First, we explain the Yanai-Kim theory by employing statistical neurodynamics. This theory involves macrodynamical equations for the dynamics of a network with serial delay elements. Next, considering the translational symmetry of the explained equations, we rederive macroscopic steady-state equations of the model by using the discrete Fourier transformation. The storage capacities are analyzed quantitatively. Furthermore, two types of synaptic prunings are treated analytically: random pruning and systematic pruning. As a result, it becomes clear that in both prunings, the storage capacity increases as the length of delay increases and the connectivity of the synapses decreases when the total number of synapses is constant. Moreover, an interesting fact becomes clear: the storage capacity asymptotically approaches 2/pi due to random pruning. In contrast, the storage capacity diverges in proportion to the logarithm of the length of delay by systematic pruning and the proportion constant is 4/pi. These results theoretically support the significance of pruning following an overgrowth of synapses in the brain and may suggest that the brain prefers to store dynamic attractors such as sequences and limit cycles rather than equilibrium states.

Amygdala and hippocampal volumes in Turner syndrome: a high-resolution MRI study of X-monosomy

Turner syndrome (TS) results from partial or complete X-monosomy and is characterized by deficits in visuospatial functioning as well as social cognition and memory. Neuroimaging studies have demonstrated volumetric differences in the parietal region of females with TS compared to controls. The present study examined amygdala and hippocampus morphology in an attempt to further understand the neural correlates of psychosocial and memory functioning in TS. Thirty females with TS age 7.6-33.3 years (mean = 14.7 +/- 6.4) and 29 age-matched controls (mean age = 14.8 +/- 5.9; range = 6.4-32.7) were scanned using high resolution MRI. Volumetric analyses of the MRI scans included whole brain segmentation and manual delineation of the amygdala and hippocampus. Compared to controls, participants with TS demonstrated significantly larger left amygdala gray matter volumes, irrespective of total cerebral tissue and age. Participants with TS also showed disproportionately reduced right hippocampal volumes, involving both gray and white matter. Amygdala and hippocampal volumes appear to be impacted by X-monosomy. Aberrant morphology in these regions may be related to the social cognition and memory deficits often experienced by individuals with TS. Further investigations of changes in medial temporal morphology associated with TS are warranted.

Synapse efficiency diverges due to synaptic pruning following overgrowth

In the development of the brain, it is known that synapses are pruned following overgrowth. This pruning following overgrowth seems to be a universal phenomenon that occurs in almost all areas-visual cortex, motor area, association area, and so on. It has been shown numerically that the synapse efficiency is increased by systematic deletion. We discuss the synapse efficiency to evaluate the effect of pruning following overgrowth, and analytically show that the synapse efficiency diverges as O(|ln c|) at the limit where connecting rate c is extremely small. Under a fixed synapse number criterion, the optimal connecting rate, which maximizes memory performance, exists.

Effects of X-monosomy and X-linked imprinting on superior temporal gyrus morphology in Turner syndrome

BACKGROUND

Turner syndrome (TS) results from complete or partial monosomy X. The cognitive phenotype of TS involves preservation of verbal skills with visuospatial functioning deficits. The superior temporal gyrus (STG), which is involved in language capacities, has not been investigated in TS.

METHODS

The STG was measured in 30 female subjects (mean age = 14.73 +/- 6.41; range = 7.56-33.30) with TS and 30 age-matched control subjects (mean age = 14.63 +/- 5.90; range = 6.35-32.65) using volumetric magnetic resonance imaging analyses.

RESULTS

-Right STG, including both gray and white matter volumes, was significantly larger in TS compared with control subjects. Overall left STG volume was not significantly different between groups, although left white matter volume was increased in the TS subjects. The TS subgroup with a maternally derived X chromosome (Xm) demonstrated more aberrant STG volumes compared with subjects with a paternally (Xp) derived X and control subjects. The difference in STG volumes between Xm and control subjects involved both white and gray matter. The Xm subjects differed from Xp subjects only in terms of gray matter.

CONCLUSIONS

These findings suggest that X-monosomy and X-linked imprinting negatively affect STG development, possibly by disrupting neural pruning mechanisms.

