Eun Sook Chung

Transient Receptor Potential Vanilloid Subtype 1 Mediates Cell Death of Mesencephalic Dopaminergic Neurons In Vivo and In Vitro

Intranigral injection of the transient receptor potential vanilloid subtype 1 (TRPV1; also known as VR1) agonist capsaicin (CAP) into the rat brain, or treatment of rat mesencephalic cultures with CAP, resulted in cell death of dopaminergic (DA) neurons, as visualized by immunocytochemistry. This in vivo and in vitro effect was ameliorated by the TRPV1 antagonist capsazepine (CZP) or iodo-resiniferatoxin, suggesting the direct involvement of TRPV1 in neurotoxicity. In cultures, both CAP and anandamide (AEA), an endogenous ligand for both TRPV1 and cannabinoid type 1 (CB1) receptors, induced degeneration of DA neurons, increases in intracellular Ca2+ ([Ca2+]i), and mitochondrial damage, which were inhibited by CZP, the CB1 antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) or the intracellular Ca2+ chelator BAPTA/AM. We also found that CAP or AEA increased mitochondrial cytochrome c release as well as immunoreactivity to cleaved caspase-3 and that the caspase-3 inhibitor z-Asp-Glu-Val-Asp-fmk protected DA neurons from CAP- or AEA-induced neurotoxicity. Additional studies demonstrated that treatment of mesencephalic cultures with CB1 receptor agonist (6aR)-trans 3-(1,1-dimethylheptyl)-6a,7,10,10a-tetrahydro-1-hydroxy-6,6-dimethyl-6H-dibenzo[b,d] pyran-9-methanol (HU210) also produced degeneration of DA neurons and increases in [Ca2+]i, which were inhibited by AM251 and BAPTA/AM. The CAP-, AEA-, or HU210-induced increases in [Ca2+]i were dependent on extracellular Ca2+, with significantly different patterns of Ca2+ influx. Surprisingly, CZP and AM251 reversed HU210- or CAP-induced neurotoxicity by inhibiting Ca2+ influx, respectively, suggesting the existence of functional cross talk between TRPV1 and CB1 receptors. To our knowledge, this study is the first to demonstrate that the activation of TRPV1 and/or CB1 receptors mediates cell death of DA neurons. Our findings suggest that these two types of receptors, TRPV1 and CB1, may contribute to neurodegeneration in response to endogenous ligands such as AEA.

Inhibition of thrombin-induced microglial activation and NADPH oxidase by minocycline protects dopaminergic neurons in the substantia nigra in vivo

The present study shows that activation of microglial NADPH oxidase and production of reactive oxygen species (ROS) is associated with thrombin‐induced degeneration of nigral dopaminergic neurons in vivo. Seven days after thrombin injection in the rat substantia nigra (SN), tyrosine hydroxylase immunocytochemistry showed a significant loss of nigral dopaminergic neurons. This cell death was accompanied by localization of terminal deoxynucleotidyl transferase‐mediated fluorecein UTP nick‐end labelling (TUNEL) staining within dopaminergic neurons. This neurotoxicity was antagonized by the semisynthetic tetracycline derivative, minocycline, and the observed neuroprotective effects were associated with the ability of minocycline to suppress NADPH oxidase‐derived ROS production and pro‐inflammatory cytokine expression, including interleukin‐1β and inducible nitric oxide synthase, from activated microglia. These results suggest that microglial NADPH oxidase may be a viable target for neuroprotection against oxidative damage.

Neuroprotective effect of nicotine on dopaminergic neurons by anti-inflammatory action.

Epidemiological studies have reported that smoking is associated with a lower incidence of Parkinsons disease (PD), leading to theories that smoking in general and nicotine in particular might be neuroprotective. Recent studies suggested cholinergic anti‐inflammatory pathway‐regulating microglial activation through α7 nicotinic receptors. In the present study, we used lipopolysaccharide (LPS)‐induced in vitro and in vivo inflammation models to investigate whether nicotine has a protective effect on the dopaminergic system through an anti‐inflammatory mechanism. Nicotine pretreatment considerably decreased microglial activation with significant reduction of tumour necrosis factor (TNF)‐α mRNA expression and TNF‐α release induced by LPS stimulation. In co‐cultures of microglia and mesencephalic neurons, nicotine pretreatment significantly decreased the loss of tyrosine hydroxylase‐immunopositive (TH‐ip) cells, approximately twice more than the LPS‐only treatment. α‐Bungarotoxin, an α7 nicotinic acetylcholine receptor subunit‐selective blocker, considerably blocked the inhibitory effects of nicotine on microglial activation and TH‐ip neuronal loss. Chronic nicotine pretreatment in rats showed that TH‐ip neuronal loss induced by LPS stimulation in the substantia nigra was dramatically decreased, which was clearly accompanied by a reduction in the formation of TNF‐α. The present study demonstrated that nicotine has a neuroprotective effect on dopaminergic neurons via an anti‐inflammatory mechanism mediated by the modulation of microglial activation. Along with various neuroprotective effects of nicotine, the anti‐inflammatory mechanism of nicotine could have a major therapeutic implication in the preventive treatment of PD.

