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O Steward

Publications and source records attributed to O Steward.

At least 109 records · Page 6Linked to original sources
PubMed1987

Ganglioside treatments reduce locomotor hyperactivity after bilateral lesions of the entorhinal cortex.

The present study evaluated the effects of exogenous gangliosides (membrane glycolipids) on open-field locomotor activity after bilateral lesions of the entorhinal cortex. Saline-treated rats showed a dramatic increase in activity followed by a time-dependent recovery (i.e. return toward control levels). Ganglioside-treated (50 mg/kg total gangliosides; i.m.) rats exhibited a similar pattern of changes in activity, except that their level of hyperactivity at 2-6 days postlesion was reduced. Thus, exogenous gangliosides lessened the severity of the lesion-induced hyperactivity, but did not enhance the total net recovery.

Animals↗
PubMed1987

Increases in ribosomal RNA within the denervated neuropil of the dentate gyrus during reinnervation: evaluation by in situ hybridization using DNA probes complementary to ribosomal RNA.

Previous studies have revealed that there are increases in the incorporation of [3H]amino acids into protein in the denervated neuropil of the dentate gyrus during periods of reactive synaptogenesis. The present study evaluates whether the increase in incorporation reflects an increase in protein synthetic machinery (ribosomes) in the denervated zone. We evaluated the distribution of ribosomal RNA (rRNA) in the denervated dentate gyrus 2-14 days after unilateral destruction of the entorhinal cortex using DNA probes complementary to rRNA for in situ hybridization. Animals with comparable lesions were injected with [3H]leucine 30 min prior to sacrifice and prepared for autoradiography in order to define the extent of protein synthesis within the denervated neuropil. Quantitative analyses revealed that the increases in [3H]leucine incorporation were accompanied by increases in labeling with the rRNA probe. In both cases, the increases were first apparent at 2 days postlesion, reached a peak on day 6, and then declined between 8 and 14 days postlesion. Plots of grain density across the neuropil revealed that the increases in rRNA, like the increases in amino acid incorporation, occurred selectively within the denervated portion of the neuropil. We propose that increased incorporation of protein precursor is the result of an increase in protein synthetic machinery within the denervated neuropil. These increases may reflect in part the increases that we have previously noted in polyribosomes under dendritic spines.

Animals↗
PubMed1986

Chronic epileptogenesis induced by kindling of the entorhinal cortex: the role of the dentate gyrus.

The role of the dentate gyrus (DG) in the development and maintenance of kindling induced by periodic electrical stimulation of the entorhinal cortex (EC) was evaluated in rats. Colchicine, a selective neurotoxin for granule cells of the DG, was injected into the DG: prior to kindling and after the development of kindling. Prior destruction of the DG delayed the development of afterdischarge (AD) induced by EC stimulation, but kindling proceeded at normal rates after the first AD was induced. Destruction of the DG after kindling did not abolish the kindled seizures. Thus, the DG was not required for either the development or maintenance of kindling by EC stimulation, but an intact perforant path input from the EC to DG facilitated the emergence of epileptogenesis by kindling of the EC. The results suggest that kindling develops and is maintained in a network of multiple pathways which are related to the site of stimulation. The DG appears to be the site of a temporally specific alteration which facilitates the development of kindling by EC stimulation.

Animals↗
PubMed1986

Emergence of the capacity for LTP during reinnervation of the dentate gyrus: evidence that abnormally shaped spines can mediate LTP.

The present study analyzes how the capacity for LTP emerges during lesion-induced sprouting of the crossed temporo-dentate (CTD) pathway of the rat. Adult rats received unilateral entorhinal lesions and were allowed to survive for intervals from 6 to 40 days. The CTD pathway was then studied using conventional acute neurophysiological procedures. Extracellular field potentials were used to measure the synaptic efficacy of the CTD pathway before and after 400 Hz conditioning stimulation (the typical regimen for inducing LTP in the temporo-dentate system). The normal CTD pathway does not exhibit LTP, as noted in previous studies. LTP was first observed in animals recorded at 8-10 days post-lesion, although the increases in synaptic efficacy were not statistically significant until days 12-16 post-lesion. Electron microscopic analyses of the spine and synapse population of the dentate molecular layer at 8 days post-lesion reveal that spines on the postsynaptic cells are structurally immature when the capacity for the LTP first appears. These results are discussed as they relate to the postulated role of the CTD in behavioral recovery following entorhinal cortical lesions, and the potential cellular mechanisms of LTP.

