Scaling Behavior of Quasi-One-Dimensional Vortex Avalanches in Superconducting Films.

Scaling behaviour of dynamically driven vortex avalanches in superconducting YBa2Cu3O7-δ films deposited on tilted crystalline substrates has been observed using quantitative magneto-optical imaging. Two films with different tilt angles are characterized by the probability distributions of avalanche size in terms of the number of moving vortices. It is found in both samples that these distributions follow power-laws over up to three decades, and have exponents ranging between 1.0 and 1.4. The distributions also show clear finite-size scaling, when the system size is defined by the depth of the flux penetration front - a signature of self-organized criticality. A scaling relation between the avalanche size exponent and the fractal dimension, previously derived theoretically from conservation of the number of magnetic vortices in the stationary state and shown in numerical simulations, is here shown to be satisfied also experimentally.

Selective positive modulation of the SK3 and SK2 subtypes of small conductance Ca2+-activated K+ channels.

BACKGROUND AND PURPOSE: Positive modulators of small conductance Ca(2+)-activated K(+) channels (SK1, SK2, and SK3) exert hyperpolarizing effects that influence the activity of excitable and non-excitable cells. The prototype compound 1-EBIO or the more potent compound NS309, do not distinguish between the SK subtypes and they also activate the related intermediate conductance Ca(2+)-activated K(+) channel (IK). This paper demonstrates, for the first time, subtype-selective positive modulation of SK channels. EXPERIMENTAL APPROACH: Using patch clamp and fluorescence techniques we studied the effect of the compound cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA) on recombinant hSK1-3 and hIK channels expressed in HEK293 cells. CyPPA was also tested on SK3 and IK channels endogenously expressed in TE671 and HeLa cells. KEY RESULTS: CyPPA was found to be a positive modulator of hSK3 (EC(50) = 5.6 +/- 1.6 microM, efficacy 90 +/- 1.8 %) and hSK2 (EC(50) = 14 +/- 4 microM, efficacy 71 +/- 1.8 %) when measured in inside-out patch clamp experiments. CyPPA was inactive on both hSK1 and hIK channels. At hSK3 channels, CyPPA induced a concentration-dependent increase in the apparent Ca(2+)-sensitivity of channel activation, changing the EC(50)(Ca(2+)) from 429 nM to 59 nM. CONCLUSIONS AND IMPLICATIONS: As a pharmacological tool, CyPPA may be used in parallel with the IK/SK openers 1-EBIO and NS309 to distinguish SK3/SK2- from SK1/IK-mediated pharmacological responses. This is important for the SK2 and SK1 subtypes, since they have overlapping expression patterns in the neocortical and hippocampal regions, and for SK3 and IK channels, since they co-express in certain peripheral tissues.

Monolayer to bilayer transition in a dipolar system.

We study the transition from a one-dimensional magnetic dipolar monolayer to a bilayer as it is compressed beyond the close-packed condition. The pressure in a close-packed monolayer is found to be nearly independent of the number of dipoles. In the case of weak dipolar interactions, our experimental results indicate that the bilayer formation is governed by short-range steric and electrostatic repulsion, whereas for strong dipolar interactions the bilayer formation is governed by long-range dipolar repulsion.

Selective block of recombinant glur6 receptors by NS-102, a novel non-NMDA receptor antagonist.

The diversity of neuronal glutamate receptors continues to increase with the discovery of multiple subunits and subunit families. The significance of this potential receptor heterogeneity is unknown because pharmacological tools that could clearly distinguish between different structural isoforms have not yet been identified. A novel glutamate receptor antagonist, 5-nitro-6,7,8,9-tetrahydrobenzo[g]indole-2,3-dione-3-oxime (NS-102), has been shown previously to selectively block the low affinity [3H]kainate binding site in rat brain. We have examined the effect of NS-102 on receptors expressed in fibroblasts from either glur6 subunits or a combination of glurB and glurD (glurB/D receptors). NS-102 (3 microM) reduced currents mediated by glur6 receptors and had very little effect on currents mediated by glurB/D receptors. The binding of [3H]kainate to glur6 receptors showed properties similar to those of the brain low affinity [3H]kainate binding site, and NS-102 inhibited specific binding to glur6 receptors with a potency nearly identical to those sites in brain membranes. Our findings suggest that NS-102 will be useful in identifying the functional role of native receptors containing a glur6 subunit.

