At least that's what Wöhr et al say in a new paper. They started from the fact that when you tickle rats, some of them seem to enjoy it, and express this by making 50 kHz squeaks of joy. But other rats don't like it, and they make a different sound, much lower at 22 kHz. (These sounds are all too high for most humans to hear, but they can be recorded electronically.)
Whether a given rat is cool with being tickled seems to be a fairly stable individual trait. Some do, some don't. And rats which don't like being tickled tend to be generally anxious and neurotic in lots of other ways. They're the Woody Allens of the rat world, maybe.
Wöhr et al decided to see whether these personality differences were related to the rate of hippocampal neurogenesis in the brain. Stress is known to decrease the rate of neurogenesis, and it may be reduced in clinical depression, while antidepressants increase it (more).After much tickling (ten rats for five days at ten minutes per day), they did indeed find a pretty strong correlation between tickle-liking and hippocampal neurogenesis. What does this mean? Hard to say. The problem is that rates of neurogenesis were measured after all the tickling was over. (This is because you can only measure it by killing the animals and dissecting their brains. Sorry, rat lovers.) Given that the rats who didn't like being tickled probably must have found the procedure very stressful, and given that we know that stress strongly reduces neurogenesis, maybe it doesn't mean very much. But it must have been a lot more fun than most animal experiments.
Before you rush to submit this paper to NCBI ROFL, I already did. Finally, here's a video of someone tickling a rat: YouTube really does have everything.
Wöhr M, Kehl M, Borta A, Schänzer A, Schwarting RK, & Höglinger GU (2009). New insights into the relationship of neurogenesis and affect: Tickling induces hippocampal cell proliferation in rats emitting appetitive 50-kHz ultrasonic vocalizations. Neuroscience PMID: 19638303
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