Phytochemistry, Pharmacology and Molecular Mechanisms of Herbal Bioactive Compounds for Sickness Behaviour

The host’s response to acute infections or tissue injury is a sophisticated and coordinated adaptive modification called sickness behaviour. Many herbs have been studied for their ability to protect animals against experimentally induced sickness behaviour. However, there is a lack of knowledge and experimental evidence on the use of herbal bioactive compounds (HBACs) in the management of sick behaviour. The goal of this review is to provide a concise summary of the protective benefits and putative mechanisms of action of phytochemicals on the reduction of lipopolysaccharide (LPS)-induced sickness behaviour. Relevant studies were gathered from the search engines Scopus, ScienceDirect, PubMed, Google Scholar, and other scientific databases (between 2000 and to date). The keywords used for the search included “Lipopolysaccharide” OR “LPS” OR “Sickness behaviour” OR “Sickness” AND “Bioactive compounds” OR “Herbal medicine” OR “Herbal drug” OR “Natural products” OR “Isolated compounds”. A total of 41 published articles that represented data on the effect of HBACs in LPS-induced sickness behaviour were reviewed and summarised systemically. There were 33 studies that were conducted in mice and 8 studies in rats. A total of 34 HBACs have had their effects against LPS-induced changes in behaviour and biochemistry investigated. In this review, we examined 34 herbal bioactive components that have been tested in animal models to see if they can fight LPS-induced sickness behaviour. Future research should concentrate on the efficacy, safety, and dosage needed to protect against illness behaviour in humans, because there is a critical shortage of data in this area.


Introduction
Sickness behaviour is a complex and coordinated adaptive change initiated by the host to respond to acute infections or tissue injury [1][2][3][4]. Malaise, hyperalgesia, fever, lethargy, social withdrawal, inhibition, decreased locomotor activity, exploration, grooming, loss of libido, anhedonia, sleepiness, anorexia weight loss, disturbed concentration, and anxiety are part of the typical sickness behavioural pattern [3,5]. Even though neuronal receptors for bacteria and viruses do not exist, the presence of these microbes might elicit sick behaviour [6,7]. The immune system possesses receptors that can detect pathogens, which send a message to the brain via chemical massagers and cause altered behaviour in sick individuals [6,8].
Endocrine, autonomic, and behavioural alterations mediated by soluble proteins released at the site of infection or injury, such as proinflammatory cytokines, describe sickness behaviour [7,9]. Interleukin (IL)-1, IL-6, and tumour necrosis factor (TNF) are among the vital proinflammatory cytokines that activate immune cell (macrophages and Figure 1. Role of the host immune system and proinflammatory cytokines in sickness behaviour. Acute infection or tissue injury acts as a trigger for the innate immune system. Dendritic cells and macrophages accumulate at the site of the infection or injury. The activated dendritic cells and macrophages release proinflammatory cytokines (PICs). The peripheral cytokines activate the vagal nerve, and some of the cytokines cross the blood-brain barrier and activate additional cytokine release from the brain. The brain is signalled by the released cytokines to start a series of behaviours (i.e., sickness behaviours). Figure  Centrally generated cytokines are believed to alter brain structures that control thermoregulation, metabolism, and behaviour via volume transmission [7,9]. As a result, the brain-based elements of the immune system are developed. The expression and actions of cytokines in the brain and other tissues are regulated by the adrenal cortex's synthesis of glucocorticoids in response to the effects of cytokines on the hypothalamus [7]. Repetitive or chronic stimulation of the cytokine system can contribute to the development of mood disorders caused by cytokine-induced alterations in tryptophan metabolism [3,4,10]. Figure 2 depicts the bidirectional linkages between immunological events and psychoneuroendocrine states.
Sickness can be managed for a social animal by increasing sensitivity to dangerous social events and boosting approach-related behaviour toward close others who might be able to help [9]. The connection between the immune system and the CNS is a key aspect of host defence. Sickness behaviour affects the immune system and improves recovery [6]. Because inflammation is a powerful organiser of social behaviour, it has an impact on immune system management [6]. In reaction to various forms of social separation, the immune system upregulates proinflammatory response genes to prepare the body for more sensitive settings [6].
The above findings throw light on the mechanisms and methods for managing nonspecific symptoms of sickness, which can occur in a range of diseases linked to inflammation and lead to pathological mood and altered cognition [9]. Even though sickness behaviour is part of the immune reaction for the better healing of infection or injury, the sufferers feel discomfort if it persists for a longer time. Hence, sick behaviour should be treated to overcome the social, cognitive, and mental alterations in sufferers.
Many herbal medicines have been evaluated for their protective action against experimentally induced sickness behaviour in animals [1,11]. Nonetheless, there is a scarcity of knowledge and experimental data on the use of herbal bioactive compounds (HBACs) in sickness behaviour. The purpose of this review was to look at the present scientific literature on HBACs that protect experimental animals against sickness caused by lipopolysaccharide.

