Toxoplasma gondii infection and behaviour - location, location, location?

Abstract

Parasite location has been proposed as an important factor in the behavioural changes observed in rodents infected with the protozoan Toxoplasma gondii. During the chronic stages of infection, encysted parasites are found in the brain but it remains unclear whether the parasite has tropism for specific brain regions. Parasite tissue cysts are found in all brain areas with some, but not all, prior studies reporting higher numbers located in the amygdala and frontal cortex. A stochastic process of parasite location does not, however, seem to explain the distinct and often subtle changes observed in rodent behaviour. One factor that could contribute to the specific changes is increased dopamine production by T. gondii. Recently, it was found that cells encysted with parasites in the brains of experimentally infected rodents have high levels of dopamine and that the parasite encodes a tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of this neurotransmitter. A mechanism is proposed that could explain the behaviour changes due to parasite regulation of dopamine. This could have important implications for T. gondii infections in humans.

Fig. 1.

Bradyzoite (cyst) stage Toxoplasma gondii-infected cells and dopamine. (A) Rat primary cortical neuron cultures (11 days old) containing GFP-tagged T. gondii bradyzoites (4 days old) viewed by (from left to right) composite, light and fluorescence microscope. (B) Dopamine antibody staining (green, fluorescein secondary antibody) of T. gondii tissue cysts (red, lectin-TRITC) in mouse brain (various regions). The two images on the right are 3D projections of a Z-stack reconstruction of serial images.