Tuesday, November 8, 2011

Parasite affects brain chemistry to modify behaviour

T. gondii, a parasite found in mammals, is able to alter brain chemistry by increasing the levels of dopamine. This chemical messenger has a lot of functions in the brain, and the increase caused by the parasite correspondingly alters host behaviour. It can cause reduced fear in animals, especially rodents, which causes them to be eaten more frequent than animals who are not infected by of T. gondii. The findings are relevant because the parasite is also found in humans.

T. gondii
British researchers discovered that T. gondii uses the dopamine system in the brain to modify the behaviour of its host. An infection in the dopamine-producing cells causes a severalfold increase in dopamine levels in the brain, which seems the causative factor for the behavioural changes. Effectively, rodents with the increased dopamine levels do not longer fear the typical cat scent, and instead are drawn towards it. It is likely that there is an additional mechanism involved, as there are no other alterations in fears found, though dopamine has a very broad spectrum of actions on the brain.

The parasite's main goal of modifying rodent's behaviour is to let them get eaten by cats, so T. gondii eventually ends up living there. In the cat, sexual reproduction can take place, which explains why evolution favored the behaviour-altering capabilities of the micro-organism. Exposure in humans happens on 'accident', but nevertheless, 10 to 20 percent of the UK population is allegedly infected, and in the US a massive 22 percent of the population is found to be carrier. On average, one in three people worldwide are infected, the highest percentage being found in developing countries that have a lower hygienic standard. Infection can take place through cat faeces.
The life cycle of T. gondii.
It was not previously known how behavioural modification was achieved in rodents, and this mechanism could be relevant to discover the effect of the parasite on humans. Often, a cyst is formed which renders T. gondii unable to assert harmful effects, but in some cases it can cause toxoplasmosis, which can be quite dangerous and even lethal if the patient is immunocompromised. Infection with T. gondii was already associated with schizophrenia-like symptoms and hallucinations, and this recent study provides a biological basis for this link.

Dopamine is used for many functions in the brain, including motor control and the reward system of our brain. Lack of dopamine is found in parkinson's patients, that have reduced numbers of dopamine-producing cells, so-called dopaminergic neurons. Lack of dopamine is also associated with altered human behaviour. Because of the symptoms, it was already suggested that dopamine plays a role in the disease process, and treating patients by limiting dopamine production could prove to be an effective therapy for non-acute infections with T. gondii. 

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