Antidepressant effect of licorice total flavonoids and liquiritin: A review

Abstract

With the development of society and changes in lifestyle, major depressive disorder (MDD) has become a significant disease that plagues many people. Licorice, an excellent natural medicine with a long history of cultivation and application, is found in classical antidepressant prescriptions such as Chaihu Shugan Powder, Ganmai Dazao Decoction, Suanzaoren Decoction, etc. Licorice mainly contains triterpenoids and flavonoids, among which licorice total flavonoids (LF) and liquiritin are the main active components with good antidepressant effects. The pharmacological effects of licorice have been extensively investigated in current studies. However, a review of the antidepressant effects of LF and liquiritin has not been conducted. This article reviews the antidepressant effects of LF and liquiritin, including the biological characteristics of licorice and the pharmacological mechanism of LF and liquiritin in treating MDD. Studies have shown that LF and liquiritin can exert their antidepressant effects by improving depressive behavior, regulating endocrine and hypothalamic-pituitary-adrenal (HPA) axis function, affecting the brain-derived neurotrophic factor (BDNF)/tyrosine kinase B (TrkB) signaling pathway, enhancing synaptic plasticity, increasing monoamine neurotransmitter levels, protecting nerve cells, reducing inflammation, preventing apoptosis, reducing oxidation and other ways. This lays a theoretical foundation for the development of antidepressant drugs.

1. Introduction

Major depressive disorder (MDD) is a severe mood disorder characterized by anhedonia, insomnia or hypersomnia, psychomotor agitation or retardation, fatigue, feelings of worthlessness or guilt, difficulty concentrating, and recurrent thoughts of suicide or death [1]. MDD is a significant public health concern [2]. According to a report by the World Health Organization (WHO), over 350 million people worldwide suffer from MDD, accounting for approximately 4.4 % of the global population. The report also stated that by 2020, MDD would become the second largest disease after cardiovascular and cerebrovascular diseases [3]. Approximately 15 % of patients with MDD choose to commit suicide each year. In the 2010 Global Burden of Disease Study, MDD was considered the leading cause of suicide and ischemic heart disease and ranked second in the world [4]. It has been reported that individuals, including COVID-19 patients, medical staff, and ordinary people, are under both physical and psychological stress during the pandemic, with many developing MDD or anxiety [5]. MDD has become a major challenge to public health and medical efforts.

The pathogenesis of MDD is still unclear. However, the most studied hypotheses include monoamine neurotransmitters and their receptors, inflammatory response hypothesis, hypothalamic-pituitary-adrenal (HPA) axis dysfunction hypothesis, neurotrophic factor hypothesis, and multifactorial hypothesis [6].

Antidepressants are categorized by the Anatomical Therapeutic Chemical classification system into tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs), and other antidepressants [7]. Behavioral therapy can also be used as an adjunct to antidepressants in treating MDD [8]. Although these drugs can improve MDD symptoms to varying degrees, their side effects can be severe. These side effects mainly include apathy, fatigue, sleep disorders, cognitive dysfunction, sexual dysfunction, etc [7]. At the same time, studies have also confirmed that using antidepressants increases the risk of stroke recurrence [7,9]. Chinese herbal medicine has more advantages than Western medicine in preventing and treating MDD due to its multitargeting nature, high efficacy, low toxicity, minimal side effects, low drug resistance, and more stable efficacy [5].

Licorice is the dried root and rhizome of leguminous plants such as Glycyrrhiza uralensis Fisch., Glycyrrhiza inflata Bat., or Glycyrrhiza glabra L., which has the effects of improving interior conditions and promoting Qi movement, relaxing spasms and relieving pain, clearing heat and detoxifying, resolving phlegm and relieving coughs, and reconciling medicinal properties [10]. Modern studies have confirmed that licorice has many pharmacological effects such as anti-oxidation, anti-inflammation, anti-depression activity, and other activities [11].

