Biomedical subjects

W T Greenough

Publications and source records attributed to W T Greenough.

At least 19 recordsLinked to original sources

PubMed1997

Fragile X mental retardation protein is translated near synapses in response to neurotransmitter activation.

Local translation of proteins in distal dendrites is thought to support synaptic structural plasticity. We have previously shown that metabotropic glutamate receptor (mGluR1) stimulation initiates a phosphorylation cascade, triggering rapid association of some mRNAs with translation machinery near synapses, and leading to protein synthesis. To determine the identity of these mRNAs, a cDNA library produced from distal nerve processes was used to screen synaptic polyribosome-associated mRNA. We identified mRNA for the fragile X mental retardation protein (FMRP) in these processes by use of synaptic subcellular fractions, termed synaptoneurosomes. We found that this mRNA associates with translational complexes in synaptoneurosomes within 1-2 min after mGluR1 stimulation of this preparation, and we observed increased expression of FMRP after mGluR1 stimulation. In addition, we found that FMRP is associated with polyribosomal complexes in these fractions. In vivo, we observed FMRP immunoreactivity in spines, dendrites, and somata of the developing rat brain, but not in nuclei or axons. We suggest that rapid production of FMRP near synapses in response to activation may be important for normal maturation of synaptic connections.

Amino Acid Sequence↗

PubMed1997

Abnormal dendritic spines in fragile X knockout mice: maturation and pruning deficits.

Fragile X syndrome arises from blocked expression of the fragile X mental retardation protein (FMRP). Golgi-impregnated mature cerebral cortex from fragile X patients exhibits long, thin, tortuous postsynaptic spines resembling spines observed during normal early neocortical development. Here we describe dendritic spines in Golgi-impregnated cerebral cortex of transgenic fragile X gene (Fmr1) knockout mice that lack expression of the protein. Dendritic spines on apical dendrites of layer V pyramidal cells in occipital cortex of fragile X knockout mice were longer than those in wild-type mice and were often thin and tortuous, paralleling the human syndrome and suggesting that FMRP expression is required for normal spine morphological development. Moreover, spine density along the apical dendrite was greater in the knockout mice, which may reflect impaired developmental organizational processes of synapse stabilization and elimination or pruning.

Animals↗

PubMed1997

Learning-dependent synaptic modifications in the cerebellar cortex of the adult rat persist for at least four weeks.

Several experiments have demonstrated increased synapse number within the cerebellar cortex in association with motor skill learning but not with motor activity alone. The persistence of these synaptic changes in the absence of continued training was examined in the present experiment. Adult female rats were randomly allocated to either an acrobatic condition (AC) or a motor activity condition (MC). The AC animals were trained to traverse a complex series of obstacles, and each AC animal was pair-matched with an MC animal that traversed an obstacle-free runway. These animals were further assigned to one of three training conditions. Animals in the EARLY condition were trained for 10 consecutive days before being killed, animals in the DELAY, condition received the same 10 d of training followed by a 28 d period without training, and animals in the CONTINUOUS condition were trained for the entire 38 d. Unbiased stereological techniques were used to obtain estimates of the number of synapses per Purkinje cell within the cerebellar paramedian lobule. Results showed the AC animals to have significantly more synapses per Purkinje cell than the MC animals in all three training conditions. There were no differences in the number of synapses per Purkinje cell among the EARLY, DELAY, and CONTINUOUS conditions. These data demonstrate that both the motor skills and the increases in synapse number presumed to support them persist in the absence of continued training.

Animals↗

PubMed1997

Learning-dependent dendritic hypertrophy of cerebellar stellate cells: plasticity of local circuit neurons.

Recent work has shown that motor learning, but not mere motor activity, changes the morphology of Purkinje cells, the major projection neurons of the cerebellar cortex. In the present study we examined how motor skill learning affects the dendritic morphology of the stellate local circuit neurons. Adult female rats were either trained to complete a complex motor learning task or forced to traverse a flat, obstacle-free runway. Golgi impregnated stellate cells were then traced via camera lucida and their dendritic arborizations examined with a concentric ring analysis. Results showed the motor learning animals to have significantly greater stellate cell dendritic arborizations than the activity controls. Thus these local circuit neurons exhibit morphological plasticity.

Animals↗

PubMed1997

Induction of multiple synapses by experience in the visual cortex of adult rats.

