Researchers have long believed that areas of the brain evolved over time to motivate animals, including humans, to behave in a way that would encourage survival and reproduction. Specifically, the mesocorticolimbic pathway in the brain is often referred to as the “reward pathway” in that it is involved in inducing pleasurable effects that make one “want” to perform the behaviour. Commonly, these rewards are associated with behaviours such as eating, drinking, and sex; however, there is mounting evidence to suggest that social interactions can also trigger a reward in this manner. Unfortunately, the way in which the brain encodes social interactions as a reward is not yet well understood.
Previous studies have shown that there is a link between the social interactions of prairie voles and an increase in activity of the chemical oxytocin in the nucleus accumbens of the brain. The nucleus accumbens, considered a key part of the mesocorticolimbic pathway, has been associated with pleasure, including laughter and reward. Meanwhile, oxytocin is a chemical that transmits signals around the brain, influencing behaviours such as orgasm and social recognition. However, more research was needed to figure out the complexities involving social behaviours and rewards.
A recent study by researchers in the US has shed some further light on this process by using mice to see if oxytocin was a requirement for animals to be “rewarded” by social interactions. To begin, the mice were injected with either saline or an oxytocin receptor antagonist. The oxytocin receptor antagonist served to block the ability of oxytocin to be active within the brain, effectively making it so that oxytocin could not play a role in social reward. The researchers then used a technique called conditioned place preference, where they tested to see whether the mice would prefer to be in a location where they had previously been exposed to social encounters or not. They found that mice who had just received a saline injection preferred to be in locations where they had previously been exposed to social encounters, while the mice that had their oxytocin activity blocked showed no preference at all. This suggests that oxytocin does indeed play an active role in social reward. In addition, the researchers were able to specifically identify that the oxytocin receptor antagonist was concentrated in the nucleus accumbens, thereby supporting the idea that it is oxytocin activity in the nucleus accumbens that is required for social reward.
Venturing further into the social reward system, the researchers also found that it was likely that oxytocin activity in the nucleus accumbens was not directly responsible for social rewards. Instead, they found that the chemical serotonin was also involved in the social reward process. Serotonin, like oxytocin, transmits signals around the brain, and has been associated with regulating aggression, feelings of well-being, and happiness. Oxytocin was shown to lead to serotonin release and activity within the nucleus accumbens. They focused specifically on the serotonin 1B receptor, an object that becomes activated by serotonin in the brain, and found that activating this 1B receptor in the nucleus accumbens induced social rewards. These results suggest that both oxytocin activity and serotonin activity are necessary chemicals in this pathway.
This research shows that there are two primary chemicals working in tandem to make us “want” to interact with others. Not only are these results important for helping us to understand what makes us interact socially with one another, they also have potential medical implications. Knowledge of the underlying functions of social interactions within the brain could potentially yield new treatments and therapies for diseases that impair social interactions, such as autism.
Source:
Dolen, G., Darvishzadeh, A., Huang, K.W., and Malenka, R.C. (2013). Social reward requires coordinated activity of nucleus accumbens oxytocin and serotonin. Nature, 501: 179-184.
