1,000 research outputs found
Activity-dependent regulation of NMDA receptors in substantia nigra dopaminergic neurones.
N-Methyl-d-aspartate receptors (NMDARs) are Ca(2+)-permeable glutamate receptors that play a critical role in synaptic plasticity and promoting cell survival. However, overactive NMDARs can trigger cell death signalling pathways and have been implicated in substantia nigra pars compacta (SNc) pathology in Parkinson's disease. Calcium ion influx through NMDARs recruits Ca(2+)-dependent proteins that can regulate NMDAR activity. The surface density of NMDARs can also be regulated dynamically in response to receptor activity via Ca(2+)-independent mechanisms. We have investigated the activity-dependent regulation of NMDARs in SNc dopaminergic neurones. Repeated whole-cell agonist applications resulted in a decline in the amplitude of NMDAR currents (current run-down) that was use dependent and not readily reversible. Run-down was reduced by increasing intracellular Ca(2+) buffering or by reducing Ca(2+) influx but did not appear to be mediated by the same regulatory proteins that cause Ca(2+)-dependent run-down in hippocampal neurones. The NMDAR current run-down may be mediated in part by a Ca(2+)-independent mechanism, because intracellular dialysis with a dynamin-inhibitory peptide reduced run-down, suggesting a role for clathrin-mediated endocytosis in the regulation of the surface density of receptors. Synaptic NMDARs were also subject to current run-down during repeated low-frequency synaptic stimulation in a Ca(2+)-dependent but dynamin-independent manner. Thus, we report, for the first time, regulation of NMDARs in SNc dopaminergic neurones by changes in intracellular Ca(2+) at both synaptic and extrasynaptic sites and provide evidence for activity-dependent changes in receptor trafficking. These mechanisms may contribute to intracellular Ca(2+) homeostasis in dopaminergic neurones by limiting Ca(2+) influx through the NMDAR
Yukawa Textures and the mu-term
We show how with an anomaly-free U(1), simple assumptions concerning the
origin of Yukawa textures and the Higgs mu-term lead to the prediction of a new
physics scale of 10^8GeV and automatic conservation of baryon number.Comment: 12 pages, uses Harvmac (option "b"
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Our changing Sun
Stellar astronomy tells us much about the long-term evolution of our Sun while forensic evidence (for example, cosmic-ray products in ice cores) gives us indications of its fluctuations over the last millennium. However, such studies do not give us a sufficiently detailed understanding of solar change over the last century to allow us to detect and quantify any role that the Sun might have played in the observed rise in average surface temperatures on Earth. This paper describes recent research that has filled this gap by applying advances in our understanding of the effects and structure of the solar wind to historical data on the Earth's magnetic field
Qudit surface codes and gauge theory with finite cyclic groups
Surface codes describe quantum memory stored as a global property of
interacting spins on a surface. The state space is fixed by a complete set of
quasi-local stabilizer operators and the code dimension depends on the first
homology group of the surface complex. These code states can be actively
stabilized by measurements or, alternatively, can be prepared by cooling to the
ground subspace of a quasi-local spin Hamiltonian. In the case of spin-1/2
(qubit) lattices, such ground states have been proposed as topologically
protected memory for qubits. We extend these constructions to lattices or more
generally cell complexes with qudits, either of prime level or of level
for prime and , and therefore under tensor
decomposition, to arbitrary finite levels. The Hamiltonian describes an exact
gauge theory whose excitations
correspond to abelian anyons. We provide protocols for qudit storage and
retrieval and propose an interferometric verification of topological order by
measuring quasi-particle statistics.Comment: 26 pages, 5 figure
The oldest X-ray supernovae: X-ray emission from 1941C, 1959D, 1968D
We have studied the X-ray emission from four historical Type-II supernovae
(the newly-discovered 1941C in NGC 4631 and 1959D in NGC 7331; and 1968D, 1980K
in NGC 6946), using Chandra ACIS-S imaging. In particular, the first three are