Lipopolysaccharide (LPS)-Induced Sickness Behaviour Model
Several animal models are used for the preclinical evaluation of the effect of drugs on sickness behaviour. Among others, lipopolysaccharide (LPS)-induced sickness behaviour in rodents is most utilised in preclinical research. Rats and mice are widely utilised as experimental animals in the study of sickness behaviour.
LPS, a component of the cell walls of Gram-negative bacteria, is crucial for hostpathogen interactions with the innate immune system during infection [12]. Injection of LPS into rodents mimics the imperative aspects of Gram-negative bacterial infections, such as activating the Toll-like receptor 4 (TLR-4, pattern recognition receptor) [13]. Hence, LPS is often used to induce sickness behaviour in animals, which mimics sickness behaviour in humans [13][14][15]. By attaching to immune cells, LPS functions as a pathogen-associated molecule pattern (PAMP) [11] and activates nuclear factor κB (NFκB) to increase the expression of TNF-α, IL-6, and IL-1β [10,11]. In the CNS, microglia and macrophages generate cytokines and induce neuroinflammation and sickness behaviour [14,15]. In CNS, peroxides and reactive oxygen species (ROS) are produced in large numbers as a result of a rapid inflammatory response initiated by LPS [14,15]. When the levels of peroxides and ROS exceed the natural antioxidant defences, oxidative stress-mediated disease results [14,15]. In the brain, lipid peroxidation targets polyunsaturated fatty acids [10,11,16]. The detailed physiological, behavioural, and biochemical alterations in LPS-induced sickness behaviour in rodents are shown in Figure 3.

Methods
The relevant studies were gathered from the search engines Scopus, ScienceDirect, PubMed, Google Scholar, and other scientific databases (between 2000 and to date). The keywords used for the search included "Lipopolysaccharide" OR "LPS" or "Sickness behaviour" OR "Sickness" and "Bioactive compounds" OR "Herbal medicine" OR "Herbal drug" OR "Natural products" OR "Isolated compounds".
Articles published only in the English language were considered, and conference abstracts and articles other than English were excluded. Studies conducted only on pure bioactive compounds were included in this review. Duplicate studies were deleted from the various databases. This review included a total of 41 published articles after applying the inclusion and exclusion criteria.

Ursolic Acid
Ursolic acid is a pentacyclic triterpenoid that can be found in the leaves, flowers, berries, and fruits of many medicinal plants, including apples, bilberries, cranberries, elder flower, peppermint, lavender, oregano, thyme, hawthorn, and prunes [54]. Ursolic acid has antioxidant, anti-inflammatory, antibacterial, and antifungal properties. Wang and colleagues looked at how ursolic acid affected LPS-induced cognitive impairments in mice [47]. Ursolic acid protects animals from cognitive deficits induced by LPS. Ursolic acid protects mice by inhibiting p38/NF-B-driven inflammatory pathways in the brain [47].

