Social group size alters social behavior and dopaminergic and serotonergic systems.

Social behavior is affected by social structure type, but how neural function changes with social type remains unclear. We investigated whether social group size affects social behaviors based on dopamine (DA) and serotonin (5-HT) systems. Four-week-old male mice were housed under different social group sizes: one, two, four, and eight mice per cage (1mpc, 2mpc, 4mpc, 8mpc, respectively). After 4 weeks, social preference, social interaction, and forced swim tests were performed to test sociability and anxiety, respectively, followed by analysis of DA and 5-HT and their metabolites (3,4-dihydroxyphenylacetic acid [DOPAC], 3-methoxytyramine [3-MT], norepinephrine, and 5-hydroxyindoleacetic acid [5-HIAA]). Social interactions and anxious behavior decreased with increased social group size. DA, 3-MT, and 5-HT levels decreased with increasing social group size, whereas DOPAC and 5-HIAA levels increased in the extended mesocorticolimbic system, including the dorsal striatum. Moreover, the increased social group size resulted in increased DOPAC/DA and 5-HIAA/5-HT ratios, accompanied by a decrease in the 3-MT/DA ratio within the extended mesocorticolimbic system. Linear regression analysis also revealed that social group size affects DA and 5-HT turnover. These suggest that social group size may influence behavior and monoamine levels, potentially contributing to DA- and 5-HT-related psychiatric disorders.