**1. Introduction**

Major depressive disorder (MDD) or depression, which has an estimated global prevalence of 4.7%, is a mental disorder that affects human thoughts, mood, and physical health. It can occur at as early as 3 years of age, and appears across all world regions [1,2]. The global burden of disease (GBD) in 2010 identified MDD as a leading contributor to the global disease burden [3]. Similar to anxiety disorder, MDD has been reported to cause brain injury, especially damaging the hippocampus region of the brain, causing neuronal dysfunction or affecting neural plasticity [4]. Brain-derived neurotrophic factor (BDNF) plays a critical neurotrophic role in neural plasticity [5,6]. In prior studies, *bdnf* has been identified as a target gene in depression treatment [5,7,8]. Adachi et al. [9] reported that the loss of BDNF in the hippocampal tissue contributed to increasing vulnerability to depression, whereas upregulation mediated antidepressant efficacy.

MDD has been thought to be a heterogeneous disease with diverse etiological and multifactorial pathogenesis [10,11]. Modern medicine has primarily focused on symptomatic treatment, and drugs are mainly applied in single-target and single-factor therapy. Traditional Chinese medicine (TCM), whose treatments are designed systematically based upon the constitution of patients, has a long history and the potential to treat many diseases including depressive-like syndromes [10,12]. Chinese herbal medicine is the major form of prescription of TCM. Chinese herbal formulas (CHFs), which aim to help patients re-achieve the Yin–Yang balance of the body, usually consist of multiple herbs that synchronize with one other when administered. Considerable progress has occurred not only in basic research, but also in clinical research, with a better understanding of the underlying neurobiological basis of CHFs. It is believed that a multi-component and multi-target mechanism may be the essential mechanism through which CHF achieve holistic effects [13].

The most common traditional method of preparing CHFs is through the water decocting method. In brief, prepared slices of medical herbs are boiled in hot water, and the decoction administrated orally [14]. Aiming to seek CHFs that have the potential to serve as alternatives for MDD treatment, Infinitus (China) Co., Ltd. offered 22 modified CHFs for regulating mood. We then explored whether the decoction of them had protective effects on hippocampal neuronal cell line (HT22) against corticosterone (CORT)-induced apoptosis (Supplementary Table S1). We found that the formula WXJ-17-001, which comprises *Panax ginseng, Angelica ginensis, Polygala tenuifolia* Willd, and *Ziziphi spinosae* Semen, had the best protection effect among them. We then renamed the formula according to its composing as PAPZ.

Each single herb of PAPZ is beneficial for central nervous system disorders and widely used in an Asian-medicated diet. *P. ginseng* is a widely-used medicinal plant in Asian countries; it has various pharmacological effects such as neuroprotection, endocrine modulation, cardiovascular protection, and immunoregulation [10]. Many research articles have reported that *P. ginseng* or its components are beneficial to the central nervous system, and have therapeutic effects on neuropsychiatric disorders [15]. *A. sinensis*, known as *Dong-Gui*, is served as a healthy food and is also a common traditional medicine used for the treatment of cerebrovascular diseases [16,17]. A study showed that the formula *Dang-Gui-Shao-Yao-San*, of which the major active component is *A. sinensis*, had pharmaceutical effects on depression [18]. *P. tenuifolia* Willd is used as an expectorant, tonic, tranquillizer, or antipsychotic agent in TCM [19]. *Senegenin*, extracted from *P. tenuifolia* Willd, shows an antidepressant effect by inhibiting the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) pathway [20]. *Z. spinosae* Semen is the mature seed of sour jujube, and is used as a sedative agent in TCM [21–23]. *Z. spinosae* Semen could improve learning and memory of mice [24]. It has been reported to have an antidepressant effect in a rat depressive model [25]. However, whether a formula of these herbs would have antidepressant effects, and the underlying molecular mechanism, remains unclear.

In the present study, chronic treatment with corticosterone (CORT), which is a common inducer of depression-related behavior both in rats and mice [26,27], was used to establish a depressive-like mice model. A hippocampus-derived HT22 cell line was applied to in vitro experiments. Our results showed that administration of PAPZ ameliorated depressive-like phenotypes by improving the protein expression of BDNF and the activity of superoxide dismutase (SOD) of hippocampal tissue in mice. PAPZ also attenuated CORT-induced apoptosis in HT22 cell in vitro. Taken together, the results demonstrated the neuroprotective effect of PAPZ, which suggested PAPZ is a good candidate for treatment of depressive disorders.
