The present study demonstrated that the administration of soyo-san reduced the depressive behavior and pro-inflammatory cytokine expression. These results suggest that repeated immobilization stress in the ovariectomized female rats is an immune challenge capable of inducing depressive-like behavior, promoting exaggerated glucocorticoid responses to stress, and changing cytokine transcription in the serum and the brain.
Depression is defined clinically as a pathological complex of psychological, neuroendocrine and somatic symptoms that cannot be reproduced in animals. To date, few models are commonly used for screening antidepressant effects or studying the mechanisms of action of these molecules. The forced swimming test (FST) was discussed the main parameters that influence the sensitivity, specificity and reliability of this model. Porsolt et al. (1977)  described “a new behavioural method for inducing a depressed state”. The idea arose out of some learning experiments they were doing with rats in a water maze. Most rats were finding the exit within 10 min but they noticed that other rats ceased struggling altogether and remained floating passively . The duration of immobility occurring in each minute was scored. A wide variety of antidepressants, and compounds with potential antidepressant activity, reduce the duration of immobility in the forced swimming test [14–16]. The present study confirms the previously reported antidepressant like effect of soyo-san. The reduction in immobility induced by soyo-san was accompanied by increased swimming, while that of soyo-san occurred with an increase in climbing behavior. The behavioral profiles observed in the present study with soyo-san, in ovariectomized female rats, are similar to those reported by other authors in male rats [17–21]. Hence, the finding of Chen et al.  can also be extended to repeated stress induced depression animal model. In a Previous study, we also demonstrated that Bupleuri radix, one of the most important constituents of soyo saan, caused a significant anti-immobility effect on the FST and reversed the CMS-induced reduction of sucrose consumption . In agreement with these findings, the previous studies on major depressive disorder have reported the elevation of IL-1β production during a depressive state . This study also showed the repeated immobilization stress induced a broad spectrum of behavioral changes that might be regarded as depression-like symptoms, including reduction of preferences for sucrose solution, inhibition of locomotor activities, and induction of learned helplessness behavior.
Several lines of evidence have indicated that menopause is associated with increased susceptibility to neurological disorders , also the mechanisms were elucidated [25–27]. Previous studies revealed a substantial involvement of endogenous estrogen in neuroinflammatory processes and provide novel mechanisms for hormone action in the brain [25–27]. Another study reported that the estrogen replacement therapy (ERT) was related to exacerbate the memory impairment induced by the chronic neuroinflammation associated with Alzheimer’s disease . In this study, our aim is to investigate whether OVX animals are more vulnerable to the inflammatory and behavioral effects of chronic stress. Many studies proved that the repeated stress accelerated expression of pro-inflammatory cytokines in the brain [28–30]. Pro-inflammatory cytokines play a key role in determining the nature and strength of immune responses . Previous studies have demonstrated that the induction of pro-inflammatory cytokines activated under stress condition. Stress (physical, psychological or both of them) can increase an inflammatory responses. The relationship between stressful events and the onset of inflammatory related diseases (injury, ischemia, Alzheimer’s disease) is well documented . Stress response is very similar to the inflammatory process generated in an organism when invaded by certain microorganisms or after injury or tissue damage as oxidative stress. In rodents, unavoidable physical stresses are the most extensively used: immobilization, exposure to extreme temperatures, fasting, immersion or electric foot shock. Various studies have provided inputs on the molecular signaling that links stress-induced neuro-endocrine-immune interactions, suggesting a bi-directional crosstalk which is based on the secretion of cytokines, glucocorticoids, neurotransmitters and neuropeptides . The previous study proved that the sex difference in novelty exposure observed in the form of a greater hypothalamic-pituitary-adrenal (HPA) axis response in female ICR mice is controlled by ovary-derived factors in adults . Ovariectomy expresses high levels of adrenal steroid receptors and is susceptible especially to damage as a result of stress . Women have a higher incidence of some stress-related disorders, such as depression, while men are generally at a greater risk for cardiovascular diseases, such as myocardial infarction and hypertension, than age-matched, premenopausal women . The higher incidence and severity of depression are associated with the presence or absence of ovarian hormones . The present study showed hyperactivity of the pro-inflammatory cytokine after stress. Pro-inflammatory cytokine’s overproduction could be integrated in the inflammatory response system, which is activated during depression, and this is consistent with the shift of the pro-/anti-inflammatory cytokine mechanism. Buchanan et al. reported that psychological stress increases the brain levels of inflammatory cytokines such as interleukin-1β (IL-1β), IL-6 and tumor necrosis factor α (TNF-α) . Zili et al. detected depression like behavior in a rat animal model which was induced inflammation in the spleen and brain by chronic mild stress (CMS) . In the present study, we observed that the expression of IL-1β immunoreactive cells was up-regulated in paraventricular nucleus (PVN), motor trigeminal nucleus (MTN), hippocampus after repeated immobilization stress. The results from depression-like animal model were consistent with previous reports, which indicated that life events and depressive symptoms are associated with the rise of central cytokine such as IL-1β in human MDD patents [40–47] and stress-treated animals [48–50]. However, the administration of soyo-san significantly reduced the expression of IL-1β. Menthae herbam, a component of soyo-san modified prescription on plasma metabolomics of rats with chronic immobilization stress . Angelicae gigantis radix, a component of soyo-san, exhibited an anti-inflammatory effect in vitro, in vivo and inhibits the stress-induced pathophysiological changes in the central nervous system . Also, paeoniflorin has neuroprotective effect and inhibition of neuroinflammation .