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The Scicurious Brain

The Scicurious Brain

The Good, Bad, and Weird in Physiology and Neuroscience

To Calm a Rat with Tickling

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"for here comes the trout that must be caught with tickling"

-Twelfth Night

Generally, I don't think of being tickled as a particularly pleasurable or calming activity. Most people who are ticklish go immediately on the defensive and tense up, and I always got the impression that most people prefer NOT to be tickled rather than otherwise.

However, that's just us. And we're not rats. And it turns out, you can calm a rat with tickling.

(Source)

Note: I'll admit, I'm covering this paper primarily because I love the idea of tickling rats. ADORABLE!

Hori et al. "Effects of repeated tickling on conditioned fear and hormonal responses in socially isolated rats" Neuroscience Letters, 2013.

Life is stressful. Whether it's running from predators, meeting tight deadlines, or trying to keep fed, there's a lot that seems to bring us down.

What saves us from tearing our hair out? Well, the happy things in life. Tasty food, friends, hugs, puppies. You know, the good stuff. These things elicit positive feeling, and positive feeling have been linked to protecting us from stress.

Of course, in humans, it's easy to say that a positive outlook on life makes someone resistant to stress...but is it really true? They may co-occur, but do positive feelings really decrease stress? If you want to get at causes, one of the best ways is to use an animal model. But how do you come up with an animal model for...happiness?

Well, you can tickle rats.

As you can see in the video above, rats like to be tickled. They even respond with "laughter"! Of course, it's not laughter as we know it, or even something we can hear. Instead, these are ultrasonic vocalizations at a specific frequency (50 kilohertz). Scientists figured they must be pleasure-sounds because rats make them when they play with other rats. And it turns out that rats make the same noise at the same frequency when they get tickled!

A rat's happiness at being tickled presents a good possible model for how positive interventions (like tickling), could change their response to stress, and could therefore help answer the question of whether positive things really CAN change stress response.

And of course, it also means you get to tickle rats daily. Who could argue with that? It's like "oh DARN, I have to go snuggle some kitties today! My life SUCKS"*

For the "tickling treatment", rats were tickled once daily, in two sessions of two minutes each, for two weeks. The rats were housed individually otherwise, something which is pretty stressful to rats.

Here you can see the responses of the rats being tickled. After a little bit of training, the rats recognize what's going on, and head right over to the experimenter's hand to get tickled as soon as he puts his hand in the cage. They also make those ultrasonic vocalizations associated with little rat happiness.

After half of the rats had been tickled for two weeks, the authors gave them something called fear conditioning. It's where you put a rat in a chamber, and pair the sound of a tone with a painful shock. It's a memorable experience, when you expose the rat to the tone again, the rats will freeze in place waiting for the shock, even though there is no shock coming at all. This test is often used as a test for memory, but it also works well when looking at stress, because if you give treatments that reduce stress, you can reduce the freezing response after training.

So did tickling reduce stress?

You can see on the left the test for fear two days after the training. There's no difference. But on the right, you can see the test for fear 5 days after training. While the untickled rats are still freezing a lot, the tickled ones are freezing significantly less. It appears that the tickling helped with the painful memory of the shock.

The authors also wanted to look at the biological aspects of stress response. They looked at two neurotransmitters, adrenaline (epinephrine), and noradrenaline (norepinehprine), two chemicals in the body that respond particularly to stress.

They were able to show that tickling the rats resulting in less adrenaline and noradrenaline release following fear testing. They also looked at corticosterone, a hormone very much involved in the stress response, but couldn't find any differences.

But it looks like tickling rats can protect against some of the effects of stress. The intervention doesn't appear to be big, as there aren't changes in corticosterone, but it's enough to produce a behavioral effect.

It'd be interesting to see how this compares to other things that can ease rat stress, like say, group housing in your buddies, enrichment of other kinds (rat playgrounds), or drug interventions (like Prozac). And it would also have been good to see how the rats being tickled responded in other measures of anxiety.

It's too bad that they didn't get any differences in plasma corticosterone. Increases in plasma corticosterone during and after stress are often the best indicators (or at least, they are the ones taken more often) of a stress response to something like fear conditioning. But I have to wonder if they didn't get anything here because of timing. They were testing memory retrieval, not the initial stress response itself. And so I wonder if the corticosterone response to the initial stress (right after the shocks had been given) might have been lower in the tickled rats.

But regardless, it's an interesting model of how the good things in life could help mitigate the bad ones. And while a lot more remains to be done, and we humans may not love getting tickled like rats do, it may mean that other positive feelings could have some effects on our stress responses, too. So take a deep breath, find a friend, and tickle a rat. It certainly wouldn't do anyone any harm.

*Seriously, rats get a bad rap. Most domestic rats (experimental and fancy pet style) are charming and very snuggly and friendly. Well worth daily tickling.

The views expressed are those of the author and are not necessarily those of Scientific American.

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