A novel non-NMDA receptor antagonist shows selective displacement of low-affinity [3H]kainate binding.

5-Nitro-6,7,8,9-tetrahydrobenzo[G]indole-2,3-dione-3-oxime (NS-102), a new competitive glutamate receptor antagonist displaced binding to non-N-methyl-D-aspartate (non-NMDA) binding sites with no activity at the NMDA and strychnine-insensitive glycine binding sites. Under experimental conditions in which both high- and low-affinity sites were labelled, NS-102 only partially inhibited the binding of [3H]kainate. Studies of NS-102 displacement of high-affinity versus low-affinity [3H]kainate binding showed a high selectivity of NS-102 for the low-affinity [3H]kainate binding site (Ki = 0.6 microM) compared to the high-affinity [3H]kainate binding site (Ki > 10 microM). NS-102 was a relatively weak inhibitor of 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) binding (IC50 = 7.2 microM). NS-102 and related compounds with similar pharmacological profiles may become valuable tools in the characterization of the functional importance of the low-affinity [3H]kainate binding site.

Paramagnetic beads surfing on domain walls.

Paramagnetic beads electrostatically stabilized in aqueous solution are attracted toward domain walls in magnetic films. The position above the domain wall can be destabilized by realigning the beads magnetic moment with an external magnetic field. The destabilization may result in a steady state dissipative mode, where the beads surf on the slope of the moving domain wall. The technique could be an alternative route to probe electrostatic and hydrodynamic interactions between particles and interfaces, and could also serve as a model system for studying motion in a one-dimensional potential.

Adsorption and diffusion in a one-dimensional potential well.

We investigate the adsorption and diffusion of colloidal particles at an interface containing a one-dimensional potential well. It is observed how the adsorption kinetics onto the wall is altered with time, and one observes the formation of a particle chain. We find that the time it takes for a bead to penetrate into the chain depends strongly on the particle density, and beyond a critical value this time diverges. We also study diffusion within the well, and find that at low particle densities the short time behavior is governed by normal Fickian diffusion.

Lightning in superconductors.

Crucially important for application of type-II superconductor films is the stability of the vortex matter--magnetic flux lines penetrating the material. If some vortices get detached from pinning centres, the energy dissipated by their motion will facilitate further depinning, and may trigger a massive electromagnetic breakdown. Up to now, the time-resolved behaviour of these ultra-fast events was essentially unknown. We report numerical simulation results revealing the detailed dynamics during breakdown as within nanoseconds it develops branching structures in the electromagnetic fields and temperature, with striking resemblance of atmospheric lightning. During a dendritic avalanche the superconductor is locally heated above its critical temperature, while electrical fields rise to several kV/m as the front propagates at instant speeds near up to 100 km/s. The numerical approach provides an efficient framework for understanding the ultra-fast coupled non-local dynamics of electromagnetic fields and dissipation in superconductor films.

Using symmetry breaking for directed transport of paramagnetic colloids on garnet films.

The transport behavior of paramagnetic particles on top of a ferrimagnetic garnet film is investigated in a modulated external magnetic field. Broken symmetries are required to direct the transport of the particles. We provide such symmetry breaking by tilting the external field modulation with respect to the normal direction of the garnet film and by the intrinsic geometrical symmetry breaking of the garnet film magnetic pattern. The interplay of both symmetry breaking mechanisms causes a rich variety in transport behavior and direction. We corroborate our experimental transport directions by comparing experimental with theoretical transport phase diagrams. Directing the transport of paramagnetic colloids will be useful when they are loaded with biomedical cargo on a magnetic lab-on-a-chip device.

Dramatic role of critical current anisotropy on flux avalanches in MgB2 films.

Anisotropic penetration of magnetic flux in MgB(2) films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, the anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate perpendicular to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.

Interaction between superconducting vortices and a Bloch wall in ferrite garnet films.

