"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," explains first author Mark Howe, a former graduate student with Graybiel who is now a postdoc in the Department of Neurobiology 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."
Gradual increase in dopamine
From previous work, the researchers expected that they might see pulses of dopamine released at different times in the trial, "but in fact we found something much more surprising," Graybiel says: The level of dopamine increased steadily throughout each trial, peaking as the animal approached its goal as if in anticipation of a reward.
The rats' behavior varied from trial to trial some runs were faster than others, and sometimes the animals would stop briefly but the dopamine signal did not vary with running speed or trial duration. Nor did it depend on the probability of getting a reward, something that had been suggested by previous studies.
"Instead, the dopamine signal seems to reflect how far away the rat is from its goal," 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.
In some t
|Contact: Kimberly Allen|
Massachusetts Institute of Technology