Thinking globally, acting locally: steroid hormone regulation of the dendritic architecture, synaptic connectivity and death of an individual neuron

Steroid hormones act via evolutionarily conserved nuclear receptors to regulate neuronal phenotype during development, maturity and disease. Steroid hormones exert 'global' effects in organisms to produce coordinated physiological responses whereas, at the 'local' level, individual neurons can respond to a steroidal signal in highly specific ways. This review focuses on two phenomena-the loss of dendritic processes and the programmed cell death (PCD) of neurons-that can be regulated by steroid hormones (e.g. during sexual differentiation in vertebrates). In insects such as the moth, Manduca sexta, and fruit fly, Drosophila melanogaster, ecdysteroids orchestrate a reorganization of neural circuits during metamorphosis. In Manduca, accessory planta retractor (APR) motoneurons undergo dendritic loss at the end of larval life in response to a rise in 20-hydroxyecdysone (20E). Dendritic regression is associated with a decrease in the strength of monosynaptic inputs, a decrease in the number of contacts from pre-synaptic neurons, and the loss of a behavior mediated by these synapses. The APRs in different abdominal segments undergo segment-specific PCD at pupation and adult emergence that is triggered directly and cell-autonomously by a genomic action of 20E, as demonstrated in cell culture. The post-emergence death of APRs provides a model for steroid-mediated neuroprotection. APR death occurs by autophagy, not apoptosis, and involves caspase activation and the aggregation and ultracondensation of mitochondria. Manduca genes involved in segmental identity, 20E signaling and PCD are being sought by suppressive subtractive hybridization (SSH) and cDNA microarrays. Experiments utilizing Drosophila as a complementary system have been initiated. These insect model systems contribute toward understanding the causes and functional consequences of dendritic loss and neurodegeneration in human neurological disorders.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Morphometry of E-PTA stained synapses at the periphery of pathological lesions

We carried out a novel application of the disector sampling and counting method, in a biopsy material from the pathologic human brain, to estimate the synaptic structural dynamics, quantitatively. Parietal cortex biopsies of adult (mean age: 40.0 years) and old (mean age: 66.2 years) patients having undergone surgical intervention were investigated. The tissue samples were excised at the periphery of meningioma masses. Synaptic contact zones were stained en bloc by the ethanol phosphotungstic acid (E-PTA) preferential technique which selectively enhances both the pre- and post-synaptic paramembranous material separated by a sharp cleft against a very faint background, thus facilitating and objectifying synaptic morphometry. The disector method, associated with currently used morphometric formulas, enabled us to measure the number of synapses/m3 of tissue (numeric density: Nv); the total area of the synaptic contact zones/m3 of tissue (surface density: Sv) and the average synaptic size (S). In old vs. adult patients, Nv decreased by 7.5% (Mean (SEM): Adult 2.0040(0.0452); Old 1.6780(0.0623)), while S increased by 17.5% (Adult 0.0203(0.0026); Old 0.0246(0.0035)). Sv did not show any age-related difference. The same negative correlation between Nv and S has also been reported in physiological aging, and this suggests the active presence of age-related synaptic restructuring mechanisms in the nervous tissue surrounding a tumoral mass.

Glia-derived signals induce synapse formation in neurones of the rat central nervous system

1. To study the effects of glial cells on synapse formation, we established microcultures of purified rat retinal ganglion cells (RGCs) and monitored synapse (autapse) development in single neurones using electrophysiological recordings, FM1-43 labelling and immunocytochemistry. 2. Solitary neurones grew ramifying neurites, but formed only very few and inefficient excitatory autapses, when cultured for up to 2 weeks in defined medium and in the absence of glial cells. 3. Treatment of glia-free microcultures of RGCs with glia-conditioned medium (GCM) increased the number of autapses per neurone by up to 10-fold. This was indicated by a similar increase in the frequency of spontaneous events and the number of FM1-43-labelled functional release sites and of puncta, where pre- and postsynaptic markers colocalized. 4. In addition, GCM treatment enhanced the efficacy of presynaptic transmitter release as indicated by lower failure rates of stimulation-induced excitatory autaptic currents, a 200-fold increase in the frequency of asynchronous release and an accelerated stimulation-induced FM1-43 destaining. Furthermore, GCM induced an increase in the quantal size. 5. GCM affected autaptic activity not immediately, but with a delay of 24 h, and the effects on stimulation-induced autaptic currents occurred before changes in the frequency of spontaneous events indicating an early strengthening of existing autapses followed by a later increase in autapse number. 6. The observed effects were mediated by proteinase K-sensitive factors in GCM and occurred independently of electrical activity. 7. These results suggest that soluble glia-derived signals induce synapse formation and maturation in neurones of the central nervous system (CNS).

