INTRODUCTION

Ginseng is any one of tire perennial plants, which are included in genus Panax and family Aralliaceae. Among eleven different species of ginseng commonly called as ginseng, three species of ginseng, i.e., Panax ginseng (commonly called as ginseng or Korean ginseng), P. quinquefolius (commonly called as American ginseng) and P. notoginseng (commonly called as Chinese no- toginseng or Sanchi) are the three most commonly used ginseng herbs at present [1]. Ginsenoside is believed to be the active compounds of ginseng herbs and widely used for the pharmacological examination of effects of ginseng ranging from the role as a traditional nourishing stimulant to anticancer reagent. Ginsenosides are classified into three categories based on their structural differences. The panaxadiol group includes Rbl, Rb2, Rb3, Re, Rd, Rg3, Rh2, and Rsl and panaxatriol group includes Re, Rf, Rgl, Rg2, and Rhl. Ro is classified as oleanolic acid group [2]. The investigation on die beneficial and sometimes harmful effects of ginseng in various pathological conditions keeps growing to previously unexpected extents and for example, it has been suggested that the combination of red ginseng with highly active anti-viral therapy may improve die clinical outcome of human immunodeficiency virus type 1-infected patients[3]. Growing number of publications are also attributed to tire effects of ginseng and ginsenosides on central nervous system (CNS) disorders. These disorders in CNS are not only confined to the traditional target diseases such as stress and ischemia but also expand into more recently acknowledged neurological and psychiatric disorders including Alzheimer's disease (AD) and attention deficit hyperactivity disorder (ADHD).

Recently, several reviews are published about the role of ginseng or ginsenosides on neurological functions or possible target proteins including different types of ion channels. For example, ginseng or ginsenoside affects nemotransmission of acetyl choline and gamma- aminobutyric acid (GABA) by mechanism involving the regulation of the expression of synthetic enzyme, neurotransmitter release as well as the signaling pathways involved in tire particular nemotransmitter systems. Ginseng and ginsenosides improve learning and memory as determined by behavioral analysis by mechanism involving alteration of synaptic plasticity and increase in neurogenesis, thereby affecting neuronal density in hippocampus. Ginsenosides affect voltage and ligandgated ion channels including K⁺, Na⁺, and Ca²⁺ channels as well as N-methyl-D-aspartate (NMDA)-, nicotine-, and serotonin-gated ion channels (for a review, see [4]). Recently, growing expectations about tire role of ginseng and ginsenosides in the regulation of stem cell proliferation and differentiation as well as the more comprehensive understanding of classical neurotrophic role of ginseng may confer enthusiasm in investigating the role of ginseng in CNS disorders such as stroke, AD, Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). At tire moment, the most extensively studied effects of ginseng and related compounds me their effects on stroke and AD. In addition to tire descriptive studies depicting the efficacy of ginseng and ginsenosides in annual models of disease as well as their nemoprotective role in terms of anti-oxidant, antiinflammatory, and anti-apoptotic activity, more specific cellular and moleculm mechanisms me now revealed, which dictates close overviews on the role of ginseng and ginsenosides in CNS disorders. However, extensive reviews on the role of ginseng on fire neurological and psychiatric diseases me scarce not to mention fire mechanism of ginseng mediating fire regulatory role in those diseases. In this review, we will summarize the works conducted regarding the effects of ginseng (fire three conventional ginseng species excluding Siberian or Indian ginseng) and ginsenosides in various CNS diseases using cell culture and annual models as well as human clinical studies in an hope to systematically understand the effects and mechanism of ginseng in CNS disorders.

ALZHEIMER'S DISEASE

With the in-depth understanding of moleculm and cellular pathophysiology and neurobiology of AD during last couple of decades, many plausible targets to treat AD have been suggested which include but not restricted to followings: 1) increase in fire uptake of choline in central nervous system, 2) release of acetylcholine from hippocampus, 3) increased activity or expression of choline acetyltransferase, 4) protection against the Aß or tan protein-induced neurotoxic effects by several mechanisms including inhibition of neuroinflammation, increased production of neurotrophic factor, and regulation of apoptotic processes, 5) repair of Aß-dmnaged neuronal networks by increased nemogenesis and synaptic plasticity, and 6) reducing fire level of Aß by decreased production or increased elimination. Actually, many of these pathophyiological targets and tire effects of ginseng and ginsenosides on them me subjected to tire intense investigation and covered by several recent reviews [5,6].

Regulation of neurite outgrowth and synaptic plasticity by ginseng

Based on tire fact that protopanaxadiol-type saponins were active constituents mediating neurite outgrowth in human nemoblastoma SK-N-SH [7], it has been demonstrated that ginsenoside Rbl and its intestinal metabolic compounds Ml recovered the unpaired spatial memory and expression levels of phosphorylated NF-H and syn- aptophysin in an annual model of AD induced by intra- cerebroventriculm injection of Aß(25-35) [8]. In cultmed cortical neurons, Ml increased axonal outgrowth even after substantial progress of neurite degeneration, suggesting a nemo-regenerative potential of tins compound. Similar neurite outgrowth potential of crude ginseng saponin was also reported in cultmed rat cerebral cortical neurons [9]. In addition, ginsenoside Rbl potentiated the nerve growth factor (NGF)-mediated neurite outgrowth of cultured chick embryonic dorsal root ganglia [10]. Interestingly, the memory impairment induced by intracerebroventriculm injection of Aß(25-35) as well as synaptic marker protein expression is only marginally increased by donepezil, if any, consistent with its primary mechanism of action, i.e., tire inhibition of acetylcholine esterase [8]. In addition, tire memory improving effects of Rbl or Ml were maintained even after the discontinuation of Rbl and Ml administration suggesting they induced a long lasting and probably structural reorganization of tire damaged brain circuits, which is ideal for tire treatment of neurodegenerative conditions including AD although further verification in oilier annual models of AD is needed in tire future. Neurite outgrowth is one of the foremost phenotypical changes in network reorganization, which might be one of the cellular and molecular changes happening during synaptic plasticity, which suggests that the observed neurite extension effects of ginseng may underlie the memory enhancing effects of ginseng in both normal and pathological conditions, although clinical studies produced both positive and negative results (see below).

In tire dentate gyrus of anesthetized rats, i.c.v. injection of ginsenoside Rbl (10, 100 nM) inhibited the induction phase of long term potentiation (LTP) and accelerated tire maintenance phase of LTP induced by high frequency stimulation in a dose-dependent manner [11]. In addition, treatment with nonsaponin fraction of ginseng significantly ameliorated deficits in place-navigation learning in tire aged rats in the place learning task along with significant augmentation in the increase in population spike amplitudes in the CA3 subfield after LTP induction in vilro [12]. Similarly, ginsenoside Rgl inhibited morphine induced spatial memory deficit which was determined by Morris water maze test and restored LTP impaired by morphine in both freely moving and anaesthetized rats, which was examined by electrophysiological recording after implantation of electrodes in vivo [13]. The electrophysiological recording in vitro also demonstrated that Rgl restored the LTP in slices from the rats treated with morphine without affecting LTP in the slices from normal rats. The restoration LTP is inhibited by NMDA receptor antagonist MK801 suggesting the involvement of NMDA receptor activation in the Rgl-induced recovery of LTP [13]. Using acutely isolated rat hippocampal CA3 pyramidal neurons with a conventional whole-cell patchclamp teclmique, it has been suggested that Compound K, a metabolite of Rbl, enhances spontaneous GABA release by increasing intraterminal Ca²⁺ concentration via Ca²⁺ release from pre-synaptic Ca²⁺ stores, although how tliese findings are related to the regulation of hippocampal excitability and learning and memory processes remains to be determined [14].

