Clues from planarians about interleukin-17A and stress that result from light avoidance: IL-17A antagonists reduce defensive responding in flatworms.

Emerging evidence links interleukin-17A (IL-17A) to anxiety and stress. Circulating levels of IL-17A are elevated in patients with anxiety disorders, and pharmacological blockade of IL-17 signaling or genetic deletion of IL-17 reduces anxiety-like behaviors in mice. Given that IL-17 is one of the most conserved cytokines among animal phyla, we tested the hypothesis that anti-IL-17 treatments reduce defensive responding in planarians, the simplest animal with bilateral symmetry and a CNS with cephalization. The endpoint selected was light avoidance, which is a common phenotype of planarians and rodents and an index of defensive responding that is reduced by anxiolytic compounds in both species. Planarians were placed at the midline of a Petri dish containing water or test solution that was equally split into light and dark halves. Planarians exposed to a selective IL-17A antibody (0.1, 1, 10 pM) over a 5-min interval spent more time in the light than water-exposed planarians. Cyanidin (0.01, 0.1 1, 10 µM), an anti-inflammatory flavonoid and non-selective IL-17A inhibitor, also increased time spent in the light. Motility was not affected by IL-17A antibody or cyanidin at concentrations that reduced light avoidance, although higher concentrations reduced motility (>10 µM). Our results show that IL-17A antagonists reduce defensive responding in planarians and suggest conservation of IL-17A effects on aspects of anxiety-related behaviors.