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Developmental plasticity in vestibulo-spinal pathways during xenopus (xenopus laevis) metamorphosis: anatomical and functional aspects
In vertebrates, vestibulo-spinal pathways generate spinal motor commands that trigger vestibulo-spinal reflexes responsible for the body stabilization in space during passive and active head movements. This thesis studies the morpho-functional organization of the vestibulo-spinal pathways involved in postural control in the xenopus (xenopus laevis), an anuran amphibian exclusively aquatic with a highly conserved vestibular system similar to other vertebrates. This work was performed through a multi-methodological approach, combining neuroanatomy and intra- and extracellular electrophysiological techniques in order to characterize neuronal properties and vestibulo-spinal networks and reflexes organization. During the metamorphosis, the complete remodelling of the posturo-locomotor system, switching from the axial-undulatory system in larva to an appendicular system in adult, implies the anatomical and functional adaptation of these vestibulo-motor pathways. In the adult xenopus, the dorsal muscles (dorsalis trunci), innervated by the thoracic motoneurons, are only postural, unlike the great majority of vertebrates where the postural muscles are also involved in propulsion. Therefore, this amphibian model allows to isolate the postural component in vestibular control of posturo-locomotor behaviour. In the first part of my thesis, I demonstrated the existence of a double vestibular command on thoracic motoneurons, innervating the postural back muscles, at rest. A first direct vestibular pathway involves bilateral descending projections from the central vestibular nuclei (LVST and tangential nucleus). This first vestibular input appears to be responsible for trunk postural adjustments in response to the head position signal. The second, indirect vestibulo-spinal pathway, involving an ascending lumbo-thoracic relay, ensures the reflexive activity coordination of the thoraco-lumbar motor networks, in response to head velocity signal. Preliminary data at larval stage, suggest the existence of functional connections from the vestibular nuclei with the rostral axial motoneurons, which from thoracic motoneurons in adult derive partially, and caudal that disappear during the metamorphosis. In parallel of the network organization, the intracellular recording (patch-clamp) of vestibulo-spinal neurons, more specifically those from the LVST on a brainstem slice revealed three distinct electrophysiological phenotypes: phasic, tonic, and intermediate. During the metamorphosis, the phasic/tonic ration is reversed, from a majority of tonic neurons in larva to a majority of phasic neuron in juvenile. In addition, the expression of an ID potassium conductance carried by the Kv1.1 subunit appears to play an important role in the establishment of phasic and intermediate phenotypes. These results allowed establishing a link between the expression of neuronal phenotype in vestibular nuclei to the dynamic of vestibulospinal reflexes they produce, in relation with different posturo-locomotor swimming behaviours expressed between these two stages. My PhD work brings the opportunity to elaborate ongoing studies that will investigate more precisely the maturation processing in vestibulospinal pathways during the metamorphosis, in term of cellular plasticity, circuit reorganization and balance control adaptation.
Keywords: vestibular, posture, xenopus laevis, thoracic motoneuron, intrinsic membrane properties, vestibulo-spinal reflex
A. Olechowski-Bessaguet, R. Grandemange, L. Cardoit, E. Courty, F.M. Lambert, D. Le Ray (2020). «Functional organization of vestibulospinal inputs on thoracic motoneurons responsible of trunk postural control in Xenopus». The journal of physiology, 598(4) ,817-838.
Mme FENELON Valérie : Président
Mme LOHOF Ann : Rapporteur
BERANECK Mathieu: Rapporteur
CHAGNAUD Boris : Examinateur
Mme THOBY-BRISSON Muriel : Invité
LE RAY Didier : Invité
LAMBERT François : Directeur de thèse
Equipe : Organisation et Adaptabilité des Systèmes Moteurs
Directeur de thèse : François Lambert