We're Alive! Let's Kiss! Toxo Love Party

You know that scene in a movie when the hero and heroine dive out of the way of gunfire, or outrun an explosion, or in other way avoid imminent demise. They end up in each other's arms, a tender smile, then the kiss. Of course it's all neuroscience and pheromones at that point.

Maurice Jarre, Lawrence of Arabia

Sex and fear live in an area rather deep in the brain called the amygdala. (This name led to no end of snickering among us medical students when Star Wars Episode 1 came out with the character of Padme Amidala. Nerds.)

From Wikimedia Commons

Fire up the amygdala, and you can be more inhibited and fearful. Repress it, and you might find yourself on one of those Girls Gone Wild videos. If you remove the amygdala, animals will get hypersexual, fearless, and hyperoral "in which inappropriate objects are placed in the mouth."(Don't be afraid to follow that link. It's just to the wikipedia article.)

Depressed folks can show increased activity in the amygdala (which could explain the decreased sexual drive and increased paranoia and fearfulness in some depressed individuals). On the opposite spectrum, say you've just survived a fearful situation, and an over activated amygdala is suddenly released? We're alive! Let's get busy, baby.

Okay, what does all this sex and fear have to do with a Toxoplasma love party? An interesting paper came out last month called "Toxoplasma on the brain: understanding host-pathogen interactions in chronic CNS infections." That title might not make you think about sex right off the bat, but the paper is a nice review and has all sorts of titillating information.

Toxoplasma gondii is a little parasite that currently infects more than 1/3 of the world's human population and is often caused by ingestion of cat feces. Alarmingly, the little infectious toxo cysts can persist for a year in the environment and be passed along through contaminated food and water supplies, or through eating infected animals.

Flickr Creative Commons

Once you are infected, the rapidly spreading "tachzoite" stage is followed by a more or less chronic "bradyzoite" stage where slow-growing cysts can form in your brain. These cysts are quite sturdy enough to survive your gut and also protect the parasite from your immune system and from drug treatments. Toxo infections are often discovered incidentally via MRI, or when you have immune compromise (such as HIV infection), or if the cysts grow enough to cause seizures. Chronic inflammation caused by the presence of the infection (or the infection itself) may lead to behavioral and mood problems in humans (see the posts linked below for more details).

Franz Schubert Serenade

Where, then, is the love party? Well, at least in rats, toxo cysts seem to preferentially take up residence in the amygdala. This location will tend to make the rats less fearful and more sexual. In fact, some studies have shown that the infection makes rats more sexually attracted to cats, which sounds like a doomed relationship if I've ever heard one. Cats, eating the infected rats, will pass on the infection to everyone else. Toxo party! It's unclear if the infection similarly causes loosening of inhibitions in humans.  (Pretty sure it's the alcohol that causes those Girls Gone Wild videos, but an observational study checking for incidence of toxo infection in participants might be interesting.)

One final interesting snippet. Toxo (either by the host inflammatory response or by direct release) seems in increase the amount of dopamine in the brain. This finding may also explain why toxo infection is linked with schizophrenia, disinhibition, and paranoia. (Haloperidol, an antipsychotic medication, and valproate, a mood stablizer, have been shown to storngly inhibit the growth of toxo in vitro, but not in vivo.) Testosterone may enhance the growth of toxo, which may be part of why men are more vulnerable to schizophrenia than women. A lot of speculation, really, but very interesting. How much of our behavior is caused or influenced by the little beasts that live within us?

Other toxo-related posts on Evolutionary Psychiatry

Toxo and Suicide in Women
Schizophrenia and Infection
Depression, a Deal With the Devil?

* Roger Ebert is where I first heard the "We're Alive, Let's Kiss" phrase applied to movies.