| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
FJ
EXPRESS SUMMARY ARTICLE The Full-length version of this article is also available, published online January 20, 2004 as doi:10.1096/fj.03-0983fje. |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
,2
* Laboratory of Pharmacology and Chemistry, National Institute of Environmental Health Science, Research Triangle Park, North Carolina, USA;
Department of Neurology, First Clinical Hospital, and
Department of Physiology, Dalian Medical University, Dalian, China;
Neurochemistry Laboratory, National Center for Toxicological Research/FDA, Jefferson, Arkansas, USA; and
Department of Pharmacodynamics, College of Pharmacy, University of Florida, Gainesville, Florida, USA
2Correspondence: Department of Pharmacodynamics, College of Pharmacy, P. O. Box 100487 HSC, University of Florida, Gainesville, FL 32610, USA. E-mail: liu{at}cop.ufl.edu
SPECIFIC AIMS
This study determined the neuroprotective effect of dextromethorphan (DM) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson’s disease (PD). Using wild-type and gene knockout mice, the role of oxygen free radical producing NADPH oxidase in the DM-afforded neuroprotection was examined.
PRINCIPAL FINDINGS
1. DM protects substantia nigra pars compacta (SNpc) dopaminergic neurons against MPTP-induced degeneration
We used a subchronic dosing regiment of MPTP injections to induce lesions in the nigrostriatal dopaminergic pathway. C57BL/6J mice received daily subcutaneous (s.c.) injections of vehicle or MPTP (15 mg free base/kg body weight) for 6 consecutive days. Six or 21 days later, the mice were killed. We quantified the MPTP-induced loss of SNpc tyrosine hydroxylase-immunoreactive (TH-ir) neurons and the decrease in striatal catecholamine content. To determine the effect of DM on the MPTP-induced dopaminergic neuronal lesions, mice were given twice-daily s.c. injections of DM (10 mg/kg body weight) in the first 6 days and once a day thereafter. Mice were killed 6 days after the last MPTP injection. As shown in Fig. 1
, MPTP induced a significant loss of SNpc TH-ir neurons compared with vehicle (saline) -injected control mice. Administration of DM markedly reduced the MPTP-induced loss of SNpc TH-ir neurons (Fig. 1)
. No significant difference was observed in the number of SNpc TH-ir neurons between mice that received saline injections and those receiving injections of DM alone (Fig. 1)
. These results demonstrated that administration of DM rendered SNpc TH-ir neurons resistant to MPTP-induced degeneration.
|
2. DM inhibits MPTP-induced production of reactive oxygen species (ROS) in neuron-glia cultures
We have recently reported that DM is capable of inhibiting bacterial endotoxin lipopolysaccharide-induced rat microglial activation and their production of superoxide free radicals. Microglial activation and free radical generation have also been shown to play a role in MPTP-induced dopaminergic neurodegeneration. In cultures, MPTP-induced reactive microgliosis has been associated with increased release of superoxide and elevated levels of intracellular ROS. Therefore, we set out to determine whether DM had any effect on the MPTP-induced free radical generation in mouse neuron-glia cultures. First, we tested the effect of DM on superoxide production. Treatment of cultures with 1 µM DM almost completely inhibited the MPTP (1 µM) -induced superoxide production (data not shown). Second, measurement of the levels of intracellular ROS indicated that DM significantly reduced the MPTP-induced elevations of intracellular ROS (data not shown). Hence, these results demonstrated that DM decreased the MPTP-induced reactive activation of microglia and especially their production of superoxide.
3. DM is ineffective in affording neuroprotection in NADPH oxidase-deficient mice
The observation that DM effectively inhibited the MPTP-induced production of ROS prompted us to speculate that inhibition of ROS production was a major underlying mechanism of action for the observed neuroprotective activity. Since the primary machinery for the production of ROS in a variety of cells (macrophages, neutrophils, and microglia) is NADPH oxidase (also called phagocyte oxidase, or PHOX), we hypothesized that the NADPH oxidase activity was vital to the neuroprotective effect of DM. Indeed, mice lacking the functional NADPH oxidase (due to the deletion of the gp91 subunit of the enzyme complex) were significantly less sensitive to MPTP-induced loss of SNpc TH-ir neurons compared with wild-type control mice (Fig. 2
). More important, DM was ineffective in protecting the SNpc TH-ir neurons of NADPH oxidase-deficient mice from the MPTP-induced degeneration (Fig. 2)
. These results demonstrated that the neuroprotective effect of DM depended on the normal function of NADPH oxidase. Hence, NADPH oxidase appeared to be a critical mediator of DM’s neuroprotective activity.
