Neuromodulation commonly enables one neuronal circuit to generate many different activity patterns. We have found a counter-example while characterizing the actions of the neuropeptide pevpyrokinin (PPK) on the gastric mill (chewing) and pyloric (filtering chewed food) motor circuits in the stomatogastric ganglion (STG) of the crab Cancer borealis. Superfusion of PPK1 (10-6M) to the isolated STG activates a gastric mill rhythm (GMR) that is nearly identical to the one elicited by selective stimulation of the identified projection neuron MCN1 (Hertzberg et al., SFN Abstr 33:604.14). This occurs despite the facts that MCN1 neither exhibits PPK-like immunolabeling nor requires PPK to elicit this rhythm, and PPK application does not activate MCN1. Moreover, MCN1 releases its cotransmitters rhythmically in the STG while PPK is tonically present in the bath. Despite the similarity in the evoked GMRs, MCN1 (Bartos et al., 1999 JNeurosci) and PPK1 use distinct mechanisms to activate this rhythm. For example, the p...