RADIATION-INDUCED APOPTOSIS IN DORSAL-ROOT GANGLION NEURONS

Ionizing radiation (IR) results in apoptosis in a number of actively p roliferating or immature cell types. The effect of IR on rat dorsal ro ot ganglion (DRG) neurons was examined in dissociated cell cultures. A fter exposure to LR, embryonic DRG neurons, established in cell cultur e for six days, underwent cell death in a manner that was dose-depende nt, requiring a minimum of 8 to 16 Gy Twenty-live per cent cell loss o ccurred in embryonic day 15 (E-15) neurons, grown in cell culture for 6 days ('immature'), and then treated with 24 Gy IR. In contrast, only 2% cell loss occurred in E-15 neurons maintained in culture for 21 da ys ('mature') and then treated with 24 Gy IR Staining with a fluoresce nt DNA-binding dye demonstrated dumping of the nuclear chromatin typic al of apoptosis. Initiation of the apoptosis occurred within 24 h afte r exposure to IR. Apoptosis was prevented by inhibition of protein syn thesis with cycloheximide-Apoptosis induced by IR occurred more freque ntly in immature than in mature neurons. Immature DRG neurons have a l ower concentration of intracellular calcium ([Ca2+](i)) than mature ne urons. Elevation of [Ca2+](i) by exposure to a high extracellular pota ssium ion concentration (35 mu M) depolarizes the cell membrane with a resultant influx of calcium ions. The activation of programmed cell d eath after nerve growth factor (NGF) withdrawal is inversely correlate d with [Ca2+](i) in immature DRG neurons. When treated with high extra cellular potassium, these immature neurons were resistant to IR exposu re in a manner similar to that observed in mature neurons. These data suggest that [Ca2+](i) modulates the apoptotic response of neurons aft er exposure to LR in a similar manner to that proposed by the ''Ca2+ s etpoint hypothesis'' for control of NGF withdrawal-induced apoptosis.