Male but not female Wistar rats show increased anxiety-like behaviour in response to bright light in the defensive withdrawal test.

The defensive withdrawal test (DWT) is used to model anxiety-like behaviour in rats. The aim of this study was to investigate whether an aversive stimulus, bright light, affects the behaviour in this test. Additionally, the effect of habituation to the apparatus was studied. Both male and female Wistar rats were used to study whether sex differences exist in the DWT, as reported for other tests of anxiety. On day 1 half of the rats were tested under low light and half under bright light. Two to seven days after trial one the same rats were repeatedly tested under the same light condition for five consecutive days. The male rats showed a higher degree of anxiety-like behaviour when tested under bright light than under low light. In contrast, the behaviour of the female rats was not affected by changes in illumination. Male rats also exhibited elevated anxiety-like behaviour compared to female rats under bright light, whereas under low light conditions no sex difference was seen. Males in low light habituated much faster than males tested under bright light, whereas in females there was little difference in habituation between low and bright light. In summary, we found that bright light is aversive for male but not female Wistar rats in the DWT. Whether this is due to sex differences in light sensitivity or if females respond with a different behavioural strategy in response to bright light, which could not be detected in the DWT, remains to be elucidated.