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At least 1,621 records · Page 90Linked to original sources

PubMed2006

Bovine tuberculosis in southern African wildlife: a multi-species host-pathogen system.

This review examines the current situation of bovine tuberculosis (bTB) in southern African savannah systems, and uses theory on multi-species host-pathogen systems to suggest possible options for future research and management. In southern Africa, the buffalo (Syncerus caffer) and the Kafue lechwe [Marsh antelope] (Kobus leche) have been found to be maintenance hosts for this disease, but the importance of other host species is becoming apparent. The role of other host species in the maintenance and spread of the disease varies, depending on the spatial distribution and resource utilization patterns of the species, disease susceptibility, transmission modes and the ecology of both host(s) and vector(s). Future research needs to identify the pathogenicity of bTB in each of the host species, and the mechanisms and rates of inter- and intra-specific transmission among different species, in order to develop multi-host models to understand the development and spread of the disease.

Africa, Southern↗

PubMed2000

Constant holdup times in gas chromatography by programming of column temperature and inlet pressure

The requirements for attaining constant holdup times in capillary gas chromatography (GC), while linearly programming both column temperature and inlet pressure, are illustrated. A scheme, which is based on a series of responses from isothermal and isobaric conditions, is proposed. The responses from two commonly used GC instruments, one with a mass-selective detector operating at near-vacuum pressure and another with a flame-ionization detector at ambient pressure, are analyzed. A significant deviation from Poiseuille flow is noted due to the use of helium as a carrier gas. Nonetheless, the experimental holdup times are approximately constant over a range of temperatures and pressures. Theoretical analysis reveals the spatial and temporal dependence of flow rates inside the column during the programmed runs.

Journal Article↗

PubMed2005

Diffusion of the synthetic pyrethroid permethrin into bed-sediments.

Bed-sediments are a sink for many micro-organic contaminants in aquatic environments. The impact of toxic contaminants on benthic fauna often depends on their spatial distribution, and the fate of the parent compounds and their metabolites. The distribution of a synthetic pyrethroid, permethrin, a compound known to be toxic to aquatic invertebrates, was studied using river bed-sediments in lotic flume channels. trans/cis-Permethrin diagnostic ratios were used to quantify the photoisomerization of the trans isomer in water. Rates were affected by the presence of sediment particles and colloids when compared to distilled water alone. Two experiments in dark/light conditions with replicate channels were undertaken using natural sediment, previously contaminated with permethrin, to examine the effect of the growth of an algal biofilm at the sediment-water interface on diffusive fluxes of permethrin into the sediment. After 42 days, the bulk water was removed, allowing a fine sectioning of the sediment bed (i.e., every mm down to 5 mm and then 5-10 mm, then every 10 mm down to 50 mm). Permethrin was detected in all cases down to a depth of 5-10 mm, in agreement with estimates by the Millington and Quirk model, and measurements of concentrations in pore water produced a distribution coefficient (Kd) for each section. High Kd's were observed for the top layers, mainly as a result of high organic matter and specific surface area. Concentrations in the algal biofilm measured at the end of the experiment under light conditions, and increases in concentration in the top 1 mm of the sediment, demonstrated that algal/ bacterial biofilm material was responsible for high Kd's at the sediment surface, and for the retardation of permethrin diffusion. This specific partition of permethrin to fine sediment particles and algae may enhance its threat to benthic invertebrates. In addition,the analysis of trans/cis-permethrin isomer ratios in sediment showed greater losses of trans-permethrin in the experiment under light conditions, which may have also resulted from enhanced biological activity at the sediment surface.

Bacteria↗

PubMed1984

Design and synthesis of naltrexone-derived affinity labels with nonequilibrium opioid agonist and antagonist activities. Evidence for the existence of different mu receptor subtypes in different tissues.

