Protective effects of ginseng on neurological disorders

Ginseng (Order: Apiales, Family: Araliaceae, Genus: Panax) has been used as a traditional herbal medicine for over 2000 years, and is recorded to have antianxiety, antidepressant and cognition enhancing properties. The protective effects of ginseng on neurological disorders are discussed in this review. Ginseng species and ginsenosides, and their intestinal metabolism and bioavailability are briefly introduced. This is followed by molecular mechanisms of effects of ginseng on the brain, including glutamatergic transmission, monoamine transmission, estrogen signaling, nitric oxide (NO) production, the Keap1/Nrf2 adaptive cellular stress pathway, neuronal survival, apoptosis, neural stem cells and neuroregeneration, microglia, astrocytes, oligodendrocytes and cerebral microvessels. The molecular mechanisms of the neuroprotective effects of ginseng in Alzheimer’s disease (AD) including β-amyloid (Aβ) formation, tau hyperphosphorylation and oxidative stress, major depression, stroke, Parkinson’s disease and multiple sclerosis are presented. It is hoped that this discussion will stimulate more studies on the use of ginseng in neurological disorders.


Introduction
Ginseng (Order: Apiales, Family: Araliaceae, Genus: Panax) roots, stems, and leaves have been used as traditional herbal medicine for over 2000 years. In Korea, China and Japan, ginseng is the most valuable of all medicinal herbs. Its anti-anxiety, antidepressant and cognition-enhancing effects has been recorded by Shi-Zhen Li in Ben Cao Gang Mu ( ) which is the most comprehensive pre-modern herbal text, compiled during the Ming Dynasty in China. Panax ginseng ( ) is mostly cultivated in Korea, the Manchuria region of China (''dong bei'') and the coastal region of Siberia due to its sensitivity to temperature and soil conditions. Panax quinquefolium L (American ginseng, ) is cultivated in southern Canada and the USA, and Panax notoginseng ( ) is grown in the Yunnan and Guangxi provinces of China. These three herbs represent the most extensively investigated species of Panax. The latter means ''cure all'', and constituents of ginseng root produce adaptogenic, restorative, immunomodulatory, vasodilatory, anti-inflammatory, antioxidant, anti-aging, anticancer, anti-fatigue, anti-stress and anti-depressive effects in rodents and humans (Attele et al., 1999;Shin et al., 2000;Chang et al., 2003;Choo et al., 2003;Cheng et al., 2005;Wang et al., 2009). The bioactive ingredients in ginseng root include more than 60 ginsenosides, e.g., Rb1, Rb2, Rb3, Rc, Rd, Re, Rg1, Rg2, and Rg3, as well as polysaccharides, fatty acids, oligopeptides and polyacetylenic alcohols (Qi et al., 2010). The purpose of this review is to discuss the effects of ginseng on the normal brain, and its protective effects in neurological disorders such as Alzheimer's disease (AD), major depression, stroke, Parkinson's disease and multiple sclerosis. It is hoped that will stimulate more studies on the use of ginseng in neurological disorders.

