Introduction
The persistence and control of rodent host by Toxoplasma gondii is attributed to brain encystment by trachyzoites derived from the definitive host. Its proliferation into latent brain cysts (bradyzoites) in rodents limits the host’s physiological functioning and induces behavioural changes that favour felid-vectored transmission (Gatkowska et al. 2012; Webster 2001). Mice response to parasite-mediated behavioural change has been known to differ between strains (Holland & Cox 2001; Saeij et al. 2005). This paper compares the responses of Balb/c and c57 strains to T. gondii infection on different behavioural tests.
T. gondii Behavioural Tests
Open Field Test
Open-field (OF) tests measure exploratory, locomotor, and compulsive behaviour of mice models (Havlícek et al. 2001). An intraperitoneal administration of T. gondii cysts (20 per rodent) was found to cause a significant reduction in the exploratory behaviour (F = 69.77, p<0.001), rearing, self-grooming, and locomotor activity of chronically infected male C57BL mice (Gatkowska et al. 2012). In contrast, Balb/c infected with two T. gondii strains exhibited a limited aversion to cat urine smeared to a section of an enclosure compared to healthy controls (Webster & McConkey 2010). Increased parasite cysts in hippocampus of infected Balb/c strain could explain the suppressed conditioned aversion displayed by the infected mice (da Silva & Langoni 2009; Johnson, Suzuki & Mack 2002). In this regard, infected Balb/c and C57BL mouse strains show diminished activity in OF tests. However, Balb/c mice also show reduced feline aversion and no place preference in enclosures.
Elevated Plus Maze
T. gondii-infected mice display diminished short-term memory in learning the elevated Plus Maze (PM) (Korte & De Boer 2003; Wang et al. 2012). In one experiment, Balb/c strain inoculated with 10 cysts showed impaired short-term memory up to 140 days post-infection compared to uninfected controls (Carola et al. 2002). The study found a significant difference (p<0.05) between T. gondii-infected Balb/c and non-infected ones at 40dpi (Carola et al. 2002). Anxiety-related behaviours in the PM differ between strains. O’leary, Gunn, and Brown (2013) found significant strain differences between C57BL/6 and Balb/c mice in the duration spent in the illuminated region of the PM. Overall, C57BL/6 spent more time in the lighted section, indicating that it was more anxious than Balb/c mice. PM measures trait anxiety typical of a specific strain (Blanchard, Griebel & Blanchard 2003). The elevated anxiety of the C57BL/6 strain limits its free-exploratory behaviour in PM tests.
Passive Avoidance
T. gondii-infected mice show a significantly higher number of cysts than non-infected ones (Wang et al. 2010). They display impaired memory, which is modulates passive avoidance. A study by Okvak, Nevalainen, and Pokk (2013) evaluated passive avoidance in 40 male Balb/c mice that received intraperitoneal inoculums of 10 T. gondii cysts. The mice showed reduced passive avoidance of darkness. In addition, Balb/c mice have been shown to display suppressed aversion to cat scent when infected with T. gondii (Queiroz et al. 2013). In contrast, C57BL/6 mice infected with 10 T. gondii cysts exhibited high passive-avoidance learning by spending more time in light than in the dark. As Webste and McConkey (2010) explain, since rodents innately prefer dark areas to well-lit zones, they tend avoid the aversive stimulus (darkness) if they remember it. Thus, infected C57BL/6 mice’s memory allows them to avoid the aversive stimulus (Lindova, Novotna & Havlícek 2006) or anxiety-causing objects (Nasello et al. 2008). In contrast, infected Balb/c mice have no memory of the stimulus, which explains its suppressed aversion or passive avoidance.
Novel Object T-Maze
The aim of this behavioural test is to evaluate the performance of the mice’s working memory. Thirteen male C57BL/6 mice injected with 10 cysts of T. gondii intraperitoneally showed less novel object discrimination than the controls as exhibited by the speed at which the mice ran to the T-maze (Heyser & Chemero 2012). The suppressed sensitization could be attributed to the abnormal dopamine neurotransmission caused by T. gondii brain encystment (Prandovszky et al. 2011; Skallova et al. 2006).
Improved sensorimotor capability was observed in male Balb/c mice infected with 20 T. gondii cysts through intraperitoneal method (Webster 2007). The mice displayed prolonged novel object exploration through climbing, touching, and gnawing. Thus, the Balb/c mice show a rapid habituation with novel objects compared to C57 mice. However, repeated exposure to the object reduces the play-like behaviours of the Balb/c mice (Haroon et al. 2012). Thus, the novelty-induced behaviours in Balb/c mice are greater in Balb/c than in C57 mice, making them useful models for studying exploratory behaviours in mice.
Marble Burying Social Approach
Marble burying is an anxiety-related behaviour. The repetitive digging response is a defensive burying trait that is genetically determined. Repetitive burying has been shown to decrease in mice receiving serotonin inhibitors (Hrda et al. 2000). The compulsive-like behaviour is suppressed in toxoplasmosis-infected mice. An introperitoneal dose of 20 cysts significantly decreased the compulsive-behaviour, including marble burying, in male C57BL mice (Gatkowska et al. 2012). Similarly, Balb/c mice showed diminished novelty burying behaviour but prolonged object exploration after intraperitoneal infection with 20 cysts (Webster & McConkey 2010). Therefore, marble burying behaviour varies between the two strains and is a reflection of the innate digging trait in mice. It differs from exploratory activity, as it is a repetitive response.
Feline Attraction Test
Mice have an innate aversion to the scent of feline predators, including cat odours. However, T. gondii-infected mice display no aversive behaviour or avoidance of cat odour (Kaushik, Knowles & Webster 2014). Instead, they tend to be attracted to cat odour, a phenomenon called fatal feline attraction (Vyas et al. 2007). Infected male Balb/c mice showed no aversion to bobcat smeared in a section of a cage compared to the controls (Ingram et al., 2013). da Silva and Langoni (2009) explain that hippocampal dysfunction due to T. gondii encystment suppresses the infected mice’s innate aversion to cats. In particular, research associates increased toxoplasmosis-related lesions in the amygdala, a brain area that controls a ‘fight-or-flight response’ in animals, with the infected mice’s attraction to feline odour (Berdoy et al. 2000; Berenreiterova et al. 2011; McConkey et al. 2013).
C57 mice infected with T. gondii also show attraction to feline odour. Inbred C57 mice were found to prefer feline odour zones to regions with rabbit (non-predator) scents (Berdoy, Webster & Macdonald 2000; Cox & Holland 2001; Vyas & Sapolsky 2010). In this respect, the limited aversion to feline odours indicates amygdalar dysfunction leading to impaired emotions, such as fear and anxiety. Moreover, the impairment of hippocampal regions involved in memory contributes to the absence of aversion to cats by infected mice. A summary of the parameters used in the experiments is given in table 1 below.
Table 1: A summary of experimental parameters.
Conclusion
Balb/c and C57BL mice genotypes display subtle differences in behaviour when infected with T. gondii. Research evidence shows that T. gondii encystment of the brain reduces exploratory behaviour, emotional responses, and short-term memory of both subtypes, affecting their performance in various behavioural tests. Overall, Balb/c mice are better for use in T. gondii experiments than C57BL mice because it shows low feline aversion and passive avoidance and novelty-induced burying behaviours. These attributes are consistent with the neurological impairments associated with toxoplasmosis.
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