Fluoxetine prevents MPTP-induced loss of dopaminergic neurons by inhibiting microglial activation

Parkinsons disease (PD) is characterized by degeneration of nigrostriatal dopaminergic (DA) neurons. Mice treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) exhibit microglial activation-induced oxidative stress and inflammation, and nigrostriatal DA neuronal damage, and thus serve as an experimental model of PD. Here, we report that fluoxetine, one of the most commonly prescribed antidepressants, prevents MPTP-induced degeneration of nigrostriatal DA neurons and increases striatal dopamine levels with the partial motor recovery. This was accompanied by inhibiting transient expression of proinflammatory cytokines and inducible nitric oxide synthase; and attenuating microglial NADPH oxidase activation, reactive oxygen species/reactive nitrogen species production, and consequent oxidative damage. Interestingly, fluoxetine was found to protect DA neuronal damage from 1-methyl-4-phenyl-pyridinium (MPP(+)) neurotoxicity in co-cultures of mesencephalic neurons and microglia but not in neuron-enriched mesencephalic cultures devoid of microglia. The present in vivo and in vitro findings show that fluoxetine may possess anti-inflammatory properties and inhibit glial activation-mediated oxidative stress. Therefore, we carefully propose that neuroprotection of fluoxetine might be associated with its anti-inflammatory properties and could be employed as novel therapeutic agents for PD and other disorders associated with neuroinflammation and microglia-derived oxidative damage.

Fluoxetine prevents LPS-induced degeneration of nigral dopaminergic neurons by inhibiting microglia-mediated oxidative stress.

Lipopolysaccharide (LPS)-induced microglial activation causes degeneration of nigral dopaminergic (DA) neurons. Here, we examined whether fluoxetine prevents LPS-induced degeneration of DA in the rat substantia nigra (SN) in vivo. Seven days after LPS injection into the SN, immunostaining for tyrosine hydroxylase (TH) revealed a significant loss of nigral DA neurons. Parallel activation of microglia (visualized by OX-42 and ED1 immunohistochemistry), production of reactive oxygen species (ROS) (assessed by hydroethidine histochemistry), and degeneration of nigral DA neurons were also observed in the SN. Western blot analyses and double-label immunohistochemistry showed an increase in the expression of inducible nitric oxide synthase (iNOS) within activated microglia. LPS also induced translocation of p67(phox), the cytosolic component of NADPH oxidase, to the membrane of SN microglia, indicating activation of NADPH oxidase. The LPS-induced loss of nigral DA neurons was partially inhibited by fluoxetine, and the observed neuroprotective effects were associated with fluoxetine-mediated suppression of microglial NADPH oxidase activation and iNOS upregulation, and decreased ROS generation and oxidative stress. These results suggest that fluoxetine and analogs thereof may be beneficial for the treatment of neurodegenerative diseases, such as PD, that are associated with microglia-derived oxidative damage.

Cannabinoids prevent lipopolysaccharide-induced neurodegeneration in the rat substantia nigra in vivo through inhibition of microglial activation and NADPH oxidase

We investigated the effects of synthetic cannabinoids, WIN55,212-2 and HU210, on LPS-injected rat substantia nigra in vivo. Intranigral injection of LPS resulted in a significant loss of nigral dopaminergic (DA) neurons, as determined by Nissl staining and TH immunohistochemistry. LPS-induced neurotoxicity was accompanied by microglial activation, as demonstrated by OX-42 immunohistochemistry. In parallel, Western blot analysis, ELISA assay and hydroethidine histochemistry revealed activation of NADPH oxidase, as demonstrated by increased translocation of the cytosolic proteins p47(phox) and p67(phox), generation of reactive oxygen species (ROS) and increased level of proinflammatory cytokines (TNF-α and IL-1β), where degeneration of nigral DA neurons was evident. Interestingly, WIN55,212-2 and HU210 increased the survival of nigral DA neurons at 7days post-LPS treatment. Consistent with these results, cannabinoids inhibited activation of NADPH oxidase, ROS production and production of proinflammatory cytokines in the rat SN. The present data suggest that cannabinoids may be beneficial for the treatment of neurodegenerative diseases, such as PD, that are associated with microglial activation.