A series of beta-funaltrexamine (2, beta-FNA) analogues (3-14) were synthesized that contain a variety of electrophilic groups attached at the 6 beta-position of the opiate. The opioid agonist and antagonist activities of these ligands were evaluated in the guinea pig ileum (GPI) and mouse vas deferens (MVD) in vitro assays. Several of the compounds behaved like beta-FNA in that they exhibited reversible agonist activity at kappa opioid receptors and irreversible antagonist activity at mu opioid receptors. The rank order of irreversible antagonism for a series of related Michael acceptors did not parallel their intrinsic chemical reactivity, confirming that the degree of covalent binding is in part dependent on the spatial disposition of the electrophilic center relative to the receptor nucleophile (secondary recognition). The maleimidoacetamide 8 behaved very differently from beta-FNA in that it exhibited considerably greater irreversible mu antagonism in MVD relative to the mu blockage in the GPI. This suggests that different proportions of mu receptor subtypes exist in the two tissues. Several of the agents tested, including some nonreactive control compounds, displayed an unusual type of persistent kappa agonist activity in the GPI. This activity, which was reversed by addition of naloxone, reappeared upon washing. Receptor models have been presented to explain this effect. A few of the reactive ligands displayed a true nonreversible kappa agonist activity, suggesting a covalent association with the receptor. Of note in this regard was the propiolamide 6, which appeared to be an irreversible mixed agonist-antagonist at kappa and mu receptors.

Affinity Labels↗

PubMed2006

Dimensional dependence of electronic structure of fullerene polymers.

Fullerene polymers made of C(60) are systematically investigated by means of a first-principles pseudopotential approach within the local density approximation of the density functional theory. We assume 10 different structures of fullerene polymers. The first three are C(60) polymer networks cross-linked by [2+2] cycloadditional four-membered rings, and the other seven are composed of peanut-shaped fused C(60) polymer chains cross-linked by either seven-membered rings or eight-membered rings. Owing to the overlap of wave functions as well as the hybrid networks of sp(2)-like (3-fold coordinated) and sp(3)-like (4-fold coordinated) carbon atoms, the electronic structure is considerably different from each other. We find that the resulting electronic structure is either semiconductor or semimetal depending on the spatial dimensionality of materials.

Journal Article↗

PubMed2005

Influence of surface chemistry on the SAXS response of polymer-based activated carbons.

Small-angle X-ray scattering (SAXS) measurements using contrast variation are reported for activated carbons prepared from poly(ethyleneterephthalate). The carbon surfaces are functionalized to different degrees by cold and hot nitric acid treatment. The latter treatment reduces the surface area by 75%, but the pore size distribution in the micropore range is hardly affected. Seven liquids, n-hexane, i-octane, i-propanol, cyclohexane, toluene, alpha-pinene, and nitrobenzene, in addition to water vapor, were used as contrast modifiers. Although the values of the specific surface area S(X) deduced from these measurements are relatively insensitive to the solvent, the detailed SAXS spectra reveal interactions occurring on different spatial scales that depend on the surface chemistry of the carbon and on the physicochemical properties of the solvent. In the most heavily oxidized sample, the amphiphilic molecule i-propanol stabilizes the surface structure, whereas nonpolar molecules make the rough surface smoother. In the untreated and the lightly functionalized carbons, water vapor at 50% relative humidity condenses only partially in the micropores at room temperature, whereas in the heavily treated sample condensation in the micropores is practically complete.

Journal Article↗

PubMed1999

A strong constitutive positive element is essential for the ammonium-regulated expression of a soybean gene encoding cytosolic glutamine synthetase.

In order to identify important promoter elements controlling the ammonium-regulated expression of the soybean gene GS15 encoding cytosolic glutamine synthetase, a series of 5' promoter deletions were fused to the GUS reporter gene. To allow the detection of positive and negative regulatory elements, a series of 3' deletions were fused to a -90 CaMV 35S promoter fragment placed upstream of the GUS gene. Both types of construct were introduced into Lotus corniculatus plants and soybean roots via Agrobacterium rhizogenes-mediated transformation. Both spectrophotometric enzymatic analysis and histochemical localization of GUS activity in roots, root nodules and shoots of transgenic plants revealed that a strong constitutive positive element (SCPE) of 400 bp, located in the promoter distal region is indispensable for the ammonium-regulated expression of GS15. Interestingly, this SCPE was able to direct constitutive expression in both a legume and non-legume background to a level similar to that driven by the CaMV 35S full-length promoter. In addition, results showed that separate proximal elements, located in the first 727 bp relative to the transcription start site, are essential for root- and root nodule-specific expression. This proximal region contains an AAAGAT and two TATTTAT consensus sequences characteristic of nodulin or nodule-enhanced gene promoters. A putative silencer region containing the same TATTTAT consensus sequence was identified between the SCPE and the organ-specific elements. The presence of positive, negative and organ-specific elements together with the three TATTTAT consensus sequences within the promoter strongly suggest that these multiple promoter fragments act in a cooperative manner, depending on the spatial conformation of the DNA for trans-acting factor accessibility.