Animals↗
PubMed1986

Quantitative analysis of synaptic potentiation during kindling of the perforant path.

Synaptic transmission was studied during the development of kindling in the pathway from entorhinal cortex (EC) to dentate gyrus (DG) of unrestrained unanesthetized rats using chronic neurophysiological techniques. Extracellular field potentials were recorded from the DG in response to activation of the perforant pathway with 0.1-ms constant current square-wave pulses. The evoked field potentials consisted of a population EPSP (a reflection of excitatory synaptic activation) and a population spike (a measure of synchronous postsynaptic discharge of granule cells). Synaptic efficacy was quantitated in this pathway by measurement of the population EPSP slope and population spike amplitude across a range of stimulus intensities from threshold to maximal evoked response. Input-output relationships for population EPSP and population spike were determined at regular intervals during the course of kindling, corresponding to the stages of evoked behavioral seizures. Increases in the population EPSP and population spike were observed after a single kindling stimulus that evoked afterdischarge (AD) when behavioral seizures were minimal. Evaluation of the input-output relationships for the group of kindled animals at the various stages of evoked behavioral seizure activity revealed that increases in the population EPSP continued to slowly evolve with repeated stimulations but that increases in the population spike were maximal after one or at most a few stimulations that evoked AD. The increases in both population EPSP and population spike persisted for the duration of the recording, i.e., through induction of generalized motor convulsions. To evaluate the translation of synaptic activation into cell discharge during kindling, we made use of the population spike/population EPSP ratio across a range of stimulus intensities. The spike/EPSP ratios revealed a dissociation of the population spike and population EPSP early in the course of kindling during class 1 seizures. Specifically, after induction of an AD, an extracellular population EPSP of a given size evoked a larger population spike than an EPSP of comparable size before the induction of an AD by kindling stimulation. The development of generalized motor convulsions (class 5 seizures) was associated with a reduction in the spike/EPSP ratio. The mechanism of this reduction in spike/EPSP ratio is uncertain, but since synaptic activation (as reflected by population EPSP) did not decline during class 5 seizures, the reduction in the spike/EPSP ratio could be consistent with increased inhibition after generalized motor convulsions, or could reflect a decrease in granule cell excitability.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Polyribosomes associated with synaptic specializations on axon initial segments: localization of protein-synthetic machinery at inhibitory synapses.

Previous studies have revealed a selective association between polyribosomes and axospinous synapses in a variety of brain regions. The present study evaluates whether polyribosomes are also associated with the symmetrical and presumably inhibitory synaptic connections found on the initial segment of axons of some neurons in the CNS. The initial segments of pyramidal neurons in the sensorimotor cortex of the monkey and of granule cells in the hippocampus of the rat were examined. The initial segments of these cell types are contacted by GABAergic terminals that form symmetrical synaptic connections. In the present study, these initial segments were found to contain polyribosomes that tended to be selectively localized beneath the synaptic specializations. Both the synaptic connections and the polyribosomes were localized to the initial segment; after the point at which the axon became myelinated, neither synapses nor polyribosomes were found. The association between polyribosomes and synapses was also suggested by the position of the polyribosomes with respect to the synapse. In each cell type, the majority of the polyribosomes that were present in the initial segments appeared to be localized preferentially beneath synaptic sites, although some polyribosomes were also present in the core of the axon. These data reveal that polyribosomes are not peculiar to spine synapses, but rather are ubiquitous components of the subsynaptic region of many types of synapses. We propose that neurons may regulate their innervation by positioning protein-synthetic machinery (and appropriate mRNA molecules) at particular locations in order to construct particular types of synapses at defined positions on the postsynaptic cells' receptive surface.

Animals↗
PubMed1986

Protein-synthetic machinery at postsynaptic sites during synaptogenesis: a quantitative study of the association between polyribosomes and developing synapses.

