PROTECTION BY DIPHENYLIODONIUM AGAINST GLUTAMATE NEUROTOXICITY DUE TOBLOCKING OF N-METHYL-D-ASPARTATE RECEPTORS

The protective effect of diphenyliodonium, known as an inhibitor of fl avin enzymes including nitric oxide synthases, was examined against th e neurotoxicity of excitatory amino acids on cultured spinal neurons o f the rat. Diphenyliodonium reduced the neuronal damage induced by 15- min exposure to glutamate or N-methyl-D-aspartate in a dose-dependent manner: half effective concentrations (EC(50)) were about 3 mu M For b oth. Protection was only observed when diphenyliodonium was added into the exposure medium: Diphenyliodonium showed no effect on the toxicit y induced by 24 h exposure to non-N-methyl-D-aspartate receptor agonis ts. Using a microfluorometry technique with Fura 2, we observed that d iphenyliodonium reversibly inhibited the N-methyl-D-aspartate-evoked i ntracellular Ca2+ elevation. The amount of Ca-45(2+) influx induced by N-methyl-D-aspartate was also inhibited by diphenyliodonium in a dose -dependent manner; EC(50) was about 3 mu M. Furthermore, we examined t he effect of diphenyliodonium on an opening activity of the N-methyl-D -aspartate receptors estimated by binding of dizocilpine maleate to me mbrane fractions from whole brain of adult rat and from cultured spina l neurons. Diphenyliodonium inhibited the binding of dizocilpine malea te dose-dependently; EC(50) was 5-8 mu M. These results suggest that d iphenyliodonium is a new antagonist to the N-methyl-D-aspartate recept ors and that diphenyliodonium protects neurons against glutamate toxic ity due to a direct blocking of the Ca2+ influx. This conclusion is su pported by the similarity of the stereochemical structures predicted b y computer between diphenyliodonium and dizocilpine maleate. Copyright (C) 1996 IBRO.