Depression and Alzheimer's disease: is stress the initiating factor in a common neuropathological cascade?

The existence of a high co-morbidity between Alzheimer's disease (AD) and depression has been known for a long time. More interesting though are recent studies indicating that depression and number of depressive episodes earlier in life is associated with increased risk of AD development. This suggests the existence of common neuropathological mechanisms behind depression and AD. Here we propose that the brain changes associated with depressive episodes that compromise the brain's ability to cope with stress may constitute risk factors for development of AD. Furthermore, in individuals with a genetic linkage to depression, there may be an increased vulnerability towards the initiation of a detrimental neurodegenerative cascade. The following review will deal with the various observations reported within the different neurobiological systems known to be involved and affected in depression, like serotonergic and cholinergic system, hypothalamic-pituitary-adrenal axis and brain derived neurotrophic factor, and discussed in relation to AD.