Astrocytes are more vulnerable than neurons to cellular Ca2+ overload induced by a mitochondrial toxin, 3-nitropropionic acid.

The differential effects of 3-nitropropionic acid on cultured neurons and astrocytes (of cortical and striatal origin) were examined by studying intracellular Ca2+ changes using imaging techniques with fura-2. The neurons and astrocytes whose intracellular Ca2+ concentration was recorded were identified later by immunocytochemical staining for microtubule-associated protein 2 and glial fibrillary acidic protein, respectively. 3-Nitropropionic acid (1.7 mM) irreversibly increased intracellular Ca2+ in astrocytes (27%) and, to a significantly smaller extent, in neurons (10%). The latency to onset of the intracellular Ca2+ increase was longer in neurons (45 min) than in astrocytes (29 min). Thus, a differential susceptibility of astrocytes and neurons was observed. The 3-nitropropionic acid-induced astrocytic and neuronal Ca2+ accumulations were both due to influx of Ca2+, as the increases were absent in Ca2+-free medium. An inhibitor of the Na+-Ca2+ exchanger (2',4'-dichlorobenzamil), greatly reduced the intracellular Ca2+ increase in astrocytes, but not in neurons. This indicates that the intracellular Ca2+ increase in astrocytes is primarily mediated by a reverse operation of the Na+-Ca2+ exchange system, whereas in neurons it is mediated by a different mechanism. In addition, we noted that astrocytic cell death occurred in 9% of cells at 60 min or more after the start of a 40 min perfusion with 3-nitropropionic acid, while only 4% of neurons died. In astrocytes, cell death was preceded by blebbing of the cell membrane, and by a sustained increase in intracellular Ca2+ followed by an abrupt further elevation occurring just before cellular collapse. The present results indicate that astrocytes are more vulnerable than neurons to 3-nitropropionic acid-induced cellular Ca2+ overload and toxicity, and hence support the hypothesis that, in part, 3-nitropropionic acid-induced neurotoxicity could be secondary to astrocytic cell death caused by Ca2+ overload.