A theoretical model for how Bloch walls occurring in in-plane magnetized ferrite garnet films can serve as efficient magnetic micromanipulators is presented. As an example, the walls' interaction with Abrikosov vortices in a superconductor in close contact with a garnet film is analyzed within the London approximation. The model explains how vortices are attracted to such walls, and excellent quantitative agreement is obtained for the resulting peaked flux profile determined experimentally in NbSe(2) using high-resolution magneto-optical imaging of vortices. In particular, this model, when generalized to include charged magnetic walls, explains the counterintuitive attraction observed between vortices and a Bloch wall of opposite polarity.

Onset of dendritic flux avalanches in superconducting films.

We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

Colloidal crystallization and transport in stripes and mazes.

We study guided crystallization and transport of paramagnetic spheres on top of a magnetic film that arranges its domains into stripes or mazes. In the absence of liquid flow, the paramagnetic spheres are confined within the magnetic domains, and it is shown how the particles self-assemble into several interesting phases depending on the complexity of the domain patterns. We also find that colloids guided through a complex maze exhibit structured patterns that can be controlled by an external magnetic field. The results presented here could help us understand both static and dynamic properties of pattern formation in confined geometries of tunable complexity.

Domain wall tip for manipulation of magnetic particles.

We demonstrate a method for manipulation of single magnetic microparticles based on a domain wall tip displaced in a controlled manner. By applying an external magnetic field, the tip can either drag or push magnetic particles. This kind of tweezers has potential applications in probing and manipulating colloidal systems.

Interactions among GYKI-52466, cyclothiazide, and aniracetam at recombinant AMPA and kainate receptors.

We examined the actions of cyclothiazide, aniracetam, and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI-52466) on recombinant alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate receptors. Receptors expressed in Xenopus oocytes or human embryonic kidney 293 cells were characterized using voltage and patch-clamp electrophysiology. Aniracetam and cyclothiazide potentiated AMPA receptor currents by slowing or blocking desensitization. Cyclothiazide was more potent at receptors consisting of flip subunits compared with receptors consisting of flop subunits, whereas aniracetam appeared to be more efficacious at flop receptors. The potency of GYKI-52466 did not differ in heteromeric flip or flop containing AMPA receptors, but GYKI-52466 was less potent at homomeric GluRAi and GluRDi receptors. At heteromeric AMPA receptors, 50 microM cyclothiazide increased the IC50 value for GYKI-52466 significantly. The increase was largest in GluRBi/Di receptors where the IC50 value shifted from 21.9 microM (95% confidence interval, 12.0-39.8 microM) to 126 microM (95% confidence interval, 72.4-214 microM) in the presence of cyclothiazide. In contrast, 100 microM GYKI-52466 did not alter the EC50 of cyclothiazide at GluRBi/Di receptors nor did it markedly change the maximal potentiation induced by cyclothiazide. At GluRBi/Di receptors transiently expressed in human embryonic kidney 293 cells, 30 microM GYKI-52466 inhibited the steady state and the peak current evoked by 300 microns L-glutamate to the same extent (34.5 +/- 12% and 27.3 +/- 13.0%, respectively; five experiments), and GYKI-52466 did not alter the apparent rate of desensitization (tau = 15.7 +/- 4.7 and 17.5 +/- 8.3 msec in the absence and presence of GYKI-52466, respectively; five experiments). GYKI-52466 inhibited L-glutamate currents in the presence and absence of 10 microM cyclothiazide, but GYKI-52466 never restored the desensitization that was blocked by cyclothiazide. Furthermore, GYKI-52466 inhibited L-glutamate currents mediated by homomeric Glu6 receptors, which are not potentiated by cyclothiazide. Our data suggest that the effect of cyclothiazide on the affinity of GYKI-52466 for its binding site is allosteric and that the positive modulatory effect of cyclothiazide and the negative modulatory effect of GYKI-52466 result from binding to separate sites on recombinant subunits.

Characterization of the binding of [3H]NS 257, a novel competitive AMPA receptor antagonist, to rat brain membranes and brain sections.

