Social isolation increases impulsive choice with minor changes on metabolic function in middle-aged rats

The effects of social isolation (SI) during middle age remain unclear, so we tested the hypothesis that SI would lead to an increase in impulsive choice (IC), anxiety-like behavior, and metabolic dysfunction in middle-aged rats. Male and female rats were housed individually or in groups of four with same-sex housing mates at 11 months of age. Two months later, IC behavior was assessed using a delay-discounting task and anxiety-like behavior through a novelty-suppressed feeding (NSF) task. Lastly, glucose tolerance and insulin sensitivity following exposure to a high-fat diet were assessed using an oral glucose tolerance test (OGTT) and an insulin tolerance test (ITT). The results showed that socially isolated rats displayed more IC behavior than did group-housed rats of both sexes. However, no significant effect of housing was evident in the NSF task, OGTT, or ITT. Male rats had a higher plasma insulin concentration and insulin resistance index compared to females. Our findings demonstrate that SI in middle age is sufficient to increase IC behavior and highlight inherent sex-specific differences in metabolic profiles. These findings underscore the importance of investigating mechanisms that underlie the effects of social isolation during different stages of life.