Correlations in spiking neuronal networks with distance dependent connections

Can the topology of a recurrent spiking network be inferred from observed activity dynamics? Which statistical parameters of network connectivity can be extracted from firing rates, correlations and related measurable quantities? To approach these questions, we analyze distance dependent correlations of the activity in small-world networks of neurons with current-based synapses derived from a simple ring topology. We find that in particular the distribution of correlation coefficients of subthreshold activity can tell apart random networks from networks with distance dependent connectivity. Such distributions can be estimated by sampling from random pairs. We also demonstrate the crucial role of the weight distribution, most notably the compliance with Dales principle, for the activity dynamics in recurrent networks of different types.     

Suppressing the excitability of spinal motoneurons by extracellularly applied electrical fields: insights from computer simulations.

The effect of extracellularly applied electrical fields on neuronal excitability and firing behavior is attributed to the interaction between neuronal morphology and the spatial distribution and level of differential polarization induced by the applied field in different elements of the neuron. The presence of voltage-gated ion channels that mediate persistent inward currents (PICs) on the dendrites of spinal motoneurons enhances the influence of electrical fields on the motoneuronal firing behavior. The goal of the present study was to investigate, with a realistic motoneuron computer model, the effects of extracellularly applied electrical fields on the excitability of spinal motoneurons with the aim of reducing the increased motoneuronal excitability after spinal cord injury (SCI). Our results suggest that electrical fields could suppress the excitability of motoneurons and reduce their firing rate significantly by modulating the magnitude of their dendritic PIC. This effect was achieved at different field directions, intensities, and polarities. The reduction in motoneuronal firing rate resulted from the reduction in the magnitude of the dendritic PIC reaching the soma by the effect of the applied electrical field. This reduction in PIC was attributed to the dendritic field-induced differential polarization and the nonlinear current-voltage relationship of the dendritic PIC-mediating channels. Because of the location of the motoneuronal somata and initial segment with respect to the dendrites, these structures were minimally polarized by the applied field compared with the extended dendrites. In conclusion, electrical fields could be used for suppressing the hyperexcitability of spinal motoneurons after SCI and reducing the level of spasticity.     

The cadherin superfamily in neuronal connections and interactions

Neural development and the organization of complex neuronal circuits involve a number of processes that require cell-cell interaction. During these processes, axons choose specific partners for synapse formation and dendrites elaborate arborizations by interacting with other dendrites. The cadherin superfamily is a group of cell surface receptors that is comprised of more than 100 members. The molecular structures and diversity within this family suggest that these molecules regulate the contacts or signalling between neurons in a variety of ways. In this review I discuss the roles of three subfamilies - classic cadherins, Flamingo/CELSRs and protocadherins - in the regulation of neuronal recognition and connectivity.     

Cortical neuronal connections and reconstruction of visual space

We investigated distribution of retrograde-labelled cells in cortical areas 17, 18, and the transition zone 17/18 of both hemispheres in cats after microiontophoretic horseradish peroxidase (HRP) injections into the single cortical columns of area 17, 18, 19 or 21a. On the base of clustered pattern of intrinsic labelling, asymmetric location of labelled callosal cells that was associated with the appropriate pattern of labelling in layers A and A1 of lateral geniculate nucleus, we suggest that cortical neuronal connections are eye-specific and may provide for each eye a separate binding of visual hemifields. After HRP injections into columns of area 19 or 21a, the disparate inputs from areas 17, 18 and transition zone 17/18 were revealed. Such connections may provide a local depth information and the selection of stereoscopic surfaces in central sectors of visual space.     

Morphology of neurosecretory cells delineated with cobalt applied extracellularly to the cephalic aorta of the insect Rhodnius prolixus

Cobalt applied extracellularly to the cephalic aorta in Rhodnius prolixus filled neurosecretory cells (NSCs) located in the brain, the retrocerebral complex, and the suboesophageal ganglion (SOG). Axons of these cells converged over the corpora cardiaca and corpus allatum and merged into a large tract before travelling posteriorly along the ventral side of the aorta. Cobalt-filled cells in the posterior margins of the brain and the retrocerebral complex lacked extensive dendritic arborizations, suggesting that their cell bodies and/or axonal processes in the retrocerebral complex are directly involved with integrative processes determining hormone release. Cobalt-filled cell bodies in the anterior region of the brain were closely associated with the ocellar nerve, and the cobalt-filled cells in the SOG formed extensive dendritic arborizations in the neuropile, suggesting the involvement of sensory cells in regulation of their electrical activity. The ability to fill NSCs with cobalt applied to the aorta demonstrates that the cephalic aorta of R. prolixus is an important neurohaemal region.     

