Researchers at the McGovern Institute for Brain Research at MIT led by NARSAD Distinguished Investigator Grantee, Ann Graybiel, Ph.D., have used a new technology to uncover how the brain concentrates to achieve long-term goals. In partnership with 2007 NARSAD Young Investigator Grantee, Paul Phillips, Ph.D., from the University of Washington, the team worked with animal models and used fast-scan cyclic voltammetry (FSCV) in which tiny, implanted, carbon-fiber electrodes allow continuous measurements of dopamine concentration. Their work was reported online August 4th in the journal Nature.
It is known that dopamine, a neurotransmitter in the brain, is connected to rewards―when an animal receives an unexpected reward, dopamine neurons show brief bursts of activity. This is believed to be important for reinforcement learning, where animals learn the behaviors that lead to rewards. In this new work, the research team wanted to see if and how dopamine changes during a task involving delayed gratification. To study this, they trained rats to navigate a maze to reach a reward and tracked the dopamine levels in their brains with FSCV.
The team found that the dopamine signals increased as the animals navigated to reach remote rewards. The level of dopamine increased steadily, peaking as the animal approached its goal―as if in anticipation of a reward. “The dopamine signal seems to reflect how far away the rat is from its goal,” Dr. Graybiel explains. “The closer it gets, the stronger the signal becomes.” The researchers also found that the size of the signal was related to the size of the expected reward: When rats were trained to anticipate a larger gulp of chocolate milk, the dopamine signal rose more steeply to a higher final concentration.
The researchers conclude that such prolonged dopamine signaling could provide sustained motivational drive, something that can be impaired in a range of neurologic and neuropsychiatric disorders.