the oldest ever found in the X-ray band, and provide constraints on the
properties of the stellar wind and circumstellar matter encountered by the
expanding shock at more advanced stages in the transition towards the remnant
phase. We estimate emitted luminosities ~ 5 x 10^{37} erg/s for SN 1941C, ~ a
few x 10^{37} erg/s for SN 1959D, ~ 2 x 10^{38} erg/s for SN 1968D, and ~ 4 x
10^{37} erg/s for SN 1980K, in the 0.3-8 keV band. X-ray spectral fits to SN
1968D suggest the presence of a harder component, possibly a power law with
photon index ~ 2, contributing ~ 10^{37} erg/s in the 2-10 keV band. We
speculate that it may be evidence of non-thermal emission from a Crab-like
young pulsar.Comment: 6 pages, accepted by ApJ. Revised version with a couple of added
references. Thanks to A. Kong and E. Schlegel for their comments. Credit to
Holt et al. (2003) for the X-ray discovery of SN 1968D, overlooked in other
recent catalog
Gauge singlet renormalisation in softly-broken supersymmetric theories
We consider the renormalisation of a softly-broken supersymmetric theory with
singlet fields and a superpotential with a linear term. We show that there
exist exact beta-functions for both the linear term in the superpotential and
the associated linear term in the Lagrangian. We also construct exact
renormalisation group invariant trajectories for these terms, corresponding to
the conformal anomaly solution for the soft masses and couplings.Comment: 13 pages, Plain TeX, uses Harvmac. Typos corrected and minor
clarification adde
Adaptive Optics for Astronomy
Adaptive Optics is a prime example of how progress in observational astronomy
can be driven by technological developments. At many observatories it is now
considered to be part of a standard instrumentation suite, enabling
ground-based telescopes to reach the diffraction limit and thus providing
spatial resolution superior to that achievable from space with current or
planned satellites. In this review we consider adaptive optics from the
astrophysical perspective. We show that adaptive optics has led to important
advances in our understanding of a multitude of astrophysical processes, and
describe how the requirements from science applications are now driving the
development of the next generation of novel adaptive optics techniques.Comment: to appear in ARA&A vol 50, 201
Simultaneous Measurements of Dinitrogen Fixation and Denitrification Associated With Coral Reef Substrates : Advantages and Limitations of a Combined Acetylene Assay
Nitrogen (N) cycling in coral reefs is of key importance for these oligotrophic ecosystems, but knowledge about its pathways is limited. While dinitrogen (N-2) fixation is comparably well studied, the counteracting denitrification pathway is under-investigated, mainly because of expensive and relatively complex experimental techniques currently available. Here, we combined two established acetylene-based assays to one single setup to determine N-2-fixation and denitrification performed by microbes associated with coral reef substrates/organisms simultaneously. Accumulating target gases (ethylene for N-2-fixation, nitrous oxide for denitrification) were measured in gaseous headspace samples via gas chromatography. We measured N-2-fixation and denitrification rates of two Red Sea coral reef substrates (filamentous turf algae, coral rubble), and demonstrated, for the first time, the co-occurrence of both N-cycling processes in both substrates. N-2-fixation rates were up to eight times higher during the light compared to the dark, whereas denitrification rates during dark incubations were stimulated for turf algae and suppressed for coral rubble compared to light incubations. Our results highlight the importance of both substrates in fixing N, but their role in relieving N is potentially divergent. Absolute N-2-fixation rates of the present study correspond with rates reported previously, even though likely underestimated due to an initial lag phase. Denitrification is also presumably underestimated due to incomplete nitrous oxide inhibition and/or substrate limitation. Besides these inherent limitations, we show that a relative comparison of N-2-fixation and denitrification activity between functional groups is possible. Thus, our approach facilitates cost-efficient sample processing in studies interested in comparing relative rates of N-2-fixation and denitrification.Peer reviewe
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