Role of serotonin in modulating the development and function of adult-born neurons in the olfactory bulb

The neuromodulatory transmitter serotonin (5-hydroxytryptamine, 5-HT) is synthesized by neurons located in the brainstem, which project more or less densely to the entire central nervous system (Charnay and Leger, 2010). Serotonin regulates a variety of physiological functions, including food intake, reward, reproduction, sleep-wake cycle, memory, cognition, emotion, and mood (Charnay and Leger, 2010). Consistently, dysfunctions of the serotonergic system are involved in the development or progression of mental disorders including autism, insomnia, anxiety, depression, schizophrenia, Parkinson’s disease, or Alzheimer’s disease (Charnay and Leger 2010). Many of these diseases (e.g., autism, schizophrenia, depression, Parkinson’s disease, Alzheimer’s disease) present with concomitant impairment of olfaction (and memory), often accompanied by a reduced volume of the olfactory bulb (OB; Figure 1A) and hippocampus. These functional impairments may result from distorted adult neurogenesis in the respective neurogenic niches, as OB and hippocampal dentate gyrus are the two major areas of the adult mammalian brain where adult-born cells are generated throughout life. A wide range of studies documents the involvement of adult-born cells in short- and long-term olfactory memory; perceptual, associative, and fear learning, etc. (summarized in Lepousez et al., 2015; Fomin-Thunemann et al., 2020).