Intestinal Metabolism and Actions of Ginseng
Ginsenosides are triterpene saponins that have a common four ring hydrophobic steroid-like structure with sugar moieties (monomeric, dimeric, or trimeric) attached mainly at carbons 3, 6 and 20 (Zhu et al., 2004;Nah et al., 2007). Ginsenosides are divided into two different structural classes: the 20 (S)protopanaxadiol (PD) group of ginsenosides Ra1, Ra2, Ra3, Rb1, Rb2, Rb3, Rc, Rd, Rg3 and Rh2 and the 20 (S)-protopanaxatriol (PT) group ginsenosides Re, Rf, Rg1, Rg2, and Rh1 (Baek et al., 2012). Figure 1 shows the chemical structures of the two ginsenosides Rb1 and Rg1. In addition, several new ginsenosides such as 25-OH-pPD and 25-OH-pPT have been isolated and characterized from the fruit. 25-OH-pPD shows strong cancer preventing activity in vitro (Wang et al., 2007). Four malonyl derivatives of ginsenosides Rb1, Rb2, Rc and Rd have also been described (Fuzzati, 2004). The different sugar moieties in ginsenosides have been proposed to provide specificity for the therapeutic effects of ginsenosides (Zhu et al., 2004;Nah et al., 2007). The most commonly studied ginsenosides are Rb1, Rg1, Rg3, Re, and Rd. Intact ginsenosides are absorbed FIGURE 1 | Chemical structure of ginsenosides Rb1 and Rg1. Ginsenoside Rb1 is an example for 20 (S)-PD and ginsenoside Rg1 is an example for 20 (S)-PT type of ginsenosides. only from the intestines (the absorption rate is as low as 1 to 3.7%) and most ginsenosides are metabolized in the stomach (acid hydrolysis) and in the intestine (bacterial hydrolysis) or transformed to other ginsenosides. In vitro studies have indicated that ginsenoside Rb1 is metabolized by human intestinal bacteria. The metabolism is initiated by ginsenoside Rd (Rd) pathway at the C-20 glucose (Rb1 → Rd → ginsenoside F2 (F2) → Compound K (Cpd K) (Hasegawa et al., 1997;Hasegawa and Uchiyama, 1998;Tawab et al., 2003;Hasegawa, 2004;Bae et al., 2006;Qian et al., 2006;Chen et al., 2008b;Hou et al., 2012;Jung et al., 2012;Zhao et al., 2012). Collective evidence suggests that the metabolism and transformation of intact ginsenosides is an important process, playing an important role not only in bioavailability but also in the potential health benefits of ginseng (Chen et al., 2008a), such as anti-tumor (Xu et al., 2007;Lee et al., 2009), antioxidant, anti-inflammatory (Keum et al., 2003Bae et al., 2006), anti-fatigue and angio-suppressive effects (Yue et al., 2006). Thus, ginseng is a natural remedy which not only enhances phagocytosis, natural killer cell activity, psychological function, cardiac function, and exercise performance, but also improves immune function by enhancing the production of interferons and increasing resistance to stress (Kiefer and Pantuso, 2003).
Many of the beneficial effects of ginseng are mediated through modulation of enzymes involved in signal transduction processes ( Table 1). Administration of ginseng extract over 10 days inhibited peroxisome proliferator-activated receptor-α function, and decreased the expression of several genes involved in lipid and lipoprotein metabolism, but increased serum concentrations of total cholesterol, high-density lipoprotein cholesterol and triglycerides (Yoon et al., 2003). Jiannaoning, a Chinese herbal formula that includes ginseng leaves, improved memory function in rats subjected to cerebral ischemia (Song et al., 2006). It is proposed that Jiannaoning acts by regulating the levels of proinflammatory cytokines-interleukin-2 (IL-2), IL-6 and neuropeptide Y in the rat brain. Moreover, ginsenosides inhibited the tumor necrosis factor-alpha (TNF-α) induced phosphorylation of IkappaB-α (IκB-α) kinase (IKK) and the subsequent phosphorylation and degradation of IκB-α. TNFα-induced phosphorylation of mitogen-activated protein kinase kinase 4 (MKK4) and subsequent activation of the c-Jun Nterminal kinases activator-protein 1 (JNK-AP-1) pathway was also suppressed (Choi et al., 2007). Ginsenosides from ginseng leaves and stems upregulated the level of glucocorticoid receptor (GR) in the brain cytosol of heat-injured rats (Li et al., 2006). Interactions of ginsenoids or glucocorticoids with GR triggered cellular responses involving activation of phosphatidylinositol-3 kinase (PI3K)/Akt pathway and increased nitric oxide (NO) production in human umbilical vein endothelial cells (Lee et al., 1997;Dancey, 2004). Ginsenosides may also regulate ion channels via their ligand-binding sites or channel pore sites (Nah et al., 2007). They inhibited voltage-dependent Ca 2+ , K + , and Na + channel activities in a stereospecific manner  and blocked some subtypes of nicotinic acetylcholine (ACh) and 5-hydroxytryptamine type 3 receptors . Ginsenosides facilitated neurotransmission in the brain (Xue et al., 2006;Liu et al., 2010) but inhibited glutamate  Kim et al. (2007) excitotoxicity (Kim et al., 2002). In addition, ginsenosides have been reported to improve central cholinergic function, and are used to treat memory deficits in humans (Rudakewich et al., 2001). Ginsenosides increased levels of dopamine and norepinephrine in the cerebral cortex (Itoh et al., 1989) and have beneficial effects on attention, cognitive processing, sensorimotor function and auditory reaction time in healthy subjects (D'Angelo et al., 1986).

