Abstract: Stressful events engender multiple neurochemical and depressive-like behavior alteration as well as immune activation. Administration of lipoposaccharide (LPS), which is known to activate immune functions and to induce the release of several cytokines both in the periphery and brain, also could engender depressive-like behavior alteration as same as stress. Many researchers paid attention to the effect of stress on immune activation. But few researches were related to the effect of activated–immune on the depressive-like behavior induced by stress. To further understand the role of immunity-activation induced cytokines in behavioral depression, the purpose of the present study was to investigate the effect of single administration of LPS prior to stress on depressive-like behavior induced by forced swim stress in rats. Forty-two rats were randomly divided into four groups: LS group (swim + LPS, n=12), LC group (LPS + control, n=10), AS group (saline +swim, n=10), AC group (saline + control, n=10). On the first experiment day, according to groups, rates were injected intraperitoneally with LPS 50ug/kg and saline respectively. After 2h, LS and AS group rats were conducted forced swim stress for 14 days. On the 2end day, the 14th day, the 21st day and the 28th day, rats were tested with saccharin preference test, open field, elevated-plus maze. The results showed that saccharin preference percent, locomotion in open field, closed arms entries in elevated-plus maze in LS group were significant lower than that in AS group on the 2end day. On the 14th day, LS and AS group rats had fewer saccharin preference percent and locomotion than AC group rats. On the 21st day, LS group rats had significant decrease compared with AC group in saccharin preference percent and locomotion behavior. These results demonstrate that a series of easily onset and longer duration of depressive-like behavior was observed in LS group rats. It suggested that LPS-induced pro-inflammation cytokines may have synergistic and additive effects on depressive-like behavior induced by stress, there may be a cross-sensitization between them to cause depression.

Key words: lipoposaccharide, forced swim stress, depressive-like behavior, saccharin preference test, open field, elevated-plus maze