Electrotonic Signals along Intracellular Membranes May Interconnect Dendritic Spines and Nucleus

Synapses on dendritic spines of pyramidal neurons show a remarkable ability to induce phosphorylation of transcription factors at the nuclear level with a short latency, incompatible with a diffusion process from the dendritic spines to the nucleus. To account for these findings, we formulated a novel extension of the classical cable theory by considering the fact that the endoplasmic reticulum (ER) is an effective charge separator, forming an intrinsic compartment that extends from the spine to the nuclear membrane. We use realistic parameters to show that an electrotonic signal may be transmitted along the ER from the dendritic spines to the nucleus. We found that this type of signal transduction can additionally account for the remarkable ability of the cell nucleus to differentiate between depolarizing synaptic signals that originate from the dendritic spines and back-propagating action potentials. This study considers a novel computational role for dendritic spines, and sheds new light on how spines and ER may jointly create an additional level of processing within the single neuron

Solitonic conduction of electrotonic signals in neuronal branchlets with polarized microstructure

A model of solitonic conduction in neuronal branchlets with microstructure is presented. The application of cable theory to neurons with microstructure results in a nonlinear cable equation that is solved using a direct method to obtain analytical approximations of traveling wave solutions. It is shown that a linear superposition of two oppositely directed traveling waves demonstrate solitonic interaction: colliding waves can penetrate through each other, and continue fully intact as the exact pulses that entered the collision. These findings indicate that microstructure when polarized can sustain solitary waves that propagate at a constant velocity without attenuation or distortion in the absence of synaptic transmission. Solitonic conduction in a neuronal branchlet arising from polarizability of its microstructure is a novel signaling mode of electrotonic signals in thin processes (<0.5 μm diameter)

Analysis of spine plasticity in CA1 hippocampal pyramidal neurons employing live cell nanoscopic imaging

In der Großhirnrinde von Säugetieren befindet sich die Mehrheit erregender Synapsen auf Dornfortsätzen, kleinen dendritischen Ausbuchtungen, die in Größe und Form stark variieren. Die Auslösung aktivitätsabhängiger synaptischer Langzeitplastizität geht mit strukturellen Veränderungen dendritischer Dornen einher. Da das beugungsbegrenzte Auflösungsvermögen konventioneller Lichtmikroskope nicht ausreicht um die Morphologie der Dornen verlässlich zu untersuchen, stellte die Elektronenmikroskopie bisher das wichtigste bildgebende Verfahren zur Erforschung von struktureller Plastizität dar, blieb dabei jedoch auf die Betrachtung fixierter Gewebeproben beschränkt. Die Anwendung hochauflösender Laser-Raster-Mikroskopie mit Stimulierter-Emissions-Auslöschung hat es mir möglich gemacht, die Dynamik dendritischer Dornenmorphologie in lebenden Zellen zu studieren. Die N-Methyl-D-Aspartat-Rezeptor-abhängige Langzeitpotenzierung von Pyramidenzellen der Cornu-Ammonis Region 1 des Hippocampus bildete dabei den Mechanismus, welcher plastische Veränderungen hervorrief. Nach Potenzierung exzitatorischer Synapsen durch die lokale Ultraviolett-Photolyse von caged-Glutamat wurde ein starker, vorübergehender Anstieg des Anteils dendritischer Dornen mit sichelförmigen Köpfen und ein leichter, anhaltender Zuwachs an pilzförmigen Dornfortsätzen über einen Zeitraum von 50 Minuten beobachtet. Meine Untersuchungen ergänzen frühere Studien zur Wechselbeziehung zwischen synaptischer Potenzierung und struktureller Plastizität dendritischer Dornen und korrespondieren mit dem aktuellen Kenntnisstand der zu Grunde liegenden molekularen Mechanismen.The majority of excitatory synapses in the cortex of mammalian brains is situated on dendritic spines, small protrusions, heterogeneous in size and shape. The induction of activity-dependent long-term synaptic plasticity has been associated with changes in the ultrastructure of spines, particularly in size, head shape and neck width. Since the dimensions of dendritic spines are at the border of the diffraction-limited resolving power of conventional light microscopes, until recently, electron microscopy on fixed tissue constituted the primary method for investigations on spine morphology. I have employed live cell stimulated emission depletion imaging to analyse spine motility and structural transitions in response to n-methyl-d-aspartate receptor dependent long-term potentiation over time at super-resolution in Cornu Ammonis area 1 pyramidal neurons of the hippocampus. Local induction of long-term potentiation via ultraviolet photolysis of caged glutamate facilitated a strong transient increase in the proportion of spines with curved heads and a subtle persistent growth in the amount of mushroom spines over a time course of 50 minutes. My findings reinforce previous investigations on the relation of synaptic potentiation and spine motility, and are in good agreement with the current knowledge of the molecular mechanisms underlying long-term plasticity

