Serotonin’s Role in Brain is Complex
For years, there have been conflicting findings on what effect serotonin has on the brain. Does it increase pleasure or anxiety? Does it promote or inhibit locomotion? According to new research from Stanford University, it can do all of the above. Researchers found that the serotonin system is made up of at least two (and possibly more) parallel subsystems that work together to affect the brain in different and sometimes opposing ways.
“The field’s understanding of the serotonin system was like the story of the blind men touching the elephant,” says Liquan Luo, a professor at Stanford University’s School of Humanities and Sciences.
“Scientists were discovering distinct functions of serotonin in the brain and attributing them to a monolithic serotonin system, which at least partly accounts for the controversy about what serotonin actually does,” Luo explains. “This study allows us to see different parts of the elephant at the same time.”
Researchers invented and used neuroanatomical methods to find these subsystems. They focused on the dorsal raphe, a region of the brainstem which contains the largest single concentration of mammalian brain neurons that transmit signals by releasing serotonin. Dorsal raphe neurons send out a sprawling network of connections to critical forebrain areas that are related to thought, memory and regulation of moods and bodily functions.
Using mice, the researchers injected viruses that infect the serotonin neuron connections, or axons, in these regions. They then created a visual map of projections between the serotonin-releasing neurons in the brainstem to various regions of the forebrain that they influence. The map showed two distinct groups of serotonin-releasing neurons in the dorsal raphe that connected to cortical and subcortical regions in the brain. This means that each subsystem is projecting to its own set of organized, related places in the brain.
Researchers also ran a series of behavioral tests on the mice, proving that serotonin neurons from the two subsystems can respond differently to stimuli. For example, neurons in both systems reacted similarly when the mice received rewards, such as sips of sugar water, but the systems exhibited opposite responses to punishments like mild foot shocks.
“We now understand why some scientists thought serotonin neurons are activated by punishment, while others thought it was inhibited by punishment. Both are correct—it just depends on which subtype you’re looking at,” Luo explains.
The findings were published in the journal Cell.