Previous studies have revealed dramatic accumulations of polyribosomes under growing spine synapses, suggesting a critical role for protein synthesis at the postsynaptic site during synaptogenesis. The present study quantitatively analyzes the distribution of polyribosomes under synapses during developmental synaptogenesis in the rat's dentate gyrus. The middle molecular layer of the suprapyramidal blade of the dentate gyrus was examined electron-microscopically at 1, 4, 7, 10, 15, 20, and 28 d of age. At each age, we evaluated (1) synapse density (the number of synapses/100 micron2 of neuropil), (2) the width of the molecular layer, (3) the proportion of spine synapses with underlying polyribosomes, and (4) the number of polyribosome-containing synapses/1000 micron2 of neuropil. From the first two measures, an estimate was obtained of the total extent of synaptogenesis, taking into account both the increase in synapse density and the increase in total area of neuropil. At 1 d of age, very few synapses were found in the molecular layer of the dentate gyrus, and those that were present were quite immature in appearance. Synapse density increased about 140-fold between 1 and 28 d of age, from an average of 0.36 synapses/100 micron2 at 1 d of age to 49 synapses/100 micron2 at 28 d of age. An inverse relationship was found between synapse density and the proportion of synapses with polyribosomes. Between 1 and 7 d of age, about 60% of the spine synapses had one or more polyribosomes under the spine base. Thereafter, the proportion of spines with polyribosomes decreased as synapse density increased. Similarly, the proportion of shaft synapses with underlying polyribosomes was greatest between 1 and 7 d postnatal, and decreased thereafter. While the proportion of synapses with polyribosomes was greatest between 1 and 7 d, the actual number of polyribosome-containing synapses/1000 micron2 of neuropil was negligible at 1 d, increased to a peak at 7 d of age, and then decreased as synapse density increased. Qualitatively, the most dramatic accumulations of polyribosomes were also found at 7 d of age. We conclude that spine-associated polyribosomes represent a structural specialization of dendrites at sites of synapse construction and as such may represent a marker for growing synapses. We propose that these elements produce protein(s) that are critically involved in the formation of the synaptic contact.

Animals↗
PubMed1985

Afferent influences on brain stem auditory nuclei of the chicken: cessation of amino acid incorporation as an antecedent to age-dependent transneuronal degeneration.

Previous studies of the avian auditory system have revealed that removal of the peripheral receptor (the cochlea) leads to a transneuronal degeneration of auditory relay neurons in nucleus magnocellularis (NM) of the brain stem. An early manifestation of the degeneration which can be observed within 12 hours is a decrease of histochemical staining for RNA (Nissl staining); such a decrease could reflect an alteration in protein synthetic activity within the NM neurons. The present study evaluates this possibility by determining whether the cochlea removal led to an alteration incorporation of protein precursors in the target neurons which exhibit transneuronal degeneration and if so, how early the changes appeared. The cochlea was removed unilaterally in seventeen 10-day-old chicks and two 66-week-old mature chickens, and incorporation of protein precursors was evaluated in the neurons of NM at 0.5, 1.5, 3, 6, 12, and 24 hours following the cochlea removal. Each chick received an intravenous injection of 3H leucine, and was allowed to survive for 30 minutes after the injection of precursor. The brains were then prepared for autoradiography. The extent of incorporation by neurons in NM was determined by counting grains overlying each cell body and determining grain density/micrometers2 of neuron cross-sectional area. We found that auditory relay neurons whose synaptic inputs have been silenced exhibit dramatic decreases in protein synthesis within 30 minutes after removal of the cochlea; leucine incorporation was reduced by about 50%. In chicks sacrificed 3 to 24 hours after removal of the cochlea, some neurons (about 1/3) were entirely unlabeled despite heavy labeling of their neighbors and heavy labeling of all NM neurons on the opposite side of the brain. The remaining neurons exhibited about a 15% reduction in incorporation in comparison with the cells in the contralateral (control) NM. While the decreases in incorporation were apparent at all survival intervals, there was no consistent decrease in Nissl staining until 6 hours after cochlea removal. There were no changes in protein precursor incorporation following removal of the cochlea in adult birds, a result which is in keeping with the relative absence of transneuronal degeneration following removal of the cochlea at maturity. The results suggest a very rapid transneuronal regulation of protein metabolism within target neurons in young animals, perhaps by activity-related events.

Amino Acids↗
PubMed1985

Polyribosomes under developing spine synapses: growth specializations of dendrites at sites of synaptogenesis.

