Higher-order control of the feeding network in Lymnaea.

A novel excitatory higher-order input (named Input-HO) to the feeding network of the pond snail, Lymnaea stagnalis, as well as a reliable procedure that makes it possible to initiate and maintain the Input-HO, are described. The Input-HO is primarily targeted to two bilaterally symmetrical pairs of extrabuccal projecting interneurons, the serotonergic Cerebral Giant Cells (CGCs or C1) and FMRFamidergic Pleural-Buccal cells (PlBs). Judging from the results of transection experiments, Input-HO originates in the paired cerebral ganglia where its left and right sources coexist. These sources discharge independently or, sometimes, one after the other. Both inputs project to the ipsilateral CGC/PlB only. In turn, the activity of PlB decreases the frequency of Input-HO bouts. We suggest that the Input-HO represents a prominent part of the mechanism controlling feeding behavior in Lymnaea.