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

Gabrielly Lisboa Da Silva Soares,Felipe Tuji De Castro Franco,Naiana De Paula Tavares,Ellen Rose Leandro Ponce De Leão,Luma Cristina Ferreira Guerreiro,Ana Karyssa Mendes Anaissi,Daniel C Anthony,Rafaela Dos Anjos Pinheiro Bogoevich Morais,Maria Vitória Nascimento Costa,Sanderson Corrêa Araújo,Gabriel Castro De Menezes,Alexa Camila Lopes De Assis,Cristovam Wanderley Picanço Diniz,Sinara Franco Freitas,Daniel Guerreiro Diniz,Raissa Maria Carvalho Alves,Jhonnathan Henrique Palheta De Oliveira,Ediclei Lima Do Carmo,José Antônio Picanço Diniz ,Heloísa Marceliano Nunes ,Sâmia Demachki

doi:10.3389/fcimb.2022.812152

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.

Highlights

Toxoplasma gondii is an intracellular parasite that can infect most warm-blooded animals, where it may invade the central nervous system and provoke neuroinflammation and behavioral changes
Using ocular conjunctival instillation of T. gondii tachyzoites, we extended previous observations by demonstrating that the infection of BALB/c mice with T. gondii alters the way in which the animals explore new environments and assess risk; these changes were associated with tachyzoite differentiation into bradyzoites in-enclosed cysts within the brain and retina and with an altered microglial response in the molecular layer of the dentate gyrus
Because the olfactory discrimination test suggested that infected mice spent a greater amount of time in the feline’s odor compartment, we reasoned that increased risk assessment for predation unlikely to account for the open-field results

Summary

Introduction

Toxoplasma gondii is an intracellular parasite that can infect most warm-blooded animals, where it may invade the central nervous system and provoke neuroinflammation and behavioral changes (for recent reviews, see Mendez and Koshy, 2017; Martinez et al, 2018; Chen et al, 2019; de Haan et al, 2021; Nasuhidehnavi and Yap, 2021; Nayeri et al, 2021; Postolache et al, 2021). Boillat and colleagues demonstrated that T. gondii lowers general anxiety in infected mice and that the lack of aversion of mice infected with T. gondii extends to predators other than felids (Boillat et al, 2020). They have shown that cyst load in the host is associated with transcriptional changes in markers of inflammation (Boillat et al, 2020). Other studies suggest that the behavioral changes might be associated with impaired long-term fear memory consolidation through dysfunctional cortical and amygdaloid circuits of infected mice (Ihara et al, 2016). Following T. gondii infections, the molecular layer of the dentate gyrus (MolDG) may be a specific target (van Groen et al, 2003) leading to memory dysfunction (Berenreiterová et al, 2011)

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