Hopping the Hurdle: Strategies to Enhance the Molecular Delivery to the Brain through the Blood-Brain Barrier.

Modern medicine has allowed for many advances in neurological and neurodegenerative disease (ND). However, the number of patients suffering from brain diseases is ever increasing and the treatment of brain diseases remains an issue, as drug efficacy is dramatically reduced due to the existence of the unique vascular structure, namely the blood-brain barrier (BBB). Several approaches to enhance drug delivery to the brain have been investigated but many have proven to be unsuccessful due to limited transport or damage induced in the BBB. Alternative approaches to enhance molecular delivery to the brain have been revealed in recent studies through the existence of molecular delivery pathways that regulate the passage of peripheral molecules. In this review, we present recent advancements of the basic research for these delivery pathways as well as examples of promising ventures to overcome the molecular hurdles that will enhance therapeutic interventions in the brain and potentially save the lives of millions of patients.

Activation of NMDA receptors in brain endothelial cells increases transcellular permeability.

Neurovascular coupling is a precise mechanism that induces increased blood flow to activated brain regions, thereby providing oxygen and glucose. In this study, we hypothesized that N-methyl-D-aspartate (NMDA) receptor signaling, the most well characterized neurotransmitter signaling system which regulates delivery of essential molecules through the blood-brain barrier (BBB). Upon application of NMDA in both in vitro and in vivo models, increased delivery of bioactive molecules that was mediated through modulation of molecules involved in molecular delivery, including clathrin and caveolin were observed. Also, NMDA activation induced structural changes in the BBB and increased transcellular permeability that showed regional heterogeneity in its responses. Moreover, NMDA receptor activation increased endosomal trafficking and facilitated inactivation of lysosomal pathways and consequently increased molecular delivery mediated by activation of calmodulin-dependent protein kinase II (CaMKII) and RhoA/protein kinase C (PKC). Subsequent in vivo experiments using mice specifically lacking NMDA receptor subunit 1 in endothelial cells showed decreased neuronal density in the brain cortex, suggesting that a deficiency in NMDA receptor signaling in brain endothelial cells induces neuronal losses. Together, these results highlight the importance of NMDA-receptor-mediated signaling in the regulation of BBB permeability that surprisingly also affected CD31 staining.

Brain Endothelial Cells Utilize Glycolysis for the Maintenance of the Transcellular Permeability.

Among the components of the blood-brain barrier (BBB), endothelial cells (ECs) play an important role in supplying limited materials, especially glucose, to the brain. However, the mechanism by which glucose is metabolized in brain ECs is still elusive. To address this topic, we assessed the metabolic signature of glucose utilization using live-cell metabolic assays and liquid chromatography-tandem mass spectrometry metabolomic analysis. We found that brain ECs are highly dependent on aerobic glycolysis, generating lactate as its final product with minimal consumption of glucose. Glucose treatment decreased the oxygen consumption rate in a dose-dependent manner, indicating the Crabtree effect. Moreover, when glycolysis was inhibited, brain ECs showed impaired permeability to molecules utilizing transcellular pathway. In addition, we found that the blockade of glycolysis in mouse brain with 2-deoxyglucose administration resulted in decreased transcellular permeability of the BBB. In conclusion, utilizing glycolysis in brain ECs has critical roles in the maintenance and permeability of the BBB. Overall, we could conclude that brain ECs are highly glycolytic, and their energy can be used to maintain the transcellular permeability of the BBB.

Spike Proteins of SARS-CoV-2 Induce Pathological Changes in Molecular Delivery and Metabolic Function in the Brain Endothelial Cells.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes the coronavirus disease (COVID-19), is currently infecting millions of people worldwide and is causing drastic changes in people's lives. Recent studies have shown that neurological symptoms are a major issue for people infected with SARS-CoV-2. However, the mechanism through which the pathological effects emerge is still unclear. Brain endothelial cells (ECs), one of the components of the blood-brain barrier, are a major hurdle for the entry of pathogenic or infectious agents into the brain. They strongly express angiotensin converting enzyme 2 (ACE2) for its normal physiological function, which is also well-known to be an opportunistic receptor for SARS-CoV-2 spike protein, facilitating their entry into host cells. First, we identified rapid internalization of the receptor-binding domain (RBD) S1 domain (S1) and active trimer (Trimer) of SARS-CoV-2 spike protein through ACE2 in brain ECs. Moreover, internalized S1 increased Rab5, an early endosomal marker while Trimer decreased Rab5 in the brain ECs. Similarly, the permeability of transferrin and dextran was increased in S1 treatment but decreased in Trimer, respectively. Furthermore, S1 and Trimer both induced mitochondrial damage including functional deficits in mitochondrial respiration. Overall, this study shows that SARS-CoV-2 itself has toxic effects on the brain ECs including defective molecular delivery and metabolic function, suggesting a potential pathological mechanism to induce neurological signs in the brain.

