Parasite/Host Interaction and the Neurobiology of Fear

The behavioral manipulation hypothesis posits that parasites can change the behavior of hosts to increase the reproductive fitness of the parasite. The protozoan parasite Toxoplasma gondii fits this description well. Toxoplasma shuttles between an asexual life-cycle in rodents and a sexual life-cycle in cat intestine. In rodents, parasites preferentially localize to the brain. The parasite has evolved the capacity to, once having formed cysts in the brain, abolish the innate fear that rodents have of the odors of cats, and to convert that fear into an attraction; this increases the likelihood of the rodent being predated, thereby completing the parasite's life cycle. <P> Our prior work has shown the robustness of this behavioral phenomenon, the preferential formation of Toxoplasma cysts in the amygdala, and the capacity of the parasite to reduce cell number and dendrite length in the basolateral amygdala. The present grant further explores the neurobiology of Toxoplasma's behavioral effects, in order to better understand both normal and pathological fear. <P> Specific Aim 1 identifies brain regions that are candidates for mediating the behavioral effects of Toxoplasma. A time course study will indicate brain regions where Toxoplasma cysts form prior to the first emergence of the behavioral effects of the parasite. In addition, we will construct a map of brain regions activated in response to cat odor in control rats (using cFos expression as a marker), and then determine whether Toxoplasma blocks such activation in any of those regions. The candidate brain regions identified in this Specific Aim will be studied in the subsequent Aims. <P> Specific Aim 2 studies whether Toxoplasma causes inflammation in local brain regions and whether that contributes to the behavioral syndrome. It will also investigate whether the parasite changes the number of cells (both neurons and glia) in different brain regions, and/or changes the complexity of dendritic processes in neurons. Finally, it will examine whether Toxoplasma blocks the effects of cat odor on extracellular concentrations of glutamate, GABA and dopamine in those candidate brain regions. <P> Specific Aim 3 will explore the reductive bases of Toxoplasma's actions. The first part will be an analysis of protein expression profiles in brain regions implicated in the parasite's behavioral effects, and in the parasite as well. The second will be a conditioned medium experiment, determining whether the medium in which cultured Toxoplasma grows can have the same effects in the brain as the parasite itself. This will be a first step in determining whether a factor secreted by the parasite mediates the behavioral effects. <P>PUBLIC HEALTH RELEVANCE Experiments proposed in this grant explore an unlikely intersection of parasitology and neurobiology, that is, loss of innate fear in rodents due to Toxoplasma infection. Pathological fear is central to several psychiatric disorders. Understanding how Toxoplasma abolishes fear will shed light on how fear is generated in the first place and how we can manage pathological fear.