Keywords

1 Traumatic Brain Injury

Traumatic brain injury (TBI) is a debilitating health concern as well as a growing financial burden on health care systems. TBI is a main cause of deaths and disabilities worldwide and is mostly reported to occur in military situations, sports practice, and vehicle accidents (Peeters et al. 2015; Phillips and Woessner 2015; Wojcik et al. 2010). The pathological features of TBI are usually localized to the brain area affected by the injury (focal or widespread damage). In addition, TBI cases can be classified according to the severity of the injury, ranging from mild, moderate to severe. Most severe cases include an impact with penetrating object, such as a bullet that penetrates the skull (Mena et al. 2011). Importantly, TBI consists of two phases: (1) the primary phase of the injury which occurs as a result of an external mechanical insult such as acceleration, deceleration or rotational forces and (2) a secondary phase (McKee and Daneshvar 2015). The secondary phase of TBI is highlighted by a series of pathological events that start following the initial mechanical impact. These events encompass oxidative stress, excitotoxicity, mitochondrial dysfunction, inflammation, edema, and neuronal cell death (Kaur and Sharma 2018). The strength of the primary insult applied to the brain alongside its magnitude determines how the secondary injury cascades proceed. The severity of the secondary injury usually determine the intensity of the clinical manifestations that follow TBI. As such, TBI can remain undiagnosed and thus can be life-threatening, since it later manifests as long-term complications and other neurological problems in patients that do not seek medical assistance (Nasser et al. 2016; Scheff and Ansari 2017; Tabet et al. 2020).

The switch toward anaerobic fermentation at an early stage of TBI due to ischemia creates an energy crisis in brain cells (Carre et al. 2013). Along this process, adenosine triphosphate (ATP)-dependent ion pumps start to fail due to the depletion of energy stores (de Lores Arnaiz and Ordieres 2014; Silva et al. 2011). As a result, the cell membrane becomes more permeable, leading to an increased inflow of calcium ions (Ca2+) into neurons of the injured brain. This causes prolonged membrane depolarization and excessive release of excitatory glutamate neurotransmitters. The massive increase in the excitatory glutamate following TBI increases intracellular Ca2+ levels by stimulating the Ca2+-permeable N-methyl-d-aspartate (NMDA) and the AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors among others in neuronal cells (de Lores Arnaiz and Ordieres 2014; Kambe et al. 2011). The disruption of Ca2+ homeostasis is associated with the generation and activation of potent inducers of cell damage such as free radicals, proteases, and phospholipases. Mitochondrial dysfunction also takes place and is another mechanism that aggravates the situation. Apart from its role in cellular respiration and ATP production, mitochondria are a main regulator of Ca2+ levels where it can sequester the excessive intracellular Ca2+ (Kambe et al. 2011). In fact, mitochondrial impairment occurs quite rapidly following TBI due to Ca2+ overload. This leads to further generation and release of reactive oxygen species (ROS) and proapoptotic factors into the cell (Dekmak et al. 2018; Scheff and Ansari 2017).

Dysfunctional mitochondria are a main source of ROS. Mitochondrial produced ROS promote the state of oxidative stress in the injured brain, mainly created by overproduction of ROS and RNS (reactive nitrogen species). The state of elevated oxidative stress can cause direct damage to numerous cellular constituents of the brain including lipids, carbohydrates, proteins, and nucleic acids. Oxidative stress is also reported to disrupt the blood–brain barrier (BBB) thus inducing cerebral edema (Toklu and Tümer 2015). This BBB breach allows peripheral immune and inflammatory cells and factors to access the brain (Kelso and Gendelman 2014). As a result, resident microglial cells are activated. These cells proliferate and migrate to the site of injury to get rid of altered cellular structures and to remove cellular debris in order to promote brain repair. Even though the primary role of these cells is to correct the damage, their long-lasting activation can lead to detrimental effects on brain health. This is because microglia, as part of their response to injury, usually secrete various proinflammatory mediators including tumor necrosis factor-α (TNF-α), interleukin- 6 (IL-6), IL-1β, and interferon-γ, thus favoring deleterious neuroinflammation on the long run (Donat et al. 2017). Additionally, leakage of the BBB enables brain-specific proteins to reach the peripheral blood flow. The leaked brain-specific proteins can act as antigens of certain B cells that in turn produce autoantibodies directed against brain components (Kobeissy and Moshourab 2015). Other glial cells, namely astrocytes, are known to perform structural (cellular hypertrophy and processes elongation) as well as functional (altered molecular signaling and gene expression) modifications to ensure brain recovery during TBI. When activated, astrocytes exert antioxidant, anti-inflammatory, and anti-apoptotic activities. They also produce neuroprotective factors to enhance several processes such as neurogenesis, synaptogenesis, and BBB repair. Over time, the persistent oxidative stress, inflammation, and excitotoxicity lead to impairment of the mechanism of astroglial activation. Not only that, but the impaired astrocytes can produce neurotoxic factors that mediate harmful adverse effects following TBI (Cheng et al. 2019; Raad et al. 2014; Scheff and Ansari 2017; Zhou et al. 2020).

2 Conventional Management of Traumatic Brain Injury

The secondary neurological injury that follows the primary traumatic insult evolves over minutes, days, and even months. It usually entails metabolic, cellular, and neurochemical changes. These changes are usually the reason for the wide range of neurologic deficits that manifest post-TBI (Loane et al. 2015). This delay in symptom manifestation proposes that there is a time window for possible therapeutic intervention (Synnot et al. 2018). In fact, improved patient prognosis has been observed when secondary TBI is prevented or mitigated by treatments (Vankipuram et al. 2020). This can impede the progress of brain damage and recover brain functions after brain injury (Loane et al. 2015). Currently there is no FDA-licensed treatment of TBI and most of the available interventions try to alleviate the consequences of TBI. Current conventional management of moderate-to-severe TBI involves ventilation and oxygenation approaches, fluid management, hypothermia stimulation, intracranial pressure (ICP) management, cerebral perfusion pressure (CPP), blood pressure (BP) management, nutrition and glucose level regulation, and surgery. Pharmacological therapies (those that are not elsewhere defined) are limited and have not been FDA approved (Bragge et al. 2016). Recently, stem cells and regenerative medicine therapies have been trialed for the management and repair of the neurological damage that follows TBI (Tabet et al. 2020; Zhou et al. 2019).

