An approach to evaluate the ability of rats to discriminate different levels of illumination in the plus maze test: Effects of scopolamine

Previous research has shown that the visual system is important for rats to establish the arm preference in the elevated plus maze (EPM), an animal model of anxiety. This study aims at evaluating whether a gradient of illumination between the enclosed arms of the maze (E/EΔLux) could be a reliable approach to detect drugs-induced harmful effect on visual discrimination of rats. Four EPM configurations with different E/EΔLux (8, 41 and 85 lx) were used to demonstrate that as E/EΔLux increases, rats avoid to explore the light enclosed arm, which characterizes the animal ability to discriminate the most illuminated area within the protected environment of the maze. The establishment of either 41 or 85 E/EΔLux failed to alter the traditional spatial-temporal variables in the EPM. In addition, systemic treatment with midazolam (MDZ; 1.0 mg kg−1, a classical anxiolytic) induced anxiolysis in rats tested in 41 and 85 E/EΔLux EPM, with no change in the visual discrimination, when evaluated by the level of light enclosed arm exploration. Systemic treatment with scopolamine (SCP; 1.0, 2.0 and 8.0 mg kg−1), a drug endowed with harmful properties upon the visual system, did not change either the open arm avoidance or the visual discrimination at the low doses, but induced increased light enclosed arm (visual discrimination deficit) and open arm exploration (anxiolytic like effect) at a higher dose. We propose that the incorporation of an E/EΔLux in the EPM may reinforce the predict validity of the test since it enables to evaluate whether a visual discrimination deficit can be confounded with an anxiolytic-like effect, thus establishing a false positive detection.