Effects of serotonin on the generation of the motor program for swimming by the medicinal leech

Abstract

The authors have examined the effects of serotonin on the generation of the motor program for swimming by the medicinal leech. Hirudo medicinalis, using both intact animals and isolated nerve cords. In normal saline, isolated nerve cords rarely produced episodes of the motor program for swimming except in response to electrical stimulation of swim-initiating interneurons or segmental nerves; saline containing micromolar concentrations of serotonin caused isolated nerve cords to produce episodes of the swim motor program in the absence of electrical stimulation. Stimulation of serotonin-containing neurons also caused isolated nerve cords to produce the swim motor program but only if the volume of saline around the ganglia containing the stimulated neurons was small (less than 50 μl); under such conditions, serotonin was detected in the saline bathing the stimulated neurons. Serotonin also was found in blood from the sinus that contains the nerve cord of the leech. The concentrations of serotonin were correlated with the behavior of the animals whose blood was sampled; animals with the highest levels swam significantly more than did animals with low or intermediate levels; animals with the lowest levels of serotonin in their blood swam very little and were generally inactive. The concentrations of serotonin found in leech blood (10 to 100 nM) were similar to the lowest concentrations of serotonin that elicited swimming from isolated nerve cords. Experimentally elevating the concentration of serotonin in leech blood caused increased swimming activity. These observations suggest that serotonin is a modulator of neuronal control of swimming by the medicinal leech.