A meta-analysis found that intensive glucose control (IGC) therapy (maintaining glucose levels between 80 and 110 mg/dL) may play a protective role post-TBI (Zhu et al. 2018). It was found that IGC can reduce mortality following TBI, improve neurological outcomes, and reduce the length of stay at the ICU, compared with patients receiving conventional glucose control (CGC) therapy (administering insulin when glucose levels exceed 200–220 mg/dL) (Zhu et al. 2018). IGC may be of particular interest in the management of TBI given the robust increase in glucose utilization following TBI, which impedes the ability of neurons to use ketone bodies as energy substrates (Robertson et al. 1991).

The reduction of body temperature after TBI has also been tested as a TBI management option. For instance, reduction of body temperature, through physical and chemical cooling techniques, to 35–37.5 °C (modest cooling) was reviewed by Saxena and colleagues. They concluded that no satisfactory research has been done about the subject and as a result no recommendation can be made about the use of modest cooling as an intervention following TBI (Saxena et al. 2014). As for hypothermia (reduction of body temperature to 32 °C), a meta-analysis suggested that although risks of unfavorable functional outcome after TBI is significantly reduced following hypothermia therapeutic management, a pronounced increase in pneumonia risk is accompanied with the hypothermia (Chen et al. 2019). Hyperbaric oxygen therapy (HBO2) is another therapeutic intervention that is currently being considered. It can target TBI-induced ischemia, leading to an increase in O2 plasma concentration and thus, can increase O2 delivery to the injured brain tissues (Daly et al. 2018). Huge controversy exists in the literature regarding the safety and efficacy of this therapy (Crawford et al. 2017; Daly et al. 2018). Currently, a rigorous Phase II trial, HOBIT trial (Hyperbaric Oxygen Brain Injury Treatment), is underway, with 200 TBI patients being recruited to evaluate the efficacy of HBO2 (clinicaltrials.gov; trial: NCT02407028) (Hennepin Healthcare Research Institute 2020). It is estimated that the study will be completed by September 2022, and hopefully, will help resolve the controversy surrounding the usage of HBO2 as a management regimen for TBI.

In the past few years stem cell therapy has been extensively explored to treat neurological impairment following TBI. These attempts included mesenchymal stem cells (MSCs), neural stem cells (NSCs), multipotent adult progenitor cells, and endothelial progenitor cells (Cox Jr. 2018; Tabet et al. 2020). These cell populations have been shown to be efficacious in preclinical models and some clinical trials of TBI. They act by modifying the regional response to injury, preventing cell death, and participating in tissue reconstruction; resulting in improved functional outcomes in treated patients (Schepici et al. 2020). Nonetheless, future studies that consider refined cellular products, specific dosing regimens, and larger sample sizes are needed (Schepici et al. 2020; Tabet et al. 2020; Zhou et al. 2019).

Importantly, the clinical use of pharmaceutical drugs following TBI has considerably grown in the past 30 years, where substantial research has been done in an attempt to develop neuroprotective treatments that improve symptoms, functions, and consequences after TBI (Kim et al. 2019; Synnot et al. 2018). Many drugs have been tested for their therapeutic efficacy including antioxidants, free radical scavengers, mannitol, magnesium, excitatory amino acids (EAA) inhibitors, corticosteroids, calcium channel blockers, progesterone, monoaminergic agonists, recombinant factor VIIa, erythropoietin (Epo), NMDA receptor antagonists, cyclosporine, as well as lipid peroxidation inhibitors (Abdelmalik et al. 2019; Xiong et al. 2009). Some compounds, like pegylated superoxide dismutase (PEG- SOD), nimodipine, in addition to triamcinolone showed encouraging results in early stage studies, but later were not able to show significance at primary endpoints of phase III clinical studies (Loane et al. 2015; Xiong et al. 2009). As such, the effectiveness of available treatments for TBI remains uncertain, with optimal treatment guidelines not yet developed due to insufficient evidence. Presently, no pharmacological drugs have been FDA approved for the treatment of TBI (Diaz-Arrastia et al. 2014). In this context, recovery of patients suffering from functional deficits following TBI is thought to be facilitated by treatment with antidepressants, psychostimulants, anticonvulsants, and other agents (Talsky et al. 2011). However, most data on these treatments originate from nonrandomized trials and the evidence remains somewhat limited (Kim et al. 2019; Synnot et al. 2018). For decades, corticosteroids were employed to manage head injuries because they were believed to decrease ICP (Xiong et al. 2009). However, recent data show that patients treated with corticosteroids have a higher risk of death than those in the placebo group, as is their risk of severe disability (Alderson and Roberts 2005; Edwards et al. 2005). To add, treatment with mannitol significantly decreases neuroinflammatory responses in rats, and is sometimes efficient at lessening brain swelling in human post-TBI. However, its use has been discontinued since 2016 following the Brain Neurotrauma Foundation clinical practice guidelines after evidence that it has a less significant effect than hypertonic saline and excessive administration may be harmful due to increased ICP (Abdelmalik et al. 2019; Xiong et al. 2009). Nonetheless, many other pharmaceutical treatments for TBI are still currently being investigated and many preclinical studies have inspired optimism. This optimism should be taken with caution since high-quality data remain scarce, relying mostly on case studies, small population randomized controlled trials, and systematic reviews that have not yet yielded definitive recommendations for TBI treatment regimens (Anghinah et al. 2018; Kim et al. 2019). For these reasons and several more, research into the use of natural compounds, including phytochemicals, as therapeutic interventions following TBI has flourished in recent years. These phytochemicals, being natural, are characterized by their low toxicity and are mostly considered safe. However, more research into their safety is needed and awareness should be raised regarding their safety.

Many phytochemicals have been reported to have neuroprotective abilities, reduced interaction with other pharmaceuticals, an ability to modulate brain functions, antioxidant capacities, and an anti-inflammatory potential; making them attractive and important options as alternative interventions for TBI management (Scheff and Ansari 2017).

