3-Nitropropionic acid exacerbates N-methyl- d-aspartate toxicity in striatal culture by multiple mechanisms

Abstract

We examined the effects of 3-nitropropionic acid-induced succinate dehydrogenase inhibition on neuronal ATP content, N-methyl- d-aspartate-induced neuronal death, resting membrane potential, and N-methyl- d-aspartate-induced changes in cytosolic calcium concentration ([Ca 2+] c) in cultured rat striatal neurons. Exposure of cultures to 3 mM 3-nitropropionic acid for 3 h did not cause overt toxicity, but reduced ATP content by 35%. Treatment with 3-nitropropionic, or removal of Mg 2+ from the medium, enhanced subsequent N-methyl- d-aspartate toxicity, reducing the LC 50 from 250 μM to 12 μM or 30 μM, respectively. Even after Mg 2+ removal, enhancement of N-methyl- d-aspartate toxicity by 3-nitropropionic acid remained pronounced, with the LC 50 further decreasing to 3 μM. The mean resting membrane potential of neurons treated with 3-nitropropionic acid was −37 mV, while that in control neurons was −61 mV. Treatment with 3-nitropropionic did not affect baseline [Ca 2+] c as determined by fura-2 microfluorimetry. N-methyl- d-aspartate (30 μM) caused a rapid rise in [Ca 2+] c, the initial magnitude of which was not affected by 3-nitropropionic acid. However, after a 1-h treatment, [Ca 2+] c was dramatically higher in 3-nitropropionic acid-treated neurons. This increased calcium load was washed out slowly and only partially, although calcium in control neurons washed out rapidly and almost completely. These results suggest that in striatal neurons, the enhancement of N-methyl- d-aspartate toxicity caused by succinate dehydrogenase inhibition may be due to synergism between partial relief of the Mg 2+ blockade of the N-methyl- d-aspartate receptor and other mechanisms, including disruption of neuronal calcium regulation. This synergism may be relevant to the neuronal death observed in neurodegenerative disorders.