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Why do we choose the friends that we do? In humans, making friends would seem to be a process that is most influenced by geographical location. At a young age, kids interact with classmates and form bonds based on shared interests or inherit similarities. In reality, the link friends share may be reflective of more than a shared interest in a professional sports team. 

A study published in Molecular Psychiatry suggests that certain enzymes in the brain may have a previously unrecognized link to how certain animals form friendships. The researchers wanted to test whether or not there is a genetic basis for the tendency of humans to choose relationships based on similarities, called homophily. 

To do this, they studied the three regions of the brain most responsible for social interaction: the hippocampus (center for memory), the cerebral cortex (center responsible for consciousness and self-control), and the limbic system (the area of the brain responsible for emotion and motivation). These researchers believe that a link found within the mice subjects can be extrapolated to explain human emotional interaction and connection.

Michy P. Kelly, the study leader, found that an enzyme released in these parts of the brain, PDE11A-cAMP-CREB (shortened to PDE11a) is related to which mice spend time together. PDE11a is found within the hippocampus and is also known to regulate oxytocin levels, a hormone that is involved in pair-bonding. It is also linked to Major Depressive Disorder.

The study was set up by putting a mouse in a 3-compartment chamber with a plexiglass cylinder on each end of the chamber. In one cylinder, a novel object was placed, while the other cylinder housed either a mouse that produces the PDE11A enzyme or one that doesn’t. For five minutes, the mouse was prevented from accessing the two cylinders using a divider and allowed to roam in the chamber. After those five minutes were up, the dividers were lifted and the mouse was allowed to roam in and out of the cylinders for another five minutes.

Experimental set-up. Image recreated to represent study authors’ drawings, by Brendan Feller, Muhammad Hameed, and Sciworthy.

When faced with the two choices, mice with the PDE11A enzyme explored the other mouse for significantly longer than their littermates without the enzyme, which showed no preference between the object and the mouse.

The results of the experiment show how the mice that did not possess any PDE11A would  prefer to interact with other mice without the enzyme. Mice with PDE11A prefer to interact with each other as well. Changes made to the concentration of the enzyme by researchers would also affect the mice and their interaction. 

The researchers explained that it is still unknown how the mice detect these enzyme changes. So far, the most probable explanation involves the way mice communicate, and how they respond to the way other mice look. Kelly found that if a mouse finds another mouse whose behavior and facial expressions are similar to theirs, they start to grow a bond with them almost immediately. This is similar to how you can feel an instant connection with someone as soon as you’d meet them.

As scientists and researchers develop a better understanding of PED11A, the scientific community as a whole will have a better understanding of the complex processes that contribute to social interaction. For example, they suggest that a better understanding of PED11A can facilitate better patient-provider relationships and provide better health outcomes and improve psychiatric care for patients whose conditions lead them to struggle with social interaction. 

Next time you find yourself wondering how you ended up with the friends you have, it may just be a result of biochemical similarity.

Study Information

Original study: A genetic basis for friendship? Homophily for membrane associated PDE11A-cAMP-CREB signaling in CA1 of hippocampus dictates mutual social preference in male and female mice

Study published on: 28 July 2021

Study author(s): Abigail J. Smith, Reagan Farmer, Katy Pilarzyk, Latarsha Porcher and Michy P. Kelly

The study was done at: University of Maryland School of Medicine (USA)

The study was funded by: University of Maryland School of Medicine (USA), The NIH Brain Initiative (NIMH) (USA), and National Institute on Aging (USA)

Raw data availability:

Featured image credit: Wikimedia commons Source