Power has its perks, even for laboratory-housed monkeys. When moved from individual to group cages, socially dominant male monkeys exhibit a brain-chemistry change that fosters resistance to using drugs such as cocaine, a new study finds.
This alteration increases the amount of so-called dopamine D2 receptors, a molecular gateway on brain cells controlled by the chemical messenger dopamine.
Earlier studies implicated these receptors in pleasurable responses to drugs and other stimuli.
In contrast, male monkeys at the bottom of the social pecking order display no boost in the D2 receptors when housed with other monkeys, say neuroscientist Michael A. Nader of Wake Forest University School of Medicine in Winston-Salem, N.C., and his colleagues. Unlike their more dominant cage mates, the low-ranking monkeys readily self-administer large amounts of cocaine.
These findings raise the possibility that a person’s vulnerability to drug abuse can be influenced by brain-altering environmental factors, Nader’s group concludes in an upcoming Nature Neuroscience.
“This is the first demonstration in primates that a social stressor, such as a dominance hierarchy, can regulate levels of dopamine D2 receptors,” remarks psychiatrist Nora D. Volkow of Brookhaven National Laboratory in Upton, N.Y. “It provides a potential biological mechanism to explain why people in lower social classes are generally at higher risk for drug abuse.”
Nader and his coworkers used a scanning technique called positron emission tomography (PET) to study D2 receptors in the brains of 20 male macaque monkeys that had been housed in individual cages for 1 years. PET scans were repeated after the monkeys were moved into larger cages, grouping four animals per cage, and given time to establish social hierarchies.
The scans revealed comparably low numbers of dopamine D2 receptors in all individually housed monkeys and in low-ranking monkeys in the groups. In dominant monkeys, D2-receptor numbers increased sharply. These animals also displayed relatively low concentrations of dopamine in the junctions, or synapses, between brain cells.
Excess synaptic dopamine leads to an oversensitivity of the brain’s reward pathway and creates a susceptibility to drug abuse, the researchers theorize.
Loss of control over environmental factors may have triggered such a dopamine pattern in low-ranking monkeys, they hold. When the scientists implanted intravenous lines, subordinate animals quickly learned to press a lever to receive infusions of cocaine in increasing doses and largely ignored a lever controlling delivery of saline solution.
In dominant monkeys, the surge in dopamine D2 receptors indicates they use dopamine efficiently for cell-to-cell communication, Nader’s group contends. These monkeys showed no preference for receiving intravenous cocaine over a saline solution.
Whether these findings have correlates among people remains an open question, the researchers note. In line with the new study, earlier PET data indicated that people with low dopamine D2-receptor numbers report more pleasurable responses to stimulant drugs than those with high D2 numbers do. “We’re going to have to start paying much closer attention to the social rank of individuals in studies of the biology of drug abuse,” Volkow says.