Dopamine sheds new light on drug addiction

A paper published in the current issue of Science has challenged beliefs about the role of dopamine in the brain, which could lead to new treatments for Parkinson’s disease, schizophrenia and drug addiction. The research suggests that dopamine has a far wider, less specialised role than previously hypothesised.

The paper, written by experts at the University of Sheffield and Macquarie University in Sydney, focused on the role of dopamine as a neurotransmitter. Researchers sought to determine the information that dopamine containing cells pass on to the structures in the brain that receive their message.

Dr Paul Overton, co-author of the study explains, “Less than one per cent of the neurons in the brain use dopamine as a neurotransmitter, but any change in its levels can have devastating effects. Too much dopamine can lead to schizophrenia, whereas not enough causes Parkinson’s Disease. Drug addicts also have a malfunction in their dopamine systems, so learning more about the way this chemical works could lead to better treatments for a range of conditions.”

“The dominant theory in the field suggested that the dopamine message carries information specifically about reward. Brain systems involved in reward fulfil a vital function in that they underpin the motivation to seek out food, water and all of the other things that we need to survive, and play a central role in learning. We tend to do more of the things that lead to nice outcomes – that is, we learn to according to the effect that our actions have”.

“Much of the previous work concerning dopamine and reward has used visual ‘stimuli’. So, our team focused on the visual responsiveness of dopamine cells and, rather than decoding the dopamine message itself, we looked at the message that the dopamine cells were being given by other bits of the brain to pass on”.

“We found that the visual information that the dopamine message carries is provided by the superior colliculus, which is one of the oldest parts of the visual system in terms of evolution. The colliculus is the brain’s ‘burglar alarm’, and hence the visual response properties of its cells are fairly primitive. We concluded that it is unlikely that such a visually primitive structure would be allowed (by evolution) to provide the sensory input to a system which was involved in something as vital as reward. Instead, we propose that, as a burglar alarm, the colliculus provides information to dopamine cells about the occurrence of biologically important stimuli, which then pass this ‘salience’ based signal to other brain areas. In other words, the remit of the dopamine message goes beyond reward to include all stimuli which are pertinent to the animal’s survival”.

“This conclusion has particular relevance to people suffering from drug addiction. It could explain why a dopamine malfunction can make it so hard for addicts to stay ‘on the wagon’ when they are around their drug of choice. Basically, the dopamine malfunction, enhancing stimulus salience, could cause the brain to be unable to ignore stimuli associated with the drug, causing relapse and cravings.”

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