Abstract
The locus coeruleus (LC) is the brain's major source of the neuromodulator norepinephrine, and is also profoundly vulnerable to the development of Alzheimer's disease (AD)-related tau pathology. Norepinephrine plays a role in neuroprotective functions that may reduce AD progression, and also underlies optimal memory performance. Successful maintenance of LC neurochemical function represents a candidate mechanism of protection against the propagation of AD-related pathology and may facilitate the preservation of memory performance despite pathology. Using [F]Fluoro-m-tyrosine ([F]FMT) PET imaging to measure catecholamine synthesis capacity in LC regions of interest, we examined relationships among LC neurochemical function, AD-related pathology, and memory performance in cognitively normal older adults (n = 49). Participants underwent [C]Pittsburgh compound B and [F]Flortaucipir PET to quantify β-amyloid (n = 49) and tau burden (n = 42) respectively. In individuals with substantial β-amyloid, higher LC [F]FMT net tracer influx (Ki) was associated with lower temporal tau. Longitudinal tau-PET analyses in a subset of our sample (n = 30) support these findings to reveal reduced temporal tau accumulation in the context of higher LC [F]FMT Ki. Higher LC catecholamine synthesis capacity was positively correlated with self-reported cognitive engagement and physical activity across the lifespan, established predictors of successful aging measured with the Lifetime Experiences Questionnaire. LC catecholamine synthesis capacity moderated tau's negative effect on memory, such that higher LC catecholamine synthesis capacity was associated with better-than-expected memory performance given an individual's tau burden. These PET findings provide insight into the neurochemical mechanisms of AD vulnerability and cognitive resilience in the living human brain.