August 5, 2013
Brain’s Long-Term Reward System Relies On Dopamine
Brett Smith for redOrbit.com - Your Universe Online
From driving across country to graduating from college, long-term goals are often difficult to stay focused on when an immediate reward isn't within sight.
The joint team's research builds on previous studies that have linked the neurotransmitter dopamine to the brain's reward system. While most previous studies have involved looking at dopamine with respect to an immediate reward, the new study found increasing levels of dopamine as laboratory rats approached an expected reward after delayed gratification.
To measure levels of dopamine in the rats' brains, the team used a system developed by UW behavioral scientist Paul Phillips called fast-scan cyclic voltammetry (FSCV) that involves small, implanted electrodes that continuously record dopamine concentration by looking for its electrochemical signature.
"We adapted the FSCV method so that we could measure dopamine at up to four different sites in the brain simultaneously, as animals moved freely through the maze," said co-author Mark Howe, currently a post-doctoral neurobiologist at Northwestern University. "Each probe measures the concentration of extracellular dopamine within a tiny volume of brain tissue, and probably reflects the activity of thousands of nerve terminals."
The scientists started by training rats to find their way through a maze in search of a reward. During each rat's run through the maze, a tone would sound instructing it to turn right or left at an intersection in pursuit of a chocolate milk reward.
The research team said they expected to see pulses of dopamine being released by the rats' brain at periodic intervals during the trials. However, they found that levels of the neurotransmitter steadily rose throughout the experiment -- culminating in a peak level as the rodent neared its reward. While the rats' behavior during each trial varied, their dopamine levels reliably rose despite running speed or probability of reward.
"Instead, the dopamine signal seems to reflect how far away the rat is from its goal," said Ann Graybiel, who runs a brain research laboratory at MIT. "The closer it gets, the stronger the signal becomes."
The team also discovered that the magnitude of the dopamine signal was associated with the size of the expected reward. When rats were conditioned to expect a larger serving of chocolate milk, their dopamine levels rose more rapidly to a higher peak.
Researchers varied the experiment by extending the maze to a more complex shape that made the rats run farther and make additional turns to reach the prize. During these longer trials, the dopamine signal increased more gradually, but eventually reached the same level as in the previous maze.
"It's as if the animal were adjusting its expectations, knowing that it had further to go," Graybiel said.
She suggested that future studies should look into this same phenomenon in humans.
"I'd be shocked if something similar were not happening in our own brains,â Graybiel said.