A neurobehavioural assessment of adaptive responses to altered gravitational environments in ground-based hypergravity and microgravity models

Altered gravitational environment represents a unique challenge for terrestrial biological systems. The investigation of the human and animal response to unphysiological gravity, such as the permanency in space or in conditions that in some way mimic weightlessness, represents an opportunity to understand the phenomena underlying tolerance and adaptation to environmental conditions. The initial and adaptive responses to these challenges are reflected in behavioural endpoints: a careful study of these responses enables to evaluate and understand coping strategies, mechanisms underlying neuroplasticity phenomena, and the individual vulnerability to stress. Furthermore, understanding the individual responses to stress and the identification of predictive indicators of susceptibility may effectively mitigate the negative effects of altered gravity and indeed they represent a relevant emerging topic for the future of human space exploration. Reproducing environments that recreate weightlessness on the Earth is practically impossible. Among the ground-based models that are extensively employed to simulate altered gravity, hypergravity induced by rotation, the hindlimb unloading model, and the bed rest paradigm are well-established methodologies. In this conceptual and methodological framework, the aim of the present work was to investigate the neurobehavioural profile of mice exposed to hypergravity or microgravity simulated conditions in order to define possible behavioural indicators of susceptibility to un-physiological gravity. Moreover, neuroinflammatory biomarkers of resilience or vulnerability were also evaluated in human volunteers subjected to bed rest. In animal studies, behavioural adaptive responses took place during the exposure to altered gravity and the behavioural repertoire resulted compromised when returned to normogravity. Interestingly, the behavioural items manifested during the first days in the centrifuge or upon suspension could predict susceptibility of the experimental subjects to these altered environments. Moreover, evaluation of individual profile on bed rest evidenced that active coping strategy in resilient individuals is characterized by an anticipatory role of biomarkers to counteract the environmental challenge. In line with the personalized medicine approach in space, data obtained from such studies could lead to new strategies to reduce health problems associated with spaceflight.