Emerging pathogenic role of peripheral blood factors following BBB disruption in neurodegenerative disease.

The responses of central nervous system (CNS) cells such as neurons and glia in neurodegenerative diseases (NDs) suggest that regulation of neuronal and glial functions could be a strategy for ND prevention and/or treatment. However, attempts to develop such therapeutics for NDs have been hindered by the challenge of blood-brain barrier (BBB) permeability and continued constitutive neuronal loss. These limitations indicate the need for additional perspectives for the prevention/treatment of NDs. In particular, the disruption of the blood-brain barrier (BBB) that accompanies NDs allows brain infiltration by peripheral factors, which may stimulate innate immune responses involved in the progression of neurodegeneration. The accumulation of blood factors like thrombin, fibrinogen, c-reactive protein (CRP) and complement components in the brain has been observed in NDs and may activate the innate immune system in the CNS. Thus, strengthening the integrity of the BBB may enhance its protective role to attenuate ND progression and functional loss. In this review, we describe the innate immune system in the CNS and the contribution of blood factors to the role of the CNS immune system in neurodegeneration and neuroprotection.

Treatment with AAV1-Rheb(S16H) provides neuroprotection in a mouse model of photothrombosis-induced ischemic stroke.

We recently reported that upregulation of the constitutively active ras homolog enriched in brain [Rheb(S16H)], which induces the activation of the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, can protect adult neurons, mediated by the induction of neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), in animal models of neurodegenerative diseases. Here we show that neuronal transduction of Rheb(S16H) using adeno-associated virus serotype 1 provides neuroprotection in a mouse model of photothrombosis-induced ischemic stroke. Rheb(S16H)-expressing neurons exhibited neurotrophic effects, such as mTORC1 activation, increases in neuronal size, and BDNF production, in mouse cerebral cortex. Moreover, the upregulation of neuronal Rheb(S16H) significantly attenuated ischemic damage and behavioral impairments as compared to untreated mice, suggesting that Rheb(S16H) upregulation in cortical neurons may be a useful strategy to treat ischemic stroke.

Induction of GDNF and GFRα-1 Following AAV1-Rheb(S16H) Administration in the Hippocampus in vivo.

The activation of neurotrophic signaling pathways following the upregulation of glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-ß family, has a potential neuroprotective effect in the adult brain. Herein, we report that hippocampal transduction of adeno-associated virus serotype 1 (AAV1) with a constitutively active form of ras homolog enriched in brain [Rheb(S16H)], which can stimulate the production of brain-derived neurotrophic factor (BDNF) in hippocampal neurons, induces the increases in expression of GDNF and GDNF family receptor α-1 (GFRα-1), in neurons and astrocytes in the hippocampus of rat brain in vivo. Moreover, upregulation of GDNF and GFRα-1 contributes to neuroprotection against thrombin-induced neurotoxicity in the hippocampus. These results suggest that AAV1-Rheb(S16H) transduction of hippocampal neurons, resulting in neurotrophic interactions between neurons and astrocytes, may be useful for neuroprotection in the adult hippocampus.

Neurotrophic interactions between neurons and astrocytes following AAV1-Rheb(S16H) transduction in the hippocampus in vivo.

