Serotonin Degeneration and Amyloid- Deposition in Mild Cognitive Impairment: Relationship to Cognitive Deficits

Background: Neuropathological and neuroimaging studies have demonstrated degeneration of the serotonin system in Alzheimer's disease (AD). Neuroimaging studies have extended these observations to the preclinical stages of AD, mild cognitive impairment (MCI). Serotonin degeneration has been observed also in transgenic amyloid mouse models, prior to widespread cortical distribution of amyloid-beta (A beta). Objective: The present study evaluated the regional distribution of the serotonin transporter (5-HTT) and of A beta in individuals with MCI and healthy older controls, as well as the contribution of 5-HTT and A beta to cognitive deficits. Methods: Forty-nine MCI participants and 45 healthy older controls underwent positron emission tomography (PET) imaging of 5-HTT and A beta, structural magnetic resonance imaging and neuropsychological assessments. Results: Lower cortical, striatal, and limbic 5-HTT and higher cortical A beta was observed in MCIs relative to healthy controls. Lower 5-HTT, mainly in limbic regions, was correlated with greater deficits in auditory-verbal and visual-spatial memory and semantic, not phonemic fluency. Higher cortical A beta was associated with greater deficits in auditory-verbal and visual-spatial memory and in semantic, not phonemic fluency. When modeling the association between cognition, gray matter volumes and A beta, inclusion of 5-HTT in limbic and in select cortical regions significantly improved model fit for auditory-verbal and visual-spatial memory and semantic, but not phonemic fluency. Conclusions: These results support the role of serotonin degeneration in the memory and semantic fluency deficits observed in MCI.