10,330 research outputs found
Altered muscarinic and nicotinic receptor densities in cortical and subcortical brain regions in Parkinson's disease
Muscarinic and nicotinic cholinergic receptors and choline acetyltransferase activity were studied in postmortem brain tissue from patients with histopathologically confirmed Parkinson's disease and matched control subjects. Using washed membrane homogenates from the frontal cortex, hippocampus, caudate nucleus, and putamen, saturation analysis of specific receptor binding was performed for the total number of muscarinic receptors with [3H]quinuclidinyl benzilate, for muscarinic M1 receptors with [3H]pirenzepine, for muscarinic M2 receptors with [3H]oxotremorine-M, and for nicotinic receptors with (-)-[3H]nicotine. In comparison with control tissues, choline acetyl-transferase activity was reduced in the frontal cortex and hippocampus and unchanged in the caudate nucleus and putamen of parkinsonian patients. In Parkinson's disease the maximal binding site density for [3H]quinuclidinyl benzilate was increased in the frontal cortex and unaltered in the hippocampus, caudate nucleus, and putamen. Specific [3H]pirenzepine binding was increased in the frontal cortex, unaltered in the hippocampus, and decreased in the caudate nucleus and putamen. In parkinsonian patients Bmax values for specific [3H]oxotremorine-M binding were reduced in the cortex and unchanged in the hippocampus and striatum compared with controls. Maximal (-)-[3H]nicotine binding was reduced in both the cortex and hippocampus and unaltered in both the caudate nucleus and putamen. Alterations of the equilibrium dissociation constant were not observed for any ligand in any of the brain areas examined. The present results suggest that both the innominatocortical and the septohippocampal cholinergic systems degenerate in Parkinson's disease.(ABSTRACT TRUNCATED AT 250 WORDS
Terguride stimulates locomotor activity at 2 months but not 10 months after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment of common marmosets
The mixed dopamine (DA) agonist/antagonist terguride acts as a DA antagonist on normosensitive receptors but shows DA agonistic properties at supersensitive DA receptors. Such a compound could offer an alternative to the treatment of Parkinson's disease with indirect or direct DA agonists. The present study compares the actions of terguride, 4-12 mg/kg i.p., in naive common marmosets with its effects in animals rendered parkinsonian by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 2 months or 10 months previously, in order to test its antiparkinsonian efficacy. Terguride reduced locomotor activity in naive common marmosets, similar to its effects in rodents and in line with the DA antagonistic activity of the compound. In marmosets treated with MPTP 2 months previously and exhibiting pronounced behavioural motor deficits, terguride stimulated locomotor activity, showing DA agonistic properties under these conditions. In contrast, the locomotor activity of animals that had recovered from MPTP treatment 10 months previously was not altered by terguride. It is concluded that terguride has anti-akinetic efficacy in this primate model of Parkinson's disease. In addition, terguride offers a unique opportunity to differentiate, pharmacologically, the extent of dopaminergic recovery from MPTP treatment in this primate species
Weak localization in mesoscopic hole transport: Berry phases and classical correlations
We consider phase-coherent transport through ballistic and diffusive
two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show
that intrinsic heavy-hole light-hole coupling gives rise to clear-cut
signatures of an associated Berry phase in the weak localization which renders
the magneto-conductance profile distinctly different from electron transport.
Non-universal classical correlations determine the strength of these Berry
phase effects and the effective symmetry class, leading even to
antilocalization-type features for circular quantum dots and Aharonov-Bohm
rings in the absence of additional spin-orbit interaction. Our semiclassical
predictions are quantitatively confirmed by numerical transport calculations
Fluctuation-Driven Molecular Transport in an Asymmetric Membrane Channel
Channel proteins, that selectively conduct molecules across cell membranes,
often exhibit an asymmetric structure. By means of a stochastic model, we argue
that channel asymmetry in the presence of non-equilibrium fluctuations, fueled
by the cell's metabolism as observed recently, can dramatically influence the
transport through such channels by a ratchet-like mechanism. For an
aquaglyceroporin that conducts water and glycerol we show that a previously
determined asymmetric glycerol potential leads to enhanced inward transport of
glycerol, but for unfavorably high glycerol concentrations also to enhanced
outward transport that protects a cell against poisoning.Comment: REVTeX4, 4 pages, 3 figures; Accepted for publication in Phys. Rev.