In clinical practice, licorice is often combined with other traditional Chinese medicines (TCM) to make various antidepressant TCM prescriptions such as Chaihu Shugan Powder, Ganmai Dazao Decoction, Suanzaoren Decoction for treating MDD. This indicates that licorice is an excellent TCM. And studies have confirmed the licorice total flavonoids (LF) and liquiritin are the material basis of licorice ’s antidepressant effect [[24], [25], [26], [27], [28], [29], [30]].

However, a review of the antidepressant effects of LF and liquiritin has not been conducted. This article reviews the antidepressant effects and pharmacological mechanisms of LF and liquiritin. LF and liquiritin can exert their antidepressant effects by improving depressive behavior, regulating endocrine and HPA axis, influencing BDNF/TrkB signaling pathway, enhancing synaptic plasticity, increasing monoamine neurotransmitter levels, protecting nerve cells, reducing inflammation, preventing apoptosis, reducing oxidation and other ways. Looking forward to more researchers utilize modern scientific methodologies to study licorice and its prescriptions in depth and transform them into modern drugs, in order to facilitate their application into the clinical therapy and provide a second solution beyond Western medicine treatment for MDD.

2. Biological characteristics of licorice

2.1. Plant characteristics of licorice

Licorice is a perennial herb that is 30–70 cm high. The cross-section of the root is pale yellow or yellow and has a sweet taste. Stem erect, plumose compound leaves. Corolla butterfly-shaped, light blue purple or purplish red. Seeds 2–8, oblate or kidney-shaped, black [17]. Licorice flowers from June to August and bears fruit from July to October.

2.2. Bioactive components

Licorice contains over 400 identified compounds, including triterpenoid saponins and flavonoids [18], coumarin, phenolic compounds, pterocarpin, and other compounds [19]. Additionally, 300 flavonoids with C6–C3–C6 basic skeletons, including flavones, flavonols, chalcones, and isoflavones, have been extracted from licorice [[20], [21], [22]]. LF is a general term for compounds with a flavonoid backbone in licorice, mainly including flavanone, flavone, flavonol, chalcone, isoflavone, isoflavanone, isoflavone, isoflavene, as shown in Fig. 1. In Western nations, licorice is primarily used in non-medicinal forms [23]. Numerous studies have demonstrated that LF and liquiritin are the material basis for licorice's antidepressant effects [[24], [25], [26], [27], [28], [29], [30]], and licorice can exert its antidepressant effects through a variety of mechanisms (Fig. 2).

Fig. 1.

(a) Flavanone, flavone, flavonol, and chalcone structure from licorice. (b) Isoflavone, isoflavanone, isoflavan, and isoflavene structure from licorice. Note: Based on the pictures provided by Wu et al., 2022, Hindawi. https://doi.org/10.1155/2022/9523071.

Fig. 2.

The antidepressant mechanism of Licorice. NF-κB: nuclear factor-k-gene binding; BDNF: brain-derived neurotrophic factor; TrkB: tyrosine kinase receptor B; pTrkB: protein tyrosine kinase receptor B; CRH: corticotropin releasing hormone; ACTH: adrenocorticotropic hormone; CORT: Corticosterone; 5-HT: 5-hydroxytryptamine; 5-HIAA: 5-hydroxyindole acetic acid; NE: Norepinephrine; SYP: synapsin; PSD-95: PSD-95 Polyclonal Antibody; IL-1β: interleukin-1β: IL-6: interleukin-6; SOD: superoxide dismutase; MDA: malonaldehyde; FGF-2: fibroblast growth factor 2; TNF-α: tumor necrosis factor-α; Bcl-xl: B-cell lymphoma-xl; Bcl-2: B-cell lymphoma-2; Bax: Bcl-2 associated protein X; iNOS: inducible nitric oxide synthase (enzyme); COX-2: Cyclooxygenase-2; PPAR-γ: peroxisome proliferators-activated receptor-γ.

