Serotonin in Space: Understanding Single Fibers.

All brain regions contain fibers that store and release 5-hydroxytryptamine (5-HT, serotonin). Since these fibers often do not have well-defined trajectories, most studies have focused on their overall densities, a measure that can be associated with local 5-HT tone in heathy and diseased brains. However, the observed fiber densities are the consequence of the behavior of single fibers. Evidence is presented as to why understanding single-fiber trajectories is important for basic and clinical neuroscience. In particular, serotonergic fibers can be viewed as natural, readily detectable stochastic probes that sample the invisible microarchitecture of brain tissue.