Loss of predator aversion in female rats after Toxoplasma gondii infection is not dependent on ovarian steroids
Introduction
Male rats infected with protozoan parasite Toxoplasma gondii (henceforth Toxoplasma) exhibit reduced aversion to cat odor (Berdoy et al., 2000, Vyas et al., 2007). This behavioral change is presumed to reflect a parasitic manipulation because cats are definitive host for the parasite. It remains undetermined if the infection indeed leads to increased predation by cats under naturalistic circumstances (Worth et al., 2014). Meanwhile neuroendocrine mechanisms of the host behavioral change have been extensively studied in male laboratory rats (Hari Dass and Vyas, 2014b, House et al., 2011, Lim et al., 2013, Vyas, 2013). Several hypotheses have been proposed to account for loss of aversion to predator odor in the infected males (Gaskell et al., 2009, McConkey et al., 2013, Prandovszky et al., 2011, Vyas, 2015b). The most parsimonious of the three posits that toxoplasma infection in male rats results in an increase in testicular testosterone synthesis (Lim et al., 2013, Vyas, 2015a). The testosterone then crosses the blood-brain barrier and causes hypomethylation of the arginine vasopressin gene within the posterodorsal medial amygdala (Auger et al., 2011). This subsequently leads to increased arginine vasopressin production which then drives the behavioral change (Hari Dass and Vyas, 2014a, Hari Dass and Vyas, 2014b). This narrative has a significant limitation in that the proposed mechanism includes nodes that are sexually dimorphic. Testosterone is a predominantly male reproductive hormone that is produced in relatively smaller quantities in females (Goymann and Wingfield, 2014). Similarly medial amygdala arginine vasopressin is testosterone dependent for its transcription (Bluthe et al., 1990, Dantzer and Bluthe, 1991, Dantzer et al., 1988). This suggests that either the behavioral change is absent in females; or that it is mediated by analogous mechanisms involving steroids synthesized by ovaries. The later assumption is plausible because much of testosterone in males gets locally aromatized to estrogen after entry into the brain. We investigated these possibilities.
Section snippets
Animals and infection
Female rats of Wistar Hans strain were used, procured from InVivos Singapore. Animals were 7–8 weeks at start of experiments. A type 2 Toxoplasma strain, Prugniaud, was used for the infection at dose of 5 million lab-grown tachyzoites per animals (i.p.). Corresponding control animals were injected with sterile buffered saline. Routine management of animals and parasites was similar to earlier studies (Hari Dass and Vyas, 2014b). All experimental procedures were reviewed and approved by local
Results and discussion
We randomly divided 24 adult female rats in two experimental groups. Each group received either 5 × 106 Toxoplasma tachyzoites (Prugniaud strain) suspended in 500 μl phosphate buffered saline or buffered saline alone (i.p.). Infection did not result in sickness behavior or decrease in weight gain during the experimentation. Aversion to cat odor was quantified after establishment of chronic infection (>7 weeks, confirmed by presence of anti-Toxoplasma IgG in blood). Toxoplasma-infected female rats
Acknowledgments
This work was financially supported by Ministry of Education, Singapore (grant RG136/15).
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