2.3. Nomenclature of licorice

Licorice is the dried root and rhizome of the leguminous plant Glycyrrhiza uralensis Fisch., Glycyrrhiza inflata Bat. or Glycyrrhiza glabra L [12]. There are about 29 species and six varieties of Glycyrrhiza in China. Only Glycyrrhiza uralensis Fisch., Glycyrrhiza inflata Bat., and Glycyrrhiza glabra L. are listed in the national pharmacopeia [13]. Licorice is distributed in the northeast, north, and northwest of China. It is a belt distribution with long east-west and narrow north-south [14]. Glycyrrhiza is China's most frequently prescribed natural medicine and has been used for more than 2000 years [15].

The information and data obtained through experimental and clinical studies have shown that licorice possesses antiviral, antimicrobial, antitussive, and other activities. In addition, gastroprotective, hepatoprotective, anticonvulsant, and other pharmacological effects can be elicited by licorice and its constituents [16,[31], [32], [33]].

2.4. Extraction methods and quality control

Common methods for extracting active ingredients from licorice include reflux extraction, ultrasonic extraction, and microwave extraction. Qualitative and quantitative analysis of glycyrrhizic acid and liquiritin primarily involves the use of high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), gas chromatography (GC) [34], and ultraviolet spectrophotometry (UV). Some researchers also employ the aluminum chloride colorimetric method or gravimetric method to analyze the content of glycyrrhizic acid [34,35]. HPLC and GC are the most commonly used methods for quality control of traditional Chinese medicine (TCM) [36,37].

According to the provisions of the 2020 edition of the Chinese Pharmacopeia, the moisture content of medicinal materials should not exceed 12.0 % (the second method of general rule 0832), the total ash content should not exceed 7.0 % (general rule 2302), and the acid insoluble ash content should not exceed 2.0 % (general rule 2302). The content of liquiritin and glycyrrhizic acid was determined by HPLC. The content of liquiritin (C₂₁H₂₂O₉) should not be less than 0.50 %, and glycyrrhizic acid (C₄₂H₆₂O₁₆) should not be less than 2.0 % [12].

3. The pharmacological mechanism of LF and liquiritin in the treatment of MDD

3.1. Improvement of depressive behaviors

Katz first proposed the chronic stress model in 1981 [38]. Over the course of more than 40 years, this model has been refined and is now widely used in scientific research on MDD [[39], [40], [41], [42], [43], [44]]. Chronic unpredictable mild stress (CUMS) is currently the most commonly used, reliable, and effective rodent model of MDD [45]. The principle behind this model is to induce behavioral changes in rats that are similar to the clinical manifestations of MDD by subjecting them to chronic mild and unpredictable stimulation. According to Willner, the chronic mild stress model is often used as an effective model for studying the pathogenesis of MDD and the mechanisms of antidepressant drugs due to its rationality as an animal model and its similarity to the mental symptoms of MDD and response to antidepressant drugs [[46], [47], [48]].

Guo et al. conducted a study on the antidepressant effects of LF by establishing a CUMS model in adult rats. They used an open field test (OFT), forced swimming test (FST), tail suspension test (TST), and serum corticosterone detection. The results showed that LF (30,100,300 mg/kg) increased the number of uprights and grid crossings in the OFT of rats and reduced feces production. Additionally, high doses of LF reduced immobility time caused by CUMS in the classical antidepressant screening model forced swimming and tail suspension experiments (Table 1). Similar findings were reported by Hua et al. and Fan et al. [28,29]. Compared to the positive drug fluoxetine group, LF had no central excitatory effect on the donor mice [46]. Cheng et al. investigated the antidepressant effects and possible mechanisms of the extracted parts of licorice by administering LF to mice and using a combination of FST, TST, ART (antagonism of reserpine induced symptoms test), and HTT (head thrust test induced by 5-hydroxytryptophan). The results showed that compared to normal controls, LF reduced immobility time in swimming and tail hanging mice and effectively antagonized ptosis and motor inability induced by rifampicin administration for 1 h but did not decrease body temperature induced by mice after 4 h [26].

