Endogenous mechanisms of neuroprotection: role of zinc, copper, and carnosine

doi:10.1016/S0006-8993(99)02215-5

Brain Research

Volume 852, Issue 1, 3 January 2000, Pages 56-61

https://doi.org/10.1016/S0006-8993(99)02215-5 Get rights and content

Abstract

Zinc and copper are endogenous transition metals that can be synaptically released during neuronal activity. Synaptically released zinc and copper probably function to modulate neuronal excitability under normal conditions. However, zinc and copper also can be neurotoxic, and it has been proposed that they may contribute to the neuropathology associated with a variety of conditions, such as Alzheimer's disease, stroke, and seizures. Recently, we demonstrated that carnosine, a dipeptide expressed in glial cells throughout the brain as well as in neuronal pathways of the visual and olfactory systems, can modulate the effects of zinc and copper on neuronal excitability. This result led us to hypothesize that carnosine may modulate the neurotoxic effects of zinc and copper as well. Our results demonstrate that carnosine can rescue neurons from zinc- and copper-mediated neurotoxicity and suggest that one function of carnosine may be as an endogenous neuroprotective agent.

Introduction

Zinc and copper are neuroactive substances that are present in many regions of the CNS; they are found in particularly high concentrations in the olfactory bulb (OB) and hippocampus 16, 21. Localized to synaptic terminals, these metals can be released by membrane depolarization and can reach extracellular/synaptic concentrations of 100–300 μM 2, 19, 22, 23. Substantial evidence supports their role as modulators of both excitatory 18, 39, 40, 42, 43 and inhibitory 28, 39, 40 synaptic transmission.

However, zinc and copper have also been implicated in diseases with neuropathological components, including Alzheimer's disease, Menkes disease, Wilson's disease, Pick's disease, amyotrophic lateral sclerosis–parkinsonism–dementia, stroke, and seizures 12, 14, 15, 20, 24, 29, 34, 41. Although the basis for the pathology induced by these transition metals remains under investigation, several mechanisms have been proposed. Some investigators have proposed that although these substances may not initiate damage, changes in the levels of these substances may contribute to damage already underway [14]. Other investigators have proposed that ischemic damage, such as stroke, may cause excessive release of zinc (and/or copper) from neurons which results in neurotoxicity (e.g., Ref. [24]). Furthermore, the laboratory of Koh et al. [24] has demonstrated that such neuronal degeneration can be protected against in vivo by the application of CaEDTA (a metal chelator) prior to the transient ischemic insult.

In the present report, we examine the potential neuroprotective role of the endogenous chelator/antioxidant, carnosine (Beta-alanyl-l-histidine). This dipeptide is expressed in glial cells throughout the brain as well as in neuronal pathways of the visual and olfactory systems 5, 31, 36. In the olfactory system, carnosine is expressed in high concentrations (2–5 mM) in the olfactory sensory neurons and their terminals in the glomerular layer of the OB 17, 35. Previous results from our lab demonstrating that carnosine can modulate the effects of zinc and copper on amino acid receptors and synaptic transmission [39] led us to hypothesize that carnosine may provide protection from zinc- and copper-mediated neurotoxicity. The results from the present experiments suggest that carnosine does protect cultured neurons from the toxic effects of zinc and copper. These results may imply a neuroprotective function of carnosine in vivo.

Section snippets

Materials and methods

Dissociated OB neurons, from E18 to P3 Sprague–Dawley rat pups, were plated on a confluent layer of previously prepared OB astrocytes using standard tissue culture methods [38].

Toxicity experiments were conducted after the neurons were in culture for 2–18 days. Neuronal counts were made at randomly marked portions of 35 mm culture dishes before the application of metal, carnosine, or CaEDTA. Exposure of cells to zinc or copper (as a chloride salt), or an agent combination consisting of either

Neurotoxic effects of zinc and copper

As shown in Fig. 1A, application of either 10–100 μM zinc or copper for 1 h was associated with neuronal death, even at concentrations approaching an order of magnitude below previously reported estimates of extracellular or synaptic concentrations of these transition metals 2, 23. Differences between the control and all of the experimental groups were statistically significant (p<0.001).

The effects of concentration of zinc or copper on the rate of cell death

The rate of cell death increased with increases in the concentration of zinc or copper to which cells were

Discussion

Our finding that zinc is neurotoxic to cultured OB neurons is consistent with the results of previous experiments examining the effects of zinc on hippocampal and cortical neurons. Furthermore, our results extend these observations by demonstrating that short-term (1 h) exposure to copper also is neurotoxic, and that both zinc and copper can be neurotoxic at concentrations of 10–30 μM. This is significant as these concentrations are an order of magnitude less than those that have been proposed

Conclusions

The present results indicate that much lower concentrations of zinc and copper than described in most earlier work can be neurotoxic. The contrast in these results suggests that exposure duration is a critical parameter in determining toxicity. These results also suggest that sufficient endogenous buffering must normally be present to prevent even synaptically released concentrations from causing neurotoxicity. Furthermore, these findings may suggest the need to combat metal-mediated toxicity,

Acknowledgements

The authors thank Dr. Joseph Travis for assistance with the statistical analyses. This work has been supported in part by the National Institute for Deafness and other Communicative Disorders and the National Institutes of Health.

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Research reportEndogenous mechanisms of neuroprotection: role of zinc, copper, and carnosine

Abstract

Introduction

Section snippets

Materials and methods

Neurotoxic effects of zinc and copper

The effects of concentration of zinc or copper on the rate of cell death

Discussion

Conclusions

Acknowledgements

Chem. Biol. Interact.

J. Biol. Chem.

Neuroscience

Neuroscience

J. Biol. Chem.

Neurobiol. Dis.

Biochim. Biophys. Acta

Brain Research

Int. Rev. Neurobiol.

Brain Res.

Neurosci Lett.

Brain Res.

J. Neurol. Sci.

Biomed. Pharmacother.

Biochim. Biophys. Acta

Neurosci. Lett.

Toxicology

Brain Res.

Neuron

Release of endogenous Zn2+ from brain tissue during activity

Nature

The galvanization of biology: a growing appreciation for the roles of zinc

Science

Biochemical and physiological evidence that carnosine is an endogenous neuroprotector against free radicals

Cell Mol. Neurobiol.

Research report
Endogenous mechanisms of neuroprotection: role of zinc, copper, and carnosine

Release of endogenous Zn²⁺ from brain tissue during activity