Le noyau réticulaire thalamique (RE) est une structure qui engendre des fuseaux, une oscillation bioélectrique de marque pendant les stades précoces du sommeil. De multiples propriétés neuronales, intrinsèques et synaptiques, sont impliquées dans la génération, la propagation, le maintien et la terminaison des ondes en fuseaux. D’un autre côté, ce rythme constitue un état spécial de l’activité du réseau qui est généré par le réseau lui-même et affecte les propriétés cellulaires du noyau RE. Cette étude se concentre sur ces sujets: comment les propriétés cellulaires et les propriétés du réseau sont inter-reliées et interagissent pour engendrer les ondes fuseaux dans les neurones du RE et leurs cibles, les neurones thalamocorticaux.

La présente thèse fournit de nouvelles évidences montrant le rôle fondamental joué par les neurones du noyau RE dans la genèse des ondes en fuseaux, dû aux synapses chimiques établies par ces neurones. La propagation et la synchronisation de l’activité sont modulées par les synapses électriques entre les neurones réticulaires thalamiques, mais aussi par les composantes dépolarisantes secondaires des réponses synaptiques évoquées par le cortex. De plus, la forme générale et la terminaison des oscillations thalamiques sont probablement contrôlées en grande partie par les neurones du RE, lesquels expriment une conductance intrinsèque leurs procurant une membrane avec un comportement bistable. Finalement, les oscillations thalamiques en fuseaux sont aussi capables de moduler les propriétés membranaires et l’activité des neurones individuels du RE.

The thalamic reticular nucleus (RE) is a key structure related to spindles, a hallmark bioelectrical oscillation during early stages of sleep. Multiple neuronal properties, both intrinsic and synaptic, are implicated in the generation, propagation, maintenance and termination of spindle waves. On the other hand, this rhythm constitutes a special state of network activity, which is generated within, and affects single-cell properties of the RE nucleus. This study is focused on these topics: how cellular and network properties are interrelated and interact to generate spindle waves in the pacemaking RE neurons and their targets, thalamocortical neurons.

The present thesis provides new evidence showing the fundamental role played by the RE nucleus in the generation of spindle waves, due to chemical synapses established by its neurons. The propagation and synchronization of activity is modulated by electrical synapses between thalamic reticular neurons, but also by the secondary depolarizing component of cortically-evoked synaptic responses. Additionally, the general shaping and probably the termination of thalamic oscillations could be controlled to a great extent by RE neurons, which express an intrinsic conductance endowing them with membrane bistable behaviour. Finally, thalamic spindle oscillations are also able to modulate the membrane properties and activities of individual RE neurons.

Thalamic reticular (RE) neurons, recorded and stained intracellularly in vivo , displayed spontaneously occurring spikelets, characteristic of central neurons coupled electrotonically via gap junctions. They were significantly different from excitatory postsynaptic potentials (EPSPs) and also distinct from synaptically generated fast prepotentials. Spikelets were strongly reduced by halothane, a blocker of gap junctions. Multi-site extracellular recordings performed before, during and after administration of halothane demonstrated a role for electrical coupling in the synchronization of spindling activity within the RE nucleus. Finally, computational models of RE neurons predicted that gap junctions between these neurons could mediate the spread of low-frequency activity at great distances.

A subgroup (20%) of RE neurons revealed the presence of membrane bistability, which consisted of two alternate membrane potentials, separated by ~17-20 mV. It was strongly voltage-dependent and only expressed under resting conditions. Addition of QX-314 in the recording micropipette either abolished or disrupted membrane bistability. Thalamocortical cells presented various patterns of spindling that reflected the membrane bistability in RE neurons. Finally, computer simulations predicted a role for RE neurons’ membrane bistability in inducing various patterns of spindling in target thalamocortical cells.

To assess the consequences of spindle oscillations on the RE nucleus, recordings were performed during periods of intense synaptic activity as represented by spindle waves. RE neurons presented low-pass filter properties, strongly dumping frequencies higher than 10 Hz, the main component of spindle waves. During spindles, membrane potential was depolarized, membrane fluctuations increased in one order of magnitude, and the apparent input resistance decreased up to 80% in a cyclic way. Both synaptic and intrinsic responsiveness were enhanced during active network states.

We analyzed both the characteristics and possible mechanisms underlying a secondary component of the cortically elicited depolarization in RE neurons. Cortical stimulation evoked fixed and short-latency EPSPs. The evoked EPSPs included a secondary depolarizing component, which occurred as an all-or-none event at ~5 ms after the peak of the initial component that was voltage-dependent and sensitive to QX-314 in the recording micropipette. This late component affected the integrative properties of RE neurons, including their spiking output and temporal summation of incoming cortical inputs.

A subgroup of RE neurons (30%) presented prolonged hyperpolarizing potentials preceding spindles. These hyperpolarizations were present just before the onset of spontaneously occurring spindle waves, and could also be evoked by corticothalamic volleys. A drop in the apparent input resistance was associated with these hyperpolarizing potentials, precluding disfacilitation processes. The reversal potential was with the activation of slow K⁺ conductances. QX-314 decreased both the amplitude and incidence of hyperpolarizations. Simultaneous extracellular and intracellular recordings in the RE nucleus demonstrated that some RE neurons discharged during the hyperpolarizations. These data support the idea that spindles are initiated in the pacemaking RE nucleus.

The following thesis is presented in the form of a collection of scientific articles, either submitted or accepted for publication. The general introduction describes the theoretical context and experimental strategies of the studies described in the articles. The articles have been submitted to different scientific journals, and therefore the format of bibliographical citations changes from one to another. However, the other aspects concerning the presentation of results have been kept consistent. A general discussion of the results finalizes the thesis with the presentation of a model which integrates them in a coherent mode. The bibliography used for both introduction and discussion, is presented at the very end of the manuscript.

The thesis is mostly related to the pacemaking activity of RE nucleus in the generation of thalamic oscillations. However, at the end of the manuscript two studies related to neocortical neurons and their integrative properties have been also included.

I would like to use this occasion to express my gratitude to my thesis supervisor Dr. Mircea Steriade for the opportunity of working in his laboratory. Without his critics, discussions, corrections and support the present study would not have been possible.

I also would like to show my gratitude to Dr. Igor Timofeev for his help, especially for the experimental techniques and his scientific advises.

Finally, I would like to thank Mr. Pierre Guiguère for his technical and moral support in every moment.

This thesis is the result of a close collaboration during a period of over three years between Dr. Mircea Steriade, Dr. Igor Timofeev and the author. I would like also to express my gratitude to Dr. Sylvain Crochet, Dr. Terrence J. Sejnowski and Dr. Maxim Bazhenov, co-authors in some of the studies.

I would like to take the opportunity also to thank the organisms which provided financial support for the development of this project: Canadian Institute for Health Research, Human Frontier Research Program, National Institute of Neurological Disorders and Stroke, and Fonds de la Recherche en Santé de Québec.

Following, is the list of scientific articles where I have participated during the progress of my Ph.D. program: