Depressed with a failing reward system: social stress disrupts brain waves
Social interactions and the brain
Depression and social rewards
In humans, social interactions are extremely important to emotional well-being. Social interactions activate a reward system in the brain, making us feel good. If this reward system doesn't work well, you'd feel less pleasure from social interactions, and have a higher risk of getting depressed. Dr. Ann Mary Iturra-Mena has investigated this system, together with her colleagues from Chile and the United States.
Reward system and depression
The reward system can be found in a specific area of the brain, called the nucleus accumbens (NAc). Among other things, this area motivates us to have social interactions, which in turn increases brain activity. It seems that during reward processing, it also produces gamma waves.
Furthermore, there seems to be a link between changes in gamma waves and major depression. However, it is not yet known what role those waves play in depressed and healthy individuals during social interaction.
Gamma waves and social stress
Social stress severely decreases the activity of the reward system, which might cause an inability to feel pleasure at all. In this study, researchers aimed to find out the effect of chronic social defeat stress (CSDS) on gamma waves during social interaction. They expected the gamma waves to be disrupted during social interaction, and therefore they measured brain waves of non-stressed and stressed rats in social situations.
Social behavior experiments
Social stress exposure
For the behavioral component, two groups of rats were tested, one of which was exposed to social stress while the other was not (control group). The stress was inflicted by placing the stressed rats in the home cage of a fellow rat daily for 7 days, which usually resulted in some form of defeat or surrender of the intruding rat.
Social preference-avoidance test
To test the influence of social stress on social behavior, a rat was placed in an open field arena containing a transparent, perforated box. The rat was allowed to explore for 2.5 minutes, during which the time spent in the interaction zone was recorded.
Then, another rat was placed inside the small box, and the experimental rat could explore for an additional 15 minutes. Total interaction time with the new rat was measured using Noldus EthoVision XT, and heat maps were generated to visualize the areas in which the experimental rats spent most of their time. Stressed rats that had a very significantly shorter interaction time than the rats in the non-stressed group were chosen for further experimentation.
Spontaneous social interaction
Using a second open field test, gamma waves were measured through brain implants in the rats' NAc. The goal was to test the influence of the social distress on social interactions and on the brain waves.
After habituation, the rats were allowed to explore the open arena for 5 minutes (non-social condition). Then, a small transparent cube was introduced as a novel object and subsequently removed after 5 more minutes. Lastly, a second rat was introduced in the arena, and both could interact freely for another 5 minutes (social condition). Noldus EthoVision XT was used to track and analyze the rats’ movements.
Links between gamma waves and social behavior
Dr. Iturra-Mena and her team aimed to find an electrophysiological link between social interaction and a reward processing brain region like the NAc. This link now seems to be found, and consists of high-gamma waves. In non-stressed rats, these waves increased during social interactions. However, stressed rats showed no differences in high-gamma waves between social and non-social interactions.
Social reward brain region as a switchboard
Earlier research suggests that the brain region NAc might be a “switchboard”, capable of regulating signal exchange and “selecting” a dominant input. This could mean that, when creating high-gamma waves in non-stressed rats, other brain regions are synchronized to the NAc, and are thus capable of influencing the rats’ behavior as well.
Furthermore, it is possible that distress is capable of influencing the connection between NAc and the brain cortex, resulting in a decrease of gamma power in stressed organisms. This could potentially lead to failure of processing of social behavior input in the NAc.
Dopamine and gamma waves
Normally, it seems, dopamine in the social reward brain region generates changes in gamma frequency during social interactions. Based on this theory, scientists propose that dopamine levels decrease in stressed rats whenever they interact with an unfamiliar fellow rat. This then leads to alterations in their brain which cause a decrease of gamma waves in the NAc during social interaction.
Other research suggests that social interaction does not cause much dopamine release towards the NAc in stressed mice compared to non-stressed mice. However, dopamine concentrations in NAc have not yet been measured during social interaction in stressed rodents, so that puzzle piece is still missing.
As found in previous research, Iturra-Mena’s study suggests that high-gamma waves are involved in the rewarding effect caused by a fellow rat being present, and socially interacting with it. Furthermore, the waves can be disrupted by chronic social stress, resulting in stress-related disorders like anxiety and major depression.
Iturra-Mena, A. M.; Aguilar-Rivera, M.; Arriagada-Solimano, M.; Pérez-Valenzuela, C.; Fuentealba, P. & Dagnino-Subiabre, A. (2019). Impact of stress on gamma oscillations in the rat nucleus accumbens during spontaneous social interaction. Frontiers in behavioral neuroscience, 13, 151.
This blog has appeared earlier on Noldus.com