The functional organisation of basal ganglia inputs

The basal ganglia allow organisms to adjust their behaviour according to changes in their internal state or their environment. One essential prerequisite for the selection and execution of appropriate movements is the convergence of inputs from various sources, conveying sensory information, motor commands, reward value, and more. These diverse inputs are integrated in the striatum, the input structure of the basal ganglia. In the last decades, numerous striatal cell types have been identified, their molecular profiles have been extracted and their local connectivity has been revealed. However, relatively little is known about the functional organisation of striatal inputs innervating these different neuron populations. The aim of this thesis is to examine how striatal inputs are integrated by the main cell types of this microcircuit. In Paper I, we uncover the mechanisms underlying sensory deficits in a mouse model of Parkinson’s disease. We show that one type of striatal projection neurons encodes the laterality of somatosensory inputs better than the other output neuron in healthy mice and that this encoding is lost in the dopamine-depleted state. In Paper II, we map the excitatory synaptic pathways of five striatal input structures (ipsi- and contralateral somatosensory and motor cortex, and the parafascicular nucleus) onto five different classes of striatal neurons. The study characterises the synaptic strength, receptor composition, and shortterm plasticity of each pathway with an unprecedented level of detail and comparability, thereby contributing to the understanding of the role of different striatal cell types. In Paper III, we create an in silico model of the striatum that integrates data from the subcellular to the microcircuit level. This model will be publicly available for testing new hypotheses and continuously updated with novel findings. In summary, the work presented in this thesis provides a further step in untangling the heterogeneous excitatory inputs that drive the activity of the primarily inhibitory microcircuit of the striatum and thus basal ganglia. We show that each striatal input targets a different set of striatal neurons and that the intricate organisation of these afferents is a function of both the presynaptic region and the postsynaptic cell type. Ultimately, knowledge of the functional connectivity of cortico- and thalamostriatal pathways as well as their synaptic properties will be essential for understanding and modelling the cortico- and thalamo-basal ganglia network in health and disease

Microfluidics based techniques for electrophysiological studies of cells

This thesis work investigates the application of microfluidics to perform electrophysiological studies on cells, including investigations of the effect of cholesterol on the dynamic ion permeability of TRPV1 ion channels, and the application of a microfluidic device, the multifunctional pipette, in electrophysiological studies on brain slices. In the first part of this thesis, Chinese hamster ovary (CHO) cells overexpressing the TRPV1 ion channel were used in a dynamic ion permeability study, where the activation properties of the TRPV1 ion channel were investigated using the patch clamp technique after depletion of membrane cholesterol. The dynaflow system, an open-volume multichannel microfluidic system, and the multifunctional pipette, a freestanding microfluidic device utilizing hydrodynamically confined flow for spatially confined solution exchange, were used to deliver chemical stimuli exclusively to the patched cell. The result showed that the depletion of membrane cholesterol impaired the dynamic permeability of large cations in TRPV1 in low calcium solutions. The second project focused on the application of the multifunctional pipette in neuropharmacological studies of the brain slices. We developed an experimental setup, performed feasibility studies, characterized the device performance and compared it with common superfusion techniques, using extra- and intracellular electrophysiological recordings of pyramidal cells in hippocampal and prefrontal cortex brain slices from rats. The multifunctional pipette was used in these experiments for highly localized delivery of the competitive AMPA receptor antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) to selected locations on the slices. By applying multifunctional pipette, we achieved a multifold gain in solution exchange time and more efficient drug delivery compared to whole slice perfusion. The amount of drugs required in the microfluidics-supported experiments was by several orders of magnitude smaller. The multifunctional pipette enabled selective perfusion of a single dendritic layer in the CA1 region of hippocampus with CNQX, without affecting other layers in this region

A model for cerebral cortical neuron group electric activity and its implications for cerebral function