Table 1.

Experiment of LF and liquiritin in MDD model mice.

Experimental objectives	Experimental method	Experimental model	Outcomes	References
Investigating the antidepressant effect of LF	FST, TST, OFT, Immunohistochemical method	CUMS mice	All dose groups of LF improved the depressive behavior of CUMS mice; CORT↓	[46]
Investigating the antidepressant mechanism of LF	FST, TST, OFT, Immunohistochemical method, Western Blot, qRT-PCR	CUS mice	LF 100, 300 mg/kg improved the depressive behavior of CUS mice; SYP↑, PSD-95↓; the mRNA expression of SYP↑, the mRNA expression of PSD-95↓	[28]
Investigating the antidepressant effects and neuroprotective effects of LF	FST, TST, OFT, RIA, Cell proliferation was detected by BrdU	CUS mice	LF 300 mg/kg improved the depressive behavior of CUS mice; newborn BrdU progenitor cells↑; CORT↓	[29]
Investigating the antidepressant effects of LF	FST, TST, OFT, ELISA, qRT-PCR, Immunohistochemical method, Western Blot,	CUS mice	All dose groups of LF improved the depressive behavior of CUS mice; BDNF positive cells↑, TrkB positive cells↑, pTrkB positive cells↑; medium and high dose LF: BDNF↑, the mRNA expression of BDNF and TrkB↑	[27]
Investigating the effect on apoptosis-related proteins in hippocampal neurons of LF	FST, TST, OFT, Western Blot	CUS mice	LF improved the depressive behavior of CUS mice; the mRNA expression of Bcl-xl↑, the expression of Caspase-3↓	[25]
Investigating the effects on weight and the behavior of depressed rats of liquiritin	Sucrose consumption test, FST	CUMS mice	No significant change in weight; syrup↑; immobility time↓	[49]

Investigating the pharmacodynamics and mechanism of liquiritin	FST, TST, OFT, ART, Water maze experiment, HPLC-ECD, Immunohistochemical method	CUMS mice	All dose groups of liquiritin improved the depressive behavior of CUMS mice, antagonized ptosis, akinesia, and hypothermia induced by reserpine; weight↑, syrup↑, learning and memory skills↑; 20 mg/kg liquiritin: 5-HT↑, NE↑, IL-1β↓, CORT↓	[50]
Investigating the anti-depressant effects of liquiritin	FST, TST, EPM, UV, ELISA, Western Blot, Immunohistochemical method	Behavioral despair model mice, CUMS mice	40 mg/kg liquiritin improved the depressive behavior of CUMS mice, SOD↑, MDA↓, weight↑, syrup↑, IL-1β↓, IL-18↓, NLRP3↓, Caspase-1↓, ASC↓	[51]
Investigating the antidepressant-like effects of liquiritin and isoliquiritin in FST and TST	FST, TST, HPLC-ECD	Behavioral despair model mice	All dose groups of liquiritin and isoliquiritin improved the depressive behavior of model mice; 5-HT↑, NE↑, 5-HIAA/5-HT↓	[52]
Investigating the association between the antidepressant activity of liquiritin and FGF-2 regulation	Western Blot, Immunofluorescence staining, qRT-PCR	LPS-induced depression model in ICR mice	FGF-2↑, the mRNA expression of IL-1β, IL-6, TNF-α↓	[53]
Investigating the effect of liquiritin on BDNF, Bax, and Bcl-2 protein expression in the prefrontal cortex of PSD rats.	Immunofluorescent staining, Western Blot	Focal cerebral ischemia mice	BDNF↑, Bcl-2↑, Bax↓	[54]