Curcumin
Curcumin (diferuloylmethane) is the main active constituent of Curcuma longa (Family: Zingiberaceae) [56]. Wang et al. evaluated the antidepressant activity of curcumin in LPS-treated mice. Treatment with curcumin attenuates iNOS, cytokines, and the expression of COX-2 mRNA via the NF-κB signalling pathway and protects animals from LPS-induced depressive-like behaviour [48]. In addition, Sorrenti et al. evaluated curcumin in acute neuroinflammation and long-term memory impairment in LPS-treated mice [42]. According to the authors, curcumin protects rats from LPS-induced memory loss and acute neuroinflammation [42]. Piperine, a main alkaloidal of Piper nigrum, inhibits glucuronidation and improves the bioavailability of curcumin [57,58]. Jangra et al. reported that piperine enhances the efficacy of curcumin in protecting neurobehavioral and neurochemical impairments in LPS-treated mice [26]. Piperine increases curcumin bioavailability, which improves its biological performance in LPS-treated mice [26].

Honokiol
Honokiol is a polyphenolic compound that can be obtained from Magnolia grandiflora (Family: Magnoliaceae) [59]. Honokiol offers antianxiety, antipain, and anti-epileptic properties [59]. Sulakhiya et al. reported the abrogative effect of honokiol in depressiveslike behaviour in LPS-treated rats by reducing neuroinflammation and oxido-nitrosative stress in mice [44]. In addition, as per the study conducted by Sulakhiya et al., honokiol offers beneficial effects on anxiety and liver damage in LPS-treated mice [24]. In mice, Honokiol had a protective effect against anxiety-like behaviour and liver damage caused by LPS. Honokiol inhibits cytokine generation, oxidative stress, and the loss of brain-derived neurotrophic factor (BDNF) [24].

Esculetin
Esculetin is a coumarin derivative found in Artemisia scoparia, Artemisia capillaries, Ceratostiggma willmottianum, and Citrus limonia [61]. Esculetin is well known for its pleiotropic biological activity, which includes antioxidant, inhibition of xanthine oxidase, platelet aggregation, and anticancer activities [61]. Sulakhiya et al. evaluated the antianxiety and antidepressant action of esculetin in LPS-treated mice [62]. Esculetin alleviated LPS-induced anxiety and sadness in rats by reducing neuroinflammation, oxidative stress, and plasma cortisol levels [62]. Esculetin reduced LPS-induced neuroinflammatory processes and depressive-like behaviour in mice [53]. According to the author, the impact of esculetin may be attributed to the suppression of the NF-B pathway and the stimulation of BDNF/TrkB signalling [53].

Caffeic Acid
Caffeic acid is a polyphenolic compound found in a wide range of plants and foods, such as coffee, wine, and tea [63]. Caffeic acid has antioxidant, anti-inflammatory, and anticarcinogenic properties [63]. Mallik et al. evaluated caffeic acid on sickness behaviour in LPS-treated mice [37]. Caffeic acid (30 mg/kg) protected from LPS-induced sickness behaviour and neuroinflammation in mice [37]. Caffeic acid reduced peripheral and central cytokine levels as well as the oxidative stress caused by LPS [37].

Embelin
Embelin is alkyl-substituted hydroxyl benzoquinone found in Embelia ribes Burm [64]. Embelin possesses neuroprotective effects against experimentally induced neurotoxicity in animals [64,65]. Shaikh et al. reported the beneficial effect of embelin in sickness behaviour in LPS-treated mice [11]. The authors reported that the antioxidant properties of embelin are responsible for its protective action against LPS-induced sickness behaviour [11].
3.2.9. Gomisin N Gomisin N is a lignan extracted from Schisandra chinensis (Family: Schisandraceae) Baill's dried fruits [66]. Schisandra chinensis has long been used in traditional Chinese and Kampo medicine for liver disorders. Gomisin N has antioxidant, anti-inflammatory, and hepatoprotective effects in vivo and in vitro. Gomisin N reduces depressive-like behaviour and interest loss caused by LPS. Gomisin N's anti-inflammatory and antineuronal actions are most likely due to the reduction in neural activation and inflammation in the PVN and CeA [18].

