The abundance of microtubules in receptive dendrites suggests they may function in sensory transduction. Responses of frog muscle spindle receptors and joint receptors is inhibited within 25 minutes by 50 mM colchicine, a microtubuledisrupting agent. The inhibition is reversible upon removal of colchicine, and the time course of recovery is comparable to that of inhibition. Frog olfactory responses are briefly inhibited by washing the olfactory mucosa with perfusion fluid. Colchicine accentuates the inhibition and substantially retards the rate of recovery in a dose-dependent fashion. Colchicine does not affect axonal conduction, nor the oxygen uptake of isolated crab or frog leg nerves. The inhibitory action of colchicine is therefore an effect on the electrical excitability of the receptive dendrites or soma, and not an effect on axonal conduction.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

No description available.

Homeostasis within the neuro-glial unit is essential to the longevity of neurons. Conversely, loss of homeostasis, particularly of Ca2+ levels, of redox balance and of ATP, contribute to neuronal loss and dysfunction in many neurodegenerative and neurological disorders. This thesis is centred on better understanding the vulnerability of neurons to stress, as well as adaptive responses to these stresses. Since neurodegenerative conditions associated with Ca2+, redox and bioenergetic dyshomeostasis are often characterised by early dendritic pathology, I first studied dendritic vs. somatic responses of primary cortical neurons to these types of challenges in real-time. Using a wide range of genetically-encoded probes to measure Ca2+, ATP, NADH, glutathione and glutamate, I show that dendrites are selectively vulnerable to oxidative stress, excitotoxicity as well as to metabolic demand induced by action potential (AP) burst activity. However, I provide evidence that neurons undergoing energetically demanding AP burst activity can adjust their metabolic output by increasing mitochondrial NADH production in a manner dependent on the mitochondrial calcium uniporter (MCU), as well as increase their capacity to buffer their intracellular redox balance. Finally, I have studied transcriptional programs in astrocytes triggered by neurons and neuronal activity to better understand adaptive signaling between different cell types in the neuro-glial unit. I developed a novel system combining neurons and astrocytes from closely-related species, followed by RNA-seq and in silico read sorting. I uncovered a program of neuron-induced astrocytic gene expression which drives and maintains astrocytic maturity and neurotransmitter uptake function. In addition I identified a novel form of synapse-to-nucleus signaling, mediated by glutamatergic activity and acutely regulating diverse astrocytic genes involved in astrocyte-neuron metabolic coupling. Of note, neuronal activity co-ordinately induced astrocytic genes involved in astrocyte-to-neuron thyroid hormone signaling, extracellular antioxidant defences, and the astrocyte-neuron lactate shuttle, suggesting that this non cell-autonomous signaling may form part of the homeostatic machinery within the neuro-glial unit.

Schizophrenia is a prevalent neurological disorder characterized by disrupted neuronal circuitry. Antipsychotic drugs (APDs) are capable of ameliorating the symptoms of schizophrenia with varying efficacy. Clozapine, the "gold-standard" for antipsychotic drug treatment, has been shown by this lab to induce the outgrowth of mediodorsal thalamic (MDT) dendritic arbor in rodents, a brain region which has altered function and decreased regional volume in schizophrenic patients. These studies further explored the ability of APD treatment to restructure dendrite arbor and the mechanisms of clozapine's ability to elaborate MDT arbor. Additionally, glutamate hypofunction is thought to contribute to the schizophrenic disease state. Using a novel model of perinatal glutamate hypofunction, we examined the long-term effects on dendritic architecture of developmental glutamate signaling disruption. MDT dysfunction is hypothesized to contribute to cognitive symptoms of schizophrenia. Clozapine has increased efficacy in ameliorating these symptoms. To further understand clozapine’s actions to remodel MDT dendritic architecture, we examined whether clozapine-induced morphological alterations are limited to the thalamus or if they also occur in additional regions associated with cognitive schizophrenic pathology, the hippocampus and striatum. We found that clozapine can induce dendritic remodeling in the hippocampus, but the not to the amplitude of remodeling seen in the thalamus, indicating that the MDT is uniquely altered by clozapine treatment and may be an important locus of clozapine's action. The mechanisms of clozapine's remodeling of MDT arbor, we examined changes to mRNA and miRNA expression and calcium dynamics in the MDT in response to APD treatment. Clozapine-treatment altered the expression of genes involved in cytoskeletal remodeling, external membrane receptors, and calcium dynamics, as well as increased the rate of calcium influx into thalamic neurons. Disruption to glutamate signaling has been hypothesized to contribute to schizophrenic pathology. Disruption to perinatal vesicular glutamate packaging along the corticolimbic axis has long term effects for neuronal morphology and function. Interestingly, we find that disruption along the corticolimbic axis also has downstream effects on MDT dendritic architecture. These studies show that the MDT is an important locus of action for clozapine and is capable of remodeling dendritic architecture in response to afferent circuitry dysfunction. / acase@tulane.edu