|
CONCLUSIONS AND SIGNIFICANCE
In this study, we have demonstrated that DM significantly protected the SNpc dopaminergic neurons in the mouse MPTP PD model. The protective effect of DM was related to its ability to inhibit MPTP-induced ROS production in activated microglia. This conclusion was supported by the observation that DM abolished the MPTP-induced production of superoxide free radicals and significantly suppressed the MPTP-induced elevation in intracellular ROS. The neuroprotective effect of DM appeared to be closely related to NADPH oxidase, the primary enzymatic system in microglia for ROS production. Because of the particular vulnerability of dopaminergic neurons to oxidative insult, inhibition of NADPH oxidase-mediated ROS production may be a primary mechanism of action responsible for the neuroprotective effect of DM (Fig. 3
).
|
PD is a devastating movement disorder characterized by progressive degeneration of the nigrostriatal dopaminergic pathway. The etiology and the precise underlying mechanism of the progressive neurodegeneration remain poorly understood. Recent studies have demonstrated that the progressive dopaminergic neurodegeneration in PD patients and PD models induced by agents such as MPTP involves active participation of the brain’s resident immune cells, microglia. Hence, inhibition of microglial activation and production of proinflammatory and neurotoxic factors may be a highly effective strategy to slow down or even halt the neurodegenerative process. The ability of DM to inhibit ROS production in activated microglia and to protect dopaminergic neurons against MPTP-induced degeneration may be particularly relevant to this microglia-related neuroprotective strategy. DM, a dextrorotatory morphinan, has been widely used as a non-opioid antitussive agent with a track record of minimum side effects in short and long-term use in humans. Its newly discovered capacity to reduce microglial activation may pave a new path for its potential use in treating neurodegenerative diseases such as PD.
FOOTNOTES
1 To read the full text of this article, go to http://www.fasebj.org/cgi/doi/10.1096/fj.03-0983fje; 
This article has been cited by other articles:
|
|
 |
|
  K. Bedard and K.-H. Krause The NOX Family of ROS-Generating NADPH Oxidases: Physiology and Pathophysiology Physiol Rev, January 1, 2007; 87(1): 245 - 313. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 W. Zhang, E.-J. Shin, T. Wang, P. H. Lee, H. Pang, M.-B. Wie, W.-K. Kim, S.-J. Kim, W.-H. Huang, Y. Wang, et al. 3-Hydroxymorphinan, a metabolite of dextromethorphan, protects nigrostriatal pathway against MPTP-elicited damage both in vivo and in vitro FASEB J, December 1, 2006; 20(14): 2496 - 2511. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-L. Liu, Y.-H. Li, G.-Y. Shi, Y.-H. Chen, C.-W. Huang, J.-S. Hong, and H.-L. Wu A Novel Inhibitory Effect of Naloxone on Macrophage Activation and Atherosclerosis Formation in Mice J. Am. Coll. Cardiol., November 7, 2006; 48(9): 1871 - 1879. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. Zhan, F. Serrano, P. Fenik, R. Hsu, L. Kong, D. Pratico, E. Klann, and S. C. Veasey NADPH Oxidase Mediates Hypersomnolence and Brain Oxidative Injury in a Murine Model of Sleep Apnea Am. J. Respir. Crit. Care Med., October 1, 2005; 172(7): 921 - 929. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S.-H. Choi, D. Y. Lee, S. U. Kim, and B. K. Jin Thrombin-Induced Oxidative Stress Contributes to the Death of Hippocampal Neurons In Vivo: Role of Microglial NADPH Oxidase J. Neurosci., April 20, 2005; 25(16): 4082 - 4090. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
m). These perturbations lead to damages to the oxidative stress-sensitive dopaminergic neurons. Injured neurons in turn trigger a reactive activation of microglia and astroglia, through a mechanism not yet defined. Activated glia produce a variety of proinflammatory and neurotoxic factors, including nitric oxide (NO) and superoxide (O2•-), which can form the more toxic intermediate peroxynitrite (ONOO•). The barrage of these glia-originated toxic factors eventually leads to the demise of dopaminergic neurons. Through inhibition of the NADPH oxidase-mediated production of reactive oxygen species as well as the production of NO, dextromethorphan (DM) protects dopaminergic neurons against MPTP neurotoxicity.