SEROTONIN PROTECTS C6 GLIOMA-CELLS FROM GLUTAMATE TOXICITY

It was recently shown that addition of L-glutamate in millimolar amoun ts to a culture of C6 glioma cells induced cell death within 24 h. The mechanism for glutamate toxicity in the C6 glioma cells is linked to the inhibition of cystine uptake, leading to glutathione depletion thr ough the cystine/glutamate antiporter (X (c) over bar) system. In the present study, neurotransmitters, whose receptors were localized on th e glioma (glial) cells, were evaluated for their ability to protect C6 cells from glutamate toxicity through this amino acid antiporter. Amo ng them, only 100 mu M serotonin suppressed cell death by glutamate in a constant co-existence culture. The suppressive dose of serotonin wa s relatively low and the half-effective dose was about 35 mu M. 8-Hydr oxy-2-(DL-n-propylamino)tetralin, a specific serotonin(1A) agonist, sh owed a comparable suppression to glutamate damage, while 1-(2,5-dimeth oxy-4-iodophenyl)-2-aminopropane, a specific serotonin(2) agonist, and quipazine, a non-selective serotonin(1B) agonist, did not suppress it . Furthermore, propranolol and pindolol significantly blocked the sero tonin effect, but spiperone, mianserin and ketanserin did not block it . These results strongly indicate that this protective action of serot onin to glutamate toxicity was receptor (serotonin(1A)) mediated. Sero tonin did not protect the C6 cells from glutathione depletion by gluta mate. The cellular level of glutathione was depleted even under the co -existence of serotonin and glutamate. Serotonin induced a significant inhibition of lipid peroxide accumulation in the C6 glioma cells to g lutamate exposure and the low rate of lipid peroxide accumulation was controlled. These results suggest that the serotonin action is not due to restoration of cellular glutathione, but due to suppression of som e generating processes of lipid peroxide accumulation. This protective function of serotonin against glutamate toxicity could not be seen in the primary cortical and cerebellar neuronal culture to glutamate tox icity, which is thought to be triggered through the N-methyl-D-asparta te receptor. This novel function of serotonin on the glioma (glial) ce ll death by glutamate was characterized pharmacologically and its cell ular mechanism for protection was discussed in comparison with that of a powerful antioxidant, vitamin E.