The binding of [3H]NS 257 (1,2,3,6,7,8-hexahydro-3-(hydroxyimino)-N,N-[3H]dimethyl-7-methyl- 2- oxobenzo[2,1-b:3,4-c']dipyrrole-5-sulfonamide) to rat cortical membranes was characterized in the absence and presence of thiocyanate. Specific [3H]NS 257 binding was saturable and reversible, and the stimulating effect of thiocyanate on binding was optimal at 100 mM. In the presence of thiocyanate [3H]NS 257 bound to a single population of binding sites with an affinity of 225 +/- 8 nM and a binding site density of 0.61 +/- 0.04 pmol/mg of original tissue. Thiocyanate increased the affinity of the binding site labeled by [3H]NS 257 for both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and L-glutamate by a factor of 20 and 5, respectively. However, the affinity of the agonist domoate and the antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)-quinoxaline (NBQX) was decreased in the presence of thiocyanate. Apparently, the affinities of antagonists as well as agonists for the AMPA receptor can be either increased or decreased by thiocyanate. The rank order of potency of the putative agonists quisqualate > AMPA > L-glutamate > domoate > kainate and of the antagonists NBQX > CNQX is consistent with the labeling of AMPA receptors. Autoradiographic studies showed that the distribution of [3H]NS 257 binding sites in rat brain was similar to that of [3H]AMPA binding sites. NS 257 is the first AMPA antagonist to be described showing an increased affinity for the AMPA receptor in the presence of thiocyanate.

Hydrodynamic instability of the flux-antiflux interface in type-II superconductors.

A possible mechanism of the macroturbulence instability observed in fluxline systems during remagnetization of superconductors is proposed. It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion. This condition occurs at the interface owing to the anisotropy of the viscous motion of vortices. The phenomenon is similar to the instability of the tangential discontinuity in classical hydrodynamics. The obtained results are supported by magneto-optical observations of flux distribution on the surface of a YBCO single crystal with twins.

Assembling and manipulating two-dimensional colloidal crystals with movable nanomagnets.

We study crystallization of paramagnetic beads in a magnetic field gradient generated by one-dimensional nanomagnets. The pressure in such a system depends on both the magnetic forces and the hydrodynamic flow, and we estimate the flow threshold for disassembling the crystal near the magnetic potential barrier. A number of different defects have been observed which fluctuate in shape or propagate along the crystal, and it is found that the defect density increases away from the nanomagnet. We also study the melting of the crystal/fluid system after removal of the nanomagnet and demonstrate that the bond-oriental order parameter decreases with time. The nanomagnet can be moved in a controlled manner by a weak external magnetic field, and at sufficiently large driving velocities we observe self-healing crack formation characterized by a roughening of the lattice as well as gap formation. Finally, when confined between two oscillating nanomagnets, the colloidal crystal is shown to break up and form dipolar chains above a certain oscillation frequency.

Anisotropic origin of the bending instability of the flux-antiflux interface in type-II superconductors.

The physical nature of the macroturbulence in vortex matter in YBCO superconductors is investigated by means of a magneto-optic study of the instability in a single crystal prepared especially for this purpose. The instability develops near those sample edges where the oppositely directed flow of vortices and antivortices, guided by twin boundaries, is characterized by the discontinuity of the tangential component of the hydrodynamic velocity. This fact indicates that the macroturbulence is analogous to the instability of fluid flow at a surface of a tangential velocity discontinuity in classical hydrodynamics and is related to the anisotropic flux motion in the superconductor.

Synthesis and receptor binding affinity of new selective GluR5 ligands.

Two hybrid analogues of the kainic acid receptor agonists, 2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA) and (2S,4R)-4-methylglutamic acid ((2S,4R)-4-Me-Glu), were designed, synthesized, and characterized in radioligand binding assays using cloned ionotropic and metabotropic glutamic acid receptors. The (S)-enantiomers of E-4-(2,2-dimethylpropylidene)glutamic acid ((S)-1) and E-4-(3,3-dimethylbutylidene)glutamic acid ((S)-2) were shown to be selective and high affinity GluR5 ligands, with Ki values of 0.024 and 0.39 microM, respectively, compared to Ki values at GluR2 of 3.0 and 2.0 microM. respectively. Their affinities in the [3H]AMPA binding assay on native cortical receptors were shown to correlate with their GluR2 affinity rather than their GluR5 affinity. No affinity for GluR6 was detected (IC50 > 100 microM).