Light-Dark Open Field (LDOF): A novel task for sensitive assessment of anxiety

Background: Pre-clinical studies of psychiatric disorders often include a measure of anxiety-like behavior. Several tasks exist that serve this purpose, but because anxiety is complex with a myriad of anxiogenic stimuli, researchers are often compelled to use multiple tasks. The Light-Dark Open Field (LDOF) combines concepts from two such tasks, Light-Dark Box and Open Field, into one task with the synergistic effect of enhanced discrimination of anxiety-like behavior. New methods: Our goal was to increase the sensitivity of the Open Field task with the addition of a shadow, conceptually similar to the Light-Dark Box, to detect concealed differences even under bright light, which is highly anxiogenic. The resulting LDOF allows assessment of anxiety due to bright light and open space simultaneously, while retaining the ability to assess the impact of each with custom indices. In addition, it maintains all the advantages and measures of the Open Field. Results: Using custom created indices from measures collected in the LDOF one can assess anxiety induced by light, open space, or light and open space combined and thus elucidate anxiety-inducing factors. Using two strains of rats: an outbred strain, Sprague-Dawley (SD), and a strain that exhibits high trait anxiety, Lewis rats, we found that increased discrimination for anxiety-like behavior can be achieved with the Light-Dark Open Field. Comparison with existing models: The LDOF allows researchers to extract the traditional measures of an Open Field, including valuable information about locomotion and habituation while adding a further layer of discrimination with the light-dark component. Because the LDOF is a combination of two different tests, it saves time compared to running multiple experiments in series that then need to be counterbalanced to reduce artefacts and task order effects. In addition, it detects differences even when traditional tasks of anxiety have reached their ceiling sensitivity (i.e. EPM under bright light conditions). Conclusion: We present the Light-Dark Open Field: a simple modification of an existing Open Field apparatus that incorporates aspects of the Light-Dark Box with the addition of a shadow. The shadow (Dark Perimeter) allows for increased discrimination in detecting anxiety-like behaviors. Comparison of anxiety-like behavior between Lewis and SD rats allowed us to develop the construct and face validity of the LDOF as well as demonstrate its sensitivity even under bright light conditions. In addition, we developed 3 indices that allow one to parse out, from one set of data, the effect of two anxiogenic stimuli- bright light and open space.