The role of parasite-produced dopamine in Toxoplasma gondii-altered host behaviour.

Certain parasites can manipulate host behaviour for their own benefit, but the mechanisms remain largely unknown. Toxoplasma gondii, the agent of the toxoplasmosis, is a canonical example, altering behaviour in rodents and other hosts, including humans. Dopamine dysregulation has been suggested as a mechanism, with parasite-encoded tyrosine hydroxylases (TgTH) proposed as a direct source of dopamine, though their role is debated. Here, using Rattus norvegicus as a model, with subtle and specific behavioural and biostatistical assays and analyses, we examine the contribution of TgTH to behavioural change. Two engineered T. gondii Prugniaud lines with moderate and high TgTH overexpression (OE) are compared to wild-type and recombinant wild-type parasites, alongside uninfected controls. All genetically modified lines induce weaker behavioural changes than true wild-type, but changes correlate with TgTH expression levels. Our findings provide empirical support that TgTH contributes to T. gondii-associated behavioural alterations, highlighting both theoretical significance and applied implications.