Showing papers on "Developmental plasticity published in 1997"

Developmental plasticity in neural circuits for a learned behavior

Abstract: The neural substrate underlying learned vocal behavior in songbirds provides a textbook illustration of anatomical localization of function for a complex learned behavior in vertebrates. The song-control system has become an important model for studying neural systems related to learning, behavior, and development. The song system of zebra finches is characterized by a heightened capacity for both neural and behavioral change during development and has taught us valuable information regarding sensitive periods, rearrangement of synaptic connections, topographic specificity, cell death and neurogenesis, experience-dependent neural plasticity, and sexual differentiation. The song system differs in some interesting ways from some well-studied mammalian model systems and thus offers fresh perspectives on specific theoretical issues. In this highly selective review, we concentrate on two major questions: What are the developmental changes in the song system responsible for song learning and the restriction of learning to a sensitive period, and what factors explain the highly sexually dimorphic development of this system? We discuss the important role of sex steroid hormones and of neurotrophins in creating a male-typical neural song circuit (which can learn to produce complex vocalizations) instead of a reduced, female-typical song circuit that does not produce learned song.

Activity-dependent regulation of NMDAR1 immunoreactivity in the developing visual cortex.

Abstract: NMDA receptors have been implicated in activity-dependent synaptic plasticity in the developing visual cortex. We examined the distribution of immunocytochemically detectable NMDAR1 in visual cortex of cats and ferrets from late embryonic ages to adulthood. Cortical neurons are initially highly immunostained. This level declines gradually over development, with the notable exception of cortical layers 2/3, where levels of NMDAR1 immunostaining remain high into adulthood. Within layer 4, the decline in NMDAR1 immunostaining to adult levels coincides with the completion of ocular dominance column formation and the end of the critical period for layer 4. To determine whether NMDAR1 immunoreactivity is regulated by retinal activity, animals were dark-reared or retinal activity was completely blocked in one eye with tetrodotoxin (TTX). Dark-rearing does not cause detectable changes in NMDAR1 immunoreactivity. However, 2 weeks of monocular TTX administration decreases NMDAR1 immunoreactivity in layer 4 of the columns of the blocked eye. Thus, high levels of NMDAR1 immunostaining within the visual cortex are temporally correlated with ocular dominance column formation and developmental plasticity; the persistence of staining in layers 2/3 also correlates with the physiological plasticity present in these layers in the adult. In addition, visual experience is not required for the developmental changes in the laminar pattern of NMDAR1 levels, but the presence of high levels of NMDAR1 in layer 4 during the critical period does require retinal activity. These observations are consistent with a central role for NMDA receptors in promoting and ultimately limiting synaptic rearrangements in the developing neocortex.

Distinct forms of short-term plasticity at excitatory synapses of hippocampus and neocortex

Abstract: The expression of short- and long-term synaptic plasticity varies strongly across pathways in the central nervous system. Differences in the properties of transmitter release may underlie some of this variability. Here we compared the short-term plasticity displayed by a neocortical and a hippocampal pathway in vitro, and observed dramatic differences. Conditions known to increase transmitter release probability were more effective in hippocampus, while conditions known to decrease release probability were similarly effective in both pathways. The effects of the irreversible open-channel blocker of N-methyl-d-aspartate receptors, MK-801, implied that synapses in the neocortical pathway have a relatively high probability of transmitter release as compared with synapses in the hippocampal pathway. Differences in release probability may explain the pathway-specific variance in short- and long-term synaptic plasticity.

Anatomical and synaptic substrates for avian song learning.

Abstract: In songbirds, vocal learning occurs during periods of major cellular and synaptic change. This neural reorganization includes massive synaptogenesis associated with the addition of new neurons into the vocal motor pathway, as well as pruning of connections between song regions. These observations, coupled with behavioral evidence that song development requires NMDA receptor activation in specific song nuclei, suggest that experiences associated with vocal learning are encoded by activity driven, Hebbianlike processes of synaptic change akin to those implicated in many other forms of developmental plasticity and learning. In this review we discuss the hypothesis that develpmental and/or seasonal changes in NMDA receptor function and the availability of new synapses may modulate thresholds for plasticity and thereby define sensitive periods for vocal learning.

Ontogenetic phenotypic plasticity during the reproductive phase in Arabidopsis thaliana (Brassicaceae).

Abstract: While phenotypic plasticity has been the focus of much research and debate in the recent ecological and evolutionary literature, the developmental nature of the phenomenon has been mostly overlooked. A developmental perspective must ultimately be an integral part of our understanding of how organisms cope with heterogeneous environments. In this paper I use the rapid cycling Arabidopsis thaliana to address the following questions concerning developmental plasticity. (1) Are there genetic and/or environmental differences in parameters describing ontogenetic trajectories? (2) Is ontogenetic variation produced by differences in genotypes and/or environments for two crucial traits of the reproductive phase of the life cycle, stem elongation and flower production? (3) Is there ontogenetic variability for the correlation between the two characters? I found genetic variation, plasticity, and variation for plasticity affecting at least some of the growth parameters, indicating potential for evolution via heterochronic shifts in ontogenetic trajectories. Within-population differences among families are determined before the onset of the reproductive phase, while among-population variation is the result of divergence during the reproductive phase of the ontogeny. Finally, the ontogenetic profiles of character correlations are very distinct between the ecologically meaningful categories of early- and late-flowering "ecotypes" in this species, and show susceptibility to environmental change.