BACKGROUND AND PURPOSE: We recently reported that AAV1-Rheb(S16H) transduction could protect hippocampal neurons through the induction of brain-derived neurotrophic factor (BDNF) in the rat hippocampus in vivo. It is still unclear how neuronal BDNF produced by AAV1-Rheb(S16H) transduction induces neuroprotective effects in the hippocampus and whether its up-regulation contributes to the enhance of a neuroprotective system in the adult brain. EXPERIMENTAL APPROACH: To determine the presence of a neuroprotective system in the hippocampus of patients with Alzheimer's disease (AD), we examined the levels of glial fibrillary acidic protein, BDNF and ciliary neurotrophic factor (CNTF) and their receptors, tropomyocin receptor kinase B (TrkB) and CNTF receptor α(CNTFRα), in the hippocampus of AD patients. We also determined whether AAV1-Rheb(S16H) transduction stimulates astroglial activation and whether reactive astrocytes contribute to neuroprotection in models of hippocampal neurotoxicity in vivo and in vitro. KEY RESULTS: AD patients may have a potential neuroprotective system, demonstrated by increased levels of full-length TrkB and CNTFRα in the hippocampus. Further AAV1-Rheb(S16H) transduction induced sustained increases in the levels of full-length TrkB and CNTFRα in reactive astrocytes and hippocampal neurons. Moreover, neuronal BDNF produced by Rheb(S16H) transduction of hippocampal neurons induced reactive astrocytes, resulting in CNTF production through the activation of astrocytic TrkB and the up-regulation of neuronal BDNF and astrocytic CNTF which had synergistic effects on the survival of hippocampal neurons in vivo. CONCLUSIONS AND IMPLICATIONS: The results demonstrated that Rheb(S16H) transduction of hippocampal neurons could strengthen the neuroprotective system and this intensified system may have a therapeutic value against neurodegeneration in the adult brain.

Therapeutic Potential of AAV1-Rheb(S16H) Transduction Against Alzheimer's Disease.

We recently reported that adeno-associated virus serotype 1-constitutively active Ras homolog enriched in brain [AAV1-Rheb(S16H)] transduction of hippocampal neurons could induce neuron-astroglia interactions in the rat hippocampus in vivo, resulting in neuroprotection. However, it remains uncertain whether AAV1-Rheb(S16H) transduction induces neurotrophic effects and preserves the cognitive memory in an animal model of Alzheimer's disease (AD) with characteristic phenotypic features, such as ß-amyloid (Aß) accumulation and cognitive impairments. To assess the therapeutic potential of Rheb(S16H) in AD, we have examined the beneficial effects of AAV1-Rheb(S16H) administration in the 5XFAD mouse model. Rheb(S16H) transduction of hippocampal neurons in the 5XFAD mice increased the levels of neurotrophic signaling molecules, including brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF), and their corresponding receptors, tropomyosin receptor kinase B (TrkB) and CNTF receptor α subunit (CNTFRα), respectively. In addition, Rheb(S16H) transduction inhibited Aß production and accumulation in the hippocampus of 5XFAD mice and protected the decline of long-term potentiation (LTP), resulting in the prevention of cognitive impairments, which was demonstrated using novel object recognition testing. These results indicate that Rheb(S16H) transduction of hippocampal neurons may have therapeutic potential in AD by inhibiting Aß accumulation and preserving LTP associated with cognitive memory.

Perspective: Therapeutic Potential of Flavonoids as Alternative Medicines in Epilepsy.

Epilepsy is a chronic neurological disorder that affects many people worldwide. Temporal lobe epilepsy is the most common and most studied type of epilepsy, but the pathological mechanisms underlying this condition are poorly understood. More than 20 antiepileptic drugs (AEDs) have been developed and used for the treatment of epilepsy; however, 30% of patients still experience uncontrolled epilepsy and associated comorbidities, which impair their quality of life. In addition, various side effects have been reported for AEDs, such as drowsiness, unsteadiness, dizziness, blurred or double vision, tremor (shakiness), greater risk of infections, bruising, and bleeding. Thus, critical medical needs remain unmet for patients with uncontrolled epilepsy. Flavonoids belong to a subclass of polyphenols that are widely present in fruits, vegetables, and certain beverages. Recently, many studies have reported that some flavonoids elicit various beneficial effects in patients with epilepsy without causing the side effects associated with conventional medical therapies. Moreover, flavonoids may have a property of regulating microRNA expression associated with inflammation and cell survival. These findings suggest that flavonoids, which are more effective but impose fewer adverse effects than conventional AEDs, could be used in the treatment of epilepsy.

Beneficial Effects of Hesperetin in a Mouse Model of Temporal Lobe Epilepsy.

Abnormal reorganization of the dentate gyrus and neuroinflammation in the hippocampus represent characteristic phenotypes of patients suffering from temporal lobe epilepsy. Hesperetin, a flavanone abundant in citrus fruit, is known to have protective effects by preventing inflammation and oxidative stress in neuronal cultures and in the adult murine brain. However, the protective effects of hesperetin against epileptic seizures in vivo remain unclear, despite one study reporting anticonvulsant effects in vitro. In this study, we report that oral administration of hesperetin not only delays the onset of seizures triggered by kainic acid (KA) but also contributes to the attenuation of granule cell dispersion in the KA-treated hippocampus. Moreover, we observed that hesperetin administration inhibited the expression of pro-inflammatory molecules produced by activated microglia in the hippocampus. Thus, administration of hesperetin might be beneficial for preventing epileptic seizures.

