Humans, like most mammals, need social interactions in order to develop and learn. However, what prompts us to make this movement to connect, and how the brain encodes the strengthening properties of social interaction, is not well understood. A team from the University of Geneva (UNIGE) studied the neurobiological mechanisms involved when two mice come into contact while learning a task. They observed that the motivation to invest in a social interaction is closely linked to the reward system, via the activation of dopaminergic neurons.
The results of their studies, they suggest, will make it possible to study a possible dysfunction of these neurons in diseases such as autism, schizophrenia or depression, which affect social interactions. “We will now be able to use these neurons as targets to find treatments for these diseases,” said Benoit Girard, PhD, researcher in the Department of Basic Neurosciences. “In addition, the reward system is the basis for the occurrence of addictive behaviors,” noted Camilla Bellone, PhD, professor in the Department of Fundamental Neurosciences at the Faculty of Medicine of UNIGE and director of the National Center for Competence in research (NCCR). Synapsia. “That the excessive use of social networks can hijack the dopaminergic system and be the basis of inappropriate behavior towards social networks is an interesting hypothesis which can now be tested. “
Bellone, Girard and their colleagues reported on their findings in Neuroscience of nature, in an article entitled “VTA dopaminergic neuron activity encodes social interaction and promotes reinforcement learning through social prediction error”.
Social interaction is an integral part of our daily life, although the intention to interact with others requires effort to act. So why are we doing it? “Humans and animals are highly motivated to interact with their peers,” the authors wrote. “Indeed, social interactions offer adaptive benefits, such as security from predators, access to mates and cooperation… By increasing the physical fitness of animals, social behaviors are motivated by their adaptive benefits, and many social interactions are considered rewarding experiences for both animals and humans. . “
But what is the mechanism behind the motivation we feel to engage with others? The authors added that “Although several studies have investigated the brain mechanisms underlying the reward processes associated with food and drug consumption, the mechanisms of social reward remain largely unknown. “
The clinical and preclinical evidence that has accumulated over the past decade suggests that social interactions are rewarding experiences reinforced by social cues. To identify which neurobiological circuit is at the base of social interaction, UNIGE researchers observed what happens in the brains of mice seeking contact with a congener. “In order to observe which neurons are activated during social interactions, we have taught mice to perform a simple task that allows them to come into contact with their peers,” Bellone explained.
Two mice were placed in two different compartments and separated by a door. When the first mouse pressed a lever, the door opened temporarily, allowing social contact with the second mouse through a grid. “As the experiment progressed, the mouse understood that it had to press the lever to join its congeners”, continues Clément Solié, PhD, researcher in the Bellone team. “With this task, we can measure how much effort mice are willing to put into interacting with their peers.”
Using electrodes, the scientists measured the activation of neurons in the ventral tegmental area (VTA) of the brain. “We found that the interaction between two mice, much like other natural rewards, led to the activation of dopaminergic neurons, which are located in the reward system,” Bellone said.
These neurons release dopamine – the so-called pleasure molecule – which is crucial for many motivated behaviors. “What is even more interesting is that if during the first sessions, the dopaminergic neurons are activated when the mice interact with the congener, as soon as the mouse learns the association between the pressure of the lever and the interaction, the ‘dopaminergic neuron activity precedes reward,’ added Girard. Likewise, if the mouse presses the lever but the door ultimately does not open, there is a sudden drop in the activity of dopaminergic neurons, a sign of great disappointment in mice, ”notes Bellone. “This predictive signal is the neural substrate for learning and is crucial for social motivation.” Interestingly, other experiments indicated that the optogenetic inhibition of VTA DA neurons in mice undergoing the task was sufficient to interfere with social reinforcement learning.
In addition, studies demonstrated a general habituation of VTA DA neuron activity when mice were repeatedly exposed to the same congener, although there was great heterogeneity in individual neuronal responses. “Indeed, although some neurons only responded to the novelty, others were activated even when the stimulus became familiar,” the team wrote. “Our data shows that different VTAs
DA neurons are recruited to encode the novelty and value of different social contacts. On the whole, the great heterogeneity of the responses of VTA DA neurons, depending on whether the contacts are passive, unilateral or reciprocal, by exposure to a congener, reflects the great complexity of social behavior. The collective data, the authors pointed out, “suggest that individual neuronal populations within the VTA may contribute to the various building blocks that shape social interaction.”
Several psychiatric illnesses such as autism, schizophrenia or depression are characterized by social dysfunctions and social motivation deficits are described in some of these patients. The results of the study indicated that these difficulties can result from dysfunctions within the reward system, and more specifically at the level of dopaminergic neurons. The authors finally concluded: “Together, these findings provide new insight into the neural dynamics underlying social interaction and motivation and further suggest that VTA DA neurons may be the neural substrate for social learning. . “