The activities of pairs of mammalian motor neurons (MNs) receiving varying degrees of common excitatory synaptic input were simulated to study the relationship between nearly-coincident spiking and common excitatory drive. The somatic membrane potential of each MN was modeled using a single compartment model. Each MN was modeled toreceive synaptic contacts from hundreds of pre-synaptic fibers. The percentage of pre-synaptic fibers that diverged to supply both MNs of a pair with common synaptic input could be varied from 0 (no common inputs) to 100% (all common inputs). Spikes trains on separate re-synaptic fibers were independent of one another and were modeled as realizations of renewal processes with mean firing rates (10 - 50Hz) resembling that associated with supra-spinal input. Maximum synaptic conductances and time constants were varied across synapsesto match experimentally observed somatic EPSPs. The number of active pre-synaptic fibers to each MN was adjusted in order that the firingrates of MNs were between 8 and 15 Hz. For each common input condition, 100 s of concurrent spiking activity of the MNs was usedto construct cross-correlation histograms. The sizes of the central peaks in the histograms were quantified using both the k' (Ellaway and Murthy 1985) and CIS (Nordstrom et al. 1992) indices ofsynchrony. Monotonically increasing linear relationships between the proportion of common synaptic input and the magnitude of synchronywere observed for both indices. For example, the model predicted that 10% common input would yield a CIS value of 0.27 whereas 100% commoninput would yield a CIS value of 1.5. These values are within the range of values observed experimentally. These results, therefore,provide a means to translate measures of nearly-coincident spiking into plausible renditions of synaptic connectivity.

The work in this thesis concerns mechanisms of excitability of neurons. Specifically, it deals with how neurons respond to input, and how their response is controlled by ion channels and other active components of the neuron. I have studied excitability in two systems of the nervous system, the hippocampus which is responsible for memory and spatial navigation, and the peripheral C–fibre which is responsible for sensing and conducting sensory information to the spinal cord. Within the work, I have studied the role of excitability mechanisms in normal function and in pathological conditions. For hippocampus the normal function includes changes in excitability linked to learning and memory. However, it also is intimately linked to pathological increases in excitability observed in epilepsy. In C–fibres, excitability controls sensitivity to responses to stimuli. When this response becomes enhanced, this can lead to pain. I have used computational modelling as a tool for studying hyperexcitability in neurons in the central nervous system in order to address mechanisms of epileptogenesis. Epilepsy is a brain disorder in which a subject has repeated seizures (convulsions) over time. Seizures are characterized by increased and highly synchronized neural activity. Therefore, mechanisms that regulate synchronized neural activity are crucial for the understanding of epileptogenesis. Such mechanisms must differentiate between synchronized and semi synchronized synaptic input. The candidate I propose for such a mechanism is the fast outward current generated by the A-type potassium channel (KA). Additionally, I have studied the propagation of action potentials in peripheral axons, denoted C–fibres. These C–fibres mediate information about harmful peripheral stimuli from limbs and organs to the central nervous system and are thereby linked to pathological pain. If a C–fibre is activated repeatedly, the excitability is altered and the mechanisms for this alteration are unknown. By computational modelling, I have proposed mechanisms which can explain this alteration in excitability. In summary, in my work I have studied roles of particular ion channels in excitability related to functions in the nervous system. Using computational modelling, I have been able to relate specific properties of ion channels to functions of the nervous system such as sensing and learning, and in particular studied the implications of mechanisms of excitability changes in diseases. / <p>QC 20102423</p>

