Synaptic reorganization in developing and adult nervous systems

Mechanisms and adaptations that shape division of labour in stingless bees

Stingless bees are a diverse and ecologically important group of pollinators in the tropics. Division of labour allows bee colonies to meet the various demands of their social life, but has been studied in only ∼3% of all described stingless bee species. The available data suggest that division of labour shows both parallels and striking differences compared with other social bees. Worker age is a reliable predictor of worker behaviour in many species, while morphological variation in body size or differences in brain structure are important for specific worker tasks in some species. Stingless bees provide opportunities to confirm general patterns of division of labour, but they also offer prospects to discover and study novel mechanisms underlying the different lifestyles found in eusocial bees.

Brain size and behavioral specialization in the jataí stingless bee (Tetragonisca angustula)

Social insects are instructive models for understanding the association between investment in brain size and behavioral variability because they show a relatively simple nervous system associated with a large set of complex behaviors. In the jataí stingless bee (Tetragonisca angustula), division of labor relies both on age and body size differences among workers. When young, both minors and soldiers engage in intranidal tasks and move to extranidal tasks as they age. Minors switch to foraging activities, while soldiers take over defensive roles. Nest defense performed by soldiers includes two different tasks: (1) hovering around the nest entrance for the detection and interception of heterospecific bees (a task relying mostly on vision) and (2) standing at the nest entrance tube for inspection of returning foragers and discrimination against conspecific non-nestmates based on olfactory cues. Here, using different-sized individuals (minors and soldiers) as well as same-sized individuals (hovering and standing soldiers) performing distinct tasks, we investigated the effects of both morphological and behavioral variability on brain size. We found a negative allometric growth between brain size and body size across jataí workers, meaning that minors had relatively larger brains than soldiers. Between soldier types, we found that hovering soldiers had larger brain compartments related to visual processing (the optic lobes) and learning (the mushroom bodies). Brain size differences between jataí soldiers thus correspond to behavioral specialization in defense (i.e., vision for hovering soldiers) and illustrate a functional neuroplasticity underpinning division of labor.

Cerebellar Volume Is Associated with Cognitive Decline in Mild Cognitive Impairment: Results from ADNI

Alzheimer's disease (AD) is a disease with dysfunctional brain network. Previous studies found the cerebellar volume changes over the course of AD disease progression; however, whether cerebellar volume change contributes to the cognitive decline in AD, or its earlier disease stage (i.e., mild cognitive impairment [MCI]) remains unclear. In ADNI, cognitive function was assessed using Alzheimer's Disease Assessment Scale-Cognitive Behavior section (ADAS-Cog). We used linear regression and linear mixed effects models to examine whether cerebellar volume is associated with either baseline cognition or with cognitive changes over time in MCI or in AD. We used logistic regression to assess the relationship between cerebellar volume and disease progression to MCI and AD. We found that cerebellar volume is associated with cognition in patients with MCI, after adjusting for age, gender, education, hippocampal volume, and APOE4 status. Consistently, cerebellar volume is associated with increased odds of the disease stages of MCI and AD when compared to controls. However, cerebellar volume is not associated with cognitive changes over time in either MCI or AD. In summary, cerebellar volume may contribute to cognition level in MCI, but not in AD, indicating that the cerebellar network might modulate the cognitive function in the early stage of the disease. The cerebellum may be a potential target for neuromodulation in treating MCI.

Developmental neuroplasticity and the origin of neurodegenerative diseases

OBJECTIVES

Neurodegenerative diseases like Alzheimer's and Parkinson's Disease, marked by characteristic protein aggregations, are more and more accepted to be synaptic disorders and to arise from a combination of genetic and environmental factors. In this review we propose our concept that neuroplasticity might constitute a link between early life challenges and neurodegeneration.

METHODS

After introducing the general principles of neuroplasticity, we show how adverse environmental stimuli during development impact adult neuroplasticity and might lead to neurodegenerative processes.

RESULTS

There are significant overlaps between neurodevelopmental and neurodegenerative processes. Proteins that represent hallmarks of neurodegeneration are involved in plastic processes under physiological conditions. Brain regions - particularly the hippocampus - that retain life-long plastic capacities are the key targets of neurodegeneration. Neuroplasticity is highest in young age making the brain more susceptible to external influences than later in life. Impacts during critical periods have life-long consequences on neuroplasticity and structural self-organization and are known to be common risk factors for neurodegenerative diseases.

CONCLUSIONS

Several lines of evidence support a link between developmental neuroplasticity and neurodegenerative processes later in life. A deeper insight into these processes is necessary to design strategies to mitigate or even prevent neurodegenerative pathologies.

