EFFECTS OF ACUTE TILT FROM ORTHOSTATIC TO HEAD-DOWN ANTIORTHOSTATIC RESTRAINT AND OF SUSTAINED RESTRAINT ON THE INTRA-CEREBROVENTRICULAR PRESSURE IN RATS

The tail-cast suspension rat model was developed to explore in ground laboratories the physiological effects of some of the stresses prevail ing during space flight including and among them those of the headward s body fluid shifts. We recently showed in rats that an acute head-dow n tilt (45 degrees) from tail-cast orthostatic (OR) to antiorthostatic restraint (AOR) induced within 30 min and for 2 to 4 h an acute stres s-like surge in plasma ACTH and corticosterone levels. Considering the proximity of the CRF producing neurons with the 3rd ventricle, we dec ided to explore the acute and longer-term effects of the OR/AOR tilt o n the intra-cerebroventricular pressure (P-icv) measured with an indwe lling sensor-transmitter catheter stereotaxically implanted in the 3rd ventricle. At 1- or 10-min intervals the unit sent radiotelemetric si gnals for both P-icv and motor activity (MA) to a receiver coupled wit h an automatic data analyser. The acute AOR-tilt induced within 10 min and for 60 min a 2.5-fold rise in P-icv which receded to baseline bet ween 60 and 90 min. During this time, the normally close correlation b etween P-icv and MA was lost, as assessed by Spearman's rank coefficie nt. In a long-term experimental series we explored the evolution of bo th P-icv and MA in individual rats subjected successively to a 7 day c ontrol phase (C), 7 days OR, and 3 days AOR. After the 1-h-long post-t ilt rise of the P-icv, the mean P-icv levels measured for the next 3 d ays decreased significantly vs. both the preceding OR phase (-30%) and the initial C phase (-40%). The circadian pattern of the diurnal P-ic v profile was impaired, as evidenced by a significant fall (i) in the night/day ratio (-25% vs. C), and (ii) even more in the spectral power of the circadian 1 c/24 h frequency (-85% vs. C). The simultaneously recorded MA fluctuations similarly displayed an altered diurnal patter n with a spectral power of the circadian frequency reduced to 7% of co ntrols. However, contrary to the short-term experiment, in the long-te rm study the large alterations to both P-icv and MA were strongly corr elated, as during the control phase. The mechanisms involved in the sw ift post-tilt rise in the P-icv together with an aroused corticotropic axis, and in the impact of sustained head-down restraint on CNS-contr olled adaptive regulations including their circadian rhythms remain un known.