Effect of different illumination levels on rat behavior in the elevated plus-maze.

The present study addressed the role of environmental light intensity on the exploratory behavior of rats in the elevated plus-maze, with the specific goal of determining the light intensity threshold for triggering the aversion to the open arms. Male Wistar-derived rats were tested in the elevated plus-maze under different illumination levels: 0, 1, 3, 10, 30, 100 and 300 lx. Exploratory behavior occurring in the open arms (e.g., entries and time spent in these arms) was more intense under 0 and 1 lx than under the other illumination levels, which did not differ among themselves; on the other hand, locomotor behavior (as indicated by frequency of entries and distance run in the closed arms) was not altered under all illumination conditions. The data indicated that vision is important in triggering aversion to the open arms of the elevated plus-maze. They also indicated that the threshold of such aversion was found between 1 and 3 lx environmental illumination and that the phenomena is not intensity-dependent but rather of an all-or-none type. It should be emphasized that these conclusions only stand for unfamiliar environments. The role of light in familiar environments is currently under investigation in our laboratory.