New projectome captures ‘complex beast’ of serotonin system in mice

The mouse brain’s expansive serotonin system—which plays a role in everything from mood to movement—is composed of five distinct groups of neurons that target functionally related brain regions, according to a study published last month in Cell .

The research provides the first “projectome,” or whole-brain map of connections, among serotonin neurons in a vertebrate, offering new insights into the system’s underlying organization. An independent team mapped the entire serotonin system in a roundworm in 2023.

The field has long known that serotonin neurons vary, but understanding how and where they connect within the brain was “pretty hazy,” says Jeremiah Cohen , professor of neuroscience at the University of Minnesota, who was not involved in the study.

The new map reveals the nature and extent of these connections, he says, serving as “the anatomical scaffold that we can all use to try to understand this system more deeply.”