Neurotransmitters play a crucial role in facilitating signal transmission between neurons and effectors within the body (Hu and Wu, 2023). Monoamine neurotransmitters are central neurotransmitters, including catecholamine and indoleamine, the former is a neurotransmitter based on catecholamine, including norepinephrine (NE), epinephrine and dopamine (DA), while the latter is composed of indole and ethylamine, which mainly refers to 5-hydroxytryptamine (5-HT) (Wu, 2023). The occurrence of depression is associated with the modulation of monoamine neurotransmitter levels, and when a decrease in the levels of 5-HT, NE, and DA may have an impact on the emotional state of individuals (El et al., 2010; Perez-Caballero et al., 2019).

Catalpol, the chemical metabolites, found in R. glutinosa, possesses a significant concentration of iridoids and displays a multitude of pharmacological properties, including but not limited to antidepressant, cognitive-enhancing, and neuroprotective effects (Zhang and Liu, 2019). Treatment with catalpol (5, 10, or 20 mg/kg) for 14 days reduced mice’s depressive-like behavior in a depression model, and it was found that catalpol increased the content of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in mice’s brains, while exhibiting minimal influence on the levels of NE and DA. This study to indicate that catalpol has an antidepressant-like effect and that its action may be mediated by the central serotonergic system (Wang et al., 2014). The phenylethanol glycoside acteoside, extracted from Radix Rehmanniae, exhibits many pharmacological properties, such as antidepressant properties, antitumor properties, anti-inflammatory properties, neuroprotective properties, etc (Ge et al., 2023). Current studies have have demonstrated that acteoside substantially elevates the serum concentrations of 5-HT, GABA, and DA in depressed mice, and the mechanism underlying the antidepressant effects of acteoside is believed to involve the augmentation of monoamine neurotransmitters, the attenuation of pro-inflammatory cytokines, and the restoration of neurotransmitter levels (Xue X. et al., 2022). Recent studies have revealed that the primary metabolites of Lilium are saponins, which exhibit antidepressant, antioxidant, anti-inflammatory, antibacterial, and regulatory properties on the cerebral and gut axis (Sun et al., 2022). Administration of intragastric Lilium saponins to mice with depression resulted in a reduction of depressive-like behavior and a decrease in body temperature (Wang, 2014). Moreover, the administration of Lilium extracts resulted in elevated levels of DA and 5-HT, thereby restoring the function of monoamine neurotransmitters in rats with depression (Guo et al., 2009). Gallic acid, a ubiquitous phenolic acid in nature, is a crucial bioactive metabolites of Lilium, exhibiting anti-aging, antioxidant, anti-inflammatory, and other therapeutic properties (Zhang and Ma, 2020; Wang P. et al., 2023). Administration of 60 mg/kg gallic acid reduced depressive behavior in depressive model mice, and its antidepressant action is mediated by increasing not only 5-HT levels in the synaptic space, but also catecholamine levels in the brain (Can et al., 2017). Berberine, the principal bioactive metabolites of Lilium alkaloids, exhibits a range of pharmacological effects including antibacterial, anti-inflammatory, antiviral, lipid-modulating, hypoglycemic, antiarrhythmic, antihypertensive, immunomodulatory, and antitumor properties (Chen et al., 2023; Zhao L. et al., 2023; Tan et al., 2023). Berberine has been observed to significantly decrease the resting time of TST and FST in mice with depression, while concurrently elevating the levels of NE and 5-HT in the hippocampus and prefrontal cortex, and the mechanism of action of berberine is believed to be linked to the regulation of monoamine neurotransmitters in the brain (Peng et al., 2007) (Figure 1; Table 4).5-HT may contribute to the pathophysiology of depression via the cAMP/PKA/CREB signaling pathway, which is mediated by the 5-HT1A receptor (Brites and Fernandes, 2015). The elevated concentration of the psamine transporter (DAT) enhances the reuptake rate of DA at synaptic terminals, resulting in a reduction of DA levels in the synaptic cleft and subsequently contributing to depressive symptoms (Zaaijer et al., 2015). 5-HT and NE interact with G protein-coupled receptors (GPCRs) to facilitate neural transmission and generate electrical signals that modulate emotional responses (Yang et al., 2018). Consequently, R. glutinosa-Lilium may modulate depressive symptoms by enhancing the secretion and synthesis of neurotransmitters such as 5-HT, DA, and NE.

