Dendritic Topology Research Articles

Relaxation of Copolymeric Dendrimers Built from Alternating Monomers

AbstractSummary: In this study we extend our previous work concerning the Rouse dynamics of linear alternating copolymers (Macromolecules 2003, 36, 486) to tree‐like structures and focus on copolymeric dendrimers built from monomers of two kinds A and B; as before, we let the monomers differ in their interaction with the solvent. In the framework of generalized Gaussian structures (GGS), we consider alternating arrangements of monomers over the dendritic structures. We develop a semi‐analytical method to determine for such structures (of arbitrary functionality, f, and number of generations, g), the eigenfrequencies (relaxation times). The method allows us to compute readily the storage, [G′(ω)] and the loss, [G″(ω)] moduli. These quantities show a multitude of features which mainly depend on the difference in the mobilities, or, equivalently, in the friction coefficients ζA and ζB of the A‐ and B‐beads. These features range from the presence of large plateau‐type regions in [G′(ω)] to the appearance of double‐peaks in [G″(ω)]. In contrast to linear alternating copolymers, the behavior of the dynamic moduli of copolymeric systems with dendritic topology can shed light into their composition, i.e. into the relative numbers of A‐ and B‐beads. We discuss these aspects in view of their experimental relevance.A system under study: a dendrimer of third generation (g = 3) with functionality f = 3, composed of alternating beads.imageA system under study: a dendrimer of third generation (g = 3) with functionality f = 3, composed of alternating beads.

Regulation of dendritic length and branching by semaphorin 3A.

The molecular mechanisms that control dendritic development are still unclear. Semaphorin 3A, a class III semaphorin, has been shown to regulate the radial orientation of pyramidal neurons in the developing neocortex. Here, we investigate the effects of Sema3A on the development of dendritic topology. Neocortical slices from Sema3A null mutant mice were cultured and neurons were transfected with GFP, reconstructed, and compared with neurons from wild-type and heterozygote littermates. We also added exogenous Sema3A to cultured wild-type neocortical slices to further test its effects on dendritic development. We document reductions in dendritic length and branching in Sema3A null mice and increases in dendritic length and branching after the addition of exogenous Sema3A to wild-type neurons. We conclude that Sema3A is necessary for the elaboration of second and third order dendritic branches in pyramidal neurons.

Brownian Dynamics Simulations of Dendrimers under Elongational Flow: Bead−Rod Model with Hydrodynamic Interactions

Computer simulations of perfectly branched dendrimers up to the sixth generation have been performed under the influence of uniaxial elongational flow for the first time for a model with explicit dendritic topology. The Brownian dynamics simulation technique has been applied to a freely jointed bead−rod model with excluded volume both with and without hydrodynamic interactions. The dependence of conformational properties and the intrinsic elongational viscosity on the flow rate were obtained. The coil−stretch transition was observed for dendrimers of all generations with it being less pronounced than the same type of transition observed for a linear polymer chain. Hydrodynamic interactions shift the onset of this transition to higher elongational rates. The transition is observed to occur in two stages as it was for a linear polymer. The dendrimer first orients at low flow rate as a whole along the flow axis without significant deformation and local orientation. Increasing flow rate leads to local orientation on the level of the monomer leading to significant global deformation of the dendrimer. The monomers belonging to inner generations are oriented stronger relative to outer monomers at all flow rates. The intrinsic elongational viscosity of the dendrimer increases with flow and plateaus at high rates. The limiting value of intrinsic elongational viscosity for a model with hydrodynamic interactions at high flow rates is less than its value for a model without these interactions. The onset of the coil−stretch transition occurs at lower elongational rates as the number of monomers, N, within the dendrimer increases. The dependence of the onset of this transition with N is less pronounced than for a linear chain. It becomes less steep with N and is not described by a power law.

The effect of dendritic topology on firing patterns in model neurons

Neuronal firing patterns are influenced by both membrane properties and dendritic morphology. Distinguishing two sources of morphological variability—metrics and topology—we investigate the extent to which model neurons that have the same metrical and membrane properties can still produce different firing patterns as a result of differences in dendritic topology. Within a set of dendritic trees that have the same number of terminal segments and the same total dendritic length, we show that firing frequency strongly correlates with topology as expressed by the mean dendritic path length. The effect of dendritic topology on firing frequency is bigger for trees with equal segment diameters than for trees whose segment diameters obey Rall's 3/2 power law. If active dendritic channels are present, dendritic topology influences not only firing frequency but also type of firing (regular, bursting).

Influence of dendritic topology on firing patterns in model neurons

Neuronal electrophysiology is influenced by both channel distribution and morphology. Distinguishing two sources of morphological variability—metrics and topology—we show that model neurons sharing the same channel densities and anatomical size can derive functional differentiation from their dendritic topology. Firing frequencies in these metrically reduced neurons show a strong correlation with both mean path length and total electrotonic transformed size of the dendritic tree. This dependency of spiking behaviour is robust to different modes of stimulation, different tapering powers, and the absence of active dendritic channels.

A quantitative Golgi study in the occipital cortex of the pyramidal dendritic topology of old adult rats from social or isolated environments

It has been previously established that the housing condition influences the appearance of the dendritic tree in young animals. The purpose of this investigation was to determine the extent of the influence that the housing condition exerts on morphometric studies involving old animals. Specifically, we found superficial pyramidal cells from the visual cortex of socially aged 20-month-old rats had more extensive oblique dendritic trees than neurons from isolated rats of the same age. The basal dendritic tree of neurons from socially reared rats was denser nearest layer IV while the basal dendritic tree of neurons from isolated rats appeared to shift away from layer IV. Our results indicate that the external environment is an independent variable and cortical dendritic morphology is a dependent variable in studies on old adult rats. Thus, the external environment should be considered in reporting results of aging studies on dendritic topology.

Transplantation of embryonic occipital cortex to the brain of newborn rats: a Golgi study of mature and developing transplants.

Segments of the occipital cortex were taken from rat embryos (E16-E19) and transplanted to the cerebral cortex or the tectal region of a newborn rat host. With the aid of Golgi impregnation techniques, neuron morphology was studied in cortical transplants which had survived for 1 week or more in the host brain. In mature transplants (greater than 4 weeks) three main groups of neurons, termed groups I-III, were identified. Group I neurons resembled pyramidal neurons of the intact cerebral cortex. No preferential orientation of either soma or dendrites of group I neurons was observed in the transplants, and some group I neurons had curved apical dendrites. Group II neurons had predominantly stellate form and their dendrites were densely covered with spines. Paucity or absence of dendritic spines characterized group III neurons which exhibited various dendritic topologies. Different neuron types were also recognized in immature transplants growing for 1 and 2 weeks in the host brain. The sequence of dendritic maturation of transplanted cortical neurons is similar to that seen in intact cortex, although the stage reached related more to the actual age of the transplant than to that of the host. Thus, group I neurons in the 1-week-old transplants taken from E16 embryos had not attained the same complexity of branching as pyramidal neurons in the surrounding host cortex, but rather resembled slightly younger cells more like those found in the cerebral cortex of the newborn rat. These results show, therefore, that at least the basic cell classes identified in intact visual cortex can also be recognized in the cortical transplants. This will provide a foundation for studies defining which cells project to the host brain and which are involved in particular intrinsic connections.