Beneficial Effects of Silibinin Against Kainic Acid-induced Neurotoxicity in the Hippocampus in vivo.

Silibinin, an active constituent of silymarin extracted from milk thistle, has been previously reported to confer protection to the adult brain against neurodegeneration. However, its effects against epileptic seizures have not been examined yet. In order to investigate the effects of silibinin against epileptic seizures, we used a relevant mouse model in which seizures are manifested as status epilepticus, induced by kainic acid (KA) treatment. Silibinin was injected intraperitoneally, starting 1 day before an intrahippocampal KA injection and continued daily until analysis of each experiment. Our results indicated that silibinin-treatment could reduce seizure susceptibility and frequency of spontaneous recurrent seizures (SRS) induced by KA administration, and attenuate granule cell dispersion (GCD), a morphological alteration characteristic of the dentate gyrus (DG) in temporal lobe epilepsy (TLE). Moreover, its treatment significantly reduced the aberrant levels of apoptotic, autophagic and pro-inflammatory molecules induced by KA administration, resulting in neuroprotection in the hippocampus. Thus, these results suggest that silibinin may be a beneficial natural compound for preventing epileptic events.

Pathogenic Upregulation of Glial Lipocalin-2 in the Parkinsonian Dopaminergic System.

UNLABELLED: Lipocalin-2 (LCN2) is a member of the highly heterogeneous secretory protein family of lipocalins and increases in its levels can contribute to neurodegeneration in the adult brain. However, there are no reports on the role of LCN2 in Parkinson's disease (PD). Here, we report for the first time that LCN2 expression is increased in the substantia nigra (SN) of patients with PD. In mouse brains, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment for a neurotoxin model of PD significantly upregulated LCN2 expression, mainly in reactive astrocytes in both the SN and striatum. The increased LCN2 levels contributed to neurotoxicity and neuroinflammation, resulting in disruption of the nigrostriatal dopaminergic (DA) projection and abnormal locomotor behaviors, which were ameliorated in LCN2-deficient mice. Similar to the effects of MPTP treatment, LCN2-induced neurotoxicity was also observed in the 6-hydroxydopamine (6-OHDA)-treated animal model of PD. Moreover, treatment with the iron donor ferric citrate (FC) and the iron chelator deferoxamine mesylate (DFO) increased and decreased, respectively, the LCN2-induced neurotoxicity in vivo In addition to the in vivo results, 1-methyl-4-phenylpyridinium (MPP(+))-induced neurotoxicity in cocultures of mesencephalic neurons and astrocytes was reduced by LCN2 gene deficiency in the astrocytes and conditioned media derived from MPP(+)-treated SH-SY5Y neuronal enhanced glial expression of LCN2 in vitro Therefore, our results demonstrate that astrocytic LCN2 upregulation in the lesioned DA system may play a role as a potential pathogenic factor in PD and suggest that inhibition of LCN2 expression or activity may be useful in protecting the nigrostriatal DA system in the adult brain. SIGNIFICANCE STATEMENT: Lipocalin-2 (LCN2), a member of the highly heterogeneous secretory protein family of lipocalins, may contribute to neuroinflammation and neurotoxicity in the brain. However, LCN2 expression and its role in Parkinson's disease (PD) are largely unknown. Here, we report that LCN2 is upregulated in the substantia nigra of patients with PD and neurotoxin-treated animal models of PD. Our results suggest that LCN2 upregulation might be a potential pathogenic mechanism of PD, which would result in disruption of the nigrostriatal dopaminergic system through neurotoxic iron accumulation and neuroinflammation. Therefore, inhibition of LCN2 expression or activity may be useful in protecting the nigrostriatal dopaminergic projection in PD.

Induction of microglial toll-like receptor 4 by prothrombin kringle-2: a potential pathogenic mechanism in Parkinson's disease.