Bioavailability of Ginseng
The oral bioavailability of ginsenosides is very low. The poor bioavailability of ginsenosides is not only because of low membrane permeability and active biliary excretion, but also due to biotransformation ). As mentioned above, deglycosylated products of ginsenosides are formed by their bacterial metabolism in the gut lumen. These products are more permeable and bioactive than glycosylated ginsenosides (Hasegawa and Uchiyama, 1998).
Intestinal bacterial metabolites of ginseng are absorbed into the bloodstream. Quantitative and statistical analysis of ginsenosides in plasma indicates that protopanaxadiol (PD) saponins exhibit higher concentration and longer half-life than protopanaxatriol (PT) saponins   . Studies on concentrations of ginsenosides in the brain at different time intervals after dosing show that ginsenosides enter into the brain rapidly, but concentrations decline rapidly with time. Ginsenosides with higher concentrations in the brain are Rg1, Re, Rb1 and Rc . Rg1 and Re may be the main components directly affecting CNS neurons, due to their better brain distribution. On the other hand, ginsenosides of PD saponins may protect the brain mostly through a peripheral effect, due to their longer time in the circulation.

Molecular Mechanisms of Effects of Ginseng on the Brain Effect on Glutamatergic Neurotransmission
Ginsenosides have a general stimulatory effect on the brain (Radad et al., 2011). Ginsenoside Rb1 increased glutamate release in neurons via the PKA-dependent signaling pathway, whereas ginsenoside Rg1 induced glutamate release in a calcium/calmodulin-dependent protein kinase II (CaMKII) dependent manner . Ocotillol is a derivative of pseudoginsenoside-F11, a ginsenoside found in American ginseng, and displayed excitatory effect on spontaneous action potential firing and depolarized the membrane potential of mitral cells. This effect was abolished by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist 6-cyano-7nitroquinoxaline-2,3-dione (CNQX) and the N-methyl-Daspartate receptor (NMDAR) receptor antagonist D-AP5, suggesting a role for glutamatergic transmission in the effects of ocotillol (Wang et al., 2011b). Transmission of action potentials transmission in the somatosensory cortex was facilitated by ginsenoside Rb3 perfusion (Cui et al., 2012). Ginsenoside Rg1 supplementation improved the performance of aged mice in behavioral tests and significantly upregulated the expression of synaptic plasticity-associated proteins in the hippocampus, including synaptophysin, NMDAR subunit 1, postsynaptic density protein 95 (PSD95) and CaMKII alpha via the mammalian target of rapamycin (mTOR) pathway . Notoginsenoside R1, a major saponin isolated from Panax notoginseng, increased membrane excitability of CA1 pyramidal neurons in hippocampal slices by lowering the spike threshold, possibly through inhibition of voltage-gated K + currents. It also reversed Aβ 1-42 oligomer-induced impairments in longterm potentiation (Yan et al., 2014). Other studies showed that ginsenoside Rb1 inhibited the activity of L-type voltage gated calcium channels, without affecting N-type or P/Q-type Ca 2+ channels in hippocampal neurons (Lin et al., 2012a). Gintonin is a newly identified compound from ginseng that is found to activate G protein-coupled lysophosphatidic acid (LPA1) receptors with high affinity (Hwang et al., 2012a;Nah, 2012). Gintonin stimulated neurotransmitter release (Hwang et al., 2015) and promoted synaptic enhancement via LPA1 receptor activation in central synapses . Gintonin contains two components: ''ginseng major latex-like protein 151'' (GLP) and ginseng ribonucleaselike storage protein; and recent studies have determined the structure of ginseng major latex-like protein 151 and its proposed lysophosphatidic acid binding mechanism . The LPA1 receptor is localized in dendritic spines of cultured hippocampal neurons (Pilpel and Segal, 2006) that are sites of glutamatergic synapses, and further study is necessary to elucidate a possible role of gintonin and LPA receptor signaling, in the CNS effects of ginseng.

Effect on Monoamine Neurotransmission
Ginsenosides could have effects on monoamine signaling. Total saponins extracted from Panax notoginseng increased the levels of 5-hydroxytryptamine, dopamine and noradrenaline, suggesting that it may have antidepressant effects by modulation of brain monoamine levels (Xiang et al., 2011).

Effect on Estrogen Signaling
Improved object recognition and decreased immobility time in the forced swim test have been reported following ginsenoside Rb1 treatment. Pre-treatment with an estrogen receptor antagonist clomiphene blocked these effects, indicating that they are dependent on the estrogen receptor (Hao et al., 2011). Red ginseng also prevented downregulation of the ERβ estrogen receptor in immobilization-stressed mice (Kim et al., 2013b).

