Influence of social rank on the development of long-term ethanol drinking trajectories in cynomolgus monkeys.

Background: Chronic stress contribute to the development of alcohol use disorder (AUD). However, characterizing the role of chronic social stressors in the development of problematic drinking trajectories in humans is complicated by practical and ethical constraints. Group-housed nonhuman primates develop social dominance hierarchies that represent a continuum of social experiences from enrichment in higher-ranked (dominant) monkeys to chronic social stress in lower-ranked (subordinate) individuals. This framework provides a translationally relevant model of chronic social stress that can be used to characterize its effects on vulnerability to AUD.

Methods: Twelve male cynomolgus monkeys living in three social groups with established social dominance hierarchies were provided access to ethanol and water for 22 h/day, 4-5 days/week, for 1 year. Ethanol-free periods (2- or 3-day "weekends" or longer periods up to 10 days) were spent in social groups to maintain the stability of the social hierarchies. Observational studies conducted 6 months into the year of drinking assessed signs of ethanol withdrawal. After 1 year, monkeys were individually housed 24 h/day, 7 days/week for four consecutive weeks to examine the effect of eliminating the "weekends" spent socially housed.

Results: Subordinate monkeys had significantly higher mean daily ethanol intakes than dominant monkeys across 1 year of open access. Subordinates also had higher intakes on the first day back drinking following ethanol-free periods of 9-10 days. Moreover, during the last 4 weeks of open access, intakes on the first drinking day after an ethanol-free weekend increased significantly in subordinate monkeys. This effect diminished when all monkeys were individually housed for 4 weeks, indicating that the increased intake in subordinates was driven by the social environment.

Conclusions: These data demonstrate that social subordination, which is associated with chronic social stress, results in increased vulnerability to the development and maintenance of heavy drinking trajectories.