One of the most difficult questions facing researchers in drug abuse is why some people are more likely than others to become addicted to a drug. It seems likely that the difference is in some way due to brain chemistry, and in particular to variations in the dopaminergic system, which is thought to mediate reward, but exactly what these variations are and how they arise is unknown.

Of course, it is very difficult to study these questions in humans. A new study in monkeys might give us some clues about where to look. Morgan et al. carried out a long-term experiment in which they looked at the possible roles of dopaminergic function and social factors in vulnerability to drug abuse, and found that social dominance can influence both changes in dopaminergic function and self-administration of cocaine in monkeys.

First, the monkeys were studied while housed individually for a year and a half, and their brains were imaged using positron emission tomography (PET). Then they were put into social groups. After three months of living in these groups, the PET scans of the monkeys that had become dominant showed a significant increase in binding of a radioactive ligand, [18F]fluoroclebopride (FCP), to dopaminergic receptors in the midbrain. This means that either there was an increase in the number of dopamine receptors or that there was a decrease in the amount of extracellular dopamine in the midbrains of these monkeys. Subordinate monkeys, by contrast, showed no changes in FCP binding.

These differences in dopaminergic function were associated with differences in cocaine self-administration rates. Subordinate monkeys reliably self-administered cocaine, but dominant monkeys did not, suggesting that they were resistant to the reinforcing effects of cocaine.

Several previous studies have indicated that social rank might relate to dopaminergic function and might influence the effects of drugs, such as cocaine, in monkeys, but this is the first to show that becoming dominant after being individually housed can cause rapid changes in the midbrain dopaminergic system. If the differences in cocaine self-administration are the result of variations in the dopamine system, then they also result from differences in the social status of the monkeys.

Of course, it is a long way from this kind of model to human addicts. But greater understanding of the effects of social context on dopaminergic function and drug-related behaviour in monkeys could help us to understand whether similar forces affect the human dopaminergic system and influence the propensity of some people to become addicted, and could also provide a new model for testing methods of interfering with the reinforcing properties of addictive drugs.