3,120 research outputs found
Quantum initial value representations using approximate Bohmian trajectories
Quantum trajectories, originating from the de Broglie-Bohm (dBB) hydrodynamic
description of quantum mechanics, are used to construct time-correlation
functions in an initial value representation (IVR). The formulation is fully
quantum mechanical and the resulting equations for the correlation functions
are similar in form to their semi-classical analogs but do not require the
computation of the stability or monodromy matrix or conjugate points. We then
move to a {\em local} trajectory description by evolving the cumulants of the
wave function along each individual path. The resulting equations of motion are
an infinite hierarchy, which we truncate at a given order. We show that
time-correlation functions computed using these approximate quantum
trajectories can be used to accurately compute the eigenvalue spectrum for
various potential systems.Comment: 7 pages, 6 figure
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Evolution of abilities to regenerate neurons in central nervous systems
This work was supported by a Kappa Kappa Gamma Graduate Fellowship Award to C. E. H. and NIH grant NS-11861 and RCDA NS-00070 to G. D. B.Neuroscienc
Car-Parrinello Molecular Dynamics on excited state surfaces
This paper describes a method to do ab initio molecular dynamics in
electronically excited systems within the random phase approximation (RPA).
Using a dynamical variational treatment of the RPA frequency, which corresponds
to the electronic excitation energy of the system, we derive coupled equations
of motion for the RPA amplitudes, the single particle orbitals, and the nuclear
coordinates. These equations scale linearly with basis size and can be
implemented with only a single holonomic constraint. Test calculations on a
model two level system give exact agreement with analytical results.
Furthermore, we examined the computational efficiency of the method by modeling
the excited state dynamics of a one-dimensional polyene lattice. Our results
indicate that the present method offers a considerable decrease in
computational effort over a straight-forward configuration interaction
(singles) plus gradient calculation performed at each nuclear configuration
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Facilitation of transmitter release at squid synapses
Facilitation is shown to decay as a compound exponential with two
time constants (T1, T2) at both giant and non-giant synapses in squid steilate ganglia
bathed in solutions having low extracellular calcium concentrations ([Ca++]o).
Maximum values of facilitation (F~) were significantly larger, and T1 was significantly
smaller in giant than non-giant synapses. Decreases in [Ca++]o or increases in
[Mn++]o had variable effects on T1 and F1, whereas decreases in temperature
increased T~ but had insignificant effects on/'1. The growth of facilitation during
short trains of equal interval stimuli was adequately predicted by the linear
summation model developed by Mallart and Martin (1967.J. Physiol. (Lond.). 193:
676-694) for frog neuromuscular junctions. This result suggests that the underlying
mechanisms of facilitation are similar in squid and other synapses which release
many transmitter quanta.This work was supported by National Science Foundation research grant GB-36949, National
Research Council (Canada) and Grass Fellowships to Dr. Charlton, and a National Institutes of
Health career award (NS-00070) to Dr. Bittner.Neuroscienc
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Presynaptic facilitation at the crayfish neuromuscular junction: Role of calcium-activated potassium conductance
Membrane potential was recorded intracellularly near presynaptic
terminals of the excitor axon of the crayfish opener neuromuscular junction (NMJ),
while transmitter release was recorded postsynaptically. This study focused on the
effects of a presynaptic calcium-activated potassium conductance, gK(Ca), on the
transmitter release evoked by single and paired depolarizing current pulses.
Blocking gK(Ca) by adding tetraethylammonium ion (TEA; 5-20 mM) to a solution
containing tetrodotoxin and aminopyridines caused the relation between presynaptic
potential and transmitter release to steepen and shift to less depolarized
potentials. When two depolarizing current pulses were applied at 20-ms intervals
with gK(Ca) not blocked, the presynaptic voltage change to the second (test) pulse was
inversely related to the amplitude of the first (conditioning) pulse. This effect of the
conditioning prepulse on the response to the test pulse was eliminated by 20 mM
TEA and by solutions containing 0 mM Ca2+/1 mM EGTA, suggesting that the
reduction in the amplitude of the test pulse was due to activation of gK(Ca) by calcium
remaining from the conditioning pulse. In the absence of TEA, facilitation of
transmitter release evoked by a test pulse increased as the conditioning pulse grew
from -40 to -20 mV, but then decreased with further increase in the conditioning
depolarization. A similar nonmonotonic relationship between facilitation and the
amplitude of the conditioning depolarization was reported in previous studies using
extracellular recording, and interpreted as supporting an additional voltagedependent
step in the activation of transmitter release. We suggest that this result
was due instead to activation of a gK(Ca) by the conditioning depolarization, since
facilitation of transmitter release increased monotonically with the amplitude of the
conditioning depolarization, and the early time course of the decay of facilitation
was prolonged when gK(Ca) was blocked. The different time courses for decay of the
presynaptic potential (20 ms) and facilitation (> 50 ms) suggest either that residual free calcium does not account for facilitation at the crayfish NMJ or that the
transmitter release mechanism has a markedly higher affinity or stoichiometry for
internal free calcium than does g K(Ca). Finally, our data suggest that the calcium
channels responsible for transmitter release at the crayfish NMJ are not of the L, N,