Brain-derived neurotrophic factor (BDNF) is a crucial neurotrophic factor in the brain that plays a pivotal role in the growth, survival, and synapse formation of neurons that are linked to emotional and cognitive functions (Lima et al., 2019). According to a study of depression patients, the level of BDNF decreased with increasing severity of the disease, and the more serious the condition, the lower the level of BDNF (Du et al., 2023; Lee et al., 2007). There are two receptors for BDNF, one high-affinity receptor that binds tyrosine kinase receptor B and another low-affinity receptor that binds neurotrophic factor (p75 NTR). BDNF participates in the pathophysiological process of depression mainly through the induction of intracellular tyrosine residue autophosphorylation and receptor dimerization by binding to TrkB (Liu and Wang, 2015). The primary routes through which phosphorylated TrkB initiates downstream signaling cascades predominantly encompass the PI3K/AKT pathway, MAPK pathway, and PLCγ/PKC pathway. These pathways facilitate enhanced synaptic plasticity, improved neuronal growth and survival, and ultimately provide neuroprotection and nutritional support to the nerves (Wang et al., 2020; Mosiolek et al., 2021).

According to the experiment, catalpol significantly increased PI3K, Akt, Nrf2, HO-1, TrkB, BNDF, and other gene and protein expression in rats modeled by CUMS, and confirmed that PI3K/Akt/Nrf2/HO-1 signaling pathways were upregulated by catalpol’s antidepressant mechanism on depression, improving hippocampal neuroprotection and antioxidant levels (Wang J et al., 2021). Phenolic acids are one of the main active metabolites of lilies, in which Regaloside A in Lilium saponins plays a role in various antidepressant compound prescriptions (Luo et al., 2017). Following treatment with Regaloside A, there was an increase in the cell survival rate and phosphorylation levels of BDNF, TrkB, PI3K, and Akt, which is postulated that Regaloside A exerts antidepressant effects via the BDNF-TrkB pathway (Yuan et al., 2021). Contemporary pharmacological research has determined that Rehmanin D possesses the capability to mitigate PC-12 cell impairment caused by elevated levels of cortisol, and the effect is attributed to its potential to augment BDNF expression and elicit anti-apoptotic responses via the BDNF-TrkB pathway, ultimately safeguarding nerve cells and manifesting antidepressant properties (Zhang et al., 2022). After chlorogenic acid treatment, the nerve damage score and brain water content of mice decreased, BDNF, NGF, 5-HT, and 5-HIAA proteins were upregulated, while pro-inflammatory cytokines iNOS, IL-6, TNF-α, NLRP3, and IL-1β were significantly downregulated (Liu et al., 2021) (Figure 2; Table 5). BDNF specifically binds to the TrkB receptor, thereby activating downstream signaling pathways such as PI3K/Akt, MAPK, and cAMP, among others (Fries et al., 2023). This interaction enhances the release of presynaptic neurotransmitters, facilitating nerve signal transmission (Li et al., 2024). Consequently, R. glutinosa-Lilium has been shown to elevate BDNF levels, thereby supporting normal neuronal function and promoting emotional recovery as well as the improvement of cognitive function.

Oxidative stress refers to the state of cellular imbalance resulting from the overproduction of reactive oxygen species (ROS) and the insufficient antioxidant capacity of cells (Sies, 2015). This condition can lead to various pathological processes, including but not limited to inflammation, neurodegeneration, tissue damage, and cell death, when ROS production exceeds the antioxidant response (Bhatt et al., 2020; Redza-Dutordoir and Averill-Bates, 2016; Xing et al., 2024a). The pathophysiology of depression is strongly influenced by oxidative stress and inflammation, the body is stimulated by stress, the redox balance is broken, and the body’s antioxidant enzyme function changes abnormally, producing excess ROS, pro-inflammatory factors are released, and the inflammatory response is activated, ultimately leading to disturbances in the structure and function of biological macromolecules and proteins in nerve cells, culminating in the manifestation of depression (Kohler et al., 2016; Pandey et al., 2018; Vavakova et al., 2015; Zhang, 2018). Clinical studies have provided empirical evidence indicating that individuals suffering from inflammatory diseases are more likely to experience depression (Beurel et al., 2020). Furthermore, an elevation in the levels of pro-inflammatory cytokines, specifically IL-1β, IL-6, and TNF-α, is positively correlated with the severity of depressive symptoms (Neupane et al., 2022; Boucas et al., 2022). Moreover, inhibiting ROS and malondialdehyde (MDA) and increasing antioxidant enzymes like superoxide dismutase and catalase (CAT) can alleviate depression symptoms (Lindqvist et al., 2017).

