Social Status in Monkeys: Effects of Social Confrontation on Brain Function and Cocaine Self-Administration

Individual differences in response to social stress and environmental enrichment may contribute to variability in response to behavioral and pharmacological treatments for drug addiction. In monkeys, social status influences the reinforcing effects of cocaine and the effects of some drugs on cocaine self-administration. In this study, we used male cynomolgus macaques (n=15) living in established social groups to examine the effects of social confrontation on the reinforcing effects of cocaine using a food-drug choice procedure. On the test day, a dominant or subordinate monkey was removed from his homecage and placed into another social pen; 30 min later he was studied in a cocaine-food choice paradigm. For the group, following social confrontation, sensitivity to cocaine reinforcement was significantly greater in subordinate monkeys compared with dominant animals. Examining individual-subject data revealed that for the majority of monkeys (9/15), serving as an intruder in another social group affected cocaine self-administration and these effects were dependent on the social rank of the monkey. For subordinate monkeys, sensitivity to the reinforcing effects of cocaine increased while sensitivity decreased in dominant monkeys. To investigate potential mechanisms mediating these effects, brain glucose metabolism was studied in a subset of monkeys (n=8) using [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) with positron emission tomography. Dominant and subordinate monkeys displayed distinctly different patterns of brain glucose metabolism in their homecage, including areas associated with vigilance and stress/anxiety, respectively, and during social confrontation. These data demonstrate that, depending on an individual's social status, the same social experience can have divergent effects on brain function and cocaine self-administration. These phenotypic differences in response to social conditions support a personalized treatment approach to cocaine addiction.