BDNF and major depressive disorder: Thus, there is a need for future studies that (1) assess the effect of EE on other parameters of neuroplasticity, e.g., adult hippocampal neurogenesis, and (2) correlate it with neurotrophin levels (e.g., BDNF) in saliva, serum and locally in the brain, and (3) correlate it with well-validated behavioural outcome measures of anxiety, depression and cognition in pigs.