Behavioural Brain Research 173 (2006) 153–157 Short communication Sand rats see the light: Short photoperiod induces a depression-like response in a diurnal rodent Haim Einat a,∗ , Noga Kronfeld-Schor b , David Eilam b a University of Minnesota, College of Pharmacy, Duluth, MN 55812, USA b Department of Zoology, Tel-Aviv University, Ramat-Aviv 69978, Israel Received 2 May 2006; received in revised form 1 June 2006; accepted 6 June 2006 Available online 10 July 2006 Abstract The present study examined the effect of simulated change in day length on the behavior of a diurnal rodent, the fat sand rat (a species of gerbil). Animals were housed under a short photoperiod (5/19 light/dark cycle) for 3 weeks and compared with controls living under a 12/12 light/dark cycle. All sand rats then underwent the forced swim test for depression-like behavior, and the open-field test for overall activity. Sand rats exposed to the short photoperiod displayed a significantly earlier sinking in the swim test, but there was no difference between their open-field activity compared with controls. Taking these responses as indicative of depression-like behavior, we suggest that a short photoperiod may induce affective-like changes, and that the sand rat may thus offer an appropriate animal model to explore the effect of photoperiod on normal, and perhaps also abnormal, seasonal mood changes (e.g., SAD), which in humans is a prevalent disorder, with winter depression episodes and spring/summer remissions. © 2006 Elsevier B.V. All rights reserved. Keywords: Animal model; Forced swim test; Psammomys obesus; Circadian rhythms; Photoperiod; Seasonal affective disorder (SAD) The present study examines the possibility of using a diurnal rodent, the fat sand rat (Psammomys obesus), as an animal model for photoperiod-dependent affective-like changes. Numerous studies have dealt with seasonal acclimation in animals and the role of photoperiod in seasonal acclimation, focusing on physi- ological aspects. The length of the day, which is a reliable cue, allows animals to anticipate environmental changes and respond to them appropriately. Indeed, physiological parameters such as enzymatic activity, immune function, thermogenesis capac- ity, storage or mobilization of energy reserves and more, have to be adjusted well before the expected environmental changes actually take place. Furthermore, behavioral timing in feeding, reproduction, migration, etc., often precedes the external events [9]. Several studies have addressed seasonally induced changes in emotional and affective-like behavior in animals (for review see Ref. [28]). Indeed, past studies revealed that a short day- light induced anxiogenic and depressive effects in male but not in female hamsters [32]; that a long photoperiod regime may ∗ Corresponding author. Tel.: +1 218 726 6029; fax: +1 218 726 6500. E-mail address: heinat@d.umn.edu (H. Einat). produce an antidepressant effect in male rats [29]; that a single day of constant light can protect against the induction of behav- ioral despair in rats [40]. In the same vein, the present study is aimed at studying the effect of a shorter daylight period on sand rat behavior for its potential application to human affective disorder. Emotional and affective seasonal changes are recognizable in humans. For example, seasonal affective disorder (SAD; [23,26]) is a depression-spectrum disorder with repeated win- ter episodes followed by spring/summer remissions, affecting 1–3% of adults in temperate climates, with higher prevalence in women and in areas with reduced light hours [22,23,26]. However, measuring emotional or mood changes in animals is challenging since these properties are not directly assessable. To date, the ‘Porsolt forced swim test’ (FST) is probably the most frequently used model to test for depression-like symptoms in rats and mice [32]. In this test, animals are forced to swim in a cylinder from which they cannot escape. After an initial period of vigorous activity, animals adopt an immobile posture, which is selectively sensitive to antidepressant treatments; immobility is consequently considered as a reflection of a state of depres- sion [32]. Subsequent studies have found that behavior in the 0166-4328/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2006.06.006