Molecular mechanisms including lower levels of a protein in the brain may be the reason why addicts choose alcohol over an alternative reward, finds a study that may help improve treatment for alcohol dependence.

The study suggested that the level of a transporter protein GAT-3, situated in the amygdala region of the brain -- responsible for emotional reactions -- was lower among the alcohol addicts.

"We have to understand that a core feature of addiction is that you know it is going to harm you, potentially even kill you, and nevertheless something has gone wrong with the motivational control and you keep doing it," said Markus Heilig from the Linkoping University in Sweden.

In the study, published in the journal Science, the team used a mouse model to measure the expression of hundreds of genes in five areas of the brain.

The team investigated the role of reduced GAT-3 levels in rats that initially preferred sweetened water over alcohol. After the reduction, they were again presented with the choice between alcohol and sugar.

They found that 15 per cent of the outbred rats chose alcohol over a high-value reward.

"Decreasing the expression of the transporter had a striking effect on the behaviour of these rats. Animals that had preferred the sweet taste over alcohol reversed their preference and started choosing alcohol," said lead investigator Eric Augier.

Further, the team analysed GAT-3 levels in brain tissue from deceased humans who had documented alcohol addiction. They found lower levels of the protein in them than in the control individuals.

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