GoKawiilWhen neuroscientists gather in the Spanish city of Seville in May for the annual Dopamine Society meeting, one discussion could be unusually lively. Session 31 will feature a debate between researchers who fundamentally disagree about the role dopamine has in the brain.
Untangling the connection between dopamine and ADHD
Dopamine is one of the most extensively studied neurotransmitters, chemicals that convey signals from cell to cell. It’s the one with the highest profile outside neuroscience: often known as the ‘pleasure chemical’, it’s depicted as the hit of reward that people get from recreational drugs or scrolling through social media.
That’s a gross simplification of what dopamine does; on that, researchers agree. But beyond that, where once there was a simple model that explained how dopamine works in the brain, now there are challenges that seek to amend the theory — or even to overturn it.
This could have implications not only for basic neuroscience, but also for clinicians trying to explain and treat conditions such as attention deficit hyperactivity disorder (ADHD) and addiction. If the model is wrong or needs modification, then so might some of the assumptions about what drives these disorders and the best way to treat them.
The classic idea, known as the reward prediction error (RPE) hypothesis, is that bursts of dopamine in the brain link stimuli to rewards, helping to reinforce associations that fulfil a need for an animal or a person. The model has dominated and guided research in the field for decades, offering a mathematical framework to interpret data from animal experiments, and it does a good job of explaining behaviour.
This was a valuable rarity for researchers struggling to overlay simple theories onto the intense complexity of the brain. “Dopamine was the one field of neuroscience where we had a computational model that explained what the signal was and what it was computing,” says Mark Humphries, a neuroscientist at the University of Nottingham, UK. People in the field knew that some of the assumptions involved in the RPE model were simplistic. But as a working understanding of part of the brain, it was seen as a major step forwards.
In the past few years, that primacy has begun to slip. About a decade ago, experimental techniques emerged that made it easier to monitor the release of dopamine from neurons in animal experiments. This threw the field wide open, with more laboratories able to gather and analyse data. And many of the data from these studies suggested that dopamine has functions in the brain that go way beyond reward, suggesting roles in cognitive functions such as attention, working memory and even social behaviours. Other studies showed that dopamine neurons can respond to new stimuli, threats and movement. The original model is no longer sufficient to explain all of this, Humphries says.
That leaves the field wrestling with a question, one that those who attend Session 31 in Seville will address: is this the end of the road for neuroscience’s most cherished model? Or is the idea, and the way it has been adapted by clinicians trying to understand ADHD, schizophrenia and addiction, now too big to fail? “I do think that the framework is insufficient,” says Kauê Costa, a neuroscientist at the University of Alabama at Birmingham. “But you know, if you aim at the king, you better not miss.”
Predicting reward
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