A role for inhibition in deafness-induced plasticity of the avian auditory brainstem

To better understand the effects of deafness on the brain, these experiments examine how disrupted balance between excitatory and inhibitory neurotransmission following the loss of excitatory input from the auditory nerve alters the central auditory system. In the avian cochlear nucleus, nucleus magnocellularis (NM), deprivation of excitatory input induced by deafness triggers neuronal death. While this neuronal death was previously accredited to the loss of excitatory drive, the present experiments examine an alternative hypothesis: that inhibitory input to NM, which may also be affected by deafness, contributes to neuronal death in NM. Using an in vitro slice preparation in which excitatory input from the auditory nerve is absent, we pharmacologically altered GABA receptor activation in NM, and assayed an early marker of neuronal health, antigenicity for the ribosomal antibody Y10B (Y10B-ir). We found that GABA decreases Y10B-ir, and that GABAA activation is necessary for the GABA-induced effect. We further found that endogenous GABAA activation similarly decreases Y10B-ir and this decrease requires extracellular Ca(2+). Our results suggest that, in the absence of excitatory input, endogenous activation of ionotropic GABAA receptors is detrimental to NM neurons.

Selected gene profiles of stressed NSC-34 cells and rat spinal cord following peripheral nerve reconstruction and minocycline treatment

The present study was conducted to investigate the effects of minocycline on the expression of selected transcriptional and translational profiles in the rat spinal cord following sciatic nerve (SNR) transection and microsurgical coaptation. The mRNA and protein expression levels of B cell lymphoma-2 (Bcl-2), Bcl-2-associated X protein (Bax), caspase-3, major histocompatibility complex I (MHC I), tumor necrosis factor-α (TNF-α), activating transcription factor 3 (ATF3), vascular endothelial growth factor (VEGF), matrix metalloproteinase 9 (MMP9), and growth associated protein-43 (GAP-43) were monitored in the rat lumbar spinal cord following microsurgical reconstruction of the sciatic nerves and minocycline treatment. The present study used semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. As a PCR analysis of spinal cord tissue enabled the examination of the expression patterns of all cell types including glia, the motorneuron-like NSC-34 cell line was used to investigate expression level changes in motorneurons. As stressors, oxygen glucose deprivation (OGD) and lipopolysaccharide (LPS) treatment were performed. SNR did not induce significant degeneration of ventral horn motorneurons, whereas microglia activation and synaptic terminal retraction were detectable. All genes were constitutively expressed at the mRNA and protein levels in untreated spinal cord and control cells. SNR significantly increased the mRNA expression levels of all genes, albeit only temporarily. In all genes except MMP9 and GAP-43, the induction was seen ipsilaterally and contralaterally. The effects of minocycline were moderate. The expression levels of MMP9, TNF-α, MHC I, VEGF, and GAP-43 were reduced, whereas those of Bax and Bcl-2 were unaffected. OGD, but not LPS, was toxic for NSC-34 cells. No changes in the expression levels of Bax, caspase-3, MHC I or ATF3 were observed. These results indicated that motorneurons were not preferentially or solely responsible for SNR-mediated upregulation of these genes. MMP9, TNF-α, VEGF and Bcl-2 were stress-activated. These results suggest that a substantial participation of motorneurons in gene expression levels in vivo. Minocycline was also shown to have inhibitory effects. The nuclear factor-κB signalling pathway may be a possible target of minocycline.

Neuroprotective effect of tempol (4 hydroxy-tempo) on neuronal death induced by sciatic nerve transection in neonatal rats

Peripheral nerve injury in newborn rats triggers extensive neuronal death within the spinal cord. Because most neurodegeneration is related to oxidative stress and apoptosis, the use of antioxidants may be of therapeutic interest. Tempol is promising because of its ability to chelate reactive oxygen species and to minimize or even prevent tissue damage. Here, we evaluated neuroprotective effects of tempol following neonatal sciatic nerve transection. Two-day-old pups underwent sciatic nerve axotomy followed by tempol (12, 24 and 48 mg/kg) treatment (i.p.) at 10 min, 6 h, and every 24 h up to 1 week after injury. The rats were then killed for lumbar intumescence analysis. Nissl staining, TUNEL, synaptophysin immunolabeling and qRT-PCR (Caspase 3, Bax and Bcl2) were carried out. The results indicated that tempol treatment, at 24 mg/kg, increased up to 21% spinal cord motoneuron survival (p<0.001), also preserving pre-synaptic terminals in the neuropile. Likewise, the TUNEL-positive cell number decreased in tempol-treated animals. qRT-PCR results indicated differential increase in Caspase 3 (3-fold), Bax (13-fold) and Bcl2 (28-fold) gene expression, after 12 h following axotomy and tempol treatment. In conclusion, tempol administration has proven to be neuroprotective after neonatal nerve injury, leading to improved motoneuron survival, synapse preservation and minimizing apoptosis.