We have previously reported that there is a dramatic increase in polyribosomes associated with dendritic spines during periods of synapse growth induced by denervating lesions. We suggested that polyribosomes at the postsynaptic site may somehow be involved in the growth of synapses. To evaluate this hypothesis further, the present study determines whether synapses which are growing in the developmental period also have accumulations of polyribosomes. We examined the dentate gyrus of the developing rat electron microscopically at 7, 10, 15, 20, and 28 days of age, which spans the major period of synaptogenesis in this structure. Qualitative observations revealed dramatic accumulations of polyribosomes under spine synapses in the youngest animals (7 and 10 days of age). With synapse development, the accumulations of polyribosomes became less dramatic, so that by 28 days of age, the neuropil of the dentate gyrus appeared qualitatively mature. To determine the relationship between polyribosomes under spine synapses and synapse development, quantitative electron microscopic methods were use to evaluate synapse density (number of synapses/100 micron 2), and the incidence of polyribosome-containing spines (proportion of spine synapses with underlying polyribosomes) in the neuropil of the dorsal blade of the dentate gyrus at each age. An inverse relationship was found between synapse density and the proportion of spines with polyribosomes. Synapse density increased in an almost linear fashion between 7 and 28 days of age to levels which were actually somewhat higher than in mature rats, whereas the incidence of polyribosome-containing spines was highest at the youngest ages and decreased with development. Thus, polyribosomes were most prominent under spine synapses during the period of maximal synapse growth. These results, together with our previous observations of increased numbers of polyribosomes under spines during lesion-induced growth, suggest that the polyribosomes represent a structural specialization of dendrites at sites of synapse construction. We propose that they produce protein(s) that are involved in synapse growth.

Animals↗
PubMed1985

Transplantation of embryonic chick spinal cord into transection adult chicken spinal cords: a useful model for transplantation research.

Adult chicken spinal cords were completely transected under direct visualization. One week later, embryonic chick spinal cords 6 days, 9 days, or 11 days old were implanted into the site of transection. The animals were allowed to survive for 2 months and then killed and the spinal cords were prepared for histologic analysis. The survival and outgrowth of the embryonic transplants were compared with regard to the age of the implant, and its effects on the host tissue. Six-day embryonic tissue survived and integrated far better than 9-day or 11-day, and adult spinal cord subjacent to the early age embryo showed fewer degenerative changes. The chick embryo may provide a model for use in CNS transplantation.

Aging↗
PubMed1985

Axonal degeneration induced by experimental noninvasive minor head injury.

Minor head injury or concussion was produced in experimental animals by an acceleration-deceleration non-impact injury. The animals sustained a brief loss of consciousness and no sequelae were observed. The brains were examined at 7 days by means of the Nauta and Fink-Heimer techniques. Degenerating axons were noted in the inferior colliculus, pons, and dorsolateral medulla. Degeneration was not seen in the subcortical white matter, thus suggesting a primary brain-stem locus for concussion. These findings also suggest that, in some instances, minor head injury or concussion can be associated with organic damage to the central nervous system.

Animals↗
PubMed1984

Neurotoxicity of colchicine and other tubulin-binding agents: a selective vulnerability of certain neurons to the disruption of microtubules.

Colchicine and certain other agents which disrupt microtubules and interfere with axonal and dendritic transport are highly toxic to certain CNS neurons. The present chapter summarizes our knowledge about this selective neurotoxicity. Injections of colchicine into several bran regions lead to the death of selected populations of neurons within those regions. Intra-hippocampal injections selectively destroy granule cells of the dentate gyrus; hippocampal pyramidal cells are essentially unaffected. Injections into the cerebellum, olfactory bulb, and caudate nucleus also destroy resident neurons. In these areas several cell types are vulnerable. Neurons of the cerebral cortex appear to be much less affected by colchicine, although some neurons of paleocortical regions are vulnerable. Colchicine does not appear to be an excitotoxin like kainic acid. The neurotoxicity of colchicine appears to be related to the destruction of microtubules, since other agents which disrupt microtubules have similar toxic effects, and since analogs of colchicine which do not disrupt microtubules are non-toxic. Colchicine may induce an autotoxic response which leads to neuronal death in certain populations due to the accumulation of some toxic cellular product which is normally transported by a microtubule-dependent process. The selective vulnerability of neurons to the neurotoxic effects of colchicine may be a model for system degenerations of the central nervous system in which certain subpopulations of neurons are selectively vulnerable to abnormal accumulations of metabolic products.

Animals↗
PubMed1984

MAP2 is localized to the dendrites of hippocampal neurons which develop in culture.

