Gliotoxic actions of excitatory amino acids

Cultures of neonatal Type I astrocytes of the rat were exposed to a series of excitatory amino acid analogs to identify those compounds that were gliotoxic. In addition to L-alpha-aminoadipate, a previously identified gliotoxin, L-homocysteate, L-serine-O-sulfate, L-alpha-amino-4-phosphonobutyrate and L-alpha-amino-3-phosphono-propionate were also found to induce a sequence of degenerative events that led to the lysis of the astrocytes. Cellular injury was assessed by quantifying the activity of lactate dehydrogenase present in the surviving astrocytes. Prior to lysis, the cells went through a succession of distinctive morphological changes, the most prominent of which involved nuclear alterations. The nuclei appeared swollen, contained "pale" or "watery" nucleoplasm and exhibited a very prominent nuclear membrane and obvious nucleoli. These astrocytes appeared quite similar in appearance to the Alzheimer's Type II astrocytes, principally associated with the pathology of hepatic encephalopathy. The nuclear anomalies, which are thought to be indicative of cellular damage and compromised function, were also produced by the endogenous transmitters L-glutamate and L-aspartate, although with time, the affected astrocytes appeared to recover and return to normal morphology, without lyzing. These findings suggest that excessive levels of excitatory amino acids may induce cellular damage to astrocytes, as well as neurons. Once damaged, the resulting reductions in astrocyte function may further contribute to CNS losses and the overall pathology attributed to the excitatory amino acids.