Morphologically, Rgl increased chronic mild stress induced decrease in dendritic spine number and hippocampal neurogenesis [15], which also suggests the modulation of processes involved in synaptic plasticity by ginsenosides.

Ginsenoside Rgl and Rblincreased proliferation and differentiation of neural progenitor cells in dentate gyrus of hippocampus of normal adult mice and global ischemia model in gerbils. In addition, Rgl increased expression of brain derived neurotrophic factor, Bcl-2 and antioxidant enzyme and increased the number of synapses and mossy fiber sprouting in CA3 regions of hippocampus suggesting the role of Rgl in the modulation of synaptic plasticity and possibly to the increased cognitive function in AD [16]. Similarly, when the mixture of brain-derived neurotrofic factors (BDNF) and ginsenosides Rgl and Rbl was treated to human neural stem cell during the differentiation procedure, it promoted cell survival and enhanced neurite outgrowth and the expression of synaptic marker proteins, which was evidenced by tune lapse microscopy, immunostaining, and Western blot [17].

Neuroprotection

Ginsenosides have direct neuroprotective effects against glutamate or Aß stimulation. In cultured PC 12 cells, glutamate decreased cell viability and increased intracellular calcium concentration and lipid peroxidation, which is evidenced by the excessive production of malondialdehyde and nitric oxide (NO) [18], all of which are prevented by ginsenosides. Whether die neuroprotective effects against glutamate has been related with die reported antagonistic activity of many ginsenoside such as Rg3 and R112 against NMDA receptors remains to be determined (for a review, see [19]). The antioxidant effects of ginseng has also been implicated in a study showing die neuroprotective effects of ginseng extracts in human neuroblastoma SY-5Y cells [20]. Treatment of SY-5Y cells with cyclosporine A inhibited calcineurin activity, which results in hyperphosphorylation of tan protein. Pretreatment of ginseng extracts effectively enhanced calcineurin activity, which ameliorates tan phosphorylation providing possible neuroprotective activity [20]. Treatment of ginsenoside Rg2 effectively and significantly attenuated the glutamate-induced toxicity and changes in above mentioned factors. In addition, ginsenoside Rg2 decreased the level of glutamate-induced increased protein expression such as calpain II, caspase-3, and Aß(l- 40) in PC12 cells. In addition, treatment of water extracts of American ginseng significantly attenuated the cellular apoptosis of SH5-SY cells induced by Aß(25-35), which was determined by staining with Hoechst 33258 [21]. Treatment of 50 pM Aß(25-35) for 48 h also produced cell toxicity in PC 12 cells and treatment of ginsenoside Rgl inhibited ß-secretase activity in vitro and protected PC12 cells from cell death along with inhibition of NO release, lipid peroxidation, reactive oxygen species (ROS) production, and elevation of intracellular calcium concentration. Similar neuroprotective action of Rbl in PC 12 cells treated with Aß(25-35) has been reported by a separate group of researchers [22], which is also related to the increased ratio of anti-apoptotic/apoptotic protein Bcl-2/Bax . Not only neuron but also astrocytes is protected by ginsenosides [23]. In cultured type I rat brain astrocytes (RBA), ginsenoside Rh2 inhibited Aß-induced inhibition of RBA growth, which is mediated by induction of pituitary adenylate cyclase-activating polypeptide (PACAP) and PACAP receptor PAC1 [24].

Anti-inflammatory effects

Due to the chronic neuroinflammatory responses occurring in AD and other neurodegenerative diseases, researchers investigated the anti-inflammatory effects of ginseng in brain. Several ginsenosides such as Rd, Rgl, Re, Rg3, Rlr2, and Rbl has been suggested to modulate neuroinflammatory responses in cultured microglia or stimulated brain [25-28]. In a mouse model of nemoinflammation induced by intracerebroventricular injection of Aß(l-42), ginsenoside Rbl reduced neuroinflammation as detected by cyclooxygenase-2 and inducible nitric oxide synthase (iNOS) expression. Remarkably, 4 wk treatment of Rbl at Are stage of eminent nemoinflammation by Aß(l-42) (2 wk after treatment), still effectively reduced inflammation along with the improvement of cognitive function as determined by Morris water maze test [29]. The anti-inflammatory effects of Rbl seem to involve NF-кВ pathway as evidenced by the modulation of Are level of IkB. More recently, strong anti-inflammatory effects of ginseng saponin metabolite, compound К [20-0-d-glucopyranosyl-20(S)-protopanaxadiol], have also been reported in two different in vivo models of neuroinflammation, i.e., sepsis and ischemia [30]. Not only the production of inflammatory mediators but also the recruitment of inflammatory cells into stimulated brain might be modulated by ginseng saponins, which might be regulated by Are modulation of expression of intercellular adhesion molecule-1 [31]. In rat model of inflammation, Rgl stimulated the production of NO and pro- inflammatory cytokines (interleukin [Ik]-Iß, IL-6, and tumor necrosis factor [TNF]-a), whereas Rbl exerted a significant inhibitory effect on the same proinflammatory mediators [32], which might suggest flrat Are regulation of inflammatory response by ginseng and ginsenosides is critically dependent on Are relative concentration and kinetics of treatment of individual components of ginseng.

The anti-inflammatory effects might be related to Are antioxidant property of ginseng. In a 7 mo chronic treatment experiment, it has been suggested Aral 100 or 200 mg/kg/d treatment of ginsenosides through drinking water unproved memory loss in senescence accelerated mice (SAMP8) along with increase antioxidant levels in serum [33]. The levels of plasticity related proteins including p-CAMKII, pCREB, and BDNF are increased in Are hippocampus of ginsenoside-treated mice aidrough it should be determined whether Are increased antioxidant activity has causal relationship with Are increased expression of plasticity related proteins as well as die improvement of memory in senescence accelerated mice.

Although much of the anti-inflammatory action of ginsenosides is dependent on the modulation of NF- кВ activity [26,29], anti-inflammatory activity of some of the ginsenosides such as R112 has been reported to be dependent on signaling pathways such as activator protein 1 and protein kinase A (PKA) [28,34] but not by NF-кВ [28]. In addition, regulation of inflammatory mediators such as cytokines and NO seems to be regulated by different signaling pathways, for example, mitogen- activated protein kinase and NF-кВ pathways in different cell lines [27,35]. These results suggest Aral Are regulation of cellular inflammatory response by ginseng and ginseng saponins is regulated by cell type and particular ginsenoside-specific manners, which necessitates fine dissection of die cellular signaling pathways involved.