Effect of changes in resource level on age and size at metamorphosis in Hyla squirella

Abstract: Recent experiments suggest that timing of metamorphosis is fixed during development in some anurans, insects, and freshwater invertebrates. Yet, these experiments do not exclude a growth rate optimization model for the timing of metamorphosis. I manipulated food resources available to larvae of squirrel treefrogs (Hyla squirella) to determine if there is a loss of plasticity in duration of larval period during development and to critically test growth rate models for the timing of metamorphosis. Size-specific resource levels for individual tadpoles were switched from low to high or high to low at three developmental stages spaced throughout larval development. The effects of changes in resource availability on larval period and mass at metamorphosis were measured. Switching food levels after late limb bud development did not significantly affect larval period in comparison to constant food level treatments. Therefore, developmental rate in H. squirella is better described by a fixed developmental rate model, rather than a growth rate optimization model. The timing of fixation of developmental rate in H. squirella is similar to that found in other anuran species, suggesting a taxonomically widespread developmental constraint on the plasticity of larval period duration. Mass at metamorphosis was not significantly affected by the timing of changes in food levels; the amount of food available later in development determined the size at metamorphosis. Larval period and mass at metamorphosis were negatively correlated in only one of two experiments, which contrasts with the common assumption of a phenotypic trade-off between decreased larval period and increased mass at metamorphosis.

N‐methyl‐D‐aspartate subunit R1 involvement in the postnatal organization of the primary visual cortex of Callithrix jacchus

Abstract: It has been demonstrated that the primary visual cortex is highly sensitive to manipulations of the visual environment during a specific, early, postdevelopmental period: the critical period. Pharmacological studies have shown that N-methyl-D-aspartate (NMDA) receptors are involved in the plasticity of the visual cortex just as they are involved in the induction of long-term potentiation (LTP), another activity-dependent form of plasticity. The setting up of synaptic connectivity in the neocortex may rely on LTP-like mechanisms. By using immunohistochemistry techniques, we tested the hypothesis of the role of subunit R1 of NMDA (NMDAR1) receptors in the thalamocortical afferent segregation into ocular-dominance columns in the New World monkey,Callithrix jacchus.We employed early and short (2 weeks) monocular-deprivation periods at different ages of postnatal development (17, 46, 67, 107, and 188 postnatal days). We observed heterogeneous distribution of NMDAR1 in the layer IVC receiving the thalamic inputs if the deprivation was realized between the ages of 46 and 107 days. Layers IVCa and IVCb were involved differently as a function of the deprivation age. The striped pattern lost its differential intensity with the postnatal age. These results are compared with the ocular-dominance pattern evolution described in other works on this primate. They provide evidence of the NMDAR1 role in the modular organization, within time limits, during the postnatal development of the primary visual cortex. J. Comp. Neurol.

27 Neural Plasticity

Abstract: I INTRODUCTION: CONNECTIVITY AND PLASTICITY Neural plasticity refers to functional changes in the nervous system and therefore encompasses a range of phenomena from changes at synapses observed on a microscopic scale to changes in behavior observed in the whole animal These diverse phenomena are related since changes in synapses are believed to underlie changes in an animal’s behavior (Greenough and Bailey 1988) Ideally, both the physical changes to the nervous system and the resultant behavioral changes could be identified and studied together to yield an integrated understanding of nervous system structure and behavior Nervous systems were once thought to be “hardwired” during development In most vertebrate central nervous systems, cell proliferation occurs during embryogenesis and new neurons are not added to the mature nervous system (Jacobson 1991) An unusual exception to this rule is found in some species of birds in which neurons are added to the brains of juveniles to accommodate song learning (Paton and Nottebohm 1984) Sensory systems such as the visual cortex form their functional connectivity during a limited “critical period” during development, and new connections in most systems are not made after this period (Hubel and Wiesel 1970) This stability was thought to be an essential requirement for reliable processing of sensory information However, this general stability has been shown to have its exceptions The nervous system of even the dimmest among us can learn new tasks and thereby reveals a degree of plasticity in our brains Even the inability of the brain to produce new

[Neuronal growth-associated proteins in neural plasticity and brain aging].

Abstract: This review summarizes current issues on neuronal structural plasticity and its molecular marker genes in the aged brain. Neuronal growth-associated proteins (nGAPs) are introduced as potential markers of neuronal structural plasticity in the brain. The expression of genes encoding nGAPs such as GAP-43, SCG10 and stathmin is increased following striatal and hippocampal neuronal deafferentation lesions in the adult rat brain. In aged brains, the magnitude of neuronal plasticity is reduced and nGAP gene induction is limited at least in part, while the kinetics of the response remains unchanged in comparison to young brains. These results suggest that nGAPs serve as molecular markers of neuronal structural plasticity in the aging brain and that neuronal plasticity decreases, at least in certain neuronal circuits, during aging. The expression of genes encoding SCG10, stathmin and GAP-43 is also altered in age-related neurodegenerative conditions such as Alzheimer's disease in humans. Thus, studies of nGAPs are important for the elucidation of the mechanisms of the reduction of neuronal structural plasticity in aged brains.

Cortical plasticity and LTP

Abstract: In the developing and adult cortex, just as in the adult hippocampus, LTP is unable to account for a variety of types of functional plasticity.

Chemical integration in mechanisms of brain plasticity

Abstract: The paper analyzes investigations on brain functions from the viewpoint of P. K. Anokhin's theory of functional systems. Brain plasticity is postulated to be the result of convergence of various excitations on single brain neurons and of chemical integration of brain transmitters and modulators of peptide origin in the realization of biological motivations. Numerous findings of duplication of various neurotransmitters and oligopeptides in the central mechanisms of biological motivations, chemical rearrangement in the recovery period after the lateral hypothalamic syndrome when bilateral motivational centers of the hypothalamus were coagulated suggests that there is chemical compensation in the brain activity for homeostatic maintenance. The plasticity of extrahypothalamic brain structures are postulated to be more expressed as compared to the specific activities of hypothalamic, brain stem, and spinal cord structures.