3 Phytochemicals as Alternative Therapeutic Interventions

Traditional medicine as well as ethnomedicine, described to be the study of traditional therapies practical by different ethnic groups, have developed throughout human history. Historically, traditional medicinal systems were based on natural products that have been in use for the management of diseases. In this regard, plant products and herbs, generally defined as any form of plant or plant product, formed the basis of early traditional therapeutic systems in different cultures (Tachjian et al. 2010). Many of the current conventional drugs were, in fact, originally obtained from plant or herbal origins; famous examples comprise aspirin of Salix alba tree, ephedrine of Ephedra sinica, taxol of Taxus brevifolia, and many others (Cragg and Newman 2013; Frishman et al. 2009; Harvey 2000). Herbal medicine, also referred to as phytotherapy, phytomedicine or healing with herbs, is nowadays an integral constituent of CAM (complementary and alternative medicine). CAM is an important arm of modern therapeutic habits where, for example, among patients of cardiovascular diseases herbal preparations are the main used kind of CAM (Tachjian et al. 2010; Yeh et al. 2006). Phytochemicals are plant-derived secondary metabolites that are highly represented in herbal extracts and herbal bioactives. Related to this chapter is that dietary phytochemicals are an important arm of micronutrients which are broadly defined as the essential dietary elements that are needed in only small amounts to exert their physiological roles.

Owing to the plethora of its beneficial effects, CAM has always received attention as alternative therapeutic options. In particular, the use of phytochemicals and plants for their medicinal benefits has become more appealing in recent years due to the bountiful of bioactives present in these plants. Indeed, many of these phytochemicals have been extensively studied in the context of several diseases such as age-related and cardiovascular diseases.

Phytochemicals are plant secondary metabolites and include several classes such as flavonoids, polyphenols, terpenoids, vitamins, alkaloids, steroidal saponins, and organosulphur compounds (Forni et al. 2019). A multitude of data is available in the literature to support the protective, preventive, and ameliorative functions of these compounds in several diseases. Indeed, increasing evidence suggests that a variety of phytochemicals/bioactives that are present in various plant preparations exert a varied range of pharmacological actions. These bioactive constituents have been extensively described to have antiviral, antioxidant, antihistaminic, anti-inflammatory, anti-cancerous as well as hepatoprotective effects (Bishayee et al. 2010; Dar et al. 2005; Ganjali et al. 2017; He et al. 2017; Mirzaei et al. 2017; Rahmani et al. 2014; Rauf et al. 2018; Ryu et al. 2017; Wang 2000; Zhu et al. 2015). Moreover, some of these phytochemicals exhibit immunomodulatory, anti-tumorigenic, or cholesterol-lowering activities. In addition, phytochemicals have been employed in the management of gastrointestinal diseases and in reducing pain (Dykes 2019; Kim et al. 2015). Moreover, several bioactives such as licorice (Guo et al. 2015; Rebhun et al. 2015; Wu et al. 2013), quercetin (Thent and Das 2015; Thent et al. 2012; Vessal et al. 2003), ginseng (Carella et al. 2017), limonene (Murali and Saravanan 2012), resveratrol (Bagul and Banerjee 2015), naringin (Pari and Chandramohan 2017), procyanidins (Gonzalez-Abuin et al. 2015) as well as ginger (Arablou et al. 2014) can help in reducing the complications of diabetes. Other studies have shown that several phytochemicals are involved in wound healing, skin protections, anti-infection, and anti-aging activities (Houston et al. 2017; Kalyana Sundaram et al. 2018; Sarandy et al. 2017).

In recent years, neurological diseases (ND) have become a major debilitating issue worldwide. Due to the aforementioned potential therapeutic effects of phytochemicals on several cardiovascular and age-related diseases, phytochemicals can be ideal potential therapeutics in the case of ND (Joseph et al. 2009). Indeed, a mounting evidence indicates that several types and classes of these bioactive molecules can help in the prevention or management of several neurological and neurodegenerative diseases such as Alzheimer’s disease (AD), Huntington’s disease, Parkinson’s disease (PD), multiple sclerosis, depression symptoms, headaches, and strokes (Ayaz et al. 2019; Bakoyiannis et al. 2019; Flanagan et al. 2018; Williams and Spencer 2012).

There are several mechanisms through which these phytochemicals produce their neuroprotective activities, one such mechanism is to prevent protein misfolding and aggregation, key pathological aspects of ND. Such a mechanism is employed by some phytochemicals such as apigenin, isoquercetin, morin, berberine, and linalool (Carmona et al. 2020; Dey et al. 2017; Kim et al. 2014; Kovacs 2014; Sabogal-Guáqueta et al. 2016). Other bioactive molecules exhibit antioxidant, anti-inflammatory, and anti-apoptotic activities thus reducing neuroinflammation and protecting neurons from oxidative damage in animal models of neurodegenerative disorders. In fact, apigenin (Dey et al. 2017), ginsenosides, naringenin (Shal et al. 2018), rutin (Sivanantham et al. 2018), linalool (Sabogal-Guáqueta et al. 2016), and berberine (Kim et al. 2014) can lower the levels of both ROS and proinflammatory mediators. Similarly, other plants such as Picrorhiza kurroa Royle and Centella asiatica were utilized owing to their antioxidant, anti-inflammatory as well as neuroprotective properties in ND (Krupashree et al. 2014; Lee et al. 2007; Pointel et al. 1987; Veerendra Kumar and Gupta 2002). Moreover, one of the extensively studied polyphenols, quercetin, was shown to significantly reduce oxidative stress thus inhibiting cytotoxicity and apoptosis (Ansari et al. 2009) and improve cognitive and emotional functions as well as inhibit proinflammatory mediators in neurodegenerative animal models (de Andrade Teles et al. 2018; Sabogal-Guáqueta et al. 2015).

Evidence has shown that resveratrol has potential prophylactic or therapeutic effects in ND. Indeed, it has a neuroprotective role in both cell culture and in vivo experiments via several mechanisms (Capiralla et al. 2012; Granzotto and Zatta 2011; Yan et al. 2018a, b). In this regard, in animal models of PD as well as AD, resveratrol was found to reduce cognitive impairment, protect neurons against induced neurotoxicity, and decrease the release of proinflammatory factors (Guo et al. 2016; Porquet et al. 2013; Zhang et al. 2010).