Synapse plasticity in motor, sensory, and limbo-prefrontal cortex areas as measured by degrading axon terminals in an environment model of gerbils (Meriones unguiculatus)

Still little is known about naturally occurring synaptogenesis in the adult neocortex and related impacts of epigenetic influences. We therefore investigated (pre)synaptic plasticity in various cortices of adult rodents, visualized by secondary lysosome accumulations (LA) in remodeling axon terminals. Twenty-two male gerbils from either enriched (ER) or impoverished rearing (IR) were used for quantification of silver-stained LA. ER-animals showed rather low LA densities in most primary fields, whereas barrel and secondary/associative cortices exhibited higher densities and layer-specific differences. In IR-animals, these differences were evened out or even inverted. Basic plastic capacities might be linked with remodeling of local intrinsic circuits in the context of cortical map adaptation in both IR- and ER-animals. Frequently described disturbances due to IR in multiple corticocortical and extracortical afferent systems, including the mesocortical dopamine projection, might have led to maladaptations in the plastic capacities of prefronto-limbic areas, as indicated by different LA densities in IR- compared with ER-animals.

A model for cortical rewiring following deafferentation and focal stroke

It is still unclear to what extent structural plasticity in terms of synaptic rewiring is the cause for cortical remapping after a lesion. Recent two-photon laser imaging studies demonstrate that synaptic rewiring is persistent in the adult brain and is dramatically increased following brain lesions or after a loss of sensory input (cortical deafferentation). We use a recurrent neural network model to study the time course of synaptic rewiring following a peripheral lesion. For this, we represent axonal and dendritic elements of cortical neurons to model synapse formation, pruning and synaptic rewiring. Neurons increase and decrease the number of axonal and dendritic elements in an activity-dependent fashion in order to maintain their activity in a homeostatic equilibrium. In this study we demonstrate that synaptic rewiring contributes to neuronal homeostasis during normal development as well as following lesions. We show that networks in homeostasis, which can therefore be considered as adult networks, are much less able to compensate for a loss of input. Interestingly, we found that paused stimulation of the networks are much more effective promoting reorganization than continuous stimulation. This can be explained as neurons quickly adapt to this stimulation whereas pauses prevents a saturation of the positive stimulation effect. These findings may suggest strategies for improving therapies in neurologic rehabilitation.

Inverse relationship between adult hippocampal cell proliferation and synaptic rewiring in the dentate gyrus

Adult neurogenesis is a key feature of the hippocampal dentate gyrus (DG). Neurogenesis is accompanied by synaptogenesis as new cells become integrated into the circuitry of the hippocampus. However, little is known to what extent the embedding of new neurons rewires the pre-existing network. Here we investigate synaptic rewiring in the DG of gerbils (Meriones unguiculatus) under different rates of adult cell proliferation caused by different rearing conditions as well as juvenile methamphetamine treatment. Surprisingly, we found that an increased cell proliferation reduced the amount of synaptic rewiring. To help explain this unexpected finding, we developed a novel model of dentate network formation incorporating neurogenesis and activity-dependent synapse formation and remodelling. In the model, we show that homeostasis of neuronal activity can account for the inverse relationship between cell proliferation and synaptic rewiring.

MHC class I upregulation is not sufficient to rescue neonatal alpha motoneurons after peripheral axotomy

Associated with neuronal death, profound synaptic changes occur in the spinal cord during the apoptotic process triggered after axotomy in neonatal rats. With respect to this, the major histocompatibility complex of class I (MHC class I) has recently emerged as a new mechanism related to synaptic stripping and plasticity. The present study investigated the impact of upregulating MHC class I expression by treatment with beta interferon (beta INF) on motoneuron survival, synaptic plasticity and astrogliosis after neonatal sciatic nerve injury. P2 rats were subjected to unilateral axotomy followed by three days of beta INF treatment. The results were analyzed by counting Nissl stained motoneurons, immunohistochemistry (anti-synaptophysin, MHC class I, GFAP and Iba-1) and transmission electron microscopy. INF treatment induced an increased expression of MHC class I, which resulted in a stronger synaptic elimination process in the spinal cord, as seen by the synaptophysin labeling. GFAP and Iba-1 upregulation were not significantly altered by the INF treatment, displaying the same degree of enhanced reactivity as compared to the placebo group. The ultrastructural analysis showed that, apart from the overall reduction of inputs in the neuropil, no statistical differences were present when comparing the INF and placebo treated animals. Also, neuronal survival was not altered by cytokine administration. The present results provide evidence that MHC class I upregulation after neonatal injury does not change the fate of lesioned motoneurons. In this way, the lack of neurotrophic support may cause broader synaptic loss, which superposes the more subtle effects of the upregulation of MHC class I.