Geniposide, an iridoid discovered in R. glutinosa, also has demonstrated antidiabetic, antioxidant, antidepressant, and neuroprotective properties (He et al., 2023; Kimura et al., 2023; Li et al., 2020). The current investigation provides evidence that the regulation of GLP-1R/AKT by geniposide effectively mitigates depressive behavior induced by repeated inhibitory stress (RRS) and hippocampal neuronal apoptosis in mice, concomitantly decreasing the content of pro-inflammatory cytokines IL-1β and TNF-α (Zhao et al., 2018). The ethanol extract derived from Lilium exhibits a specific inhibitory effect on the nuclear factor κ B (NF- κ B) signal pathway, which is induced by inhibitor kappa B kinase β (IKK β), thereby exerting an anti-inflammatory effect and the main bioactive metabolites in the Lilium alcohol extract were identified as quercetin, luteolin, and kaempferol through high performance liquid chromatography (HPLC) (Han et al., 2018). This observation indicates that lilies may mitigate oxidative stress by modulating glutamate metabolism, which subsequently activates the Nrf-2 signaling pathway (Xing et al., 2024b). Quercetin, luteolin, and kaempferol are flavonoids in Lilium, which have potent antioxidant activity (Xu et al., 2022). The administration of Quercetin has been observed to yield a significant reduction in anxiety and depression in mice that have been subjected to chronic unpredicted stress (CUS)-induced depression and quercetin has been observed to decrease the expression of oxidative stress markers and pro-inflammatory cytokines in hippocampal neurons, thereby conferring protection to the mouse brain against oxidative and inflammatory stress (Mehta et al., 2017). The administration of luteolin to CUMS mice leads to a significant increase in the activation of SOD and GSH-Px in brain tissue, a reduction in MDA levels, and inhibition of neuronal oxidative stress (Liu et al., 2013). Kaempferol activates the AKT/catenin cascade in the prefrontal cortex of CSDS mice, thereby augmenting its antioxidant and anti-inflammatory properties (Gao et al., 2019). Notably, a neuroinflammatory response is triggered by the activation of microglia and subsequent release of pro-inflammatory cytokines (Zhang et al., 2016). Under normal circumstances, microglia (M0) in the central nervous system are in a quiescent state and play the role of “immune surveillance,” while microglia in the pathological state come to life and release a series of cytokines, which participate in the occurrence and development of neuroinflammation (Xu et al., 2020). The continuous activation of classical activated microglia (M1) will produce excessive inflammatory factors and oxidative stress, causing damage to nerve cells and leading to aggravation of the disease, while alternative activated microglia (M2) can promote tissue repair and regeneration and play a neuroprotective role (Kwon and Koh, 2020; Wolf et al., 2017; Chen et al., 2024). Thus, the inhibition of M1 microglia proliferation can lead to an improvement in depressive symptoms and a prevention of neuroinflammation. Compared with the model group, the Rehmannia glycoside D-group exhibited a reduction in the levels of pro-inflammatory cytokines IL-6 and IL-1β released by M1 microglia, and an increase in the levels of anti-inflammatory cytokines IL-4 and IL-10 released by M2 microglia, which may be attributed to the inhibition of microglial transformation from M2 to M1 (Wang H. H. et al., 2021). As measured by CUMS, Catalpol not only increased hippocampal SOD, CAT, GSH-Px, GST, GST and GSH levels in rats, but also inhibit microglial polarization of the M1 phenotype and reduce the expression of IL-1β, TNF-α and iNOS (Wang Y. T. et al., 2021) (Figure 3; Table 6). Oxidative stress can enhance the activity of the rate-limiting enzyme in tryptophan metabolism by promoting neuroinflammation, leading to increased production of quinolinic acid and stimulating microglia to express kynurenine-3-monooxygenase (KMO, resulting in the conversion of kynurenine to the neurotoxic metabolite quinolinic acid (Vaglio-Garro et al., 2024; Qin and Zhang, 2020; Sipahi et al., 2023). Empirical studies have demonstrated that the activation of NF-κB and the presence of pro-inflammatory cytokines such as IL-1β and TNF-α can enhance the expression and activity of iNOS, promotes the production of nitric oxide (NO), induces the release of glutamate-containing vesicles from astrocytes, inhibits the reuptake of glutamate, and consequently elevates extracellular glutamate concentrations (Ida et al., 2008; Olivenza et al., 2000; Du et al., 2022). Consequently, oxidative stress interacts with inflammation to exacerbate depressive symptoms and R. glutinosa-Lilium may exert an antidepressant effect by reducing the levels of pro-inflammatory cytokines IL-1β and TNF-α and modulating the tryptophan-kynurenine pathway (Guo et al., 2024).