Roles of Transient Receptor Potential Vanilloid Subtype 1 and Cannabinoid Type 1 Receptors in the Brain: Neuroprotection versus Neurotoxicity

Transient receptor potential vanilloid subtype 1 (TRPV1), also known as vanilloid receptor 1 (VR1), is a nonselective cation channel that is activated by a variety of ligands, such as exogenous capsaicin (CAP) or endogenous anandamide (AEA), as well as products of lipoxygenases. Cannabinoid type 1 (CB1) receptor belongs to the G protein-coupled receptor superfamily and is activated by cannabinoids such as AEA and exogenous Δ-9-tetrahydrocannabinol (THC). TRPV1 and CB1 receptors are widely expressed in the brain and play many significant roles in various brain regions; however, the issue of whether TRPV1 or CB1 receptors mediate neuroprotection or neurotoxicity remains controversial. Furthermore, functional crosstalk between these two receptors has been recently reported. It is therefore timely to review current knowledge regarding the functions of these two receptors and to consider new directions of investigation on their roles in the brain.

Interactions between CB1 receptors and TRPV1 channels mediated by 12-HPETE are cytotoxic to mesencephalic dopaminergic neurons

We recently proposed the existence of neurotoxic interactions between the cannabinoid type 1 (CB1) receptor and transient receptor potential vanilloid 1 (TRPV1) channels in rat mesencephalic cultures. This study seeks evidence for the mediator(s) and mechanisms underlying the neurotoxic interactions between CB1 receptors and TRPV1 in vitro and in vivo.

Neuro-protective effects of bee venom by suppression of neuroinflammatory responses in a mouse model of Parkinson's disease: Role of regulatory T cells

In the present study, we sought to determine whether bee venom (BV) promotes the survival of dopaminergic (DA) neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinsons disease (PD). Treatment with BV prevented degeneration of DA neurons in the substantia nigra (SN). This neuro-protective effect of BV was associated with microglial deactivation and reduction of CD4 T cell infiltration. Additionally, BV treatment significantly increased the proportion of CD4(+)CD25(+)Foxp3(+) Tregs in vivo and in vitro. The increased proportion of Tregs by BV treatment remained suppressive ex vivo. Interestingly, BV treatment did not prevent MPTP neurotoxicity in mice depleted of Tregs by anti-CD25 antibody injection. Therefore, our present studies suggest that modulation of peripheral immune tolerance by Treg may contribute to the neuroprotective effect of BV in the MPTP model of Parkinsons disease.

Prothrombin kringle-2 induces death of mesencephalic dopaminergic neurons in vivo and in vitro via microglial activation

We have shown that prothrombin kringle‐2 (pKr‐2), a domain of human prothrombin distinct from thrombin could activate cultured rat brain microglia in vitro. However, little is known whether pKr‐2‐induced microglial activation could cause neurotoxicity on dopaminergic (DA) neurons in vivo. To address this question, pKr‐2 was injected into the rat substantia nigra (SN). Tyrosine hydroxylase (TH) immunohistochemistry experiments demonstrate significant loss of DA neurons seven days after injection of pKr‐2. In parallel, pKr‐2‐activated microglia were detected in the SN with OX‐42 and OX‐6 immunohistochemistry. Reverse transcription PCR and double‐label immunohistochemistry revealed that activated microglia in vivo exhibit early and transient expression of inducible nitric oxide synthase (iNOS), cyclooxygenase‐2 (COX‐2) and several proinflammatory cytokines. The pKr‐2‐induced loss of SN DA neurons was partially inhibited by the NOS inhibitor NG‐nitro‐L‐arginine methyl ester hydrochloride, and the COX‐2 inhibitor DuP‐697. Extracellular signal‐regulated kinase 1/2, c‐Jun N‐terminal kinase and p38 mitogen‐activated protein kinase were activated in the SN as early as 1 hr after pKr‐2 injection, and localized within microglia. Inhibition of these kinases led to attenuation of mRNA expression of iNOS, COX‐2 and several proinflammatory cytokines, and rescue of DA neurons in the SN. Intriguingly, following treatment with pKr‐2 in vitro, neurotoxicity was detected exclusively in co‐cultures of mesencephalic neurons and microglia, but not microglia‐free neuron‐enriched mesencephalic cultures, indicating that microglia are required for pKr‐2 neurotoxicity. Our results strongly suggest that microglia activated by endogenous compound(s), such as pKr‐2, are implicated in the DA neuronal cell death in the SN.