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JACKI LYDEN, host:

This is ALL THINGS CONSIDERED from NPR News. I'm Jacki Lyden.

A new twist on an age-old story is the subject of today's Science Out of the Box.

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LYDEN: If you've ever seen your cat catch a mouse, you might think, wow, what a clever cat I have. It turns out you may not have such a clever cat after all. You may be seeing the work of a clever parasite. NPR's David Kestenbaum reports on some new research you may find unsettling.

DAVID KESTENBAUM: To understand this story, think of yourself not as the cat or the mouse. Put yourself in the shoes of a tiny single-celled parasite.

Dr. ROBERT SAPOLSKY (Neuroscientist, Stanford University): Toxoplasma gondii, not as in Mahatma, but spelled somewhat different than that.

KESTENBAUM: This is Robert Sapolsky, a neuroscientist at Stanford University. Toxo, as he calls it, has a strange life. It spends part of its time riding around inside a rat or a mouse that it has infected, but here's the tricky part - this furry lodging? Not a place to breed.

Dr. SAPOLSKY: They can only reproduce sexually in the gut of cats.

KESTENBAUM: In the guts of cats? That's the place where it…

Dr. SAPOLSKY: Yes. Who knows what that one was about, but that's where it has sex.

KESTENBAUM: Okay, so you're the parasite. You're happily freeloading off a mouse or a rat. But you need to get yourself into a cat to reproduce. So how do you pull that one off?

Dr. SAPOLSKY: Toxo does something incredibly elegant that it migrates up to the brain of the rat…

KESTENBAUM: Where, somehow, it changes the way the rat behaves so that the rat, when it smells a cat or cat urine, does not do the normal thing and run like hell.

Dr. SAPOLSKY: Instead, it makes the rodents decide the cat's smell is kind of nice, and they go right up to it.

KESTENBAUM: The cat pounces, catches the rat. The cat thinks, darn, I'm good. The rat thinks, what I am doing? And the Toxoplasma thinks, mission accomplished. A few chews by the cat and Toxoplasma gondii is home free in the gut of the cat, where it can have sex. Its offspring catches a ride out of the cat on cat feces. Another rat or mouse picks them up, and the whole cycle begins again.

Dr. SAPOLSKY: If you really reflect on it, you get, sort of, the heebie-jeebies.

KESTENBAUM: Sapolsky read about all this five years ago in an obscure journal, and the question he had was this - what was the parasite doing to the rat's brain, so that the rats, instead of fearing the smell of a cat, are drawn to it. Sapolsky's lab bought some Toxoplasma, bought some rats, bought some bobcat urine…

Dr. SAPOLSKY: It never occurred to me, you could buy bobcat urine, but it turns out you can.

KESTENBAUM: And they tried to find out. They wanted to know how clever was the parasite. Was it making surgical strikes on specific circuits in the brain, or just generally wreaking havoc? Sapolsky's team tested infected rats. They seemed remarkably normal. Sense of smell, fine. Could run a maze, fine.

Dr. SAPOLSKY: They're still anxious about other things that rats are anxious about.

KESTENBAUM: Just that one thing had changed - the rats had an unusual fondness for bobcat urine. How was Toxo doing it? The researchers studied the rats' brains. It's known that Toxoplasma travels through the brain and makes cysts.

Dr. SAPOLSKY: Where do you get the most cysts? And it turned out, if you had to the design the perfect hamster(ph) and this was the part of the brain called the amygdala. Amygdala, you learn anything about fear and anxiety in an introductory neuroscience class, and the first part of the brain that I could mention is the amygdala. And there's twice as many of these cysts in the amygdala as anywhere else in the brain.

KESTENBAUM: That could explain why the rats aren't afraid of cat smell. He had no idea how the parasite creates an attraction. Sapolsky says if you asked him to make a drug that would be this specific, he'd have no idea how to do it.

Dr. SAPOLSKY: You go to one of these neuroscience meetings where 25,000 neuroscientists running around like headless chickens. And, like, hundreds, thousands, of them study fear and phobias and anxiety, and this protozoan parasite knows more about anxiety than we do. It's totally amazing.

KESTENBAUM: What does this little thing look like?

Dr. SAPOLSKY: Under a microscope, it looks like a little squiggle. That is about.

KESTENBAUM: The little squiggle is doing all that.

Dr. SAPOLSKY: It's a very evolved little squiggle.

KESTENBAUM: The squiggles are not just in rats. Toxo is pretty common, and there are decent odds that you are, or were at some point, infected with Toxoplasma. Some studies indicate it may have some subtle behavioral effects on us. The research appeared this month in the journal proceedings of the National Academy of Sciences.

David Kestenbaum, NPR News.

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