Modulatory actions of octopamine and serotonin on the contraction of buccal muscles in Rapana thomasiana—II. Inhibition of contraction in radula retractor

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1. 1. L-Glutamate and acetylcholine evoked contraction in the radula retractor of Rapana thomasiana and at high concentrations glutamate induced larger tension than acetylcholine. 2. 2. Twitch contraction was blocked by high-Mg 2+, depressed by curare but not potentiated by eserine. 3. 3. Octopamine and serotonin hyperpolarized muscle fibre membrane and depressed excitatory junctional pot potential. These amines also depressed twitch contraction and contraction by low concentration of glutamate. The contraction by low concentration of acetylcholine was depressed by serotonin but enhanced by octopamine. 4. 4. The extract of retractor relaxed catch tension in the Mytilus ABRM. Neither mersalyl nor haloperidol blocked the relaxation. 5. 5. In the retractor, L-glutamate may be an excitatory transmitter and octopamine and serotonin may be inhibitory transmitters.