Duke University neuroscientists reported in an eLife paper published earlier this week that they have pinpointed a single type of neuron deep within the brain that serves as a "master controller" of habits.
"This cell is a relatively rare cell but one that is very heavily connected to the main neurons that relay the outgoing message for this brain region," said Nicole Calakos, an associate professor of neurology and neurobiology at the Duke University Medical Center. "We find that this cell is a master controller of habitual behavior, and it appears to do this by re-orchestrating the message sent by the outgoing neurons."
The team initiated their habit formation study in 2016 when they trained otherwise healthy mice to receive a tasty treat every time they pressed a lever. Many mice developed a lever-pressing habit, continuing to press the lever even when it no longer dispensed treats. The team then compared the brain activity of mice who had developed a lever-pressing habit with those who hadn't. They focused on the striatum, which contains two sets of neural pathways: a "go" pathway, which incites an action, and a "stop" pathway, which inhibits action. They found that both the go and stop pathways were stronger in habit-driven mice.
In the current study, the team wanted to understand the circuitry that coordinates these various long lasting changes in the brain. They had a hunch that the fast-spiking interneuron (FSI) might serve as master conductor of the widespread changes in the outgoing neurons' activity. The FSI belongs to a class of neurons responsible for relaying messages locally between other types of neurons in a particular brain region. Though FSIs make up about only 1% of the cells in the striatum, they grow long branch-like tendrils that link them up to the 95% of neurons that trigger the stop and go pathways.
Justin O'Hare, a graduate student in Calakos' lab, led the effort to take a closer look at the brain activity in lever-pressing mice. He found that forming a habit appeared to make the FSIs more excitable. He then gave the mice a drug that decreases the firing of FSIs, and found that the stop and go pathways reverted to their "pre-habit" brain activity patterns, and the habit behavior disappeared.
"Some harmful behaviors like compulsion and addiction in humans might involve corruption of the normally adaptive habit-learning mechanisms." Calakos said, "Understanding the neurological mechanisms underlying our habits may inspire new ways to treat these conditions."
Caption: A highly magnified view of the striatum of a mouse brain reveals a relatively rare type of cell called the fast-spiking interneuron (purple), which is responsible for orchestrating the brain circuits that control our habits. Image courtesy of Justin O'Hare, Duke University.