Combined effects of illumination, closed wall type and extramaze space size on the anxiety-related behavioral baseline of rats submitted to the elevated plus-maze

The elevated plus-maze (EPM) is currently used to study the neural basis of anxiety as well as to assess the anxietymodulating activity of various pharmacological agents. Although the anxiogenic stimulus is supposed to be the open space in this test, the anxiety-related behavioral baseline of rats is sensitive to numerous methodological-linked parameters [1]. In order to improve the quality of behavioral measurements, some modifications have been performed on both original apparatus or procedures initiated by Pellow et al. [2]. Two modifications are often reported: 1) rats are tested under a red light in order to mimic the dark phase of the cycle, and 2) some laboratories used a modified EPM with translucent closed walls to increase the accuracy of the behavioral scoring. At present, the combined effects of these two modifications on spontaneous rat behavior in the EPM have not been sufficiently investigated. Besides, the effect of extramaze space size (i.e. size of the experimental room) on this baseline remains untested. However, an inadequate behavioral baseline may reduce the efficacy of the EPM for measuring anxiety variations in animals. Thus, this study aimed to clarify the respective effects of illumination (ILL), closed-wall type (CWT) and extramaze space size (ESS) on the anxiety-related behavioral baseline of rat in the EPM.