Evaluation of heating effects on atoms trapped in an optical trap

We solve a stochastic master equation based on the theory of Savard et al. [T. A. Savard. K. M. O'Hara, and J. E. Thomas, Phys, Rev. A 56, R1095 (1997)] for heating arising from fluctuations in the trapping laser intensity. We compare with recent experiments of Ye et al. [J. Ye, D. W. Vernooy, and H. J. Kimble, Phys. Rev. Lett. 83, 4987 (1999)], and find good agreement with the experimental measurements of the distribution of trap occupancy times. The major cause of trap loss arises from the broadening of the energy distribution of the trapped atom, rather than the mean heating rate, which is a very much smaller effect

Realization of an Excited, Strongly-Correlated Quantum Gas Phase

Ultracold atomic physics offers myriad possibilities to study strongly correlated many-body systems in lower dimensions. Typically, only ground state phases are accessible. Using a tunable quantum gas of bosonic cesium atoms, we realize and control in one dimensional geometry a highly excited quantum phase that is stabilized in the presence of attractive interactions by maintaining and strengthening quantum correlations across a confinement-induced resonance. We diagnose the crossover from repulsive to attractive interactions in terms of the stiffness and the energy of the system. Our results open up the experimental study of metastable excited many-body phases with strong correlations and their dynamical properties

Phase-coherent detection of an optical dipole force by Doppler velocimetry

We report phase-coherent Doppler detection of optical dipole forces using large ion crystals in a Penning trap. The technique is based on laser Doppler velocimetry using a cycling transition in $^{9}$Be$^{+}$ near 313 nm and the center-of-mass (COM) ion motional mode. The optical dipole force is tuned to excite the COM mode, and measurements of photon arrival times synchronized with the excitation potential show oscillations with a period commensurate with the COM motional frequency. Experimental results compare well with a quantitative model for a driven harmonic oscillator. This technique permits characterization of motional modes in ion crystals; the measurement of both frequency and phase information relative to the driving force is a key enabling capability -- comparable to lockin detection -- providing access to a parameter that is typically not available in time-averaged measurements. This additional information facilitates discrimination of nearly degenerate motional modes.Comment: Related manuscripts at http://www.physics.usyd.edu.au/~mbiercuk

Tribology studies of the natural knee using an animal model in a new whole joint natural knee simulator

The successful development of early-stage cartilage and meniscus repair interventions in the knee requires biomechanical and biotribological understanding of the design of the therapeutic interventions and their tribological function in the natural joint. The aim of this study was to develop and validate a porcine knee model using a whole joint knee simulator for investigation of the tribological function and biomechanical properties of the natural knee, which could then be used to pre-clinically assess the tribological performance of cartilage and meniscal repair interventions prior to in vivo studies. The tribological performance of standard artificial bearings in terms of anterior–posterior (A/P) shear force was determined in a newly developed six degrees of freedom tribological joint simulator. The porcine knee model was then developed and the tribological properties in terms of shear force measurements were determined for the first time for three levels of biomechanical constraints including A/P constrained, spring force semi-constrained and A/P unconstrained conditions. The shear force measurements showed higher values under the A/P constrained condition (predominantly sliding motion) compared to the A/P unconstrained condition (predominantly rolling motion). This indicated that the shear force simulation model was able to differentiate between tribological behaviours when the femoral and tibial bearing was constrained to slide or/and roll. Therefore, this porcine knee model showed the potential capability to investigate the effect of knee structural, biomechanical and kinematic changes, as well as different cartilage substitution therapies on the tribological function of natural knee joints

Distinct roles of NMDA receptors at different stages of granule cell development in the adult brain.

NMDA receptor (NMDAR)-dependent forms of synaptic plasticity are thought to underlie the assembly of developing neuronal circuits and to play a crucial role in learning and memory. It remains unclear how NMDAR might contribute to the wiring of adult-born granule cells (GCs). Here we demonstrate that nascent GCs lacking NMDARs but rescued from apoptosis by overexpressing the pro-survival protein Bcl2 were deficient in spine formation. Insufficient spinogenesis might be a general cause of cell death restricted within the NMDAR-dependent critical time window for GC survival. NMDAR loss also led to enhanced mushroom spine formation and synaptic AMPAR activity throughout the development of newborn GCs. Moreover, similar elevated synapse maturation in the absence of NMDARs was observed in neonate-generated GCs and CA1 pyramidal neurons. Together, these data suggest that NMDAR operates as a molecular monitor for controlling the activity-dependent establishment and maturation rate of synaptic connections between newborn neurons and others