Effects on overall Aß production

Using cell based assay system, which is a Chinese hamster ovary (CHO) cell line stably transfected with human APP 695wt, to detect the accumulation of Aß, Chen et al. demonstrated that certain ginsenosides lowered Aß concentration secreted into the culture supernatants in a dose-dependent manner [36]. The strongest effects were observed with ginsenoside Rg3 having an approximate IC₅₀ of under 25 pM against Aß42. Among the various ginsenosides tested to be positive in the reduction of Aß, ginsenoside Rgl, Rg3, and RE resulted in significant reductions in die amount of Aß detected in die brains of an animal model of AD, which is Swedish mutant model of familial AD (Tg2576 line), after single oral administration, albeit small in extent compared with a known y-secretase inhibitor LY411575. Unfortunately, the authors can not provide what would be die actual molecular mechanism of the observed Aß-lowering effects of ginseng and ginsenosides.

As well as protecting cultured PC 12 cells from Aß- induced toxicity, a flavonol ganglioside isolated from roots of P notoginseng (RNFG) inhibited the aggregation of Aß in an in vitro assay in a dose-dependent manner [37]. It has been suggested that specific sugar moiety is required for the neuroprotective activity because the flavonol backbone was devoid of neuroprotective activity. No structural similarities to the currently available aggregation inhibitors have been suggested, which gives another level of complexity for tire understanding of tire possible protective mechanism of RNFG.

Effects on ß-secretase activity

The level of ß-site APP-cleaving enzyme 1 (BACE1) is responsible for tire elevation of Aß peptides in tire brain of AD patients and the expression of BACE1 can be regulated by peroxisome proliferator-activated receptor-y (PPARy) by binding to its promoter region. In N2a- APP695 cells, Rgl decreased tire levels of secreted Aß(l- 40) and Aß(l-42) as well as ß-CTFs, a cleaved C-tenni- nal fragment of APP by BACE1. The expression levels of both BACE1 mRNA and protein were decreased in cells treated with Rgl. Rgl induced activation of PPARy was demonstrated by tire nuclear translocation of PPARy and Rgl is suggested to have PPARy agonist activity like rosiglitazone [38]. In addition, Rbl inhibited BACE1 activity in vitro and rescue PC 12 cells from Aß(25-35)- induced toxicity as determined by LDH release, NO release, ROS production, lipid peroxidation, intracellular calcium elevation, and apoptosis [23]. Interestingly, Rgl treatment also inhibited activity of y-secretase in transgenic AD mice over-expressing APP/Aß (Tg mAPP) as well as В103-APP cells [39]. In addition, Rgl enhanced activation of PKA/CREB pathway suggesting the multitarget action of Rgl against AD. In a more recent study using human platelets, it has been demonstrated that Rgl promoted a-secretase cleavage of APP via estrogen receptor (ER) extranuclear signaling pathway suggesting Rgl behaves as phytoestrogen, which might have important implications considering estrogen withdrawal as a risk factor for AD [40,41].

Aß clearance

Although most of tire studies dealing with tire role of ginseng on glial cells have focused on tire anti-inflammatory activity, Joo and Lee [42] focused on tire phagocytic role of microglia and found that Rg3, a by-product of red ginseng, enhances tire microglial Aß uptake, internalization, and digestion. The increased clearance of Aß by microglia by Rg3 seems to be mediated by tire increased expression of macrophage scavenger receptor type A, which may provide an additional therapeutic target of AD by ginseng.

In SK-N-SH cells transfected with SweAPP, ginsen- oside Rg3 reduced tire level of AMO and Aß42, probably due to the increase in gene expression of neprilysin [43], which has been suggested to be tire rate-limiting enzyme in the Aß degradation in tire brain [44]. Whether other proteases such as martix metalloproteinase, angiotensin converting enzyme, and tissue plasminogen activator, which has been implicated in Aß clearance, were also affected by ginsenoside remains to be investigated. At present, the data investigating whether ginseng and gin- senosides may affects Aß-clearance through transport across tire blood-brain barrier by modulating tire regulator molecules such as receptor for advanced glycation end products and lipoprotein receptor-related protein are not available, which may need further investigation.

Effects on tau phosphorylation

Increased phosphorylation of tau protein can results in self aggregation, which is involved in tire pathogenesis of AD. After exposure to Aß(25-35), the levels of tau protein phosphorylation in hippocampal neurons at the sites of Thr205, Ser396, and Ser404 were increased as well as the level of p25. Pretreatment with ginsenoside Rbl, at least in part, reversed these changes [45]. Similarly, Rbl reduced tire phosphorylation of tau induced by Aß(l-42), which is regulated by a cascade of signaling pathways comprised with PI3K-Akt-GSK3ß [46]. Total ginsenosides extracts also partially inhibited cyclosporine-induced increase in tau phosphorylation in SY-5Y cells [20]. Using tire slice culture of brain obtained from 5 weeks old Wistar rats, it has also been suggested that ginsenoside Rglsignificantly inhibited okadaic acid- induced phophorylation of tau [47]. The same group of researchers also reported that pretreatment of ginsenoside Rd inhibited okadaic acid-induced tau phosphorylation in cultured neuron as well as in brain of male Sprague Dawley rats bilaterally micro-infused with okadaic acid into tire cerebral ventricle, which might be mediated by tire activation of PP-2A [48].

Effects on cholinergic system

Loss of cholinergic neurons in cerebral cortex and hippocampus is closely associated with AD. Panaxynol, one of the compounds isolated from tire lipophilic fraction of P nologinseng. concentration-dependently up-regulated tire number of M, receptor in CHO cells transfected with human m, subtype gene [49]. Panaxynol caused a significant stimulation of cAMP accumulation and tire increase in M| receptor number was blocked by a PKA inhibitor RP-cAMP. In PC 12 cells it has been suggested that Rbl and Rgl increased neurotransmitter release by modulating the phosphorylation of synapsin in PKA dependent and independent manner, respectively [50].

Ginsenosides also modulate acetylcholine release and the level of choline acetyl transferase (ClrAT). Rbl and Rgl can modulate acetylcholine release and reuptake determined with rat hippocampal slices [51] and the number of choline uptake sites determined by [³H] hemicholinium-3 binding experiments, especially in tire hippocampus and to a lesser extent in cortex when it administered at least three days [52]. They also increased choline acetyltransferase levels in rodent brains which was determined by in situ hybridization in basal forebrain [53] as well as the level of acetylcholine in the brain [54], although other reports suggested Rgl did not show any effects on ClrAT activity [51]. Interestingly, Aß(25-35)- induced suppression of the К-evoked [³H]-acetylcholine release from the rat hippocampal slices was effectively reversed by Rbl but not by Rgl in cholinergic synapse [55]. Taken together, these data suggest that Rbl is the principal components regulating acetylcholine dynamics in brain.

