Controversial Effects of Diverse Types of Toxoplasma gondii on the Anxiety-like Behavior and Cognitive Impairments in the Animal Model of Alzheimer's Disease

Toxoplasma gondii is a neurotropic parasite with lifelong persistence in the host brain. Many researchers suggested toxoplasmosis as a risk factor for the development of Alzheimer's disease (AD); however, the link between them has not been fully elucidated.

The present study was designed to investigate the effects of chronic toxoplasmosis infection with Types I (RH), II (PRU), and III (VEG) strains alone and in combination on cognitive impairments in Alzheimer's rat model.

Seven months after the inoculation of the strains, AD was induced bilaterally in rats by injecting human amyloid beta 1-42 (Aβ1-42) peptide into the brain hippocampus. Behavioral tests, including the elevated plus maze (EPM) and Morris water maze (MWM) were conducted 10 days after the AD induction.

Our findings showed that chronic infection with RH strain increased anxiety-like behavior in the Alzheimer's rats in the EPM. In agreement with EPM findings, rats infected with the RH strain exacerbated spatial learning disorders in the MWM test; however, it did not affect the spatial memory. Conversely, infection with the PRU strain significantly enhanced spatial learning without being able to improve memory impairments in the Alzheimer's rat model. Improvement in spatial learning and memory impairments were also observed in rats infected with PRU and VEG strains in combination.

Taken together, our findings suggest that chronic infection with PRU strain, as well as PRU and VEG strains in combination, can significantly improve cognitive deficits induced by Aβ1-42 in Alzheimer's rats, while RH strain plays a detrimental role in AD pathogenesis.