Monday, May 18, 2009

Parasites and Mental Health

After sitting through three early morning lectures at this year’s Society of Biological Psychiatry conference, I had decided I needed a break and was going to skip the Presidential Lecture (this choice was a little bizarre since I had been looking forward to the talk. Monotony breeds indifference). Shortly after making this decision, I ran into my PI (who was a little surprised to see me at a conference I wasn’t registered for. Come for the food, stay for the learning.), who informed me the next speaker was well worth seeing.

The title of the presentation was, “Does a Parasite Know More about Our Brains than We Do?” The presenter was one Robert Sapolsky, a person whom I had been ashamed of by my PI for not worshipping. But after hearing his lecture, I am motivated to purchase one of his books. For those of you who are familiar with Sapolsky, you know he has a very engaging manner of speaking (at one point, he referred to the frontal cortex as the part of the brain that “keeps you from being a serial murder.” Other references were made to Brittney Spears and the urine of grad students).

I myself became way too excited; I swear I almost ran up to the podium to kiss him, when he began talking about parasitic life cycles and how they have very specific ways of manipulate the host’s behaviour in order to ensure their own propagation. At this point, I had to stop to ask myself, “Is it normal for someone to get turned on by parasites?”

I answer, emphatically, YES. Anything that can survive through such elegantly executed means deserves at least some reverence.

Examples of parasitic manipulated host behaviour outlined by Sapolsky were an explanation of a particular barnacle (sacculina) that attaches itself to crabs and then alters the crabs reproductive anatomy and behaviour. When attached to a male crab, the barnacle will sterilise the crab by injecting it with estrogen, which leads to other female adaptations as well. When attached to a female crab, the barnacle causes ovaries to atrophy. The result in both cases is that the crab will engage in nesting behaviour, but will be unable to lay eggs; the crab digs a nest for the parasite.

(There are many more examples, all very interesting and I encourage you to look some of them up).

The talk segued into the life cycle toxoplasma gondii. As you may well know, toxo lives in cat intestines, and it is only in cat intestines that toxo can reproduce. So when a toxo finds itself outside of a cat, in a rodent who came a little to close to a cat’s feces, it needs to finds its way back inside a cat.

This it does by making the rat, instead of deterred by, attracted specifically to cat pheromones, not dog, rabbit, or human smells. The stress responses of the rat are not affected; it can still be conditioned to fear cats.

Parasite ridden rats are cute and all, but, besides fetal damage, toxo also has some effects on humans. It seems that a toxo infected human is at increased risk for schizophrenia. There is some mild neuropsychiatric disinhibition and a person is 2-4 more times likely to die as a result of speeding while driving. As well, treatment with haloperidol eliminated infection induced behaviour.

The hypothesis for these outcomes is because toxo preferentially goes to the amygdala where it blocks the glucocorticoid response of the brain. Toxo contains two genes for tyrosine hydroxylase which is the rate limiting step in the production of dopamine.

For much more detailed descriptions of toxo induced behaviour, check out some of the references.

Also, here's a video interview with Sapolsky.

References: 1, 2, 3, 4, 5
The lovely life cycle chart came from here.

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