Short day lengths alter stress and depressive-like responses, and hippocampal morphology in Siberian hamsters Joanna L. Workman a, ⁎, Natalie Manny a , James C. Walton b , Randy J. Nelson a, b, c a Department of Psychology, The Ohio State University, Columbus, OH 43201, USA b Department of Neuroscience, The Ohio State University, Columbus, OH 43201, USA c Institute for Behavioral Medicine Research, The Ohio State University, Columbus, OH 43201, USA abstract article info Article history: Received 5 April 2011 Revised 29 July 2011 Accepted 31 July 2011 Available online 7 August 2011 Keywords: Hippocampus Depression Stress Photoperiod Seasonality Plasticity Seasonal affective disorder Many psychological disorders comprise a seasonal component. For instance, seasonal affective disorder (SAD) is characterized by depression during autumn and winter. Because hippocampal atrophy may underlie the symptoms of depression and depressive-like behaviors, one goal of this study was to determine whether short days also induce structural changes in the hippocampus using photoperiod responsive rodents — Siberian hamsters. Exposure to short days increases depressive-like responses (increased immobility in the forced swim test) in hamsters. Male hamsters were housed in either short (LD 8:16) or long days (LD 16:8) for 10 weeks and tested in the forced swim test. Brains were removed and processed for Golgi impregnation. HPA axis function may account for photoperiod-related changes in depressive-like responses. Thus, stress reactivity was assessed in another cohort of photoperiod-manipulated animals. Short days reduced soma size and dendritic complexity in the CA1 region. Photoperiod did not induce gross changes in stress reactivity, but an acute stressor disrupted the typical nocturnal peak in cortisol concentrations. These data reveal that immobility induced by exposure to short days is correlated with reduced CA1 cell complexity (and perhaps connectivity). This study is the first to investigate hippocampal changes in the context of short-day induced immobility and may be relevant for understanding psychological disorders with a seasonal component. © 2011 Elsevier Inc. All rights reserved. Introduction In seasonally-changing environments, small mammals must alter physiology and behavior in order to coordinate endogenous processes with ambient conditions. Winter is a particularly challenging time to survive and reproduce. In order to reliably coordinate physiological processes with environmental conditions, many rodents monitor day length, a precise and relatively noise-free cue. Day length (photope- riod) information is encoded physiologically through the duration of nighttime melatonin secretion, which is inversely proportional to day length (Reiter, 1993). In addition to coordinating reproductive processes, day length alters many nonreproductive behavioral and physiological changes. Seasonally-breeding rodents, for instance, undergo changes in brain morphology dependent on photoperiod. Wild-caught rodents have reduced skull size (Pucek, 1963), whole brain weights and hippocampal weight in winter (Yaskin, 1984). Exposure to short days in the laboratory reduces whole brain and hippocampal volume in white-footed mice (Peromyscus leucopus) (Perrot-Sinal et al., 1998; Pyter et al., 2005) suggesting that changes in brain volume in the wild may reflect changes induced by decreased day length. The brain requires a disproportionate amount of energy compared to other tissues, consequently slight reductions in brain volume may be adaptive because they help conserve energy in harsh environments. Seasonally-breeding rodents can be used in the laboratory to isolate the effects of day length on brain morphology; research in this area may reveal mechanisms of how day length alters the brain (and subsequently mental processes) in humans. SAD is one disorder with a strong seasonal component, and is a subtype of major depressive disorder that encompasses similar symptoms to major depression (e.g., depressed mood, anhedonia, guilt), but some atypical symptoms as well (e.g., hypersomnia, hyperphagia). These symptoms remit in the summer and some individuals may become hypomanic. SAD has been associated with an inability to inhibit morning melatonin secretion in the winter (Lewy et al., 1999; Lewy and Sack, 1988; Rosenthal et al., 1984; Wehr et al., 2001) suggesting that shortened day length, and in turn extended melatonin secretion, may induce symptoms in vulnerable individuals. Currently, there is no animal model of SAD but several species exhibit greater depressive-like responses after exposure to short days. The forced swim test (FST) is a well-validated behavioral measure used to Hormones and Behavior 60 (2011) 520–528 ⁎ Corresponding author at: The University of British Columbia, 2136 West Mall, Vancouver, Canada BC V6T 1Z4. E-mail address: joanna@psych.ubc.ca (J.L. Workman). 0018-506X/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2011.07.021 Contents lists available at ScienceDirect Hormones and Behavior journal homepage: www.elsevier.com/locate/yhbeh