Neuroimaging and autopsy studies found that depression patients’ plasma, cerebrospinal fluid, and brain glutamate (Glu) concentrations were higher, and serum Glu levels were positively correlated with MDD severity (Zhang et al., 2013; Yan, 2022). With ketamine, a glutamate receptor (NMDAR) antagonist, as a quick-acting antidepressant, the role of glutamatergic nervous system in depression has received widespread attention (Murrough et al., 2017). Glu homeostasis is maintained by the glutamate-glutamine cycle in the central nervous system, and neurons and astrocytes provide a strong guarantee of neuronal activity (Eid et al., 2016; Mahmoud et al., 2019). A high concentration of Glu damages nerve cells and overstimulates glutamate receptors (NMDAR, etc.), which may contribute to depression (Wu et al., 2014). In the rat model of depression induced by CUMS, the levels of NMDAR phosphorylation and subunit NR1/NR2B protein increased significantly, the abnormal concentration of Glu in synaptic space led to the overactivation of extra synaptic NMDAR, and a large amount of Ca²⁺ influx led to intracellular Ca²⁺ overload, resulting in nerve cell death (Li et al., 2018; Beneyto et al., 2007).

Modern studies have found that catalpol in R. glutinosa can significantly reverse the decrease of cell survival rate and muscarinic receptor density induced by L-Glu, suggesting that catalpol may have neuroprotective effects by regulating the cholinergic nervous system (Wang et al., 2008). Acubin inhibits glutamate receptor NMDAR1 and oxidative stress, thereby improving Glu excitotoxicity, and improving PC-12 cell damage induced by Glu, which has potential activity in the treatment of depression (Lu et al., 2022). The phenylethanol glycoside compound echinacoside exhibits neuroprotective, antiinflammatory, antioxidant, antiviral, cardiac activity, and many other biological properties (Liu et al., 2018). Moreover, echinacoside crosses the blood-brain barrier, suggesting potential clinical application for neurological diseases (Zhu et al., 2013). Echinacea glycoside may improve Glu-induced PC-12 cell damage by reducing intracellular Ca²⁺ accumulation, inhibiting NMDAR1 protein expression and antioxidation (Lu et al., 2021). The consumption of berberine significantly decreased ROS production, lipid peroxidation, and DNA fragmentation in glutamate-damaged hippocampal cells, increasing glutathione content and SOD activity, and the anti-apoptotic effect of berberine was demonstrated by reducing the overexpression of Caspase-3 and Bax/Bcl-2 induced by glutamate (Xue, 2021) (Figure 4; Table 7). In vivo, elevated glutamate levels lead to the over-activation of NMDA receptors, causing a substantial influx of Ca2+ and subsequent Ca2+ overload (Zhou et al., 2024). This cascade triggers downstream pathways that result in the production of reactive oxygen species and mitochondrial dysfunction, ultimately leading to neuronal damage (Wang Y. L et al., 2021). Rehmannia glutinosa-Lilium may exert an antidepressant effect by mitigating Ca2+ accumulation within cells and inhibiting the expression of glutamate receptors.

Depression can be caused by excessive excitation of the HPA neurohormone, which regulates stress states in the body (Juruena et al., 2018; Tang et al., 2019). The hypothalamus begins with adrenocorticotropic hormone-releasing hormone (CRH) secreted by the paraventricular nucleus and then stimulates the pituitary gland, where the anterior lobe releases adrenocorticotropic hormone (ACTH), which in turn induces adrenal gland secretion of CORT. In turn, CORT regulates the stress response by reducing its own secretion by sending feedback signals to the hypothalamus and pituitary to reduce the production of CRH and ACTH (Sarno et al., 2021; Plotsky et al., 1998). It was found that the secretion and response of CORT, the level of CRH in cerebrospinal fluid and inflammation increased in patients with severe depression (Zunszain et al., 2011; Amasi-Hartoonian et al., 2022; Horowitz et al., 2020). The increase of CORT level caused by imbalance of HPA axis was directly related to depressive symptoms (Wang et al., 2022). Behavioral test results of mice after injection of CORT suggested depression-like behavior, and serum levels of CORT, ACTH, and CRH increased dose-dependently and over time (Chen et al., 2021; Mikulska et al., 2021; Lok et al., 2012; Joseph and Golden, 2017).

