ELSEVIER SSDI 0031-9384(95)02182-5 Physiology & Behavior, Vol. 59, Nos. 4/5, pp. 735-739, 1996 Copyright © 1996 Elsevier Science Inc, Printed in the USA. All rights reserved 0031-9384/96 $15.00 + .00 Changes in Daily Rhythms of Body Temperature and Activity After a Single Social Defeat in Rats P. MEERLO, l S.F. DE BOER, J. M. KOOLHAAS, S. DAAN AND R. H. VAN DEN HOOFDAKKER Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands Received 15 May 1995 MEERLO, P., S. F. DE BOER, J. M. KOOLHAAS, S. DAAN AND R. H. VAN DEN HOOFDAKKER. Changes in daily rhythms of body temperature and activity after a single social defeat in rats. PHYSIOL BEHAV 59(4/5) 735-739, 1996.--The long-term consequences of social stress on daily rhythms of body temperature and activity in rats were studied by means of radiotelemetry with intraperitoneally implanted transmitters. Rats were subjected to a single social defeat by placing them into the territory of a male conspecific for l h. Social defeat caused a sharp subsequent reduction in the amplitude of the daily temperature rhythm, which lasted for at least 4 days. The reduced amplitude was mainly due to higher temperatures during the circadian rest phase, i.e., the light period. Movement activity was less affected, but the decrease in activity during the dark phase after defeat correlated significantly with the temperature increase during the light phase. The stress-induced changes in daily rhythms of body temperature and activity are discussed in terms of their relevance to the role of rhythm-disturbances in the pathogenesis of affective disorders. Depression Stress Social defeat Aggressive interaction Body temperature Activity Telemetry Circadian rhythms STRESSFUL life events are considered to be an important factor in the development of human pathologies, including affective disorders like anxiety and depression (2,3,6,11,18). Stress-in- duced pathologies are often characterized by alterations in circa- dian rhythms. In depression, for example, severe disturbances of the sleep-wake rhythm, and changes in body temperature rhythm and hormone profiles have been reported in numerous studies [for review, see (7)]. It has been suggested that changes in circadian rhythmicity are not only an epiphenomenon, but actually play a causal role in the pathogenesis of affective disorders (12,24). However, because of the constraints on experimental manipula- tion that characterize clinical research, such theories have been difficult to prove (25). In animals, uncontrollable stressors have been shown to in- duce changes in a wide variety of behavioral parameters, includ- ing decreases in general locomotor activity and explorative be- havior, impairment of sexual behavior, feeding, and drinking (28,29). Such stress-induced behavioral changes have been con- sidered to provide one of the most valid animal models of human affective disorders (28). The available data on circadian rhythms in animals subjected to stress are very limited (20). Changes in daily rhythms of body temperature and heart rate under conditions of chronic stress have been found in rats (15) and tree shrews (27). A recent study in rats showed profound decreases in the amplitude of the rhythms of body temperature and heart rate, which persisted for 10 days after the last of five daily confrontations with a dominant resident (22). Such profound effects on rhythmicity, in relation to both short- and long-term behavioral and physiological consequences of social defeat, may provide a useful model to study chronobio- logical aspects of stress pathology. Social defeat induces classical stress responses with an acute and strong cardiovascular and neuroendocrine activation (5,17). Most of these responses diminish within 1 or 2 h after the aggressive interaction. However, a number of reports have indi- cated much longer lasting effects on various parameters at the level of both physiology and behaviour (16,17). Recent work in our lab showed that social defeat induces changes in open field behaviour, social behavior, food intake, and growth, which last for a period of 2 to 7 days (Meerlo, in preparation). The present experiment describes the effects of a single social defeat on daily rhythms of body temperature and activity in rats. The use of radio telemetry with implantable transmitters allowed continuous moni- toring of activity and body temperature for a long period of time in rats subjected to a single conflict, but otherwise left undis- turbed in their home cages. METHOD Animals and Housing The experiment was performed with 21 male Tryon Maze Dull $3 rats of 4 months of age. This strain is well known for its i To whom requests for reprints should be addressed. 735