Modulation of neuromuscular transmission by serotonin in the molluscan radular muscles: Involvement of cyclic nucleotides

Abstract

1. The mechanisms of modulatory action of serotonin in neuromuscular transmission in the radular muscles of a mollusc, Rapana thomasiana, were investigated using a sucrose gap method. 2. In the radular protractor, one action of serotonin, the enhancement of the electrical and mechanical responses to electrical stimulations, was blocked by methysergide, whereas the other action of serotonin, the hyperpolarization of muscle membrane, was not blocked. 3. In the radular retractor, the depression of responses to electrical stimulation was blocked by methysergide but the hyperpolarization was not. 4. Dibutyryl- cAMP enhanced the responses to electrical stimulations in the protractor and depressed them in the retractor, similar to the actions of serotonin. 5. Dibutyryl- cGMP also appeared to enhance the responses in the protractor, while the responses in the retractor were quite variable. 6. In the protractor, the responses to electrical stimulations were enhanced by theophylline and the enhancement was greater when both theophylline and serotonin were applied to the muscle. 7. In the retractor, IBMX depressed the responses and the application of IBMX together with serotonin gave rise to further depression of the responses. 8. In spontaneously active muscle, cyclic nucleotide derivatives or phosphodiesterase inhibitors stopped the firing and hyperpolarized the membrane by a few millivolts, although they produced no appreciable change in membrane potential of the resting muscle. 9. It is assumed that these drugs hyperpolarized the membrane of the firing muscle by activating the electrogenic Na pump. which was inactivated in the firing muscle, although usually activated in the resting muscle. 10. A schematic model illustrating the action of serotonin in the radular protractor is presented.