New research shows how a brain parasite can manipulate rodent fear responses for the parasite's own benefit. The study, authored by Patrick House and Dr. Robert Sapolsky of Stanford University and released this week in PloS One, addressed how the single-celled parasite Toxoplasma gondii makes infected rodents more likely to spend time near cat odors. The study finds Toxoplasma-infected male rats have altered activation in brain regions involved in fear and increased activation of brain regions involved in sexual attraction after exposure to cat odors. The findings may help explain the biological bases of innate fear and sexual attraction.
Toxoplasma requires the cat digestive system for sexual reproduction. Infected rodents, with reduced fear response to cat odors, are presumably more susceptible to predation by cats, thereby enabling completion of the parasite lifecycle. Toxoplasma is manipulating the fear response specifically to the urine of cats -- infected rats behave normally on anxiety, fear, social and memory tasks, and retain fear behavior to non-feline predator odors.
"These findings support the idea that in the rat, Toxoplasma is shifting the emotional salience of the detection of the cat. They also suggest that fear and attraction might lie on the same spectrum, or at least that the emotional processing of fear and attraction are not entirely unrelated," House said.
The study does not advance evidence for how Toxoplasma is altering the brain, only evidence that it does. Previous research showed that Toxoplasma invades the brain of the host and settles near the amygdala, a region involved in a wide range of fear and emotional behaviors. This study extends these findings by showing that not only is Toxoplasma found in the amygdala of infected male rat hosts, but it also changes the way certain subregions of the amygdala respond to cat odor - specifically, by increasing neural activity in the presence of cat odor in regions normally activated by exposure to a female rat.
Up to a third of humans test positive for Toxoplasma, due largely to the consumption of undercooked meat or contact with cat litter. In humans, Toxoplasma exposure is most dangerous to developing fetuses and pregnant women. However, many recent studies find Toxoplasma exposure linked with schizophrenia, a disease noted for amygdala dysfunction and improper emotional response, compelling further investigation into what exactly Toxoplasma is doing in the host brain.
Citation: House PK, Vyas A, Sapolsky R (2011) Predator Cat Odors Activate Sexual Arousal Pathways in Brains of Toxoplasma gondii Infected Rats. PLoS ONE 6(8): e23277. doi:10.1371/journal.pone.0023277
Funding: This work was supported by the National Institutes of Health (5R01 MH079296) and The Stanley Medical Research Institute (06R-1463). The funders had no role in study design, data collection and analysis, decision to publish, or preparation
Competing Interests: The authors have declared that no competing interests exist.