PIRTLE, T.J.*; SATTERLIE, R.A.; Arizona State University; Arizona State University: Modulation of Swimming Speed in Clione Limacina

An animal must modify its behavior to a changing environment. Neuromodulatory mechanisms intrinsic to the nervous system shape the cellular properties of neurons to ultimately adjust behavior. Modulation of neurons comprising the swim system of the pteropod mollusk, Clione limacina, illustrates some of the mechanisms of neuromodulation. Swimming in Clione is usually characterized as an unvarying slow upward movement. However, certain conditions, will initiate fast swimming in Clione. Centrally, serotonin mediates the swim speed changes observed in Clione by affecting the cellular properties of swim interneurons through the modulation of specific ionic currents. Here we report that serotonin modulates an I_h-like current characteristic of swim interneurons to increase the cycle frequency of swimming. Application of serotonin enhances the sag potential mediated by I_h. Furthermore, the effects of the I_h antagonist, ZD7288, when added to the Clione preparation in the presence of serotonin significantly reduced swim interneuron cycle frequency suggesting a role of I_h in the generation of fast swimming in Clione. Serotonin also modulates an I_A-like current and an I_K(Ca)-like current of swim interneurons to cause an increase in swim speed. Application of 4-aminopyridine (a blocker of I_A) suggests that reduction in the I_A-like current decreases the latency to peak postinhibitory rebound. Application of apamin (a blocker of I_K(Ca)) in the presence of serotonin inhibits spike narrowing in swim interneurons and subsequently decreases swim cycle frequency.