Alzheimer's Disease Pathologies Affect Dopaminergic Neural Mechanisms of Memory.

In young adults, higher dopamine synthesis capacity was associated with superior overall memory performance.

• Study used [18F]Fluoro-L-m-tyrosine (FMT) PET to measure dopamine synthesis capacity
• Sample included n=80 young and older adults
• Memory was assessed using functional magnetic resonance imaging during a reward-based memory task
• This positive dopamine-memory association was observed in young adults but not independently in older adults

In young adults, higher dopamine synthesis capacity was associated with greater temporal lobe activation.

• Temporal lobe activation was measured using fMRI during the memory task
• The association between dopamine synthesis capacity and temporal lobe activation was present in young adults
• This relationship was disrupted in older adults with AD pathology
• Dopamine synthesis capacity was measured using [18F]Fluoro-L-m-tyrosine PET

In older adults, neither dopamine synthesis capacity nor AD pathology independently predicted overall memory performance.

• AD-related pathology was measured using [11C]Pittsburgh Compound B (amyloid-β) and [18F]Flortaucipir (tau) PET
• Dopamine synthesis capacity was measured using FMT PET
• Neither measure alone was a significant predictor of memory in the older adult group
• This contrasts with the significant independent association observed in young adults

In older adults, higher dopamine synthesis capacity was associated with memory biases favoring stimuli associated with rewards rather than losses.

• Memory bias toward reward-associated stimuli was the specific cognitive outcome linked to dopamine in older adults
• This reward-specific memory benefit was observed even when overall memory was not predicted by dopamine synthesis
• The task was designed to assess memory for rewarding events specifically

Interactions between dopamine synthesis capacity and AD pathology predicted memory performance in older adults, such that only those with minimal pathology showed preservation of positive dopamine-memory associations.

• Significant interaction effects were observed between dopamine synthesis capacity and AD pathology (amyloid-β and/or tau)
• Older adults with minimal AD pathology retained dopamine-memory relationships resembling those in young adults
• Older adults with higher AD pathology did not show this positive association
• This interaction pattern argues against a simple resilience account of elevated dopamine synthesis in aging

The presence of AD pathology disrupted and even reversed relationships between dopamine synthesis, memory, and temporal lobe activation in older adults.

• In older adults with AD pathology, the direction of the dopamine-memory and dopamine-temporal lobe activation relationship was reversed compared to young adults and low-pathology older adults
• This reversal suggests that elevated dopamine synthesis may not confer resilience when AD pathology is present
• AD pathology was indexed by both amyloid-β ([11C]Pittsburgh Compound B) and tau ([18F]Flortaucipir) PET measures
• Authors describe this as AD pathological processes 'acutely altering the mechanisms by which elevated dopamine synthesis supports optimal memory performance'

Dopamine synthesis capacity is upregulated in older age and was studied as a potential source of resilience to age-related neural losses.

• Prior evidence cited in the paper suggests dopamine synthesis capacity is upregulated in older age
• The study was designed in part to test whether elevated dopamine synthesis could impart resilience to age-related neural losses
• Results instead indicated that AD pathology undermines this potential resilience mechanism
• Both male and female human participants were included in the study