Like resveratrol, curcumin is extensively reported as a neuroprotective agent in many cell and animal models of several ND. In different cell lines, curcumin was shown to inhibit proapoptotic signals, decrease cytotoxicity, and enhance cell viability (Dutta et al. 2009; Wang et al. 2010). It also showed the same neuroprotective effects in different in vivo models of PD and AD. These effects were mediated via controlling the pathogenic inflammatory and oxidative mechanisms (Hatcher et al. 2008). Curcumin suppressed neuroinflammation, in an AD mouse model, by reducing the production of proinflammatory mediators and mitigating memory impairments in these mice (Liu et al. 2016). In a mouse model of PD, curcumin inhibited lipid peroxidation and increased the activity of several antioxidant enzymes, thus suppressing neuroinflammation and restoring motor deficits in these mice (Khatri and Juvekar 2016). All of these data can justify the traditional use of Curcuma longa (turmeric) in the management of several mental disorders (Xia et al. 2007)

Other significantly and broadly researched phytochemicals are ginsenosides. Several studies in cell culture and animal models of AD and PD have reported that ginseng, rich in ginsenosides, improved memory and learning skills and protected mice from induced toxicity, promoted neural growth and exerted an anti-apoptotic effect as well as prevented oxidative damage and neuroinflammation (Ardah et al. 2015; Wu et al. 2009; Zhao et al. 2013; Zhou et al. 2016). In addition to chronic ND, ginseng was also shown to exhibit neuroprotective effects in acute ND. Ginsenosides improved neurobehavioral functioning and reduced brain edema in several thromboembolic strokes and hemorrhage rat and patient models (Chen et al. 2016; Li et al. 2011; Yoshikawa et al. 2008). Furthermore, ginseng improved neurological deficits and neuronal damage after focal cerebral ischemia, reduced brain infarct volume, and enhanced regeneration of the central nervous system in adults (Wang et al. 2009; Xie et al. 2015; Zheng et al. 2011). More interestingly, the neuroprotective activities of ginseng against stress and depression have been thoroughly described in the literature. Ginseng decreased the levels of stress markers, elevated the level of certain neuromodulators, and improved memory and learning abilities in several rat and mouse stress models (Dong et al. 2013; Lee et al. 2006a,b). In addition, ginseng acted as an anti-apoptotic factor, decreased reactive oxygen species, increased neuronal viability and mitochondrial stability in cells subjected to stress (Liang et al. 2013). Interestingly, ginseng acted as an antidepressant in several studies of rats and depression patients (Jeong et al. 2015; Jin et al. 2017; Lee and Ji 2014).

In addition, there is supporting evidence to show that phytochemicals can participate in the therapeutic management of ND by a rather different mechanisms that involve inhibition of NLRP3 (nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3) inflammasome. The NLRP3 inflammasome complex is composed of caspase-1 protease, NLRP3 sensor protein, and apoptosis associated speck-like protein which contains an adaptor protein with caspase recruitment domain (de Zoete et al. 2014). NLRP3 inflammasome activation is related and implicated in the pathology of several types of diseases ND (Balashov et al. 1999; Losy and Niezgoda 2001; Shao et al. 2018). Numerous in vitro and in vivo reports of acute and chronic brain injuries demonstrated that some phytochemicals such as resveratrol, curcumin, baicalein, and sulforaphane can suppress NLRP3 inflammasome activation and improve the cognitive functions and spatial learning and memory through different mechanisms of action (Dong et al. 2015; He et al. 2017, 2018; Qi et al. 2019; Rui et al. 2020; Yu et al. 2017).

Taken together, the above studies and evidence suggest that several classes of phytochemicals can act as neuroprotective agents. These bioactives appear to have potential as candidate therapeutic drugs and interventions for several ND including TBI. In the next section we discuss in detail the involvement of several phytochemicals in the management of the adverse outcomes of TBI.

4 Phytochemicals in TBI Management

The literature is packed with a huge number of reports on the use of many phytochemicals with claimed neuroprotective effects following experimental TBI. Most of these phytochemicals were able to modulate the pathological events that follow secondary injury following TBI. In this context, the majority of these phytochemicals were shown to reduce oxidative stress or neuroinflammation. For example, resveratrol treatment has shown significant enhancement cognitive functions post-TBI by modulating several modes of action of secondary injury like decreasing oxidative stress and neuroinflammation or reducing of edema and lesion volume (Scheff and Ansari 2017).

Phytochemicals reported to exert beneficial outcomes in experimental models of TBI include: allicin, apocynin, baicalein, caffeine, coumarin, crocin, curcumin, 7,8-Dihydroxyflavone, ellagic acid, epigallocatechin-3-gallate, formononetin, luteolin, gallic acid, genistein, ginkgo biloba extract, ginseng, quercetin, resveratrol, and many others (Scheff and Ansari 2017). In this chapter the discussion will be focused on six important phytochemicals, namely: ginseng, curcumin, coumarin, genistein, apocynin, and baicalein. These phytochemicals were chosen because they have shown therapeutic potential in neurological diseases other than TBI.

4.1 Ginseng

Ginseng has been in use in traditional medicine since more than 20 centuries ago (Kim 2012), yet its usage in western medicine started only in the early twentieth century (Shih-Chen et al. 1973). Ginseng mainly grows in Asian countries such as China, Korea, Japan, and Vietnam and some North American countries (predominantly Canada and the United States). Several species of ginseng exist and are of medical use: Panax ginseng C.A. Mey (Korean red ginseng), Panax notoginseng (Chinese ginseng), Panax quinquefolium L. (American ginseng), and Japanese ginseng (Panax japonicas).

Presently, ginseng is utilized either in liquid (tea or oil extracts) or solid forms (dried roots, tablets, or capsules) (Valli and Giardina 2002). It is reported that extracts of ginseng root, berry, and leaf have a myriad of beneficial effects that include: antihypertensive, anti-obesity, antihyperglycemic, and antihyperlipidemic activities (Kim 2012).

Ginseng extracts have a rich repertoire of isolated bioactives which have been reported to exceed 300. The major constituents isolated from ginseng extracts are ginsenosides, triterpene saponins (Mahady et al. 2000). Of the 40 ginsenosides identified from ginseng extracts, Rg1, Rb1, Rg3, Re, and Rd. are the most commonly studied (Lee and Kim 2014). As of recent, the study of individual ginsenosides instead of total ginseng extracts has lately drawn interest (Kim 2012).

