Persistence of tight junctions and changes in apical structures and protein expression in choroid plexus epithelium...

Major alterations of choroidal cell polarity and protein expression were previously shown to be induced in rats by long-term adaptation to space flight (14 days aboard a space shuttle) or anti-orthostatic suspension (14 and 28 days) performed by tilting rats head-down (i.e. using a ground-based model known to simulate several effects of weightlessness). In rabbits, it was hypothesized that the blood-CSF barrier was opened in choroid plexus, after a short head-down suspension. To understand the early responses to fluid shifts induced by head-down tilts and evaluate the tightness of the choroidal junctions, we have investigated the effects of acute adaptations to anti-orthostatic restraints, using hindlimb-suspended Sprague-Dawley and Wistar rats. Ultrastructural and immunocytochemical studies were performed on choroid plexuses from lateral, third and fourth ventricles, after 30, 90 and 180 minutes of head-down tilt. Alterations were not perceptible at the level of choroidal tight junctions, as shown by freeze-fracture, claudin-1 and ZO-1 immunolocalizations and conventional electron microscopy, after intravenous injection of cytochrome C. The apical surface of choroidal cells was clearly more affected. Microvilli were longer and thinner and ezrin was over-expressed during all the periods of time considered, showing an early cytoskeletal response. Several proteins involved in the choroidal production of cerebrospinal fluid (sodium-potassium ATPase, carbonic anhydrase II, aquaporin 1) appeared first increased (30 minutes after the tilt), and then, returned to the control level or were lowered (after a 3-hour head-down suspension). Although head-down tilts do not seem to damage the blood-cerebrospinal fluid barrier in choroid plexus, it seemed that the expression of several apical proteins is affected very early.