Anxiety disorders are thought to reflect a maladaptive state characterised by an exaggerated fear response, but are poorly understood. To investigate the mechanisms involved in these common disorders, Mitra et al. studied fear reduction in rats infected with Toxoplasma gondii. This protozoan parasite abolishes the fear normally experienced by rodents in response to cat odour to ensure its transmission from rats to cats, where it sexually reproduces. T. gondii infection of rats resulted in dendritic retraction of neurons in the basolateral amygdala, a brain region that regulates emotions, and reduced secretion of corticosterone stress hormones. These findings support the hypothesis that these two components of the fear response form a positive feedback loop that regulates fear. Further studies in this model system could guide the search for new anxiolytic therapies. Page 516
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