Final Report Summary - SVETA (Vestibular System, Cognition and Vegetative Regulations)
The vestibular system, and more specifically its otolithic part (specialized in detecting gravity and inertial linear acceleration), has long been recognized for its role in spatial orientation and postural equilibrium. Its involvement in the regulation of other physiological systems (respiratory and cardiovascular systems, circadian regulation, food intake, bone mineralization) has been made clear only recently. Besides an increase in basic scientific knowledge, studying the biological impact of the vestibular system is also crucial for space exploration because it could participate in some harmful effects of prolonged exposure to weightlessness - such as cardiovascular and sensory-motor deconditioning, bone loss, and hormonal changes. Vestibular system dysfunction could also be implicated in common pathological conditions such as orthostatic hypotension, sleep disruption, bone loss…
Studying the biological effects of the otolith system can be performed by removing it or, more physiologically, by changing the level of gravity. Hypergravity can be produced by centrifuges while reduced gravity can only be obtained during parabolic or space flights. There are few ground alternatives, such as head-down bed rest and dry immersion, for studying some aspects of the effect of weightlessness on physiological systems. These facilities are scattered throughout Europe, Russia and the USA and no single scientific team has the expertise in all the main scientific fields relevant to gravitational physiology: neurosciences, musculo-skeletal physiology, endocrinology, chronobiology, and cardiovascular physiology. Thus, ambitious scientific research on the implication of vestibular system in gravitational physiology is not conceivable without international cooperation.
The general aim of SVETA project was to establish long-term research cooperation on vestibular system and gravitational physiology at an international level, implicating the main research facilities in Europe, Russia and the USA, and to create new research opportunities in this area. This project had several specific aims.
The first one was to the study the effect of short term vestibular stimulation on cardiovascular and brain functions. Parabolic flight experiments have shown that the vestibular system not only has a direct impact on limb arterial circulation but also indirectly modulates cardiovascular system by altering baroreflexes. In a second experiment, we tested the hypothesis that the effect of the vestibular system on the cardiovascular system does not result from reflexes but is mediated by an internal representation of gravity. Indeed, we observed that, when submitted to an ambiguous movements, subjects have a stronger cardiovascular response when perceiving tilt than when perceiving translation although vestibular stimulation is exactly the same.
The second aim was to study physiological and cognitive impairments induced by vestibular loss or long-term vestibular hyperstimulation. Several experiments have been achieved in France, New-Zealand and the USA. We have explored the connections between the vestibular inner ear and the hippocampus and identified some neural pathways through which vestibular information is transmitted. We have also shown that a bilateral vestibular loss (BVL) leads to spatial learning impairments that are not offset by visual information. In another study we have observed that BVL disrupts the daily rhythms of temperature and locomotor activity in rats probably because of a loss of vestibular input on the suprachiasmatic nucleus (the so-called “biological central clock”). We have also explored long-term effects of BVL on daily rhythms and bone density in mice and rats and we have shown that some of these effects are compensated with time.
The third aim was to determine whether abnormal sensory integration could disrupt autonomic functions. To that end we have characterized the effects of Head Down Bed Rest (HDBR) and Dry Immersion (DI) – 2 procedures known to disrupt cardiovascular regulation - on the peripheral and central processing of vestibular information and to evaluate the impact of these alterations on cardiovascular control, circadian regulation and cognitive functions. Experiments are still ongoing and will be finished in April 2016. However, we have already found important results: i) HDBR and DI alter peripheral and central processing of vestibular information; ii) Modification of peripheral vestibular function is caused partly by an increase in endolymphatic pressure; iii) Change in vestibular processing alters the influence of vestibular and neck afferences on cardiovascular regulations; iv) Vestibular-induced modification of cardiovascular regulation contributes to cardiovascular deconditioning; v) Change in vestibular processing alters spatial learning and memory tasks;
Our findings support the hypothesis that the vestibular system has an influence on cognitive and autonomous functions on Earth, and raise the question of whether those functions might be altered due to vestibular pathology in humans. They also raise the question of whether vestibular system dysfunction could participate in some harmful effects of long-term weightlessness observed in astronauts. In order to answer those two questions, teams of the SVETA project have submitted joint proposals, some of them are already selected.
Studying the biological effects of the otolith system can be performed by removing it or, more physiologically, by changing the level of gravity. Hypergravity can be produced by centrifuges while reduced gravity can only be obtained during parabolic or space flights. There are few ground alternatives, such as head-down bed rest and dry immersion, for studying some aspects of the effect of weightlessness on physiological systems. These facilities are scattered throughout Europe, Russia and the USA and no single scientific team has the expertise in all the main scientific fields relevant to gravitational physiology: neurosciences, musculo-skeletal physiology, endocrinology, chronobiology, and cardiovascular physiology. Thus, ambitious scientific research on the implication of vestibular system in gravitational physiology is not conceivable without international cooperation.
The general aim of SVETA project was to establish long-term research cooperation on vestibular system and gravitational physiology at an international level, implicating the main research facilities in Europe, Russia and the USA, and to create new research opportunities in this area. This project had several specific aims.
The first one was to the study the effect of short term vestibular stimulation on cardiovascular and brain functions. Parabolic flight experiments have shown that the vestibular system not only has a direct impact on limb arterial circulation but also indirectly modulates cardiovascular system by altering baroreflexes. In a second experiment, we tested the hypothesis that the effect of the vestibular system on the cardiovascular system does not result from reflexes but is mediated by an internal representation of gravity. Indeed, we observed that, when submitted to an ambiguous movements, subjects have a stronger cardiovascular response when perceiving tilt than when perceiving translation although vestibular stimulation is exactly the same.
The second aim was to study physiological and cognitive impairments induced by vestibular loss or long-term vestibular hyperstimulation. Several experiments have been achieved in France, New-Zealand and the USA. We have explored the connections between the vestibular inner ear and the hippocampus and identified some neural pathways through which vestibular information is transmitted. We have also shown that a bilateral vestibular loss (BVL) leads to spatial learning impairments that are not offset by visual information. In another study we have observed that BVL disrupts the daily rhythms of temperature and locomotor activity in rats probably because of a loss of vestibular input on the suprachiasmatic nucleus (the so-called “biological central clock”). We have also explored long-term effects of BVL on daily rhythms and bone density in mice and rats and we have shown that some of these effects are compensated with time.
The third aim was to determine whether abnormal sensory integration could disrupt autonomic functions. To that end we have characterized the effects of Head Down Bed Rest (HDBR) and Dry Immersion (DI) – 2 procedures known to disrupt cardiovascular regulation - on the peripheral and central processing of vestibular information and to evaluate the impact of these alterations on cardiovascular control, circadian regulation and cognitive functions. Experiments are still ongoing and will be finished in April 2016. However, we have already found important results: i) HDBR and DI alter peripheral and central processing of vestibular information; ii) Modification of peripheral vestibular function is caused partly by an increase in endolymphatic pressure; iii) Change in vestibular processing alters the influence of vestibular and neck afferences on cardiovascular regulations; iv) Vestibular-induced modification of cardiovascular regulation contributes to cardiovascular deconditioning; v) Change in vestibular processing alters spatial learning and memory tasks;
Our findings support the hypothesis that the vestibular system has an influence on cognitive and autonomous functions on Earth, and raise the question of whether those functions might be altered due to vestibular pathology in humans. They also raise the question of whether vestibular system dysfunction could participate in some harmful effects of long-term weightlessness observed in astronauts. In order to answer those two questions, teams of the SVETA project have submitted joint proposals, some of them are already selected.