Human clinical trials

In healthy volunteers, acute or chronic treatment of ginseng extracts produced both positive and negative results in cognitive functional tests (for a review, see [6,56]), preventing a generalized conclusion on the psychoactive properties of ginseng. Nevertheless, a few clinical trials have been conducted regarding the efficacy of ginseng or ginsenosides on AD. In 12 wk, open-label clinical trial, Heo et al. [57] treated AD patients with low or high concentration of Korean red ginseng extracts for 12 wk. The congnitive function of patients was assessed using the Alzheimer's Disease Assessment Scale, Korean version of the Mini-Mental Status Examination and Clinical Dementia Rating scale at the end of the 12 wk study period and high concentration of Korean red ginseng extracts treated group showed either a tendency or statistically significant improvement as compared with control group. The same group of researchers elaborated further with more patients and reported that 12 wk treatment of encapsulated P. ginseng powder (4.5 g/d) increased cognitive performance and tire discontinuation of ginseng gradually decreased tire congnitive performance back to control level [58]. Although the number of subjects is relatively small, tire results suggest tire potential of therapeutic significance of ginseng compounds in AD.

Currently available drugs and many of the experimental reagents against AD are generally symptomatic treatment and disease modifying medication is eagerly anticipated. In general, aggregation blockers and reagents effective for the Aß dynamics in brain as well as drugs targeting Aß production and clearance are expected to possess disease modifying potentials in some extents. Some of ginseng components are suitable for this criteria but the investigation in this field still in its infancy albeit numerous reports suggesting the beneficial effects on cognition and neuroprotection. In addition, some ginsenosides seemingly have multiple site of action, which make them ideal candidates for tire next generation AD therapeutics. Future will tell, with unceasing efforts from existing researchers as well as new investigators in this field, whether the multi-faceted effects of ginsenosides on AD will be proven successful in the long run (Fig. 1).

PARKINSON'S DISEASE

Mainly due to the strong antioxidant effects and neu- roprotective effects of ginseng and related compounds, many researchers investigated the effect of ginseng on neuroprotection in culture or annual models of PD. Some of the early studies using cultured neuronal cell lines such as PC 12 suggested ginseng and ginsenosides provided anti-oxidant effects thereby protecting the cells from oxidant injury induced by dopamine [59], hydrogen peroxide [60] or l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP). Treatment of dopamine in a concentration of 0.15 to 0.45 mM for 24 h induced apoptotic cell death of PC 12 cells and tire pretreatment of ginsenoside Rgl protected apoptotic cell death. Ginsenoside Rgl reduced tire generation of dopamine-induced reactive oxygen species, iNOS induction and NO release, and the release of mitochondrial cytochrome c into tire cytosol, and subsequently inhibited tire activation of caspase-3 [59].

Some researchers also used primary dopaminergic cultures obtained from embryonic mouse mesencephalon and showed partial neuroprotective effects of ginsenoside Rgl and Rbl as evidenced by inhibition of decreased neurite length or number induced by a toxic metabolite of MPTP, l-methyl-4-phenylpyridinium (MPP) treatment [61]. Somewhat bigger neuroprotective effects of ginsenoside Rgl has also been demonstrated in reducing MPTP-induced substantia nigra neuronal loss in С57/ BL6 mice, possibly via increased level of glutathione and superoxide dismutase activity [62]. Rgl also inhibited MPTP-induced overload of iron and modulation of the expression of iron transporters such as divalent metal transporter and ferroportinl in tire substantia nigra [63]. The activity and level of iron transporters is regulated by the IRE dependent transcriptional control as well as inhibition of iron regulator protein [64], which seems to be mediated by the regulation of ROS-NF-кВ pathway [65]. Consistent with these results, it has been reported that panaxatriol saponins extracted from P. notoginseng induced thioredoxin-1 and protect PC 12 cells or Kunming mice from MPP⁺ or MPTP-induced toxicity, which was determined by MTT assay, LDH release and behavioral assays such as locomotor activity test and fraction test [66,67]. Similar neuroprotective effects was also reported using rotenone model of neurotoxicity using primary dopaminergic neuron [68], which involves inhibition of mitochondrial apoptotic pathway, possibly via the modulation of glucocorticoid receptor and PI3K/Akt pathway. Involvement of Akt pathway in the modulation of neuronal survival by Rgl against 6-hydroxydopamine (6-OHDA)-induced cell death was also reported in a study using MES23.5 cells [69]. Rgl modulated 6-OH- DA-induced loss of mitochondrial membrane potential and apoptotic protein expression in human neuroblastoma SK-N-SH cells via modulation of estrogen receptor as evidenced by increased estrogen responsive element (ERE)-luciferase activity by Rgl treatment in cells transfected with the ERE-luciferase reporter construct [70], suggesting the phytoestrogen-like effect of Rgl. Pharmacological study also suggested the involvement and possible crosstalk between ER and insulin-like growth factor receptor pathways in Rgl-mediated nemoprotection in 6-OHDA treated SK-N-SH cells [70]. Similar findings were confirmed in vivo using 6-OHDA lesion model by unilaterally injecting 6-OHDA into the medial forebrain bundle of ovariectomized Wistar rats [71].

In spite of seemingly strong evidence that ginsenoside Rgl shows strong antioxidant effects in these PD models, a recent study suggested that individual ginsenosides such as Rbl, Rgl, and Re, have stimulus-dependent effects on tire regulation of CD40 expression and virtually have no effects on NO production in N9 microglia, while P. notoginseng extracts showed strong inhibitory effects on CD40 expression, cytokine expression, NO production, and chemokine receptor expression regardless of the identity of immunological stimulus either by TRL ligands such as LPS and poly I:C, or by cytokine such as IFN-y, suggesting it may be tire combination of different ginsenosides but not a single ginsenosides that provide strong anti-inflammatory efleets against TRL ligand or cytokine stimulation of microglia [72]. In this study, it is also suggested that tire anti-inflammatory effects of P notoginseng extracts is not mediated by glucocorticoid receptor given that a pharmacological inhibitor of glucocorticoid receptor, RU-486, failed to reverse the antiinflammatory effects of P notoginseng extracts [72].

Animals received oral administration of P ginseng extract Gil5 10 d prior to and/or following exposure (total 20 d) to Are MPTP in mice, or MPP⁺ in rats showed significantly fewer tyrosine hydroxylase (TH)-positive cell loss along with the reduction of apomorphine-induced contralateral rotations [73]. The lowest effective dose was 75 mg/kg/d and 200 or 500 mg/kg/d almost completely abolished tire dopaminergic neural cell death in substantia nigra. Interestingly, tire administration of Gil 5 can be delayed until 2 d after MPP⁺ administration, which still showed modest protective effects [73]. The delayed protection conferred by Gil5 is not likely due to blockade of neurotoxin uptake. Rather, the authors suggested the neurotrophic effects of ginseng such as tire induction of NGF [53,74] may underlie tire observed neuroprotective effects.

