Ferulic acid as a therapeutic agent in depression: Evidence from preclinical studies

Abstract Depression is a common but severe mood disorder with a very high prevalence across the general population. Depression is of global concern and poses a threat to human physical and mental health. Ferulic acid (FA) is a natural active ingredient that has antioxidative, anti‐inflammatory, and free radical scavenging properties. Furthermore, studies have shown that FA can exert antidepressant effects through a variety of mechanisms. The aim of the review was to comprehensively elucidate the mechanisms in FA that alleviate depression using animal models. The in vivo (animal) studies on the mechanism of FA treatment of depression were searched in PubMed, Chinese National Knowledge Infrastructure, Baidu academic, and Wan fang databases. Thereafter, the literature conclusions were summarized accordingly. Ferulic acid was found to significantly improve the depressive‐like behaviors of animal models, suggesting that FA is a potential natural product in the treatment of depression. The mechanisms are achieved by enhancing monoamine oxidase A (MOA) activity, inhibiting microglia activation and inflammatory factor release, anti‐oxidative stress, promoting hippocampal nerve regeneration, increasing brain‐derived neurotrophic factor secretion, regulating gut microbiome, and activating protein kinase B/collapsin response mediator protein 2 (AKT/CRMP2) signaling pathway. Ferulic acid produces significant antidepressant effects in animal depression models through various mechanisms, suggesting its potential value as a treatment of depression. However, clinical research trials involving FA are required further to provide a solid foundation for its clinical application.

largest burden worldwide by 2030. 4 The pathogenesis of depression is still controversial, and is considered to result from the combination of genetic, environmental, psychological, and biological factors. 5 Several widely accepted theories for its development include reduced serum serotonin (5-HT) levels, reduction in brain-derived neurotrophic factor (BDNF) release, hyperfunction of hypothalamic pituitary adrenal (HPA), and inflammatory responses. [6][7][8] Given the gradual increase of in-depth depression studies, it has been found that mitochondrial metabolic disorders also contribute to the pathogenesis of depression. 9,10 Despite the significant progress made in the treatment of depression over the past few decades, most clinically developed antidepressant drugs have been associated with unsatisfactory therapeutic effects. Furthermore, these drugs generally have long treatment cycles, severe side effects, and an increased risk of drug dependence. 11 Therefore, the development of highly efficient antidepressant drugs with minimal side effects has become pertinent to the treatment of depression. According to the WHO, approximately 80% of the global population still relies on plant medicines. 12 Phytomedicines are significant gifts from nature. Many natural products that have been isolated from phytomedicines, such as paclitaxel, artemisinin, ginkgolide B, and curcumin, have been structurally modified to yield great medical value. 13 Therefore, it is important to investigate natural products that can serve as potential drugs in the treatment of depression.
The chemical name of FA is 4-hydroxy-3-methoxycinnamic acid, which is one of the derivatives of cinnamic acid (also known as cinnamic acid, 3-phenyl-2-acrylic acid). Originally found in the seeds and leaves of plants, FA is a phenolic acid that is present widely in plants. Ferulic acid combines with polysaccharides and proteins to form the skeleton of the cell wall. 14 A typical Mediterranean diet is rich in plant-based foods (including breads, grains, vegetables, and fruits) and low to moderate in fish, red meat and wine, with daily FA intake of about 150-250 mg (16-24 mol/kg body weight). 15 However, FA intakes are only theoretical (Table 1), as they vary according to dietary habits and the number of vegetables/fruits per day. [16][17][18][19] Furthermore, FA is prevalent in Chinese herbal medicines Currently, significant evidence indicates that FA has various functions such as anti-oxidation, 22 anti-apoptosis, 23 inhibition of neuroinflammation, 24 scavenging free radicals, 25 and promoting nerve regeneration. 26 Therefore, FA can be used to treat a variety of diseases, including dementia, Parkinson's disease, 27 cardiovascular disease, 28 diabetes, 29 and cirrhosis. 30 Notably, FA has shown significant results in antidepression. 31 However, research that systematically explains the antidepressant mechanism of FA is limited. Therefore, this review aims to explore the mechanism of FA that alleviates depression using animal depression models (in vivo). Furthermore, this review aims to provide a useful experimental basis for clinical research and explore innovative ideas for the development of new antidepressants.

