In the brain, dopamine contributes to series of processes that control our behaviour. Actions such as eating, winning a competition, having sex or taking a narcotic drug increase dopamine release. Scientists think that dopamine helps motivate us to repeat actions that have previously been associated with reward.
"Control mechanisms in the brain help keep the dopamine signal in balance so we can register the tiny deviations that signal reward and punishment. We discovered while trying to describe these control mechanisms that our model can be used to examine the influence of Ritalin, for example, on the signal. Suddenly we could see that different pathways of the reward system are affected to different degrees by the medicine, and we could calculate at what dosage different parts of the signal would be changed or destroyed," says Jakob Kisbye Dreyer.
Different dosage, different effect
Drugs such as Ritalin can have paradoxical effects: high dosage increases the patient's activity while low dosage reduces it. Therefore it can be a laborious process to find the right dosage for a patient.
"We can explain this double effect using our theory. The dopamine signal in the part of the brain that controls our motor behaviour is only affected at a higher dose that the dose usually prescribed for treatment. Also, our model shows that the threshold between a clinically effective dose and too high a dose is very low. That may explain why the small individual differences between patients have a big impact on treatment," says Jakob Kisbye Dreyer.
In the long term, the scientists hope that their new insight will help doctors determine the correct dose for each patient. The model can also help us understand what signals in the brain affect not only ADHD, but schizophrenia, Parkinson's disease and drug abuse as well.
|Contact: Postdoc Jakob Kisbye Dreyer|
University of Copenhagen