Antagonistic responses of the radular protractor and retractor to the same putative neurotransmitters.

1. Mechanisms of antagonistic responses of the radular protractor and the retractor to the same neurotransmitters, octopamine and serotonin, were investigated in a mollusc, Rapana thomasiana. 2. Hyperpolarization induced by octopamine or serotonin was found to depend mainly on the increase in Cl- conductance of the membrane in both the radular protractor and the retractor. 3. In the protractor, the enhancement of acetylcholine depolarization during the application of octopamine or serotonin was exhibited even when the membrane potential was varied by current application. Spontaneous firing was blocked by these amines, but larger spikes with higher frequency were elicited on giving a depolarizing current above threshold. 4. In the retractor, the inhibition of glutamate depolarization during the application of octopamine or serotonin was observed irrespective of membrane potential. Spontaneous spikes were stopped by these amines but reappeared on depolarization above threshold. Reappeared spikes during serotonin application were smaller than control spikes, while those during octopamine application were about the same as or larger than control ones. 5. The modes of actions of octopamine and serotonin on both muscles were considered.