The Aplysia Mytilus inhibitory peptide-related peptides: Identification, cloning, processing, distribution, and action

Neuropeptides are a ubiquitous class of signaling molecules. In our attempt to understand the generation of feeding behavior in Aplysia, we have sough t to identify and fully characterize the neuropeptides operating in this sy stem. Preliminary evidence indicated that Mytilus inhibitory peptide (MIP)- like peptides are present and operating in the circuitry that generates fee ding in Aplysia. MIPs were originally isolated from the bivalve mollusc Myt ilus edulis, and related peptides have been identified in other invertebrat e species, but no precursor has been identified. In this study, we describe the isolation and characterization of novel Aplysia MIP-related peptides ( AMRPs) and their precursor. Several AMRPs appear to have some structural an d functional features similar to vertebrate opioid peptides. We use matrix- assisted laser desorption/ionization time-of-flight mass spectrometry to co nfirm that all 14 AMRPs predicted by the precursor are processed in isolate d neurons. Northern analysis, whole-mount in situ hybridization, and immuno histochemistry are used to map the abundant expression of these peptides in the CNS and peripheral tissues such as the digestive tract, vasculature, a nd the reproductive organs. Physiological studies demonstrate that the rank order of the inhibitory actions of these peptides is different for three t arget muscles. These results underscore the importance of using a multidisc iplinary approach to identifying and characterizing the actions of neuropep tides in an effort to gain understanding of their role in systems of intere st. The widespread distribution of the AMRPs indicates that they may be ope rating in many different systems of Aplysia.