The dual role of serotonin in leech swimming

The swimming behavior of the leech depends upon the level of serotonin in its blood stream: the higher the level, the more likely the leech is to swim. Serotonin is contained in Retzius cells, segmentally paired neurons, which release enough of this substance to affect its blood level and thus the swimming behavior of the animal. Retzius cells appear not to have synaptic effects on swim-related neurons. Pairs of other serotonin-containing neurons (cells 21 and 61) are also present in each ganglion. Stimulation of these cells can initiate swimming within seconds. In contrast to Retzius cells, neurons 21 and 61 make synaptic connections onto swim-related neurons; for example, cell 61 makes what appears to be a conductance decrease excitatory synapse onto the pattern-generating interneuron cell 208. These results suggest that the two classes of serotonin-containing neurons control the swimming behavior of the leech through two different mechanisms: Retzius cells exert a neurohormonal action whereas cells 21 and 61 act synaptically.