NMDA receptor delayed maturation and schizophrenia

This paper presents the hypothesis that NMDA receptor delayed maturation (NRDM) may lead to the pathogenesis of schizophrenic psychotic symptoms. This hypothesis is further analyzed in the language of a neural modeling formulation. This formulation points to a possible chain of pathological events, leading from molecular-level NRDM to over-increased synaptic plasticity, and to the formation of pathological attractors, a putative macroscopic-level correlate of schizophrenic positive symptoms. The relations of the NRDM hypothesis to other alterations which are assumed to take place in schizophrenia are discussed, together with possible ways to test this hypothesis.

Neuronal regulation: A mechanism for synaptic pruning during brain maturation

Human and animal studies show that mammalian brains undergo massive synaptic pruning during childhood, losing about half of the synapses by puberty. We have previously shown that maintaining the network performance while synapses are deleted requires that synapses be properly modified and pruned, with the weaker synapses removed. We now show that neuronal regulation, a mechanism recently observed to maintain the average neuronal input field of a postsynaptic neuron, results in a weight-dependent synaptic modification. Under the correct range of the degradation dimension and synaptic upper bound, neuronal regulation removes the weaker synapses and judiciously modifies the remaining synapses. By deriving optimal synaptic modification functions in an excitatory-inhibitory network, we prove that neuronal regulation implements near-optimal synaptic modification and maintains the performance of a network undergoing massive synaptic pruning. These findings support the possibility that neural regulation complements the action of Hebbian synaptic changes in the self-organization of the developing brain.

Age-changes of brain synapses and synaptic plasticity in response to an enriched environment

Numerical synaptic density and synaptic vesicle density in rat frontal cortex were examined by electron microscopy as a function of age. The density of axospinous synapses, a major population of synapses, was found to peak at age 1 month, and to gradually decrease with aging. The synaptic vesicle density in axospinous synapses was shown to rapidly increase to a peak during the first 3 weeks and then decrease to the adult level, which remained unchanged in senescence. The time course of synaptic changes in aging is presented in this study. In a previous report (Saito et al. [1994] J. Neurosci. Res. 39:57-62), we showed that enriched rearing conditions restored the age-related decrease of synaptophysin contents. This might be due to increased numerical synaptic density or enhanced packing density of synaptic vesicles in synapses. The results of the present study support the latter explanation; that is, synaptic vesicle contents were increased without changes in synaptic density. Synaptic plasticity induced by environmental stimulation is shown to relate with synaptic strengthening, but not with the formation of new synapses.

Synaptic pruning in development: a computational account

Research with humans and primates shows that the developmental course of the brain involves synaptic overgrowth followed by marked selective pruning. Previous explanations have suggested that this intriguing, seemingly wasteful phenomenon is utilized to remove, "erroneous" synapses. We prove that this interpretation is wrong if synapses are Hebbian. Under limited metabolic energy resources restricting the amount and strength of synapses, we show that memory performance is maximized if synapses are first overgrown and then pruned following optimal "minimal-value" deletion. This optimal strategy leads to interesting insights concerning childhood amnesia.

Gangliosides in cerebrospinal fluid in children with autism spectrum disorders

Gangliosides are sialic acid-containing glycolipids found in all cells, especially abundant in nerve cells and mainly situated on outer-membrane surfaces. The aim of this study was to provide data on the concentration of gangliosides in the CSF of children and adolescents with autism spectrum disorders (ASD) - 66 with autistic disorder, and 19 with other autism spectrum disorders. The comparison group consisted of 29 children and adolescents, whose CSF had been sampled to exclude acute infectious CNS disorder. The concentrations of the gangliosides GM1, GD1a, GD1b, and GT1b were determined using a microimmunoaffinity technique. The ASD group had a significantly higher concentration of ganglioside GM1 compared with the comparison group. The GM1 increase could not be explained as secondary to other clinical factors. Mean ganglioside levels did not differentiate subgroups with autistic disorder and those with a more atypical clinical picture, nor subgroups with known medical disorders and those with idiopathic autism. Altered patterns of gangliosides in the CNS might reflect important correlates of pathogenesis in autism.

