Diversity and abundance: the basic properties of neuropeptide action in molluscs

Neuropeptides, the most diverse group of signaling molecules, are responsible for regulating a variety of cellular and behavioral processes in all vertebrate and invertebrate animals. The role played by peptide signals in information processing is fundamentally different from that of conventional neurotransmitters. Neuropeptides may act as neurotransmitters or neuromodulators and are released at either synaptic or non-synaptic sites. Peptide signals control developmental processes, drive specific behaviors or contribute to the mechanisms of learning and memory storage. Co-transmission within or across peptide families, and between peptide and non-peptide signaling molecules, is common; this ensures the great versatility of their action. How these tasks are fulfilled when multiple neuropeptides are released has become an important topic for peptide research. Although our knowledge concerning the physiological and behavioral roles of most of the neuropeptides isolated from molluscs is incomplete, this article provides examples to address the complexity of peptide signaling.