Reciprocal moderation by Toxoplasma gondii seropositivity and blood phenylalanine – tyrosine ratio of their associations with trait aggression

We previously reported that trait aggression, proposed as an endophenotype for suicidal behavior, is positively associated with ( ) seropositivity in females, but not in males. Additionally, older males seropositive for had lower scores on measures of trait aggression, including self-aggression. Trait aggression may be influenced by dopaminergic signaling, which is known to be moderated by gender and age, and potentially enhanced in positives through the intrinsic production of dopamine by the microorganism. Therefore, we investigated associations between trait aggression and interactions between enzyme-linked immunoabsorbant assay (ELISA) IgG titer-determined seropositivity and high-performance liquid chromatography- (HPLC-) measured blood levels of dopamine precursors phenylalanine (Phe), tyrosine (Tyr), and their ratio in a sample of 1000 psychiatrically healthy participants. Aggressive traits were assessed using the questionnaire for measuring factors of aggression (FAF), the German version of the Buss-Durkee hostility questionnaire. We found that 1) the decrease in trait aggression scores in positive older males was only present in individuals with a low Phe:Tyr ratio, and 2) that there was a positive correlation between Phe:Tyr ratio and total aggression and selected subscales of aggression in -positive males, but not in -negative males. These findings point toward a gender-specific reciprocal moderation by Phe:Tyr ratio and seropositivity of their associations with aggression scores, and lead to experimental interventions geared to manipulating levels of dopamine precursors in selected positive individuals with increased propensity for aggression.