Anatomy of the serotonin transporter

Serotonin plays an important role in the regulation of various physiological functions, including mood, social and sexual behaviors, appetite, digestion, sleep, and memory. Therefore, the extracellular levels of serotonin must be finely controlled and regulated. Indeed, imbalance in serotonin homeostasis produces various physiological alterations, including anxiety, depression, anxiety, poor mood, sexual dysfunction, compulsivity, or cognitive deficits. One of the main actors of this regulation is the serotonin transporter (5-HTT), which reuptakes the extracellular serotonin back in serotonin neurons for further recycling and storage into the synaptic vesicles. This allows for a rapid termination of serotonin signaling and ensures constant levels of serotonin in subsequent vesicular releases. The 5-HTT is a solute carrier family 6 member 4 protein that is primarily located in the enterochromaffin cells of the intestines, the serotonergic neurons of the brain, and the blood platelets. This chapter describes the anatomical expression, distribution, and subcellular localization of the 5-HTT along the serotonin innervation of both the central and peripheral nervous system, and in nonserotonergic neurons, as well as its transient expression during the development. Finally, the segregation of the 5-HTT within different subsets of serotonin axons is discussed in the context of the functional heterogeneity of serotonin neurons.