229 research outputs found
Configurational temperatures and interactions in charge-stabilized colloid
We demonstrate that the configurational temperature formalism can be derived
from the classical hypervirial theorem, and introduce a hierarchy of
hyperconfigurational temperature definitions, which are particularly well
suited for experimental studies. We then use these analytical tools to probe
the electrostatic interactions in monolayers of charge-stabilized colloidal
spheres confined by parallel glass surfaces. The configurational and
hyperconfigurational temperatures, together with a novel thermodynamic sum
rule, provide previously lacking self-consistency tests for interaction
measurements based on digital video microscopy, and thereby cast new light on
controversial reports of confinement-induced like-charge attractions. We
further introduce a new method for measuring the pair potential directly that
uses consistency of the configurational and hyperconfigurational temperatures
as a set of constraints for a model-free search.Comment: 15 pages, 12 figures, submitted to J. Chem. Phy
Blunted Cystine–Glutamate Antiporter Function in the Nucleus Accumbens Promotes Cocaine-induced Drug Seeking
Repeated cocaine alters glutamate neurotransmission, in part, by reducing cystine–glutamate exchange via system xc−, which maintains glutamate levels and receptor stimulation in the extrasynaptic compartment. In the present study, we undertook two approaches to determine the significance of plasticity involving system xc−. First, we examined whether the cysteine prodrug N-acetylcysteine attenuates cocaine-primed reinstatement by targeting system xc−. Rats were trained to self-administer cocaine (1 mg/kg/200 μl, i.v.) under extended access conditions (6 h/day). After extinction training, cocaine (10 mg/kg, i.p.) primed reinstatement was assessed in rats pretreated with N-acetylcysteine (0–60 mg/kg, i.p.) in the presence or absence of the system xc− inhibitor (S)-4-carboxyphenylglycine (CPG; 0.5 μM; infused into the nucleus accumbens). N-acetylcysteine attenuated cocaine-primed reinstatement, and this effect was reversed by co-administration of CPG. Secondly, we examined whether reduced system xc− activity is necessary for cocaine-primed reinstatement. To do this, we administered N-acetylcysteine (0 or 90 mg/kg, i.p.) prior to 12 daily self-administration sessions (1 mg/kg/200 μl, i.v.; 6 h/day) since this procedure has previously been shown to prevent reduced activity of system xc−. On the reinstatement test day, we then acutely impaired system xc− in some of the rats by infusing CPG (0.5 μM) into the nucleus accumbens. Rats that had received N-acetylcysteine prior to daily self-administration sessions exhibited diminished cocaine-primed reinstatement; this effect was reversed by infusing the cystine–glutamate exchange inhibitor CPG into the nucleus accumbens. Collectively these data establish system xc− in the nucleus accumbens as a key mechanism contributing to cocaine-primed reinstatement
Anomalous tumbling of colloidal ellipsoids in Poiseuille flows
Shear flows cause aspherical colloidal particles to tumble so that their
orientations trace out complex trajectories known as Jeffery orbits. The
Jeffery orbit of a prolate ellipsoid is predicted to align the particle's
principal axis preferentially in the plane transverse to the axis of shear.
Holographic microscopy measurements reveal instead that colloidal ellipsoids'
trajectories in Poiseuille flows strongly favor an orientation inclined by
roughly relative to this plane. This anomalous observation is
consistent with at least two previous reports of colloidal rods and dimers of
colloidal spheres in Poiseuille flow and therefore appears to be a generic, yet
unexplained feature of colloidal transport at low Reynolds numbers.Comment: 5 pages, 4 figure
Repeated \u3cem\u3eN\u3c/em\u3e-Acetylcysteine Administration Alters Plasticity-Dependent Effects of Cocaine
Cocaine produces a persistent reduction in cystine–glutamate exchange via system xc− in the nucleus accumbens that may contribute to pathological glutamate signaling linked to addiction. System xc− influences glutamate neurotransmission by maintaining basal, extracellular glutamate in the nucleus accumbens, which, in turn, shapes synaptic activity by stimulating group II metabotropic glutamate autoreceptors. In the present study, we tested the hypothesis that a long-term reduction in system xc− activity is part of the plasticity produced by repeated cocaine that results in the establishment of compulsive drug seeking. To test this, the cysteine prodrug N-acetylcysteine was administered before daily cocaine to determine the impact of increased cystine–glutamate exchange on the development of plasticity-dependent cocaine seeking. Although N-acetylcysteine administered before cocaine did not alter the acute effects of cocaine on self-administration or locomotor activity, it prevented behaviors produced by repeated cocaine including escalation of drug intake, behavioral sensitization, and cocaine-primed reinstatement. Because sensitization or reinstatement was not evident even 2–3 weeks after the last injection of N-acetylcysteine, we examined whether N-acetylcysteine administered before daily cocaine also prevented the persistent reduction in system xc− activity produced by repeated cocaine. Interestingly, N-acetylcysteine pretreatment prevented cocaine-induced changes in [35S]cystine transport via system xc−, basal glutamate, and cocaine-evoked glutamate in the nucleus accumbens when assessed at least 3 weeks after the last N-acetylcysteine pretreatment. These findings indicate that N-acetylcysteine selectively alters plasticity-dependent behaviors and that normal system xc− activity prevents pathological changes in extracellular glutamate that may be necessary for compulsive drug seeking
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
Theory of Double-Sided Flux Decorations
A novel two-sided Bitter decoration technique was recently employed by Yao et
al. to study the structure of the magnetic vortex array in high-temperature
superconductors. Here we discuss the analysis of such experiments. We show that
two-sided decorations can be used to infer {\it quantitative} information about
the bulk properties of flux arrays, and discuss how a least squares analysis of
the local density differences can be used to bring the two sides into registry.
