Modulation of fictive feeding by dopamine and serotonin in Aplysia

The buccal ganglia of Aplysia contain a central pattern generator (CPG) tha t mediates rhythmic movements of the buccal apparatus during feeding. Activ ity in this: CPG is believed to be regulated, in parr, by extrinsic seroton ergic inputs and by an intrinsic and extrinsic system of putative dopaminer gic cells. The present study investigated the roles of dopamine (DA) and se rotonin (5-HT) in regulating feeding movements of the buccal apparatus and properties of the underlying neural circuitry. Perfusing a semi-intact head preparation with DA (50 mu M) or the metabolic precursor of catecholamines (L-3-4-dihydroxyphenylalanine, DOPA, 250 mu M) induced feeding-like moveme nts of the jaws and radula/odontophore. These DA-induced movements were sim ilar to bites in intact animals. Perfusing with 5-HT (5 mu M) also induced feeding-like movements, but the 5-HT-induced movements were similar to swal lows. In preparations of isolated buccal ganglia, buccal motor programs (BM Ps) that represented at least two different aspects of fictive feeding (i.e ., ingestion and rejection) could be recorded. Bath application of DA (50 m u M increased the frequency of BMPs, in part, by increasing the number of i ngestion-like BMPs. Bath application of 5-HT (5 mu M) did not significantly increase the frequency of BMPs nor did it significantly increase the propo rtion of ingestion-like BMPs being expressed. Many of the cells and synapti c connections within the CPG appeared to be modulated by DA or 5-HT. For ex ample, bath application of DA decreased the excitability of cells B4/5 and B34, which in turn may have contributed to the DA-induced increase in inges tion-like BMPs. In summary, bite-like movements were induced by DA in the s emi-intact preparation, and neural correlates of these DA-induced effects w ere manifest as an increase in ingestion-like BMPs in the isolated ganglia. Swallow-like movements were induced by 5-HT in the semi-intact preparation . Neural correlates of these 5-HT-induced effects were not evident in isola ted buccal ganglia, however.