Although the majority of PD etiology is believed to be sporadic, genetic defects also contribute in the pathogenesis of PD and tire annual models based on the genetic modulation of key genes involved in PD pathogenesis such as LRRK2 and a-synuclein found in autosomal- dominant PD as well as parkin, DJ-1, and PINK1 responsible for autosomal-recessive PD, provided insight into tire molecular mechanisms of PD and new opportunities to design and develop new therapeutic agents for this devastating disease [75]. The investigation of the therapeutic effects and molecular and cellular mechanisms of ginseng on these animal models will provide further evidence of the efficacy of ginseng in PD.

OTHER NEURODEGENERATIVE DISORDERS

Huntington's disease

HD is an autosomal dominant, inherited disorder characterized by progressive nemodegeneration resulting in motor abnormalities, including chorea and psychiatric disturbance with gradual dementia. The characteristic pathological feature of HD is a polyglutamine expansion in huntingtin and tire loss of medium spiny neurons in the striatum, although tire exact mechanism of pathophysiology needs to be clarified. HD is an incurable disease, which needs the development of effective therapeutic reagents. Using in vitro culture of medium spiny neuron of striatum obtained from fire YAC128HD mouse model, Wu et al. [76] tested ginsenosides such as Rbl, Re, Rd, Re, Rg3, Rg5, Rhl as well as Re and Rd mixture, Rkl and Rg5 mixture, and Rh4 and Rk3 mixture. The authors reported that nanomolar concentrations of ginsenoside Rbl and Rc and micromolar concentration of Rg5 effectively protected TAG 128 medium spiny neurons from glutamate-induced toxicity, which was mediated by regulation of intracellular calcium concentration. Other ginsenosides were without effects. Unfortunately, follow up studies examining the effects of those ginsenosides on other models of HD as well as the effects in animal models are not available yet, which will substantiate fire efforts to develop HD medication based on ginseng and related products.

Amyotrophic lateral sclerosis

ALS is a degenerating motor neuron disease characterized by muscle weakness and paralysis in affected individuals. The correct pathophysiology is currently unknown with which most of fire cases are sporadic with a few identified genetic risk factors. Albeit these lack of pathophiological information of the disease, it has been reported that mutations in fire gene that produces tire Си/ Zn superoxide dismutase (SOD1) enzyme were highly associated with familial cases of ALS and experimental animals with SOD1 mutation showed ALS-like phenotypes. Using B6SJL-TgN(SODl-G93A)lGm transgenic mice, the efficacy of ginseng extracts, which was given in drinking water, was tested on onset of motor symptoms as well as survival rates of tire mice [77]. Although there was no dose-response relationship (40 and 80 mg/ kg), ginseng extracts provided weak but significant delay in Are onset of ALS symptoms when it administered from age 30 d and onwards [77]. Whether the protective effects are related to the anti-oxidant effects (and thereby nemoprotection) or neurotrophic effects of ginseng remains to be determined.

CEREBRAL ISCHEMIA

Protective effects of ginseng and ginsenosides

There are dozens of studies indicating ginseng extracts or ginsenosides obtained from various ginseng species provide functional as well as neuroanatomical protection from ischemic stroke (Table 1). Various experimental models has been used including middle cerebral artery occlusion (focal transient ischemia), four-vessel or bilateral common carotid artery occlusion (global ischemia)

Table 1. Effects of ginseng or ginsenosides on experimental ischemic stroke

The effects of Panax ginseng or P notoginseng either in extracts or total saponin forms as well as ginsenosides obtained from them on animal or culture model of ischemia were summarized. Experimental scheme was categorized as focal and global ischemia as well as in vitro culture model. The treatment protocols of ginseng and experimental end points in each experiment were also summarized with relevant references.

MCAO, middle cerebral artery occlusion; SD, Sprague-Dawley rats; PNS, P notoginseng saponins; KRG, Korean red ginseng; TNF, tumor necrosis factor; IL, interleukin; Ch AT, choline acetyl transferase; nNOS; neuronal nitric oxide synthase; PGE, P ginseng extracts; SHR-SP, spontaneously hypertensive rats-stroke prone; PNE, P notoginseng extracts; PGS, P ginseng saponins; 4-VO, four-vessel occlusion; BCCaO, bilateral (common) carotid artery occlusion; OGD, oxygen glucose deprivation in rats and mice [78-80], transient forebrain ischemia by short-term transient bilateral common carotid artery ligation in gerbil [81], and hypoxic-ischemic brain injury model in neonatal rats [82], in addition to tire in vitro oxygen glucose deprivation or glutamate-induced toxicity model [83,84]. For example, ginsenosides Rd [85], Rg3 [86], Rb and Ro [80,87] as well as ginsenoside Rh2 reduced ischemic brain injury in rats or mice [88]. In 5-min transient forebrain ischemia model of gerbil, ginsenoside Rbl significantly prolonged the response latency of ischemic gerbils in passive avoidance test and rescued a significant number of ischemic CAI pyramidal neurons, whereas ginisenosides Rgl and Ro were ineffective.

Anti-ischemic mechanism of ginseng and ginsenosides

Although part of the reason that much of the pharmacological intervention study against neurological diseases using ginseng and/or ginsenosides are focused on cerebral ischemia are related to tire ginseng's potent antiinflammatory and anti-oxidative activity in microglia and astrocytes as well as in insulted brain [30,80,101,112], many other studies suggest that other mechanisms are also involved in the potent neuroprotective or neurorestorative actions such as regulation of channel proteins including TRM, ASIC and NMDA receptors [91], Potassium channel [113], Na channel [114], inhibition of mitochondrial permeability transition pores [86], regulation of anti-apoptotic proteins [87], changes in receptor binding activity including NMDA and GABA receptors [115], defense against excessive endoplasmic reticulum stress [116], regulation of angiogenesis and the expression of vascular endothelial growth factor [99,117,118], cerebral vasorelaxation possibly via NO pathway [119], and induction of hypoxia inducible factor-la [120], which may affects cell survival, angiogenesis, and nemogenesis after ischemic injury.

In one study, rats were subjected to 45 min of myocardial ischemia followed by 120 min of reperfusion and 10 min preconditioning with ginsenoside Rbl right before tire induction of ischemia increased Akt phosphorylation, probably through the activation of the PI3K pathway [121]. РІЗК/Akt pathway has been shown to be essential in the regulation of cell survival as well as infarct size
reduction, which seems to be plausible molecular switch mediating the effects of ginseng in ischemic stroke in brain as well.

Excessive sodium influx is one of the factors involved in neuronal damage in ischemic conditions. In tsA201 cells transfected with cDNA expressing a subunits of tire Brain_2a Na⁺ channel, the whole-cell patch clamp experiments revealed that American ginseng (P. quinquefolius) extract or ginsenoside Rbl, tonically and reversibly blocked the channel in a concentration- and voltagedependent manner [114]. Ginsenoside Rbl stimulated tire expression of tire mitochondrion-associated antiapoptotic factor Bcl-x(L) in vitro as well as in stroke prone spontaneously hypertensive rats (SP-SHR) subjected to permanent focal cerebral ischemia [87]. Interestingly, Stat5 responsive element in tire bcl-x promoter became active in response to Rbl treatment suggesting tire possible role of Rbl in tire regulation of cell death pathway thereby providing nemoprotection in ischemic condition [87].

