Major Depression as a Neuroprogressive Prelude to Dementia: What Is the Evidence?

Epidemiological studies implicate chronic depression as a predisposing factor for dementia in later life. However, the link is incompletely understood and controversial. The aim of this review is to consider some of the biological factors that contribute to neuroprogressive brain dysfunction in late life as a consequence of prolonged, low-grade inflammation in the course of depressive episodes. As chronic inflammation is known to precipitate increased apoptosis of neurons and astrocytes, this could be a contributing factor to brain dysfunction. In addition, certain proinflammatory cytokines activate the neurotoxic derivatives of the tryptophan-kynurenine pathway. This results in the synthesis of the NMDA glutamate agonist, quinolinic acid, and kynurenine metabolites which initiate oxidative stress and insulin receptor resistance. As a consequence of these changes, combined with a structural and functional defect in brain mitochondria, glucose transport into the brain is affected. Due to the ensuing reduction in the metabolic energy needed to sustain brain function, brain cells die prematurely. These changes could provide a link between chronic inflammation and dementia, at least in some patients with recurrent and chronic depression. This outcome may be particularly true in poor responders and treatment-resistant depression.