Behavioral and Neuropathological Changes After Toxoplasma gondii Ocular Conjunctival Infection in BALB/c Mice

Abstract

Ocular infection with Toxoplasma gondii causes toxoplasmosis in mice. However, following ocular infection with tachyzoites, the cause of the accompanying progressive changes in hippocampal-dependent tasks, and their relationship with the morphology and number of microglia, is less well understood. Here, in 6-month-old, female BALB/c mice, 5 μl of a suspension containing 48.5 × 106 tachyzoites/ml was introduced into the conjunctival sac; control received an equal volume of saline. Before and after instillation, all mice were subject to an olfactory discrimination (OD) test, using predator (cat) feces, and to an open-field (OF) task. After the behavioral tests, the animals were culled at either 22 or 44 days post-instillation (dpi), and the brains and retinas were dissected and processed for immunohistochemistry. The total number of Iba-1-immunolabeled microglia in the molecular layer of the dentate gyrus was estimated, and three-dimensional reconstructions of the cells were evaluated. Immobility was increased in the infected group at 12, 22, and 43 dpi, but the greatest immobility was observed at 22 dpi and was associated with reduced line crossing in the OF and distance traveled. In the OD test, infected animals spent more time in the compartment with feline fecal material at 14 and at 43 dpi. No OD changes were observed in the control group. The number of microglia was increased at 22 dpi but returned to control levels by 44 dpi. These changes were associated with the differentiation of T. gondii tachyzoites into bradyzoite-enclosed cysts within the brain and retina. Thus, infection of mice with T. gondii alters exploratory behavior, gives rise to a loss in predator’s odor avoidance from 2 weeks after infection, increased microglia number, and altered their morphology in the molecular layer of the dentate gyrus.

Chronic infection by atypical Toxoplasma gondii strain induces disturbance in microglia population and altered behaviour in mice

“…In our study of T. gondii chronic infection, it was found a dramatic impact on microglia population in mice infected with either ME49 or CK2 strains, when compared to non-infected mice. A recent study highlighted the ability of T. gondii infection to induce the increase of microglia on the eyes of BALB/c mice after instillation of tachyzoites in the conjunctival sac ( Soares et al, 2022 ). However, the increased microglia population was identified based only in the immunohistochemical staining for Iba-1, which does not allow a real differentiation among microglia and infiltrated inflammatory monocytes ( Amici et al, 2017 ; Jurga et al, 2020 ).…”

Contrasting Disease Progression, Microglia Reactivity, Tolerance, and Resistance to <em>Toxoplasma gondii</em> Infection in Two Mouse Strains

“…Microglia respond to T. gondii infection by producing immune mediators and interacting with lymphocytes, contributing to acquired immunity [48]. Moreover, chronic T. gondii infection elicits morphological and numerical changes in microglia, associated with altered behavioral responses [49].…”

“…However, the effect size for BALB/c was approximately half of what was observed in albino Swiss mice. For a more detailed analysis of the numerical changes in microglia within the BALB/c lineage, please refer to the study published elsewhere [49].…”

“…For each mouse, we used the contrast index to assess the relative occupation of the three compartments in the olfactory test and the periphery vs central areas in the open field task [49][50]. This index provides a valuable method for normalizing the data.…”

“…-Olfactory Discrimination Test Following the previous protocol, the olfactory discrimination (O.D.) test was conducted within a three-compartmental box [49]. The experiment involved placing mice in a test apparatus consisting of three interconnected compartments.…”