This study examined experience effects upon the formation of multiple synaptic contacts among individual dendritic and axonal elements. Axonal boutons and dendritic spines forming contacts with more than one process were assessed within layer IV of the visual cortex in adult rats following 60 days of housing in standard laboratory cages (IC) or in complex environments (EC). Multiple synaptic boutons (MSBs) that formed synaptic contacts with both a dendritic spine and a dendritic shaft were found to be markedly increased in number per neuron in EC rats in comparison to those in IC rats. In contrast, single-synaptic contacts were not increased, indicating that the formation of new single-synaptic boutons is, at most, merely sufficient to replace boutons that may have been recruited into the population of MSBs. This apparent tendency to reutilize presynaptic processes may indicate a constraint upon the formation of neural circuitry and a fundamental form of plastic synaptic change.

Animals↗

PubMed1996

Synaptogenesis and dendritic growth in the cortex opposite unilateral sensorimotor cortex damage in adult rats: a quantitative electron microscopic examination.

Unilateral lesions of the forelimb area of the sensorimotor cortex in adult rats resulted in time-dependent increases in the number of synapses per neuron and the volume and membrane surface area of dendritic processes per neuron within layer V of the contralateral motor cortex in comparison to sham-operated rats. Based on previous findings of a behavioral relationship with increased dendritic arborization, these changes may be related to lesion-induced compensatory changes in the use of the non-impaired (ipsilateral to the lesion) forelimb.

Age Factors↗

PubMed1996

Synaptogenesis and Fos expression in the motor cortex of the adult rat after motor skill learning.

Recent work has suggested that changes in synapse number as well as changes in the expression of the Fos protein may occur within the motor cortex in association with motor learning. The number of synapses per neuron and the percentage of Fos-positive neurons within layer II/III of the rat motor cortex was measured after training on a complex motor learning task. Adult female rats were allocated randomly to either an acrobatic condition (AC), a motor control condition (MC), or an inactive control condition (IC). AC animals were trained to traverse a complex series of obstacles, and each AC animal was pair matched with an MC animal that traversed an obstacle-free runway. IC animals received no motor training. Animals from each condition were killed at various points during training, and unbiased stereological techniques were used to estimate the number of synapses per neuron and the percentage of Fos-positive cells within layer II/III of the motor cortex. AC animals exhibited an overall increase in the number of synapses per neuron in comparison to MC and IC animals at later stages of training. AC animals also had a significantly higher overall percentage of Fos-positive cells in comparison to both controls, with a trend for the increase to be greater during the acquisition versus the maintenance phase. These data suggest that Fos may be involved in the biochemical processes underlying skill acquisition and that motor learning, as opposed to motor activity, leads to increases in synapse number in the motor cortex.

Animals↗

PubMed1996

Ultrastructural evidence for increased contact between astrocytes and synapses in rats reared in a complex environment.

Rats raised from weaning in a complex environment have an increased number of synapses per neuron in the visual cortex in comparison to animals housed in standard laboratory cages. Previous research has suggested that experience-dependent synaptic changes may be coordinated with changes in astrocytes. The present study used electron microscopy to examine astrocytic processes in the visual cortex of rats raised in a complex environment (EC) or in standard laboratory cages, either individually (IC) or in pairs (social condition, SC). Measurements of the surface density of astrocytic membrane in direct apposition to synaptic elements revealed that astrocytic processes have increased contact with synaptic elements within the visual cortex of EC rats in comparison to SC and IC animals. In contrast, other astrocytic size variables revealed no significant change in astrocytic processes per unit volume of tissue. Previous work has indicated no significant differences in synaptic density in these subjects. The specific increase in the contact between astrocytes and synapses suggests an experience-related enhancement of the astrocytic involvement in synaptic activity.

Animals↗

PubMed1996

Increased density of multiple-head dendritic spines on medium-sized spiny neurons of the striatum in rats reared in a complex environment.

It has generally been assumed that new synapses added to various brain regions in response to experience are equivalent to those already in existence. Theorists have recently posited that synaptic configurations involving multiple associated contacts may facilitate plastic change. The number of multiple-headed dendritic spines on medium-sized spiny neurons in the rat dorsolateral corpus striatum was determined following rearing in environments differing in complexity. Postweaning rats were either housed as a group in a toy- and object-filled environment or housed individually in standard laboratory cages for 30 days. Dendritic segments of Golgi-Cox impregnated Type I spiny neurons of the complex environment housed rats had approximately 60% more multiple-head spines than those of the individually caged animals. Multiple-head spines may reflect parallel synaptic contacts that modify relative strengths of existing connections or connections with a novel presynaptic terminal that alter the neuron's pattern of connections.

Animals↗

PubMed1996

Motor-skill learning: changes in synaptic organization of the rat cerebellar cortex.

