Depression is a prevalent and debilitating mental health disorder (Monroe and Harkness, 2022). Current antidepressants often have delayed effects, limited efficacy, and some patients are treatment-resistant. This highlights the need for better treatment. Ketamine can rapidly improve symptoms and induce synaptic remodeling (Chen et al., 2023; Kim et al., 2023; Krystal et al., 2023a). However, its risk of abuse and toxicity restricts long-term clinical use (Schifano et al., 2021), emphasizing the need for safer, rapid-acting antidepressants.

Traditional Chinese Medicine (TCM) has long been valued for its significant therapeutic benefits and fewer side effects compared to conventional antidepressants (Feng et al., 2016; Zeng et al., 2025; Zhuang et al., 2023). Yueju, a classic TCM formulation used for over 800 years, shows rapid-onset antidepressant effects in both clinical and preclinical studies (Ren and Chen, 2017; Tang et al., 2015b; Zhang et al., 2021). A single dose of Yueju produces rapid antidepressant effects in animal models, similar to ketamine (Ren et al., 2016; Tang et al., 2015a; Xia et al., 2016; Xue et al., 2013).

Gardenia jasminoides J. Ellis is the principal herb in Yueju responsible for its rapid antidepressant response (Zhang et al., 2015). Among its active components, shanzhiside methyl ester (SM) at a high dose induces a rapid antidepressant response, whereas geniposide (GP) alone is effective (Zhang et al., 2015; Zhao et al., 2018). Extracts often work better than isolated constituents due to synergistic interactions among their constituents (Rasoanaivo et al., 2011). Combining SM and GP lowers the required SM dose for rapid antidepressant action (Zhang et al., 2022). A single co-treatment of SM and GP (GS) induces rapid and sustained antidepressant effects, unlike either compound alone, highlighting GS as a promising strategy for rapid-onset depression treatment.

A single dose of GS increases pituitary adenylate cyclase activating polypeptide (PACAP) expression in the hippocampus, a neuropeptide implicated in mood regulation and synaptic plasticity. PACAP, a member of the secretin-glucagon-vasoactive intestinal polypeptide family, is widely presented in the nervous system (Vaudry et al., 2009). PACAP-ergic signaling is crucial for stress regulations and neuroprotection (Lutfy and Shankar, 2019). Dysregulation of PACAP and its receptors is linked to mood disorders (Kormos and Gaszner, 2013; Pinhasov et al., 2011), and the antidepressant effects of both Yueju and ketamine (Zhang et al., 2024). Preclinical studies show that Yueju rapidly increases PACAP levels, activates PAC1 receptors and enhances synaptic plasticity (Ren et al., 2023; Xue et al., 2016). GS co-treatment synergistically maintains elevated PACAP levels, supporting synaptic plasticity and prolonged antidepressant effects (Zhang et al., 2022), indicating the role of hippocampal PACAP signaling in the actions of Yueju and GS.

Chronic antidepressant treatment is essential for long-term recovery and relapse -prevention. However, it remains unclear whether prolonged administration with high-dose of GS can confer benefits comparable to conventional antidepressants, such as fluoxetine, particularly with respect to enhancing hippocampal plasticity. This study aims to evaluate the effects and optimal dosage of chronic GS treatment in ameliorating depressive phenotypes and its molecular signaling pathway.