Trajectories of late-life depression: insights from molecular imaging.

Late-life depression is associated with greater disability, suicide risk and mortality than depression in mid-life, and is a risk factor/prodrome for neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Many depressed, older adults fail to respond to first line antidepressant treatment, experience relapse and exhibit persistent symptoms, including anxiety, apathy and cognitive impairment, that may reflect underlying neurodegenerative processes. Advances in molecular imaging, particularly positron emission tomography (PET) allow direct in-vivo investigation of neurobiological mechanisms underlying late-life depression symptom trajectories, treatment response and the potential links to neurodegenerative disease. Molecular imaging studies in late-life depression have revealed alterations across neurotransmitter systems and Alzheimer's disease pathology (beta-amyloid and Tau) and a potential role of neuroinflammation. In late-life depression, variability in symptom presentation and treatment response arises from interacting neurotransmitter, inflammatory, and neurodegenerative processes and potentially other molecular mechanisms that impair synaptic plasticity. Future directions include the application of next-generation PET tracers targeting glutamatergic signaling, mitochondrial function, histone deacetylase activity, and cell-type-specific inflammation, along with multi-modal image analysis methods to test mechanistic models . Molecular imaging holds significant promise for guiding the development of targeted, mechanism-based treatments that reduce the burden of late-life depression and its associated vulnerability to neurodegenerative disease.