Microglia-mediated neuroinflammation may play an important role in the initiation and progression of dopaminergic (DA) neurodegeneration in Parkinson's disease (PD), and toll-like receptor 4 (TLR4) is essential for the activation of microglia in the adult brain. However, it is still unclear whether patients with PD exhibit an increase in TLR4 expression in the brain, and whether there is a correlation between the levels of prothrombin kringle-2 (pKr-2) and microglial TLR4. In the present study, we first observed that the levels of pKr-2 and microglial TLR4 were increased in the substantia nigra (SN) of patients with PD. In rat and mouse brains, intranigral injection of pKr-2, which is not directly toxic to neurons, led to the disruption of nigrostriatal DA projections. Moreover, microglial TLR4 was upregulated in the rat SN and in cultures of the BV-2 microglial cell line after pKr-2 treatment. In TLR4-deficient mice, pKr-2-induced microglial activation was suppressed compared with wild-type mice, resulting in attenuated neurotoxicity. Therefore, our results suggest that pKr-2 may be a pathogenic factor in PD, and that the inhibition of pKr-2-induced microglial TLR4 may be protective against degeneration of the nigrostriatal DA system in vivo.

Roles of Rheb(S16H) in substantia nigra pars compacta dopaminergic neurons in vivo.

Although there are ongoing intensive research efforts, no effective pharmacological therapies for Parkinson's disease (PD) have been developed thus far. However, with the development of efficient gene delivery systems, gene therapy for PD has become a focus of research and increasing evidence suggests that continuous production of neurotrophic factors play a significant role in the functional restoration of the nigrostriatal dopaminergic (DA) system. Our recent study reported that the transduction of DA neurons with ras homolog enriched in brain, which has an S16H mutation [Rheb(S16H)], protected the nigrostriatal DA projection in a neurotoxin model of PD in vivo. In addition, Rheb(S16H) expression significantly increased the levels of glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor, which contributed to the neuroprotective effects of Rheb(S16H) in DA neurons in the adult brain, indicating that the activation of the signaling pathways involved in cell survival by a specific gene delivery, such as Rheb(S16H) to adult neurons, may be a useful strategy to protect neural systems in the adult brain. In the present study, a brief overview of our recent studies is provided, which demonstrates the neuroprotective mechanisms of Rheb(S16H) on the nigrostriatal DA projection in the adult brain.

In vivo AAV1 transduction with hRheb(S16H) protects hippocampal neurons by BDNF production.

Recent evidence has shown that Ras homolog enriched in brain (Rheb) is dysregulated in Alzheimer's disease (AD) brains. However, it is still unclear whether Rheb activation contributes to the survival and protection of hippocampal neurons in the adult brain. To assess the effects of active Rheb in hippocampal neurons in vivo, we transfected neurons in the cornu ammonis 1 (CA1) region in normal adult rats with an adeno-associated virus containing the constitutively active human Rheb (hRheb(S16H)) and evaluated the effects on thrombin-induced neurotoxicity. Transduction with hRheb(S16H) significantly induced neurotrophic effects in hippocampal neurons through activation of mammalian target of rapamycin complex 1 (mTORC1) without side effects such as long-term potentiation impairment and seizures from the alteration of cytoarchitecture, and the expression of hRheb(S16H) prevented thrombin-induced neurodegeneration in vivo, an effect that was diminished by treatment with specific neutralizing antibodies against brain-derived neurotrophic factor (BDNF). In addition, our results showed that the basal mTORC1 activity might be insufficient to mediate the level of BDNF expression, but hRheb(S16H)-activated mTORC1 stimulated BDNF production in hippocampal neurons. These results suggest that viral vector transduction with hRheb(S16H) may have therapeutic value in the treatment of neurodegenerative diseases such as AD.

Induction of GDNF and BDNF by hRheb(S16H) transduction of SNpc neurons: neuroprotective mechanisms of hRheb(S16H) in a model of Parkinson's disease.

The transduction of dopaminergic (DA) neurons with human ras homolog enriched in brain, which has a S16H mutation [hRheb(S16H)] protects the nigrostriatal DA projection in the 6-hydroxydopamine (6-OHDA)-treated animal model of Parkinson's disease (PD). However, it is still unclear whether the expression of active hRheb induces the production of neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF), which are involved in neuroprotection, in mature neurons. Here, we show that transduction of nigral DA neurons with hRheb(S16H) significantly increases the levels of phospho-cyclic adenosine monophosphate (cAMP) response element-binding protein (p-CREB), GDNF, and BDNF in neurons, which are attenuated by rapamycin, a specific inhibitor of mammalian target of rapamycin complex 1 (mTORC1). Moreover, treatment with specific neutralizing antibodies for GDNF and BDNF reduced the protective effects of hRheb(S16H) against 1-methyl-4-phenylpyridinium (MPP(+))-induced neurotoxicity. These results show that activation of hRheb/mTORC1 signaling pathway could impart to DA neurons the important ability to continuously produce GDNF and BDNF as therapeutic agents against PD.

