Social-cooperation differs from individual behavior in hypothalamic and striatal monoamine function: Evidence from a laboratory rat model

Explanations and models of cooperation usually focus on the economics of an individual's invested effort and outcomes while down-playing social dimensions of naturally occurring cooperation. This study examined whether cooperative and individual behaviors differ in monoaminergic function in a manner that may explain the reported 'bias for cooperation' even under conditions where no immediate economic gains exist. Cooperation, represented by pairs of rats reinforced for coordinated shuttles within a shared chamber (COOP), was compared with rats shuttling for reinforcements individually (IND), and behaviorally naïve rats (NAïVE). Following training, the hypothalamus and striata were sampled and the activity patterns of the noradrenergic, serotonergic and dopaminergic systems were assessed using HPLC analyses. By matching the proportions of reinforced individual shuttles for COOP and IND rats the economic differences of invested effort (shuttles) and outcomes (obtained reinforcements) were neutralized. Nevertheless, differences were evident in monoaminergic functions. In comparison with IND rats, COOP rats showed significantly higher hypothalamic norepinephrine levels and exhibited a trend toward higher striatal serotonin levels. Differences in levels of dopaminergic metabolites were restricted to the right striatum; compared to IND rats, COOP rats exhibited significantly higher levels of HVA, whereas NAïVE rats exhibited significantly higher DOPAC levels.Since economic differences between cooperative and individual shuttling were neutralized, the results demonstrate a relationship between social cooperation and a distinct activity pattern in brain mechanisms that were related with arousal, goal directed behaviors and motivation and further highlight the key role of social behaviors in the reported 'bias for cooperation'.