LF: licorice total flavonoids; FST: forced swimming test; TST: tail suspension test; OFT: open filed test; CUMS: chronic unpredictable mild stress; CUS: chronic unpredictable stress; CORT: Corticosterone; qRT-PCR: Quantitative Real-time PCR; SYP: Synaptophysin; PSD-95: Postsynaptic Density Protein 95; RIA: Radioimmuno-assay; BrdU: Bromodeoxyuridine; ART: Antagonism of reserpine induced symptoms test; HTT: head thrust test induced by 5-hydroxytryptophan; KM: Kun Ming; ELISA: enzyme linked immunosorbent assay; BDNF: brain-derived neurotrophic factor; TrkB: tyrosine kinase B; pTrkB: phospho-tyrosine kinase B; Bcl-xl: B-cell lymphoma-xl; Bcl-2: B-cell lymphoma-2; HPLC-ECD: High performance liquid phase electrochemical detection; 5-HT: 5-hydroxytryptamine; NE: Norepinephrine; IL-1β: interleukin-1β; IL-18: interleukin-18: IL-6: interleukin-6; EPM: elevated plus-maze test; SOD: superoxide dismutase; MDA: malonaldehyde; NLRP3: NOD-like receptor protein 3; ASC: apoptosis-associated speck-like protein containing; 5-HIAA: 5-hydroxyindole acetic acid; FGF-2: fibroblast growth factor 2; LPS: lipopolysaccharide; TNF-α: tumor necrosis factor-α; Bax: Bcl-2 associated protein X; PSD: postsynaptic density.

Zhao et al. found that liquiritin effectively shortened forced swimming immobility time and increased sugar and water consumption in rats with MDD [49]. Single administration of 40 mg/kg, 20 mg/kg, and 10 mg/kg of liquiritin significantly shortened tail suspension and swimming immobility time in mice. Each dose group of liquiritin significantly increased body weight, sugar and water consumption, grid crossings, and uprights in OFT rats and improved their memory. Liquiritin also had a significant anti-lipoprotein effect [50]. Liu et al. investigated the antidepressant effect of liquiritin and found that it reduced immobility time in TST and FST in despair model mice without affecting autonomic activity. Liquiritin shortened immobility time in TST and FST in CUMS mice, increased dwell time in EPM, and the number of times they entered the open arm. Liquiritin (40 mg/kg, once daily for four weeks) significantly increased sucrose consumption and body weight in CUMS mice [51]. These studies show that LF and liquiritin have significant antidepressant effects and can effectively improve depressive behavior in MDD model mice.

3.2. Endocrine and HPA axis regulation

Stress hormones such as corticosterone (CORT) play a crucial role in MDD development. Chronic CORT administration can induce HPA axis dysfunction leading to MDD [55]. Hyper-functional HPA axis increases corticotropin-releasing hormone (CRH) secretion, leading to increased plasma CORT levels. The high level of CORT in plasma further damages the hippocampus, which contains many glucocorticoid receptors, forming a vicious cycle. In animal experiments for antidepressant drug screening, CORT level detection has become an objective index to determine MDD model establishment and antidepressant pharmacodynamics [46,56,57]. LF can reduce serum CORT level increase caused by chronic stress stimulation, suggesting that LF can improve CUMS behavioral indexes by reducing serum CORT level [46]. Additionally, the medium dose group of liquiritin (20 mg/kg) could reverse serum CORT increase [50]. In summary, both LF and liquiritin can effectively reduce serum or plasma CORT levels. Since serum CORT level increase is closely related to HPA axis dysfunction or hyperactivity, it can be inferred that LF and liquiritin's antidepressant effect may be achieved by regulating the HPA axis.

