A common parasite call Toxoplasma gondii, which typically infects cats, is famous among scientists for its clever tactic of jumping from one cat to another by infecting rats and altering their behavior to make them more likely to be eaten by another cat, thus transferring the parasite to a new host.
Flegr discovered that the behaviors that toxo provokes in rats in order to get them eaten—slowed reaction times, lethargy, reduction in fear—also show up in infected humans. But until very recently, scientists knew little about how toxo might be doing this.
Enter Antonion Barragan, a researcher at the Center for Infectious Medicine at Karolinska Institute in Sweden. As Barragan and his team examined toxo in the blood of mice, they found the parasite living in a surprising place: inside the immune cells designed to kill them, a type of white blood cell called a "dendritic cell," after its treelike appearance. "These are the gatekeepers of the immune system," he said. "And we wondered, maybe the parasite is using these cells to get around." Using the cells as Trojan horses. As it turned out, he was right. Toxo was using the immune system cells to travel through the body and get to the host's brain. But how? The immune cells need to be stimulated in order to move—and the toxo itself obviously wasn't getting them going; the cells didn't even seem to know they'd been infected. What was agitating the dendritic cells?
And then they found it: a neurotransmitter called GABA. "It didn't make any sense," Barragan said. "GABA operates in the brain. What's it doing in the immune system?" But there it was. Barragan was seeing something nobody had seen before. Toxo appeared to be inducing GABA production inside the dendritic cells, which excited GABA receptors on the outside of the very same dendritic cells, and sent them zooming through the body, and to the brain. Now, here's the fascinating part: Disturbances in GABA are commonly seen in many psychiatric disorders, including schizophrenia. And elevated GABA levels, Barragan says, "are associated with decreases in fear and anxiety."
Still, Flegr cautions that this discovery doesn't tell the whole story. "I still think the most important molecule is dopamine," he said. "But this GABA mechanism is brand new and very interesting."
And perhaps not surprisingly, given all we've learned about toxo so far, he said, "It's very, very clever."