Neuronal lysosome accumulation in degrading synapses of sensory-motor and limbic subsystems in the duck Anas platyrhynchos: indication of rearrangements during avian brain development?

Computational benefits of structural plasticity, illustrated in songbirds

The plasticity of nervous systems allows animals to quickly adapt to a changing environment. In particular, the structural plasticity of brain networks is often critical to the development of the central nervous system and the acquisition of complex behaviors. As an example, structural plasticity is central to the development of song-related brain circuits and may be critical for song acquisition in juvenile songbirds. Here, we review current evidences for structural plasticity and their significance from a computational point of view. We start by reviewing evidence for structural plasticity across species and categorizing them along the spatial axes as well as the along the time course during development. We introduce the vocal learning circuitry in zebra finches, as a useful example of structural plasticity, and use this specific case to explore the possible contributions of structural plasticity to computational models. Finally, we discuss current modeling studies incorporating structural plasticity and unexplored questions which are raised by such models.

Synaptic remodeling in the dentate gyrus, CA3, CA1, subiculum, and entorhinal cortex of mice: effects of deprived rearing and voluntary running

Hippocampal cell proliferation is strongly increased and synaptic turnover decreased after rearing under social and physical deprivation in gerbils (Meriones unguiculatus). We examined if a similar epigenetic effect of rearing environment on adult neuroplastic responses can be found in mice (Mus musculus). We examined synaptic turnover rates in the dentate gyrus, CA3, CA1, subiculum, and entorhinal cortex. No direct effects of deprived rearing on rates of synaptic turnover were found in any of the studied regions. However, adult wheel running had the effect of leveling layer-specific differences in synaptic remodeling in the dentate gyrus, CA3, and CA1, but not in the entorhinal cortex and subiculum of animals of both rearing treatments. Epigenetic effects during juvenile development affected adult neural plasticity in mice, but seemed to be less pronounced than in gerbils.

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.

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.

C. elegans RPM-1 regulates axon termination and synaptogenesis through the Rab GEF GLO-4 and the Rab GTPase GLO-1

C. elegans RPM-1 (for Regulator of Presynaptic Morphology) is a member of a conserved protein family that includes Drosophila Highwire and mammalian Pam and Phr1. These are large proteins recently shown to regulate synaptogenesis through E3 ubiquitin ligase activities. Here, we report the identification of an RCC1-like guanine nucleotide exchange factor, GLO-4, from mass spectrometry analysis of RPM-1-associated proteins. GLO-4 colocalizes with RPM-1 at presynaptic terminals. Loss of function in glo-4 or in its target Rab GTPase, glo-1, causes neuronal defects resembling those in rpm-1 mutants. We show that the glo pathway functions downstream of rpm-1 and acts in parallel to fsn-1, a partner of RPM-1 E3 ligase function. We find that late endosomes are specifically disorganized at the presynaptic terminals of glo-4 mutants. Our data suggest that RPM-1 positively regulates a Rab GTPase pathway to promote vesicular trafficking via late endosomes.

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.

Passive immunization against beta-amyloid peptide protects central nervous system (CNS) neurons from increased vulnerability associated with an Alzheimer's disease-causing mutation

To characterize the effects of the familial Alzheimer's disease-causing Swedish mutations of amyloid precursor protein (SwAPP) on the vulnerability of central nervous system neurons, we induced epileptic seizures in transgenic mice expressing SwAPP. The transgene expression did not change the seizure threshold, but consistently more neurons degenerated in brains of SwAPP mice as compared with wild-type littermates. The degenerating neurons were stained both by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling and by Gallyas silver impregnation. A susceptible population of neurons accumulated intracellular Abeta and immunoreacted with antibodies against activated caspase-3. To demonstrate that increased Abeta levels mediated the increased vulnerability, we infused antibodies against Abeta and found a significant reduction in neuronal loss that was paralleled by decreased brain levels of Abeta. Because the SwAPP mice exhibited no amyloid plaques at the age of these experiments, transgenic overproduction of Abeta in brain rendered neurons susceptible to damage much earlier than the onset of amyloid plaque formation. Our data underscore the possibility that Abeta is toxic, that it increases the vulnerability of neurons to excitotoxic events produced by seizures, and that lowering Abeta by passive immunization can protect neurons from Abeta-related toxicity.

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.

Pharmacologically induced neural plasticity in the prefrontal cortex of adult gerbils (Meriones unguiculatus)

Using a selective antibody serum against glutaraldehyde-conjugated gamma-aminobutyric acid (GABA), GABAergic neurons were identified in the medial prefrontal cortex of young adult gerbils (Meriones unguiculatus) following a single non-invasive dose of methamphetamine (25 mg/kg i.p.) applied at the age of 90 days. GABA-immunoreactive profiles were electron microscopically counted in a defined test field (0.875 mm2) covering the prefrontal prelimbic area after a single dose of either methamphetamine or saline. Within 30 days following the drug challenge the density of GABAergic innervation significantly increased by about 20%. Several lines of previous investigation indicate that a single dose of methamphetamine is an appropriate stimulus to cause selective autotoxic destruction of certain prefrontal dopamine fibres due to drug-induced hyperactivation. There is further indication of postsynaptic and transneuronal neuroplasticity since the densities of dendritic spines on prefrontal pyramidal cells went through a significant sequence of post-drug gain and loss. These structural dynamics resemble typical alterations seen after classical mechanical or chemical lesioning in other regions of the brain. The present results on drug-induced reactive neuroplasticity are discussed together with the current understanding of stimulus-induced adaptive reorganization in the mammalian central nervous system.

Development of human lateral geniculate nucleus: an electron microscopic study

A qualitative and quantitative ultrastructural study has been carried out on lateral geniculate nuclei (LGN) of 21 human fetuses ranging in gestational age from 13-14 to 34-35 weeks. At the early age period of 13-15 weeks, LGN is characterized by immature cells with indented nuclei possessing multiple nucleoli and by a sparse neuropil. During the subsequent age periods studied progressive maturational changes lead to neurons having round nuclei with a single nucleolus and well-developed cytoplasmic organelles as well as to an elaborate neuropil. Synaptic contacts which are seen for the first time at the age of 13-14 weeks are of retinogeniculate type. They show features of immature synapses and are located mainly on the juxtasomatic parts of dendrites. With increasing gestational age, the synapses increase in size, maturity, types and complexity; an acquisition of complex synaptic arrangement (triad) occurs by 20-21 weeks. Excitatory synapses appear earlier than do the inhibitory ones. Formation of retinogeniculate contacts precedes that of the corticogeniculate type. The synaptic density and total synapse number show a progressive increase with increasing gestational age. The age period of 15-20 weeks of gestation is marked by presence of organelles suggestive of a high rate of metabolic activity, significant increase in synaptogenesis, presence of transient contacts on soma and large number of free postsynaptic membrane densities (PSD). The period thereby represents a critical period in the development of synapses in LGN. The numerical values obtained by by the age of 34-35 weeks are still low as compared to the values reported for other areas of brain.