Stress-induced levels of plasma glucocorticoid hormones are known to modulate leukocyte function. These experiments examined the effects of a social stressor on the responsiveness of peripheral immune cells. Male mice experienced six evening cycles of social disruption (SDR), in which an aggressive male intruder was placed into their home cage for 2 h. Although circulating corticosterone was elevated in SDR mice, they had enlarged spleens and increased numbers of splenic leukocytes. Splenocytes from SDR and control mice were cultured with lipopolysaccharide and corticosterone. Cells from SDR mice exhibited decreased sensitivity to the antiproliferative effects of corticosterone, suggesting that the peripheral immune cells were resistant to glucocorticoids. In addition, SDR cells produced more interleukin (IL)-6. To determine which cell population was affected, we used antibody-labeled magnetic beads to deplete splenocyte suspensions of B cells or macrophages. Depletion of macrophages from SDR cultures, but not depletion of B cells, abolished both the corticosterone resistance and enhanced IL-6 secretion. These findings demonstrate that a psychosocial stressor induced glucocorticoid resistance in mouse splenic macrophages.

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The experimental model, social disruption (SDR), is a model of social stress in which mice are repeatedly attacked and defeated in their home cage by an aggressive conspecific. In terms of the impact of this stressor on the immune response, SDR has been reported to cause hyperinflammation and glucocorticoid insensitivity. To this point however, the behavioral consequences of SDR have not been thoroughly characterized. Because social defeat has been reported to cause anxiety-and depressive-like behaviors, the current study was designed to assess whether SDR also causes anxietyand depressive-like behaviors. Using the light/dark preference test and the open field test as tools to measure behaviors characteristic of anxiety, the data showed that C57BL/6 and CD-1 male mice subjected to SDR displayed increased anxiety-like behavior. The increase in anxiety-like behaviors persisted for at least one week after the cessation of the stressor. In contrast, depressive-like behaviors were not elicited by SDR as assessed by the forced swim test or the tail suspension test. These data indicate that social disruption stress causes an increase in anxiety-like behaviors, but not depressivelike behaviors.

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A BSTRACT : Various medical conditions that involve activation of the immune system are associated with psychological and neuroendocrine changes that resemble the characteristics of depression. In this review we present our recent studies, designed to investigate the relationship between the behavioral effects of immune activation and depressive symptomatology. In the first set of experiments, we used a double-blind prospective design to investigate the psychological consequences of illness in two models: (1) vaccination of teenage girls with live attenuated rubella virus, and (2) lipopolysaccharide (LPS) administration in healthy male volunteers. In the rubella study, we demonstrated that, compared to control group subjects and to their own baseline, a subgroup of vulnerable individuals (girls from low socioeconomic status) showed a significant virus-induced increase in depressed mood up to 10 weeks after vaccination. In an ongoing study on the effects of LPS, we demonstrated significant LPSinduced elevation in the levels of depression and anxiety as well as memory deficits. These psychological effects were highly correlated with the levels of LPS-induced cytokine secretion. In parallel experiments, we demonstrated in rodents that immune activation with various acute and chronic immune challenges induces a depressive-like syndrome, characterized by anhedonia, anorexia, body weight loss, and reduced locomotor, exploratory, and social behavior. Chronic treatment with antidepressants (imipramine or fluoxetine) attenuated many of the behavioral effects of LPS, as well as LPS-induced changes in body temperature, adrenocortical activation, hypothalamic serotonin release, and the expression of splenic TNF-␣ mRNA. Taken together, these findings suggest that cytokines are involved in the etiology and symptomatology of illness-associated depression.