Scary situations for mammals become less scary when they have a mate nearby. A new study in mice reveals the specific brain wiring responsible for this increase in social courage. Scientists have discovered that social interactions change the firing patterns of dopamine-producing neurons, making us less sensitive to risk and stimulating our sense of exploration. The results were published in a magazine neuron.
Exploration is a basic biological drive. Animals must venture into the unknown to secure food, find shelter, and find mates. This important behavior carries inherent risks, and animals must constantly weigh resource benefits against the threat of predators and physical harm.
Psychological research has proven that social engagement can promote exploratory behavior. Animals often explore in groups to share the burden of vigilance among multiple mates. The precise neural circuits linking social friendships and decision-making mechanisms in these risky scenarios have remained largely hidden from neuroscientists.
Dopamine is generally widely recognized as the basic reward chemical. In fact, this molecule serves a variety of functions throughout the brain, including motor control, risk assessment, and promoting volitional behavior. An area located deep in the center of the brain called the ventral tegmental area serves as a major hub for dopamine production.
Qiaowen Zhen, a researcher at Xi’an Jiaotong University in China, led a study to map the biological relationship between this dopamine hub and courage. Zheng worked collaboratively with a large team of colleagues, including corresponding author Changhe Wang. The researchers suspected that dopamine may serve as a physical bridge between socialization and risk calculation.
The researchers designed a series of behavioral tests to observe how mice avoid environmental risks. First, the mice were trained to associate a specific room with a mild foot shock. When these conditioned mice were then placed alone in the test apparatus, they completely avoided the dangerous chamber. They instead decided to stay in a safe corner of the enclosure.
Next, the researchers placed a familiar cagemate in the enclosure alongside the conditioned mouse. In the presence of a partner, the conditioned mice significantly increased their visits to the dangerous room.
To see if this courage extended to innate fear, the scientists simulated a sudden attack by a predator using a toy snake held onto a rat’s tail. They also exposed mice to chemicals extracted from fox urine. This chemical is known to cause an automatic and overwhelming fear response in rodents.
In all experimental scenarios, the presence of a social partner increased the amount of time mice spent exploring the dangerous area. The researchers also allowed the test mice to interact with a peer immediately before exploring the arena alone. Their temporary friendships boosted their courage during their solo trials in dangerous conditions. This particular finding proved that the mice were not simply imitating the movement patterns of their active partners, but were experiencing physiological changes in motivation.
To find out what was happening in the brain, the research team used a technique called fiber photometry. This method uses emitted light to track calcium signals within specific nerve cells. This technique provides a continuous surrogate of neural activity because calcium rushes into the cell as it fires.
The scientists monitored dopamine neurons in the ventral tegmental area across multiple trials. When a lone mouse approached the danger zone, these dopamine neurons fired rapidly. This rapid electrical activity, known as phasic firing, was quantitatively correlated with exploration depth. The closer the rat was to danger, the louder the electrical burst became, showing that staged ignition serves to encode risk assessment.
With the introduction of companions, this electrical behavior has completely changed. Dopamine neurons stopped producing large spikes in response to environmental risk. Instead, they maintained a higher and more stable baseline of activity. This steady electrical rhythm is known as tonic firing.
The research team used advanced experimental techniques such as optogenetics and chemical genetics to artificially control the firing patterns of living mice. Optogenetics uses artificial light-sensitive proteins to issue instructions to brain cells, while chemical genetics uses synthetic molecules to achieve similar control. When the researchers stimulated the cells to create a stable tonic rhythm, the solitary mice boldly explored the danger zone as if they had a friend. When the researchers generated rapid phasic bursts in neurons, the highly socialized mice became denerved and avoided dangerous areas.
The scientists then tracked where these dopamine signals travel. They mapped two distinct pathways originating from the original dopamine hub. Both pathways ultimately lead to the basolateral amygdala, an almond-shaped structure deeply involved in emotional processing and encoding the severity of external threats.
The first pathway goes directly to the amygdala. The second pathway makes a pit stop inside the prefrontal cortex. This prefrontal cortex is widely known for handling complex decision-making and emotional regulation.
Direct and indirect pathways work together in a competitive manner to ultimately determine behavioral decisions. They also utilize completely different mobile devices to read incoming dopamine signals.
The direct pathway targets specialized cellular structures called D1 receptors. Activating these receptors requires a large dopamine wash. As a result, the animal primarily responds to large transient bursts of firing, which subsequently trigger the animal’s avoidance response.
The indirect pathway targets different structures in the prefrontal cortex called D2 receptors. This receptor is highly sensitive to dopamine. It responds perfectly to the low, continuous drip of dopamine brought on by tonic firing. Activating this indirect pathway promotes motivated exploration and overcomes fear.
Social interactions effectively scale between these two routes. By shifting the brain into a state of tonic dopamine release, peer interaction activates indirect pathways that promote exploration. At the same time, the avoidance-promoting direct pathway remains relatively quiet because no large dopamine burst occurs.
The researchers found that both of these pathways are concentrated in the exact same group of downstream neurons in the amygdala. Structural convergence allows the amygdala to integrate contradictory information streams. It seamlessly weighs the biological motivations facilitated by the presence of friends and the innate vigilance necessary to avoid predators and stay alive.
The authors noted that the study had certain limitations. All experiments were performed in mice. Although rodent brains share many basic circuits with human brains, they are unable to reproduce the immense social complexity of human interactions.
The researchers also noted that the biological sequence of events immediately preceding dopamine changes remains unclear. The precise sensory network that recognizes friends and directs the ventral tegmental area to alter its electrical firing has not yet been identified. Future studies will need to map these upstream connections to complete the anatomical picture.
The study, “Concentration of dopamine pathways in basolateral amygdala neurons encodes exploration decisions,” was authored by Chaowen Zheng, Xiaoying Liu, Anqi Wei, Bing Liu, Qianyun Zhang, Junjie Jiang, Xiaofeng Gao, Hong Fan, Anran Zhao, Xueting Duan, Xu Cheng, Haiyao Liu, Niki Gooya, and Fenghan. Mao, Aomei An, Shuaijie Zhong, Jie Jian, Wenxin Shen, Xingyao Dong, Kaikai Yang, Bianbian Wang, Ziyang Li, Jingxiao Huo, Jingyu Yao, Weiwei Li, Yu Lu, Junxi Kang, Kai Huang, Nan Dong, Yang Chen, Qian Song, Zigang Huang, Rong Huang, Zhenli Xie, Yan Li, Shuqin Zhan, Han Xu, Yong Jiang, Chunxiang Zhang, Dan Xu, Haowen Liu, Jinhong Ma, Yuqing Zhang, Huadong Xu, Xinjiang Kang, Changhe Wang.