Several reports have shown that the ginseng medicinal herb is useful in the treatment of ND (reviewed in (Hou et al. 2020)) and traumatic brain injury (TBI) (Fig. 14.1) (Ji et al. 2005; Kumar et al. 2014; Xia et al. 2012). In context, Ginseng Total Saponins (GTS) extracts improve neurological functions and attenuate the secondary injury in rat models of TBI through their anti-inflammatory, antioxidant, and anti-apoptotic actions. More specifically, GTS is reported to downregulate the activity of nitric oxide synthase, release of inflammatory mediators (IL-6, IL-1β, and TNF-α), and levels of apoptotic proteins (caspase-3 and Bax). On the other hand, this biologically active substance increases the activity of the antioxidative enzyme superoxide dismutase, together with the anti-inflammatory cytokines (interleukin-10) and the anti-apoptotic factors (Bcl-2). GTS also appears to trigger tissue repair and regeneration of neurons through the production of nerve growth-related factors, as well as the stimulation of cell division of NSCs, and neural progenitor cells. Besides, this traditional treatment option is demonstrated to reduce the volume of cerebral edema and cerebral contusion. Of interest, it is believed that ginseng might ameliorate learning and memory deficits following TBI. Also, its anti-inflammatory actions are linked to the inhibition of the activation of brain microglia. This is of therapeutic benefit because suppression of neuroinflammation is associated with a reduction of neuronal loss and an improvement of cognitive functions (Hu et al. 2014; Kumar et al. 2014; Xia et al. 2012). Interestingly, a recent report showed that the combination of Korean red ginseng and Angelica sinensis can alleviate ischemia brain injury by decreasing the activation of NLRP3 complex, reducing pyroptosis of microglia and partially suppressing Drp1-mediated mitochondrial fission (Hu et al. 2020). This study opens the door for combination therapies using phytochemicals in the treatment of TBI. Combination therapy may be of value in the management of TBI since the complexity of the secondary injury may need a multi-faceted therapeutic intervention that can target different cellular pathways at the same time (Scheff and Ansari 2017). However, it has to be warned that due to diverse pharmaceutical interactions, multiple drug combinations do not usually work in concert and may result in adverse physiological conditions (Scheff and Ansari 2017).

Fig. 14.1
figure 1

Different pathological mechanisms participate in the progression of TBI. Several of these mechanisms are a target of phytochemicals. Most phytochemicals are antioxidants or ROS scavengers. For example, curcumin can inhibit lipid peroxidation. Another example is apocynin which can inhibit NOX activity. Other phytochemicals such as baicalein, genistein, and curcumin can activate Nrf2-dependent antioxidant transcriptional programs. Phytochemicals can also activate Nrf2 causing the production of anti-inflammatory agents leading to neuronal survival. Oxidative stress can also cause mitochondrial dysfunction and possibly eventual apoptosis. Genistein can inhibit mitochondria-induced apoptosis. Neuroinflammation is another pathological event during TBI. Phytochemicals can inhibit Myd88/NFkB-dependent and NRLP3-dependent production of proinflammatory mediators. Other phytochemicals such as genistein and curcumin can activate survival signaling pathways in neurons, for example, the BDNF/Akt signaling pathway

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Finally, the available research data indicate that inhibition of oxidative stress and neuroinflammation, two main components of the secondary injury phase of TBI, are the main ways by which ginseng may mitigate the outcomes of TBI. More studies are warranted to reveal the exact mechanisms that underlie the neuroprotective effects of ginseng following TBI (Hu et al. 2014; Ji et al. 2005; Kumar et al. 2014; Xia et al. 2012). Importantly, well-designed and controlled clinical studies are required to support the neuroprotective roles of ginseng post-TBI.

4.2 Curcumin

Curcumin phytochemical is a flavonoid found in curry spice and is isolated from the rhizomes of the plant Curcuma longa (Samini et al. 2013; Wu et al. 2006). Increasing proof backs the potential use of curcumin as a treatment option in TBI patients (Fig. 14.1), since it acts on different levels to improve the neurological status and to mitigate the severity score of the secondary brain injury. Curcumin particularly acts through inhibition of neuroinflammation, oxidative stress, cerebral edema, and neuronal apoptosis (Dai et al. 2018; Dong et al. 2018; Huang et al. 2018; Samini et al. 2013; Wu et al. 2011). In fact, curcumin possesses well-proven antioxidant properties. Curcumin can decrease the levels of malondialdehyde (MDA), a major biomarker of marker of oxidative stress where its levels increase because of lipid peroxidation (a consequence of TBI associated cellular injury) (Lu et al. 2014). At the same time, curcumin can maintain high levels of antioxidant enzymes activity by triggering the Nrf2 (nuclear factor erythroid 2-related factor 2) pathway (Dai et al. 2018; Dong et al. 2018; Huang et al. 2018; Samini et al. 2013; Wu et al. 2006).

Along the same lines, curcumin treatment can also mitigate the inflammatory response post-TBI by decreasing neutrophil infiltration and the rates of microglia activation. More specifically, curcumin can act on the TLR4/MyD88/NF-κB (Toll-like receptor 4/Myeloid differentiation primary response 88/Nuclear factor kappa B) cascade to suppress microglial activation and to lower the number of activated macrophages. This eventually leads to decreased availability of inflammatory mediators that usually contribute to neuronal apoptosis and cerebral edema (Dong et al. 2018; Zhu et al. 2014). Of note, it appears that inhibition of activation of microglia and subsequent reduction of production of proinflammatory mediators attenuate aquaporin-4 expression and thus cerebral water content and edema (Laird et al. 2010). Moreover, this phytochemical is reported to reduce cell death, axonal injury, and brain lesion size (Huang et al. 2018; Samini et al. 2013).

Finally, it is of importance to indicate that dietary curcumin supplementation was demonstrated to ameliorate neuronal and synaptic plasticity impairments as well as locomotor and cognitive deficits in the injured brain through different mechanisms: a) normalization of neuronal signaling, b) inhibition of oxidative stress after modulating the BDNF system, c) restoration of plasma membrane integrity, and d) regulation of different molecular processes to re-establish energy homeostasis (Sharma et al. 2009, 2010; Wu et al. 2006, 2011). Consistently, a recent report demonstrated that curcumin administration can mitigate the spatial memory deficits, in a TBI mouse model, through a reduction in chronic neuroinflammation, and an enhancement of hippocampal neurogenesis where the BDNF/Trkb/PI3K/Akt pathway maybe involved (Sun et al. 2020).

