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Myomodulin A (MMA) application or stimulation of neuron B16, which releases MMA, increases cAMP levels in the accessory radula closer (ARC) muscle of Aplysia. MMA application also increases cAMP-dependent protein kinase (cAPK) activity in one subcellular compartment of the muscle. These results suggest that at least part of MMA's effects in this system are mediated via the cAPK signal transduction pathway. Since the effects of the small cardioactive peptides (SCPs) on ARC muscle contraction are similar to those of MMA, our results suggest that the convergent physiological effects of MMA and SCPB in this system may be due, in part, to the two peptide neuromodulators utilizing the same signal transduction pathway.

Application of small cardioactive peptide (SCP) or stimulation of motorneuron B15 increases the level of activated cAMP-dependent protein kinase (cAPK) in the ARC muscle. SCP application also appears to induce a translocation of cAPK between different subcellular compartments of the ARC muscle and this translocation is also induced by cAMP addition to muscle homogenates. These results suggest that the actions of SCP in the Aplysia ARC neuromuscular system are mediated via the cAPK signal transduction pathway.

The marine mollusc Aplysia, whose simple nervous system facilitates study of the neural basis of behavior, was used to investigate the role of peptidergic cotransmission in feeding behavior. Several novel modulatory neuropeptides were purified and localized to identified cholinergic motoneurons. Physiological and biochemical studies demonstrated that these peptides are released when the motoneurons fire at frequencies that occur during normal behavior, and that the peptides modify the relationship between muscle contraction amplitude and relaxation rate so as to maintain optimal motor output when the intensity and frequency of feeding behavior change.

Despite their ubiquitous presence in the central and peripheral nervous systems, the behavioral functions of peptide co-transmitters remain to be elucidated. The marine mollusc Aplysia, whose simple nervous system facilitates the study of the neural basis of behavior, was used to investigate the role of peptidergic co-transmission in feeding behavior. Several novel modulatory neuropeptides were purified, and localized to identified cholinergic motorneurons. Physiological and biochemical studies demonstrated that these peptides are released when the motorneurons fire at frequencies that occur during normal behavior, and that the peptides modify the relationship between muscle contraction amplitude and relaxation rate so as to maintain optimal motor output when the intensity and frequency of feeding behavior change.