Effect on Nitric Oxide Production
Ginsenosides have been shown in many studies to inhibit inducible nitric oxide synthase (iNOS). Rg3 at 20 and 30 mg/kg oral doses modulated an increase in TNF-α, IL-1β and IL-6 mRNA after lipopolysaccharide (LPS) injection in mice (Park et al., 2012b). Protective effects of Re treatment occurred via the phospho-p38, iNOS, and cyclooxygenase-2 (COX-2) signaling pathways in LPS stimulated BV-2 microglial cells . A bacterial metabolite of Rg5 which is the main constituent of heat-processed ginseng, Rh3, also decreased the expression of iNOS and TNF-α and IL-6, and increased the expression of heme oxygenase −1(HO-1) in LPS stimulated BV-2 microglial cells . As with iNOS, neuronal nitric oxide synthase (nNOS) is inhibited by ginseng. Ginseng saponins are transformed by intestinal microflora, and a product Rh2 decreased the expression of nNOS in the hippocampus, while another product Rg3 decreased its expression in the neocortex (Jang et al., 2004). Ginsenoside Rg1 has neuroprotective effects on ischemia-reperfusion injury in cultured hippocampal cells by blocking calcium influx into neuronal cells and decreasing nNOS activity . In contrast to iNOS, and nNOS, ginsenosides appear to stimulate endothelial nitric oxide synthase (eNOS) and may have beneficial effects on the circulation. Ginsenoside-Rg1 increased the phosphorylation of GR, PI3K, Akt/PKB and eNOS, leading to NO production in human umbilical vein endothelial cells. Knockdown of GR modulated the Rg1 induced NO production. Results indicate that Rg1 can act as an agonist ligand for GR, and that activated GR can induce rapid NO production from eNOS via the PI3K/Akt pathway .

Effect on the Keap1/Nrf2 Adaptive Cellular Stress Pathway (Figure 2)
This is an important adaptive pathway in response to oxidative stress, and many studies have pointed to this pathway as a target of dietary phytochemicals (Reviewed in . Under physiological conditions, Keap1 keeps the Nrf2 transcription factor in the cytoplasm, allowing it to be ubiquitinated and degraded by proteasomes; but when cells are exposed to neurodegenerative disease-mediated oxidative stress, a signal involving phosphorylation and/or redox modification in Keap1 blocks the enzymatic activity of the Keap1-Cul3-Rbx1 E3 ubiquitin ligase complex, leading to decrease in Nrf2 ubiquitination and degradation. As a result, free Nrf2 translocates into the nucleus, where it transactivates the antioxidant response element (ARE) of many cytoprotective genes (Figure 2). Rg1 pretreatment induced Nrf2 nuclear translocation and activation of the PI3K/Akt pathway, which could antagonize iron-induced increase in reactive oxygen species (ROS) and decrease in mitochondrial transmembrane potential in cultured neurons (Du et al., 2013). Likewise, ginsenoside Rh3 increased Nrf2 DNA-binding activity and level of sirtuin 1 (SIRT1) resulting in inhibition of NF-κB in cultured cells . Protopanaxtriol extracted from Panax ginseng increased Nrf2 entering the nucleus, and induced the expression of phase II antioxidant enzymes, including HO-1 and nicotinamide adenine dinucleotide phosphate (NADPH) quinone oxidase 1, after 3-nitropropionic acid induced injury (Gao et al., 2015). Notoginsenoside R1 isolated from Panax notoginseng increased nuclear translocation of Nrf2 and ARE activity, and upregulated the expression and activity of HO-1, NADPH quinone oxidase 1 and gamma-glutamylcysteine synthetase (γ-GCSc) in PC12 cells (Meng et al., 2014). Together, results indicate an important role of the Keap1/Nrf2 pathway in neuroprotective effects of ginseng.

Effect on Neuronal Cell Survival
Oral administration of ginsenoside Rb1 significantly increased cell survival but not proliferation in the hippocampus, that may be related to its effects on learning and memory .

Effect on Apoptosis
Ginsenoside Rg induced downregulation of calpain I and caspase-3 and attenuated neuronal apoptosis caused by cerebral ischemia-reperfusion injury . Ginsenoside Rb1 is reported to suppress the activation of endoplasmic reticulum stress-associated proteins including protein kinase RNA (PKR)like ER kinase (PERK) and C/EBP homology protein (CHOP) and downregulation of Bcl-2 induced by high glucose . Panax notoginseng saccharides increased Bcl-2/Bax ratio and had anti-apoptotic and neuroprotective effects after cerebral ischemia in rats .