Liquiritigenin
Liquiritigenin is a flavanone identified from Glycyrrhiza uralensis and found in many plants, including Glycyrrhiza glabra [67]. Su et al. discovered that liquiritigenin protects mice from depressive-like behaviour caused by LPS [43]. Liquiritigenin's anti-inflammatory properties and impact on the BDNF/TrkB signalling pathway are thought to be the cause of its antidepressant effects [43].

Trans-Astaxanthin
Algae, plants, a few fungi, and bacteria all contain large amounts of the red carotenoid pigment trans-astaxanthin [28]. Trans-astaxanthin has been shown to have neuroprotective properties in a variety of neurodegenerative illnesses [28]. Trans-astaxanthin has been shown in animal studies to reduce LPS-induced neuroinflammation and depressive-like behaviour [69]. Trans-astaxanthin inhibited iNOS, nNOS, and COX-2 expression as well as NO levels in the hippocampus and prefrontal cortex by regulating NF-κB [69]. Furthermore, it has been observed that trans-astaxanthin has an antidepressant-like impact on the serotonergic system [28].

Ginsenoside Rg3
Ginsenoside Rg3 is a tetracyclic triterpenoid and a glycoside found in Panax ginseng (red ginseng, Family: Araliaceae), and it has antioxidant, anti-inflammatory, and immunomodulatory properties [71]. Kang and colleagues found that ginsenoside Rg3 suppressed depression-like behaviour and neuroinflammation produced by LPS in mice [30]. The protective effect was achieved by inhibiting neuroinflammatory disturbances and regulating TRP-KYN metabolism in both the brain and the peripheral nervous system [30].

Methyl Jasmonate
Methyl jasmonate is a hormone initially isolated from Jasmonium grandiflorum (Family: Oleaceae) essential oil [73]. Methyl jasmonate is known to have anti-amnesic, antinociceptive, adaptogenic, and antidepressant properties [17]. Adebesin et al. reported the antidepressant effect of methyl jasmonate in LPS-treated mice [17]. The authors reported that the observed effect of methyl jasmonate was attributed to the suppression of oxidative stress and TNF-α release [17].

Gentiopicroside
Gentiopicroside is an iridoid glucoside and one of the primary compounds enriched in Gentiana Macrophylla Pall roots (Family: Gentianaceae) [15]. Gentiopicroside has been shown to possess analgesic, anti-inflammatory, anticancer, lipid regulating, and antidepressant properties. Deng et al. reported the gentiopicroside abrogates depressive-like behaviour in mice induced by LPS [21]. The abrogative effect of gentiopicroside mediates through the tryptophan-degrading pathway [21].

Selanylimidazopyridine
Selanylimidazopyridine has received a lot of interest lately because of its antioxidant properties and potential to guard against depression-like behaviours [76]. According to Domingues et al., selanylimidazopyridine targets neurotrophins and inflammatory/oxidative mediators to prevent LPS-induced depressive-like behaviour in mice [22].

Macranthol
Macranthol is a triphenyl lignan derived from the plant Illicium dunnianum (Family: Schisandraceae) [78]. Macranthol has been reported to possess antidepressant action in a preclinical study [79]. Weng et al. reported the attenuating action of macranthol in depressive-like behaviours in LPS-treated mice [50]. The antidepressant action of macranthol is mediated by inhibiting neuroinflammation in the prefrontal cortex [50]. According to another study, macranthol stimulates hippocampal neuronal development in mice via the BDNF-TrkB-PI3K/Akt signalling pathway [80].

Hesperidin
Hesperidin is a bioflavonoid found primarily in citrus fruit, such as lemon, grapefruit, orange, and tangerine [81]. Hesperidin has several pharmacological properties, including antihyperlipidemic, cardioprotective, antihypertensive, and antidiabetic effects. [81]. According to a study conducted by Kwatra et al., hesperidin was found to be protective against LPS-induced hippocampus and frontal brain damage in mice [32]. The authors reported that the TLR4/NF-κB, p38 MAPK/JNK, and Nrf2/ARE signalling pathways play important roles in the activity of hesperidin [32].

