Neuromodulation commonly enables one CPG circuit to generate many different activity patterns. We have found a counter-example while characterizing the actions of the neuropeptide pyrokinin (PK) on the gastric mill (chewing: cycle period ~10s) and pyloric (filtering chewed food: cycle period ~1s) motor circuits in the stomatogastric ganglion (STG) of the crab Cancer borealis. PK (10 -6 M) superfusion to the isolated STG activates a gastric mill rhythm (GMR) that is very similar to the one elicited by selective stimulation of the projection neuron MCN1 (Hertzberg et al., SFN Abstr 33:604.14). This occurs despite the facts that MCN1 neither exhibits PK-like immunolabeling nor requires PK to elicit this rhythm. Moreover, MCN1 releases its cotransmitters rhythmically in the STG (Nusbaum et al., TINS 2001) while PK is tonically present in the bath. Despite the similarity in the evoked GMRs, and the inclusion of the reciprocally inhibitory pattern generator neurons Int1 and LG in both rhythms, MCN1 (Bartos et al...