Tickled pink: Neuroscientists tickle rats to find brain’s play centre

Scientific experiments are rarely amusing for rats, but new research by German neuroscientists involved tickling rats to investigate the brain mechanisms behind play.

The study published in Neuron identifies a specific part of the brain called the peri-aqueductal gray (PAG) as the driver of playfulness.

Professor Michael Brecht from the Humboldt University of Berlin, an author of the study, says the findings are significant because while play is critical for humans and many animals the associated brain mechanisms are poorly understood. 

“For example, we do not know why young animals and humans are very playful, whereas older individuals lose playfulness,” he says.

In a series of experiments investigating the neuroscience of play, scientists tickled rats, invited them to ‘chase the hand’ and checked to see if the animals were having fun by listening for rat vocalisations.

“We chose rats because they are very playful and ticklish animals. In this regard they differ from mice (the subject of most cellular neuroscience) that are not very playful or ticklish,” Brecht says.

When rats are enjoying themselves, they emit a high-pitched squeak at 50kHz, inaudible to humans, the paper explains.

During the experiments the researchers used high resolution neuron recordings enabling them to measure activity in different subregions of the rats’ brains.

“When we found cells in the lateral column of the PAG that responded very strongly to tickling and play, we had a first hint that we were recording a play-related structure,” Brecht says.

The researchers compared rat vocalisations during tickling and game-play under relaxed conditions, and after blocking activity in the PAG brain region with an injection of muscimol (a psychoactive drug which can induce anxiety) and separately of lidocaine (a local anaesthetic).

“These drugs (muscimol and lidocaine) block brain activity in different ways,” says Brecht. 

“We later on did additional blocking experiments, where we blocked specifically the lateral column of the PAG (where we had observed many play-activated cells).”

Rats’ vocal responses to tickling and play in the rats was reduced when the lateral column of the PAG was blocked.

The scientists also tested the rats’ responses under relaxed and anxiety-inducing conditions in which rats were placed on an elevated platform under bright light. 

They found the stressful conditions also suppressed the responses in the PAG part of the brain.

Around 20 rats were used across the different experiments. 

It wasn’t all fun and games for the rats. Afterwards, Brecht says, “in most cases animals were killed by an overdose of anaesthetic to perform histological analysis of their brains”.

The researchers plan to study play in other animals, particularly as there are large variations in playfulness across species. 

“Some animals, like monkeys are very playful, whereas others, like mice, are not. We want to study if this relates differences in the brain, specifically in the lateral column of the PAG,” Brecht says.