Target recognition and study on the antidepressant effect of saikosaponin A
Depression is a common mental system disease, and its incidence rate is increasing year by year. It is predicted that by 2030, depression will become the primary cause of abnormal death and disability in the world.
The traditional Chinese medicine Chaihu has the effects of soothing the liver, relieving depression, clearing heat, and relieving external heat. It is often used as the main ingredient in traditional Chinese medicine formulas for treating depression, such as Xiaochaihu Tang, Chaihu Shugan San, and Chaihu Guizhi Tang. The main active ingredient in Chaihu is the triterpenoid saponin compound saikosaponin, among which saikosaponin A (SSA), az saikosaponin B2 (SSB2), and saikosaponin D (SSD) are the main active ingredients of saikosaponin. Research has shown that SSA exerts antidepressant effects by increasing the levels of monoamine neurotransmitters in the brains of depression model rats, activating the p-CREB/BDNF pathway, and inhibiting hippocampal neuronal apoptosis. In addition, SSA exerts antidepressant effects by enhancing the BDNF TrKB signaling pathway in rat hippocampal tissue. SSA can also exert antidepressant effects by increasing the expression of proline rich transmembrane proteins (PRRT2) and elevating dopamine receptor (DA) levels in hippocampal tissue. Although the antidepressant effect of SSA has been clearly defined, previous studies have only used changes in neurotransmitter levels to characterize the antidepressant effect of SSA, with a single detection index and limited mechanism research, which cannot fully explain the target of antidepressant drugs. In order to clarify the antidepressant effect and target of SSA, this study intends to construct a chronic unpredictable mild stress (CUMS) model mouse, and use tail suspension test (TST) and forced swimming test (FST) to determine the antidepressant effect of SSA; Further utilize computer molecular docking technology to analyze the targets of SSA, and validate the targets of SSA using cellular thermal shift assay (CET-SA) and drug affinity responsive target stability assay (DARTS); And Western blotting and other techniques were used to study the antidepressant mechanism of SSA. This study will provide new methods and strategies for the discovery of small molecule drug targets.
The pathogenesis of depression is currently unclear and there are multiple hypotheses, among which the monoamine neurotransmitter disorder hypothesis plays an important role in the onset of depression. Receptors associated with neurotransmitter disorders, such as 5-hydroxytryptamine receptor (5-HT), dopamine receptor (DAR), melatonin receptor (MT), oxytocin receptor (OXTR), etc., are G protein coupled receptors; Research on patients with depression has shown a correlation between oxytocin and its receptor genes and depression. Oxytocin (OT) is a pituitary neuropeptide composed of 9 amino acids that can reduce anxiety levels, enhance sedation, and have significant antidepressant effects; Its specific targeting of OXTR expression and activation is associated with some diseases caused by depression, anxiety, and stress. Therefore, OXTR is one of the main targets of SSA’s antidepressant effect. This study aims to comprehensively analyze the G protein coupled receptors related to depression and clarify the targets of SSA’s antidepressant effects. This article aims to investigate the targets and mechanisms of SSA’s antidepressant effects, providing necessary theoretical and reference basis for clinical research and new drug development of mental disorders such as depression.

 

Depression has a significant impact on the lives and work of patients, imposing a huge burden on their families and society. The pathological mechanism of depression is complex, and most of the various antidepressant drugs currently available on the market have slow onset, poor cure rates, and cannot be fundamentally cured, with a high incidence of adverse reactions. Therefore, the development of a new antidepressant drug has important clinical value for the treatment of depression and will bring hope and good news to patients with depression.
Chaihu has various pharmacological effects, including anti-inflammatory, anticancer, antiviral, antibacterial, hepatoprotective, and immunomodulatory effects. In recent years, traditional Chinese medicine has achieved significant therapeutic effects in treating depression. Existing data indicates that one-third of antidepressant medications are based on traditional Chinese medicine formulas such as Chaihu, highlighting the importance of Chaihu in the treatment of depression. The active ingredients with antidepressant effects in Bupleurum chinense mainly include SSA, SSB2, and SSD, among which SSA and SSD have the same molecular formula and only differ in spatial structure. Therefore, studying the antidepressant mechanism of SSA is of great significance for the development and utilization of traditional Chinese medicine Bupleurum chinense. Although the beneficial effects of saikosaponin on depression have been established, its mechanism of action is still unclear.
Oxytocin is believed to play a role in intimacy, social recognition, bonding between partners, and anxiety. OXTR is also related to some symptoms caused by depression, anxiety, and stress. It can improve patients’ depressive symptoms by regulating the hypothalamic pituitary adrenal axis (HPA axis) and hippocampal neuronal targets. Therefore, OXTR is likely to become a new target for the treatment of depression.
This study is based on the results of previous cell screening, using CUMS model mice as carriers, and using TST and FST to detect the antidepressant effect of SSA. The experimental results show that SSA has significant antidepressant effects, and its antidepressant effect is comparable to the positive control Fluoxetine. Furthermore, the target of SSA was identified using computer molecular docking technology, CETSA, and DARTS.
The activation of MAPK pathway, especially MEK1/2 phosphorylation activation, is an important pathway for oxytocin to exert anti anxiety effects, and ERK1/2 is one of the main kinases in the MAPK pathway. Therefore, this study preliminarily assessed the antidepressant efficacy of small molecule compounds by detecting changes in the phosphorylation level of ERK1/2. Although molecular docking experiments, cell heat transfer assays, and drug affinity reaction target stability experiments all showed that SSA binds to OXTR, and SSA has a stronger binding ability to OXTR than SSB2. However, in the ERK1/2 phosphorylation level detection experiment, SSA did not activate the ERK1/2 pathway downstream of OXTR. The possible reason is that although SSA can bind to OXTR, its antidepressant effect after binding to OXTR does not occur through the MAPK pathway. OXTR can be coupled to various G proteins (including Gq and Gi), which may transmit OXTR signals to different intracellular pathways, leading to different cellular functions. This result is consistent with previous reports, for example, in prostate cancer cells, activated OXTR mainly relies on the Gi pathway to mediate the migration of prostate cancer cells, and the ERK1/2 pathway is not activated during this process. In addition, SSA exerts a protective effect against LPS induced endometritis by inhibiting the phosphorylation levels of nuclear transcription factor kappa B (NF – κ B) and kappa B inhibitor (I κ B α) induced by lipopolysaccharide (LPS). In the future, we can use SSA to stimulate cells that stably express OXTR, and further determine the specific mechanism by which SSA activates OXTR and exerts antidepressant effects by detecting the phosphorylation levels of its downstream signaling pathways.
Given the enormous potential of traditional Chinese medicine Chaihu in treating depression, and its advantage of fewer adverse reactions compared to Western medicine, elucidating the antidepressant mechanism of SSA plays an extremely important role in the prevention and treatment of depression. The G protein coupled receptor family has more than 800 members and is involved in regulating almost all pathological and physiological processes in the human body. Researching whether SSA targets OXTR and related cellular signaling pathways provides necessary theoretical and reference basis for clinical research, treatment, and new drug development of mental disorders such as depression.