Objective: To detect the learning and memory ability in mice model of latent Toxoplasma gondii infection with object recognition test and Morris water maze test.
Methods: Thirty-six Kunming mice were divided into control group, infection group with 6 cysts each mouse (low infection group), and infection group with 12 cysts each mouse (high infection group) averaged. Mice in the two infection groups were orally infected with T. gondii Prugniaud (PRU) low virulence strain. Object recognition test was conducted at the 63rd day after infection. After the first day of adaptation and the second day of familiarization in the test, the time expended on exploring new and familiar objects was recorded on the third day and the discrimination index (DI) was calculated. Morris water maze test was conducted at the 66th day. The ability of spatial learning, spatial memory retention and working memory capacity was evaluated by place navigation test, spatial probe test, and working memory test, respectively. The mice were sacrificed at the 74th day after infection. The left cerebral hemisphere of mice was fixed, sliced, and stained with eosin-hematoxylin for pathological examination. The right hemisphere was used to detect the activity of superoxide dismutase (SOD) and malondialdehyde (MDA) content.
Results: The results of object recognition test showed that the discrimination index of high infection group and low infection group was (14.3 +/- 5.2)% and (17.5 +/- 5.6)%, respectively, significantly lower than the control [(28.9 +/- 7.1)%] (P < 0.01). In the place navigation test, the latency to find the platform in the two infection groups was longer than the control, with significant difference on the second and third day (P<0.05). In the spatial probe test, the percentage of the distance across the platform quadrant in the total swimming distance of high infection group and low infection group were (19.9 +/- 5.0)% and (23.9 +/- 6.8)%, respectively, significantly lower than the control [(27.4 +/- 3.6)%] (P < 0.05). In the working memory test, at the fourth day of test the latency of high infection group and low infection group [(365 +/- 14.2) s and (35.3 +/- 13.7) s] was significantly longer than the control [(30.4 +/- 12.5) s] (P<0.05). In all the tests, there was no statistical significance between low infection group and high infection group (P > 0.05). The brain sections of two infection groups showed cysts of T. gondii, proliferation of glial cells, widened gap around small blood vessels, and a phenomenon of "vascular cuff". The activity of SOD in the mice brains of two infection groups was significantly lower than the control, while MDA level was significantly higher (P < 0.05). SOD and MDA showed no significant difference between two infection groups (P > 0.05).
Conclusion: Latent infection of T. gondii may lead to learning and memory impairment in mice.