Effect on Microglia
Microglia are resident innate immune cells in the CNS, which are activated in response to a variety of stimuli, such as infection, traumatic brain injury, toxic metabolites, or autoimmunity. Ginsenoside Rh1 inhibited the expression of iNOS, COX-2, and pro-inflammatory cytokines while increasing the expression of anti-inflammatory IL-10 in LPS-stimulated microglia. These effects were attenuated by inhibition of HO-1 (Jung et al., 2010). In addition, ginsenoside Rb1 reversed the changes in several direct or indirect neuroinflammatory markers in the hippocampus (Wang et al., 2011a). Ginsenoside Rb1 also decreased the expression of TNF-α, down regulated IL-6 expression, inhibited the activation of NF-κB and modulated microglial activation after brain ischemia (Zhu et al., 2012). Ginsenoside Rg1 pre-treatment of LPS induced BV-2 microglial cells activated the phospholipase C signaling pathway and modulated the expression of iNOS, COX-2, TNF-α, IL-1β and NF-κB (Zong et al., 2012). A ginseng saponin metabolite, compound K (20-O-D-glucopyranosyl-20 (S)-PD), reduced the number of Iba1-positive activated microglia, and inhibited the expression of TNF-α and IL-1β in the brain after LPS-induced sepsis (Park et al., 2012a). Together, results indicate that ginseng has anti-neuroinflammatory effects by reducing activation of microglia.

Effect on Astrocytes
Ginsenoside Rd increased levels of phosphorylated protein kinase B (PKB/Akt) and phospho-ERK1/2 (p-ERK1/2) and expression of glutamate transporter-1 (GLT-1) in astrocyte Antidepressant Upregulation of BDNF Xu et al. (2010) and Wang et al. (2013c) cultures after oxygen-glucose deprivation. The effect of Rd on GLT-1 expression was abolished by inhibition of PI3K/AKT or ERK1/2 signaling pathways . Moreover, total saponins in the leaves of Panax notoginseng reduced H 2 O 2 induced cell death in primary rat cortical astrocytes, likely through nuclear translocation of Nrf2 and upregulation of antioxidant systems including HO-1 and glutathione S-transferase (Zhou et al., 2014b). In contrast to normal or hypoxic astrocytes, ginsenoside Rg3 showed anti-cancer activities in malignant astrocytes glioblastoma multiforme cells, by induction of apoptosis via the methyl ethyl ketone (MEK) signaling pathway (Choi et al., 2013).

Effect on Oligodendrocytes and Myelination
Administration of American ginseng Panax quinquefolium to mice at a dose of 150 mg/kg body mass resulted in improvement of demyelination scores after induction of experimental autoimmune encephalomyelitis (EAE; Bowie et al., 2012).

Effect on Cerebral Microvessels
Ginsenosides may be effective in inhibiting changes to the cerebral microvasculature after cerebral ischemia. Weinaokang, a preparation consisting of active compounds extracted from Ginkgo biloba, ginseng, and Crocus sativus (saffron) inhibited the translocation of G protein-coupled receptor kinase 2 from the cytosol to the cell membrane, and reduced extracellular-signal-regulated kinases (ERK1/2) phosphorylation and matrix metalloproteinase expression in cerebral microvessels . Ginsenoside Rg1 also modulated increase in aquaporin 4 expression and blood brain barrier (BBB) disruption after cerebral ischemia in rats (Zhou et al., 2014a).

Protective Effects of Ginseng on Neurological Disorders
Neurochemical Aspects of Neurodegenerative, Neurotraumatic, and Neuropsychiatric Disorders Neurodegenerative diseases include AD, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. They are associated with progressive loss of cognitive function and motor disabilities with devastating consequences to the affected individual. Genetic factors, environmental factors and unhealthy lifestyle have been suggested to contribute to the pathogenesis of neurodegenerative processes (Farooqui, 2010;Seidl et al., 2014). Many neurodegenerative diseases are accompanied by oxidative stress and neuroinflammation, and associated with increased generation of lipid mediators, abnormal protein aggregation, slow excitotoxicity, loss of synapses and disintegration of neural networks, leading to failure of neurological functions (Jellinger, 2009;Farooqui, 2010). Neurotraumatic diseases are caused by metabolic or mechanical insult to the brain and spinal cord. They include cerebral ischemia or stroke, spinal cord injury and traumatic brain injury. Neurochemical events in neurotraumatic diseases include the release of glutamate, overstimulation of glutamate receptors, rapid calcium influx, activation of cytosolic phospholipase A 2 (cPLA 2 ), phospholipase C, COX-2 and NOS, induction of oxidative stress and neuroinflammation (Farooqui, 2010). Neuropsychiatric disorders include both neurodevelopmental disorders and behavioral or psychological difficulties such as depression, schizophrenia, and bipolar disorders. Impairment of cognitive processes results in behavioral symptoms such as abnormal thoughts or actions, delusions, and hallucinations. They involve abnormalities in the cerebral cortex, ventral striatum and other components of the limbic system (Gallagher, 2004).

