BDNF and Parkinson disease: Although a role for it in counteracting the neuronal alterations induced by weightlessness has not yet been demonstrated, several evidences have proposed irisin as a critical regulator of cognitive function [53], being able to inhibit oxidative stress [54], promote BDNF accumulation in hippocampal neurons [55,56], improve synaptic plasticity in mouse models of Alzheimer’s disease [57] and prevent neurodegeneration in mouse models of Parkinson’s disease [58].