Synaptic runaway in associative networks and the pathogenesis of schizophrenia

Synaptic runaway denotes the formation of erroneous synapses and premature functional decline accompanying activity-dependent learning in neural networks. This work studies synaptic runaway both analytically and numerically in binary-firing associative memory networks. It turns out that synaptic runaway is of fairly moderate magnitude in these networks under normal, baseline conditions. However, it may become extensive if the threshold for Hebbian learning is reduced. These findings are combined with recent evidence for arrested N-methyl-D-aspartate (NMDA) maturation in schizophrenics, to formulate a new hypothesis concerning the pathogenesis of schizophrenic psychotic symptoms in neural terms.

Memory maintenance via neuronal regulation

Since their conception half a century ago, Hebbian cell assemblies have become a basic term in the neurosciences, and the idea that learning takes place through synaptic modifications has been accepted as a fundamental paradigm. As synapses undergo continuous metabolic turnover, adopting the stance that memories are engraved in the synaptic matrix raises a fundamental problem: How can memories be maintained for very long time periods? We present a novel solution to this long-standing question, based on biological evidence of neuronal regulation mechanisms that act to maintain neuronal activity. Our mechanism is developed within the framework of a neural model of associative memory. It is operative in conjunction with random activation of the memory system and is able to counterbalance degradation of synaptic weights and normalize the basins of attraction of all memories. Over long time periods, when the variance of the degradation process becomes important, the memory system stabilizes if its synapses are appropriately bounded. Thus, the remnant memory system is obtained by a dynamic process of synaptic selection and growth driven by neuronal regulatory mechanisms. Our model is a specific realization of dynamic stabilization of neural circuitry, which is often assumed to take place during sleep.

Dynamic expression suggests multiple roles of the eph family receptor brain-specific kinase (Bsk) during mouse neurogenesis

The eph family ligands and receptors have been implicated in mediating topographic neuron-target interactions. We recently isolated Bsk, a new member of the eph family receptors, and showed that it is expressed primarily in the brain. To investigate the role of Bsk in the development of the nervous system, we examined the temporal and spatial patterns of Bsk expression using in situ hybridization. We report here that Bsk expression exhibits dynamic changes during embryogenesis. In early embryos, Bsk is widely transcribed in the nervous system, including the forebrain, midbrain, hindbrain and spinal cord. Bsk expression in the midbrain, hindbrain and spinal cord, however, gradually decreases while in the forebrain increases over time. By embryonic day 18, the most intense Bsk expression was found in the limbic system. High levels of the expression in the limbic system persisted throughout post-natal development and remained stable in the adult up to 24 month. The topography of Bsk expression is in the form of gradients in several regions of the brain, including the lateral septum, spinal cord, as well as the hippocampus. Selective expression was also observed in Purkinje cells. Our findings on the topography of Bsk expression provide support to potential roles of Bsk in topographic projection. Our analyses further suggest that there may be other novel functions of Bsk in early neurogenesis in addition to potential roles in topographic mapping.

Rapid astroglial reactions in the motor cortex of adult rats following peripheral facial nerve lesions

We report on changes in the motor cortex of adult rats that rapidly and transiently followed various types of facial nerve lesions. These reactions led to enhanced immunoreactivities of various astroglial markers: S-100 protein (a Ca2+- and Zn2+-binding protein predominantly located in the cytosol of astrocytes), glial fibrillary acidic protein (a cytoskeletal protein) and connexin 43 (the astroglial gap junction protein). Reactions could be visualized 1 h after the facial nerve lesion and disappeared within about 5 days after surgery. Combined lesions of the facial and trigeminal nerves modified the spatial pattern of the astroglial reaction, similar to intramuscular injections of botulinum toxin, which inhibits the release of acetylcholine in motor endplates. Data presented suggest that peripheral interference with muscular functions rapidly induces modifications in the motor cortex.