Behavioural Brain Research 202 (2009) 303–307 Contents lists available at ScienceDirect Behavioural Brain Research journal homepage: www.elsevier.com/locate/bbr Research report Male but not female Wistar rats show increased anxiety-like behaviour in response to bright light in the defensive withdrawal test Erika Roman a,1 , Lotta Arborelius b,∗ a Department of Pharmaceutical Biosciences, Division of Pharmacology, Uppsala University, P.O. Box 591, SE-751 24 Uppsala, Sweden b Department of Clinical Neuroscience, Section for Psychology, Karolinska Institutet, Fogdevreten 2a, SE-171 76 Stockholm, Sweden article info Article history: Received 6 February 2009 Received in revised form 15 April 2009 Accepted 16 April 2009 Available online 24 April 2009 Keywords: Emotionality Sex difference Illumination Habituation Anxiety test abstract 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Laboratory rats show an innate avoidance of unfamiliar envi- ronments, in particular open areas, but subsequently they start to explore it for possible resources [7]. This hereditary characteristic of rats is often used in rodent models of anxiety. One such model is the defensive withdrawal test (DWT) which evolved from so-called timidity tests in which the latency to emerge from a sheltered to a novel environment is recorded [2]. The DWT consists of an unfa- miliar open arena with a small, dark, familiar tube inside (see e.g. [1]). A rat treated with an anxiolytic drug leaves the sheltered tube faster and spends more time in the open arena than an untreated rat [32] [Arborelius, unpublished observations]. On the other hand, anxiogenic drugs as well as acute stress increase the latency to first enter the open arena and increase the time the animal spend with- drawn in the tube [10,29,31,32]. Based on these observations the DWT has been suggested as a test for anxiety-like behaviour in rats (see e.g. [1]). ∗ Corresponding author. Tel.: +46 8 524 82442; fax: +46 8 307298. E-mail addresses: Erika.Roman@farmbio.uu.se (E. Roman), Lotta.Arborelius@ki.se (L. Arborelius). 1 Tel.: +46 18 4174632. Nocturnal animals such as rats naturally avoid bright light. Sev- eral studies have found that high illumination is aversive and hence this can be used to induce anxiety-related behaviours in laboratory rats. For instance, increasing the illumination will decrease locomo- tion, rearing and time spent in the centre in the open field [30] and reduce the interaction between two rats in an unfamiliar environ- ment in the social interaction test [11]. However, in other tests of anxiety-like behaviour, such as the elevated plus-maze, changes in light intensity do not generally alter the behaviour (see e.g. [4]). The illumination used in the DWT varies considerably between laboratories from low, e.g. about 20 lx [20] to high intensity, e.g. 600 lx [10]. However, whether the behavioural parameters mea- sured in this test are sensitive to changes in light intensity has not yet been investigated. Hence, one aim of this study was to inves- tigate whether parameters measured in the DWT are affected by light intensity. Most, if not all of the behavioural tests for anxiety-like behaviour in rodents in use today have been evaluated and validated using male rats only. However, about twice as many women as men will develop anxiety disorders during their lifetime [25]. It has also been argued that pharmacological treatment needs to be better evalu- ated in females [5,23], thus stressing the need to evaluate animal models for anxiety also for females. There are indeed a number of animal studies showing that female rats behave differently in 0166-4328/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2009.04.019