Abstract
Depression is one of the most common and disabling neuropsychiatric complications of neurodegenerative diseases. In Alzheimer's disease (AD) and Parkinson's disease (PD), depressive syndromes affect more than one-third of patients and are associated with accelerated cognitive decline, reduced quality of life, and increased healthcare utilisation. Despite this burden, current antidepressant treatments show little or no efficacy in these populations, suggesting that depression arising in the context of neurodegeneration may reflect distinct underlying neurobiological mechanisms. Emerging evidence from molecular imaging, neuropathology, and cognitive neuroscience indicates disease-specific disruption of monoaminergic, glutamatergic, inflammatory, and reward-related circuits in AD and PD. These alterations may undermine the mechanisms of action of standard antidepressants and contribute to treatment resistance. Yet patients with cognitive impairment are routinely excluded from antidepressant trials, and few high-quality studies have evaluated novel therapies in neurodegenerative disease. In this narrative review, we synthesise current evidence on the neurobiological mechanisms underpinning depression in AD and PD and critically evaluate emerging pharmacological and neuromodulatory therapies targeting glutamatergic, serotonergic, dopaminergic, immune, and circuit-level dysfunction. Here, we highlight that depression in neurodegenerative disease offers a unique and underutilised model for mechanistically guided antidepressant development. We identify how interventions including glutamate modulators, dopaminergic agents, kappa opioid antagonists, immune-modulating therapies, and next-generation brain stimulation approaches may offer therapeutic promise. By aligning treatment development with disease-specific circuit and molecular pathology, this framework may improve outcomes for this neglected population while advancing precision psychiatry more broadly.