We are testing the hypothesis that peptide hormones modulate the actions of modulatory projection neurons onto rhythmic motor circuits both presynaptically (projection neuron level) and postsynaptically (circuit level). To this end, we are studying the influence of the hormone crustacean cardioactive peptide (CCAP) on the gastric mill (chewing) circuit in the stomatogastric ganglion (STG) of the crab Cancer borealis. In the crab, the STG is situated in the ophthalmic artery. CCAP is pumped into this artery from the heart to bathe the STG after it is released by the pericardial organs into the lumen of the heart. We showed previously that CCAP superfusion influences the actions of the projection neuron MCN1 (which does not contain CCAP) in several ways (Kirby and Nusbaum, SFN 2002). For example, during the MCN1-elicited gastric mill rhythm (GMR), CCAP (10-6–10-7M) increases the GMR cycle period (~20%) as well as the burst duration (~35%) and number of action potentials/burst (>50%) of the pattern generator ...