Rats trained on motor-skill learning tasks for 30 days were previously found to have more synapses in the volume of tissue proportional to a Purkinje cell than rats that exercised or were inactive. In the motor learning tasks, hooded rats were required to traverse an obstacle course requiring balance and coordination. Rats in two exercise groups were required to walk rapidly or allowed to run in activity wheels. Controls were relatively inactive in standard housing and handled once daily. Synapses were classified to determine which synaptic types changed in number across levels of the molecular layer in the paramedian lobule. The motor learning group had significantly more parallel fiber synapses and climbing fiber synapses per unit Purkinje cell reference volume than all other groups. There were also more synapses and more parallel fiber synapses per reference volume in the outermost than in the innermost molecular layer. The plasticity reported here occurs in vivo under normal physiological conditions. Excitatory synapses account for at least 80% of the synapses in the molecular layer. The results support prior predictions that parallel fiber synapses are modifiable during conditions of learning.

Animals↗

PubMed1996

Rapid laminar-dependent changes in GFAP immunoreactive astrocytes in the visual cortex of rats reared in a complex environment.

Neuronal changes in the visual cortex have previously been found to occur within days of housing weanling rats in a complex environment (EC) compared to rats housed in standard laboratory cages (IC). In contrast, layer IV astrocytes immunostained for glial fibrillary acidic protein (GFAP) have been found to be slow to change. Recent quantitative analysis has shown the surface density of GFAP immunoreactive (GFAP-IR) astrocytes in young rats to be significantly lower in layer IV in comparison to layer II/III. In the present study, the analysis of experience effects on GFAP-IR astrocytes was extended to include layer II/III as well as layer IV of EC and IC rats. The surface density of GFAP-IR processes was found to be significantly increased within layer II/III after 4-10 days of EC rearing in comparison to IC rats. Consistent with previous findings, housing condition did not significantly affect GFAP-IR within layer IV during these early time points. It is possible that GFAP immunocytochemistry is not a sensitive means of detecting experience-induced early changes in astrocytes within layer IV of weanling rats. The rapid astrocytic changes detected in layer II/III are suggestive of a close relationship between astrocytic plasticity and experience-induced synaptic plasticity.

Animals↗

PubMed1996

Calcium ion impedes translation initiation at the synapse.

Stimulation of synaptoneurosome suspensions by the neurotransmitter glutamate gives rise to rapid loading of ribosomes onto mRNA and increased incorporation of amino acids into trichloroacetic acid-precipitable polypeptides. Metabotropic glutamate receptors (mGluRs) are responsible for this effect. Although simultaneous Ca2+ entry and mGluR stimulation do not change the response, entry of Ca2+ 30 s or 3 min before mGluR stimulation markedly depresses the polyribosomal loading. Either NMDA or ionophore (A23187) produces the depression. A calmodulin antagonist, W7, alleviates the effect, suggesting that inactivation of phospholipase A2 by calcium-calmodulin-dependent kinase II is partially responsible for the phenomenon. Thus, interaction between different classes of glutamate receptors affects the control of protein translation at the synapse. This effect may partially explain recent observations of negative interactions between receptor classes in induction of long-term potentiation.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1995

LTP varies across the estrous cycle: enhanced synaptic plasticity in proestrus rats.

Previous studies have shown that the number of dendritic spines and synapses in hippocampal CA1 stratum radiatum decreases more than 30% between the proestrus (high estrogen) and estrus (low estrogen) phases of the rat estrous cycle [10,27]. In the present study, we investigated whether hippocampal synaptic plasticity, as measured by long-term potentiation (LTP), might also vary across the estrous cycle of the female rat. Male rats, and female rats at each phase of the estrous cycle were tested in either the morning or afternoon. There were no significant group differences in the pre-LTP I/O curves. However, females examined during the afternoon of proestrus, the phase during which prior studies indicate synapse number to be highest, demonstrated the greatest degree of potentiation. Diestrus, proestrus and estrus females tested in the morning demonstrated similar amounts of potentiation. There were also significant differences in post-LTP I/O curves between the afternoon proestrus females and males tested in the afternoon. These results suggest that gonadal hormones, interacting with the time of day, may regulate neural processes underlying learning and memory.

Analysis of Variance↗

PubMed1995

Expression of DMAP-45R in the rat visual cortex is modulated by visual experience.

Effects of visual experience upon expression of a developmentally regulated microtubule-associated protein (MAP) were studied in the visual cortex of monocularly deprived rats. The antibody Drosophila MAP-45 (DMAP-45) recognizes proteins in the developing ventral nerve cord of Drosophila and in rat brain. Monocular deprivation from day 12, before eye opening, to day 80 reduced the number of DMAP-45 immunoreactive layer V pyramidal cell apical dendrites in the monocular segment (Oc1M) of the visual cortex contralateral to the deprived eye. No significant visual deprivation effects were seen in the binocular segment (Oc1B). Immunoreactivity was restored to control levels in Oc1M of rats in which the monocular sutures were removed at day 75, subsequently allowing 5 days of exposure to light. These results indicate potential involvement of this MAP in experience-dependent structural plasticity.