Tenascin-R associates extracellularly with parvalbumin immunoreactive neurones but is synthesised by another neuronal population in the adult rat cerebral cortex

The molecular components surrounding a neurone serve as recognition cues for the nerve terminals and glial processes that contact them and the constellations formed by these inputs will therefore be determined by the blend of adhesive and repulsive components therein. Using immunohistochemical methods, we observed that the large extracellular matrix-protein, tenascin-R (Restrictin, J1-160-180, Janusin), associates preferentially with the parvalbumin-positive subpopulation of interneurones within the cerebral cortex. In situ-hybridization indicated that tenascin-R-mRNA was expressed in a subpopulation of nerve cells distinct from that containing parvalbumin, suggesting that this protein's association with the latter is receptor mediated. These nerve cells thus modulate at a distance the composition of the extracellular matrix around parvalbuminneurons.     

Post-tetanic modification of the efficacy of excitatory transmission in neuronal networks with interhemispheric connections

The modifiable reciprocal transcallosal monosynaptic excitatory connections were for the first time detected in vivo experiments in rat motor cortex using multiunit recording and crosscorrelation analysis, It was shown that high-frequency microstimulation (MCS) of a small group of cortical cells of one hemisphere produces long-term changes in the efficacy of transcallosal excitatory connections, and also ipsilateral connections in both hemispheres. The posttetanic changes appear as long-term potentiation (LTP) and long-term depression (LTD). The bursting neurons were found to have more favorable conditions for the induction of LTP of most converging inputs (in contrast to cells with other discharge patterns). Both LTP and LTD could be simultaneously induced in synapses formed by axon collaterals of a callosal cell on several neurons. LTP and LTD could be simultaneously obtained at diverse synapses of the same cell. The number of spontaneously active callosal neurons as well as the number and efficacy of transcallosal connections increased after the MCS, whereas the number and efficacy of ipsilateral connections decreased. Basing on these data we assume that the ipsilateral inhibition is more effective than the transcallosal inhibition. MCS results in the modification of the pattern of initially existing connections between numerous neurons of an ensemble including cells of both hemispheres.     

Magnetic resonance imaging of neuronal connections in the macaque monkey

Recently, an MRI-detectable, neuronal tract-tracing method in living animals was introduced that exploits the anterograde transport of manganese (Mn2+). We present the results of experiments simultaneously tracing manganese chloride and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) to evaluate the specificity of the former by tracing the neuronal connections of the basal ganglia of the monkey. Mn2+ and WGA-HRP yielded remarkably similar and highly specific projection patterns. By showing the sequential transport of Mn2+ from striatum to pallidum-substantia nigra and then to thalamus, we demonstrated MRI visualization of transport across at least one synapse in the CNS of the primate. Transsynaptic tract tracing in living primates will allow chronic studies of development and plasticity and provide valuable anatomical information for fMRI and electrophysiological experiments in primates.     

A fluorescence-based double retrograde tracer strategy for charting central neuronal connections

Microspheres (beads) tagged with different fluorescent markers can be used for double retrograde axonal tracing of CNS connections. They have several advantages over other double tracer techniques, including ease-of-use, high transport efficiency, distinctive cell labeling and the ability to produce well-defined injection sites. In this protocol we describe the basic procedure for their use, some common problems and how these can be overcome. The protocol, including animal surgery, preparation and delivery of tracer can be completed in approximately 0.5 d. Subsequent histological processing (excluding survival time) can be completed in 0.5-1 d.     

Structure of alpha-alpha-hairpins with short connections.

This paper presents the results of detailed stereochemical analysis of structures and sequences of alpha-alpha-hairpins with short connections. It is shown that alpha-alpha-hairpins of each given type have very similar patterns of hydrophobic, hydrophilic and glycine residues in their amino acid sequences. These results can be used in the prediction of alpha-alpha-hairpin conformation as well as in protein design and engineering.