PLDT (planarian light/dark test): an invertebrate assay to quantify defensive responding and study anxiety-like effects.

BACKGROUND: Planarians, like rodents, instinctively spend more time in dark versus light environments when given a choice. This behavioral phenomenon is called negative phototaxis, which may reflect defensive responding related to an anxiety-like phenotype.

NEW METHOD: We propose a planarian light/dark test, designated PLDT, to predict anxiogenic- or anxiolytic-like effects. Experimentally, we placed a planarian at the midline of a Petri dish, containing test compound or water, that was split evenly into light and dark compartments and determined time spent in the light over 10min.

RESULTS: A clinically-approved benzodiazepine agonist (clorazepate; 10μM) increased time spent in the light whereas an inverse benzodiazepine agonist (FG-7142; 1, 10μM) produced the opposite response. Fluoxetine (1μM) or ethanol (1%), as well as the 'bath salt' cathinone S-mephedrone (300μM), enhanced time spent in the light. Planarians exposed to predator (frog) odor spent more time in the dark.

COMPARISON WITH EXISTING METHODS: The light/dark box (LDB) test in rodents is used to screen putative medications for possible anxiolytic and anxiogenic effects. Our results showing that time spent in the light by planarians is enhanced by common anxiety-relieving drugs (e.g. benzodiazepine agonist, ethanol, fluoxetine) and decreased by anxiogenic substances (e.g. predator odor, benzodiazepine inverse agonist) reveal directionally similar effects in the established (LDB) and new (PLDT) assays.

CONCLUSION: Our data identify the PLDT as a cost-effective, invertebrate assay for quantifying the effects of practically any water-soluble substance on defensive responding and for studying and teaching anxiety-like responses in a living organism.