FIGURE 9.

Model showing how various inputs might modulate Melibe feeding, locomotion, and overall arousal. Open circles are excitatory, closed circles are inhibitory. During the day, which is their quiescent period, 5-HT levels are reduced and light both inhibits active behaviors and reduces the expression of several clock genes. With the onset of darkness, clock genes are upregulated and 5-HT levels rise, leading to increased feeding, locomotion, and overall arousal. 5-HT may also have effects on clock gene expression (gray circle), perhaps serving to phase advance the circadian clock. When food is ingested it causes the stomach to distend, which is communicated to the NS by the PNs that innervate the stomach. This input reduces the motivation to feed, 5-HT levels, and locomotion. In the sea slug Melibe leonina the posterior nerves communicate stomach distention to inhibit feeding and modify oral hood movements.