Neurons have the ability to dynamically adjust their own membrane channel densities to modulate the strength of communication with other neurons. This process is integral to such neuronal functions as spatial recognition and memory but has been difficult to measure experimentally. Historically, neuroscientists have used changes in voltage to infer changes in neuronal channel densities. However, voltage is difficult to measure away from the soma. Many important functions in the neuron, like synaptic integration, take place in the dendritic tree where traditional voltage measurements can not be taken. To interrogate the neuron in the dendrites, experimentalists have come to rely on calcium fluorescence based microscopy to infer qualitative information about voltage changes in the dendrites. In these experiments, intracellular calcium changes due to voltage depolarizations are recorded at spatially distributed sites on the dendrites through the binding of calcium to a fluorescent buffer. The recovery of channel densities can be posed as a parameter identification problem in a coupled nonlinear partial differential equation that relates the responses of calcium, the fluorescent buffer and voltage to neuronal stimulation. We convert temporally and spatially distributed fluorescence data into quantitative measurements of voltage sensitive channel densities by inverting slow time-scaled calcium data into fast time-scaled voltage data. Our approach is to solve four interrelated inverse problems corresponding to three different proposed experiments to go from calcium fluorescence to channel densities. In the first experiment, we use subthreshold calcium dynamics to infer the reaction kinetics between calcium arid fluorescent buffer. From these kinetics, we can use suprathreshold voltage stimulation to infer calcium channel densities and recover distributed voltage data. Finally we use the voltage data to infer potassium channel densities in the dendrites. Our algorithm has been shown to recover channel densities for several different calcium channel models and the delayed rectifying potassium channel from simulated noisy fluorescence data in morphologically realistic neurons.

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts.

With the recent discoveries of phenomena such as plateau potentials, bistability, and synaptic amplification the focus of motoneuron research has been directed to the dendritic processes giving rise to these latent behaviors. The common consensus is that the mechanism behind bistability (an L-type calcium channel generating a persistent inward current, PIC; Schwindt and Crill 1980, Hounsgaard and Kiehn 1985, 1989) is also responsible for the amplification of synaptic input in motoneurons. However, modeling studies utilizing only calcium-based PICs (Powers 1993, Booth et al. 1997, Elbasinouy et al. 2005) have been unable to reproduce the high degree of synaptic amplification observed in experimental preparations (Prather et al. 2001, Lee et al. 2003, Hultborn et al. 2003). The present work examines a theoretical amplification mechanism (electrotonic compression), based on a sodium PIC of dendritic origin, which acts to supplement the synaptic amplification due to the calcium PIC. The current goal is to test the "goodness-of-fit" of electrotonic compression with established mechanisms and behaviors. The findings of this modeling study support the concept of a dendritic sodium PIC which acts to reduce the attenuation of synaptic currents enroute to the motoneuron soma. Furthermore, it is suggested that the ratiometric expression of ion channels giving rise to this mechanism takes the form of a distribution "rule" applied ubiquitously across the dendritic tree, while the plateau-producing L-type calcium channels undergo a more discretized or regional distribution. This study demonstrates the power inherent to the controlled expansion of morphological complexity in an already complex model. While modeling studies are suitable testbeds for the evaluation of theoretical and/or experimentally intractable facets of physiology, great care and consideration should be given to the specification of models with high dimensionality. With the continual progression of our knowledge-base and computational capabilities, we can expect that more and more empirical observations will find their way into models of increasing complexity wherein the layers of embedded hypotheses are frequently implicit. It is therefore imperative that the neural modeling discipline adopt more rigorous methodologies to both accommodate and rein-in this growing complexity.

In the present study the effect of one commercially available anionic surfactant on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. Surfactants are known to increase gas hydrate formation rate. Memory water was used and the experiments were carried out at three different degrees of undercooling and two different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. Finally, the addition of 2200 ppm of SDS was found to increase the mole consumption for hydrate formation by 4.4 times.

Considerable research has been done to improve hydrate formation rate. One of the ideas is to introduce mechanical mixing which later tend to complicate the design and operation of the hydrate formation processes. Another approach is to add surfactant (promoter) that will improve the hydrate formation rate and also its storage capacity to be closer to the maximum hydrate storage capacity. Surfactant is widely known as a substance that can lower the surface or interfacial tension of the water when it is dissolved in it. Surfactants are known to increase gas hydrate formation rate, increase storage capacity of hydrates and also decrease induction time. However, the role that surfactant plays in hydrate crystal formation is not well understood. Therefore, understanding of the mechanism through morphology studies is one of the important aspects to be studied so that optimal industrial processes can be designed. In the present study the effect of three commercially available anionic surfactants which differ in its alkyl chain length on the formation/dissociation of hydrate from a gas mixture of 90.5 % methane – 9.5% propane mixture was investigated. The surfactants used were sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS). Memory water was used and the experiments for SDS were carried out at three different degrees of under-cooling and three different surfactant concentrations. In addition, the effect of the surfactant on storage capacity of gas into hydrate was assessed. The morphology of the growing crystals and the gas consumption were observed during the experiments. The results show that branches of porous fibre-like crystals are formed instead of dendritic crystals in the absence of any additive. In addition, extensive hydrate crystal growth on the crystallizer walls is observed. Also a “mushy” hydrate instead of a thin crystal film appears at the gas/water interface. Finally, the addition of SDS with concentration range between 242ppm – 2200ppm (ΔT =13.10C) was found to increase the mole consumption for hydrate formation by 14.3 – 18.7 times. This increase is related to the change in hydrate morphology whereby a more porous hydrate forms with enhanced water/gas contacts.