CHOROIDAL RESPONSES IN MICROGRAVITY - (SLS-1, SLS-2 AND HINDLIMB-SUSPENSION EXPERIMENTS)

Fluid and electrolyte shifts occuring during human spaceflight have be en reported and investigated at the level of blood, cardiovascular and renal responses. Very few data were available concerning the cerebral fluid and electrolyte adaptation to microgravity, even in animal mode ls. It is the reason why we developed several studies focused on the e ffects of spaceflight (SLS-1 and SLS-2 programs, carried on NASA STS 4 0 and 56 missions, which were 9- and 14-day flights, respectively), on structural and functional features of choroid plexuses, organs which secrete 70-90 % of cerebrospinal fluid (CSF) and which are involved in brain homeostasis. Rats flown aboard space shuttles were sacrificed e ither in space (SLS-2 experiment, on flight day 13) or 4-8 hours after landing (SLS-1 and SLS-2 experiments). Quantitative autoradiography p erformed by microdensitometry and image analysis, showed that lateral and third ventricle choroid plexuses from rats flown for SLS-1 experim ent demonstrated an increased number (about x 2) of binding sites to n atriuretic peptides (which are known to be involved in mechanisms regu lating CSF production). Using electron microscopy and immunocytochemis try, we studied the cellular response of choroid plexuses, which produ ce cerebrospinal fluid (CSF) in brain lateral, third and fourth ventri cles. We demonstrated that spaceflight (SLS-2 experiment, inflight sam ples) induces changes in the choroidal cell structure (apical microvil li, kinocilia organization, vesicle accumulation) and protein distribu tion or expression (carbonic anhydrase II, water channels,...). These observations suggested a loss of choroidal eel polarity and a decrease in CSF secretion. Hindlimb-suspended rats displayed similar choroidal changes. Ail together, these results support the hypothesis of a modi fied CSF production in rats during long-term (9, 13 or 14 days) adapta tions to microgravity.