A theoretical network model to analyse neurogenesis and synaptogenesis in the dentate gyrus

We describe a strongly biologically motivated artificial neural network approach to model neurogenesis and synaptic turnover as it naturally occurs for example in the hippocampal dentate gyrus (DG) of the developing and adult mammalian and human brain. The results suggest that cell proliferation (CP) has not only a functional meaning for computational tasks and learning but is also relevant for maintaining homeostatic stability of the neural activity. Moderate rates of CP buffer disturbances in input activity more effectively than networks without or very high CP. Up to a critical mark an increase of CP enhances synaptogenesis which might be beneficial for learning. However, higher rates of CP are rather ineffective as they destabilize the network: high CP rates and a disturbing input activity effect a reduced cell survival. By these results the simulation model sheds light on the recurrent interdependence of structure and function in biological neural networks especially in hippocampal circuits and the interacting morphogenetic effects of neurogenesis and synaptogenesis.

Postnatal development of vibrissae motor output following neonatal infraorbital nerve manipulation

Using the model of infraorbital nerve (IoN) injury, we have studied the role IoN signals have on the developing vibrissal motor system. To this end, in ten rats, the IoN was severed on the day of birth: in five rats, the IoN was repaired to promote axon regeneration (Reinnervated group) while axon regeneration was prevented in the remaining five rats (Deafferented group). In another five rats, the isolated IoN was left intact (Sham group) and still another group of five rats was left untouched (Control group). After these rats had reached adulthood, the compound muscle action potential (MAP) was recorded from the vibrissa muscle and intracortical microstimulation (ICMS)-evoked movements were mapped in the frontal cortex contralateral to the operated side. We found: (i) no difference between Control, Sham and Reinnervated groups in the integrated MAPs and in the size and excitability of the M1 vibrissal representation. (ii) the Deafferented group showed a 42.9% decrease in the integrated MAP plus a 47.2% and 36.9% reduction, respectively, in the size and excitability of the M1 vibrissae representation. We conclude that, during perinatal life, IoN signals regulate the development of both the peripheral and central vibrissal motor system and that IoN reinnervation restores sensory signals able to stabilize normal development of the vibrissal motor system.

Offer and demand: proliferation and survival of neurons in the dentate gyrus

The proliferation and survival of new cells in the dentate gyrus of mammals is a complex process that is subject to numerous influences, presenting a confusing picture. We suggest regarding these processes on the level of small networks, which can be simulated in silico and which illustrate in a nutshell the influences that proliferating cells exert on plasticity and the conditions they require for survival. Beyond the insights gained by this consideration, we review the available literature on factors that regulate cell proliferation and neurogenesis in the dentate gyrus in vivo. It turns out that the rate of cell proliferation and excitatory afferents via the perforant path interactively determine cell survival, such that the best network stability is achieved when either of the two is increased whereas concurrent activation of the two factors lowers cell survival rates. Consequently, the mitotic activity is regulated by systemic parameters in compliance with the hippocampal network's requirements. The resulting neurogenesis, in contrast, depends on local factors, i.e. the activity flow within the network. In the process of cell differentiation and survival, each cell's spectrum of afferent and efferent connections decides whether it will integrate into the network or undergo apoptosis, and it is the current neuronal activity which determines the synaptic spectrum. We believe that this framework will help explain the biology of dentate cell proliferation and provide a basis for future research hypotheses.

Synaptic lesions and synaptophysin distribution change in spinal motoneurons at early stages following sciatic nerve transection in neonatal rats

The purpose of this investigation was to evaluate the rate of synaptic stripping, changes in the synaptophysin distribution, and synapses ultrastructure of spinal motoneurons at early stages of sciatic nerve axotomy in newborn rats. Seven groups were used in the experiment, which were sacrificed after 1, 3, 6, 12, 24, 48 and 72 h. L4-L6 spinal segments from the animals of the above groups were prepared and processed for indirect immunoperoxidase. Accordingly, tissue samples were prepared from similar groups for routine electron microscopy. Synaptophysin-labeled motoneurons were classified into intact, partial, cytoplasmic and negative patterns. Synaptophysin immunoreactivity in both the axotomized and non-axotomized sides showed no negative pattern at the first three time points, while this pattern significantly increased in the 12, 24, 48 and 72 h groups at the axotomized side, moreover, there was a progressive reduction in the percentage of the intact pattern at the axotomized side. The percentages of the cytoplasmic and partial patterns also increased during the time course. The rate of synaptic stripping was five time higher in the axotomized side than that of non-axotomized side. The results of the ultrastructural study showed synaptic membranes irregularity, synaptic vesicles displacement, synaptic membrane detachment and ensheathment of degenerated synapses. The conclusion of the study was that early synaptophysin immunoreactivity changes were seen in both the axotomized and non-axotomized sides, but the rate of change in the axotomized side was more rapid than that of the non-axotomized side.

