SCP-containing R20 neurons modulate respiratory pumping in Aplysia

Respiratory pumping in Aplysia consists of transient, synchronous pumping actions of the gill, siphon, mantle shelf, and parapodia. This behavior has previously been shown to be driven by a network of coupled interneurons in the abdominal ganglion, the R25 and the L25 cells. We describe here a pair of electrically coupled cells, the R20 cells, which when active can initiate respiratory pumping or increase its spontaneous rate of occurrence. This action is mediated by a slow, long-lasting excitation of the endogenous burst mechanism of the cells in the R25/L25 network. The R20 cells, which are located in the abdominal ganglion, also make slow inhibitory connections to the RB cells and to the RG cells in that ganglion, and to the gill motoneurons in the branchial ganglion. The R20 cells are immunoreactive to SCPB, a molluscan neuropeptide. Biochemical purification studies demonstrate that each of the R20 cells synthesizes not only SCPB, but also SCPA, a closely related molecule known to be encoded by the same gene as SCPB. The R20 cells also synthesize in abundance several other low-molecular-weight, methionine-containing peptides. The excitatory actions of the R20 cells on the R25/L25 network are mimicked by SCPA and SCPB. However, the inhibitory actions of the R20 cells on the RB cells, the RG cells, and on the cells of the branchial ganglion are not mimicked by the SCPs. Thus, the data support the hypothesis that the R20 cells release SCPA and SCPB and at least one other unidentified transmitter.