Resveratrol
Resveratrol is a polyphenolic, non-flavonoid found in plants, such as rhubarb, grapes, mulberries, and peanuts [82]. Resveratrol provides numerous health benefits, including antioxidant, anti-inflammatory, antiplatelet, blood glucose-lowering, and anticancer effects [83]. A group of researchers from China reported that resveratrol reduces anxietylike behaviour in LPS-treated mice [46]. The antianxiety effect of resveratrol is attributed to its attenuating effect on YAP-mediated neuro-inflammation and promoting hippocampal autophagy [46].

Solidagenone
Solidagenone is a diterpenoid compound found in Solidago chilensis (Family: Asteraceae) that is used in folk medicine to treat pain and inflammatory diseases [84]. The aerial parts of Solidago chilensis are frequently used to treat burns and for their diuretic, analgesic, anti-inflammatory, antirheumatic, and healing properties. Solidagenone has antiinflammatory, antigastroprotective, and immunomodulatory properties [85]. According to Locateli et al., solidagenone has antidepressant-like effects in LPS-treated mice [36]. The impact of solidagenone has been linked to the control of antioxidant systems and a decrease in the inflammatory process [36].

Diallyl Disulfide
Diallyl disulfide is an organosulfur compound derived from Allium sativum (Garlic, family: Allium) [86]. Wei et al. reported that diallyl disulfide attenuates depressionlike behaviour in mice treated with LPS [49]. The observed effect was attributed to its regulating effect on neuroinflammation and oxido-nitrosative stress [49]. Lu and colleagues also reported the beneficial effect of diallyl disulfide in LPS-induced depression in mice [87].

Rosmarinic Acid
Rosmarinic acid is a polyphenol constituent identified in Rosmarinus officinalis (Family: Lamiaceae) and many culinary herbs [88]. Rosmarinic acid is an ester of caffeic acid and 2-hydroxy-dihydrocaffeic alcohol with antioxidant and anti-inflammatory properties. Thingore et al. reported the ameliorative effect of rosmarinic acid on oxidative stress and neuroinflammation in LPS-induced memory-impaired mice [38]. The increased levels of proinflammatory cytokines and apoptotic proteins were revived after pretreatment with rosmarinic acid [38].

Paeoniflorin
One of the most important bioactive components of paeony (Paeonia lactiflora, Family: Paeoniaceae) is paeoniflorin, a monoterpene glucoside [89]. Kim and Ha evaluated paeoniflorin against LPS-induced oxidative stress and lipid metabolism in rats [31]. Administration of paeoniflorin regulated the levels of lipid profile (triglyceride, total lipid, totalcholesterol, and HDL-cholesterol) levels and protected animals from oxidative stress [31]. This study demonstrates that paeoniflorin markedly ameliorated LPS-induced oxidative stress and lipid metabolism in rats [31]. Significant body weight loss is part of the typi-cal sickness behavioural pattern [3,5]. The loss of appetite and altered lipid and protein metabolism leads to a significant body weight loss during sickness behaviour [90].

Parthenolide
Parthenolide is a sesquiterpene lactone found in the herb feverfew (Tanacetum parthenium, Family: Asteraceae) [91]. A group of German researchers evaluated the effect of Parthenolide (1 mg/kg) on fever, circulating cytokines, and markers of brain inflammation in LPS-treated rats [39]. Parthenolide reduced LPS-induced fever in rats, and the authors propose that inhibition of the peripheral circulating IL-6 and TNF-α, as well as direct central action on brain cells via partial inhibition of oxidative stress, the NFκB and NF-IL6 signalling pathways, and inhibition of cytokines at the brain was attributed to its action [40].

Quercetin
Quercetin is a bioflavonoid present in a variety of plants and foods, including onions, apples, berries, green tea, and red wine, and is known to have powerful ROS-scavenging properties [40]. Three important functions of quercetin are antioxidant, anti-inflammatory, and immunomodulatory. Sah et al. studied quercetin for them LPS-induced-sickness behaviour in rats [40]. The authors conclude that administration of quercetin (2 and 25 mg/kg) significantly attenuates sickness behaviour induced by LPS by inhibiting oxidative stress and modulating cytokines production [40]. The effect of quercetin on LPS-induced abnormality was also evaluated in mice as an animal model. Liao and Lin administered quercetin intraperitoneally (0.06 µmol/mouse) to LPS-challenged mice [34]. Quercetin treatment protected mice from LPS-induced systemic inflammation [34].