Base Sequence↗

PubMed1993

Nonvisual navigation by blind and sighted: assessment of path integration ability.

Blindfolded sighted, adventitiously blind, and congenitally blind subjects performed a set of navigation tasks. The more complex tasks involved spatial inference and included retracing a multisegment route in reverse, returning directly to an origin after being led over linear segments, and pointing to targets after locomotion. As a group, subjects responded systematically to route manipulations in the complex tasks, but performance was poor. Patterns of error and response latency are informative about the internal representation used; in particular, they do not support the hypothesis that only a representation of the origin of locomotion is maintained. The slight performance differences between groups varying in visual experience were neither large nor consistent across tasks. Results provide little indication that spatial competence strongly depends on prior visual experience.

Adolescent↗

PubMed1995

Entorhinal-perirhinal lesions impair performance of rats on two versions of place learning in the Morris water maze.

The effects of entorhinal-perirhinal lesions in rats were studied with 2 versions of a place learning task in the Morris water maze. These lesions impaired performance on a multiple-trial task (3 days of 6 trials and a probe trial). This assessment was followed by a task in which rats were repeatedly trained to find novel locations with a variable delay (30 s or 5 min) imposed between each sample trial and retention test. Entorhinal-perirhinal damage produced a delay-dependent deficit in spatial memory: Rats with lesions were impaired at the 5-min delay relative to the control group and to their own performance at 30 s. These findings are discussed in relationship to memory impairment after entorhinal damage and spatial learning deficits observed after hippocampal damage.

Animals↗

PubMed1999

Visuospatial priming and stroop performance in patients with obsessive compulsive disorder.

Two laboratory measures of competitive information processing were studied in obsessive compulsive disorder (OCD) and normal control participants to assess the effects of priming and interfering information on response latency or speed. In the visuospatial priming (VSP) task, key press latency is facilitated or inhibited, depending on the spatial location of a priming cue. In the Stroop task, participants name the ink color of printed words that have an interfering semantic value. OCD participants displayed significantly slowed baseline response latency and increased facilitory priming scores in the VSP task, with no significant difference in VSP inhibition compared with control participants. Higher interference cost in the Stroop task was also observed in OCD participants. Clinical associations between VSP and Stroop performance and specific OCD symptoms were examined. Increased VSP facilitation was most pronounced in OCD participants who reported a history of violent images, tics, "just right" obsessions, or checking compulsions.

Adult↗

PubMed1985

Suppression of visible persistence.

Five experiments were conducted to examine duration of visible persistence in sequences of stimuli. The basic display consisted of a point that stepped around a circular path on the face of an oscilloscope. Observers estimated the number of points seen simultaneously. Results were compared with a control condition in which the points were plotted in random order rather than sequentially. It was found that visible persistence of a point is suppressed if other points are shown nearby and after an appropriate delay. The degree of suppression depended on the spatial proximity of successive points. It was also found that both duration of visible persistence and degree of suppression increase with eccentricity in the visual field. The results are discussed in terms of two independent processes, persistence and suppression, that operate in a hierarchically antithetical relation.

Figural Aftereffect↗

PubMed2005

Intentions determine the effect of invisible metacontrast-masked primes: evidence for top-down contingencies in a peripheral cuing task.

In 5 experiments, the authors tested whether the processing of nonconscious spatial stimulus information depends on a prior intention. This test was conducted with the metacontrast dissociation paradigm. Experiment 1 demonstrated that masked primes that could not be discriminated above chance level affected responses to the visible stimuli that masked them. Experiments 2 and 3 showed that this effect was abolished when the task instruction was changed in such a way that the primes ceased to be task relevant. Experiments 4 and 5 demonstrated that a prime's effect depended on whether it was associated with the same response as the target or with an opposite response.

Adult↗

PubMed1994

A yeast gene necessary for bud-site selection encodes a protein similar to insulin-degrading enzymes.

Cells of the yeast Saccharomyces cerevisiae choose bud sites in a non-random spatial pattern that depends on mating type: axial for haploid cells and bipolar for a/alpha diploid cells. We identified a mutant yeast, axl 1, in which the budding pattern is altered from axial to bipolar. Expression of the AXL1 gene is repressed in a/alpha diploid cells. With the ectopic expression of AXL1, a/alpha cells exhibited an axial budding pattern, thus AXL1 is a key morphological determinant that distinguishes the budding pattern of haploid cells from that of a/alpha diploid cells. AXL1 encodes a protein similar in sequence of the human and Drosophila insulin-degrading enzymes and to the Escherichia coli ptr gene product. The axial budding pattern might result from degradation of a target protein by the putative Axl1 protease.

