Neuropeptides and Their Physiological Functions in Mollusks

Abstract

Neuropeptides have essential functions in the neural regulation of physiological functions of various tissues and organs, as well as of animal behaviors. Many neuropeptides have been identified in mollusks, and investigation of their functions is currently proceeding. In this review, I attempt to give an overview of the neuropeptides in mollusks. Then, regulatory actions of neuropeptides are described with a special reference to reproduction. I chose three neuropeptides: egg-laying hormone (ELH) and caudodorsal cell hormone (CDCH), gonadotropin-releasing hormone (GnRH), and APGWamide. ELH and CDCH are well-investigated peptide hormones that trigger complex egg-laying behaviors in Aplysia and Lymnaea. GnRH, which is a key peptide that induces gonadal maturation and ovulation in mammals, also regulates gonadal maturation in bivalves and cephalopods. However, evidence suggests that GnRH also mediates other activities such as feeding and locomotion in mollusks. APGWamide, which regulates the male copulatory activity in freshwater snails, seems to have pheromonal actions in bivalves and cephalopods. These facts collectively emphasize the diverse actions of neuropeptides and peptide hormones on the regulation of reproduction in mollusks.