Treatments to stop drug addiction and over-eating must go beyond blocking the pleasure centers of the brain, say researchers.
Neuroscientist, Professor Bernard Balleine and colleagues report their study on the pleasure and reward systems in the brain in the Proceedings of the National Academy of Sciences.
It has long been known that creatures from lab rats to humans learn from experience what will give them rewards, says Balleine, of the University of Sydney’s Brain and Mind Research Institute.
For example, we seek out chocolate if we anticipate from past experience that eating it makes us feels good.
But scientists have long pondered the brain processes involved in our experience of pleasure and in our anticipation of reward.

Opiate receptors
Balleine says chocolate causes a large release of pleasure-inducing chemicals in the body called opioids, which interact with the same brain receptors as heroin.
But, he says the question is whether this is a general system, applying to all pleasurable experiences. And which parts of the brain are involved?
Scientists have debated whether it is the amygdala or the ventral striatum, two areas within the brain that both have opiate receptors that is involved.
To investigate this question, Balleine and colleagues set up an experiment involving rats and sugar.

Hungry rats
In the first experiment hungry rats were fed sugar. The scientists tested what happened when opiate receptors were blocked in different parts of the brain.
When opiate receptors were blocked in the amygdale, there was no effect on the rats.
But when opiate receptors were blocked in the ventral striatum, this decreased the rat’s pleasure in eating the sugar.
Rats usually lick their lips in pleasure when eating sugar, says Balleine, but, in this case, they didn’t.
These findings supported the idea that the ventral striatum is involved in experiencing pleasure.
But the next experiment surprised the researchers.

Anticipating Rewards
Once the rats had been taught the pleasure of eating sugar, the researchers then tested whether they would actively seek it out, in the anticipation of reward.
They found that when opiate receptors in the amygdale were blocked, the rats didn’t bother seeking out the sugar.
This suggested the part of the brain involved anticipating pleasure, is different from that involved in experiencing pleasure itself.
 
Addiction treatment research
Balleine says the findings have been implications for those researching treatments for addictive behaviors.
He says treatments usually focus on blocking the pleasurable experience of an addictive drug or food in the hope this will reduce the addict’s consumption.
But, says Balleine, treatments will have limited effect without also targeting the separate reward anticipation center in the brain.
If treatments only focus on the pleasurable center, there will be a greater risk of relapse, he says.