Posted on behalf of Ashley Yeager

The parasite Toxoplasma gondii causes rodents to enter the lion’s den: They go against their instincts and sniff out the scent of cat urine. Now, new research has identified the specific regions in the brain involved in this game of cat-and-mouse.

Toxo is a parasite that can only reproduce in the gut of a cat, so it “basically co-opts certain brain circuits in the rats’ amygdala to change their fear into a sexual attraction,” says Patrick House, a neuroscientist at Stanford University in Palo Alto, California.

The team scanned the brains of both healthy and Toxo-infected rats to see which neurons fired when the rats smelled scents classified as neutral, feline, or potential mates. The healthy rats exposed to cat odour show an increase in stress hormones. But infected rats showed a reduced hormone response. In fact, neurons in the same amygdala regions fire in an infected rat as they do in a male rat smelling a potential female mate.

Many people carry Toxo, usually contracted after eating under-cooked meat or unwashed vegetables, or through handling cat litter. The parasite is mostly dormant in humans, though House says some studies suggest that infected people take part in more risky behaviour. But “you can never tell if the behaviour causes people to pick up the parasite or if the parasite triggers the risky behaviour,” he says.

Those diagnosed with schizophrenia are also more likely to carry the parasite. It’s possible, says House, that Toxo carries a gene whose expression could trigger higher dopamine production in both rats and humans – thus leading to behavioural changes, he says.

More research needs to be done to understand the link, he said – which is why the Stanford team’s current research focuses on how dopamine production factors into rats’ attraction to cats.

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