3.3. BDNF/TrkB signaling pathway influence

Brain-derived neurotrophic factor (BDNF) is an important neurotrophic factor widely expressed in the central nervous system, with the highest levels in the hippocampus and cortical areas. BDNF enhances the autophosphorylation of tyrosine kinase B (TrkB) by binding to TrkB, which in turn activates cyclic phosphoadenosine effector binding protein (CREB) by activating the Ras-Mapk signaling pathway and finally at the serine site of CREB, which in turn enhances synaptic plasticity by increasing the expression of the BDNF gene and the anti-apoptotic protein Bcl-2. Guo used ELISA to detect changes in serum BDNF levels, immunohistochemistry to determine the protein content of BDNF, TrkB, and phospho-tyrosine kinase B (pTrkB) in the hippocampus, Western Blot to determine the content of BDNF, and qRT-PCRT to detect the mRNA expression of BDNF and TrkB. The results showed that LF effectively improved serum BDNF level, increased the number of BNDF protein-positive cells, TrkB protein-positive cells, and pTrkB protein-positive cells in hippocampal DG, CA1, CA3, and Cortex. It also increased BDNF protein content and enhanced BDNF and TrkB mRNA expression in model rat hippocampus, suggesting that LF's antidepressant mechanism might be related to enhancing BDNF and its receptor TrkB protein expression [27]. Studies on glycyrrhizin's antidepressant effects found that it can treat chronic MDD through the PI3K/Akt/mTOR mediated BDNF/TrkB pathway, similar to LF's mechanism of action [58].

3.4. Synaptic plasticity enhancement

Synaptophysin (SYP) and postsynaptic compactor protein-95 (PSD-95) are two critical proteins in synapses. SYP and PSD-95 are closely related to synaptic plasticity and MDD. Antidepressants often modulate neuroplasticity-related intracellular signal transduction pathways, suggesting that neuroplasticity mechanisms may play a vital role in MDD pathogenesis and treatment [28,[59], [60], [61], [62], [63], [64], [65]]. It was found that 100,300 mg/kg of LF significantly increased the expression of SYP protein and mRNA in the hippocampus of CUMS mice and reduced the expression of PSD-95 protein and mRNA, suggesting that LF may exert its anti-chronic unpredictable stress depression effect by regulating essential synaptic plasticity proteins’ expression [28]. Increased synaptic plasticity can further affect long-term potentiation (LTP), considered one of the main molecular mechanisms of learning and memory formation.

3.5. Central 5-HT nerve function enhancement and monoamine components regulation

The monoamine hypothesis of MDD suggests that 5-hydroxytryptamine (5-HT), norepinephrine (NE), and dopamine (DA) synergistically affect mood, forming the basis of current MDD drug therapy [66]. 5-HT is an essential neurotransmitter in many physiological processes such as platelet aggregation, pain, sleep, appetite, muscle contraction, emotions, obsessions, and compulsions [67].

Studies showed that LF 150 mg/kg and 400 mg/kg could significantly synergize to increase head shakes after 5-HT injection. Monoamine oxidase (MAO) activity in mice cortex, hippocampus, and thalamus was not different from normal controls, suggesting that LF's improvement of mice depressive behavior may be related to directly enhancing brain 5-HT neurological function [26]. The mechanism of action may be through enhancing central 5-HT or dopaminergic neurological function, increasing monoamine transmitter levels in the brain synaptic gap to achieve antidepressant effects. Wang et al. found that liquiritin and isoliquiritin effectively antagonized model mice depression-like behavior and increased hippocampus, hypothalamus, and cortex 5-HT and NE concentration in mice. 5-HT is eventually metabolized into biologically inactive 5-hydroxyindoleacetic acid (5-HIAA) in vivo. Liquiritin and isoliquiritin reduced the ratio of 5-HIAA/5-HT in mice's hippocampus, hypothalamus, and cortex and slowed down the metabolism of 5-HT [52]. Additionally, the liquiritin medium dose group (20 mg/kg) significantly increased hippocampus 5-HT and NE levels but had no noticeable effect on DA and dopamine metabolite dihydroxyphenylacetic acid (DOPAC) content. This suggests that liquiritin's antidepressant effect may be caused by increasing hippocampus, hypothalamus, and cortex 5-HT and NE concentration in mice.

