Action of 5-Hydroxytryptamine on Isolated Spinal Cord of Bullfrogs

Abstract

Abstract-Slow depolarizations of dorsal root nerve terminals and motoneurons, which were produced by 5-hydroxytryptamine (5-HT) applied directly to isolated bullfrog spinal cords, were recorded by the sucrose-gap method. These depolarizations were eliminated in the Ca-deficient Ringer's solution containing Mg, suggesting that these 5-HT depolarizations were not caused by a direct action of 5-HT on dorsal root nerve terminals or motoneurons but rather by actions of transmitters released from inter neurons. Indeed, mephenesin, which is 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 was not considered to be GABA as the 5-HT depolarization was not antagonized by picrotoxin. It would thus appear that 5-HT stimulates interneurons in the amphibian spinal cord and un known transmitters released from these interneurons depolarize the dorsal root nerve terminals. According to the present experiments, which were carried out in order to clarify the site of action of 5-HT on the amphibian spinal cord, 5-HT stimulated interneurons in the amphibian spinal cord and unknown transmitters released from these interneurons de polarized the dorsal root nerve terminals and the motoneurons. MATERIALS AND METHODSThe spinal cord of bullfrogs (Rana catesbeiana) was carefully isolated. The hemisected cord with the eighth and ninth lumbar dorsal and ventral roots intact was then placed in a trough ( Fig. 1) and continuously perfused with oxygenated Ringer's solution at 16-18'C.

Effects of Excitatory Transmitter Candidates on Frog Spinal Motor Function

“…In Call-free, Mg2+ (9 mM)-Ringer's solution, however, the depolarizing effect of acetylcholine could not be observed as has been shown in the procaine pretreated preparation (9). Serotonin had no depolarizing action on motoneurons as was also reported by Shirasawa and Koketsu (10). Depolarization by L-glutamate could be seen even in a Ca2+-free medium, indicating that the drug has a direct effect on motoneuron as reported by Constanti and Nistri (I1).…”

The actions of serotonin on frog primary afferent terminals and cell bodies

“…Since TTX or Mg 2+ treatments minimize interneuronal influences on primary afferents, this 5-HT induced primary afferent depolarization (PAD) was attributed to serotonin's direct action on afferent nerve terminals. In contrast, Shirasawa and Koketsu (1977) reported Mg 2+ blockade of the depolarizing action of 5-HT on anuran dorsal roots, an observation suggesting an interneuronal mediation of this response. Therefore, we have re-examined serotonin's purported effect on anuran primary afferents in an attempt to more accurately define its site of action.…”

“…Further studies are required to identify which of these possibilities occur. In the meantime, it must be realized that the observation of Shirasawa and Koketsu (1977), that Mg 2+ blocks 5-HT depolarizations of primary afferents, should not simply be interpreted as indicating an interneuronal site of serotonin's action. It may be that both Mg 2+ and Mn 2+ block the direct 5-HT depolarization of afferent terminals.…”