Perspective From sea level to the summit: Platelets as the environmental messengers of the neurotrophic pathway to brain plasticity Exercise and hypoxia independently stimulate platelet liberation of brain-derived neurotrophic factor and platelet factor 4 K. N. Thomas, H. G. Caldwell, D. B. Macleod, W. C. Abraham, J. M. J. R. Carr, C. Gasho, L. M. Brewster, C. D. Bruce, J. A. Monteleone, J. S. Duffy, C. A. Howe, J. C. Tremblay, E. J. Jenkins, J. S. Talbot, M. Stembridge, P. N. Ainslie, T. D.