Targeting the serotonergic system in the treatment of neurodegenerative diseases—emerging therapies and unmet challenges

More than 65 million people worldwide experience neurodegenerative diseases, such as Alzheimer disease, frontotemporal dementia, Parkinson disease, and amyotrophic lateral sclerosis. As the risk of developing these diseases increases with age, increasing life expectancy will further accelerate their prevalence. Despite major advances in the understanding of the molecular mechanisms of neurodegeneration, no curative therapy is available to date. Neurodegenerative diseases are known to be associated with alterations in serotonergic neurotransmission, which might critically contribute to the pathogenesis of these diseases. Therefore, targeting the serotonergic system appears to be a promising therapeutic approach. In this review, we provide a comprehensive overview of pathological changes in serotonergic neurotransmission in different neurodegenerative diseases and discuss novel treatment strategies based on targeted modulation of the serotonergic system. We primarily focus on the therapeutic approaches modulating serotonin homeostasis, its biosynthesis, and the modulation of defined serotonin receptors. SIGNIFICANCE STATEMENT: A common feature of multiple neurodegenerative diseases is dysregulation of the serotonergic system at the cellular, molecular, and genetic levels that strongly contributes to specific pathological phenotypes. Targeting these alterations represents a suitable therapeutic strategy to combat disease-relevant pathomechanisms, slow down disease progression, and overcome pathological consequences.