The distribution of the microtubule-associated protein MAP2 in cultured hippocampal neurons was studied using immunocytochemistry with monoclonal antibodies. MAP2 was preferentially localized to dendritic, but not axonal, processes even in single isolated cells which developed without making intercellular contacts. Hence regional differences in the molecular composition of the neuronal cytoskeleton can develop independently of cell interactions. The presence of MAP2 may be a useful marker for identifying dendrites in cell culture.

Animals↗
PubMed1984

On the role of hippocampal connections in the performance of place and cue tasks: comparisons with damage to hippocampus.

Behavioral changes following interruption of the main connections of hippocampus and closely related areas (entorhinal cortex, mammillary bodies, dentate gyrus) were determined and compared with findings of previous research that involved direct damage to hippocampus. By a within-subjects design, rats were trained to run in a radial maze with a procedure that involved two kinds of learning (place and cue) and two memory functions (working and reference memory). Rats with fimbria-fornix and entorhinal cortex lesions were impaired on both the place and the cue task. Specifically, the animals suffered a general impairment in working memory on both tasks but were impaired in reference memory only on the place task. Animals with lesions of the dentate gyrus and mammillary bodies were able to perform the complex place and cue tasks with minimal problems. In previous research it was found that direct damage to hippocampus (including all cell fields, alveus, fimbria) resulted in impaired performance only on the place task (Jarrard, 1983). Taken together, these findings indicate that interruption of hippocampal input/output pathways and/or damaging some closely related structures has a greater effect on the behaviors studied than does direct damage to hippocampus.

Animals↗
PubMed1984

Differential subcellular localization of tubulin and the microtubule-associated protein MAP2 in brain tissue as revealed by immunocytochemistry with monoclonal hybridoma antibodies.

The distribution and subcellular localization of tubulin and MAP2 in brain tissue were analyzed by immunocytochemistry with monoclonal hybridoma antibodies prepared against Chinese hamster brain tubulin and MAP2. We examined three anti-tubulin hybridoma antibodies (Tu3B, Tu9B, Tu12) specific for beta-tubulin, and two anti-MAP2 hybridoma antibodies (AP9,AP13). The specificity of each of the monoclonal antibodies was characterized by staining nitrocellulose electrophoretic blots of SDS-polyacrylamide gels of whole brain or hippocampal extracts. Each hybridoma antibody bound only its respective antigen in these preparations. Polyclonal antisera against tubulin were also examined. Sections reacted with antisera against tubulin or monoclonal antibodies against beta-tubulin revealed a wide variety of stained cellular compartments. The reaction product was found to decorate dendritic and axonal microtubles in neurons; glial cells were also stained. MAP2 immunoreactivity was found only in neurons. In the case of one of the monoclonal antibodies (AP9), staining was preferentially associated with dendritic processes. However, light but significant staining of axonal processes was seen with AP13. Within dendrites, MAP2 was found associated with dendritic microtubules and postsynaptic densities (psd), both in shaft and spine synapses. In addition, strong immunoreactivity for MAP2 was found within the cytoplasm of dendritic spines. There was little or no immunoreactivity for tubulin in the spine cytoplasm, although the psd was stained. The localization of MAP2 in dendritic spines and in the psd suggests that this protein may have a biological role independent of its association with microtubules. The observations on differential staining of the hybridoma antibodies against MAP2 suggest that there may be distinct subtypes or states of MAP2 within neurons.

Animals↗
PubMed1983

Mechanisms of colchicine neurotoxicity in the dentate gyrus: dissociation of seizures and cell death.

Extracellular field potentials and the EEG were studied in the dentate gyrus of the rat after intrahippocampal injections of colchicine, which is a relatively selective neurotoxin for dentate granule cells. Injection of colchicine (0.5 microliters of a 5-mg/ml solution of colchicine in deionized water) resulted in granule cell hyperexcitability manifested by multispike field potentials in response to stimulation of the excitatory projections from the entorhinal cortex. In anesthetized rats, this state of granule cell hyperexcitability was occasionally accompanied by interictal epileptic spiking or brief electrographic seizures, but granule cell death was observed even in the absence of epileptic activity. Injection of colchicine into the CA1 area of the hippocampus also resulted in multispike field potentials in response to stimulation of the CA3 commissural pathway, but CA1 pyramidal cells were not destroyed by colchicine. Colchicine has been reported to act as a convulsant agent in the dentate gyrus, but it is a relatively selective neurotoxin for dentate granule cells even in the absence of epileptic activity.

Animals↗