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Innervation of rat vestibular maculae in hypergravity; an in vivo and in vitro study.  S. Gaboyard, E.Scarfone, J. Lehouelleur and A. Sans. INSERM 432 Univ. Montpellier. 

                The primary captors of the peripheral vestibular system, the sensory hair cells are innervated by neurones of Scarpa’s ganglion during the perinatal period. We have studied hair cell activity as an epigenetic factor influencing development and neuronal plasticity while modifying the gravitational field, the primary stimulus.In a new 3-D co-culture preparation, explants of vestibular  maculae and ganglion from 3-day old rats are embedded in an extracellular matrix gel, permitting orientaion and process growth in three dimensions. The preparation was flown on the Photon-12 mission. The time course of neurite regeneration has been presented previously (ARO 2000, 571). We submitted the co-cultures to hypergravity (2g). Neuroflament staining showed no change in the time course of neurite growth when compared to 1g. We also studied the in vivo effects of hypergravity on afferentation. Neurofilaments were stained in preparations taken from developming rats subjected to 2g. As in vitro, in vivo studies revealed no change in the time course of innervation. The unchanged time course of development agrees with morphological studies of synapse formation following weightlessness (Ross, M.D., J. Vest. Res., 1993), and electrophysiological studies of the major potassium conductances in hair cells ex vivo (Lennan, G.W.T., 749, ARO Abstracts, 1999). However, in the microgravity study the number of synaptic bodies on hair cells was elevated, and in hypergravity the current density of the potassium current of type I cells was greatly increased. We conclude that an altered gravitational field may not influence the time course of vestibular development, but could modify the functional interactions of the signalling pathway between mechanoelectrical transduction and Scarpa’s neurones.

(Supported by CNES & EPR Languedoc-Roussillon.)