Interestingly, curcumin was tried in combination with autologous transplantation of adult NSCs for the treatment of TBI and it was revealed that the combination results in more effective repair of the injured cerebral tissue than curcumin treated only or autologous stem cell transplantation only brain-injured rats following TBI. Such therapeutic combinations maybe important for devising future therapeutic interventions for TBI (Attari et al. 2020)

Despite the proven beneficial activities of curcumin like its anti-inflammatory and antioxidant capacities, excellent biological and pharmacological properties, and low side effects, curcumin has been tested only in few preclinical trials (Dong et al. 2018; Laird et al. 2010). In fact, there exist numerous clinical studies that test curcumin in the context of different neurological diseases (Salehi et al. 2020), but not TBI (Farkhondeh et al. 2020). As such, clinical studies to test the efficacy of curcumin in the context of TBI are urgently needed. Lastly, a main limitation for the pharmaceutical use of curcumin is its low bioavailability, for this reason curcumin nanomedicine formulations are being developed to enhance its bioavailability, efficacy, and therapeutic index (Salehi et al. 2020).

4.3 Baicalein

Baicalein or 5,6,7-trihydroxyflavone is a bioactive flavonoid isolated from the roots of Scutellaria baicalensis and Scutellaria lateriflora and is extensively used in the traditional Chinese medicinal system to manage cardiovascular diseases, inflammation, and infections (Kumari Varsha et al. 2017). It is characterized by its ability to cross the blood–brain barrier (BBB) and offer neuroprotection in a number of neurological diseases including cerebral ischemia (Liang et al. 2017), temporal lobe epilepsy (Qian et al. 2019), and subarachnoid hemorrhage (Shi et al. 2017).

Baicalin can protect against the secondary injury post-TBI (Fig. 14.1). Recent studies have demonstrated that baicalin reduces inflammatory cytokine production following TBI (Chen et al. 2008) and inhibits TBI-induced neuronal apoptosis and oxidative damage, through stimulation of the Akt/Nrf2 pathway (Fang et al. 2018). Consistently, baicalin protected the brain in a mouse model of TBI by inhibition of TBI-induced apoptosis; however; autophagy was implicated in this inhibition (Fang et al. 2019). Also, baicalein significantly alleviates brain edema and reduces BBB permeability (Wang et al. 2015). As such, the usage of baicalein as a therapeutic approach to reduce the evolution of tissue damage following TBI has recently gained attention by virtue of its anti-inflammatory, antioxidative, and neuroprotective abilities (Scheff and Ansari 2017). In earlier studies, treatment of rats with baicalein post-TBI demonstrated improved functional and histological outcomes, and reduced the contusion volume and FJB staining that marks degenerating neurons (Chen et al. 2008). Baicalein also attenuated the TBI-induced elevation in proinflammatory agents TNF-α, IL-6, and IL-1β, in parallel with reduced neurological deficits and brain damage (Chen et al. 2008). All these activities were attributed to modification of the proinflammatory cascade. Interestingly, baicalin was also shown to attenuate neuroinflammation in Parkinson’s disease by blunting the NLRP3 inflammasome signaling; a similar mechanism could be implicated in TBI, but this remains to be examined (Rui et al. 2020)

More studies evaluated baicalein use in the treatment of subarachnoid hemorrhage, a condition that is often secondary to TBI. Baicalein administration was shown to significantly attenuate BBB disruption, proinflammatory cytokine release, neural cell apoptosis, and brain edema in rats with subarachnoid hemorrhage (Wang et al. 2015). Treatment with baicalein also resulted in suppression of TLR4 protein expression, degradation of IκBα, and a decrease in the translocation of p65 to the nucleus. This may be a potential mode of action of baicalein (Wang et al. 2015). Similar findings were reported in a rat model of rotenone-induced brain injury (Zhang et al. 2017). An important downstream effector of the TLR4/NF-κB pathway is matrix metalloproteinase-9 (MMP-9), which contributes to BBB disruption and formation of edema following subarachnoid hemorrhage. Wang et al. demonstrated that baicalein appears to inhibit the subarachnoid hemorrhage-induced rise in MMP-9 and restore the normal levels of tight junctional protein ZO-1, ultimately causing the restoration of the BBB (Wang et al. 2015). Additionally, baicalein was shown to stimulate Epo and VEGF expression, which are considered as hypoxia-induced neuroprotective factors; thus protecting neurons from excitotoxicity in a manner dependent on Epo/VEGF and PI3K (Sun et al. 2013).

Endoplasmic Reticulum stress has recently gained attention for its role in neuronal injury. As such, in a hippocampal neuronal murine cell line HT22, apoptotic cell death was induced triggered via thapsigargin (TG; ER stress inducer) and brefeldin A (BFA) and then evaluated following treatment with baicalein. Baicalein treatment attenuated BFA- and ER stress-triggered cell death, cell death and the stimulation of MAP kinases like p38, JNK, and ERK (Choi et al. 2010). Baicalein treatment also appears to reduce the levels of caspase-3, PARP (poly ADP-ribose polymerase), and cleaved caspase-12, and attenuate BFA and TG-stimulated ER stress signaling events, thereby providing neuroprotection against ER-induced cell death (Choi et al. 2010). Crosstalk between ER and mitochondria during ER stress causes mitochondrial damage, thereby increasing ROS and promoting cellular death. Baicalein treatment decreases TG and BFA-stimulated ROS generation in HT22 cells as early as 30 min post-treatment, suggesting that baicalein has antioxidant effects on ER stress-induced ROS production (Choi et al. 2010).

Following TBI, an increased iron accumulation leads to one type of regulatory cell death termed ferroptosis. During ferroptosis increased iron accumulation can induce lipid peroxidation, ROS generation, neuroinflammation, and mitochondrial impairment. Given that the brain has an the highest abundance of poly-unsaturated fatty acids, which are precursors for lipid peroxidation, and that lipid peroxidation is associated with neurological disorders, including TBI (Kenny et al. 2019), then this poses the prospect of targeting iron deposition and lipid peroxidation as a therapeutic approach in TBI (Tang et al. 2020). In the model of iron-induced damage using HT22 cells, baicalein treatment appears to markedly reduce ferroptotic indices and downregulate the levels of 12/15-lipoxygenase (12/15-LOX), which contributes to oxidative and inflammatory responses (Li et al. 2019). This in vitro TBI model suggests that baicalein possesses an anti-ferroptotic activity through the inhibition of 12/15-LOX.

