Parasites practicing mind control.

Zimmer points to a further installment in the fascinating story of Toxoplasma gondii parasites, who can infect any mammal or bird, but can reproduce only inside of a cat whose feces then contain cysts that infect new hosts. Infected rats and mice become unafraid of feline odor, and thus become easier prey. It turns out the parasites use a very elegant technique to alter their host's behavior: they use an enzyme to remove inhibiting methyl groups from the arginine vasopressin gene, and the resulting increase in arginine vasopressin makes rats become more fearless. The abstract:

Male rats (Rattus novergicus) infected with protozoan Toxoplasma gondii relinquish their innate aversion to the cat odors. This behavioral change is postulated to increase transmission of the parasite to its definitive felid hosts. Here, we show that the Toxoplasma gondii infection institutes an epigenetic change in the DNA methylation of the arginine vasopressin promoter in the medial amygdala of male rats. Infected animals exhibit hypomethylation of arginine vasopressin promoter, leading to greater expression of this nonapeptide. The infection also results in the greater activation of the vasopressinergic neurons after exposure to the cat odor. Furthermore, we show that loss of fear in the infected animals can be rescued by the systemic hypermethylation, and recapitulated by directed hypomethylation in the medial amygdala. These results demonstrate an epigenetic proximate mechanism underlying the extended phenotype in the Rattus novergicus – Toxoplasma gondii association.