RESPONSE OF POSTMITOTIC NEURONS TO X-IRRADIATION - IMPLICATIONS FOR THE ROLE OF DNA-DAMAGE IN NEURONAL APOPTOSIS

The molecular changes responsible for inducing neuronal apoptosis are unknown. Rat cortical neurons were treated with x-irradiation 7 d afte r isolation to test for the role of DNA damage in neuronal death. The response of neurons to x-irradiation was compared with that of astrocy tes that had been isolated 3 weeks earlier from newborn rats. At the t ime of irradiation, the neurons appeared well differentiated morpholog ically and were predominantly (90-95%) noncycling, based on flow cytom etric analysis. There was a similar, linear increase in DNA double-str and breaks with increasing radiation dose in neurons and astrocytes. H owever, whereas doses as low as 2 Gy induced typical apoptotic changes in neurons, including nuclear fragmentation and/or internucleosomal D NA fragmentation, doses as high as 32 Gy caused little or no apoptosis in astrocytes. Radiation-induced apoptosis of neurons started 4-8 hr after irradiation, was maximal at 12 hr, and was dependent on dose up to 16 Gy. It was prevented when cycloheximide, a protein synthesis inh ibitor, was added up to 6 hr after irradiation. In addition to their d istinct apoptotic response, neurons rejoined radiation-induced DNA dou ble-strand breaks more slowly than astrocytes. Treatment with benzamid e to inhibit ADP-ribosylation and strand break repair increased apopto sis; splitting the dose of radiation to allow increased time for DNA r epair decreased apoptosis. These data suggest that DNA damage may indu ce neuronal apoptosis, that the extent of damage may determine the deg ree of apoptosis induced, and that slow repair of damage may play a ro le in the susceptibility of neurons to apoptosis.