Tickling Rats Reveals a Brain Region Linked to Laughter and Play

This group of neurons could someday inform the treatment of depression and anxiety in humans, scientists say

Close-up of a rat
Rats make a high-pitched sound similar to laughter when they're being tickled. Pexels / Denitsa Kireva

Rats, they’re just like us: The oft-maligned rodents love to play games and be tickled, and they’ve even been known to let out high-pitched noises similar to laughter.

Now, researchers say they’ve identified the region of the rat brain that’s primarily responsible for the creatures’ gaiety, according to a new paper published last week in the journal Neuron. In the future, this discovery could help inform the study and treatment of depression and anxiety in humans, since play may be one of the keys to mental and emotional well-being.

More broadly, the findings add to scientists’ very limited understanding of play, which is one of the least-studied behaviors among mammals, per the paper. But play may help mammalian brains grow, and it could contribute to social and emotional development. For instance: When little kids are play-fighting with each other, they often take a break if they realize their playmate has stopped laughing, reports Holly Spanner for BBC Science Focus.

“Neuroscience tends to focus very much on aversive things,” says study co-author Michael Brecht, a neuroscientist at the Humboldt University of Berlin, to Science’s Phie Jacobs. “There’s relatively little research on positive emotions, which I tend to think is a mistake.”

To better understand the neural underpinnings of play and laughter, researchers implanted electrodes into the brains of rats. Rat laughter is too high-pitched for human ears to hear, so the scientists also rigged up an ultrasonic microphone to record any sounds the animals made.

Then, they placed the rats in a plastic box and waited for them to get acclimated to their new environment.

Once the rats were sufficiently settled, the researchers began engaging with them in several ways. They tickled the rats on their bellies, as well as on their backs, and got the animals to chase a researcher’s hand as it moved around inside their enclosure. Finally, the scientists let pairs of rats play together in the same box.

By analyzing the animals’ brain activity during tickling and play, the scientists could see that a particular area of the brain, called the periaqueductal gray (PAG), was lighting up. In contrast, when the researchers put the rats in an environment known to make them anxious—up on a platform under a bright light—PAG activity was suppressed, even when the rats were tickled.

To further confirm the brain region’s role in playfulness, the researchers used a chemical to stop the rats’ PAG neurons from working correctly. Under these conditions, when the animals were playing or being tickled, they quickly lost interest and did not emit “laughter.”

Taken together, the findings suggest the PAG plays a critical role in playfulness in rats. Naturally, the researchers wondered whether the same could be true for other animals, including humans, who have very large PAG regions in their brains. The team plans to investigate this in future studies.

In the long run, the findings could help shine a light on the importance of play for treating mood disorders and uncover a deeper understanding of depression, says Jeffrey Burgdorf, a neuroscientist at Northwestern University who was not involved in the study, to Science News’ Simon Makin.

“When you’re playing, you’re being your most creative thoughtful, interactive self,” Burgdorf tells the publication.

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