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Geometric quantum gate for trapped ions based on optical dipole forces induced by Gaussian laser beams
We present an implementation of quantum logic gates via internal state
dependent displacements of ions in a linear Paul trap caused by optical dipole
forces. Based on a general quantum analysis of the system dynamics we consider
specific implementations with alkaline earth ions. For experimentally realistic
parameters gate infidelities as low as can be obtained.Comment: 10 pages, 4 figure
Characteristic energies, transition temperatures, and switching effects in clean SNS graphene nanostructures
We study proximity effects in clean nanoscale superconductor-normal
metal-superconductor (SNS) graphene heterostructures using a
self-consistent numerical solution to the continuum Dirac Bogoliubov-de Gennes
(DBdG) equations. We obtain results for the pair amplitude and the local
density of states (DOS), as a function of doping and of the geometrical
parameters determining the width of the structures. The superconducting
correlations are found to penetrate the normal graphene layers even when there
is extreme mismatch in the normal and superconducting doping levels, where
specular Andreev reflection dominates. The local DOS exhibits peculiar
features, which we discuss, arising from the Dirac cone dispersion relation and
from the interplay between the superconducting and Thouless energy scales. The
corresponding characteristic energies emerge in the form of resonant peaks in
the local DOS, that depend strongly on the doping level, as does the energy
gap, which declines sharply as the relative difference in doping between the S
and N regions is reduced. We also linearize the DBdG equations and develop an
essentially analytical method that determines the critical temperature of
an \sns nanostructure self-consistently. We find that for S regions that occupy
a fraction of the coherence length, can undergo substantial variations as
a function of the relative doping. At finite temperatures and by manipulating
the doping levels, the self consistent pair amplitudes reveal dramatic
transitions between a superconducting and resistive normal state of the
structure. Such behavior suggests the possibility of using the proposed system
as a carbon-based superconducting switch, turning superconductivity on or off
by tuning the relative doping levels.Comment: 13 pages, figures include
Self-consistent simulation of quantum wires defined by local oxidation of Ga[Al]As heterostructures
We calculate the electronic width of quantum wires as a function of their
lithographic width in analogy to experiments performed on nanostructures
defined by local oxidation of Ga[Al]As heterostructures. Two--dimensional
simulations of two parallel oxide lines on top of a Ga[Al]As heterostructure
defining a quantum wire are carried out in the framework of Density Functional
Theory in the Local Density Approximation and are found to be in agreement with
measurements. Quantitative assessment of the influence of various experimental
uncertainties is given. The most influential parameter turns out to be the
oxide line depth, followed by its exact shape and the effect of background
doping (in decreasing order).Comment: 5 pages, 6 figures; revised figures, clarified tex
Optically excited states in positronium
Optical excitation are reported of the 1 3S-2 3P transition in positronium, and a second excitation from n=2 to higher n states. The experiment used light from two pulsed dye lasers. Changes in the positronium annihilation rate during and after the laser pulse were used to deduce the excited state populations. The n=2 level was found to be saturable and excitable to a substantial fraction of n=2 positronium to higher levels. Preliminary spectroscopic measurements were performed on n=14 and n=15 positronium
Effect of spatial inhomogeneity on the mapping between strongly interacting fermions and weakly interacting spins
A combined analytical and numerical study is performed of the mapping between
strongly interacting fermions and weakly interacting spins, in the framework of
the Hubbard, t-J and Heisenberg models. While for spatially homogeneous models
in the thermodynamic limit the mapping is thoroughly understood, we here focus
on aspects that become relevant in spatially inhomogeneous situations, such as
the effect of boundaries, impurities, superlattices and interfaces. We consider
parameter regimes that are relevant for traditional applications of these
models, such as electrons in cuprates and manganites, and for more recent
applications to atoms in optical lattices. The rate of the mapping as a
function of the interaction strength is determined from the Bethe-Ansatz for
infinite systems and from numerical diagonalization for finite systems. We show
analytically that if translational symmetry is broken through the presence of
impurities, the mapping persists and is, in a certain sense, as local as
possible, provided the spin-spin interaction between two sites of the
Heisenberg model is calculated from the harmonic mean of the onsite Coulomb
interaction on adjacent sites of the Hubbard model. Numerical calculations
corroborate these findings also in interfaces and superlattices, where
analytical calculations are more complicated.Comment: 7 pages, 6 figure
Mode Bifurcation and Fold Points of Complex Dispersion Curves for the Metamaterial Goubau Line
In this paper the complex dispersion curves of the four lowest-order
transverse magnetic modes of a dielectric Goubau line () are
compared with those of a dispersive metamaterial Goubau line. The vastly
different dispersion curve structure for the metamaterial Goubau line is
characterized by unusual features such as mode bifurcation, complex fold
points, both proper and improper complex modes, and merging of complex and real
modes
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