Transmitter-dependent switching of respiratory interneurons to the locomotor rhythm in the pulmonate mollusk Lymnaea.

Cell RPeD1 of the pond snail Lymnaea stagnalis is a dopaminergic neuron known to be involved in generating the respiratory rhythm. This report describes conditions in which RPeD1 follows the rhythm of another central generator, identified as the generator of muscular locomotion. This rhythmic activity of RPeD1 developed in isolated CNS preparations treated with 0.05 mM serotonin (5-hydroxytryptamine, 5-HT) or 0.1 mM 5-hydroxytryptophan (5-HTP). It was coordinated by cyclic synaptic currents with the activities of other pedal neurons and with trains of spikes in the motor (pedal and columellar) nerves. Only the pedal ganglia (both of them) retained the ability to generate 5-HT-dependent cyclic activity after complete isolation, indicating a pedal localization and paired nature for the central generator. In intact animals, injection of 100 microg/g of 5-HTP induced coordinated cyclic movements of the foot and shell which had similar periods and resembled movements during terrestrial (non-ciliary) locomotion. These results, along with the known role of dopamine in generating the respiratory pattern, show that switching of RPeD1 from one central rhythm to another is a transmitter-dependent phenomenon.