Amino Acid Sequence↗

PubMed1996

Conservation of hippocampal memory function in rats and humans.

The hippocampus is critical to declarative memory in humans. This kind of memory involves associations among items or events that can be accessed flexibly to guide memory expression in various and even new situations. In animals, there has been controversy about whether the hippocampus is specialized for spatial memory or whether it mediates a general memory function, as it does in humans. To address this issue we trained normal rats and rats with hippocampal damage on non-spatial stimulus-stimulus associations, then probed the nature of their memory representations. We report here that normal rats demonstrated two forms of flexible memory expression, transitivity, the ability to judge inferentially across stimulus pairs that share a common element, and symmetry, the ability to associate paired elements presented in the reverse of training order. Rats with neurotoxic damage limited to the hippocampus demonstrated neither form of flexible expression, indicating that non-spatial declarative processing depends specifically on the hippocampus in animals as it does in humans.

Animals↗

PubMed2002

Establishment of developmental precision and proportions in the early Drosophila embryo.

During embryonic development, orderly patterns of gene expression eventually assign each cell in the embryo its particular fate. For the anteroposterior axis of the Drosophila embryo, the first step in this process depends on a spatial gradient of the maternal morphogen Bicoid (Bcd). Positional information of this gradient is transmitted to downstream gap genes, each occupying a well defined spatial domain. We determined the precision of the initial process by comparing expression domains in different embryos. Here we show that the Bcd gradient displays a high embryo-to-embryo variability, but that this noise in the positional information is strongly decreased ('filtered') at the level of hunchback (hb) gene expression. In contrast to the Bcd gradient, the hb expression pattern already includes the information about the scale of the embryo. We show that genes known to interact directly with Hb are not responsible for its spatial precision, but that the maternal gene staufen may be crucial.

Animals↗

PubMed2001

Form constraints in motion binding.

Visual analyses of form and motion proceed along parallel streams. Unified perception of moving forms requires interactions between these streams, although whether the interactions occur early or late in cortical processing remains unresolved. Using rotating outlined shapes sampled through apertures, we showed that binding local motions into global object motion depends strongly on spatial configuration. Identical local motion components are perceived coherently when they define closed configurations, but usually not when they define open configurations. Our experiments show this influence arises in early cortical levels and operates as a form-based veto of motion integration in the absence of closure.

Form Perception↗

PubMed2006

Biological context networks: a mosaic view of the interactome.

Network models are a fundamental tool for the visualization and analysis of molecular interactions occurring in biological systems. While broadly illuminating the molecular machinery of the cell, graphical representations of protein interaction networks mask complex patterns of interaction that depend on temporal, spatial, or condition-specific contexts. In this paper, we introduce a novel graph construct called a biological context network that explicitly captures these changing patterns of interaction from one biological context to another. We consider known gene ontology biological process and cellular component annotations as a proxy for context, and show that aggregating small process-specific protein interaction sub-networks leads to the emergence of observed scale-free properties. The biological context model also provides the basis for characterizing proteins in terms of several context-specific measures, including 'interactive promiscuity,' which identifies proteins whose interacting partners vary from one context to another. We show that such context-sensitive measures are significantly better predictors of knockout lethality than node degree, reaching better than 70% accuracy among the top scoring proteins.

Protein Interaction Mapping↗

PubMed2004

Perceived luminance depends on temporal context.

Brightness--the perception of an object's luminance--arises from complex and poorly understood interactions at several levels of processing. It is well known that the brightness of an object depends on its spatial context, which can include perceptual organization, scene interpretation, three-dimensional interpretation, shadows, and other high-level percepts. Here we present a new class of illusion in which temporal relations with spatially neighbouring objects can modulate a target object's brightness. When compared with a nearby patch of constant luminance, a brief flash appears brighter with increasing onset asynchrony. Simultaneous contrast, retinal effects, masking, apparent motion and attentional effects cannot account for this illusory enhancement of brightness. This temporal context effect indicates that two parallel streams--one adapting and one non-adapting--encode brightness in the visual cortex.

Contrast Sensitivity↗