3.6. Neuroprotection

The subgranular zone (SGZ) of the hippocampal dentate gyrus is an important area for nerve regeneration in adulthood. The proliferation, migration, and differentiation of stem cells are closely related to MDD occurrence and have become a key target for anti-depression drugs [29,68,69]. Bromodeoxyuridine (BrdU) is a thymidine analog that incorporates into the DNA of dividing cells during the S-phase of the cell cycle and is a marker of DNA synthesis [70]. By intraperitoneal injection of BrdU, it can participate in DNA replication instead of thymine, allowing observation of the number of stem cells transformed into progenitor cells in SGZ and evaluation of SGZ nerve regeneration and recovery ability. The 300 mg/kg dose of LF reduced serum corticosterone levels and restored the number of BrdU-labeled newborn progenitor cells in rat hippocampal dentate gyrus SGZ, indicating that high doses of LF have a specific protective effect on hippocampal nerve regeneration damage caused by chronic unpredictable stress [29].

Xiao investigated liquiritin and liquiritin-containing serum's protective effects on neuronal cells using a glutamate injury model of BV2 cells. The tetramethylazole salt (MTT) method determined cell viability and the AnnexinV-FITC kit detected apoptosis. The results showed that liquiritin could delay glutamate-injured BV2 cell apoptosis and reduce damaged neuronal cell mortality to some extent. The effect of liquiritin-containing serum on neuronal cell morphology is yet to be determined, suggesting that liquiritin's antidepressant activity may be related to its protective effect on neuronal cells [71].

3.7. Anti-inflammatory effect

The release of inflammatory cytokines can lead to neuroendocrine and immune system dysfunction and induce MDD. On one hand, inflammatory cytokine IL-6 release will enhance 5-HT and DA neuron activity, accelerating 5-HT and DA reuptake and reducing their levels in the synaptic cleft. On the other hand, IL-6 will regulate HPA axis negative feedback inhibition by peripheral circulating glucocorticoids, resulting in excessive HPA axis response [72,73]. Nuclear transcription factor-κB (NF-κB) may activate hippocampal NO synthase, increasing NO synthesis and inhibiting hippocampal neurogenesis. NF-κB also leads to increased synthesis of inflammatory cytokines, reactive oxidants, and excitotoxins that cause neurodegenerative lesions, affecting central nervous system process changes, which are also thought to be closely associated with the development of MDD [74]. It was found that both LF and licorice flavonoid components licorice chalcone A and isoglycyrrhizin inhibited iNOS, cyclooxygenase-2 (COX-2) gene and protein level expression and IL-6 inflammatory mediator gene expression. They also upregulated peroxisome proliferator-activated receptor-γ (PPAR-γ) gene expression, suggesting that isoglycyrrhizin may be LF's active anti-inflammatory component [6].

Liquiritin has a similar mechanism of action to LF. Studies found that the medium dose group of liquiritin (20 mg/kg) could reverse serum IL-1β and IL-6 increase but had no significant effect on IL-2 [50]. Fibroblast growth factor 2 (FGF-2) is a protein that maintains central nervous system development and maturation. It was found that liquiritin effectively increased model mice hippocampus FGF-2 level and reduced inflammatory factor IF-1β, IL-6, and TNF-α mRNA expression levels, suggesting that liquiritin may exert its antidepressant effect by maintaining synaptogenesis and inhibiting neuroinflammation [53]. Additionally, Su et al. found that liquiritigenin effectively reduced pro-inflammatory cytokine levels and upregulated BDNF and TrkB expression by NF-κB/p-p65 and p-IκBα, suggesting that liquiritigenin's antidepressant effect may be related to its anti-inflammatory activity and enhancement of BDNF and TrkB protein expression [75].