Effects of brief seizures during development

The effects of brief seizures during development depend on multiple factors such as underlying brain pathology, specific age of occurrence and frequency. Studies in rats are frequently used to determine the consequences of seizures in the developing brain. The shorter prepubertal development and life span of the rat compared to humans may suggest that brief seizures in the rat are not necessarily equivalent to brief seizures in humans. Nevertheless, there is substantial evidence that in the rat, the consequences of seizures are age-dependent. The immature brain is relatively resistant to morphological damage, especially in the hippocampus, and functional changes as measured by electrophysiology and behavior. Developmental kindling can be used as a model to study brief seizures early in life. Kindling permanently alters the brain so that rats stimulated again in adulthood require only few kindling stimuli for fully kindled seizures to occur although there are no apparent morphological and functional changes in the hippocampus resulting from kindling early in life. The appreciation that kindling can alter brain function without any discrete (to date) morphological changes may lead to the development of effective neuroprotective strategies to alter the process, but it is not clear that all kindling-induced changes are detrimental to the brain.

Differential expression of S100B and S100A6(1) in the human fetal and aged cerebral cortex

S100B and S100A6 (calcylin) are two members of the S100 Ca(2+)-binding protein family and have been localized in the mammalian nervous system. However, information on their distribution in the human nervous system, especially in the developing human fetal brain, is scarce. In the present study, an immunocytochemical method was used to examine the spatio-temporal protein expression patterns of S100B and S100A6 in normal human fetal hippocampus, entorhinal cortex and occipital cortex. Normal aged adult human brain specimens were also included for comparison. From week 15 onwards, an increase with advancing gestation age in both the number and staining intensity of S100B positive, astrocyte-like cells was found in the pyramidal layer of the hippocampus, while both the molecular and polymorphic layers showed similar S100B immunoreactivities at all stages examined. A decrease in the immunoreactivities was found in the molecular layer of the aged adult hippocampus while other layers exhibited immunoreactivities similar to those of the late fetus. At week 15, the molecular, pyramidal and ganglionic/multiform layers of the entorhinal cortex also showed positive S100B immunoreactivities which were maintained throughout the rest of the gestation and in adult specimens. In the occipital cortex, the numbers of positive cells for all layers were about twofold higher than those found in the hippocampus and entorhinal cortex, and immunoreactivities detected in the granular layer increased from week 21, reaching a plateau at around week 27. S100B positive fibers were also found at week 30 but were not observed in aged adult specimens. S100A6 positive cells were on the whole fewer in number than those of S100B in the brain regions examined. The S100A6 immunoreactivities which were localized in some pyramidal neuron-like and some glial-like cells of the pyramidal and molecular layers of the hippocampus increased by midgestation and became weak in the late fetus and in aged adult specimens. Weakly stained S100A6 positive cells were also observed in the entorhinal cortex throughout the gestation and in aged adult cortex. S100A6 immunoreactivities were weak in the fetal occipital cortex. They were also localized in the glial-like cells of the aged adult occipital cortex. The differential spatio-temporal expression of S100B and S100A6 proteins suggests that the proteins play different roles in different brain regions during development and in adulthood.

Neuronal degeneration and reorganization: a mutual principle in pathological and in healthy interactions of limbic and prefrontal circuits

Based on developmental principles and insights from animal research about neuroplasticity in cell assemblies, this article is to propose a view of plasticity that promotes a link between hippocampal and prefrontal structure and function. Both the mitotic activity (counting of BrdU-labeled cells) in hippocampal dentatus and the maturation of dopamine fibres (quantitative immunochemistry of mesoprefrontal projection) in the prefrontal cortex proved to be a measurable combination for investigating the complex chain of events that relate activity dependent neuroplasticity to normal as well as to pathological maturational processes. With our animal model we demonstrate that both rearing conditions and neuroactive substances can effectively interfere with developmental plasticity and induce a malfunctional adaptation of prefrontal structures and neurotransmitter systems (dopamine, GABA). In the hippocampal dentatus, where ontogenetic plasticity proved to be preserved by continued neuro- and synaptogenesis, serious damage can be internalized without simultaneous disruption of neural dynamics offering an approach to reverse dysfunctional reorganization in the prefrontal cortex.

The subependymal layer in rodents: a site of structural plasticity and cell migration in the adult mammalian brain

The persistence of neurogenesis and structural plasticity was believed until recently to be restricted to lower vertebrates and songbirds. Nevertheless, it has now been ascertained that these phenomena can occur in the adult mammalian nervous system, at least in three distinct sites: the olfactory neuroepithelium of the nasal mucosa and two brain regions, namely, the hippocampal dentate gyrus and the olfactory bulb. The newly generated cells of the olfactory bulb originate from the subependymal layer, a remnant of the primitive subventricular zone persisting in the adult forebrain. Besides being characterized by high rates of cell proliferation, the subependymal layer is a site of long-distance tangential cell migration, wherein migrating cells form chains enwrapped by a particular type of astrocytes. These glial cells give rise to channels (glial tubes) that separate single chains from the surrounding mature tissue. The cellular composition and the pattern of cell migration in the mammalian subependymal layer appear to be quite different in neonatal and adult animals, changing strikingly in the postnatal period. Other features of uniqueness involve the capability of neuronal precursors to divide while undergoing migration and the presence of multipotent stem cells. Thus, the subependymal layer is an area of the adult mammalian brain endowed with a cohort of phenomena proper of neural development, persisting into (and adapted to) the fully mature nervous tissue. Such features make this system an optimal model to unravel mechanisms permitting highly dynamic structural plasticity during adulthood, in the perspective of providing strategies for possible brain repair.

