A rat model of volitional mutual social interactions.

Social interactions are essential for building societies and fostering cooperation among individuals. These behaviors are governed by complex norms and signaling mechanisms promoting mutual engagement. While animal models are often used to study social behaviors, they typically focus on one individual, overlooking the role and motivation of an otherwise passive social partner. Here, we developed a model where resident and partner rats voluntarily engage in mutual social interactions. In this model, the resident initiates interaction by pressing a lever to activate cues for the partner, who responds by pressing an additional lever, leading to social interaction. To test motivation for mutual social interaction, we increased the effort required for both residents and partners either concurrently or independently. We further investigated the mechanisms underlying these interactions by manipulating the norepinephrine system both systemically and centrally during mutual social interactions. Both male and female paired rats consistently demonstrate mutual motivation to engage in social interactions, regardless of their roles. The rats effectively coordinate their actions, showing low latency and high engagement frequency even as effort demands increase. The average social score analysis identified a significant proportion of highly motivated social pairs. Manipulating the norepinephrine system selectively disrupted the distribution of highly motivated social pairs, emphasizing its role in regulating social interactions. Ablating norepinephrine terminals had no impact on motivation for food rewards, further confirming that central norepinephrine manipulation specifically affects mutual social interactions. Our findings provide insight into the fundamental behavioral and neurobiological mechanisms underlying sociability and complex social structures in rodents.