Nobiletin protects dopaminergic neurons in the 1-methyl-4-phenylpyridinium-treated rat model of Parkinson's disease.

This study investigated the effect of nobiletin, a flavonoid found in citrus fruits, on the degeneration of dopaminergic (DA) neurons in a neurotoxin model of Parkinson's disease (PD). 1-Methyl-4-phenylpyridinium (MPP(+)) was unilaterally injected into the median forebrain bundle of rat brains (to generate a neurotoxin model of PD) with or without daily intraperitoneal injection of nobiletin. Our results showed that nobiletin treatment at 10 mg/kg bw, but not at 1 or 20 mg/kg bw, significantly protected DA neurons in the substantia nigra (SN) of MPP(+)-treated rats. In parallel to the neuroprotection, nobiletin treatment at 10 mg/kg inhibited microglial activation and preserved the expression of the glial cell line-derived neurotrophic factor, which is a therapeutic agent against PD, in the SN. These results suggest that the proper supplementation with nobiletin may protect against the neurodegeneration involved in PD.

Naringin protects the nigrostriatal dopaminergic projection through induction of GDNF in a neurotoxin model of Parkinson's disease.

This study investigated the effect of naringin, a major flavonoid in grapefruit and citrus fruits, on the degeneration of the nigrostriatal dopaminergic (DA) projection in a neurotoxin model of Parkinson's disease (PD) in vivo and the potential underlying mechanisms focusing on the induction of glia-derived neurotrophic factor (GDNF), well known as an important neurotrophic factor involved in the survival of adult DA neurons. 1-Methyl-4-phenylpyridinium (MPP(+)) was unilaterally injected into the medial forebrain bundle of rat brains for a neurotoxin model of PD in the presence or absence of naringin by daily intraperitoneal injection. To ascertain whether naringin-induced GDNF contributes to neuroprotection, we further investigated the effects of intranigral injection of neutralizing antibodies against GDNF in the MPP(+) rat model of PD. Our observations demonstrate that naringin could increase the level of GDNF in DA neurons, contributing to neuroprotection in the MPP(+) rat model of PD, with activation of mammalian target of rapamycin complex 1. Moreover, naringin could attenuate the level of tumor necrosis factor-α in microglia increased by MPP(+)-induced neurotoxicity in the substantia nigra. These results indicate that naringin could impart to DA neurons the important ability to produce GDNF as a therapeutic agent against PD with anti-inflammatory effects, suggesting that naringin is a beneficial natural product for the prevention of DA degeneration in the adult brain.

Silibinin attenuates MPP⁺-induced neurotoxicity in the substantia nigra in vivo.

Parkinson's disease (PD) is characterized by degeneration of the nigrostriatal dopaminergic (DA) pathway. The cause of neuronal death in PD is largely unknown, but it is becoming clear that inflammation plays a significant role in the pathophysiology of PD. Silibinin is a major flavonoid in milk thistle which has an anti-inflammatory activity. We investigated whether silibinin could have neuroprotective effects on DA neurons in the 1-methyl-4-phenylpyridinium ion (MPP(+))-treated animal model of PD in vivo. To address this question, animals received intraperitoneal (i.p.) injections 10, 50, or 100 mg/kg of silibinin, starting 1 day before MPP(+) injection and continued daily until 6 days post-lesion for tyrosine hydroxylase (TH) staining, or until 1 hour prior to the MPP(+) injection to examine the expression levels of inflammatory proteins. Finally, their brains were harvested at the indicated time points for the analyses. Silibinin treatment with 10 mg/kg had no significantly neuroprotective effects in the substantia nigra (SN). However, 50 and 100 mg/kg of silibinin ameliorated the MPP(+)-induced neurotoxicity in the SN in a dose-dependent manner, and the increased levels of inflammatory molecules such as tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1ß) and inducible nitric oxide synthase (iNOS) by MPP(+) treatment were attenuated by treatment with 100 mg/kg of silibinin. These results indicate that silibinin could be a useful and beneficial natural product offering promise for the prevention of DA neuronal degeneration involved in PD.