or T type.This work was partially supported by NIAAA grant AA0776 to G. D. Bittner.Neuroscienc
River Bed Response to Channel Width Variation: Theory and Experiments (HES 49)
Illinois Water Resources Center (USGS Project 04 Contract 14-08-0004-G2017unpublishednot peer reviewe
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Mechanisms for the maintenance and eventual degradation of neurofilament proteins in the distal segments of severed goldfish Mauthner axons
Cellular mechanisms that might affect the degradation of neurofilament proteins (NFPs) were examined in the distal segments
of severed goldfish Mauthner axons (M-axons), which do
not degenerate for more than 2 months after severance. Calpain
levels, as determined by reactivity to a polyclonal antibody,
remained constant for 80 d postseverance in distal segments
of M-axons and then declined from 80 to 85 d
postseverance. Calpain activity in rat brain, as determined by a
spectrophotometric assay, was much higher than calpain activity
in control and severed goldfish brain, spinal cord, muscle,
or M-axons. Calpain activity was extremely low in M-axons
compared with that in all other tissues and remained low for up
to 80 d postseverance in distal segments of M-axons. Phosphorylated
NFPs, as determined by Stains-All treatment of SDS
gels, were maintained for up to 72 d postseverance and then decreased noticeably at 75 d postseverance when NFP breakdown
products appeared on silver-stained gels. By 85 d postseverance,
phosphorylated NFPs no longer were detected, and
NFP breakdown products were the most prominent bands on
silver-stained gels. These results suggest that the distal segments
of M-axons survive for months after severance, because
NFPs are maintained in a phosphorylated state that stabilizes
and protects NFPs from degradation by low levels of calpain
activity in the M-axon; the distal segments of severed M-axons
degenerate eventually when NFPs no longer are maintained in
a phosphorylated state and become susceptible to degradation,
possibly by low levels of calpain activity in the M-axon.This work was supported by an Advanced Technology Project Grant to G.D.B.Neuroscienc
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Maintenance and degradation of proteins in intact and severed axons: Implications for the mechanism of long-term survival of anucleate crayfish axons
Protein maintenance and degradation are examined in the
severed distal (anucleate) portions of crayfish medial giant
axons (MGAs), which remain viable for over 7 months following
axotomy. On polyacrylamide gels, the silver-stained
protein banding pattern of anucleate MGAs severed from
their cell bodies for up to 4 months remains remarkably similar
to that of intact MGAs. At 7 months postseverance, some
(but not all) proteins are decreased in anucleate MGAs compared
to intact MGAs. To determine the half-life of axonally
transported proteins, we radiolabeled MGA cell bodies and
monitored the degradation of newly synthesized transported
proteins. Assuming exponential decay, proteins in the fast
component of axonal transport have an average half-life of
14 d in anucleate MGAs and proteins in the slow component
have an average half-life of 17 d. Such half-lives are very
unlikely to account for the ability of anucleate MGAs to survive
for over 7 months after axotomy.This work was supported by an ATP grant to G.D.B.Neuroscienc
Estrogenic chemicals often leach from BPA-free plastic products that are replacements for BPA-containing polycarbonate products
Background: Xenobiotic chemicals with estrogenic activity (EA), such as bisphenol A (BPA), have been reported to
have potential adverse health effects in mammals, including humans, especially in fetal and infant stages. Concerns
about safety have caused many manufacturers to use alternatives to polycarbonate (PC) resins to make hard and
clear, reusable, plastic products that do not leach BPA. However, no study has focused on whether such BPA-free
PC-replacement products, chosen for their perceived higher safety, especially for babies, also release other chemicals
that have EA.
Methods: We used two, well-established, mammalian cell-based, assays (MCF-7 and BG1Luc) to assess the EA of
chemicals that leached into over 1000 saline or ethanol extracts of 50 unstressed or stressed (autoclaving,
microwaving, and UV radiation) BPA-free PC-replacement products. An EA antagonist, ICI 182,780, was used to confirm
that agonist activity in leachates was due to chemicals that activated the mammalian estrogen receptor.
Results: Many unstressed and stressed, PC-replacement-products made from acrylic, polystyrene, polyethersulfone, and
Tritan™ resins leached chemicals with EA, including products made for use by babies. Exposure to various forms of UV
radiation often increased the leaching of chemicals with EA. In contrast, some BPA-free PC-replacement products made
from glycol-modified polyethylene terephthalate or cyclic olefin polymer or co-polymer resins did not release chemicals
with detectable EA under any conditions tested.
Conclusions: This hazard assessment survey showed that many BPA-free PC- replacement products still leached
chemicals having significant levels of EA, as did BPA-containing PC counterparts they were meant to replace. That is,
BPA-free did not mean EA-free. However, this study also showed that some PC-replacement products did not leach
chemicals having significant levels of EA. That is, EA-free PC-replacement products could be made in commercial
quantities at prices that compete with PC-replacement products that were not BPA-free. Since plastic products often
have advantages (price, weight, shatter-resistance, etc.) compared to other materials such as steel or glass, it is not
necessary to forgo those advantages to avoid release into foodstuffs or the environment of chemicals having EA that
may have potential adverse effects on our health or the health of future generations.This work was supported by the following NIH/NIEHS grants: R44 ES011469,
01–03 (CZY); 1R43/44 ES014806, 01–03 (CZY); subcontract (CZY, PI) on
an NIH Grant 01–03 43/44ES018083-01 to PlastiPure (DK, SY PIs).Neuroscienc
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