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 245-265).The electroencephalogram, or EEG, is a recording of the field potential generated by the electric activity of neuronal populations of the brain. Its utility has long been recognized as a monitor which reflects the vigilance states of the brain, such as arousal, drowsiness, and sleep stages. Moreover, it is used to detect pathological conditions such as seizures, to calibrate drug action during anesthesia, and to understand cognitive task signatures in healthy and abnormal subjects. Being an aggregate measure of neural activity, understanding the neural origins of EEG oscillations has been limited. With the advent of recording techniques, however, and as an influx of experimental evidence on cellular and network properties of the neocortex has become available, a closer look into the neuronal mechanisms for EEG generation is warranted. Accordingly, we introduce an effective neuronal skeleton circuit at a neuronal group level which could reproduce basic EEG-observable slow ( 3mm). The effective circuit makes use of the dynamic properties of the layer 5 network to explain intra-cortically generated augmenting responses, restful alpha, slow wave (< 1Hz) oscillations, and disinhibition-induced seizures. Based on recent cellular evidence, we propose a hierarchical binding mechanism in tufted layer 5 cells which acts as a controlled gate between local cortical activity and inputs arriving from distant cortical areas. This gate is manifested by the switch in output firing patterns in tufted(cont.) layer 5 cells between burst firing and regular spiking, with specific implications on local functional connectivity. This hypothesized mechanism provides an explanation of different alpha band (10Hz) oscillations observed recently under cognitive states. In particular, evoked alpha rhythms, which occur transiently after an input stimulus, could account for initial reogranization of local neural activity based on (mis)match between driving inputs and modulatory feedback of higher order cortical structures, or internal expectations. Emitted alpha rhythms, on the other hand, is an example of extreme attention where dominance of higher order control inputs could drive reorganization of local cortical activity. Finally, the model makes predictions on the role of burst firing patterns in tufted layer 5 cells in redefining local cortical dynamics, based on internal representations, as a prelude to high frequency oscillations observed in various sensory systems during cognition.by Fadi Nabih Karameh.Ph.D

Intrinsic structure of the inferior colliculus

The inferior colliculus (IC) is a major integrative center within the central auditory system, which contains three main subdivisions, i.e. the central nuclei (ICc), the external cortex (ICx) and the dorsal cortex (ICd). The central nucleus is characterized by the fibro-dendritic laminae constructed from the dendrites and axons of its two principal cell types: the disc-shaped cells and the stellate cells. Whereas in the ICc ascending inputs converge from all auditory nuclei upstream, the external and dorsal cortex converge the descending inputs from all auditory nuclei downstream. Additionally, the external cortex is receiving ascending inputs from multisensory systems. The IC plays an important functional role in the auditory system. A large amount of work has been done on the cell morphology and electrophysiology; however, less research work was done to study the intrinsic network, which is important to understand the computations performed by the inferior colliculus. Recently, the existence of excitatory and inhibitory intrinsic innervations has been proven in the ICc; however, it is still unclear whether the amount of intrinsic innervation and orientation depends on the location, the morphology of the cell or its intrinsic membrane parameters. Besides, the intrinsic inputs of other IC subdivisions are still unknown. This thesis was a phenomenological study of the intrinsic innervation of the inferior colliculus neurons. We used whole-cell patch clamp technique combined with laser scanning photostimulation of caged glutamate in order to determine the organization of intrinsic inputs of the ICc and the ICx neurons in the Mongolian gerbil. Our approach specifically aimed not only to study the intrinsic innervations and their regional differences, but also to find the correlations of intrinsic innervation with electrophysiological and morphological parameters. The excitatory and inhibitory intrinsic inputs of neurons from the ICc and ICx were measured. In addition, the electrophysiology and morphology of the neurons were recorded. We were the first to find that in case of excitation, ICc cells had two preferred orientations that were arranged both in the isofrequency axis and in the tonotopic axis, and yet the innervation of ICx cells only oriented along the outer boundary of the IC. In contrast, the inhibitory innervations of both ICc and ICx neurons dispersed in different directions. Moreover, we found that the intrinsic innervations were regionally heterogeneous in their excitation-inhibition balance. Whereas cells in the dorso-medial ICc cells had a balanced intrinsic excitation and inhibition, excitation was prevalent in the ventro-lateral ICc and in the ICx. Besides, we compared the amount of inputs in paired regions. Thus we found that in the lateral ICc, there existed a narrow region spanning along the outer boundary of the ICc where cells received significantly smaller amounts of excitatory inputs than in the adjacent region (ICx). In addition, we found that ICx cells received more excitatory inputs than the ICc cells. Moreover, we found that the amount of excitatory intrinsic innervations was correlated with the apparent surface of the cells, and the amount of inhibitory intrinsic innervations was correlated with the membrane conductance of the cells. Thus we found that the dorso-medial ICc cells varied most in their membrane conductance, whereas the ICx cells were most similar in their membrane conductance and the lateral ICc cells were most similar in their apparent cell surface

Analysis of spine plasticity in CA1 hippocampal pyramidal neurons employing live cell nanoscopic imaging

