Darkness during behavioral tests induces exploratory activity and decreases anxiety-related behavior in male mice

Light is presumed to be highly aversive to laboratory rodents. It can change their physiological responses and activity. Much behavioral experimentation is conducted under dim light, though even dim light may alter the behavior of animals. Some research facilities use a reversed light regimen and provide darkness during routine and experimental procedures. However, there are no unified guidelines of light intensity during behavioral testing, and "dim light" is being understood very broadly. It can be suggested that testing in darkness can be ethologically relevant for mice, rats, and other rodents who are known for nocturnality in laboratory conditions. We hypothesized that laboratory mice would be more active when being tested in a dark room than in an illuminated one regardless of the circadian phase. To test our hypotheses we conducted an experiment to evaluate male outbred mice behavior in four standard tests widely used in preclinical studies: the open field test, the hole-board test, the tail suspension test, and the elevated plus maze test. Locomotion in the open field test increased in darkness. The hole-board test revealed significantly reduced exploratory activity when performed in a dark room. In the elevated plus maze, mice tested in darkness behave less anxiously than those tested in light. Illumination did not affect mobility of mice in the tail suspension test. We suppose that darkness reduces anxiety and, hence, induces locomotion and exploration in mice in tests that are provided to study these traits. The tail suspension test is aversive itself, so it may mask any environmental effects.