Aug 19, 2011 09:01 GMT  ·  By
Individual Toxoplasma parasites (green) are shown invading neurons (red) grown in a petri dish in the lab
   Individual Toxoplasma parasites (green) are shown invading neurons (red) grown in a petri dish in the lab

The parasite Toxoplasma is dead serious when it comes to fulfilling its reproductive cycle. Fortunately for it, the death part applies to rats it infects in order to achieve its purpose. Infected rodents lose their fear response when exposed to cat urine, which makes them easy targets for the felines.

Whenever rats stumble across territory that has been marked by a cat, their brains start exhibiting activity in areas associated with controlling the flight of fight response. Usually, the rodents flee.

However, when Toxoplasma infects the rat brain, it forces the latter to exhibit increased levels of activity in an area that studies have associated with triggering a mating response in a male rat when it meets a female rat.

What this means is not that male rats suddenly start to view cats as attractive prospective partners, but that they lose their fear response when exposed to the felines' scent. Needless to say, this is deadly for them. However, it is extremely beneficial for the microbe.

The reason why it causes these effects is simple – Toxoplasma reproduces happily in the intestines of cats. But these creatures are not as curious as rats are, in terms of tasting or trying to eat everything they come across.

As such, the parasite took the scenic route in ensuring it infects cats. All the effects it causes in the rat brain are nothing but a scheme designed to help them make their way into a cat. This explanation is what experts call the “manipulation hypothesis.”

“Well, we see activity in the pathway that normally controls how male rats respond to female rats, so it's possible the behavior we are seeing in response to cat urine is sexual attraction behavior, but we don't know that,” researcher Patrick House says.

“I would not say that they are definitively attracted, but they are certainly less afraid. Regardless, seeing activity in the attraction pathway is bizarre,” adds the investigator, who is a PhD candidate in neuroscience at the Stanford University School of Medicine.

House is also the lead author of a new paper detailing the findings, which is published in the August 17 issue of the peer-reviewed scientific journal Public Library of Science (PLoS) ONE. The PhD student carried out the work in the lab of Stanford professor of biologym neurology and neurological sciences Robert Sapolsky.

The group explains that Toxoplasma produces cysts in the rat brain that – through some yet-unidentified mechanism – cause neural activity when pathways associated with fear should be activated. The activity manifests itself in the other region of the brain, which makes the rat lose fear.

“Toxoplasma is altering these circuits in the amygdala, muddling fear and attraction,” House concludes. Funds for the new research were provided by the US National Institutes of Health (NIH) and the Stanley Medical Research Institute.