Information about the tilt, compressional and shear moduli of bulk vortex
configurations can be extracted from these measurements.Comment: 17 pages, 3 figures not included (to request send email to
[email protected]
The Chandra Source Catalog
The Chandra Source Catalog (CSC) is a general purpose virtual X-ray
astrophysics facility that provides access to a carefully selected set of
generally useful quantities for individual X-ray sources, and is designed to
satisfy the needs of a broad-based group of scientists, including those who may
be less familiar with astronomical data analysis in the X-ray regime. The first
release of the CSC includes information about 94,676 distinct X-ray sources
detected in a subset of public ACIS imaging observations from roughly the first
eight years of the Chandra mission. This release of the catalog includes point
and compact sources with observed spatial extents <~ 30''. The catalog (1)
provides access to the best estimates of the X-ray source properties for
detected sources, with good scientific fidelity, and directly supports
scientific analysis using the individual source data; (2) facilitates analysis
of a wide range of statistical properties for classes of X-ray sources; and (3)
provides efficient access to calibrated observational data and ancillary data
products for individual X-ray sources, so that users can perform detailed
further analysis using existing tools. The catalog includes real X-ray sources
detected with flux estimates that are at least 3 times their estimated 1 sigma
uncertainties in at least one energy band, while maintaining the number of
spurious sources at a level of <~ 1 false source per field for a 100 ks
observation. For each detected source, the CSC provides commonly tabulated
quantities, including source position, extent, multi-band fluxes, hardness
ratios, and variability statistics, derived from the observations in which the
source is detected. In addition to these traditional catalog elements, for each
X-ray source the CSC includes an extensive set of file-based data products that
can be manipulated interactively.Comment: To appear in The Astrophysical Journal Supplement Series, 53 pages,
27 figure
Statistical Characterization of the Chandra Source Catalog
The first release of the Chandra Source Catalog (CSC) contains ~95,000 X-ray
sources in a total area of ~0.75% of the entire sky, using data from ~3,900
separate ACIS observations of a multitude of different types of X-ray sources.
In order to maximize the scientific benefit of such a large, heterogeneous
data-set, careful characterization of the statistical properties of the
catalog, i.e., completeness, sensitivity, false source rate, and accuracy of
source properties, is required. Characterization efforts of other, large
Chandra catalogs, such as the ChaMP Point Source Catalog (Kim et al. 2007) or
the 2 Mega-second Deep Field Surveys (Alexander et al. 2003), while
informative, cannot serve this purpose, since the CSC analysis procedures are
significantly different and the range of allowable data is much less
restrictive. We describe here the characterization process for the CSC. This
process includes both a comparison of real CSC results with those of other,
deeper Chandra catalogs of the same targets and extensive simulations of
blank-sky and point source populations.Comment: To be published in the Astrophysical Journal Supplement Series (Fig.
52 replaced with a version which astro-ph can convert to PDF without issues.
A standardized terminology for describing reproductive development in fishes
19 páginas, 12 figuras, 3 tablas.-- Open access journalAs the number of fish reproduction studies has proliferated, so has the number of gonadal classification schemes
and terms. This has made it difficult for both scientists and resource managers to communicate and for comparisons to
be made among studies.We propose the adoption of a simple, universal terminology for the phases in the reproductive
cycle, which can be applied to all male and female elasmobranch and teleost fishes. These phases were chosen because
they define key milestones in the reproductive cycle; the phases include immature, developing, spawning capable,
regressing, and regenerating. Although the temporal sequence of events during gamete development in each phase
may vary among species, each phase has specific histological and physiological markers and is conceptually universal.
The immature phase can occur only once. The developing phase signals entry into the gonadotropin-dependent stage
of oogenesis and spermatogenesis and ultimately results in gonadal growth. The spawning capable phase includes (1)
those fish with gamete development that is sufficiently advanced to allow for spawning within the current reproductive
cycle and (2) batch-spawning females that show signs of previous spawns (i.e., postovulatory follicle complex) and
that are also capable of additional spawns during the current cycle. Within the spawning capable phase, an actively
spawning subphase is defined that corresponds to hydration and ovulation in females and spermiation in males. The
regressing phase indicates completion of the reproductive cycle and, for many fish, completion of the spawning season.
Fish in the regenerating phase are sexually mature but reproductively inactive. Species-specific histological criteria
or classes can be incorporated within each of the universal phases, allowing for more specific divisions (subphases) while preserving the overall reproductive terminology for comparative purposes. This terminology can easily be
modified for fishes with alternate reproductive strategies, such as hermaphrodites (addition of a transition phase) and
livebearers (addition of a gestation phase)Fish Reproduction and Fisheries (FRESH; European
Cooperation in Science and Technology Action FA0601)
and theWest Palm Beach Fishing Club (Florida) provided funding
for the gonadal histology workshops where this terminology
was developed and refined. Additionally, we thank FRESH for
travel and publication fundsPeer reviewe
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