Baicalein penetrates the BBB within 20–30 min of its application, distributing evenly across various areas of the brain (Tsai et al. 2002). Chen et al. found that one baicalein injection was enough to provide neuroprotection for as long as 28 days in rat brains subjected to controlled cortical impact injury (Chen et al. 2008). As such, baicalein possesses other advantages that facilitate its use as a therapeutic intervention post-TBI. For example, it has low levels of toxicity, does not require chronic dosing and is easy to administer in emergency rooms.

In conclusion, baicalin has proven effects against experimental TBI. It has the promise for treatment of TBI in clinical settings; however, this promise awaits testing of baicalin efficacy in well-designed and controlled clinical trials.

4.4 Apocynin

Apocynin is 4-hydroxy-3-methoxy-acetophenone, also called acetovanillone, is a natural methoxy-substituted catechol isolated from roots of the medicinal plant Kutki (Picrorhiza kurroa), a rhizome native to the mountains of Nepal, India, Pakistan, and Tibet, (Wang et al. 2006) and from the Canadian hemp Apocynum cannabinum (Scheff and Ansari 2017). Apocynin has been used for hundreds of years to reduce inflammation and manage certain infectious diseases (Stolk et al. 1994). In addition, it has been extensively used in several experimental models as an inhibitor of NADPH oxidases (NOX) (Song et al. 2013).

Notably, apocynin appears to be safe having a very low toxicity and an LD50 of 9 g/kg in mice (Stefanska and Pawliczak 2008). Even when treated with apocynin for 3 months, rabbits did not demonstrate any signs of disease when compared to control animals (James Arthur Holland 1999). When administered by nebulization in a clinical trial to test the influence of apocynin on nitrite and hydrogen peroxide concentrations in breath condensates of COPD patients, no serious or non-serious adverse events were observed (Stefanska et al. 2012). Furthermore, mutagenicity assay and sister chromatid exchange assay of Salmonella typhimurium demonstrated that apocynin does not display any genotoxic activities (Pfuhler et al. 1995). All in all, these data indicate that apocynin is safe with minimal cytotoxicity.

NADPH oxidases are a family of membrane-bound enzyme complexes composed of several subunits (p40phox, p47phox, p67phox, Rac2, and activated Rac1). Several NOX isoforms (termed NOX 1–5) exist, with varying tissue-specific expression pattern (Lu et al. 2014). Current evidence demonstrates that NOX, together with the mitochondria, participates in the production of the superoxide anion \( {\mathrm{O}}_2^{-} \) following TBI. Markedly, NOX2, whose catalysis occurs through the Gp91-phox (phagocytic oxidase) subunit, is highly localized to neurons, microglia of the cortex and hippocampus (Zhang et al. 2012). Upon cellular stress, the NOX2 complex is activated in microglia by several phox subunits and activation of Rac1, which relocalizes from the cytoplasm to the membrane. Active NOX2 complex formation elevates ROS production and contributes to neuroinflammation through the increased microglial formation of several proinflammatory and neurotoxic cytokines (Zhang et al. 2012).

Oxidative stress is of particular interest since it is known to have critical roles in the pathology of TBI, especially that ROS generation can trigger neuronal cell death and functional impairments (Wada et al. 1999). A myriad of studies showed that apocynin can protect against several aspects of the secondary injury following TBI (Fig. 14.1). Interestingly, apocynin has been demonstrated to be effective at reducing damage caused by oxidative stress post-TBI. Apocynin can inhibit NOX activation by preventing the intracellular translocation of p41-phox and p67-phox, leading to a block of assembly (Lafeber et al. 1999; Zhang et al. 2012). When administered into Nox2 knockout mice, apocynin appears to lose its neuroprotective abilities, suggesting that it exclusively targets the inhibition of NOX2 NADPH oxidase (Zhang et al. 2012). Conformingly, Choi et al., reported that apocynin prevents TBI-triggered neuronal cell death by inhibition of NOX activation (Choi et al. 2012). Nevertheless, apocynin may have NOX-2 independent antioxidant functions since apocynin has been shown to promote superoxide dismutase antioxidant activity (Lu et al. 2014). In a related study, Feng et al., reported that apocynin may exert its neuroprotective roles by modulation of the TLR4/NF-κB pathway as well as neuronal autophagy (Feng et al. 2017).

Additionally, apocynin treatment was shown to mitigate microglia activation, attenuate BBB disruption and the leakage of immunoglobulins associated with BBB impairment. Furthermore, apocynin can reduce inflammation, neuronal death, and β-amyloid-induced neurotoxicity in mouse models (Ferreira et al. 2013; Zhang et al. 2012). At TBI onset, cerebral hypoxia is elicited due to impaired blood flow, this hypoxia elicitits a shift from aerobic to anaerobic metabolism, increasing Ca2+ levels, and thus leading to increased ROS production (Angeloni et al. 2015). Relatedly, apocynin was shown to be to protective from ischemia-induced neuronal degeneration and ROS production, as well as decrease delayed neuronal death and increase neuronal density (Wang et al. 2006). Consistently, mice subjected to TBI then treated with apocynin showed a lower level of MDA than brain-injured mice but not treated with apocynin. Moreover, apocynin treatment reduced brain edema, enhanced spatial learning functions, in Morris water maze, and mitigated cognitive deficits in mice (Song et al. 2013). It is interesting to mention that apocynin was also tried in combination with tert-butylhydroquinone, a synthetic phenolic antioxidant, in different brain injury models and the combination was shown to improve the neurologic recovery, cognitive function, and reduce the secondary cortical lesion more than either molecule alone. This enhanced effect was attributed to attenuation of ROS generation by apocynin-mediated NOX2 inhibition and ROS degradation by tert-butylhydroquinone-mediated activation of the Nrf2 pathway (Parastan et al. 2020).

Altogether, these results indicate that apocynin can be an important new neuroprotective approach against TBI, mainly, by reducing TBI-induced upregulation of NOX2 protein and activity. Yet, the therapeutic potential of apocynin, demonstrated in preclinical animal models, has not been supported by controlled clinical trials.

