The researchers focused their investigation for the first time on the role of the brain's opioidergic (or endorphine) system-specifically, the connection between an individual's level of reward expectancy and the brain's ability to transmit naturally occurring opiates.
The study was conducted on 23 males with no history of substance abuse, who were administered Fluoro-ethyl-diprenorphine-a radiolabeled chemical that binds readily to the brain's naturally occurring opiate system- and then underwent a PET scan.
They then compared the scans with the results of each participant's Cloninger temperament and character inventory, a questionnaire that assesses human personality based on four dimensions: novelty seeking, harm avoidance, reward dependence and persistence.
This revealed that the binding to opiate receptors in the ventral striatum-an area of the brain known to be a central part of the reward system-narrowly linked to the individual degree of reward dependence.
Those participants, who were seen to have a high need to feel rewarded by approval, were also those with the highest uptake of opiates, or endorphins, in the reward system.
"Our main finding was that reward dependence is the only personality dimension correlated with opiate receptor binding, and that positive correlation was restricted to the ventral striatum, which is considered the key area of the human reward system and of the development of addictive behavior," said Peter Bartenstein, M.D., professor of nuclear medicine, Ludwig Maximilians-University, Munich, Germany.
He added: "This correlation means that people with high reward dependence have a high concentration of opiate receptors available in that area, while people with low dependence have fewer opiate receptors."
The finding suggests that people suffering from a relative endorphine deficit in their reward system show increased reward dependence and are probably more at risk for developing addictions.
"This is a novel finding and will provide a deeper understanding of the functional relation between human personality, neurobiology and addictive behaviour," said Mathias Schreckenberger, M.D., professor of nuclear medicine, Johannes Gutenberg-University, Mainz, Germany.
The findings of the study are published in an article in The Journal of Nuclear Medicine.