Cellular Changes Induced by Ginseng
AD is the most common form of dementia, a general term for memory loss and other intellectual abilities serious enough to interfere with daily life. It is characterized by the presence of extracellular plaques consisting of aggregated Aβ peptides and neurofibrillary tangles that contains hyperphosphorylated tau protein, as well as cerebral amyloid angiopathy, due to Aβ deposition around the walls of arterioles or capillaries in the brain (Reviewed in Cheng et al., 2005). Aβ peptides are generated from amyloid precursor protein (APP) by βsecretase (BACE1) and γ-secretase activities, and are generally considered to be toxic to neurons, while neurofibrillary tangles interfere with cellular transport processes. Treatment with Panax notoginseng flavonol glycosides inhibited the aggregation of Aβ in a dose-dependent manner and modulated the increase in Ca 2+ and cell death triggered by Aβ in cultured neurons. These flavonol glycosides also reduced memory impairment in a passive avoidance task in scopolamine-treated rats (Choi et al., 2010). Treatment of SH-SY5Y neuroblastoma cells with gintonin, a novel LPA1 receptor activating ligand decreased Aβ1-42 release, and attenuated Aβ1-40 induced toxicity. In addition, long-term oral administration of gintonin attenuated amyloid plaque deposition, and short-and long-term memory impairment in a mouse model of AD (Hwang et al., 2012b). Fermented ginseng also reduced Aβ1-42 levels in HeLa cells stably expressing the Swedish mutant form of APP and decreased memory impairment in mouse models of AD (Kim et al., 2013a).

Effect on Aβ Formation
Ginseng may have beneficial effects in AD by reducing the formation of Aβ oligomers (Figure 2). Chronic supplementation with ginsenosides for 8 months in the drinking water modulated age-related memory impairment in mice (Zhao et al., 2011). Panax notoginseng saponins reduced brain APP mRNA levels and improved learning and memory in senescence-accelerated mice (Zhong et al., 2011), and daily consumption of American ginseng helped preserve cognitive functions in senescenceaccelerated mice . Moreover, aged transgenic AD mice overexpressing APP/Aβ treated with ginsenoside Rg1 showed marked decrease in cerebral Aβ levels, reversal of neuropathological changes and protection of spatial learning abilities and memory . Ginsenosides CK, F1, Rh1 and Rh2 from Panax ginseng were found to have potential BACE1 inhibitory activities (Karpagam et al., 2013); and Panax notoginseng saponins increased non-amyloidogenic processing of APP by increasing α-secretase activity, and decreased amyloidogenic processing by decreasing BACE1 expression . Ginsenoside Rh2 improved learning and memory performance, and reduced the number of senile plaques in brains of Tg2576 mice (Qiu et al., 2014). Results suggest neuroprotective effects of ginsenosides by reducing Aβ levels via stimulation of α-secretase activity and inhibition of β-secretase activity.

Effect on tau Phosphorylation
Ginseng may also have a beneficial effect in AD by reducing tau hyperphosphorylation and neurofibrillary tangle formation. Total ginsenoside extracts from stems and leaves of Panax ginseng enhanced the phosphatase activity of purified calcineurin. This could be useful in AD, since inhibition of calcineurin leads to tau hyperphosphorylation at multiple sites in AD brains (Tu et al., 2009). Treatment with ginsenoside Rd cultured cortical neurons or AD rats (10 mg/kg for 7 days) reduced okadaic acid-induced neurotoxicity and tau hyperphosphorylation by enhancing the activities of protein phosphatase 2A (PP2A; Li et al., 2011a). Ginsenoside Rb1 reversed aluminum exposure-induced decreased PP2A level and tau phosphorylation in the cortex and hippocampus . Ginsenoside Rg1 (20 mg/kg) reversed memory impairments induced by okadaic acid by decreasing levels of phosphorylated tau and suppressing the formation of Aβ in the brains of rats (Song et al., 2013). Results suggest neuroprotective effect of ginsenosides by reducing tau hyperphosphorylation.