3.8. Anti-neuronal apoptosis effect

Bcl-xL is an essential anti-apoptotic protein closely related to synaptic plasticity regulation [76]. Bcl-xL protects against activation of the Caspase-3-independent apoptotic pathway [77], achieving a protective effect against neural cells apoptosis. Cheng et al. used Western blot to detect changes in Bcl-xL and Caspase-3 protein expression levels in CUS rat hippocampal tissues and found that Bcl-xL protein expression significantly increased and Caspase-3 protein expression significantly decreased in the hippocampus after LF intervention, suggesting that LF's antidepressant effect may be related to the neural cell apoptosis protective mechanism [25]. Additionally, it was found that liquiritin significantly increased BDNF and Bcl-2 protein expression in model rat prefrontal cortex and reduced Bax protein expression, suggesting that liquiritin's antidepressant effect may be achieved by anti-neuronal apoptosis and enhancing synaptic plasticity.

3.9. Anti-free radical effect

The decrease of superoxide dismutase (SOD) activity and increased malondialdehyde (MDA) content in the body is also related to MDD. Many free radicals produced by oxidative stress, such as lipid peroxidation in vivo, are mainly eliminated by SOD. MDA is a lipid peroxidation metabolite that can accumulate in the brain and crosslink with protein through the primary amino group reaction, destroying the cell membrane lipid bilayer, changing cell permeability, and ultimately causing neuron degeneration and apoptosis. Animal experiments found that CUS model rat SOD activity decreased and MDA content increased [6]. It was found that liquiritin could improve SOD activity, increase antioxidant enzyme activity, reduce free radicals, inhibit lipid peroxidation, reduce MDA production, and improve MDD through anti-free radical methods [30]. Additionally, liquiritigenin can also increase SOD activity and decrease MDA levels [58].

4. Limitations and future prospects

According to literature reports, long-term high-dose intake of licorice can lead to apparent mineralocorticoid excess syndrome and pseudo-hyperaldosteronism [78,79]. And similar to other TCM, licorice exhibits a complex profile characterized by multiple-constituents, multifaceted-targets, and a diverse array of mechanisms of action. This starkly contrasts with the regulatory paradigm of Western medicines, which typically feature single compounds and singular mechanisms of action. As a result, this creates difficulties in LF and liquiritin's research, extraction, purification and industrialization.

The prospect of modernizing TCM holds immense promise. By transforming licorice and its associated classic antidepressant prescriptions into preparations, akin to the approach pioneered by Japan's Tsumura, the range of drug choices for the treatment of MDD patients can be expanded. Furthermore, this endeavor serves as a catalyst for the modernization of other TCM.

5. Conclusions

Licorice is an excellent TCM with a wide range of pharmacological and significant curative effects, especially in treating MDD, highlighting TCM's characteristics. LF and liquiritin are the material basis of licorice in treating MDD. However, it remains to be further studied which signaling pathways LF and liquiritin play a role in enhancing central nervous function and regulating monoamine neurotransmitters and their receptors. Additionally, the extraction and purification process of LF and liquiritin is complex, and the extraction cost is high. Realizing its industrialization is also a critical problem to be solved in developing new drugs. Therefore, it is still necessary to further explore and study the mechanism of LF and liquiritin's antidepressant effect and their extraction and purification process to provide a theoretical basis for developing new drugs and their clinical application.

Funding

This research was funded by the Natural Science Foundation of China (No. 82004212), the Project of Shandong Traditional Chinese Medicine Plan (No. M − 2022259), and the Academic Promotion Programme of Shandong First Medical University (No. 2019LJ001).

Data availability statement

No data was used for the research described in the article.

Additional information

No additional information is available for this paper.

CRediT authorship contribution statement

Ruyu Wang: Writing – original draft, Investigation. Yiwei Chen: Writing – review & editing. Zhiying Wang: Writing – review & editing. Baorui Cao: Writing – review & editing. Jinxin Du: Writing – review & editing. Tingting Deng: Writing – review & editing. Meina Yang: Writing – review & editing, Conceptualization. Jinxiang Han: Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.