The excitotoxic hypothesis of neurodegeneration has stimulated much interest in the possibility of using compounds that will block excitotoxic processes to treat neurologic disorders. Riluzole is a neuroprotective drug that blocks glutamatergic neurotransmission in the CNS. Riluzole inhibits the release of glutamic acid from cultured neurons, from brain slices, and from corticostriatal neurons in vivo. It is thought these effects may be partly due to inactivation of voltage-dependent sodium channels on glutamatergic nerve terminals, as well as activation of a G-protein-dependent signal transduction process. Riluzole also blocks some of the postsynaptic effects of glutamic acid by noncompetitive blockade of N-methyl-D-aspartate (NMDA) receptors. In vivo, riluzole has neuroprotective, anticonvulsant, and sedative properties. In a rodent model of transient global cerebral ischemia, a complete suppression of the ischemia-evoked surge in glutamic acid release has been observed. In vitro, riluzole protects cultured neurons from anoxic damage, from the toxic effects of glutamic-acid-uptake inhibitors, and from the toxic factor in the CSF of patients with amyotrophic lateral sclerosis.
Familial amyotrophic lateral sclerosis (FALS) has been linked in some families to dominant mutations of the SOD1 gene encoding Cu,Zn superoxide dismutase (Cu,ZnSOD). We have used a transgenic model of FALS based on expression of mutant human Cu,ZnSOD to explore the etiology and therapy of the genetic disease. Expression of mutant, but not wild-type, human Cu,ZnSOD in mice places the brain and spinal cord under oxidative stress. This causes depletion of vitamin E, rather than the typical age-dependent increase in vitamin E content as occurs in nontransgenic mice and in mice expressing wild-type human Cu,ZnSOD. Dietary supplementation with vitamin E delays onset of clinical disease and slows progression in the transgenic model but does not prolong survival. In contrast, two putative inhibitors of the glutamatergic system, riluzole and gabapentin, prolong survival. However, riluzole did not delay disease onset. Thus, there was clear separation of effects on onset, progression, and survival by the three therapeutics tested. This suggests the hypothesis that oxidative damage produced by the expression of mutant Cu,ZnSOD causes slow or weak excitotoxicity that can be inhibited in part by alerting glutamate release or biosynthesis presynaptically.
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