Ginsenosides can regulate the expression of factors important in tire pathogenesis and prognosis of ischemia such as HIF-la. The induction of HIF-la by ginsenosides is independent of hypoxia, for example, ginsenoside Rgl regulated the induction of HIF-la even in nonnoxic condition [120]. Interestingly, tire induction is not regulated by transcriptional regulation evidenced by no changes in the level of HIF-la mRNA. Rather, the induction is governed by translational up-regulation of HIF-la in a mechanism dependent on tire activation of PI3K and S6K pathway. LY294002 or rapamycin, inhibitors of PI3K and S6K pathway, respectively, attenuated Rgl-mediated induction of HIF-la, suggesting tire essential role of tlrese pathways in the ginsenoside-mediated regulation of HIF-la. Considering the importance of mTOR pathw ay in several psychiatric disorders, this data is potentially interesting to investigate w hether regulation of mTOR is involved in flic regulation of psychiatric disorders by ginseng.

Ginseng and ginsenosides may affect nemogenesis in flic ischemic brain. It has been suggested that ginsenoside Rgl and Rbl increase proliferation and differentiation of neural progenitor cells in dentate gyrus of hippocampus of normal adult mice and gerbils subjected to global ischemia suggesting the usefulness of the ginsenosides in neurodegenerative diseases such as AD as well as neurological insults condition such as cerebral ischemia (for a review, see [16]). P notoginseng extracts increased neural stem cell (NSC) proliferation and tire expression of nestin/BrdU, and also enhanced Tuj-1, vimentin, and nestin mRNA expressions in hippocampal NSCs isolated from newborn rat hippocampus with similar effects in oxygen glucose deprived NSCs, suggesting possible neu- roprotective role of ginseng by providing increased neurogenesis in ischemic or neurodegenerative conditions [110]. Similarly, when total ginseng total saponin (GTS) was intraperitoneally administered from 3 d before ischemia until 14 d after it, the number of BrdU⁺ cells and BrdU⁺/NeuN⁺ cells in GTS group were significantly higher than those in saline-treated group in tire ipsilateral subventricular zone and in tire ipsilateral infarct area after MC AO [104]. Importantly, the increase of tire number of BrdU⁺/NeuN⁺ cells highly correlated with tire decrease of neurological scores, suggesting the therapeutic role of tire increased nemogenesis by GTS in ischemic stroke.

More recently, it has been suggested that tire nemopro- tective effects of Rb3 in oxygen-glucose deprived neuron might be related to their effects on GABA_a receptor [108]. It remains to be determined whether the GABAergic effects of Rb3 are dependent on the modulation of steroid binding sites on GABA_a receptor.

Unfortunately, most of the studies in this field me explanatory on the therapeutic potential of ginseng compounds with little explanation of mechanism of action, especially, the causal relationship of the molecular or biological changes induced by ginseng on therapeutic action.

Recently, Yue et al. [122] reported that eighteen proteins involved in pathways including energy metabolism, lipid metabolism, muscle contraction, heat shock stress, and cell survival and proliferation has been changed in ischemia-reperfusion induced injury model of rat treated with notoginsengnosides with or without salvianolic acids suggesting these proteins might be the possible protein targets in their cardioprotective effects. Some of these proteins me also implicated in nemoprotection as well as ischemia-induced neuronal cell death pathway, which may prompts further study using cerebral ischemia model.

Although it has been reported that ginseng regulates a set of gene which is involved in cellular physiologic response related genes such as MPHOSPH10, IMP-3 and SDCBP, in hypoxic human nemoblastoma cells SK-N- MC in an experiment using 8K human cDNA microarray analysis [109], systemic investigation trying to find out cellular responses affected by ginseng using metabolo- mics, proteomics, and genomics approach in ischemic condition is scarce. In this sense, it is noteworthy that two studies investigated the effects of ginseng in gene expression profile after immobilization stress (Gene Expression Omnibus [GEO] accession number GSE12656) and tire role of micro RNA in ginsenoside-Rgl-induced Angiogenesis (GSE17541) [117,118], with their data submitted in GEO.

Clinical trials and human study

There's not much carefully designed, controlled clinical trials using ginseng on the possible effects on ischemic stroke. Sanchi, the dried root of P notoginseng, is one of the most widely used herbal medicines for ischemic stroke in China and Chen et al. [123] examined available clinical reports about the effectiveness and safety of Sanchi and concluded that Sanchi appears to be beneficial and safe for acute ischemic stroke. However, as the authors stated in their review, the small sample size and inferior quality of study design and performance in some of those studies, prevented a definite conclusion, which dictates more well-designed randomized controlled trials. Recently, one clinical trial has suggested that со-treatment of Sanchi capsule with low concentration of aspirine significantly ameliorated neurological deficit and impairments in activities of daily living in a study conducted with 140 human patients hospitalized in four hospitals in China over the year 2004 to 2006 [124].

Kinetics and related issues

In most of the studies, ginseng extracts or ginsenosides were treated at least 30 min before the induction of ischemia [80,81,96,106]. In many cases, post ischemic treatment of ginseng or ginsenoside was not effective to rescue the ischemic brain [81], which limits the therapeutic applicability of the ginseng and related products. However, the post-ischemic infusion of ginsenoside Rgl provided neuroprotection as well as functional recovery determined by Morris water maze after permanent middle cerebral artery occlusion in SP-SHR [87]. In some cases, ginseng containing prescription such as Sheng- mai San also effectively suppressed the oxidative stress determined by TEARS formation even when it was administered after 45 min reperfusion following ischemia, although individual components of tire prescription did not provided a protective effects [125]. Recently, it was also reported that treatment of Korean red ginseng (KRG) after ischemic injury protected brain from neurological deficits [93]. In this case, the authors used adult male Sprague-Dawley rats to induce transient middle cerebral artery occlusion for two hours and then the rats were fed KRG extract (100 mg/kg/d per orally) or saline after reperfusion. After seven day treatment of KRG extracts, both brain infarct volume as well as neurological score was improved by KRG treatment and the elevated serum levels of TNF-a, IE-IP, and IL-6 were attenuated by KRG with concomitant increase in serum level of IL-10 [93], which has been argued to be responsible for the improved ischemic outcome with oral-treatment of KRG extract. In one study using transient focal cerebral ischemia (MCAO) model in rats, P notoginseng saponin (PNS) was administered at different time points after MCAO and the authors argued that the administration of PNS at 3 to 4 h after onset of ischemia significantly reduced neurological deficit score, infarct size and brain edema with decreasing effects at 5 h after tire onset of ischemia and no effects at 6 h suggesting the therapeutic window until 5 h in rat model [92].