An ultrastructural study into the effect of global transient cerebral ischaemia on the synaptic population of the cerebellar cortex in rats

The density of synapses, the shape and size of presynaptic dense projections (PDP), and the curvature of synaptic appositions in the molecular layer of the cerebellum cortex of rat at 10 min ischaemia and after 90 min, 1, 3, 7, 30 days of re-circulation were examined using quantitative ultra-structural techniques. The numerical density of mature junctions decreased significantly (44.0%) after 1 and 3 days of re-circulation, and was increased to 149.8% of the value in the control animals after 7 days of re-circulation. The restoration of the population of mature synaptic junctions was accompanied by a considerable increase of the number of immature junctions. We found a close association between the synaptic curvature and the size of PDP. The curvature of the larger junctions was consistently associated with a reduced height of PDP and a rounder shape. Synaptic curvature increased from 0.0885 (control) to 0.2041 (3 days of re-circulation) and to 0.2128 (7 days re-circulation). The maximum reduction in synaptic numerical density and larger junction curvature was found in zones of irreversibly damaged Purkinje cells. Our results revealed that the synaptic curvature and the height of the pre-synaptic dense projections undergo reciprocal changes after global transient cerebral ischemia. It is tempting to hypothesize that the positive synaptic curvature occurs as a result of changes in morphological conditions for the PDP filaments and in the shape and size of PDP and depends on the level of Ca2+ in synaptic appositions.

Non-conventional role of lysosomal acid phosphatase in olfactory receptor axons: co-localization with growth-associated phosphoprotein-43

Olfactory receptor neurons undergo a continuous turnover in adult mammals. It is largely unknown how their axons invade the olfactory bulb and induce synaptic re-organization in glomeruli. Here, the cytochemical localization of lysosomal acid phosphatase has been studied in olfactory bulbs of adult rats and mice. The enzyme has been identified by specific substrate, inhibitors and absence in lysosomal acid phosphatase-knockout mice. Lysosomal acid phosphatase is located in primary and secondary lysosomes, which are unevenly distributed in the olfactory nerve layer and among olfactory glomeruli. In consecutive sections of glomeruli, the intensity of lysosomal acid phosphatase immunoreactivity co-varied with that of growth-associated phosphoprotein. Electron microscopically, differential lysosomal acid phosphatase staining in glomeruli corresponded to different proportions of labelled and unlabelled axons. Quantification revealed that lysosomal acid phosphatase labelling was strongest in non-synaptic profiles of terminal axons, while it was weak in or even missing from most synaptic profiles. Hence, growing olfactory axons apparently carry more lysosomal acid phosphatase than those which have established synaptic contacts. Following olfactory deafferentation both lysosomal acid phosphatase activity and growth-associated phosphoprotein-43 are lost from glomeruli, suggesting that both proteins are expressed in olfactory sensory axons during growth, while lysosomal acid phosphatase is apparently not a marker of anterograde terminal degeneration.

Structural basis of information capacity changes of sensory-motor cerebral cortex of rat brain during post-resuscitation period

In a rat model of asphyxial cardiac arrest the volume of the sensory-motor cortex and the number of neurons and synapses were determined 90 min, 6 h, and 1, 3, 7, 14, and 30 days post-resuscitation. The number of synapses was determined from serial fragmental sections, using selective contrast in ethanol solution of phosphorotungsten acid (PTA), and by cytoarchitectonic analysis on medium-thick sections. The sensory-motor cortex (SMC) volume did not change significantly during the subsequent 30 days after resuscitation. The number of neurons decreased from 2.462 +/- 0.082) x 10(6) pre-insult to (1.441 +/- 0.098) x 10(6) 30 days after resuscitation. Damage was most severe in the small neuronal cell complexes of layers III-IV, which serve as an afferent cortical 'entrance'. Damage was least in the large neuronal cell complexes of layer V, which serves as an efferent cortical 'outlet'. The number of SMC synapses decreased from (5.920 +/- 0.51) x 10(9) pre-insult to (3.441 +/- 0.305) x 10(9) 30 days after resuscitation. Damage was most severe in the synaptic pool of the cortical SMC 'entrance'. An increase in the number of high-efficiency hypertrophic synaptic contacts was observed during the post-resuscitation period, which may significantly change interneuronal relationships.