La Sclérose Latérale Amyotrophique (SLA) est une maladie neurodégénérative au cours de laquelle les motoneurones meurent. Le premier dysfonctionnement des motoneurones est la rétractation de leurs jonctions neuromusculaires. La présence de vacuoles a été décrite dans l’axone et les dendrites des motoneurones avant la dénervation dans les souris SOD1G93A, modèle murin de la maladie. L’origine des vacuoles n’est pas connue. On peut toutefois se demander si elle pourrait résulter d’un stress excitotoxique. L’excitotoxicité pourrait provenir soit d’une hyperexcitabilité intrinsèque du motoneurone, soit d’une hyperexcitation (balance des entrées excitatrices et inhibitrices modifiées au profit d’une plus grande excitation). Or il a été montré que si les motoneurones sont hyperexcitables au stade embryonnaire dans les souris SOD1G93A, seuls les motoneurones résistants à la SLA (type S) sont hyperexcitables à la deuxième semaine postnatale tandis que les motoneurones vulnérables (types FF et FR) deviennent hypoexcitables avant leur dégénérescence chez l’adulte. Nous avons donc étudié les entrées synaptiques reçues par les motoneurones, pour savoir si la balance excitation/inhibition est déplacée et s’ils sont ainsi hyperexcités. Pour cela nous avons réalisé des enregistrements électrophysiologiques de motoneurones lors de la stimulation de circuits pré-moteurs, et des marquages intracellulaires de motoneurones combinés avec des marquages immunohistochimiques des boutons VGlut1, VGlut2 et Vgat. Nous avons montré que l’amplitude des PPSE monosynaptiques Ia était diminuée dans les souris SOD1, les PPSI di- et trisynaptiques étaient moins nombreux et les interneurones inhibiteurs moins excitables. Cette modification des entrées synaptiques n’était pas due à un changement du nombre de synapses. En revanche, les synapses sont particulièrement nombreuses aux domaines dendritiques qui se vacuolisent dans les souris SOD1, suggérant un lien entre l’activité synaptique et la vacuolisation. Des marquages intracellulaires de motoneurones de souris SOD1, montrent que les vacuoles grandissent avec l’évolution de la maladie, suggérant leur implication dans le processus de dégénération. Grâce à des révélations immunohistochimiques, nous avons montré que ces vacuoles apparaissent dans l’espace intermitochondrial lors de la dégénérescence des mitochondries. Le réticulum endoplasmique est également impliqué. Enfin, l’autophagie, mécanisme d’élimination des organites cellulaires, est déficient au moment de l’apparition des vacuoles, expliquant pourquoi elles s’accumulent avec le temps. Ces résultats amènent à reconsidérer l’hypothèse de l’excitotoxicité supposée comme mécanisme à l’origine de la mort des motoneurones. / Glutamate excitotoxicity arising from excessive entry of calcium in the cell, has long been suggested to contribute to the degeneration of motoneurons in Amyotrophic Lateral Sclerosis (ALS). This hypothesis is enhanced by the observation of vacuoles on motoneurones dendritic tree. Such vacuoles were previously observed on neurons under excitotoxic stress. Excitotoxicity may stem from an intrinsic hyperexcitability of the motoneurons or from a shift of the balance of excitatory / inhibitory inputs received by the motoneurons toward more excitation. Thanks to an in vivo preparation that allows us to make intracellular recordings of motoneurons in adult mice, it was shown that spinal motoneurons do not display an intrinsic hyperexcitability just prior to their degeneration in SOD1 G93A mice, the standard model of ALS. Thus, to study excitotoxicity hypothesis, we decided to study dendritic vacuoles and undersand their genesis, and then to study synaptic inputs on motoneurons, to decipher if there is a hyperexcitability. We have shown, with intracellular labelling and immunohistochemistry, that vacuoles grow with age, that they appear in the intermembrane space of mitochondria, and that deficiency in autophagy prevent their elimination. With electrophysiological recordings, we have shown that monosynaptic EPSP amplitude is reduced in SOD1 mice. IPSP were less numerous and inhibitory interneurons were less excitable. These alterations were not due to synapses numbers, however synapses are preferentially localised on dendritic places that vacuolate, suggesting a link between synaptic activity and vacuolation. These results suggest that excitotoxicity might not be the mecanism of motoneuron death.