Action of 5-Hydroxytryptamine on Isolated Spinal Cord of Bullfrogs

Abstract

Slow depolarizations of dorsal root nerve terminals and motoneurons produced by 5-hydroxytryptamine (5-HT) applied directly to isolated bullfrog spinal cords, were recorded by sucrose-gap method. Depolarizations were eliminated in the Ca-deficient Ringer''s solution containing Mg. 5-HT depolarizations were apparently caused by actions of transmitters released from interneurons. Mephenesin, a selective blocker of polysynaptic transmission in the spinal cord, inhibited more markedly the 5-HT depolarization than the L-glutamate or GABA depolarization. The transmitter directly responsible for the generation of the 5-HT depolarization of dorsal root nerve terminals apparently was not GABA. 5-HT depolarization was not antagonized by picrotoxin. 5-HT stimulates interneurons in the spinal cord. Unknown transmitters released from these interneurons depolarized the dorsal root nerve terminals.