| FA : PHYS I CO CHEMI C AL PROPERTIE S AND B IOSYNTHE S IS
The molecular formula of FA is C10H10O4, with a molecular weight of 194.18, and a melting point of 169-173°C. 32 Ferulic acid is highly soluble in methanol, ethanol, and acetone. It is soluble in hot water, slightly soluble in cold water and petroleum ether, and insoluble in benzene. Furthermore, FA can form salt compounds with metal ions to obtain good pH stability. The molecular structure of FA can be divided into two types: cis-form and trans-form. The cis-form, generally, is a yellow oily substance, whereas the trans-form is a white to yellowish square or fibrous crystals. Ferulic acid has strong antioxidant and reduction properties and is easily decomposed when exposed to light. 33 Ferulic acid can be obtained from plants in three ways: first, from binding with certain small molecules; second, from extraction from its plant cell wall; third, extraction from tissue culture. Ferulic

Significance statement
Ferulic acid (FA) is a phenolic acid extracted from the resin of Asafetida. Asafetida is a perennial herb of the Umbelliferae family. Ferulic acid is a common aromatic acid in the plant kingdom. It is a component of suberin. It rarely exists in a free state in plants and mainly forms a combined state with oligosaccharides, polyamines, lipids, and polysaccharides. It has many healthcare functions, such as scavenging free radicals, antithrombosis, antibacterial and anti-inflammatory, inhibiting tumors, preventing and treating hypertension, heart disease, and enhancing sperm motility. Ferulic acid has low toxicity and is easy to be metabolized by the human body. It can be used as a food preservative and has a wide range of uses in food and medicine.
This review aims to explore the mechanism by which ferulic acid alleviates depression in animal depression models (in vivo), provide a useful experimental basis for clinical research, and explore ideas for the development of new antidepressants. In a systematic review of previous studies, we find that FA exerts important neuroprotective effects in CNS diseases through antioxidant, anti-inflammatory, antiapoptosis, and other mechanisms. Moreover, FA was found to alleviate abnormal depressive behaviors in multiple animal depression models, underscoring the potential use of FA as a new antidepressant agent. Given the lack of knowledge explaining the antidepressant mechanism of FA, we believe that our study makes a significant contribution to the literature because prior studies confirm FAs' impact on treating dementia, Parkinson's disease, cardiovascular disease, diabetes, cirrhosis, and depression. acid in plants is crosslinked usually with polysaccharides and lignin through ester bonds or self-esterification or etherification to form di-FA. Ferulic acid can be released by breaking ester bonds by alkali and enzymatic methods, and then extracted with appropriate solvents. 34,35 The chemical synthesis method of FA uses vanillin as the basic raw material and Wittig-Horner reaction (WHR) and Knoevenagel reaction (KR) as the main organic reactions. During the first method, FA is synthesized with WHR. The WHR of triethyl phosphite acetate and acetyl vanillin combine in a strong base system followed by acidification with concentrated hydrochloric acid to obtain FA.
The method needs to first protect phenolic hydroxyl groups in advance. Due to the presence of a strong base, the formation of sodium phenol will inhibit the reaction between the carbonyl group and carbanion, which is also prone to side reactions that generate impurities. For the second method, FA is synthesized with KR. A KR between vanillin and malonic acid was conducted to produce ferulic acid by adding a small amount of organic base in pyridine solvent as a catalyst. The catalysts include piperidine and aniline. However, the method has a long reaction time of up to 3 weeks, obtained from a mixture of trans-FA and cis-FA. 36 In addition, several microorganisms can be used to convert FA precursors to FA by biosynthesis.
For example, the butanol cinnamate ester extracted from clove oil can be converted to FA. 37,38 Even though biosynthesis is a clean and effective synthesis method, it is yet to be mass-produced.

| S E VER AL PATHOG ENE S E S INVOLVED IN DEPRE SS I ON
The pathogenesis of depression is complex and involves the biological, physiological, and social environment. The monoamine hypothesis was proposed in the 20th century and prompted revolutionary progress in the treatment of depression. 39 Most clinical studies have indicated that the onset of depression is mainly due to reduced concentrations of monoamine neurotransmitters such as 5-HT and dopamine (DA). 40 This may be related to the increased activity of indoleamine 2, 3-dioxygenase, and the accelerated decomposition rate of tryptophan among patients with depression, thus inhibiting the metabolism of tryptophan to the 5-HT pathway. 41 In addition, studies have shown that the concentration of dopamine transporter (DAT) is significantly higher among patients with depression compared with the general population. Furthermore, the high concentration of DAT in the body can increase the recovery of DA at the synaptic terminal, which is manifested as a decrease in DA levels in the synaptic space, ultimately inducing depression. 42,43 TA B L E 1 Approximate amounts of ferulic acid in some foods.  45 In rats treated with GC, it was found that CRH-positive neurons in the paraventricular nucleus of the hypothalamus, adrenocorticotropic hormone (ACTH)-positive neurons in the anterior pituitary, and CRH-positive nerve fibers in the median eminence were reduced significantly, resulting in depressive behavior. 46 Furthermore, the hyperfunction of the HPA axis promotes the secretion of excessive GC, which can damage the hippocampal neurons. 47 The damaged hippocampus cannot inhibit the abnormal activity of HPA function effectively, thus forming a vicious circle that can eventually induce depression and cause cognitiverelated dysfunction.