In der Großhirnrinde von Säugetieren befindet sich die Mehrheit erregender Synapsen auf Dornfortsätzen, kleinen dendritischen Ausbuchtungen, die in Größe und Form stark variieren. Die Auslösung aktivitätsabhängiger synaptischer Langzeitplastizität geht mit strukturellen Veränderungen dendritischer Dornen einher. Da das beugungsbegrenzte Auflösungsvermögen konventioneller Lichtmikroskope nicht ausreicht um die Morphologie der Dornen verlässlich zu untersuchen, stellte die Elektronenmikroskopie bisher das wichtigste bildgebende Verfahren zur Erforschung von struktureller Plastizität dar, blieb dabei jedoch auf die Betrachtung fixierter Gewebeproben beschränkt. Die Anwendung hochauflösender Laser-Raster-Mikroskopie mit Stimulierter-Emissions-Auslöschung hat es mir möglich gemacht, die Dynamik dendritischer Dornenmorphologie in lebenden Zellen zu studieren. Die N-Methyl-D-Aspartat-Rezeptor-abhängige Langzeitpotenzierung von Pyramidenzellen der Cornu-Ammonis Region 1 des Hippocampus bildete dabei den Mechanismus, welcher plastische Veränderungen hervorrief. Nach Potenzierung exzitatorischer Synapsen durch die lokale Ultraviolett-Photolyse von caged-Glutamat wurde ein starker, vorübergehender Anstieg des Anteils dendritischer Dornen mit sichelförmigen Köpfen und ein leichter, anhaltender Zuwachs an pilzförmigen Dornfortsätzen über einen Zeitraum von 50 Minuten beobachtet. Meine Untersuchungen ergänzen frühere Studien zur Wechselbeziehung zwischen synaptischer Potenzierung und struktureller Plastizität dendritischer Dornen und korrespondieren mit dem aktuellen Kenntnisstand der zu Grunde liegenden molekularen Mechanismen.The majority of excitatory synapses in the cortex of mammalian brains is situated on dendritic spines, small protrusions, heterogeneous in size and shape. The induction of activity-dependent long-term synaptic plasticity has been associated with changes in the ultrastructure of spines, particularly in size, head shape and neck width. Since the dimensions of dendritic spines are at the border of the diffraction-limited resolving power of conventional light microscopes, until recently, electron microscopy on fixed tissue constituted the primary method for investigations on spine morphology. I have employed live cell stimulated emission depletion imaging to analyse spine motility and structural transitions in response to n-methyl-d-aspartate receptor dependent long-term potentiation over time at super-resolution in Cornu Ammonis area 1 pyramidal neurons of the hippocampus. Local induction of long-term potentiation via ultraviolet photolysis of caged glutamate facilitated a strong transient increase in the proportion of spines with curved heads and a subtle persistent growth in the amount of mushroom spines over a time course of 50 minutes. My findings reinforce previous investigations on the relation of synaptic potentiation and spine motility, and are in good agreement with the current knowledge of the molecular mechanisms underlying long-term plasticity

Mathematical Modelling of the Neurochemical Processes in Schizophrenia

Schizophrenia is an endogenous psychosis with a 1 \% prevalence in world population. Several pharmacological studies suggest that alterations in the function of different neurotransmitter systems such as dopamine or glutamate are related to schizophrenic symptoms. This thesis represents mathematical models that are constructed to investigate the dynamical behaviour of the neurochemical systems in the human brain. These models formulate the anatomical properties and physiological processes of synapses, single brain compartments and large neurochemical pathways involved in the regulation of behaviour such as the basal ganglia and the limbic system. The interaction between the neurochemical systems and the electrophysiological activities are considered by modelling in different scales. In the synaptic scale, it has been shown that the transport of neurotransmitters in the synaptic cleft is merely governed by electrical forces than diffusion. The intra-synaptic concentration of neurotransmitters is modelled using partial differential equations and is coupled to the Hodgkin-Huxley equation (neurochemical modification) to model the effect of neurotransmitter-receptor binding in the generation of post-synaptic potentials. Considering the morphological and ultra-morphological studies of brain compartments, the averaged electrophysiological activity is modelled by integral equations respecting these internal structures. A system comprised by nonlinear delay differential equations is constructed to simulate the dynamical behaviour of neurochemical concentrations, coupled to the local electrophysiological activity of the compartments, on the brain pathways. By parameter sensitivity analysis, we have also investigated qualitatively the influence of certain anti-psychotic agents. Synchronized oscillations are experienced in electrophysiological systems. The neurotransmitter concentrations also demonstrate an oscillatory behaviour. The resulting oscillatory dynamics of these processes reveals a profound view on the relation between the dynamical behaviour of the neurochemical systems and the occurrence of psychotic states. These facts led us to establish a hypothesis on this relation, called the oscillation hypothesis of psychosis. Because of the general formulation of the models, these are not only useful for schizophrenia, but also for the investigations of other neurological diseases