CONTROL OF LOCOMOTION IN THE MARINE MOLLUSK CLIONE-LIMACINA .11. EFFECTS OF SEROTONIN

The locomotor activity in the marine mollusc Clione limacina has been found to be strongly excited by serotonergic mechanisms. In the presen t study putative serotonergic cerebropedal neurons were recorded simul taneously with pedal locomotor motoneurons and interneurons. Stimulati on of serotonergic neurons produced acceleration of the locomotor rhyt hm and strengthening of motoneuron discharges. These effects were acco mpanied by depolarization of motoneurons, while depolarization of the generator interneurons was considerably lower (if it occurred at all). Effects of serotonin application on isolated locomotor and non-locomo tor pedal neurons were studied. Serotonin (5x10(-7) to 1x10(-6) M) aff ected most pedal neurons. All locomotor neurons were excited by seroto nin. This suggests that serotonergic command neurons exert direct infl uence on locomotor neurons. Effects of serotonin on nonlocomotor neuro ns were diverse, most neurons being inhibited by serotonin. Some effec ts of serotonin on locomotor neurons could not be reproduced by neuron depolarization. This suggests that, along with depolarization, seroto nin modulates voltage-sensitive membrane properties of the neurons. As a result, serotonin promotes the endogenous rhythmical activity in ne urons of the C. limacina locomotor central pattern generator.