The chapter discusses a possible role for Sl00β and serotonin (5-HT) as an astrocytic (S100β)/ neuronal (serotonin; 5-HT) link to neuropathologies associated with Alzheimer's disease. Sl00β is a protein produced and released from astrocytes in the brain. Extracellular Sl00β can function as a neurotrophic factor to select central and peripheral neurons and a gliotrophic factor, inducing mitosis and morphogenesis of astrocytes. Release of S100β from astrocytes can be stimulated by serotonin acting through 5-HT1, receptors on astrocytes, and S100β functions as a serotonergic neurite extension factor. The abnormal regulation of Sl00β levels or of serotonin-stimulated release of Sl00β in Alzheimer's disease contributes to the cascade of events that with aging lead to neuropathologies. It is also suggested that this astrocytic (S100β)/ neuronal (serotonin) link may contribute to neuropathological changes observed not only in Alzheimer's disease, but also in Down syndrome.
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