Types of food
The onset of depression is closely related to the inflammatory response of the body. 48 Excessive pro-inflammatory factors can reduce the utilization rate of tryptophan precursor and C-reactive protein, which are closely related to 5-HT synthesis. In turn, this can reduce the concentration of neurotransmitters, ultimately causing depression. 49 Furthermore, the overexpression of inflammatory factors can increase the secretion of hypothalamic regulatory hormones, which leads to the overactivation of the HPA axis, exacerbating the depression-like behavior among the rats. Thus, this suggests that inflammatory responses could be an important mechanism of depression. 50 Neurotrophic factor (NTF) plays an important role in the occurrence, development, and functional maintenance of the central nervous system (CNS). 51 Neurotrophic factor can provide nutritional support for neurons and pertinent brain regions in the CNS related to the regulation of emotional behavior. 52 Brain-derived neurotrophic factor is a common NTF that uses physiological roles to change synaptic plasticity and increase synaptic connections. 53 Studies have shown that the content of BDNF in the hippocampus and prefrontal cortex of patients with depression is significantly reduced, whereas the level of BDNF is significantly increased after antidepressant intervention in patients with depression. 54  Evidence has shown that hippocampal neurogenesis plays an important role in depression. 57 Disruption of hippocampal neurogenesis and blockage of neurogenesis using focal radiation can induce depression-like behavior and result in a reduced antidepressant treatment effect. This suggests that neurogenesis is involved in the onset of depression and resembles the therapeutic effects of antidepressants to a certain extent. 58 Mitochondria have potential roles in the complex processes of producing adenosine triphosphate (ATP), establishing membrane stability, signaling intracellular Ca 2+ , reactive oxygen species (ROS) homeostasis, and performing neurotransmission and plasticity. 9 Studies have also shown that glucose utilization decreased in the prefrontal cortex, anterior cingulate gyrus, and caudate nucleus of patients with depression. 59,60 This suggests that depression may be caused by the impairment of brain energy owing to mitochondrial dysfunction. 61,62 In recent years, the relationship between gut microbiota and diseases has attracted more and more attention. Studies have shown that the imbalance of gut microbiota is closely related to the occurrence of depression. Gut microbiota can affect the host's behavior and mood through the microbiota-gut-brain axis, and then induce depressive symptoms. 63 Compared with healthy adults, patients with depression have different gut microbiota diversity, microbial richness and evenness, and a smaller number of bacterial taxa are associated with depression. 64 The proportion of Actinobacteria in the intestine of patients with depression increased, 65  Traditional Chinese medicine (TCM) has made significant progress in the treatment of depression by delaying the onset and course of the illness, thus improving the efficacy of western medicine and reducing its side effects. 70 Currently, TCM for the treatment of depression is derived mostly from Chinese medicine and active ingredients.  3) and (6) The initial screening of the studies was based on the title and abstract of the articles. Thereafter, the publications were reviewed in their entirety.

| Inclusion and exclusion criteria
Studies on animal models of FA treatment of depression were included. These studies analyzed either the effect of FA on the depression model, FA's potential alleviating or therapeutic effect, or its mechanism through the in vivo behavior model. According to the statistical classification and writing requirements of this review, the eligibility criteria for inclusion in this systematic review were as follows: 1. All articles should be indexed and published in renowned journals with considerable impact factors.
2. Depression models should be treated with FA.
3. The studies must have a control group.
4. Research content must be associated with depression.
The exclusion criteria in this systematic review were as follows:

| Data extraction and treatment evaluation
Two authors independently collected the original literature and resolved disputes through discussion. The following information was retrieved from each study: (1) year of study publication and first author; (2)
Among the 12 studies, five studies utilized behavior test induced models, including forced swimming test (FST) and tail suspension test (TST); three studies used drugs to induce models: two studies utilized reserpine induced models, one study utilized corticosterone (CORT), and one study utilized lipopolysaccharide induced models.
F I G U R E 2 Prisma flowchart outlining the article screening process.
Another three studies utilized chronic stress-induced models and one study utilized the inevitable stress-induced model. Additional data on these studies are provided in Tables 2 and 3.

