Amyloid Hypothesis: Is There a Role for Antiamyloid Treatment in Late-Life Depression?

Abstract

Antidepressants have modest efficacy in late-life depression (LLD), perhaps because various neurobiologic processes compromise frontolimbic networks required for antidepressant response. We propose that amyloid accumulation is an etiologic factor for frontolimbic compromise that predisposes to depression and increases treatment resistance in a subgroup of older adults. In patients without history of depression, amyloid accumulation during the preclinical phase of Alzheimer disease (AD) may result in the prodromal depression syndrome that precedes cognitive impairment. In patients with early-onset depression, pathophysiologic changes during recurrent episodes may promote amyloid accumulation, further compromise neurocircuitry required for antidepressant response, and increase treatment resistance during successive depressive episodes. The findings that support the amyloid hypothesis of LLD are (1) Depression is a risk factor, a prodrome, and a common behavioral manifestation of AD; (2) amyloid deposition occurs during a long predementia period when depression is prevalent; (3) patients with lifetime history of depression have significant amyloid accumulation in brain regions related to mood regulation; and (4) amyloid deposition leads to neurobiologic processes, including vascular damage, neurodegeneration, neuroinflammation, and disrupted functional connectivity, that impair networks implicated in depression. The amyloid hypothesis of LLD is timely because availability of ligands allows in vivo assessment of amyloid in the human brain, a number of antiamyloid agents are relatively safe, and there is evidence that some antidepressants may reduce amyloid production. A model of LLD introducing the role of amyloid may guide the design of studies aiming to identify novel antidepressant approaches and prevention strategies of AD.

Keywords: Alzheimer disease; Amyloid hypothesis; Antiamyloid agents; Late-life depression.

Figure 1. Amyloid Model of Late-Life Depression

Model 1) Depression increases the risk of dementia: Pathophysiological changes occurring during depressive episodes in early life lead to frontolimbic and frontostriatal dysfunction directly and/or through amyloid accumulation. Frontolimbic and frontostriatal dysfunction predisposes to depressive syndromes in late life and worsens their outcomes. Model 2) Depression is a prodrome of dementia: Amyloid deposition during the asymptomatic phase of Alzheimer’s disease or of other amyloid-related dementias.