Chronic psychosocial stress and antidepressant treatment in tree shrews: time-dependent behavioral and endocrine effects.

Social defeat has been shown to cause a number of behavioral, physiological, and central nervous changes in male tree shrews. The present study was designed to assess: (i) a potential time lag in the occurrence of behavioral alterations (locomotor activity, self-grooming, marking behavior, food and water intake, and avoidance behavior) after stress and long-term antidepressant treatment; and (ii) to investigate potential interactions between behavioral and endocrine variables (urinary cortisol and norepinephrine). Male tree shrews were submitted to chronic psychosocial stress for 39 days. In this paradigm, the stress-induced behavioral and endocrine alterations in subordinate animals are based exclusively on the central nervous interpretation of the continuous visual presence of the dominant conspecific. During the last 29 days of stress exposure, the tricyclic antidepressant clomipramine was administered daily to the subordinate animals (50 mg/kg, p.o). Results from this group were compared with three other experimental groups: one group was just stressed, one group received only clomipramine, and one group only vehicle. To determine the time-dependent effects of psychosocial stress and clomipramine treatment, behavior was recorded immediately after and 9 h after daily social encounters. Depending on the observation time, significant differences between the effects of psychosocial stress and antidepressant treatment were found. Generally, the effect of stress on behavioral parameters tended to be less distinct immediately after the social encounter compared to the later observation time. Furthermore the drug had a time-dependent restorative influence on marking and grooming behavior, locomotor activity, avoidance behavior, as well as on urinary cortisol, and norepinephrine excretion. Correlation analysis revealed significant interdependencies between locomotor activity, marking behavior, and avoidance behavior on 1 day with cortisol and norepinephrine quantified in the morning urine of the following day. These findings indicate that experimental manipulations, stress, and psychotropic drug application have a time lag on bio-behavioral parameters which should be considered when studying animal behavior in response to stressors and/or to drug treatment.