BDNF and major depressive disorder: These different CNS changes have all been linked to MDD: inflammation is strongly linked to MDD [36] and inflammatory mediators such as IL-1 drive sickness behaviours [37,38,39] that are identical to the neurovegetative symptoms and anhedonia of MDD [3,5,40]; reduced CNS serotonin and brain-derived neurotrophic factor (BDNF) are both linked to MDD [41], and shifts in neurogenesis and BDNF signalling may mediate cognitive changes in MDD [5,42].