Effects of irradiation of Aplysia pacemaker neurons with 20-MeV electrons

Aplysia pacemaker neurons are excited by irradiation with 20-MeV electrons. The response is an increase in discharge frequency occurring immediately after exposure and decaying within a few seconds to minutes except at very high exposures. The threshold is on the order of 1000 rad, and cell inactivation occurs acutely only at doses on the order of 20,000 rad. Within these limits the excitatory effect is more or less linear with dose. The acute effect is not associated with dramatic resistance changes, although the resulting depolarization indicates an increase in Na+ permeability. Synaptic transmission in this preparation does not appear to be more sensitive than impulse propagation. At very high doses, spike generation is blocked. Neurons recorded for a number of hours following irradiation show a hyperpolarization prior to final depolarization, which suggests that one terminal event may be an accumulation of intracellular Ca2+ leading to increased K+ conductance. These studies confirm and extend previous observations on the relative radioresistance of Aplysia neurons. With respect to mammalian studies on nervous system susceptibility to high doses of radiation, no events were found which correlate in time with early transient incapacitation. However, the depressed excitability occurring several hours after exposure may correlate with the occurrence of death due to the central nervous system syndrome which is seen in higher aminals