Effect on Reactive Oxygen and Nitrogen Species
Ginsenosides are reported to have antioxidant effects. Rg1 is a potential regulator of hypoxia-inducible factor-1α (HIF-1α), and could act via this transcription factor to improve cell survival, angiogenesis and neurogenesis (Tang et al., 2011). Oral treatment with pseudoginsenoside-F11 (PF11), a component of Panax quinquefolium increased the activity of antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and is associated with improved learning and memory in a mouse model of AD (Wang et al., 2013c). Ginsenoside Rg1 treatment modulated Aβ 25-35 induced mitochondrial dysfunction, inhibition of HIF-1α expression and increase in reactive nitrogen species and protein nitrotyrosination in human endothelial cells (Yan et al., 2013).

Effect on Cholinergic and Neurotrophic Signaling
Loss of ACh is found in the brain in AD. Ginsenoside Rg5 found in Panax ginseng modulated cognitive dysfunction and neuroinflammation in the brains of streptozotocin-induced memory impaired rats. Acetylcholinesterase (AChE) activity was reduced, while choline acetyltransferase (ChAT) activity was increased in the cortex of Rg5 treated rats . Ginsenosides Re and Rd also induced the expression of ChAT/VAChT and increased the level of ACh in Neuro-2a cells .

Clinical Studies on Use of Ginseng in AD
Patients in a high-dose Korean Red Ginseng group (9 g/day) showed significant improvement on the Alzheimer's Disease Assessment Scale (ADAS), Clinical Dementia Rating (CDR) after 12 weeks of Korean Red Ginseng therapy, when compared with those in the control group (Heo et al., 2008). In the long-term evaluation of the efficacy of Korean Red Ginseng in patients after 24 weeks, the Korean version of the Mini Mental Status Examination (K-MMSE) score remained without significant decline at the 48 th and 96 th weeks, and similar results were obtained with ADAS. Together, results point to long-term beneficial effects of Korean Red Ginseng in patients with AD (Heo et al., 2011).
Major Depression (Table 2) Preclinical Studies Major depressive disorder or major depression is a mental disorder characterized by a pervasive and persistent low mood that is accompanied by low self-esteem and by a loss of interest or pleasure in normally enjoyable activities.
Ginseng total saponins at doses of 50 and 100 mg/kg for 7 days reduced the immobility time in the forced swim test, which is a mouse model of depression. They also reversed the reduction in the sucrose preference index, decrease in locomotor activity, as well as prolongation of latency of feeding in a novel environment, in a chronic stress model of depression in rats (Dang et al., 2009). Ginseng saponins also had antidepressant effects in the corticosterone-induced mouse model of depression . Mice that received total saponins extracted from Panax notoginseng at doses of 10-1000 mg/kg daily for 1, 7, and 14 days showed decreased immobility time in the forced swim test.
In the chronic mild stress model, chronic total saponin treatment (70 mg/kg) reversed depression-like behavior in rats. Levels of 5-hydroxytryptamine, dopamine, and noradrenaline were increased in the brains of treated rats, which could be the basis of the antidepressant effects (Xiang et al., 2011). Ginsenoside Rg1 upregulated BDNF signaling in the hippocampus, down regulated serum corticosterone levels, and reversed the decrease in dendritic spine density and hippocampal neurogenesis caused by chronic stress (Jiang et al., 2012b;Wang et al., 2013a). A post-metabolism compound and intestinal metabolite of ingested ginsenosides, PD, showed antidepressant effects in the tail suspension and forced swim tests, in the olfactory bulbectomy rat model of depression . Together with findings in the ''Effect on Glutamatergic Neurotransmission'' ''Effect on monoamine Neurotransmission'', ''Effect on Estrogen Signalling'', ''Effect on Neural Stem Cells and Neuroregeneration'' sections, they suggest that ginseng may have beneficial effects in major depression via multiple actions: (a) increasing AMPA receptor signaling, in a manner similar to AMPA receptor agonists or AMPAkines; (b) increasing the level of monoamines; (c) reducing stress related corticosteroid levels; and (d) increasing estrogen receptor signaling and BDNF, which are related to decreased neuronal death and improved neurogenesis.

Clinical Studies
A randomized, multicenter, double-blind parallel group study of 193 post-menopausal women treated with ginseng and 191 treated with placebo reported significant difference in depression scores in favor of ginseng compared with placebo (Wiklund et al., 1999). Clinical studies on 35 female outpatients aging from 18 to 65 years who were remitted from major depression with residual symptoms and given Korean Red Ginseng at a dose of 3 g/day reported significant decrease in depressive symptoms on the Depression Residual Symptom Scale (DRSS) and Montgomery-Åsberg Depression Rating over an 8-week period (Jeong et al., 2014). Another study of possible casual relation between hormones, energy sources, and minerals indicated that depression was related to serum lipids including cholesterol, and that fermented red ginseng had beneficial effects on depression by modulating this relation (Lee and Ji, 2014).