Structural study of the development of ocularity domains using a neural network model

We present a model for the development of ocularity domains in the visual cortex of mammals during the embryonic stage. We model the thalamo-cortical pathway with a self-organising neural network with two source layers, each of them serving different retinae, and one target layer, where the connections end. The connectivity between the source layers and the target layer is driven by Hebbian learning. In both the source layers and the target layer we assume excitatory lateral signal diffusion between proximal neurons that causes them to be correlated. According to the developmental state being modelled, we do not consider either correlation or anti-correlation between the signals originated in neurons of different retinae. The basic assumptions made are proved to be sufficient to attain a distribution of connections arranged in ocularity domains. The dependence of the geometry of the ocularity domains on the parameters of the model is analysed and a correlation between the width of the signal diffusion and the extent of the domains is found. The generality of the assumptions made allows an easy translation of the model to explain the development of other elements of the sensory nervous system.

Postnatal development of the human primary motor cortex: a quantitative cytoarchitectonic analysis

The postnatal development of the human primary motor cortex (area 4) was analyzed in 54 individuals ranging in age from birth to 90 years. Three parameters defining major cytoarchitectonic features (areal fraction, numerical density and mean area of cells) were measured in vertical columns extending from the pial surface to the border between cortex and underlying white matter. The data were compiled in profile curves that reveal or more detailed laminar pattern than the classical cytoarchitectonic descriptions. The most pronounced decreases in numerical density and areal fraction of Nissl-stained cells profiles during early postnatal ontogeny are observed in layer II. A clearly delineable layer IV, which is still recognizable in the newborn, disappears gradually during the first postnatal months. Although the width of the cortex as a whole increases during this period, layer V, the main source of pyramidal tract fibers, is the only lamina that also increases in relative thickness. The other layers remain stable or become relatively thinner. These results reveal specific laminar growth processes in area 4, which take place in parallel with the functional maturation of the cortical motor system.

Synchronous development of pyramidal neuron dendritic spines and parvalbumin-immunoreactive chandelier neuron axon terminals in layer III of monkey prefrontal cortex

Postnatal development of the primate cerebral cortex involves an initial proliferation and the subsequent attrition of cortical synapses. Although these maturational changes in synaptic density have been observed across the cortical mantle, little is known about the precise time course of developmental refinements in synaptic inputs to specific populations of cortical neurons. We examined the postnatal development of two markers of excitatory and inhibitory inputs to a subpopulation of layer III pyramidal neurons in area 9 and 46 of rhesus monkey prefrontal cortex. These neurons are of particular interest because they play a major role in the flow of information both within and between cortical regions. Quantitative reconstructions of Golgi-impregnated mid-layer III pyramidal neurons revealed substantial developmental changes in the relative density of dendritic spines, the major site of excitatory inputs to these neurons. Relative spine density on both the apical and basilar dendritic trees increased by 50% during the first two postnatal months, remained at a plateau through 1.5 years of age, and then decreased over the peripubertal age range until stable adult levels were achieved. As a measure of the postnatal changes in inhibitory input to the axon initial segment of these pyramidal neurons, we determined the density of parvalbumin-immunoreactive axon terminals belonging to the chandelier class of local circuit neurons. The density of these distinctive axon terminals (cartridges) exhibited a temporal pattern of change that exactly paralleled the changes in dendritic spine density. These results suggest that subpopulations of cortical neurons may be regulated by dynamic interactions between excitatory and inhibitory inputs during development and, in concert with other data, they emphasize the cellular specificity of postnatal refinements in cortical circuitry.

Structural dynamics of synapses and synaptic components

Learning and memory formation are apparently based on cascades of molecular and cellular processes with increasing time constants (ms to days and weeks), but even the most long-lasting effects are transient. Memory traces may permanently modify the behavior (activity patterns, gene expression) of neurons and neuronal networks. Therefore the question is raised whether our current view on the stability of synapses under normal conditions is tenable. Evidence is reviewed suggesting that as direct or indirect effects of modifications in bioelectrical activity and chemical trophicity, synapses may be remodeled and removed within days and weeks, and possibly within hours. Accordingly, species-specific connectivity patterns are not restricted to the standard architecture of the CNS, but (morpho-)genetics allow for a considerable number of alternative wiring patterns, which appear under unusual conditions during ontogenesis and in adulthood. Our present knowledge suggests that, rather than the formation of synapses, they are a selective process. Until now there is no direct method of measuring either synaptic reorganization or the average life span of synapses. Specific cases, however, allow to estimate synapse turnover during ontogenesis, at its lowest possible level. Such data suggest that each synapse is on average remodeled or replaced several to many times during normal developmental, e.g. in the cerebral cortex of Marmoset monkeys at the very least 5 to 10 times (corresponding to 250 million synapses eliminated per hour in area 17!). It is discussed how the consequences of synapse turnover could be utilized by learning processes. Conclusions are followed by an outlook.