Intravenous injection of Rbl before or with MCAO at dose range of 10 to 40 mg/kg in rat reduced infarct size and neurological deficit score [103] but not with 40 mg/ kg PNS [126], suggesting the use of correct combination or active principle of ginsenosides might provide lower therapeutic dose for tire treatment of ischemia.

Another point of concern in tire comparison of different reports regarding tire effectiveness of ginseng is the route of administration. Many of studies used IV injection of ginsenosides for tire study of short term effects of ginsenosides [80], while others used oral administration route to study the long term effects or the restorative potential of ginseng. Even in some cases, ginsenoside was administered by intracerebroventricular infusion [105]. Intranasal delivery of ginsenoside Rbl targets the brain and ameliorates cerebral ischemia/reperfusion injury in rats [95], which might be a good route of administration to overcome tire blocking effects of blood brain barrier against ginsenoside. In this study, it was argued that a local bioavailability of 10.28% to 32.48% and drug targeting index (which means tire availability of Rbl in brain compared with IV injection) of 7.35 to 23.22 in different brain regions. Intranasal Rbl was determined to be braintargeting and might be an efficient way to deliver ginsenosides in cerebral ischemia.

DEPRESSION

Several reports suggested that traditional medicinal formulation containing ginseng may be effective in ameliorating tire symptoms of depression in humans and animal models [127-129]. In addition, total ginseng extracts or total saponin preparation has been reported to be effective in combating depression-like behaviors evaluated using annual models such as tail suspension test, forced swimming test, injection of corticosterone, chronic unpredictable mild stress, and menopausal depressive-
like state in female mice induced by ovariectomy or morphine withdrawal-induced depression behavior [130- 136]. Similar anti-depressive effects of chronic oral KRG extracts administration on human postmenopausal patients with climacteric syndrome have also been reported [137].

Based on the anti-depressant effects of orally administered ginseng, Xu et al. [138] examined the effects of intestinal metabolite of ginseng, 20(S)-protopanaxadiol (code name Sill) on depressive behavior in experimental annuals, which was tested in tail suspension test and forced swimming test as well as olfactory bulbectomy depression model in rats. Interestingly, the antidepres- sant-like activity was comparable to that of fluoxetine and Sill increased monoamine neurotransmitters in the brain and showed modest inhibitory effects against neurotransmitter reuptake in vitro. In contrast to fluoxetine, Sill inhibited oxidative stress and reduced serum corticosterone level in olfactory bulbectomized animals suggesting Sill modulates depression-like behavior by regulating multiple targets including both central and peripheral inflammation, regulation of hypothalamic corticotrophin-releasing factor (CRF) and Neuropeptide Y (NPY) expression and regulation of glucocorticoid receptor and BDNF expression as well as neurofilament- L in hippocampus [133-135,139]. Many of these results suggest that ginseng and ginsenosides regulate brain neurotransmitter turnover and hypothalamic- pituitaryadrenal axis.

Ginsenosides may affect adult nemogenesis which is reminiscent of the effects of fluoxetine on hippocampal nemogenesis implicating a possible link of the ginseng and related compounds on anti-depressant action. Actually, in a recent experiment using chronic mild stress model of depression in mice, it has been reported that Rgl up-regulated the BDNF signaling pathway in the hippocampus and reversed the decrease in dendritic spine density and hippocampal nemogenesis caused by chronic mild stress without affecting Are monoaminergic system [15]. Recently, Yamada et al. [136] reported that Rg3 and compound К provided strong anti-depressant effects in ovariectomized animals which me blocked by 5-HT_2A antagonist ritanserine suggesting additional moleculm target in Are treatment of depression by ginseng.

ANXIETY

In several different experimental paradigms to detect anxiety like behaviors such as open-field, elevated plus-maze tests, conflict behavior in thirsty rats and footshock-induced fighting in paired mice, chronic oral administration of ginseng extracts but not single acute administration of ginseng produced comparable anxiolytic activity as acute diazepam [140], which is suggested to be related with the modulation of monoamine oxidase activity. Using five day-old male chick, the effects of ginsenoside Rbl on separation distress was evaluated, which provided robust effects of ginseng on anxiety level. These results led Are authors to draw the conclusion Arat nootropic hence Are memory enhancing effects of ginseng is strongly associated with the anxiolytic effects. Among several different ginsenosides tested including Rbl, Rgl, Rg3-R, and Rg3-S, and Are Rg5 and Rk, ginsenosides Rbl, Rgl, and Are Rg5 and Rk mixture showed anxiolytic-like effects in elevated plus maze test. Similar anxiolytic effects of Rbl [143] or Rg3 and Rlr2 was observed in separate sets of experiments [144]. Although the anxiolytic activity of ginsenosides may involve the action on GABA/Benzodiazepine receptors [144], ginsenosides inhibited locomotor activity to a lesser extent Aran diazepam, which may implicate Are better side/adverse effect profile of ginsenosides as anxiolytic agents [142,145].

However, negative results me also available that did not observe any effects on anxiety or depression behavior even after 3 wk chronic treatment of several ginsenosides [146], which suggests that careful interpretation and more studies might be needed using multiple annual models and test paradigms including light/dmk test, holeboard test, and isolation-induced aggressive test [145], which is reviewed in a recent article [147].

ADDICTION

Ginseng has been believed to reduce the behavioral and physiological responses against psychostimulants and other drug of abuse such as opioids and to ameliorate the withdrawal symptoms. Pseudoginsenoside-F(ll), a saponin contained in American ginseng, effectively attenuated methamphetamine-induced behavioral and nemochemical toxicides such as anxiety, depression, and memory deficits and alterations of monoamine contents in mice brain [148].

When the effects of total ginseng saponins were examined on presynaptic nicotine-induced dopamine (DA) release in Are striatum of freely moving rats using in vivo microdialysis technique and on the in vitro and in vivo binding of [³H]raclopride to DA D₂ receptors, inhibition of nicotine-induced DA release and D₂ receptor binding was observed along with behavioral inhibition of
nicotine-induced sensitization [149]. Similar results were observed with repeated cocaine administration. Pretreatment with GTS before the daily intraperitoneal injections of cocaine (15 mg/kg,) significantly inhibited die repeated cocaine-induced behavioral sensitization as well as die c-Fos expression in the core and shell of nucleus accumbens. In addition, pretreatment with GTS significantly decreased the repeated cocaine-induced increase in DA release in the nucleus accumbens as determined by in vivo microdialysis [150]. Similarly, Using real-time measurements of the extracellular DA concentrations by real-time fast-scan cyclic voltammetry in slices of rat brain nucleus accumbens, it has been demonstrated tiiat со-treatment of GTS inhibited the release enhancement of DA and subsequently prevented the rebound increase during acute withdrawal of cocaine without affecting die cocaine-mediated DA uptake [151].

In addition, morphine withdrawal-induced anxiety and depression-like behavior were effectively inhibited by wild ginseng extracts by modulating hypothalamic expression of CRF and NPY [133]. Moreover, die same group of researchers have reported diat wild ginseng extracts effectively attenuated morphine withdrawal-induced behavioral sensitization along with modulation of c-Fos expression in nucleus accumbens and TH expression in ventral tegmental area suggesting the possible involvement of ginseng in die modulation of dopaminergic nervous system activity during morphine withdrawal [152].