Stroke
Stroke or cerebrovascular event is associated with a sudden loss of brain function, due to disturbance in blood supply to the brain. Ginsenoside Rd (10-50 mg/kg) significantly decreased infarct volume and was associated with a better neurological outcome after transient middle cerebral artery occlusion (MCAO) in rats (Ye et al., 2011a). Interestingly, ginsenoside Rd demonstrated neuroprotection even when administered 4 h after recirculation of transient MCAO or after onset of ischemic stroke induced by permanent MCAO (Ye et al., 2011b). mRNA and protein expression levels of non-selective cation channels such as transient receptor potential melastatin (TRPM) and acid sensing ion channels (ASIC) were significantly increased 24 h following MCAO, and these effects were attenuated by treatment with 10 mg/kg ginsenoside Rd . High dose ginsenoside Rb1 treatment reduced neurological deficits, brain edema, BBB disruption and the number of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positive cells after hemorrhagic stroke . Significant reduction in basilar artery vasospasm and lumen thickness was also observed after Rb treatment (Li et al., 2011b). Results suggest that neuroprotective effect of ginseng in stroke via inhibition of ion channels or modulation of vasospasm. It is, however, noted that ginseng may have a drug interaction with the anticoagulant warfarin that could lead to increased risk of bleeding, and this should be taken into consideration in patients who are on warfarin.

Parkinson's Disease
Parkinson disease is a neurodegenerative disorder affecting mainly the motor system, as a result of death of dopaminergic neurons in the substantia nigra. Ginsenosides investigated as neuroprotective agents for Parkinson's disease are Rb1, Rg1, Rd and Re. These compounds exert neuroprotective effects through inhibition of oxidative stress and neuroinflammation, decrease in toxin-induced apoptosis and nigral iron levels, and regulation of N-methyl-D-aspartate receptor channel activity (reviewed by González-Burgos et al., 2014). Oral administration of the ginseng extract, G115 modulated tyrosine hydroxylase-positive cell loss in the substantia nigra, and reduced locomotor dysfunction in the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP + ) models of Parkinson's disease (Van Kampen et al., 2003). G115 also reduced dopaminergic cell loss, microgliosis, and accumulation of αsynuclein aggregates after chronic exposure to the dietary phytosterol glucoside, β-sitosterol β-d-glucoside or ''BSSG model'' of chronic Parkinson's disease (Van Kampen et al., 2014). MPTP administration resulted in behavioral impairment, dopaminergic neuronal death, and increased Cdk5 and p25 expression but decreased p35 expression in the nigrostriatal system of mice, and these effects were modulated by Korean Red Ginseng (Jun et al., 2015). Panaxatriol saponins, the main constituents extracted from Panax notoginseng provided neuroprotection against loss of dopaminergic neurons and behavioral impairment after MPTP treatment (Luo et al., 2011). Together, results suggest that ginseng may have neuroprotective effects in Parkinson's disease.

Multiple Sclerosis
Ginsenoside Rd has been studied in an animal model of multiple sclerosis, EAE. Intraperitoneally administered ginsenoside Rd at 40 mg/kg/day reduced the permeability of the BBB, regulated the secretion of interferon-gamma and IL-4, and decreased the severity of EAE in mice .

Conclusions and Future Directions
Many ginsenosides have been isolated and characterized. The molecular mechanisms associated with ginsenosides involve scavenging free radicals, inhibition of inflammation, and prevention of excitotoxicity. Animal and cell culture studies have indicated that ginsenosides have different activities in both physiological and pathologic conditions. The structure activity relationship of ginsenosides has not been fully elucidated. However, it is becoming increasingly evident that ginsenosides produce neuroprotective effects by reducing free radical production and enhancing brain function. Studies involving each ginsenoside should include mechanisms of action, specificity, structure and function relationship, detailed pharmacokinetics and toxicity studies, and therapeutic studies in animal models and humans. Further studies are necessary to examine the effects of ginseng on metabotropic glutamate receptors and transporters, and the Keap1/Nrf2 adaptive cellular pathway. The neurological disorders for which there is most evidence from preclinical and a small number of clinical studies to benefit from ginseng are AD and major depression, and clinical trials are necessary to confirm the efficacy of ginseng and ginsenosides in the prevention and treatment of these, and possibly other neurological conditions.