Geneniposide was found to restore the negative feedback between the CRH expression and HPA axis injured by CUMS, which inhibited its high activity, and a significant reduction in CORT serum levels, as well as CRH mRNA expression, was also observed, but ACTH levels were not significantly affected (Cai et al., 2015). Lilium saponins have been found to exhibit an antidepressant effect by suppressing the hyperactivity of the HPA axis, leading to a reduction in circulating levels of COR, ACTH and CRF mRNA in rats (Guo et al., 2010). In mice, the antidepressant effect of catalpol is attributed to its ability to regulate both NF-κB and Nrf2, thereby inhibiting HPA axis hyperactivity, central inflammation, oxidative damage, and depression-like behavior induced by CORT (Song et al., 2021). Berberine has the potential to induce a calming and hypnotic effect through the inhibition of the HPA axis and the augmentation of the levels of 5-HT and NE in the hypothalamus of a mouse model of insomnia induced by PCPA (Zhou et al., 2014) (Figure 5; Table 8). When 5-HT levels in the brain are low, ACTH secretion of the pituitary gland increases, resulting in increased secretion of peripheral cortisol, which suggest that the 5-HT system in the brain exerts an inhibitory effect on ACTH secretion (Zhao et al., 2021a). Following the interaction between cortisol and glucocorticoid receptors, there is an activation of tyrosine aminotransferase and tryptophan pyrroliase, which reduces the synthesis of the 5-HT and NE precursors, tyrosine and tryptophan, resulting in a decrease in the content of monoamine transmitters in the brain and worsening anxiety and depression (Arborelius and Eklund, 2007; Greenstein and Hunt, 2023). Therefore, R. glutinosa-Lilium regulates the HPA axis by reducing the content of hormones such as CORT, ACTH and CRH, thus playing an antidepressant role.

The gut-brain axis, which refers to the complex interaction between the gastrointestinal tract and the brain, has been demonstrated to exert a significant impact on emotional regulation, cognitive processes, and the central nervous system, ultimately contributing to the pathogenesis of depression (Tian et al., 2022; Kim et al., 2023; Xie et al., 2023). Recent research has demonstrated notable alterations in the intestinal microbiota of both depressed individuals (Huang Y et al., 2023) and animal models of depression (Xie et al., 2023), indicating a strong association between gut flora and depression (Zhang M. et al., 2021; Zhao N. et al., 2023). By regulating coding RNA, non-coding RNA and various signal pathways, intestinal flora can regulate not only the function of hippocampal and microglia, but also the expression level of BDNF and immune inflammatory response related to depression, which ultimately affect depression’s occurrence and development, suggesting that a potential target for treating depression could be inhibition of intestinal flora (Chen et al., 2022; Xu M. et al., 2023).

The anti-fatigue, antidepressant, antibacterial, and other effects of Lilium polysaccharides have been demonstrated in modern pharmacological studies (Gao et al., 2015). Lilium polysaccharides regulate intestinal flora imbalance by inhibiting the increase of LPS, IL-6, and TNF- α, increasing the content of secretory immunoglobulin A (SIgA) and regulating intestinal flora imbalance by cultivating beneficial bacteria and inhibiting harmful bacteria (Zhao et al., 2020). In STZ-induced diabetic mice, oral administration of 300 mg/kg R. glutinosa stachyose can significantly lower blood glucose levels, restore the number of Lactobacillus and some normal bacteria reduced by disease to a certain extent, which has the dual effect of regulating blood sugar and intestinal flora (Wang, 2013). Following the administration of kaempferol, the intestinal microbiota of mice with ulcerative colitis exhibited an increase in richness and the relative ratio of Firmicutes and Bacteroides was observed to increase, while the relative abundance of pathogenic species decreased and the abundance of probiotics increased (Qu, 2021). Berberine can significantly reduce the levels of both Trichobacterium and Clostridium diffrium in rats, increase the levels of Rumen and Lactic Acid Bacteria, and inhibit pro-inflammatory cytokines, thus inhibiting the overactivated inflammatory response by regulating rat intestinal flora (Huang D. X. et al., 2023). In mice with non-alcoholic fatty liver disease (NAFLD), the administration of chlorogenic acid resulted in an increase in intestinal flora and the secretion of glucagon-like peptide-1 (GLP-1), which is known to regulate inflammation (Shi et al., 2021) (Figure 6; Table 9). Turicibacter is an intestinal bacterium that expresses a sodium transporter-related protein exhibiting sequence and structural homology to mammalian neurotransmitters, among which DA, NE, acetylcholine, and GABA are all neurotransmitters closely associated with depression, thus causing depressive mood and behavior (Fung et al., 2019; Feng et al., 2024). We hypothesize that R. glutinosa-Lilium may influence the synthesis and transport of neurotransmitters by effectively preventing intestinal microorganisms from entering systemic circulation, inhibiting the body’s immune response, and regulating the abundance of intestinal flora, thereby achieving therapeutic effects in the treatment of depression.