| Risk of bias
The risk of bias was associated with studies that ranged from 3 to 5 out of a total of six points. Two studies scored 3 points (16.7%), nine studies scored 4 points (66.7%), and two studies scored 5 points (16.7%). The risk of bias in the 12 studies is shown in Table 4.

| Summary of the studies
The FST and TST were the two main strategies used for assessing showed that the level of 5-HT was improved from another aspect. 71 Furthermore, Zhang et al. 31 demonstrated that FA selectively inhibited 5-HT, NE, and DA reuptakes regulated the HPA axis, increased ghrelin, and stimulated jejunal contraction simultaneously.
Reserpine can affect NE of sympathetic nerve endings and lowers blood pressure and central sedation by depleting NE. 72 Reserpine has been reported to have several effects on biogenic amines. Furthermore, it was found that oxidative stress may be more conducive to initiating the onset of depression. 73

TA B L E 3
Main outcome and mechanism of ferulic acid for depression.

Chen et al. (2015) FA increased monoamine neurotransmitter levels in the hippocampus and frontal cortex
Inhibited MAO-A activity pus. 71 Similarly, other studies have reached the same conclusion.
Neuroinflammation also contributes to the pathogenesis of depression. Pro-inflammatory cytokines such as IL-1β, IL-6, and TNFα are expressed highly in the plasma or brain of depressionlike animals. 90 Activated microglia, which are a major source of pro-inflammatory cytokines in the brain, release multiple proinflammatory cytokines, induce neuroinflammation, and exacerbate TA B L E 4 Risk of bias of included studies. The neurotrophic hypothesis suggests that decreased NTF trigger neuronal atrophy, whereas decreased hippocampal synapse protein synthesis and neurogenesis trigger depression. 98 Antidepressants have been reported to improve this neurotrophic factor deficit, thus reversing cell loss. 99 Recent clinical studies reported that BDNF levels in the peripheral blood of patients with Sirtuins (SIRTs) are a family of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases that regulate many cellular physiological processes. 107 More recently, SIRT has been associated with mood disorders in mice and humans. 108 Li et al. 109  suggesting that FA selectively inhibited 5-HT reuptake in brain. FA

F I G U R E 4
Multipathway antidepressant mechanisms of FA. FA inhibits microglia, NF-κB signaling, NLRP3 inflammasome activation, IL-1β, IL-6, and TNFα expression; FA can also increase CAT, SOD, and GSH-Px activities in the blood, hippocampus, and cerebral cortex of mice, as well as inhibit ROS production; FA treatment increases BDNF levels in prefrontal cortex and hippocampus to serve anti-depressives effect; FA promotes the expression of glycolytic genes (Aldoc, Glo1, Phgdh, Pgm, and Pk), as well as mitochondrial tricarboxylic acid cycle; FA inhibits SIRT6 expression through stimulating AKT/CRMP2 signaling pathways. AKT, protein kinase B; BDNF, brain-derived neurotrophic factor; CAT, catalase; FA, ferulic acid; GSH-Px, glutathione peroxidase; IL, interleukin; NF-κB, nuclear factor-kappa B; NLRP3, NOD-like receptor family pyrin domain containing 3; ROS, reactive oxygen species; SOD, superoxide dismutase. F I G U R E 5 Regulative role of FA in gut-brain axis. Ferulic acid modulates gut microbiota diversity and subsequently involved in gut-brain axis; FA is related with the 5-HT system and ultimately contributes to the regulation between gut homeostasis and brain function. 5-HT, 5-hydroxytryptamine; FA, ferulic acid.

F I G U R E 6
Schematic representation of the neuroprotective mechanisms of ferulic acid in treating depression. Ferulic acid was shown to improve behavioral disorders of depression and exhibited regulation of monoaminergic neurotransmission, anti-inflammation, anti-oxidation, and altered gut microbiota among other mechanisms. Ferulic acid also serves as a neuroprotective agent used in depression. Red words indicate the increased expression and green words indicate the decreased expression.

AUTH O R CO NTR I B UTI O N S
Xiaoyu Dong was involved in data curation, formal analysis, investigation, methodology, and writing-original draft. Dongxue Zhao was involved in conceptualization, supervision, validation, and writingreview and editing.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they have no competing interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing not applicable to this article as no datasets were generated or analyzed during this study.