Long Road to Ruin: Noradrenergic Dysfunction in Neurodegenerative Disease

It has been known for decades that degeneration of the locus coeruleus (LC), the major noradrenergic nucleus in the brain, occurs in both Alzheimer’s disease (AD) and Parkinson’s disease (PD), but it was given scant attention. It is now recognized that hyperphosphorylated tau in the LC is the first detectable AD-like neuropathology in the human brain, α-synuclein inclusions in the LC represent an early step in PD, and experimental LC lesions exacerbate neuropathology and cognitive/behavioral deficits in animal models. The purpose of this review is to consider the causes and consequences of LC pathology, dysfunction, and degeneration, as well as their implications for early detection and treatment. A unique combination of neuroanatomy, neurochemistry, and physiology render LC neurons vulnerable to insults and degeneration. The LC is among the earliest sites of detectable pathology in AD (tau) and PD (α-synuclein), and may seed the development of aberrant protein aggregates in interconnected brain regions. In both AD and PD, LC neurons display dysfunction, fiber degeneration, and cell death. LC dysfunction and degeneration likely contribute to neuropsychiatric, neurological, and cognitive symptoms in AD and PD.