Sapolsky's group previously determined that although the parasite infects the entire brain, it shows a preference for a region of the brain called the amygdala, which is associated with various emotional states. Once in the brain, the parasite forms cysts around itself, in which it essentially lies dormant.
House was interested in how the amygdala is affected by the parasite, so he ran a series of experiments with both healthy and Toxoplasma-infected rats. He exposed each male rat to either cat urine or a female rat in heat for 20 minutes before analyzing its brains for evidence of excitation in the amygdala.
For the experiments, he used cat urine he purchased in bulk from a wholesaler. No actual cats participated in the experiments.
House analyzed certain subregions of the amygdala that focus on innate fear and innate attraction.
In healthy male rats, cat urine activated the "fear" pathway.
But in the infected rats, although there was still activity in the fear pathway, the urine prompted quite a bit of activity in the "attraction" pathway as well. "Exactly what you would see in a normal rat exposed to a female," House said.
"Toxoplasma is altering these circuits in the amygdala, muddling fear and attraction," he said.
The findings confirmed observations House made during the experiments, when he noticed that the infected rats did not run when they smelled cat urine, but actually seemed drawn to it and spent more time investigating it than they would just by chance.
Although House doesn't have the data yet to speculate on just how the cysts in the rats' brains are causing the behavioral changes, he is impressed with what Toxoplasma can accomplish.
"There are not many organisms that can get into the brai
|Contact: Louis Bergeron|