Abstract
In aging, we see marked alterations to the dopamine system, specifically with losses in the number of dopamine receptors and transporters and increases in dopamine synthesis capacity. While striatal dopamine has been a focus of research, midbrain synthesis capacity has been less thoroughly explored. Here, we developed a protocol that allowed us to accurately define and measure midbrain dopamine synthesis capacity, specifically in the substantia nigra and ventral tegmental area. Using the positron emission tomography (PET) tracer 6-[18F]fluoro-L-m-tyrosine ([18F]FMT), we measured the dopamine synthesis capacity of 50 older adults and 15 younger adults. Our older adults also completed a battery of neuropsychological tasks. We were able to establish that older adults do have an upregulation of midbrain synthesis capacity, compared to young adults. In order to examine the relationship between synthesis capacity and cognition, we created composite scores for cognitive stability and flexibility. We estimated stability performance using the Stroop task as a measure of inhibition and the Backwards Digit Span as a measure of working memory. We estimated flexibility performance using the Trail Making Task B-A and the Wisconsin Card Sorting Task (WCS) as measures of shifting. We did not find significant relationships between midbrain dopamine synthesis capacity and cognition. In addition to this, we found that instead of a tradeoff between stability and flexibility in individuals, higher stability performance predicted higher flexibility performance.