Experience-Dependent, Rapid Structural Changes in Hippocampal Pyramidal Cell Spines

Morphological changes in dendritic spines may contribute to the fine tuning of neural network connectivity. The relationship between spine morphology and experience-dependent neuronal activity, however, is largely unknown. In the present study, we combined 2 histological analyses to examine this relationship: 1) Measurement of spines of neurons whose morphology was visualized in brain sections of mice expressing membrane-targeted green florescent protein (Thy1-mGFP mice) and 2) Categorization of CA1 neurons by immunohistochemical monitoring of Arc expression as a putative marker of recent neuronal activity. After mice were exposed to a novel, enriched environment for 60 min, neurons that expressed Arc had fewer small spines and more large spines than Arc-negative cells. These differences were not observed when the exploration time was shortened to 15 min. This net-balanced structural change is consistent with both synapse-specific enhancement and suppression. These results provide the first evidence of rapid morphological changes in spines that were preferential to a subset of neurons in association with an animal's experiences

Methamphetamine-induced structural plasticity in the dorsal striatum

Repeated exposure to psychostimulant drugs produces long-lasting changes in dendritic structure, presumably reflecting a reorganization in patterns of synaptic connectivity, in brain regions that mediate the psychomotor activating and incentive motivational effects of these drugs, including the nucleus accumbens and prefrontal cortex. However, repeated exposure to psychostimulant drugs also facilitates a transition in the control of some behaviors from action–outcome associations to behavior controlled by stimulus–response (S–R) habits. This latter effect is thought to be due to increasing engagement and control over behavior by the dorsolateral (but not dorsomedial) striatum. We hypothesized therefore that repeated exposure to methamphetamine would differentially alter the density of dendritic spines on medium spiny neurons (MSNs) in the dorsolateral vs. dorsomedial striatum. Rats were treated with repeated injections of methamphetamine, and 3 months later dendrites were visualized using Sindbis virus-mediated green fluorescent protein (GFP) expression in vivo . We report that prior exposure to methamphetamine produced a significant increase in mushroom and thin spines on MSNs in the dorsolateral striatum, but a significant decrease in mushroom spines in the dorsomedial striatum. This may be due to changes in the glutamatergic innervation of these two subregions of the dorsal striatum. Thus, we speculate that exposure to psychostimulant drugs may facilitate the development of S–R habits because this reorganizes patterns of synaptic connectivity in the dorsal striatum in a way that increases control over behavior by the dorsolateral striatum.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72347/1/j.1460-9568.2007.05316.x.pd

PICK1 inhibition of the Arp2/3 complex controls dendritic spine size and synaptic plasticity

Dendritic spine remodelling involves regulated actin dynamics and is essential for synaptic plasticity. The Arp2/3 inhibitor PICK1 is here shown to regulate spine size: inducing shrinkage during long-term depression

Rapid and Long-Lasting Increase in Sites for Synapse Assembly during Late-Phase Potentiation in Rat Hippocampal Neurons

Long-term potentiation in hippocampal neurons has stages that correspond to the stages of learning and memory. Early-phase (10–30 min) potentiation is accompanied by rapid increases in clusters or puncta of presynaptic and postsynaptic proteins, which depend on actin polymerization but not on protein synthesis. We have now examined changes in pre- and postsynaptic puncta and structures during glutamate-induced late-phase (3 hr) potentiation in cultured hippocampal neurons. We find that (1) the potentiation is accompanied by long-lasting maintenance of the increases in puncta, which depends on protein synthesis, (2) most of the puncta and synaptic structures are very dynamic, continually assembling and disassembling at sites that are more stable than the puncta or structures themselves, (3) the increase in presynaptic puncta appears to be due to both rapid and more gradual increases in the number of sites where the puncta may form, and also to the stabilization of existing puncta, (4) under control conditions, puncta of postsynaptic proteins behave similarly to puncta of presynaptic proteins and share sites with them, and (5) the increase in presynaptic puncta is accompanied by a similar increase in presumably presynaptic structures, which may form at distinct as well as shared sites. The new sites could contribute to the transition between the early and late phase mechanisms of plasticity by serving as seeds for the formation and maintenance of new synapses, thus acting as local “tags” for protein synthesis-dependent synaptic growth during late-phase plasticity