Friday, September 7, 2012

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.


  1. Enjoyed the post.

    Cats have very definitely not had a good week - at least here in the UK as per the BBC headlines: 'Infection risk posed by cats revealed' -
    and a report from the Food Standards Agency on the gondii:

    Expect Garfield to plummet down the popularity ratings....

  2. This comment has been removed by the author.

  3. According to available information, the most common source of toxoplasma infection is from improperly cooked meat from infected animals.

    1. I don't recommend eating undercooked meat.

    2. LOL! That's not very paleo of you. I just couldn't help myself now. (It's not the toxo though or maybe it is. Probably too late for spiramycin. damn.) Or madcow disease from eating raw beef in England back in the early 80s... who knew?

  4. You might like this:

    The study included 266 men and women between the ages of 60 and 64 enrolled in the PATH Through Life Study, which is a longitudinal study of aging. Fasting plasma glucose and other factors were measured upon enrollment, and those with glucose levels of 6.1 micromoles per liter (110 mg/dL, classified as impaired fasting hyperglycemia by the World Health Organization) or higher were excluded. Magnetic resonance imaging (MRI) scans of the brain were conducted at the beginning of the study and four years later.

    Fasting plasma glucose levels ranged from 3.2 to 6.0 micromoles per liter (57.6 to 108 mg/dL) upon enrollment. Dr Cherbuin and his colleagues uncovered a significant association between a decline in volume in the brain's hippocampus and amygdala (cerebral structures affected by aging and neurodegeneration) and higher plasma glucose levels within this nondiabetic population. "Plasma glucose levels were found to be significantly associated with hippocampal and amygdalar atrophy and accounted for 6%–10% in volume change after controlling for age, sex, body mass index, hypertension, alcohol, and smoking," Dr Cherbuin and his associates write. "These findings suggest that even in the subclinical range and in the absence of diabetes, monitoring and management of plasma glucose levels could have an impact on cerebral health."

    When asked if there is a specific level at which plasma glucose would begin to be considered risky, Dr Cherbuin told Life Extension: "We found that the top two highest quarters (or quartiles) had significantly more shrinkage than the lowest one which might suggest a tentative cut-off of 5.1 [micromoles per liter]. However because overall the association with hippocampal atrophy across all glucose levels was relatively uniform, more evidence was needed to point to a specific cut-off."

    1. Thanks! Actually I have the PDF fired up on my monitor as the next study to write about :-)

    2. Enjoyed the post Emily. In practice I work with DiagnosTechs GI panel which has the SIgA antibody test for Toxoplasma gondii. You stated in the post that the parasite is protected from the immune system due to its encapsulation. Did you come across any information that would point to this parasites ability to mask itself from the immune system enough to avoid antibody production or if it just able to survive the immune response? Treatment for toxoplasmosis is usually only provided to patients with suppressed immune systems (or severe Sx) as it seems a competent one at least keeps the infection latent. This would be one of the countless reasons to maintain overall health of the body & psyche. Come across any research showing a healthy immune response being able to completely eradicate this bug?

  5. Emily,

    Thank you so much for your wonderful blog. I am social worker and actually impressed my supervisor with your posts on violence. Don't leave out the social workers:). We like to affect change too...

  6. Very interesting. I remember Robert Sapolsky saying that Toxo actually possesses a copy of the Tyrosine Hydroxylase gene, so it is able to increase dopamine via enzyme activity--however that works.

    This info brings a question to mind tho..
    If Toxo infection creates an excess dopamine situation and at the same time this REDUCES fear behavior, why does the MET158 Allele of the COMT gene raise dopamine and INCREASE fear behavior in humans at the same time?
    Is it the structure of the brain where dopamine is increased that counts?
    Or are the studies mixing causation and correlation?
    OR is it even further above my head-> receptor up/down-regulation?

    Where's my aspirin..


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