Neuroinflammation and amyloid-beta 40 are associated with reduced serotonin transporter (SERT) activity in a transgenic model of familial Alzheimer's disease.

Background: Discrepant and often contradictory results have accumulated regarding the antidepressant and pro-cognitive effects of serotonin transporter (SERT) antagonists in Alzheimer's disease.

Methods: To address the discrepancy, we measured the activity and density of SERT in the neocortex of 3-24-month-old APP/PS1 and wild-type littermate mice, by using [H]DASB autoradiography and the [H]5-HT uptake assay. Levels of soluble amyloid-β (Aβ), and pro-inflammatory cytokines that can regulate SERT function, such as interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor (TNF), were measured in parallel. Neuroinflammation in aging APP/PS1 mice was further evaluated by [H]PK11195 autoradiography.

Results: Decreased SERT density was observed in the parietal and frontal cortex of 18-24-month-old APP/PS1 mice, compared to age-matched, wild-type animals. The maximal velocity uptake rate (V) of [H]5-HT was reduced in neocortical preparations from 20-month-old transgenic vs. wild-type mice. The reduction was observed when the proportion of soluble Aβ in the Aβ ratio increased in the aged transgenic brain. At concentrations compatible with those measured in 20-month-old APP/PS1 mice, synthetic human Aβ, but not Aβ, reduced the baseline V of [H]5-HT by ~ 20%. Neuroinflammation in APP/PS1 vs. wild-type mice was evidenced by elevated [H]PK11195 binding levels and increased concentration of IL-1β protein, which preceded the reductions in neocortical SERT density and activity. Age-induced increases in the levels of IL-1β, IL-6, and TNF were observed in both transgenic and wild-type animals.

Conclusions: The progression of cerebral amyloidosis is associated with neuroinflammation and decreased presynaptic markers of serotonergic integrity and activity. The Aβ-induced reduction in the uptake kinetics of [H]5-HT suggests that the activity of SERT, and potentially the effects of SERT antagonism, depend on the levels of interstitial Aβ.