"This could be what underlies addiction," Surmeier said. "Dopamine released by drugs leads to abnormal strengthening of the cortical synapses driving the striatal 'go' circuits, while weakening synapses at opposing 'stop' circuits. As a result, when events associated with drug taking where you took the drug, what you were feeling occur, there is an uncontrollable drive to go and seek drugs."
"All of our actions in a healthy brain are balanced by the urge to do something and the urge to stop," Surmeier said. "Our work suggests that it is not just the strengthening of the brain circuits helping select actions that is critical to dopamine's effects, it is the weakening of the connections that enable us to stop as well. "
In the second part of the experiment, scientists created an animal model of Parkinson's disease by killing dopamine neurons. Then they looked at what happened when they simulated cortical commands to move. The result: the connections in the "stop" circuit were strengthened, and the connections in the "go" circuit were weakened.
"The study illuminates why Parkinson's patients have trouble performing everyday tasks like reaching across a table to pick up a glass of water when they are thirsty," Surmeier said.
Surmeier explained the phenomenon using the analogy of a car. "Our study suggests that the inability to move in Parkinson's disease is not a passive process like a car running out of gas," he said. "Rather, the car doesn't' move because your foot is jammed down on the brake. Dopamine normally helps you adjust the pressure on the brake and gas pedals. It helps you learn that when you see a red light at an intersection, you brake and when the green light comes on, you take your foot off the brake and depress the gas pedal to go. Parkinson's disease patients, who have lost the neurons
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