4.5 Genistein

Genistein is 5,7-Dihydroxy-3-[4-hydroxyphenyl] chromen-4-one. It is a naturally occurring isoflavone phytochemical that is widely distributed across a variety of plants such as Medicago sativa, legumes, soy, and chickpea (Scheff and Ansari 2017). It is recognized as a phytoestrogen and has been evaluated for its neuroprotective abilities in different models of TBI (Fig. 14.1). For example, it was found to normalize vasodilation, cerebral blood flow, and ICP following Moderate Fluid Percussion Injury in adult rats (Hong et al. 2001). Genistein appears to significantly improve neurological performance, motor and vestibulo-motor deficits, reduce ICP, brain edema, and BBB permeability in a closed head injury model of TBI in rats (Soltani et al. 2015). Furthermore, this natural compound has been shown to have neuroprotective abilities through the activation of Nrf2, which can activate antioxidant enzymes expression and can have beneficial effects on cell survival (Wu et al. 2018). Genistein was also shown to prevent excessive microglia activation of CA1 neurons, as well as reducing CA1 neuronal death induced by forebrain ischemia, and suppressing the effects of corticosterone (Shi et al. 2014). Lee and colleagues demonstrate that genistein participates in the inhibition of cytokine-induced overexpression of proinflammatory mediators, and thus attenuates proinflammatory pathways in human brain endothelial cells (Lee and Lee 2008).

The exact molecular and cellular modes of action of genistein are yet to be unraveled. However, Soltani and colleagues suggest that neuroprotection of genistein occurs through reduction of oxidative stress, neural apoptosis, and activation of PI3K (Soltani et al. 2015). In concert, these results suggest a neuroprotective ability of genistein in the treatment of traumatic brain injury (Lee and Lee 2008).

The clinical promise of genistein is far from being realized, but the existing experimental evidence should be able to give genistein involvement in the management of TBI a chance at clinical trials.

4.6 Coumarin

Coumarin phytochemicals are a family of benzopyrones, 2H-1-benzopyran-2-ones. They are largely distributed in nature. They are structurally composed of a benzene ring bonded to ring of α-pyrone, giving them a conjugated system rich with electrons, and a good charge transport ability (Venketeshwer Rao 2015). Coumarin compounds have been widely used throughout history as medicinal drugs. In western medicine, coumarin was first isolated in 1868 from the seed of D. Odorata and synthesized as a precursor of anticoagulant therapeutical compounds, but has since gained interest for a wide range of other applications (Venketeshwer Rao 2015). In fact, coumarins were shown to have a strong pharmacological activity, low toxicity, limited drug resistance, low side effects, broad-spectrum of activity, and high bioavailability (Venketeshwer Rao 2015). One of the natural derivatives of coumarin is Osthole (7-methoxy-8-isopentenyl coumarin). Osthole is a compound isolated from the extracts of Cnidium monnieri or Peucedanum ostruthium plants and has been found to have anti-apoptotic, antioxidative, anti-inflammatory, and neurotrophic effects (Kong et al. 2019). These properties made osthole a good candidate for the management of secondary injury post-TBI (Fig. 14.1). Recent studies show that osthole has protective abilities in several animal models of CNS diseases like autoimmune encephalomyelitis (Gao et al. 2014), cerebral ischemia (Li et al. 2016), AD (Yao et al. 2015), and TBI (Kong et al. 2019). For instance, treatment with osthole prior to TBI showed decreased neurological deficits post-TBI as compared to non-treated brain-injured rats. Also, there was a reduction of cerebral edema and an improvement of functional outcomes, in an osthole dose-dependent manner (He et al. 2012). Mice subjected to TBI, treated with Osthole, were found to have suppressed neuroinflammation levels when compared to non-treated TBI-induced mice. An attenuated release of inflammatory cytokines and reduced apoptosis were evident as well (Kong et al. 2019). Kong and colleagues demonstrated that this osthole neuroprotective effect may via attenuation of the NF-κB pathway, which explains the alleviation of the inflammatory response following TBI. Osthole was also shown to promote neuronal survival and differentiation from neuronal stem cells in cell culture experiments. This effect was Notch-dependent and involved the upregulation Hes 1 and Notch 1, self-renewal genes (Yan et al. 2018a).

Lipid peroxidation following free radical accumulation contributes to the pathogenesis of TBI. Malondialdehyde appears to significantly accumulate in rat’s post-TBI. However, this increase in MDA is attenuated in rats treated with osthole. Osthole treatment, following TBI, also suppresses the excessive formation of ROS, and ameliorates apoptosis by increasing Bcl-2/Bax ratio. (He et al. 2012). Similar findings were achieved by Xai and colleagues, who demonstrated that osthole inhibits IL-6 expression, and prevents apoptosis via decreasing the ratio of Bax to Bcl-2, as well as attenuating the expression of caspase-3, a key apoptotic effector (Xia et al. 2015). Accordingly, osthole treatment was also shown to reduce microglia and macrophage numbers in the parenchyma of the brain, by decreasing infiltrating leukocytes number and reducing proinflammatory cytokine level, granting osthole an anti-apoptotic, antioxidant, anti-inflammatory, and neurotrophic effects (Xia et al. 2015).

Another major contributor to secondary brain injury post-TBI is the remarkable increase in Ca2+ influx into neurons. Calcium influx activates an array of degenerative enzymes that bring about a breakdown of the cell membrane and cytoskeletal components, ROS generation, mitochondrial damage, DNA degradation, and, ultimately, cell death (Wojda et al. 2008). Osthole treatment appears to reduce calcium influx by inhibiting L-type calcium channels in neuronal cells, thereby suggesting that osthole affects MDA levels and superoxide dismutase activity through the blockade of Ca2+ entry (He et al. 2012).

The repertoire of beneficial effects of osthole in experimental models of TBI needs to be complemented by clinical trials to test its safety and efficacy in the treatment of TBI.

5 Conclusion

This chapter discussed the growing evidence on the molecular modes of action and the neuroprotective properties of phytochemicals post-TBI. Many of the phytochemicals were shown to target at least one of the pathologic events of the secondary injury post-TBI (Fig. 14.1). As a result it is not surprising that in many cases phytochemicals were found to exhibit neuroprotection in animal models of TBI. The hope is that similar beneficial effects of these phytochemicals will be seen in human TBI patients. However, there is a scarcity and in most cases a lack of clinical studies on the safety and efficacy of phytochemicals in the context of TBI. Hence, well-designed and controlled clinical studies are urgently needed to determine the safety and clinical benefits of phytochemicals in the context of TBI and to translate the beneficial effects of phytochemicals in experimental TBI from the bench to the bedside.