Animals↗

PubMed1995

Monocular deprivation alters the morphology of glial fibrillary acidic protein-immunoreactive astrocytes in the rat visual cortex.

Monocular deprivation was used to examine the experience-dependent structural plasticity of astrocytes in Oc1M and Oc1B visual cortex of young and adult rats. Stereological techniques were employed to assess the numerical density (Nv) of cells and surface density (Sv) of processes immunoreactive for glial fibrillary acidic protein in laminae II/III, IV, V and VI in the hemisphere opposite the deprived eye. In one group of pups eyelids were sutured on postnatal day 12 (P12) and maintained until P80 (MD), while a second group had the sutures removed at P75 followed by 5 days of light exposure (MD + L). An unoperated light experienced group was used for comparisons (L). The Sv of astrocytic processes in lamina IV but not laminae II/III, V and VI was significantly decreased in the MD group. The ratio of Sv to the Nv of neurons, an estimate of the amount of astrocytic membrane per neuron, was also significantly decreased in layer IV. The Nv of astrocytes was not significantly different among the three groups. In adults that were monocularly deprived for 5, 10 and 30 days the Nv of astrocytes and Sv of their processes were not significantly altered in layer IV. There was however an increase in the Nv of all types of glial cells combined in layer IV following 10 and 30 days. These results indicate that the structure of astrocytes is influenced by visual experience during development whereas merely altering the level of visually-driven activity in the adult was not sufficient to induce astrocytic structural change.

Age Factors↗

PubMed1995

Morphogenesis in memory formation: synaptic and cellular mechanisms.

We review some of the evidence for structural changes in synapses in response to environmental stimulation. These include changes in synapse number, in distribution of presynaptic vesicles, in synaptic bouton size, and complex changes in the shape and size of synaptic contact zones. Increased numbers of postsynaptic polyribosomal aggregates (PRA) are correlated histologically with developmental plasticity. We discuss the role that dendritically targeted mRNAs and polyribosomes might play in providing rapid, localized synthesis of proteins necessary for structural change. Using synaptoneurosomes, we have demonstrated that depolarization leads to a rapid (1-2 min) increase in PRA and in [35S]methionine incorporation into polypeptides. We have shown that this process is initiated by metabotropic glutamate receptors, which trigger phosphatidyl inositol hydrolysis, leading to release of internal Ca2+ stores and activation of protein kinase C. Entry of external Ca2+, however, seems to downregulate polyribosomal aggregation, via a calmodulin-dependent mechanism, suggesting that translation may be controlled by interaction of ionotropic receptors, voltage-dependent calcium channels, and metabotropic receptors.

Animals↗

PubMed1995

Differential rearing alters spine density on medium-sized spiny neurons in the rat corpus striatum: evidence for association of morphological plasticity with early response gene expression.

Morphological plasticity of medium-sized spiny neurons of the striatum was examined in Long-Evans hooded rats reared in complex or individual cage environments. Rat pups, aged 28-32 days, were housed either individually in standard laboratory cages or as a group in a large toy- and object-filled environment for 30 days. The spine density on dendrites of medium-sized spiny neurons in the dorsolateral striatum was then examined using the Golgi-Cox method. Rats reared in the complex environment displayed an increase of approximately 30% in spine density relative to those reared individually. These results demonstrate experience-dependent changes in neural structure in the striatum and suggest that the mechanisms for information storage in response to experience may be more widespread in the forebrain than previously believed.

Animals↗

PubMed1995

Correspondence between sites of NGFI-A induction and sites of morphological plasticity following exposure to environmental complexity.

To determine if gene regulation may play a role in behaviorally-induced morphological plasticity in the brain, we used in situ hybridization to measure levels of mRNA for the immediate early gene transcription factor NGFI-A (also known as ZENK, zif/268, egr-1 and Krox 24). Brains of periadolescent male rats exposed to 2-4 days of the following behavioral treatments were compared: (1) group housing in a complex environment (EC); (2) individual housing with daily handling (HIC); and (3) individual handling (IC). Quantitative analysis of the autoradiograms revealed that EC rats had significantly higher levels of NGFI-A than IC rats in regions of cortex previously shown to exhibit morphological plasticity (most pronounced in visual cortex), but not in frontal cortex where no dendritic changes have been detected. HIC rats were intermediate between the two groups. These data support an association between structural plasticity and altered patterns of immediate early gene expression.

Animals↗