Rotational and translational microrheology from shape-anisotropic particles.

In this work, we report measurements of the mean squared angular and translational displacements of a colloidal dumbbell immersed in a viscoelastic fluid using digital microscopy. From the mean squared displacements, we obtain the mechanical properties of the media. Both angular and translational motions provide the same viscoelastic complex modulus and agree with that obtained from the translational motion of a spherical probe particle.

AMPA receptors modulate the reorganization of F-actin in Bergmann glia cells through the activation of RhoA.

Alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid glutamate receptors have been shown to modulate the morphology of the lamelar processes of Bergmann glia cells in the molecular layer of the cerebellum. Here we suggest that reorganization of F-actin may underlay the changes in the morphology of the lamelar processes. Using the fluorescent staining of F-actin with Phalloidin and the quantification of RhoA activation through immunoprecipitation or pull-down assays, we show that RhoA is activated after stimulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors and leads to the reorganization of the actin cytoskeleton of Bergmann fibers. This reorganization of the actin cytoskeleton is reflected in the form of an increase in the intensity of the F-actin staining as well as in the loss of the number of Bergmann fibers stained with Phalloidin. Moreover, using a pharmacological approach, we show that activation of RhoA and the change in the intensity of the F-actin staining depends on the activation of PI3-K, focal adhesion kinase, and protein kinase C, whereas changes in the number of Bergmann fibers depend on external calcium in a RhoA independent manner. Our findings show that glutamate may induce a form of structural plasticity in Bergmann glia cells through the reorganization of the actin cytoskeleton. This may have implications in the way the synaptic transmission is processed in the cerebellum.

Optical microscopy measurement of pair correlation functions.

We studied the pair correlation function g(r) of silica particles with a fluorescent core and a nonfluorescent shell which were confined between two glass plates by optical video microscopy. To investigate the possible role of optical artifacts due to overlapping particle images, we compared experiments, where, first, the whole particle (white image) and then, only the fluorescent core (fluorescent image) was used for determining particle positions. While under white-image conditions the observed g(r) exhibits a main peak at about 1.2 times the particle's diameter; under fluorescent image conditions the obtained g(r) resembles a short-ranged repulsive system where the main peak is close to contact. This discrepancy points towards artifacts of video microscopy, leading to erroneous g(r) and in turn to erroneous effective-pair potentials.

Short-time dynamics in quasi-two-dimensional colloidal suspensions.

The short-time dynamic properties of colloidal particles in quasi-two-dimensional geometries are studied by digital video microscopy. We demonstrate experimentally that the effective-two-dimensional physical quantities such as the dynamic structure factor, the hydrodynamic function, and the hydrodynamic diffusion coefficients are related in exactly the same manner as their three-dimensional counterparts.

Effective pair potential between confined charged colloidal particles.

The pair correlation function g(r) between like-charged colloidal particles in quasi-two-dimensional geometries is measured by optical microscopy for a wide range of particle concentrations and various degrees of confinement. The effective pair potential u(r) is obtained by deconvoluting g(r) via Monte Carlo computer simulations. Our results confirm the existence of a long-range attractive component of u(r) and the appearance of an extra attractive term under stringent confinement.