Gypenosides
Gypenosides is saponin derived from Gynostemma pentaphyllum (Jiaogulan, Family: Cucurbitaceae) [94]. Gypenosides have been demonstrated to have anxiolytic and neuroprotective benefits in the treatment of depressive disorders [95]. Lee and a friend showed that in rats, gypenosides reduce lipopolysaccharide-induced neuroinflammation and memory loss [96]. Due to the fact of their anti-inflammatory actions and adequate modulation of NFκB/iNOS/TLR4/BDNF, gypenosides have been shown to have anxiolytic and neuroprotective benefits through increasing memory functions [96].
Carvacrol inhibits memory impairment and inflammation in LPS-treated rats, the carvacrol showed anti-inflammatory effects mediated by BDNF and TLR4 regulation [33].

Mechanism of Action(s) of HBACs against LPS-Induced Sickness Behaviour
LPS is a pathogen-associated molecular pattern (PAMP) that allows bacteria to be identified by pattern recognition receptors on certain host receptors (PRRs). LPS works as a toxin by activating the Toll-like receptors (TLRs) signalling pathway, which promotes pathogenic inflammatory responses by increasing the nuclear translocation of NF-B and triggering the production of proinflammatory cytokines, such as IL-1β, IL-6, and TNF-α. High LPS concentrations induce the production of proinflammatory mediators, which can result in oxidative stress. It is thought that ROS are involved in the mechanism of LPS toxicity. The majority of HBACs reduced the oxidative and nitrative stress and attenuated proinflammatory cytokines IL-1β, IL-6, and TNF-α, as a result, suppressing the neuronal inflammation in the LPS-treated animals. Corticoids produced in response to the hypothalamic effects of proinflammatory cytokines control cytokine expression and function [7]. Some of the HBACs (esculetin and methyl jasmonate) suppress the corticosterone and reduce the further release of proinflammatory cytokines. In addition, some HBACs (ursolic acid, curcumin and proanthocyanidin) also inhibited the COX-2 enzyme in the brain. COX-2 is thought to be involved in the inflammatory response in the neurons inhibiting it and suppresses the inflammation. BDNF is a pleiotropic protein that modulates neurotransmitters and plays a role in memory and learning [101]. BDNF is necessary for the appropriate development of various nervous system components [102]. LPS administration reduces BDNF levels in the brain and affects memory and learning in animals [101]. Some of the HBACs (mangiferin, honokiol, liquiritigenin, paeonol, gypenosides, selanylimidazopyridine, carvacrol, and hesperidin) restored BDNF levels in LPS-treated animals and protected the animals from LPS-induced abnormalities. Certain HBACs showed their actions with multiple targets and protected the animals from LPS-induced toxicities. Table 2 and Figure 7 represent the molecular mechanism of action(s) of HBACs against LPS-induced sickness behaviour in rodents. Table 2. Mechanism of action(s) of HBACs against LPS-induced sickness behaviour in rodents.

Restoration of BDNF Levels
Hesperidin

Conclusions and Future Perspectives
In this review, we looked at 34 herbal bioactive components that have been studied for their ability to combat LPS-induced illness in animal models. The majority of the researched herbal bioactive compounds induced a reduction in sickness behaviour signs in experimental animals, according to our review. Nonetheless, because most studies focused solely on its effects on sickness behaviour, the toxicological profiles of the herbal bioactive components are unknown. Furthermore, there is a severe dearth of data on the efficacy, safety, and required dosage to protect from sickness behaviour in humans, which should be the focus of future research. The potentials of herbal bioactive compounds should be studied for the development of novel medications as adjuvants or as a new armamentarium to augment sickness behaviour treatment.
Author Contributions: All authors contributed equally. All authors have read and agreed to the published version of the manuscript.