Acute Radiation Effects on Synaptic Activity in the Mammalian Spinal Cord

Abstract

A monosynaptic test reflex was studied in the spinal cord of the cat after local exposures to 0.6 to 12 Kr of x rays. Records were obtained from S1 ventral roots and with intraor extracellular electrodes. The facilitatory and inhibitory effects of conditioning shocks given at the L7 and L6 dorsal roots on the test reflex in the S1 segment were analyzed by changing time intervals between conditioning and test shocks. Immediately after exposure to 3 Kr the facilitation observed with a latency shorter than 1.0 msec was strongly enhanced. However, with longer latencies the normal facilitation was converted into depression. Larger doses (10 to 12 Kr) resuited in less short- latency facilitation, but the response was still larger than in controls. Control preparations showed no inhibition with latencies shorter than 0.8 msec. However, after a dose of 3 Kr inhibitory conditioning shocks with very short latencies produced a marked facilitation. This paradoxical effect gradually diminishes with higher radiation doses. On the other hand, inhibitory effects with latencies longer than 1.5 msec appear to be entirely unchanged even with doses of 12 Kr. It is suggested that the enhancement of short-latency facilitation and the paradoxical facilitation are due to directmore » stimulatory actions of radiation on excitatory synapses. However, inhibitory synaptic activity is not directly affected by doses of 0.6 to 12 Kr. Polysynaptic reflex activity increases with higher x ray doses and may remain enhanced for several months. This increased activity is thought to be due to an integrated direct effect of radiation on individual excitatory synapses. By means of microelectrode techniques, excitatory, and inhibitory postsynaptic potentials (EPSP and IPSP) were obtained from within single motoneurons. Immediately after exposure to 600 r both polysynaptic and monosynaptic EPSP's are appreciably enhanced. With larger doses the polysynaptic EPSP appears to be increased even more than the monosynaptic EPSP. Under these conditions the prolonged IPSP is gradually masked by the EPSP as total dose becomes greater. These results are discussed.« less