The signaling pathway leading to the decreased DA release and die neurobiological targets of ginseng mediating the up/down-regulation of proteins involved in die addiction subsequent to the withdrawal of drug of abuse remains to be determined, although regulation of intracellular ion influx including calcium seems to be one of flic plausible targets.

OTHER PSYCHIATRIC DISORDERS

In a rat model of epileptic seizure, pentylenetetrazole was injected intraperitoneally at the dose of 30 mg/kg, on alternate days to obtain generalized tonic-clonic convulsions in rats and pretreatment of P ginseng protected the rats from epileptic seizure [153]. Based on the fast inhibition of ginseng total saponins and ginsenoside Rg3 on NMDA receptor-mediated |Ca² | in cultured hippocampal neurons, Kim and Rliim [154] determined that ginseng total saponins and ginsenoside Rg3 inhibited Mg²⁺ free-induced increase of |Ca² | and spontaneous |Ca² I oscillations in cultured rat hippocampal neurons suggesting ginseng may be effective in correcting the epileptiform discharge induced by excitatory/inhibitory imbalance. In an experiments using three different chemical seizure inducing agents, i.e., pentylenetetrazole, kainic acid, and pilocarpine, American ginseng provided protective effects against seizure [155]. Albeit the root preparation or leaves/stem preparation provided a marginal range of protection, the partially purified extracts that concentrate Rb ginsenosides (Rb extract) had a dosedependent anticonvulsant effect in all three models of chemically induced seizures. Rb extracts also reduced neuronal stress induced by kainic acid as evidenced by the reduced immunohistochemical signal against heat shock protein 72 [155]. The same group of researchers expanded their efforts to identify active components in ginseng extracts and concluded that flic mixtures of purified Rbl and Rb3 provided most robust anti-epileptic activity [156]. Interestingly, no single ginsenoside provided enough anti-epileptic and neuroprotective effects, which suggest that the combination of different ginsenosides is needed for optimal results [156].

Ginseng has been suggested to be effective in schizo- pherinic patients. In a 4 wk double-blind, placebo- controlled study using HT1001, a proprietary North American ginseng extract, unproved working memory in schizophrenic patients [157]. In addition to flic verbal and visual working memory improvement, HT101 reduced extrapyramidal symptoms after 4 wk treatment suggesting HT1001 might be effective in treating schizophrenia as well as to reduce the side effects of currently available medications [157]. Similarly, P. quinquefolius showed various range of neurological effects in ketamine-induced experimental psychosis model in mice without severe extra-pyramidal side effects and catalepsy [158]. Increased glutamate contents and decreased DA and 5-HT contents are evident along with increased acetylcholine esterase and nitrate level, which may impose flic need for further study.

NEURODEVELOPMENTAL DISORDERS

A recent drastic increase in prevalence of several neu- rodevelopmental diseases including ADHD and autism spectrum disorder (ASD) along with the relative lack of safe and efficient therapeutic agents in this field albeit flic wide spread use of stimulant medication in the case of ADHD, necessitates the investigation whether ginseng has therapeutic potential in this group of diseases. Surprisingly, only a small number of investigations have been reported so far, mostly focused on ADHD. Using a combinatorial herbal medicine consists of American ginseng and Ginko biloba, 36 children ranging in age from 3 to 17 yr with ADHD was enrolled in open-label clinical study [159]. After 4 wk treatment of tire herbal mixture, parents completed revised version of Conners' Parent Rating Scale and the authors reported that 44% to 74% of patients showed some kind of improvement either in social problem attributes or DSM-IV hyperactive-impulsive attribute [159]. The study hampers with the small number of subjects, and even with the initially promising results, no follow up study has been reported so far. Up to now, contradictory results have been reported about tire possible effects of ginseng on hyperactive phenotype in experimental animals. Chronic oral administration of ginseng slightly decreased the activity in open field test while it did not produce any significant difference against hyperactivity induced by amphetamine [136], while others reported the attenuation of behavioral sensitization induced by chemical stimulants [138,139,142]. More recently, it has been also suggested that three ADHD inattentive type patients showed improvements in their hyperactivity and inattention symptoms after taking P ginseng as judged by Conners' parent ratings [160]. Obviously, clinical studies with more number of patients as well as animal studies using relevant models and behavioral paradigms might be needed in this field [161].

ASD is a neurodevelopmental disorder characterized by tire impairment in communication, social interaction, and repetitive behavior. Although it has been suggested drat P. ginseng may have beneficial effects in the treatment of ASD patients in a preliminary study with human patients [162], no definite and elaborate study has been conducted in either animal and human studies. Interestingly, one recent article suggested that the active acidic polysaccharide portion of P ginseng increased social engagement both in time and frequency in normal mice. The reason for increased engagement was majorly attributed to the increased non-aggressive engagement in mice which is interpreted being related to the anti-depressant effects of ginseng [163]. Combined with the fact that ginseng may have efficacy in seizure, anxiety and schizophrenia, investigating whether ginseng might have therapeutic effects in this devastating disorder would be helpful to devise tools for intervention against the otherwise non-curable disorder.

CONCLUSION

Ginseng and ginsenosides affect various aspects of neurodevelopmental disorder including AD and PD as well as nemopsychiatric disorders and even neurodevelopmental disorder. Even with the vast array of researches conducted during last two decades, it still needs elaboration on the molecular and cellular mechanism of action. Even in studies showing the effects on therapeutic targets, the causal relationship of the target modulation is not clear. Obviously, investigations using more pharmacological model systems as well as genetic model systems would be essential to unravel the therapeutic efficacy as well as molecular mechanisms. Recent progress in high-throughput measures to study biological processes such as omics technology would be beneficial in tire field of ginseng research as well. Unresolved questions such as tire obvious therapeutic efficacy even with tire poor kinetic profiles of some ginsenosides after oral administration, are being approached in several ways, for example, demonstrating the therapeutic efficacy of ginsenoside metabolites such as compound К and devising new way of administration including intranasal administration. It is increasingly evident that not a single molecular target can explain all the pathophyiological features of a given neurological diseases. In many cases such as AD and ischemia, hope to develop a therapeutic agent based on a single molecular targets is almost coming to an end unless tire ultimate new target is developed and available for those invincible disease. In this regard, the fact that ginseng affects seemingly a multiple number of targets regulating synaptic plasticity, neurogenesis, neuroprotection, neural transmission, and much more, is worthy of special attention. Finally, the comparison of pharmacological and therapeutic effects of different ginseng preparation needs standardization of preparation otherwise there's no way to improve our understanding of the differential effects of specific ginseng preparation on particular neurological disorders. With the unceasing efforts of the existing and forthcoming investigators in this field, future will tell us whether ginseng will provide effective tools to combat with AD, PD, ischemia and other neurological condition or disorders.

ACKNOWLEDGEMENTS

This work was supported by Konkuk University in 2012.

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