Is schizophrenia due to excessive synaptic pruning in the prefrontal cortex? The Feinberg hypothesis revisited

Several lines of evidence support the notion that a substantial reorganization of cortical connections, involving a programmed synaptic pruning, takes place during adolescence in humans. A review of neurobiological abnormalities in schizophrenia indicates that the neurobiological parameters that undergo peripubertal regressive changes may be abnormal in this disorder. An excessive pruning of the prefrontal corticocortical, and corticosubcortical synapses, perhaps involving the excitatory glutamatergic inputs to pyramidal neurons, may underlie schizophrenia. A reciprocal failure of pruning in certain subcortical structures, such as lenticular nuclei, may also occur. Several developmental trajectories, related to early brain insults as well as genetic factors affecting postnatal neurodevelopment, could lead to the illness. These models would have heuristic value and may be consistent with several known facts of the schizophrenic illness, such as its onset in adolescence and the gender differences in its onset and natural course. The relationship between these models and other etiological models of schizophrenia are summarized and approaches to test relevant hypotheses are discussed.

Pre- and postnatal development of the primary visual cortex of the common marmoset. II. Formation, remodelling, and elimination of synapses as overlapping processes

During ontogenesis changes in the numerical density of synapses are usually assumed to depend essentially on variations in the formation of synapses. Only the final adjustment to adult synapse densities is thought to include the elimination of synapses in some brain regions of certain species. Here, we focus attention on quantitative aspects of synapse elimination throughout development of area 17 of marmoset monkeys (Callithrix jacchus). Mature synapses, various precursor forms, and indicators of lysosomal degradation of synapses were quantitatively analysed by electron microscopy and morphometric methods. A total number of about 135 x 10(9) synapses was calculated for area 17 in each adult hemisphere corresponding to a volume density of 600 x 10(6) synapses/mm3. At 3 months of age, the respective values were 508 x 10(9)/area and 1,159 x 10(6)/mm3, while at birth these values were 69 x 10(9)/area and 328 x 10(6)/mm3. Consequently, at least three out of four synapses are eliminated between 3 months and adulthood. However, the real number of synapses being eliminated during development is probably much larger if the time course of lysosomal degradation is additionally taken into account. The frequency of lysosomes in presynaptic endings is highest before net-elimination of synapses occurs, i.e., between 1 and 3 months. This suggests that lysosomal degradation is not directly responsible for the majority of synapses removed during ontogenesis but apparently represents a second mechanism for synapse remodelling and elimination. Thus, it appears from this study that remodelling and elimination of synapses are quantitatively as important as their formation, and accompany synaptogenesis from its very onset onwards.

Facial nerve lesions lead to increased immunostaining of the astrocytic gap junction protein (connexin 43) in the corresponding facial nucleus of rats

After peripheral transection of the facial nerve, immunostaining of astrocytic gap junction protein changed in the corresponding brainstem nucleus of the rat. Enhanced connexin-43 immunoreactivity was restricted to the ipsilateral facial nucleus and to astrocytes surrounding lesioned motoneurons. This reaction is focally distinct, and marks only a part of the astrocytic network indicating a local plasticity of intercellular coupling. These results suggest that astrocytes work as sensors of signals which either depend on the integrity of neighboring neurons or inform about neuronal disorders.

Modulation by GABA of neuroplasticity in the central and peripheral nervous system

Apart from being a prominent (inhibitory) neurotransmitter that is widely distributed in the central and peripheral nervous system, gamma-aminobutyric acid (GABA) has turned out to exert trophic actions. In this manner GABA may modulate the neuroplastic capacity of neurons and neuron-like cells under various conditions in situ and in vitro. In the superior cervical ganglion (SCG) of adult rat, GABA induces the formation of free postsynaptic-like densities on the dendrites of principal neurons and enables implanted foreign (cholinergic) nerves to establish functional synaptic contacts, even while preexisting connections of the preganglionic axons persist. Apart from postsynaptic effects, GABA inhibits acetylcholine release from preganglionic nerve terminals and changes, at least transiently, the neurochemical markers of cholinergic innervation (acetylcholinesterase and nicotinic receptors). In murine neuroblastoma cells in vitro, GABA induces electron microscopic changes, which are similar in principle to those seen in the SCG. Both neuroplastic effects of GABA, in situ and in vitro, could be mimicked by sodium bromide, a hyperpolarizing agent. In addition, evidence is available that GABA via A- and/or B-receptors may exert direct trophic actions. The regulation of both types of trophic actions (direct, receptor-mediated vs. indirect, bioelectric activity dependent) is discussed.