© 2001 by The Society for Integrative and Comparative Biology
Serotonergic Neural System not only Activates Swimming but also Inhibits Competing Neural Centers in a Pteropod Mollusc1
1 Department of Biology, Arizona State University, Tempe, Arizona 85287-1501, and Friday Harbor Laboratories, 620 University Road, Friday Harbor, Washington 98250, USA
Initiation of a particular behavior requires not only activation of the neural center directly involved in its control but also inhibition of the neural networks controlling competing behaviors. In the pteropod mollusc, Clione limacina, many identified serotonergic neurons activate or modulate different elements of the swimming system resulting in the initiation or acceleration of the swimming behavior. Cerebral serotonergic neurons are described here, which produce excitatory inputs to the swimming system as well as inhibitory inputs to the neural centers that control competing behaviors. Whole-body withdrawal behavior is incompatible with swimming activity in Clione. The main characteristic of whole-body withdrawal is complete inhibition of swimming. Cerebral serotonergic neurons were found to produce a prominent inhibition of the pleural neurons that control whole-body withdrawal behavior. By inhibiting pleural withdrawal cells, serotonergic neurons eliminate its inhibitory influence on the swimming system and thus favor increased swimming speed. Serotonergic neurons also produce a prominent inhibition of the Pleural White Cell, which is presumably involved in reproductive or egg-laying behavior. Thus the serotonergic system directly activates swimming system and, at the same time, alters a variety of other neural systems preventing simultaneous initiation of incompatible behaviors.