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INFLUENCE OF HYPERGRAVITY ON THE POSTNATAL MATURATION OF RAT VESTIBULAR UTRICLES. S. Gaboyard, C. Chabbert, J. Lehouelleur and A. Sans, INSERM U583 Montpellier, France.

   Among the vestibular receptor organs, the utricular macula is devoted to sense the terrestrial gravity. To study whether that primary stimulus may act as an epigenetic factor that influences the development of the sensory hair cells and the formation of the synaptic contacts with their cognate afferents, we reared rats under an artificially enhanced gravity (2g) using a terrestrial centrifuge. We previously showed that hypergravity modifies their hair cell physiology and synaptogenesis maturation in utricles: the current density of the potassium current of type I cells was greatly increased (Chabbert, EJN, 2003), and the relocation of synaptic vesicles at the upper part of calices was delayed during the postnatal developmental period (Gaboyard, Dev Brain Res, 2003). The aim of the present study was to precise the modifications of these synaptic connections induced by hypergravity. Using immuno-cytochemistry we estimated the number of sensory cells labeled for calretinin. This calcium-binding protein is a reliable marker of the vestibular epithelium maturation since it evolves from a hair cell to a calyceal cytoplasmic expression during the maturation process. The afferent calices were immuno-stained for the Na⁺/K⁺ ATPase 3 subunit. This enzyme localizes in membranes of this type I connections to regulate the excitable properties. We found that the increase of the primary stimulus did not modify the maturation of the hair cell molecular marker during the first postnatal week, whereas it altered the set up of the ionic pump localisation within the calyx terminal. These results fit and confirm our previous studies suggesting that hypergravity influences the postnatal maturation of the utricular type I connections in rats: it would induce a delay in the developing functional interactions of the signalling pathway between the mechanoreceptors and their afferent neurons.

(Supported by CNES)